Patent Application
20230387417
The present disclosure relates to a binder composition for an electrochemical device, a slurry composition for an electrochemical device electrode, an electrode for an electrochemical device, and an electrochemical device.
Electrochemical devices such as lithium ion secondary batteries, electric double-layer capacitors, and lithium ion capacitors have characteristics such as compact size, light weight, high energy-density, and the ability to be repeatedly charged and discharged, and are used in a wide variety of applications.
An electrode used in an electrochemical device normally includes a current collector and an electrode mixed material layer formed on the current collector. This electrode mixed material layer is formed by, for example, applying a slurry composition containing an electrode active material, a binder-containing binder composition, and so forth onto the current collector, and then drying the applied slurry composition.
Thermal runaway may occur in an electrochemical device as a result of an internal short circuit between electrodes. For this reason, attempts have been made to inhibit heat generation and ensure safety in an electrochemical device even when there is an internal short circuit between electrodes.
For example, Patent Literature (PTL) 1 discloses a positive electrode that includes a current collector and a positive electrode mixed material layer and in which the positive electrode mixed material layer contains a positive electrode active material and a melamine-acid salt that is a salt of melamine and an acid. According to PTL 1, safety of a non-aqueous electrolyte solution can be ensured and input-output characteristics and charge-discharge efficiency can be enhanced by using this positive electrode.
PTL 1: WO2014/119315A1
However, there is room for further improvement in the conventional technique described above in terms of, on the one hand, even further inhibiting heat generation in the event of an internal short circuit of an electrochemical device, and also in terms of reducing IV resistance at low temperature (hereinafter, also referred to simply as “low-temperature IV resistance”) while also improving high-temperature storage characteristics.
Accordingly, an object of the present disclosure is to provide a new technique that can sufficiently inhibit heat generation in the event of an internal short circuit of an electrochemical device while also reducing IV resistance of the electrochemical device at low temperature and improving high-temperature storage characteristics of the electrochemical device.
The inventors conducted diligent investigation with the aim of solving the problem set forth above. The inventors discovered that by forming a battery member such as an electrode using a binder composition that contains, in addition to a binder, a thermally decomposable material having a thermal decomposition temperature within a specific range and specific particle properties, it is possible to sufficiently inhibit heat generation in the event of an internal short circuit of an electrochemical device and to reduce low temperature IV resistance of the electrochemical device while also improving high-temperature storage characteristics of the electrochemical device. In this manner, the inventors completed the present disclosure.
Specifically, the present disclosure aims to advantageously solve the problem set forth above, and a presently disclosed binder composition for an electrochemical device comprises a binder and a thermally decomposable material, wherein the thermally decomposable material includes a foaming agent, and the thermally decomposable material has a thermal decomposition temperature of not lower than 150° C. and not higher than 400° C., a number-average particle diameter of not less than 0.01 μm and not more than 10 μm, a ratio of the number-average particle diameter relative to volume-average particle diameter of not less than 0.05 and not more than 1, and a circularity of not less than 0.05 and not more than 0.95. By using a binder composition that contains a binder and a thermally decomposable material having a thermal decomposition temperature, a number-average particle diameter, a ratio of number-average particle diameter relative to volume-average particle diameter (hereinafter, also referred to simply as a “particle diameter ratio”), and a circularity that are within the ranges set forth above in this manner, it is possible to produce an electrochemical device in which heat generation in the event of an internal short circuit is sufficiently inhibited, that has low low-temperature IV resistance, and that has excellent high-temperature storage characteristics.
The “foaming agent” included in the thermally decomposable material referred to in the present disclosure is a compound that releases an incombustible gas such as nitrogen, carbon dioxide, ammonia, or water vapor through thermal decomposition.
The “thermal decomposition temperature” of the thermally decomposable material referred to in the present disclosure can be measured by a method described in the EXAMPLES section.
The “number-average particle diameter” of the thermally decomposable material referred to in the present disclosure is the particle diameter corresponding to a cumulative value of 50% in a particle size distribution (by number) measured using a laser diffraction particle size distribution analyzer.
The “volume-average particle diameter” of the thermally decomposable material referred to in the present disclosure is the particle diameter corresponding to a cumulative value of 50% in a particle size distribution (by volume) measured using a laser diffraction particle size distribution analyzer.
The “circularity” of the thermally decomposable material referred to in the present disclosure is calculated by a formula: circularity=4πS/L2, where S is the area of a two-dimensional image of the thermally decomposable material and L is the perimeter length thereof. Note that the circularity takes a value of more than 0 and not more than 1 and that the circularity of a perfect circle is 1. The value of the circularity decreases with increasing complexity of the two-dimensional image.
In the presently disclosed binder composition for an electrochemical device, it is preferable that the thermally decomposable material further includes a surfactant and that the thermally decomposable material has a core-shell structure including a core formed of the foaming agent and a shell formed of the surfactant that at least partially covers an outer surface of the core. When the thermally decomposable material has the above-described core-shell structure in which a foaming agent is covered by a surfactant, low-temperature IV resistance of an electrochemical device can be further reduced while also further enhancing high-temperature storage characteristics of the electrochemical device.
Moreover, in the presently disclosed binder composition for an electrochemical device, it is preferable that a proportion constituted by an amount of the surfactant among a total amount of the foaming agent and the surfactant is not less than 0.01 mass % and not more than 10 mass %. When the quantitative ratio of the foaming agent and the surfactant forming the above-described core-shell structure is within the range set forth above, an electrode mixed material layer formed using the binder composition can be caused to closely adhere well to a current collector (i.e., peel strength of an electrode can be increased), and heat generation in the event of an internal short circuit of an electrochemical device can be further inhibited. In addition, low-temperature IV resistance of an electrochemical device can be further reduced while also further enhancing high-temperature storage characteristics of the electrochemical device.
Note that the “proportion constituted by the surfactant among the total amount of the foaming agent and the surfactant” in the thermally decomposable material that is referred to in the present disclosure can be measured by thermogravimetric analysis, thermal decomposition GC-MS, or the like.
In the presently disclosed binder composition for an electrochemical device, it is preferable that the surfactant has a melting point of not lower than 50° C. and not higher than 350° C. When the melting point of the surfactant is within the range set forth above, heat generation in the event of an internal short circuit of an electrochemical device can be further inhibited. In addition, low-temperature IV resistance of an electrochemical device can be further reduced while also further enhancing high-temperature storage characteristics of the electrochemical device.
In the presently disclosed binder composition for an electrochemical device, it is preferable that the surfactant is an anionic surfactant. When an anionic surfactant is used as the surfactant, peel strength of an electrode can be increased, and low-temperature IV resistance of an electrochemical device can be further reduced.
In addition, in the presently disclosed binder composition for an electrochemical device, it is preferable that the surfactant is either or both of an aliphatic carboxylic acid and a salt of an aliphatic carboxylic acid. When an aliphatic carboxylic acid and/or a salt of an aliphatic carboxylic acid (hereinafter, these are also collectively referred to simply as an “aliphatic carboxylic acid (salt)”) is used as the surfactant, peel strength of an electrode can be further increased, and low-temperature IV resistance of an electrochemical device can be even further reduced.
In the presently disclosed binder composition for an electrochemical device, it is preferable that the binder is a polymer including one or more functional groups selected from the group consisting of a carboxy group, a hydroxyl group, a nitrile group, an amino group, an epoxy group, an oxazoline group, a sulfo group, an ester group, and an amide group. When a polymer that includes at least any one of the functional groups set forth above is used as the binder, peel strength of an electrode can be further increased, and low-temperature IV resistance of an electrochemical device can be even further reduced.
In the presently disclosed binder composition for an electrochemical device, it is preferable that the foaming agent is a nitrogenous foaming agent. When a nitrogenous foaming agent is used as the foaming agent included in the thermally decomposable material, heat generation in the event of an internal short circuit of an electrochemical device can be further inhibited. In addition, low-temperature IV resistance of an electrochemical device can be further reduced while also further enhancing high-temperature storage characteristics of the electrochemical device.
Note that the term “nitrogenous foaming agent” as used in the present disclosure refers to a foaming agent that releases nitrogen as an incombustible gas through thermal decomposition.
The presently disclosed binder composition for an electrochemical device may further comprise a solvent.
Moreover, the present disclosure aims to advantageously solve the problem set forth above, and a presently disclosed slurry composition for an electrochemical device electrode comprises: an electrode active material; and the binder composition for an electrochemical device set forth above that contains a solvent. Through an electrode obtained using a slurry composition that contains the presently disclosed binder composition set forth above, it is possible to produce an electrochemical device in which heat generation in the event of an internal short circuit is sufficiently inhibited, that has low low-temperature IV resistance, and that has excellent high-temperature storage characteristics.
Furthermore, the present disclosure aims to advantageously solve the problem set forth above, and a presently disclosed electrode for an electrochemical device comprises an electrode mixed material layer formed using the presently disclosed slurry composition for an electrochemical device electrode set forth above. Through an electrode that includes an electrode mixed material layer formed from the presently disclosed slurry composition set forth above, it is possible to produce an electrochemical device in which heat generation in the event of an internal short circuit is sufficiently inhibited, that has low low-temperature IV resistance, and that has excellent high-temperature storage characteristics.
Also, the present disclosure aims to advantageously solve the problem set forth above, and a presently disclosed electrochemical device comprises the presently disclosed electrode for an electrochemical device set forth above. An electrochemical device that includes the presently disclosed electrode set forth above has excellent safety because heat generation in the event of an internal short circuit is sufficiently inhibited. In addition, this electrochemical device has low low-temperature IV resistance and excellent high-temperature storage characteristics.
According to the present disclosure, it is possible to provide a binder composition for an electrochemical device, a slurry composition for an electrochemical device electrode, and an electrode for an electrochemical device that can sufficiently inhibit heat generation in the event of an internal short circuit of an electrochemical device, reduce IV resistance at low temperature, and cause the electrochemical device to display excellent high-temperature storage characteristics.
Moreover, according to the present disclosure, it is possible to provide an electrochemical device in which heat generation in the event of an internal short circuit is sufficiently inhibited, that has low IV resistance at low temperature, and that has excellent high-temperature storage characteristics.
The following provides a detailed description of embodiments of the present disclosure.
The presently disclosed binder composition for an electrochemical device is a binder composition that is used for an application of producing a device member, such as an electrode, that is a constituent of an electrochemical device. For example, the presently disclosed binder composition for an electrochemical device can be used in production of a slurry composition for an electrochemical device electrode.
Moreover, the presently disclosed slurry composition for an electrochemical device electrode is a slurry composition that is produced using the presently disclosed binder composition for an electrochemical device and can be used in formation of an electrode mixed material layer of an electrode for an electrochemical device.
Furthermore, the presently disclosed electrode for an electrochemical device is an electrode that can be used as an electrode of an electrochemical device such as a lithium ion secondary battery, an electric double-layer capacitor, or a lithium ion capacitor and that includes an electrode mixed material layer formed using the presently disclosed slurry composition for an electrochemical device electrode.
Also, the presently disclosed electrochemical device is an electrochemical device that includes the presently disclosed electrode for an electrochemical device.
(Binder Composition for Electrochemical Device)
The presently disclosed binder composition contains a binder and a thermally decomposable material, and optionally contains a solvent and/or other components. The thermally decomposable material includes a foaming agent and optionally includes a surfactant.
