Positive electrode binder, electrode mixture, electrode, and secondary battery

Abstract

To provide a binder that lowers the interface resistance between a current collector and an electrode material layer in a positive electrode of a sodium-ion battery and that has good battery properties, as well as an electrode mixture, an electrode, and a secondary battery.SOLUTION: The binder for a positive electrode of a sodium ion battery includes a copolymer having a vinylidene fluoride unit (A) and a composition unit (B) derived from a monomer selected from trifluoroethylene, hexafluoropropylene, chlorotrifluoroethylene, general formula (1), (3) and the like, but not having a tetrafluoroethylene unit.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This disclosure relates to a binder for positive electrodes, an electrode mixture, an electrode, and a secondary battery. [Background technology]

[0002] In recent years, the demand for electrochemical devices such as rechargeable batteries has rapidly increased due to the miniaturization and weight reduction of electrical products. Furthermore, as electrical products become more high-performance and equipped with previously unavailable functions, there is a growing need for electrochemical devices that can withstand longer periods of use and harsher conditions.

[0003] Furthermore, research is being conducted on sodium-ion secondary batteries that use sodium ions as the charge carrier. Sodium is attracting attention as a secondary battery that can be produced at low cost and scaled up, because it is more abundant and inexpensive than lithium. Patent documents 1 and 2 describe sodium-ion batteries that use a fluorine compound as a binder. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2012-79687 [Patent Document 2] Japanese Patent Publication No. 2014-26818 [Overview of the project] [Problems that the invention aims to solve]

[0005] This disclosure aims to provide a binder that reduces the interfacial resistance between the current collector and the electrode material layer in the positive electrode of a sodium-ion battery, and improves battery characteristics, as well as an electrode mixture, electrode, and secondary battery using the same. [Means for solving the problem]

[0006] This disclosure relates to vinylidene fluoride units (A), as well as trifluoroethylene, chlorotrifluoroethylene, and Monomers represented by the following general formula (1), monomers represented by the following general formula (2) Body A binder for the positive electrode of a sodium-ion battery comprising a copolymer having constituent units (B) derived from at least one monomer selected from the group and not having tetrafluoroethylene units, wherein the content of vinylidene fluoride units (A) is 60 to 99.5 mol% relative to the total monomer units in the copolymer.

[0007] [ka] (In the formula, Rf 1 This refers to a linear or branched fluorinated alkyl group or fluorinated alkoxy group having 1 to 12 carbon atoms. Both fluorinated alkyl groups and fluorinated alkoxy groups, if they have 2 or more carbon atoms, may contain an oxygen atom (-O-) between carbon atoms. [ka] (In the formula, Rf 2 This refers to a linear or branched fluorinated alkyl group or fluorinated alkoxy group having 1 to 12 carbon atoms. Both fluorinated alkyl groups and fluorinated alkoxy groups may contain an oxygen atom (-O-) between carbon atoms if they have 2 or more carbon atoms. )

[0008] before It is preferable that the constituent unit (B) is derived from at least one monomer selected from the group consisting of trifluoroethylene, hexafluoropropylene, chlorotrifluoroethylene, 2,3,3,3-tetrafluoropropene, perfluoro-(2,9,9-trihaloid-5-trifluoromethyl-3,6-dioxa-8-heptene) and acrylic acid. Furthermore, it is preferable that the constituent unit (B) is a constituent unit derived from 2,3,3,3-tetrafluoropropene.

[0009] The positive electrode binder may contain two or more of the copolymers. The positive electrode binder may further contain a fluorine-containing polymer other than the copolymer. The fluorine-containing polymer is preferably polyvinylidene fluoride or modified polyvinylidene fluoride.

[0010] This disclosure also relates to an electrode mixture in which the electrode active material contains a sodium composite oxide and includes a binder for the positive electrode of the sodium-ion battery. The present disclosure also relates to an electrode comprising a current collector and an electrode material layer formed from the electrode mixture, provided on one or both sides of the current collector. This disclosure also relates to a sodium-ion secondary battery equipped with the aforementioned electrodes. [Effects of the Invention]

[0011] This disclosure provides a binder that reduces the interfacial resistance between the current collector and the electrode material layer in the positive electrode of a sodium-ion battery, and improves battery characteristics. Furthermore, electrodes and secondary batteries using the binder of this disclosure have reduced interfacial resistance between the current collector and the electrode material layer in the positive electrode. In addition, secondary batteries using the binder of this disclosure have good battery characteristics. [Modes for carrying out the invention]

[0012] The details of this disclosure are described below. This disclosure relates to a binder used for forming the positive electrode of a sodium-ion battery. In recent years, the development of sodium-ion batteries (SIBs) has progressed. Because sodium ions have a larger ionic radius than lithium ions, SIBs experience more drastic changes in interlayer distance and structure due to charging and discharging compared to lithium-ion batteries (LiBs). While the conductive path can be maintained regardless of shrinkage within the electrode material layer (excluding the interface with the current collector foil) due to the binder, maintaining the conductive path is more difficult at the interface with the current collector foil because there is no binder between the active material and the current collector foil. Therefore, in SIBs, a lower interfacial resistance is required at the time of electrode fabrication.