In the presently disclosed binder composition, the aforementioned thermally decomposable material is required to have a thermal decomposition temperature of not lower than 150° C. and not higher than 400° C., a number-average particle diameter of not less than 0.01 μm and not more than 10 μm, a particle diameter ratio of not less than 0.05 and not more than 1, and a circularity of not less than 0.05 and not more than 0.95.
As a result of the thermally decomposable material that includes the foaming agent having a thermal decomposition temperature that is within the specific range set forth above and the specific particle properties set forth above in the presently disclosed binder composition, it is possible to sufficiently inhibit heat generation in the event of an internal short circuit of an electrochemical device, lower low-temperature IV resistance of the electrochemical device, and enhance high-temperature storage characteristics of the electrochemical device by using this binder composition to produce a device member such as an electrode.
<Binder>
Any polymer can be used as the binder without any specific limitations so long as it is a polymer that can display binding capacity inside of an electrochemical device. Suitable examples of polymers that may be used as the binder from a viewpoint of increasing peel strength of an electrode and further reducing low-temperature IV resistance of an electrochemical device include a polymer including mainly aliphatic conjugated diene monomer units or a hydrogenated product thereof (i.e., a diene polymer), a polymer including mainly (meth)acrylic acid ester monomer units (i.e., an acrylic polymer), a polymer including mainly (meth)acrylonitrile (i.e., a nitrile polymer), and a polymer including mainly fluorine-containing monomer units (i.e., a fluoropolymer). Of these polymers, a diene polymer, an acrylic polymer, and a nitrile polymer are more preferable, and a diene polymer is even more preferable.
Note that one type of binder may be used individually, or two or more types of binders may be used in combination in a freely selected ratio.
In the present disclosure, “(meth)acryl” indicates “acryl” and/or “methacryl”, and “(meth)acrylo” indicates “acrylo” and/or “methacrylo”.
Moreover, when a polymer is said to “include a monomer unit” in the present disclosure, this means that “a polymer obtained using that monomer includes a repeating unit derived from the monomer”.
Furthermore, when a polymer is said to “include mainly” a certain monomer unit in the present disclosure, this means that “the proportional content of that monomer unit is more than 50 mass % when the amount of all repeating units included in the polymer is taken to be 100 mass %”.
The proportional content of each type of monomer unit in a polymer referred to in the present disclosure can be measured by a nuclear magnetic resonance (NMR) method such as 1H-NMR.
<<Prescribed Functional Group>>
The binder is preferably a polymer that includes a functional group. The functional group included in the binder may be a carboxy group, a hydroxyl group, a nitrile group, an amino group, an epoxy group, an oxazoline group, a sulfo group, an ester group, or an amide group (hereinafter, these functional groups are also referred to collectively as “prescribed functional groups”). The polymer serving as the binder may include one type of prescribed functional group described above or may include two or more types of prescribed functional groups described above.
When a polymer that includes any of these prescribed functional groups is used as the binder, peel strength of an electrode can be increased, and low-temperature IV resistance of an electrochemical device can be further reduced. Moreover, from a viewpoint of further increasing peel strength of an electrode while also even further reducing low-temperature IV resistance of an electrochemical device, the polymer serving as the binder preferably includes one or more selected from the group consisting of a carboxy group, a hydroxyl group, and a nitrile group, more preferably includes either or both of a carboxy group and a nitrile group, and even more preferably includes both a carboxy group and a nitrile group.
No specific limitations are placed on the method by which any of the prescribed functional groups described above is introduced into the polymer. For example, although a polymer may be produced using a monomer that includes any of the prescribed functional groups described above (prescribed functional group-containing monomer) so as to obtain a polymer that includes a prescribed functional group-containing monomer unit or any polymer may be modified so as to obtain a polymer into which any of the prescribed functional groups described above has been introduced, the former of these methods is preferable. In other words, the polymer serving as the binder preferably includes at least any one of a carboxy group-containing monomer unit, a hydroxyl group-containing monomer unit, a nitrile group-containing monomer unit, an amino group-containing monomer unit, an epoxy group-containing monomer unit, an oxazoline group-containing monomer unit, a sulfo group-containing monomer unit, an ester group-containing monomer unit, and an amide group-containing monomer unit, more preferably includes at least any one of a carboxy group-containing monomer unit, a hydroxyl group-containing monomer unit, and a nitrile group-containing monomer unit, even more preferably includes either or both of a carboxy group-containing monomer unit and a nitrile group-containing monomer unit, and particularly preferably includes both a carboxy group-containing monomer unit and a nitrile group-containing monomer unit.
<<Carboxy Group-Containing Monomer Unit>>
Examples of carboxy group-containing monomers that can form a carboxy group-containing monomer unit include monocarboxylic acids, derivatives of monocarboxylic acids, dicarboxylic acids, acid anhydrides of dicarboxylic acids, and derivatives of dicarboxylic acids and acid anhydrides thereof.
Examples of monocarboxylic acids include acrylic acid, methacrylic acid, and crotonic acid.
Examples of derivatives of monocarboxylic acids include 2-ethylacrylic acid, isocrotonic acid, α-acetoxyacrylic acid, β-trans-aryloxyacrylic acid, and α-chloro-β-E-methoxyacrylic acid.
Examples of dicarboxylic acids include maleic acid, fumaric acid, and itaconic acid.
Examples of derivatives of dicarboxylic acids include methylmaleic acid, dimethylmaleic acid, phenylmaleic acid, chloromaleic acid, dichloromaleic acid, fluoromaleic acid, and maleic acid monoesters such as nonyl maleate, decyl maleate, dodecyl maleate, octadecyl maleate, and fluoroalkyl maleates.
Examples of acid anhydrides of dicarboxylic acids include maleic anhydride, acrylic anhydride, methylmaleic anhydride, and dimethylmaleic anhydride.
An acid anhydride that produces a carboxy group through hydrolysis can also be used as a carboxy group-containing monomer.
Of these examples, acrylic acid and methacrylic acid are preferable as carboxy group-containing monomers. Note that one carboxy group-containing monomer may be used individually, or two or more carboxy group-containing monomers may be used in combination in a freely selected ratio.
<<Hydroxyl Group-Containing Monomer Unit>>
Examples of hydroxyl group-containing monomers that can form a hydroxyl group-containing monomer unit include ethylenically unsaturated alcohols such as (meth)allyl alcohol, 3-buten-1-ol, and 5-hexen-1-ol; alkanol esters of ethylenically unsaturated carboxylic acids such as 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, di-2-hydroxyethyl maleate, di-4-hydroxybutyl maleate, and di-2-hydroxypropyl itaconate; esters of (meth)acrylic acid and polyalkylene glycol represented by a general formula CH2═CRa—COO—(CqH2qO)p—H (where p represents an integer of 2 to 9, q represents an integer of 2 to 4, and Ra represents a hydrogen atom or a methyl group); mono(meth)acrylic acid esters of dihydroxy esters 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; mono(meth)allyl ethers of halogen or hydroxy substituted (poly)alkylene glycols such as glycerin mono(meth)allyl ether, (meth)allyl-2-chloro-3-hydroxypropyl ether, and (meth)allyl-2-hydroxy-3-chloropropyl ether; mono(meth)allyl ethers of polyhydric phenols such as eugenol and isoeugenol, and halogen substituted products thereof; (meth)allyl thioethers of alkylene glycols such as (meth)allyl-2-hydroxyethyl thioether and (meth)allyl-2-hydroxypropyl thioether; and hydroxyl group-containing amides such as N-hydroxymethylacrylamide (N-methylolacrylamide), N-hydroxymethylmethacrylamide, N-hydroxyethylacrylamide, and N-hydroxyethylmethacrylamide. Note that one hydroxyl group-containing monomer may be used individually, or two or more hydroxyl group-containing monomers may be used in combination in a freely selected ratio.
In the present disclosure, “(meth)allyl” indicates “allyl” and/or “methallyl”, and “(meth)acryloyl” indicates “acryloyl” and/or “methacryloyl”.
<<Nitrile Group-Containing Monomer Unit>>
Examples of nitrile group-containing monomers that can form a nitrile group-containing monomer unit include α,β-ethylenically unsaturated nitrile monomers. Specifically, any α,β-ethylenically unsaturated compound that includes a nitrile group can be used as an α,β-ethylenically unsaturated nitrile monomer without any specific limitations. Examples include acrylonitrile; α-halogenoacrylonitriles such as α-chloroacrylonitrile and α-bromoacrylonitrile; and α-alkylacrylonitriles such as methacrylonitrile and α-ethylacrylonitrile. Note that one nitrile group-containing monomer may be used individually, or two or more nitrile group-containing monomers may be used in combination in a freely selected ratio.
<<Amino Group-Containing Monomer Unit>>
Examples of amino group-containing monomers that can form an amino group-containing monomer unit include dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, aminoethyl vinyl ether, and dimethylaminoethyl vinyl ether. Note that one amino group-containing monomer may be used individually, or two or more amino group-containing monomers may be used in combination in a freely selected ratio.
In the present disclosure, “(meth)acrylate” indicates “acrylate” and/or “methacrylate”.
<<Epoxy Group-Containing Monomer Unit>>
Examples of epoxy group-containing monomers that can form an epoxy group-containing monomer unit include monomers that include a carbon-carbon double bond and an epoxy group.
Examples of monomers that include a carbon-carbon double bond and an epoxy group include unsaturated glycidyl ethers such as vinyl glycidyl ether, allyl glycidyl ether, butenyl glycidyl ether, and o-allylphenyl glycidyl ether; monoepoxides of dienes and polyenes such as butadiene monoepoxide, chloroprene monoepoxide, 4,5-epoxy-2-pentene, 3,4-epoxy-1-vinylcyclohexene, and 1,2-epoxy-5,9-cyclododecadiene; alkenyl epoxides such as 3,4-epoxy-1-butene, 1,2-epoxy-5-hexene, and 1,2-epoxy-9-decene; and glycidyl esters of unsaturated carboxylic acids such as glycidyl acrylate, glycidyl methacrylate, glycidyl crotonate, glycidyl-4-heptenoate, glycidyl sorbate, glycidyl linoleate, glycidyl-4-methyl-3-pentenoate, glycidyl ester of 3-cyclohexenecarboxylic acid, and glycidyl ester of 4-methyl-3-cyclohexenecarboxylic acid. Note that one epoxy group-containing monomer may be used individually, or two or more epoxy group-containing monomers may be used in combination in a freely selected ratio.
<<Oxazoline Group-Containing Monomer Unit>>
Examples of oxazoline group-containing monomers that can form an oxazoline group-containing monomer unit include 2-vinyl-2-oxazoline, 2-vinyl-4-methyl-2-oxazoline, 2-vinyl-5-methyl-2-oxazoline, 2-isopropenyl-2-oxazoline, 2-isopropenyl-4-methyl-2-oxazoline, 2-isopropenyl-5-methyl-2-oxazoline, and 2-isopropenyl-5-ethyl-2-oxazoline. Note that one oxazoline group-containing monomer may be used individually, or two or more oxazoline group-containing monomers may be used in combination in a freely selected ratio.
<<Sulfo Group-Containing Monomer Unit>>
Examples of sulfo group-containing monomers that can form a sulfo group-containing monomer unit include vinyl sulfonic acid, methyl vinyl sulfonic acid, (meth)allyl sulfonic acid, styrene sulfonic acid, (meth)acrylic acid 2-sulfoethyl, 2-acrylamido-2-methylpropane sulfonic acid, and 3-allyloxy-2-hydroxypropane sulfonic acid. Note that one sulfo group-containing monomer may be used individually, or two or more sulfo group-containing monomers may be used in combination in a freely selected ratio.