[0013] Unlike LiBs, SIBs require electrodes with lower interfacial resistance. The inventors have discovered that by using a binder containing a vinylidene fluoride-containing copolymer with a specific composition and no constituent units derived from tetrafluoroethylene as a binder component during electrode fabrication, the conductive agent can be pressed more firmly onto the current collector during electrode pressing, thereby reducing the interfacial resistance between the electrode's current collector (e.g., Al) and the electrode material layer (conductive agent / Na active material / binder). Furthermore, using the binder of this disclosure improves the battery performance of sodium-ion batteries.

[0014] The binder for the positive electrode of the sodium-ion battery of this disclosure includes vinylidene fluoride (VdF) units (A) and constituent units (B) derived from at least one monomer selected from the group consisting of trifluoroethylene (TrFE), hexafluoropropylene (HFP), chlorotrifluoroethylene (CTFE), monomers represented by the following general formula (1), monomers represented by the following general formula (2), and monomers represented by the following general formula (3), and comprises a copolymer that does not contain tetrafluoroethylene units.

[0015] [ka] (In the formula, Rf 1This refers to a linear or branched fluorinated alkyl group or fluorinated alkoxy group having 1 to 12 carbon atoms. Both fluorinated alkyl groups and fluorinated alkoxy groups, if they have 2 or more carbon atoms, may contain an oxygen atom (-O-) between carbon atoms. [ka] (In the formula, Rf 2 This refers to a linear or branched fluorinated alkyl group or fluorinated alkoxy group having 1 to 12 carbon atoms. Both fluorinated alkyl groups and fluorinated alkoxy groups, if they have 2 or more carbon atoms, may contain an oxygen atom (-O-) between carbon atoms. [ka] (In the formula, R 1 , R 2 and R 3 Each of these is independently a hydrogen atom, a chlorine atom, or an alkyl group having 1 to 5 carbon atoms. X is an atomic group with a molecular weight of 500 or less, consisting of single bonds or a main chain with 1 to 20 atoms. Y represents an inorganic cation and / or an organic cation.

[0016] The fluorine-containing monomer represented by the general formula (1) above is a linear or branched fluorinated alkyl group having 1 to 12 carbon atoms, or a linear or branched fluorinated alkoxy group having 1 to 12 carbon atoms, where Rf1 is a linear or branched fluorinated alkoxy group having 1 to 12 carbon atoms. Both the fluorinated alkyl group and the fluorinated alkoxy group may contain an oxygen atom (-O-) between carbon atoms if they have 2 or more carbon atoms.

[0017] The fluorinated alkyl group Rf1 may be a partially fluorinated alkyl group in which some of the hydrogen atoms bonded to the carbon atom are substituted with fluorine atoms, or it may be a perfluorinated alkyl group in which all of the hydrogen atoms bonded to the carbon atom are substituted with fluorine atoms. Furthermore, the fluorinated alkyl group Rf1 may have hydrogen atoms substituted with substituents other than fluorine atoms, but it is preferable that it does not contain substituents other than fluorine atoms.

[0018] Further, the fluorinated alkoxy group of Rf1 may be a partially fluorinated alkoxy group in which a part of the hydrogen atoms bonded to the carbon atoms are substituted by fluorine atoms, or a perfluorinated alkoxy group in which all of the hydrogen atoms bonded to the carbon atoms are substituted by fluorine atoms. Also, the fluorinated alkoxy group of Rf1 may have a hydrogen atom substituted by a substituent other than a fluorine atom, but it is preferably free of substituents other than fluorine atoms.

[0019] The number of carbon atoms of Rf1 is preferably 1 to 10, more preferably 1 to 6, still more preferably 1 to 4, and particularly preferably 1.

[0020] As Rf1, a general formula: -(Rf 11 )m-(O)p-(Rf 12 -O)n-Rf 13 (wherein, Rf 11 and Rf 12 are each independently a linear or branched fluorinated alkylene group having 1 to 4 carbon atoms, Rf 13 is a linear or branched fluorinated alkyl group having 1 to 4 carbon atoms, p is 0 or 1, m is an integer of 0 to 4, and n is an integer of 0 to 4) is preferred.

[0021] Rf 11 [[ID=)30]]and Rf 12 The fluorinated alkylene groups of may be a partially fluorinated alkylene group in which a part of the hydrogen atoms bonded to the carbon atoms are substituted by fluorine atoms, or a perfluorinated alkylene group in which all of the hydrogen atoms bonded to the carbon atoms are substituted by fluorine atoms. Also, the fluorinated alkylene groups of Rf 11 and Rf 12 may have a hydrogen atom substituted by a substituent other than a fluorine atom, but it is preferably free of substituents other than fluorine atoms. Rf 11 and Rf 12 may be the same or different in each occurrence.

[0022] Rf 11 Examples of fluorinated alkylene groups include -CHF-, -CF2-, -CH2-CF2-, -CHF-CF2-, -CF2-CF2-, -CF(CF3)-, -CH2-CF2-CF2-, -CHF-CF2-CF2-, -CF2-CF2-CF2-, -CF(CF3)-CF2-, -CF2-CF(CF3)-, -C(CF3)2-, -CH2-CF2-CF2-CF2-, -CHF-CF2-CF2-CF2-, -CF2-CF2-CF2-CF2-, -CH(CF3)-CF2-CF2-, -CF(CF3)-CF2-CF2-, and -C(CF3)2-CF2-, among which perfluorinated alkylene groups having 1 or 2 carbon atoms are preferred, and -CF2- is more preferred.

[0023] Rf 12 Examples of fluorinated alkylene groups include -CHF-, -CF2-, -CH2-CF2-, -CHF-CF2-, -CF2-CF2-, -CF(CF3)-, -CH2-CF2-CF2-, -CHF-CF2-CF2-, -CF2-CF2-CF2-, -CF(CF3)-CF2-, -CF2-CF(CF3)-, -C(CF3)2-, -CH2-CF2-CF2-CF2-, -CHF-CF2-CF Examples include 2-CF2-, -CF2-CF2-CF2-CF2-, -CH(CF3)-CF2-CF2-, -CF(CF3)-CF2-CF2-, and -C(CF3)2-CF2-, among which perfluorinated alkylene groups having 1 to 3 carbon atoms are preferred, and -CF2-, -CF2CF2-, -CF2-CF2-CF2-, -CF(CF3)-CF2- or -CF2-CF(CF3)- are more preferred.

[0024] Rf 13 The fluorinated alkyl group may be a partially fluorinated alkyl group in which some of the hydrogen atoms bonded to the carbon atom are replaced by fluorine atoms, or a perfluorinated alkyl group in which all of the hydrogen atoms bonded to the carbon atom are replaced by fluorine atoms. Also, Rf 13The fluorinated alkyl group may have hydrogen atoms substituted with substituents other than fluorine atoms, but it is preferable that it does not contain substituents other than fluorine atoms (for example, -CN, -CH2I, -CH2Br, etc.).

[0025] Rf 13 Examples of fluorinated alkyl groups include -CH2F, -CHF2, -CF3, -CH2-CH2F, -CH2-CHF2, -CH2-CF3, -CHF-CH2F, -CHF-CHF2, -CHF-CF3, -CF2-CH2F, -CF2-CHF2, -CF2-CF3, -CH2-CF2-CH2F, -CHF-CF2-CH2F, -CF2-CF2-CH2F, -CF(CF3)-CH2F, -CH2-CF2-CHF2, -CHF-CF2-CHF2, -CF2-CF2-CHF2, -CF(CF3)-CHF2, -CH2-CF2-CF3, -CHF-CF2-CF 3. Examples include -CF2-CF2-CF3, -CF(CF3)-CF3, -CH2-CF2-CF2-CF3, -CHF-CF2-CF2-CF3, -CF2-CF2-CF2-CF3, -CH(CF3)-CF2-CF3, -CF(CF3)-CF2-CF3, -C(CF3)2-CF3, and among these, -CF3, -CHF-CF3, -CF2-CHF2, -CF2-CF3, -CF2-CF2-CF3, -CF(CF3)-CF3, -CF2-CF2-CF2-CF3, -CH(CF3)-CF2-CF3 or -CF(CF3)-CF2-CF3 are preferred.

[0026] For p, 0 is preferable.

[0027] m is preferably an integer between 0 and 2, more preferably 0 or 1, and even more preferably 0. Furthermore, when p is 0, it is preferable that m is also 0.

[0028] n is preferably an integer between 0 and 2, more preferably 0 or 1, and even more preferably 0.