<<Ester Group-Containing Monomer Unit>>
Examples of ester group-containing monomers that can form an ester group-containing monomer unit include (meth)acrylic acid ester monomers. Examples of (meth)acrylic acid ester monomers include acrylic acid alkyl esters such as methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, butyl acrylate (n-butyl acrylate, t-butyl acrylate, etc.), pentyl acrylate, hexyl acrylate, heptyl acrylate, octyl acrylate (2-ethylhexyl acrylate, etc.), nonyl acrylate, decyl acrylate, lauryl acrylate, n-tetradecyl acrylate, and stearyl acrylate; and methacrylic acid alkyl esters such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, butyl methacrylate (n-butyl methacrylate, t-butyl methacrylate, etc.), pentyl methacrylate, hexyl methacrylate, heptyl methacrylate, octyl methacrylate (2-ethylhexyl methacrylate, etc.), nonyl methacrylate, decyl methacrylate, lauryl methacrylate, n-tetradecyl methacrylate, and stearyl methacrylate. Note that one ester group-containing monomer may be used individually, or two or more ester group-containing monomers may be used in combination in a freely selected ratio.
Also note that in a case in which a certain monomer includes a prescribed functional group other than an ester group, that monomer is considered to not be included among ester group-containing monomers in the present disclosure.
<<Amide Group-Containing Monomer Unit>>
Examples of amide group-containing monomers that can form an amide group-containing monomer unit include acrylamide, methacrylamide, and vinylpyrrolidone. Note that one amide group-containing monomer may be used individually, or two or more amide group-containing monomers may be used in combination in a freely selected ratio.
When the amount of all repeating units included in the polymer serving as the binder is taken to be 100 mass %, the proportional content of prescribed functional group-containing monomer units in the polymer is preferably 10 mass % or more, more preferably 20 mass % or more, and even more preferably 30 mass % or more from a viewpoint of increasing peel strength of an electrode and further reducing low-temperature IV resistance of an electrochemical device. The upper limit for the proportional content of prescribed functional group-containing monomer units in the polymer serving as the binder is not specifically limited, but is 100 mass % or less, and can be set as 99 mass % or less, for example.
<<Other Repeating Units>>
The polymer serving as the binder may include repeating units other than the prescribed functional group-containing monomer units described above (i.e., may include other repeating units). Although no specific limitations are placed on such other repeating units, one example thereof is an aliphatic conjugated diene monomer unit in a case in which the polymer is a diene polymer.
Examples of aliphatic conjugated diene monomers that can form an aliphatic conjugated diene monomer unit include 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, and 1,3-pentadiene. One of these aliphatic conjugated diene monomers may be used individually, or two or more of these aliphatic conjugated diene monomers may be used in combination in a freely selected ratio.
Note that the term “aliphatic conjugated diene monomer unit” as used in the present disclosure is inclusive of a structural unit (hydrogenated unit) obtained through hydrogenation of a monomer unit included in a polymer that is obtained using an aliphatic conjugated diene monomer.
Of the aliphatic conjugated diene monomers described above, 1,3-butadiene and isoprene are preferable. In other words, a 1,3-butadiene unit, an isoprene unit, a hydrogenated 1,3-butadiene unit, or a hydrogenated isoprene unit is preferable as an aliphatic conjugated diene monomer unit, and a hydrogenated 1,3-butadiene unit or a hydrogenated isoprene unit is more preferable as an aliphatic conjugated diene monomer unit.
In a case in which the polymer serving as the binder includes an aliphatic conjugated diene monomer unit, from a viewpoint of increasing peel strength of an electrode and further reducing low-temperature IV resistance of an electrochemical device, the proportional content of diene monomer units in the polymer when the amount of all repeating units included in the polymer is taken to be 100 mass % is preferably more than 50 mass %, and more preferably 60 mass % or more, and is preferably 90 mass % or less, preferably 80 mass % or less, and even more preferably 70 mass %.
<<Production Method of Binder>>
No specific limitations are placed on the method by which the binder is produced. For example, the binder, which is a polymer, may be produced by polymerizing a monomer composition containing one monomer or two or more monomers in an aqueous solvent and then optionally performing hydrogenation and/or modification. Note that the proportional content of each monomer in the monomer composition can be determined based on the desired proportional content of each monomer unit in the polymer.
The polymerization method is not specifically limited and may, for example, be any of solution polymerization, suspension polymerization, bulk polymerization, and emulsion polymerization. Also, the polymerization reaction can be any of ionic polymerization, radical polymerization, living radical polymerization, various types of condensation polymerization, addition polymerization, and so forth. A known emulsifier and/or polymerization initiator may be used in the polymerization as necessary. Moreover, the hydrogenation and modification can be performed by known methods.
<Thermally Decomposable Material>
The thermally decomposable material is a component that at least includes a foaming agent that releases incombustible gas through thermal decomposition as previously described. When an electrode mixed material layer is formed using a binder composition that contains a thermally decomposable material including such a foaming agent, the foaming agent in the electrode mixed material layer decomposes (foams) and releases incombustible gas in a situation in which an electrochemical device undergoes thermal runaway and the temperature inside of the device rises. This release of incombustible gas can dilute combustible gas released by electrolyte solution decomposition or the like caused by the high temperature, and thus can prevent spreading of fire. Moreover, foaming of the thermally decomposable material and release of incombustible gas cause destruction of electrode structure (for example, detachment of an electrode active material from a current collector) and cutting of electrical conduction paths. As a result, it is possible to inhibit generation of Joule heat and suppress further temperature rise inside of the electrochemical device.
<<Foaming Agent>>
No specific limitations are placed on the foaming agent that is included in the thermally decomposable material so long as it is a compound that is caused to decompose and release incombustible gas through heat. However, from a viewpoint of further inhibiting heat generation in the event of an internal short circuit of an electrochemical device, further reducing low-temperature IV resistance, and further enhancing high-temperature storage characteristics, nitrogenous foaming agents are preferable, compounds including one or more selected from the group consisting of an amino group, an azo group, a hydrazino group, a hydrazo group, and a nitroso group, derivatives of these compounds, and salts of these compounds and derivatives thereof are more preferable, compounds including either or both of an amino group and an azo group, derivatives of these compounds, and salts of these compounds and derivatives thereof are even more preferable, and melamine compounds are particularly preferable.
Note that one foaming agent may be used individually, or two or more foaming agents may be used in combination in a freely selected ratio.
[Melamine Compound]
Examples of melamine compounds that may be used include melamine, derivatives of melamine, and salts of melamine and derivatives thereof. The melamine or derivative of melamine may, for example, be a compound represented by the following formula (I).
In formula (I), each A represents, independently of one another, a hydroxyl group or NR1R2 (R1 and R2 each represent, independently of each other, a hydrogen atom, a hydrocarbon group, or a hydroxyl group-containing hydrocarbon group; when more than one R1 is present in formula (I), each R1 may be the same or different; and when more than one R2 is present in formula (I), each R2 may be the same or different).
Note that when the hydrocarbon group and the hydroxyl group-containing hydrocarbon group of R1 and R2 include two or more carbon atoms, these groups may have one or more oxygen atoms (—O—) interposed between carbon atoms (however, when two or more oxygen atoms are interposed, these oxygen atoms are not adjacent to one another). The number of carbon atoms in the hydrocarbon group and the hydroxyl group-containing hydrocarbon group of R1 and R2 is not specifically limited but is preferably not less than 1 and not more than 5.
Moreover, examples of salts of melamine and derivatives of melamine include, but are not specifically limited to, sulfates and cyanurates.
From a viewpoint of improving peel strength of an electrode while also further reducing low-temperature IV resistance and further enhancing high-temperature storage characteristics of an electrochemical device, the melamine compound is preferably melamine, ammeline, ammelide, or a salt of any thereof with cyanuric acid, is more preferably melamine or a cyanuric acid salt of melamine (melamine cyanurate), and is even more preferably melamine cyanurate.
Note that one melamine compound may be used individually, or two or more melamine compounds may be used in combination in a freely selected ratio.
[Other Foaming Agents]
Examples of foaming agents other than the above-described melamine compounds that may be used include azobisisobutyronitrile, p-toluenesulfonyl hydrazide, 5-methyl-1H-benzotriazole, oxybis(benzenesulfonyl hydrazide), trihydrazine triazine, azodicarbonamide, hydrazodicarbonamide, dinitrosopentamethylenetetramine, p-toluenesulfonyl semicarbazide, p,p′-oxybis(benzenesulfonyl semicarbazide), and sodium hydrogen carbonate. Note that one other foaming agent may be used individually, or two or more other foaming agents may be used in combination in a freely selected ratio.
<<Thermal Decomposition Temperature>>
The thermal decomposition temperature of the thermally decomposable material is required to be not lower than 150° C. and not higher than 400° C., is preferably 200° C. or higher, and more preferably 300° C. or higher, and is preferably 380° C. or lower, and more preferably 360° C. or lower. When the thermal decomposition temperature of the thermally decomposable material is lower than 150° C., there are instances in which the thermally decomposable material unexpectedly decomposes during normal operation of an electrochemical device or during storage, low-temperature IV resistance of an electrochemical device increases, and high-temperature storage characteristics deteriorate. On the other hand, when the thermal decomposition temperature of the thermally decomposable material is higher than 400° C., it becomes harder to cause the thermally decomposable material to release incombustible gas at an appropriate timing, and an effect of inhibiting heat generation in the event of an internal short circuit that is expected through use of the thermally decomposable material cannot be sufficiently achieved.
Note that the thermal decomposition temperature of the thermally decomposable material can be adjusted by altering the type of foaming agent that is included in the thermally decomposable material, for example.
<<Particle Properties>>
The thermally decomposable material has specific particle properties. More specifically, the thermally decomposable material is required to have a number-average particle diameter of 0.01 μm to 10 μm, a particle diameter ratio of 0.05 to 1, and a circularity of 0.05 to 0.95.
As a result of the thermally decomposable material having the particle properties set forth above, it is possible to sufficiently ensure an effect of inhibiting heat generation in the event of an internal short circuit that is due to the foaming agent included in the thermally decomposable material while also realizing effects of reduction of low-temperature IV resistance and improvement of high-temperature storage characteristics. This is presumed to be because it is possible to control the behavior of the thermally decomposable material during application and drying in formation of an electrode mixed material layer from a slurry composition that contains the binder composition and because the thermally decomposable material (particularly a comparatively small thermally decomposable material) can be inhibited from excessively covering an electrode active material in the obtained electrode mixed material layer.
[Number-Average Particle Diameter]
The number-average particle diameter of the thermally decomposable material is required to be not less than 0.01 μm and not more than 10 μm, is preferably 0.05 μm or more, more preferably 0.3 μm or more, and even more preferably 0.4 μm or more, and is preferably 5 μm or less, more preferably 4 μm or less, and even more preferably 1.2 μm or less. When the number-average particle diameter of the thermally decomposable material is less than 0.01 μm, the effect of inhibiting heat generation in the event of an internal short circuit that is expected through use of the thermally decomposable material cannot be sufficiently achieved, and low-temperature IV resistance of an electrochemical device increases. On the other hand, when the number-average particle diameter of the thermally decomposable material is more than 10 μm, low-temperature IV resistance of an electrochemical device increases.
[Volume-Average Particle Diameter]
The volume-average particle diameter of the thermally decomposable material is preferably 0.01 μm or more, more preferably 0.1 μm or more, even more preferably 1 μm or more, and particularly preferably 2 μm or more, and is preferably 10 μm or less, more preferably 8 μm or less, and even more preferably 5 μm or less. When the volume-average particle diameter of the thermally decomposable material is 0.01 μm or more, low-temperature IV resistance of an electrochemical device can be further reduced, and high-temperature storage characteristics of the electrochemical device can be further improved. On the other hand, when the volume-average particle diameter of the thermally decomposable material is 10 μm or less, heat generation in the event of an internal short circuit of an electrochemical device can be further inhibited, and low-temperature IV resistance of the electrochemical device can be further reduced.
[Particle Diameter Ratio]
A ratio of number-average particle diameter relative to volume-average particle diameter for the thermally decomposable material is required to be not less than 0.05 and not more than 1, is preferably 0.2 or more, more preferably 0.3 or more, and even more preferably 0.5 or more, and is preferably 0.95 or less, and more preferably 0.8 or less. When the particle diameter ratio of the thermally decomposable material is less than 0.05, low-temperature IV resistance of an electrochemical device increases, and high-temperature storage characteristics of the electrochemical device deteriorate. On the other hand, when the particle diameter ratio of the thermally decomposable material is 0.95 or less, an effect of inhibiting heat generation in the event of an internal short circuit that is expected through use of the thermally decomposable material can be more sufficiently achieved.
[Circularity]
The circularity of the thermally decomposable material is required to be not less than 0.05 and not more than 0.95, is preferably 0.5 or more, more preferably 0.6 or more, and even more preferably 0.75 or more, and is preferably 0.9 or less, and more preferably 0.85 or less. When the circularity of the thermally decomposable material is less than 0.05, peel strength of an electrode decreases, low-temperature IV resistance of an electrochemical device increases, and high-temperature storage characteristics of the electrochemical device deteriorate. On the other hand, when the circularity of the thermally decomposable material is more than 0.95, low-temperature IV resistance of an electrochemical device increases, and high-temperature storage characteristics of the electrochemical device deteriorate.
Note that the particle properties of the thermally decomposable material described above can be controlled by altering the production conditions of the thermally decomposable material.
For example, in production of a thermally decomposable material that includes melamine cyanurate as a foaming agent, the number-average particle diameter and the volume-average particle diameter can be controlled by altering the solid content concentration in a reaction for obtaining melamine cyanurate from melamine and cyanuric acid or by performing pulverization with respect to the obtained melamine cyanurate using a bead mill or the like. Moreover, in production of a thermally decomposable material that includes melamine cyanurate as a foaming agent, for example, the circularity can be controlled by altering the pH in a reaction for obtaining melamine cyanurate from melamine and cyanuric acid.
Furthermore, particle properties of the thermally decomposable material can also be controlled by altering granulation conditions in production of the thermally decomposable material.
<<Structure>>
The thermally decomposable material having the thermal decomposition temperature and particle properties set forth above is not specifically limited so long as it includes at least a foaming agent and may be substantially composed of just the foaming agent. However, from a viewpoint of further reducing low-temperature IV resistance of an electrochemical device while also further enhancing high-temperature storage characteristics of the electrochemical device, it is preferable that the thermally decomposable material has a core-shell structure including a core formed of the foaming agent and a shell formed of a surfactant that at least partially covers an outer surface of the core.
Although it is not clear why low-temperature IV resistance of an electrochemical device can be further reduced while also further enhancing high-temperature storage characteristics of the electrochemical device through the thermally decomposable material having the above-described core-shell structure, the reason for this is presumed to be that the thermally decomposable material having a structure in which the foaming agent is covered by the surfactant does not excessively adsorb to an electrode active material, and, as a result, electrical contact among the electrode active material can be sufficiently ensured.
[Surfactant]
Examples of surfactants that can form the shell covering the foaming agent that constitutes the core include anionic surfactants, non-ionic surfactants, and cationic surfactants.
The anionic surfactant may, for example, be an aliphatic carboxylic acid (salt) such as lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid, or a metal salt thereof (sodium salt, lithium salt, potassium salt, calcium salt, magnesium salt, aluminum salt, or zinc salt); an alkyl sulfate salt such as sodium 2-ethylhexyl sulfate or sodium lauryl sulfate; a dialkyl sulfosuccinate salt such as sodium bis(2-ethylhexyl) sulfosuccinate; or an alkyl benzene sulfonic acid salt.
The non-ionic surfactant may, for example, be an ether-type non-ionic surfactant such as polyoxyethylene lauryl ether, polyoxyethylene stearyl ether, or polyoxyethylene 2-ethylhexyl ether; or an ester-type non-ionic surfactant such as polyoxyethylene monolaurate, polyoxyethylene monostearate, sorbitan monostearate, sorbitan monolaurate, sorbitan trioleate, or glyceryl stearate.
The cationic surfactant may, for example, be an amine salt-type cationic surfactant such as tetradecylamine acetate or octadecylamine acetate; or a trimethyl-type cationic surfactant such as dodecyltrimethylammonium chloride or octadecyltrimethylammonium chloride.
One surfactant may be used individually, or two or more surfactants may be used in combination in a freely selected ratio. From a viewpoint of increasing peel strength of an electrode and further reducing low-temperature IV resistance of an electrochemical device, the surfactant is preferably an anionic surfactant, more preferably an aliphatic carboxylic acid (salt), even more preferably sodium stearate or lithium stearate, and particularly preferably sodium stearate.
The molecular weight of the surfactant is preferably 50 g/mol or more, more preferably 100 g/mol or more, and even more preferably 250 g/mol or more, and is preferably 1,000 g/mol or less, more preferably 800 g/mol or less, and even more preferably 500 g/mol or less. High-temperature storage characteristics of an electrochemical device can be further improved when the molecular weight of the surfactant is 50 g/mol or more, whereas low-temperature IV resistance of an electrochemical device can be further reduced when the molecular weight of the surfactant is 1,000 g/mol or less.
In addition, the melting point of the surfactant is preferably 50° C. or higher, more preferably 200° C. or higher, and even more preferably 250° C. or higher, and is preferably 350° C. or lower, more preferably 330° C. or lower, and even more preferably 320° C. or lower. When the melting point of the surfactant is 50° C. or higher, low-temperature IV resistance of an electrochemical device can be further reduced while also further improving high-temperature storage characteristics of the electrochemical device. On the other hand, when the melting point of the surfactant is 350° C. or lower, heat generation in the event of an internal short circuit of an electrochemical device can be further inhibited.
The amount of the surfactant that is included in the thermally decomposable material having the above-described core-shell structure when the total amount of the foaming agent and the surfactant (normally the amount of the overall thermally decomposable material) is taken to be 100 mass % is preferably 0.01 mass % or more, more preferably 0.1 mass % or more, and even more preferably 1 mass % or more, and is preferably 10 mass % or less, more preferably 5 mass % or less, and even more preferably 4 mass % or less. When the proportion constituted by the surfactant among the total of the foaming agent and the surfactant is 0.01 mass % or more, low-temperature IV resistance of an electrochemical device can be further reduced while also further improving high-temperature storage characteristics of the electrochemical device. On the other hand, when the proportion constituted by the surfactant among the total of the foaming agent and the surfactant is 10 mass % or less, peel strength of an electrode can be increased while also further inhibiting heat generation in the event of an internal short circuit of an electrochemical device.
<<Quantitative Ratio of Binder and Decomposable Material>>
The amount of the thermally decomposable material contained in the binder composition is preferably 50 parts by mass or more, more preferably 70 parts by mass or more, even more preferably 90 parts by mass or more, and particularly preferably 95 parts by mass or more per 100 parts by mass of the binder, and is preferably 150 parts by mass or less, more preferably 130 parts by mass or less, even more preferably 110 parts by mass or less, and particularly preferably 105 parts by mass or less per 100 parts by mass of the binder. When the content of the thermally decomposable material in the binder composition is 50 parts by mass or more per 100 parts by mass of the binder, peel strength of an electrode can be increased while also further inhibiting heat generation in the event of an internal short circuit of an electrochemical device. On the other hand, when the content of the thermally decomposable material in the binder composition is 150 parts by mass or less per 100 parts by mass of the binder, low-temperature IV resistance of an electrochemical device can be further reduced while also further improving high-temperature storage characteristics of the electrochemical device.
<<Production Method of Thermally Decomposable Material>>
The thermally decomposable material can be produced, for example, through granulation of a composition (hereinafter, referred to as a “composition for a thermally decomposable material”) that contains at least a foaming agent and that optionally contains a surfactant and a dispersion medium.
The preferred quantitative ratio of the foaming agent and the surfactant in the composition for a thermally decomposable material may be the same as the preferred ratio of the foaming agent (core) and the surfactant (shell) in the desired thermally decomposable material.
Note that a dispersion medium can be used as appropriate depending on the method of granulation, etc. and that the type of dispersion medium can also be selected as appropriate from water and known organic solvents depending on the method of granulation.
[Granulation]
No specific limitations are placed on the method of granulation by which a thermally decomposable material is obtained from the composition for a thermally decomposable material described above so long as it is possible to obtain a thermally decomposable material having the desired particle properties. Examples of methods that may be used include spray granulation, fluidized bed granulation, coagulant precipitation, pH precipitation, dry mixing, and drying and granulation performed after wet mixing. Of these methods, spray granulation is preferable.
In spray granulation, a slurry composition (hereinafter, referred to as a “slurry composition for a thermally decomposable material”) that contains a foaming agent and a dispersion medium and that optionally contains a surfactant can be spray dried as a composition for a thermally decomposable material so as to obtain a thermally decomposable material having the desired particle properties.
The method by which the slurry composition for a thermally decomposable material is produced is not specifically limited and can be by performing mixing of the above-described components using a known mixer. Examples of known mixers that may be used include a ball mill, a sand mill, a bead mill, a pigment disperser, a grinding machine, an ultrasonic disperser, a homogenizer, and a planetary mixer. The mixing is normally performed in a range of from room temperature to 80° C. for from 10 minutes to several hours.
The slurry composition for a thermally decomposable material obtained by this mixing is sprayed using a spray dryer so as to dry droplets of the sprayed slurry composition for a thermally decomposable material inside a drying tower. This makes it possible to obtain a thermally decomposable material having a particulate form. Note that in a case in which the slurry composition for a thermally decomposable material contains a surfactant, a thermally decomposable material in which the outer surface of the foaming agent is at least partially covered by the surfactant can be obtained through the surfactant physically and/or chemically bonding to the outer surface of the foaming agent contained in the droplets. The temperature of the sprayed slurry composition for a thermally decomposable material is normally room temperature, but the slurry composition for a thermally decomposable material may be heated to a higher temperature than room temperature. Moreover, the hot air temperature during spray drying is preferably lower than the thermal decomposition temperature of the thermally decomposable material. For example, the hot air temperature may be not lower than 80° C. and not higher than 250° C., and is preferably not lower than 100° C. and not higher than 200° C.
<Solvent>
The solvent that is optionally contained in the binder composition may be water or an organic solvent, but is preferably an organic solvent. Examples of organic solvents that can be used include acetonitrile, N-methylpyrrolidone, acetylpyridine, cyclopentanone, N,N-dimethylacetamide, dimethylformamide, dimethyl sulfoxide, methylformamide, methyl ethyl ketone, furfural, and ethylenediamine. Of these organic solvents, N-methyl-2-pyrrolidone (NMP) is particularly preferable from viewpoints of ease of handling, safety, ease of synthesis, and so forth.
Note that one solvent may be used individually, or two or more solvents may be used in combination in a freely selected ratio.
<Other Components>
Besides the components described above, the presently disclosed binder composition may contain known components such as conductive materials, cross-linkers, reinforcing materials, antioxidants, dispersants, rheology modifiers, and additives for electrolyte solution having a function of inhibiting decomposition of electrolyte solution.
Examples of conductive materials that can be used include, but are not specifically limited to, conductive carbon materials such as carbon black (for example, acetylene black, Ketjenblack® (Ketjenblack is a registered trademark in Japan, other countries, or both), furnace black, etc.), single-walled or multi-walled carbon nanotubes (multi-walled carbon nanotubes are inclusive of cup-stacked carbon nanotubes), carbon nanohorns, vapor-grown carbon fiber, milled carbon fiber obtained through pyrolysis and subsequent pulverization of polymer fiber, single-layer or multi-layer graphene, and carbon non-woven fabric sheet obtained through pyrolysis of non-woven fabric formed of polymer fiber; and fibers, foils, etc. of various metals.
One other component may be used individually, or two or more other components may be used in combination in a freely selected ratio.
<Production Method of Binder Composition>
The presently disclosed binder composition set forth above can be produced by mixing the above-described binder, the above-described thermally decomposable material, and a solvent and/or other components that are used as necessary by a known method. The mixing method used to obtain the binder composition is not specifically limited and may involve using a typical mixing device such as a disper blade, a mill, or a kneader, for example.
(Slurry Composition for Electrochemical Device Electrode)
The presently disclosed slurry composition for an electrochemical device electrode is a composition in the form of a slurry that contains at least an electrode active material and the presently disclosed binder composition for an electrochemical device set forth above that contains a solvent. In other words, the presently disclosed slurry composition is normally a composition having an electrode active material, the above-described binder, the above-described thermally decomposable material, and the above-described other components that are optionally used dissolved and/or dispersed in the above-described solvent. Through an electrode that includes an electrode mixed material layer formed from the presently disclosed slurry composition, it is possible to produce an electrochemical device in which heat generation in the event of an internal short circuit is sufficiently inhibited, that has low low-temperature IV resistance, and that has excellent high-temperature storage characteristics as a result of the presently disclosed slurry composition containing the presently disclosed binder composition set forth above.
<Electrode Active Material>
The electrode active material is a material that gives and receives electrons in an electrode of an electrochemical device. In a case in which the electrochemical device is a lithium ion secondary battery, for example, the electrode active material is normally a material that can occlude and release lithium.
Although the following describes, as one example, a case in which the slurry composition for an electrochemical device electrode is a slurry composition for a lithium ion secondary battery electrode, the present disclosure is not limited to the following example.
Examples of positive electrode active materials for lithium ion secondary batteries that may be used include known positive electrode active materials such as lithium-containing cobalt oxide (lithium cobalt oxide; LiCoO2), lithium manganate (LiMn2O4), lithium-containing nickel oxide (LiNiO2), a lithium-containing complex oxide of Co—Ni—Mn (Li(Co Mn Ni)O2), a lithium-containing complex oxide of Ni—Mn—Al, a lithium-containing complex oxide of Ni—Co—Al, olivine-type lithium iron phosphate (LiFePO4), olivine-type lithium manganese phosphate (LiMnPO4), a Li2MnO3—LiNiO2-based solid solution, a lithium-rich spinel compound represented by Li1+xMn2−xO4 (0<x<2), Li[Ni0.17Li0.2Co0.07Mn0.56]O2, and LiNi0.5Mn1.5O4 without any specific limitations.
The amount and particle diameter of the positive electrode active material are not specifically limited and may be the same as those of conventionally used positive electrode active materials.
Examples of negative electrode active materials for lithium ion secondary batteries that may be used include carbon-based negative electrode active materials, metal-based negative electrode active materials, and negative electrode active materials that are combinations thereof.
A carbon-based negative electrode active material can be defined as an active material that contains carbon as its main framework and into which lithium can be inserted (also referred to as “doping”). Examples of carbon-based negative electrode active materials include carbonaceous materials and graphitic materials.
Examples of carbonaceous materials include graphitizing carbon and non-graphitizing carbon, typified by glassy carbon, which has a structure similar to an amorphous structure.
The graphitizing carbon may be a carbon material made using tar pitch obtained from petroleum or coal as a raw material. Specific examples of graphitizing carbon include coke, mesocarbon microbeads (MCMB), mesophase pitch-based carbon fiber, and pyrolytic vapor-grown carbon fiber.
Examples of the non-graphitizing carbon include pyrolyzed phenolic resin, polyacrylonitrile-based carbon fiber, quasi-isotropic carbon, pyrolyzed furfuryl alcohol resin (PFA), and hard carbon.
Examples of graphitic materials include natural graphite and artificial graphite.
Examples of the artificial graphite include artificial graphite obtained by heat-treating carbon containing graphitizing carbon mainly at 2800° C. or higher, graphitized MCMB obtained by heat-treating MCMB at 2000° C. or higher, and graphitized mesophase pitch-based carbon fiber obtained by heat-treating mesophase pitch-based carbon fiber at 2000° C. or higher.
A metal-based negative electrode active material is an active material that contains metal, the structure of which usually contains an element that allows insertion of lithium, and that has a theoretical electric capacity per unit mass of 500 mAh/g or more when lithium is inserted. Examples of metal-based active materials that may be used include lithium metal; a simple substance of metal that can form a lithium alloy (for example, Ag, Al, Ba, Bi, Cu, Ga, Ge, In, Ni, P, Pb, Sb, Si, Sn, Sr, Zn, or Ti); alloys of the simple substance of metal; and oxides, sulfides, nitrides, silicides, carbides, and phosphides of lithium metal, the simple substance of metal, and the alloys of the simple substance of metal. Of these metal-based negative electrode active materials, active materials containing silicon (silicon-based negative electrode active materials) are preferred. One reason for this is that the capacity of a lithium ion secondary battery can be increased through use of a silicon-based negative electrode active material.
Examples of silicon-based negative electrode active materials include silicon (Si), a silicon-containing alloy, SiO, SiOx, and a composite material of conductive carbon and a Si-containing material obtained by coating or combining the Si-containing material with the conductive carbon. Note that one of these silicon-based negative electrode active materials may be used individually, or two or more of these silicon-based negative electrode active materials may be used in combination.
The amount and particle diameter of the negative electrode active material are not specifically limited and may be the same as those of conventionally used negative electrode active materials.
<Binder Composition>
The presently disclosed binder composition for an electrochemical device is used as a binder composition.
The proportional content of the binder composition in the slurry composition for an electrochemical device electrode is preferably an amount such that the amount of the binder is 0.3 parts by mass or more, and more preferably 0.7 parts by mass or more per 100 parts by mass of the electrode active material, and is preferably an amount such that the amount of the binder is 5 parts by mass or less, more preferably 4 parts by mass or less, and even more preferably 3 parts by mass or less per 100 parts by mass of the electrode active material. Peel strength of an electrode can be improved through the slurry composition containing the binder composition in an amount such that the amount of the binder is 0.3 parts by mass or more. On the other hand, the proportion constituted by the electrode active material among an electrode mixed material layer can be ensured and the capacity of an electrochemical device can be sufficiently increased through the slurry composition containing the binder composition in an amount such that the amount of the binder is 5 parts by mass or less.
<Other Components>
Other components that can be contained in the slurry composition are not specifically limited and may be the same as the other components that can be contained in the binder composition set forth above. One other component may be used individually, or two or more other components may be used in combination in a freely selected ratio.
<Production Method of Slurry Composition>
The slurry composition set forth above can be produced by mixing the above-described components. Specifically, the slurry composition can be produced by mixing the above-described components and an optionally added solvent using a mixer such as a ball mill, a sand mill, a bead mill, a pigment disperser, a grinding machine, an ultrasonic disperser, a homogenizer, a planetary mixer, or a FILMIX. The solvent that is optionally added during production of the slurry composition can be any of the same solvents as described in the section relating to the binder composition.
(Electrode for Electrochemical Device)
The presently disclosed electrode for an electrochemical device includes an electrode mixed material layer formed using the presently disclosed slurry composition for an electrochemical device electrode set forth above. For example, the presently disclosed electrode may include a current collector and an electrode mixed material layer formed on the current collector, and this electrode mixed material layer may be a dried product of the presently disclosed slurry composition for an electrochemical device electrode. Note that the presently disclosed electrode for an electrochemical device may optionally include other layers (for example, an adhesive layer and/or a porous membrane layer) besides the electrode mixed material layer. The presently disclosed electrode for an electrochemical device can be used as an electrode of an electrochemical device such as a lithium ion secondary battery, an electric double-layer capacitor, or a lithium ion capacitor.
By using the presently disclosed electrode, it is possible to produce an electrochemical device in which heat generation in the event of an internal short circuit is sufficiently inhibited, that has low low-temperature IV resistance, and that has excellent high-temperature storage characteristics.
<Current Collector>
The current collector included in the electrode for an electrochemical device is not specifically limited so long as it is a material having electrical conductivity and electrochemical durability, and may be selected in accordance with the type of electrochemical device. In a case in which the electrode for an electrochemical device is an electrode for a lithium ion secondary battery, the material forming the current collector may be iron, copper, aluminum, nickel, stainless steel, titanium, tantalum, gold, platinum, or the like. Of these materials, copper foil is particularly preferable as a current collector used for a negative electrode. On the other hand, aluminum foil is particularly preferable as a material forming a current collector used for a positive electrode.
Note that one of these materials may be used individually, or two or more of these materials may be used in combination in a freely selected ratio.
<Electrode Mixed Material Layer>
The electrode mixed material layer formed using the presently disclosed slurry composition for an electrochemical device electrode may be a dried product of the slurry composition for an electrochemical device electrode, for example.
Components that are contained in the electrode mixed material layer are components that were contained in the presently disclosed slurry composition for an electrochemical device electrode, and the preferred ratio of these components is the same as the preferred ratio of the components in the presently disclosed slurry composition for an electrochemical device electrode.
<Production Method of Electrode for Electrochemical Device>
No specific limitations are placed on the method by which the electrode for an electrochemical device is produced. For example, the electrode for an electrochemical device can be produced through a step of applying the slurry composition for an electrochemical device electrode onto at least one side of the current collector (application step) and a step of drying the slurry composition for an electrochemical device electrode that has been applied onto at least one side of the current collector so as to form an electrode mixed material layer on the current collector (drying step).
<<Application Step>>
The method by which the slurry composition for an electrochemical device electrode is applied onto the current collector is not specifically limited and may be a commonly known method. Specific examples of application methods that can be used include doctor blading, dip coating, reverse roll coating, direct roll coating, gravure coating, extrusion coating, and brush coating. The thickness of the slurry coating on the current collector after application but before drying may be set as appropriate in accordance with the thickness of the electrode mixed material layer to be obtained through drying.
<<Drying Step>>
The method by which the slurry composition for an electrochemical device electrode on the current collector is dried is not specifically limited and may be a commonly known method. Examples of drying methods that can be used include drying by warm, hot, or low-humidity air; drying in a vacuum; and drying by irradiation with infrared light, electron beams, or the like. Through drying of the slurry composition for an electrochemical device electrode on the current collector in this manner, an electrode mixed material layer can be formed on the current collector to thereby obtain an electrode for an electrochemical device that includes the current collector and the electrode mixed material layer.
After the drying step, the electrode mixed material layer may be further subjected to a pressing process, such as mold pressing or roll pressing. The pressing process can improve peel strength of the electrode.
(Electrochemical Device)
A feature of the presently disclosed electrochemical device is that it includes the electrode for an electrochemical device set forth above. The presently disclosed electrochemical device may, for example, be a lithium ion secondary battery, an electric double-layer capacitor, or a lithium ion capacitor, but is not specifically limited thereto, and is preferably a lithium ion secondary battery. As a result of the presently disclosed electrochemical device including the presently disclosed electrode, heat generation in the event of an internal short circuit is sufficiently inhibited, and the electrochemical device maintains a high level of safety. In addition, the presently disclosed electrochemical device has low IV resistance at low temperature and also has excellent high-temperature storage characteristics.
Although the following describes, as one example, a case in which the electrochemical device is a lithium ion secondary battery, the present disclosure is not limited to the following example. A lithium ion secondary battery that is an example of the presently disclosed electrochemical device normally includes electrodes (positive electrode and negative electrode), an electrolyte solution, and a separator, and has the presently disclosed electrode for an electrochemical device as at least one of the positive electrode and the negative electrode.
<Electrodes>
Examples of electrodes other than the presently disclosed electrode for an electrochemical device set forth above that can be used in the lithium ion secondary battery that is an example of the presently disclosed electrochemical device include known electrodes without any specific limitations. Specifically, an electrode obtained by forming an electrode mixed material layer on a current collector by a known production method may be used as an electrode other than the electrode for an electrochemical device set forth above.
<Electrolyte Solution>
The electrolyte solution is normally an organic electrolyte solution obtained by dissolving a supporting electrolyte in an organic solvent. The supporting electrolyte may, for example, be a lithium salt. Examples of lithium salts that can be used include LiPF6, LiAsF6, LiBF4, LiSbF6, LiAlCl4, LiClO4, CF3SO3Li, C4F9SO3Li, CF3COOLi, (CF3CO)2NLi, (CF3SO2)2NLi, and (C2F5SO2)NLi. Of these lithium salts, LiPF6, LiClO4, and CF3SO3Li are preferable because they readily dissolve in solvents and exhibit a high degree of dissociation, with LiPF6 being particularly preferable. One electrolyte may be used individually, or two or more electrolytes may be used in combination in a freely selected ratio. In general, lithium ion conductivity tends to increase when a supporting electrolyte having a high degree of dissociation is used. Therefore, lithium ion conductivity can be adjusted through the type of supporting electrolyte that is used.
The organic solvent used in the electrolyte solution is not specifically limited so long as the supporting electrolyte can dissolve therein. Examples of suitable organic solvents include carbonates such as dimethyl carbonate (DMC), ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), butylene carbonate (BC), and methyl ethyl carbonate (EMC); esters such as γ-butyrolactone and methyl formate; ethers such as 1,2-dimethoxyethane and tetrahydrofuran; and sulfur-containing compounds such as sulfolane and dimethyl sulfoxide. Furthermore, a mixture of such solvents may be used. Of these solvents, carbonates are preferable due to having a high permittivity and a wide stable potential region, and a mixture of ethylene carbonate and ethyl methyl carbonate is more preferable.
The concentration of the electrolyte in the electrolyte solution can be adjusted as appropriate and is, for example, preferably 0.5 mass % to 15 mass %, more preferably 2 mass % to 13 mass %, and even more preferably 5 mass % to 10 mass %. Known additives such as vinylene carbonate, fluoroethylene carbonate, and ethyl methyl sulfone may be added to the electrolyte solution.
<Separator>
Examples of separators that can be used include, but are not specifically limited to, those described in JP2012-204303A. Of these separators, a microporous membrane formed of polyolefinic (polyethylene, polypropylene, polybutene, or polyvinyl chloride) resin is preferred since such a membrane can reduce the total thickness of the separator, which increases the ratio of electrode active material particles in the lithium ion secondary battery, and consequently increases the volumetric capacity. A functional layer-equipped separator that includes a functional layer (porous membrane layer or adhesive layer) at one side or both sides of a separator substrate may be used as the separator.
<Production Method of Lithium Ion Secondary Battery>
The lithium ion secondary battery in accordance with the present disclosure can be produced by, for example, stacking the positive electrode and the negative electrode with the separator in-between, performing rolling, folding, or the like of the resultant laminate in accordance with the battery shape, as necessary, to place the laminate in a battery container, injecting the electrolyte solution into the battery container, and sealing the battery container. In order to prevent pressure increase inside the secondary battery and occurrence of overcharging or overdischarging, an overcurrent preventing device such as a fuse or a PTC device; an expanded metal; or a lead plate may be provided as necessary. The shape of the secondary battery may be a coin type, button type, sheet type, cylinder type, prismatic type, flat type, or the like.
The following provides a more specific description of the present disclosure based on examples. However, the present disclosure is not limited to the following examples. In the following description, “%” and “parts” used in expressing quantities are by mass, unless otherwise specified.
Moreover, in the case of a polymer that is produced through copolymerization of a plurality of types of monomers, the proportion in the polymer constituted by a monomer unit that is formed through polymerization of a given monomer is normally, unless otherwise specified, the same as the ratio (charging ratio) of the given monomer among all monomers used in polymerization of the polymer.
In the examples and comparative examples, the following methods were used to measure or evaluate the thermal decomposition temperature, average particle diameter (number-average particle diameter, volume-average particle diameter, and particle diameter ratio), and circularity of a thermally decomposable material, the proportion constituted by the amount of a surfactant among the total amount of a foaming agent and a surfactant in a thermally decomposable material, the peel strength of a positive electrode, and the low-temperature IV resistance, high-temperature storage characteristics, and inhibition of heat generation in the event of an internal short circuit of a lithium ion secondary battery.
<Thermal Decomposition Temperature>
The weight of a thermally decomposable material was measured while the thermally decomposable material was heated from 25° C. to 500° C. at a heating rate of 10° C./min under a nitrogen atmosphere in thermogravimetric analysis using a thermogravimetry/differential thermal analyzer (produced by Hitachi High-Tech Science Corporation; product name: TG/DTA7200). The temperature (5% weight loss temperature) at which the measured weight reached 95% of the weight at the start of measurement (25° C.) was taken to be the thermal decomposition temperature of the thermally decomposable material.
The number-average particle diameter and volume-average particle diameter of a thermally decomposable material were measured using a laser diffraction particle size distribution analyzer (produced by MicrotracBEL Corp.; product name: MT3000II). In addition, the particle diameter ratio was calculated.
The circularity of a thermally decomposable material was evaluated by using a particle image analyzer (produced by Malvern Panalytical Ltd.; product name: Morphologi® G3 (Morphologi is a registered trademark in Japan, other countries, or both)) to measure the circularity of each of 1,000 particles and calculate an average value of these measurements.
The weight of a thermally decomposable material was measured while the thermally decomposable material was heated from 25° C. to 500° C. at a heating rate of 10° C./min under a nitrogen atmosphere in thermogravimetric analysis using a thermogravimetry/differential thermal analyzer (produced by Hitachi High-Tech Science Corporation; product name: TG/DTA7200). Weight loss other than that at the thermal decomposition temperature of a foaming agent was calculated as the content of a surfactant.
A positive electrode was cut out as 100 mm in length and 10 mm in width to obtain a test specimen. The test specimen was arranged with the surface at which the positive electrode mixed material layer was present facing downward, and cellophane tape (tape in accordance with JIS Z1522) was affixed to the surface of the positive electrode mixed material layer. One end of the current collector was pulled in a perpendicular direction at a pulling speed of 60 mm/min so as to peel off the current collector, and the stress during this peeling was measured (note that the cellophane tape was fixed to a test stage). A total of three measurements were made in this manner, and an average value of these measurements was determined and was evaluated by the following standard. A larger value for the average value of the stress indicates that the positive electrode has better peel strength and that there is stronger close adherence between the positive electrode mixed material layer and the current collector.
A lithium ion secondary battery was left at rest at a temperature of 25° C. for 5 hours after injection of electrolyte solution. Next, the lithium ion secondary battery was charged to a cell voltage of 3.65 V by a 0.2C constant-current method at a temperature of 25° C. and was then subjected to 12 hours of aging at a temperature of 60° C. The lithium ion secondary battery was subsequently discharged to a cell voltage of 3.00 V by a 0.2C constant-current method at a temperature of 25° C. Thereafter, the lithium ion secondary battery was CC-CV charged (upper limit cell voltage: 4.35 V) by a 0.2C constant-current method and CC discharged to 3.00 V by a 0.2C constant-current method. This charging and discharging at 0.2C was repeated three times. Thereafter, the lithium ion secondary battery was subjected to an operation of charging to a state of charge (SOC) of 50% at 0.2C in a 25° C. environment, was left at rest in a −10° C. environment for 3 hours, and the voltage V0 thereof was subsequently measured. The lithium ion secondary battery was subsequently subjected to a discharge operation at a discharge rate of 1C, and the voltage V1 thereof 10 seconds after the start of discharge was measured. A value for low-temperature IV resistance (mΩ) (=(V0−V1)/200×1000) was evaluated by the following standard. A smaller value indicates that the lithium ion secondary battery has better low-temperature IV resistance.
A lithium ion secondary battery was left at rest at a temperature of 25° C. for 5 hours after injection of electrolyte solution. Next, the lithium ion secondary battery was charged to a cell voltage of 3.65 V by a 0.2C constant-current method at a temperature of 25° C. and was then subjected to 12 hours of aging at a temperature of 60° C. The lithium ion secondary battery was subsequently discharged to a cell voltage of 3.00 V by a 0.2C constant-current method at a temperature of 25° C. Thereafter, the lithium ion secondary battery was CC-CV charged (upper limit cell voltage: 4.35 V) by a 0.2C constant-current method and CC discharged to 3.00 V by a 0.2C constant-current method. This charging and discharging at 0.2C was repeated three times. The discharge capacity obtained in the final repetition of charging and discharging was taken to be X1.
The lithium ion secondary battery was subsequently charged to a cell voltage of 4.35 V at 25° C. and was left in that state in an environment having a temperature of 45° C. for 10 days. Thereafter, the lithium ion secondary battery was discharged to a cell voltage of 3.00 V by a 0.2C constant-current method at 25° C. This discharge capacity was taken to be X2.
A capacity maintenance rate expressed by ΔC=(X2/X1)×100(%) was determined using the discharge capacity X1 and the discharge capacity X2 and was evaluated by the following standard. A larger value for the capacity maintenance rate ΔC indicates that the lithium ion secondary battery has better high-temperature storage characteristics.
A lithium ion secondary battery of each example or comparative example was left at rest at a temperature of 25° C. for 5 hours after injection of electrolyte solution. Next, the lithium ion secondary battery was charged to a cell voltage of 3.65 V by a 0.2C constant-current method at a temperature of 25° C. and was then subjected to 12 hours of aging at a temperature of 60° C. The lithium ion secondary battery was subsequently discharged to a cell voltage of 3.00 V by a 0.2C constant-current method at a temperature of 25° C. Thereafter, the lithium ion secondary battery was CC-CV charged (upper limit cell voltage: 4.35 V) by a 0.2C constant-current method and CC discharged to 3.00 V by a 0.2C constant-current method. This charging and discharging at 0.2C was repeated three times. The lithium ion secondary battery was subsequently charged to 4.35 V (cut-off condition: 0.02C) by a constant-voltage constant-current (CC-CV) method at a charge rate of 0.2C in a 25° C. atmosphere. Thereafter, an iron nail of 3 mm in diameter and 10 cm in length was caused to pierce the lithium ion secondary battery at an approximately central location at a speed of 5 m/min so as to cause a forced short circuit. A forced short circuit was performed for each of 5 lithium ion secondary batteries (test subjects) prepared by the same operations, and an evaluation was made by the following standard based on the number of test subjects in which rupturing and ignition did not occur. A larger number of test subjects in which rupturing and ignition do not occur indicates that the lithium ion secondary battery has better inhibition of heat generation in the event of an internal short circuit.
An autoclave equipped with a stirrer was charged with 240 parts of deionized water, 2.5 parts of sodium alkylbenzene sulfonate, 30 parts of acrylonitrile as a nitrile group-containing monomer, 5 parts of methacrylic acid as a carboxy group-containing monomer, and 0.25 parts of t-dodecyl mercaptan as a chain transfer agent in this order, and then the inside of a bottle was purged with nitrogen. Thereafter, 65 parts of 1,3-butadiene as an aliphatic conjugated diene monomer was injected, 0.25 parts of ammonium persulfate was added, and a polymerization reaction was performed at a reaction temperature of 40° C. This yielded a polymer including acrylonitrile units, methacrylic acid units, and 1,3-butadiene units. The polymerization conversion rate was 85%.
The obtained polymer was adjusted to a total solid content concentration of 12% with water, 400 mL (total solid content: 48 g) of the solution was loaded into a stirrer-equipped autoclave of 1 L in capacity, and dissolved oxygen in the solution was removed by passing nitrogen gas for 10 minutes. Thereafter, 75 mg of palladium acetate as a hydrogenation reaction catalyst was dissolved in 180 mL of deionized water to which 4 molar equivalents of nitric acid relative to Pd had been added and was added into the autoclave. The system was purged twice with hydrogen gas, and then the contents of the autoclave were heated to 50° C. in a state in which the pressure was raised to 3 MPa with hydrogen gas, and a hydrogenation reaction (first stage hydrogenation reaction) was performed for 6 hours.
Next, the autoclave was restored to atmospheric pressure, and 25 mg of palladium acetate as a hydrogenation reaction catalyst was dissolved in 60 mL of water to which 4 molar equivalents of nitric acid relative to Pd had been added and was added into the autoclave. The system was purged twice with hydrogen gas, and then the contents of the autoclave were heated to 50° C. in a state in which the pressure was raised to 3 MPa with hydrogen gas, and a hydrogenation reaction (second stage hydrogenation reaction) was performed for 6 hours to yield a water dispersion of hydrogenated nitrile rubber. An appropriate amount of NMP was added to the obtained water dispersion of the hydrogenated nitrile rubber to obtain a mixture. Thereafter, vacuum distillation was performed at 90° C. so as to remove water and excess NMP from the mixture and yield an NMP solution (solid content concentration: 8%) of the hydrogenated nitrile rubber.
Equimolar amounts of melamine (63.0 g) that had been pulverized to a volume-average particle diameter of 100 μm and cyanuric acid (64.5 g) were added into a reactor. Thereafter, 1% of potassium hydroxide was added relative to 100%, in total, of melamine and cyanuric acid, and deionized water was further added to adjust the solid content concentration to 55% and obtain a mixture. This mixture was subsequently heated to 75° C. under stirring and was stirred for 120 minutes to prepare a slurry containing melamine cyanurate (foaming agent) as a core of a thermally decomposable material. A slurry composition for a thermally decomposable material was then obtained by adding 3 parts of sodium stearate (molecular weight: 306.5 g/mol; melting point: 305° C.), which is an anionic surfactant, to the obtained slurry relative to 97 parts of melamine cyanurate, adding deionized water so as to adjust the solid content concentration to 20%, and performing 30 minutes of stirring. The obtained slurry composition was dried by spray drying at 140° C. to yield a thermally decomposable material. The thermal decomposition temperature, average particle diameter, and circularity of this thermally decomposable material and the proportion constituted by the amount of the surfactant among the total amount of the foaming agent and the surfactant in the thermally decomposable material were measured, and the particle diameter ratio of the thermally decomposable material was calculated. The results are shown in Table 1.
A binder composition having a solid content concentration of 8% was produced by mixing 100 parts (in terms of solid content) of the NMP solution of the hydrogenated nitrile rubber and 100 parts of the thermally decomposable material, and further adding NMP.
A slurry composition for a positive electrode was obtained by loading 96 parts of lithium cobalt oxide as a positive electrode active material, 2.0 parts in terms of solid content of carbon black (produced by Denka Company Limited; product name: Li-100) as a conductive material, and 2.0 parts in terms of solid content of the above-described binder composition into a planetary mixer, mixing these materials, gradually further adding NMP, and performing stirred mixing at a rotation speed of 60 rpm and a temperature of 25±3° C. to adjust the viscosity as measured by a B-type viscometer at 60 rpm (M4 rotor) and 25±3° C. to 3,600 mPa·s.
The slurry composition for a positive electrode was applied onto aluminum foil of 20 μm in thickness serving as a current collector by a comma coater such as to have a coating weight of 20±0.5 mg/cm2. The aluminum foil was conveyed inside an oven having a temperature of 90° C. for 2 minutes and an oven having a temperature of 120° C. for 2 minutes at a speed of 0.5 m/min so as to dry the slurry composition for a positive electrode on the aluminum foil and thereby obtain a positive electrode web having a positive electrode mixed material layer formed on the current collector. The positive electrode mixed material layer-side of the produced positive electrode web was subsequently roll pressed with a load of 14 t (tons) in an environment having a temperature of 25±3° C. to obtain a positive electrode having a positive electrode mixed material layer density of 3.80 g/cm3. The peel strength of the obtained positive electrode was evaluated. The result is shown in Table 1.
A 5 MPa pressure-resistant vessel equipped with a stirrer was charged with 63 parts of styrene as an aromatic vinyl monomer, 34 parts of 1,3-butadiene as an aliphatic conjugated diene monomer, 2 parts of itaconic acid as a carboxy group-containing monomer, 1 part of 2-hydroxyethyl acrylate as a hydroxyl group-containing monomer, 0.3 parts of t-dodecyl mercaptan as a molecular weight modifier, 5 parts of sodium dodecylbenzenesulfonate as an emulsifier, 150 parts of deionized water as a solvent, and 1 part of potassium persulfate as a polymerization initiator. These materials were thoroughly stirred and were then heated to a temperature of 55° C. to initiate polymerization. Cooling was performed to quench the reaction at the point at which monomer consumption reached 95.0%. The water dispersion containing a polymer that was obtained in this manner was adjusted to pH 8 through addition of 5% sodium hydroxide aqueous solution. Unreacted monomer was subsequently removed through thermal-vacuum distillation. Thereafter, cooling was performed to a temperature of 30° C. or lower to yield a water dispersion (binder composition for a negative electrode) containing a binder for a negative electrode.
A planetary mixer was charged with 48.75 parts of artificial graphite (theoretical capacity: 360 mAh/g) and 48.75 parts of natural graphite (theoretical capacity: 360 mAh/g) as negative electrode active materials and 1 part in terms of solid content of carboxymethyl cellulose as a thickener. These materials were diluted to a solid content concentration of 60% with deionized water and were then kneaded at a rotation speed of 45 rpm for 60 minutes. Thereafter, 1.5 parts in terms of solid content of the binder composition for a negative electrode obtained as described above was added and was kneaded therewith at a rotation speed of 40 rpm for 40 minutes. Deionized water was then added to adjust the viscosity to 3,000±500 mPa·s (measured at 25° C. and 60 rpm using a B-type viscometer) and thereby produce a slurry composition for a negative electrode.
The slurry composition for a negative electrode was applied onto the surface of copper foil of 15 μm in thickness serving as a current collector by a comma coater such as to have a coating weight of 11±0.5 mg/cm2. The copper foil onto which the slurry composition for a negative electrode had been applied was subsequently conveyed inside an oven having a temperature of 80° C. for 2 minutes and an oven having a temperature of 110° C. for 2 minutes at a speed of 400 mm/min so as to dry the slurry composition for a negative electrode on the copper foil and thereby obtain a negative electrode web having a negative electrode mixed material layer formed on the current collector. The negative electrode mixed material layer-side of the produced negative electrode web was subsequently roll pressed with a line pressure of 11 t (tons) in an environment having a temperature of 25±3° C. to obtain a negative electrode having a negative electrode mixed material layer density of 1.60 g/cm3.
A separator made of a single layer of polypropylene (produced by Celgard, LLC.; product name: #2500) was prepared.
The positive electrode, the negative electrode, and the separator were used to produce a stacked laminate cell (initial design discharge capacity equivalent to 3 Ah) and were arranged inside aluminum packing. Vacuum drying was performed under conditions of 10 hours at 60° C. The aluminum packing was subsequently filled with LiPF6 solution of 1.0 M in concentration (solvent: mixed solvent of ethylene carbonate (EC)/diethyl carbonate (DEC)=3/7 (volume ratio); additive: containing 2 volume % (solvent ratio) of vinylene carbonate) as an electrolyte solution. The aluminum packing was then closed by heat sealing at a temperature of 150° C. to tightly seal an opening of the aluminum packing, and thereby produce a lithium ion secondary battery. Low-temperature IV resistance, high-temperature storage characteristics, and inhibition of heat generation in the event of an internal short circuit were evaluated for the obtained lithium ion battery. The results are shown in Table 1.
A binder, a thermally decomposable material, a binder composition, a slurry composition for a positive electrode, a positive electrode, a negative electrode, a separator, and a lithium ion secondary battery were prepared and various evaluations were performed in the same way as in Example 1 with the exception that the solid content concentration of the mixture used to obtain melamine cyanurate in production of the thermally decomposable material was changed from 55% to 60% (Example 2) or 70% (Example 3). The results are shown in Table 1.
A binder, a thermally decomposable material, a binder composition, a slurry composition for a positive electrode, a positive electrode, a negative electrode, a separator, and a lithium ion secondary battery were prepared and various evaluations were performed in the same way as in Example 1 with the exception that the additive amount of potassium hydroxide in production of the thermally decomposable material was changed from 1% to 0.5% (Example 4) or 2.0% (Example 5) relative to 100%, in total, of melamine and cyanuric acid. The results are shown in Table 1.
A binder, a thermally decomposable material, a binder composition, a slurry composition for a positive electrode, a positive electrode, a negative electrode, a separator, and a lithium ion secondary battery were prepared and various evaluations were performed in the same way as in Example 1 with the exception that the amounts of melamine cyanurate and sodium stearate in production of the thermally decomposable material were changed as indicated below. The results are shown in Table 1.
Example 6: Melamine cyanurate 92 parts, sodium stearate 8 parts
Example 7: Melamine cyanurate 99.5 parts, sodium stearate 0.5 parts
A binder, a thermally decomposable material, a binder composition, a slurry composition for a positive electrode, a positive electrode, a negative electrode, a separator, and a lithium ion secondary battery were prepared and various evaluations were performed in the same way as in Example 1 with the exception that a surfactant indicated below was used instead of sodium stearate, which is an anionic surfactant, in production of the thermally decomposable material. The results are shown in Table 2.
Example 8: Sorbitan monostearate (molecular weight: 430.6; melting point: 50° C.; non-ionic)
Example 9: Tetradecylamine acetate (molecular weight: 273.5 g/mol; melting point: 65° C.; cationic)
Example 10: Lithium stearate (molecular weight: 290.4 g/mol; melting point: 220° C.; anionic)
Example 11: Sodium 2-ethylhexyl sulfate (232.3 g/mol; melting point: 150° C.; anionic)
A binder, a binder composition, a slurry composition for a positive electrode, a positive electrode, a negative electrode, a separator, and a lithium ion secondary battery were prepared and various evaluations were performed in the same way as in Example 1 with the exception that a thermally decomposable material produced as described below was used. The results are shown in Table 2.
A mixture was obtained by adding deionized water to 50.0 g of melamine as a foaming agent so as to adjust the solid content concentration to 5%. This mixture was subsequently heated to 90° C. under stirring and was stirred for 10 minutes to prepare a slurry containing melamine as a core of a thermally decomposable material. A slurry composition for a thermally decomposable material was obtained by adding 3 parts of sodium stearate (molecular weight: 306.5 g/mol; melting point: 305° C.), which is an anionic surfactant, to the obtained slurry relative to 97 parts of melamine and performing 30 minutes of stirring. The obtained slurry composition was dried by spray drying at 140° C. to yield a thermally decomposable material.
A binder, a binder composition, a slurry composition for a positive electrode, a positive electrode, a negative electrode, a separator, and a lithium ion secondary battery were prepared and various evaluations were performed in the same way as in Example 1 with the exception that a thermally decomposable material produced as described below was used. The results are shown in Table 2.
Deionized water was added to 100.0 g of azodicarbonamide as a foaming agent so as to adjust the solid content concentration to 20%, and then a Three-One Motor (produced by SHINTO Scientific Co., Ltd.; product name: BL300) was used to produce a pre-dispersion (premix). A bead mill (produced by Ashizawa Finetech Ltd.; product name: LMZ-015) was then used to treat the obtained pre-dispersion for 5 minutes under conditions of a bead diameter of 1.0 mm, a bead loading rate of 80%, and a circumferential speed of 8 m/s to thereby produce a slurry containing azodicarbonamide as a core of a thermally decomposable material. A slurry composition for a thermally decomposable material was then obtained by adding 3 parts of sodium stearate (molecular weight: 306.5 g/mol; melting point: 305° C.), which is an anionic surfactant, to the obtained slurry relative to 97 parts of azodicarbonamide and performing 30 minutes of stirring. The obtained slurry composition was dried by spray drying at 140° C. to yield a thermally decomposable material.
A thermally decomposable material, a binder composition, a slurry composition for a positive electrode, a positive electrode, a negative electrode, a separator, and a lithium ion secondary battery were prepared and various evaluations were performed in the same way as in Example 1 with the exception that a binder produced as described below was used. The results are shown in Table 3.
A reactor A having a mechanical stirrer and a condenser attached thereto was charged with 85 parts of deionized water and 0.2 parts of sodium dodecylbenzenesulfonate under a nitrogen atmosphere. These materials were subsequently heated to 55° C. under stirring, and 0.3 parts of potassium persulfate was added as a 5.0% aqueous solution to the reactor A. Next, a separate vessel B having a mechanical stirrer attached thereto was charged with 94.0 parts of acrylonitrile as a nitrile group-containing monomer, 1.0 parts of acrylamide as an amide group-containing monomer, 2.0 parts of acrylic acid as a carboxy group-containing monomer, 3.0 parts of n-butyl acrylate as an ester group-containing monomer, 0.6 parts of sodium dodecylbenzenesulfonate, 0.035 parts of t-dodecyl mercaptan, 0.4 parts of polyoxyethylene lauryl ether, and 80 parts of deionized water under a nitrogen atmosphere, and these materials were stirred and emulsified to produce a monomer mixture. The monomer mixture was added into the reactor A at a constant rate over 5 hours while in a stirred and emulsified state and was reacted until the polymerization conversion rate reached 95% to yield a water dispersion of polyacrylonitrile. An appropriate amount of NMP was added to the obtained water dispersion of polyacrylonitrile to obtain a mixture. Thereafter, vacuum distillation was performed at 90° C. so as to remove water and excess NMP from the mixture and yield an NMP solution (solid content concentration: 8%) of polyacrylonitrile.
A thermally decomposable material, a binder composition, a slurry composition for a positive electrode, a positive electrode, a negative electrode, a separator, and a lithium ion secondary battery were prepared and various evaluations were performed in the same way as in Example 1 with the exception that a binder prepared as described below was used. The results are shown in Table 3.
Polyvinylidene fluoride (produced by Kureha Corporation; product name: L #7200) was dissolved in NMP to prepare an NMP solution (solid content concentration: 8%) of polyvinylidene fluoride.
A binder, a binder composition, a slurry composition for a positive electrode, a positive electrode, a negative electrode, a separator, and a lithium ion secondary battery were prepared and various evaluations were performed in the same way as in Example 1 with the exception that a thermally decomposable material produced as described below was used. The results are shown in Table 3.
Equimolar amounts of melamine (63.0 g) and cyanuric acid (64.5 g) that had been pulverized to a volume-average particle diameter of 100 μm were added into a reactor. Thereafter, 1% of potassium hydroxide was added relative to 100%, in total, of melamine and cyanuric acid, and deionized water was further added to adjust the solid content concentration to 55% and obtain a mixture. This mixture was subsequently heated to 75° C. under stirring and was stirred for 120 minutes to prepare a slurry containing melamine cyanurate (foaming agent) as a core of a thermally decomposable material. Deionized water was added to the obtained slurry to adjust the solid content concentration to 20%, and then a bead mill (produced by Ashizawa Finetech Ltd.; product name: LMZ-015) was used to treat the slurry for 5 minutes under conditions of a bead diameter of 1.0 mm, a bead loading rate of 80%, and a circumferential speed of 8 m/s. A slurry composition for a thermally decomposable material was obtained by adding 3 parts of sodium stearate (molecular weight: 306.5 g/mol; melting point: 305° C.), which is an anionic surfactant, relative to 97 parts of melamine cyanurate and performing 30 minutes of stirring. The obtained slurry composition was dried by spray drying at 140° C. to yield a thermally decomposable material.
A binder, a binder composition, a slurry composition for a positive electrode, a positive electrode, a negative electrode, a separator, and a lithium ion secondary battery were prepared and various evaluations were performed in the same way as in Example 1 with the exception that a thermally decomposable material produced as described below was used. The results are shown in Table 3.
Equimolar amounts of melamine (63.0 g) and cyanuric acid (64.5 g) that had been pulverized to a volume-average particle diameter of 100 μm were added into a reactor. Thereafter, 1% of potassium hydroxide was added relative to 100%, in total, of melamine and cyanuric acid, and deionized water was further added to adjust the solid content concentration to 55% and obtain a mixture. This mixture was subsequently heated to 75° C. under stirring and was stirred for 120 minutes to obtain a slurry composition for a thermally decomposable material containing melamine cyanurate (foaming agent). The obtained slurry composition was dried by spray drying at 140° C. to yield a thermally decomposable material.
A binder, a binder composition, a slurry composition for a positive electrode, a positive electrode, a negative electrode, a separator, and a lithium ion secondary battery were prepared and various evaluations were performed in the same way as in Example 1 with the exception that a thermally decomposable material produced as described below was used. The results are shown in Table 3.
Deionized water was added to melamine cyanurate (produced by Nissan Chemical Industries, Ltd.; product name: MC-6000) to adjust the solid content concentration to 40% and prepare a slurry containing melamine cyanurate (foaming agent) as a core of a thermally decomposable material. A slurry composition for a thermally decomposable material was then obtained by adding 3 parts of sodium stearate (molecular weight: 306.5 g/mol; melting point: 305° C.), which is an anionic surfactant, to the obtained slurry relative to 97 parts of melamine cyanurate, adding deionized water so as to adjust the solid content concentration to 20%, and performing 30 minutes of stirring. The obtained slurry composition was dried by spray drying at 140° C. to yield a thermally decomposable material.
A binder, a binder composition, a slurry composition for a positive electrode, a positive electrode, a negative electrode, a separator, and a lithium ion secondary battery were prepared and various evaluations were performed in the same way as in Example 1 with the exception that a thermally decomposable material produced as described below was used. The results are shown in Table 3.
Equimolar amounts of melamine (63.0 g) and cyanuric acid (64.5 g) that had been pulverized to a volume-average particle diameter of 100 μm were added into a reactor. Thereafter, 1% of potassium hydroxide was added relative to 100%, in total, of melamine and cyanuric acid, and deionized water was further added to adjust the solid content concentration to 55% and obtain a mixture. This mixture was subsequently heated to 75° C. under stirring and was stirred for 120 minutes to prepare a slurry containing melamine cyanurate (foaming agent) as a core of a thermally decomposable material. The obtained slurry was dried at 80° C. for 12 hours, and the resultant dried product was subjected to spheroidization using a mechanical spheroidization device (produced by EarthTechnica Co., Ltd.; product name: KRYPTRON ORB CSH0). Deionized water was added to the dried product that had undergone spheroidization so as to adjust the solid content concentration to 40% and prepare a slurry containing melamine cyanurate (foaming agent) as a core of a thermally decomposable material. A slurry composition for a thermally decomposable material was then obtained by adding 3 parts of sodium stearate (molecular weight: 306.5 g/mol; melting point: 305° C.), which is an anionic surfactant, to the obtained slurry relative to 97 parts of melamine cyanurate, adding deionized water so as to adjust the solid content concentration to 20%, and performing 30 minutes of stirring. The obtained slurry composition was dried by spray drying at 140° C. to yield a thermally decomposable material.
In Tables 1 to 3, shown below:
It can be seen from Tables 1 to 3 that it is possible to produce an electrochemical device in which heat generation in the event of an internal short circuit is sufficiently inhibited, that has low low-temperature IV resistance, and that has excellent high-temperature storage characteristics in Examples 1 to 17 in which a binder and a thermally decomposable material including a foaming agent are included and in which the thermal decomposition temperature, number-average particle diameter, particle diameter ratio, and circularity of the thermally decomposable material are within specific ranges.
In contrast, it can be seen from Table 3 that heat generation in the event of an internal short circuit of an electrochemical device is not sufficiently inhibited, low-temperature IV resistance is high, and high-temperature storage characteristics are poor in Comparative Example 1 in which the particle diameter ratio of a thermally decomposable material is outside of a specific range.
It can also be seen from Table 3 that low-temperature IV resistance of an electrochemical device is high and high-temperature storage characteristics of the electrochemical device are poor in Comparative Example 2 in which the circularity of a thermally decomposable material is outside of a specific range.
According to the present disclosure, it is possible to provide a binder composition for an electrochemical device, a slurry composition for an electrochemical device electrode, and an electrode for an electrochemical device that can sufficiently inhibit heat generation in the event of an internal short circuit of an electrochemical device, reduce IV resistance of the electrochemical device at low temperature, and cause the electrochemical device to display excellent high-temperature storage characteristics.
Moreover, according to the present disclosure, it is possible to provide an electrochemical device in which heat generation in the event of an internal short circuit is sufficiently inhibited, that has low IV resistance at low temperature, and that has excellent high-temperature storage characteristics.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2020-183242 | Oct 2020 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2021/039536 | 10/26/2021 | WO |