[0029] As a repeating unit, -CH2-CF[-CF3]-, -CH2-CF[-CF2CF3]-, -CH2-CF[-CF2CF2CF3]-, -CH2-CF[-CF2CF2CF2CF3]-, -CH2-CF[-CF2-O-CF(CF3)-CF2-O-CHF-CF3]-, -CH2-CF[-CF2-O-CF(CF3)-CF2-O-CF2-CF3]-, -CH2-CF[-CF2-O-CF(CF3)-CF2-O-CF(CF3)-CF3]-, -CH2-CF[-CF2-O-CF(CF3)-CF2-O-CH(CF3)-CF2-CF3]-, -CH2-CF[-CF2-O-CF(CF3)-CF2-O-CF(CF3)-CF2-CF3]-, -CH2-CF[-OCF2OCF3]-, -CH2-CF[-OCF2CF2CF22OCF3]-, -CH2-CF[-CF2OCFOCF3]-, -CH2-CF[-CF2OCF2CF2CF2OCF3]-, or -CH2-CF[-O-CF2-CF3]- This is preferable. -CH2-CF[-CF3]- This is preferable.

[0030] Examples of monomers represented by general formula (1) include 2,3,3,3-tetrafluoropropene (1,2,3,4-yf), 2,3,3,4,4,4-hexafluoro-1-butene, and perfluoro-(2,9,9-trihaloid-5-trifluoromethyl-3,6-dioxa-8-heptene) (AEHF-1). Among these, 2,3,3,3-tetrafluoropropene (1,2,3,4-yf) is preferred.

[0031] The fluorine-containing monomer (2) represented by the general formula (2) above is such that Rf2 is a linear or branched fluorinated alkyl group having 1 to 12 carbon atoms, or a linear or branched fluorinated alkoxy group having 1 to 12 carbon atoms. Both the fluorinated alkyl group and the fluorinated alkoxy group may contain an oxygen atom (-O-) between carbon atoms if they have 2 or more carbon atoms.

[0032] The fluorinated alkyl group Rf2 may be a partially fluorinated alkyl group in which some of the hydrogen atoms bonded to the carbon atom are substituted with fluorine atoms, or it may be a perfluorinated alkyl group in which all of the hydrogen atoms bonded to the carbon atom are substituted with fluorine atoms. Furthermore, the fluorinated alkyl group Rf2 may have hydrogen atoms substituted with substituents other than fluorine atoms, but it is preferable that it does not contain substituents other than fluorine atoms.

[0033] Furthermore, the fluorinated alkoxy group of Rf2 may be a partially fluorinated alkoxy group in which some of the hydrogen atoms bonded to the carbon atom are substituted with fluorine atoms, or it may be a perfluorinated alkoxy group in which all of the hydrogen atoms bonded to the carbon atom are substituted with fluorine atoms. In addition, the fluorinated alkoxy group of Rf2 may have hydrogen atoms substituted with substituents other than fluorine atoms, but it is preferable that it does not contain substituents other than fluorine atoms.

[0034] The number of carbon atoms in Rf2 is preferably 1 to 10, more preferably 1 to 6, even more preferably 1 to 4, and particularly preferably 1.

[0035] For Rf2, the general formula is: -(Rf 21 )m-(O)p-(Rf 22 -O)n-Rf 23 (In the formula, Rf 21 and Rf 22 These are, independently, linear or branched fluorinated alkylene groups having 1 to 4 carbon atoms, Rf 23A preferred group is one that is linear or branched, has 1 to 4 carbon atoms, is a fluorinated alkyl group, where p is 0 or 1, m is an integer from 0 to 4, and n is an integer from 0 to 4.

[0036] Rf 21 and Rf 22 The fluorinated alkylene group may be a partially fluorinated alkylene group in which some of the hydrogen atoms bonded to the carbon atom are replaced by fluorine atoms, or it may be a perfluorinated alkylene group in which all of the hydrogen atoms bonded to the carbon atom are replaced by fluorine atoms. Also, Rf 21 and Rf 22 The fluorinated alkylene group may have hydrogen atoms substituted with substituents other than fluorine atoms, but it is preferable that it does not contain substituents other than fluorine atoms. Rf 21 and Rf 22 In each occurrence, they may be the same or different.

[0037] Rf 21 Examples of fluorinated alkylene groups include -CHF-, -CF2-, -CH2-CF2-, -CHF-CF2-, -CF2-CF2-, -CF(CF3)-, -CH2-CF2-CF2-, -CHF-CF2-CF2-, -CF2-CF2-CF2-, -CF(CF3)-CF2-, -CF2-CF(CF3)-, -C(CF3)2-, -CH2-CF2-CF2-CF2-, -CHF-CF2-CF2-CF2-, -CF2-CF2-CF2-CF2-, -CH(CF3)-CF2-CF2-, -CF(CF3)-CF2-CF2-, and -C(CF3)2-CF2-, among which perfluorinated alkylene groups having 1 or 2 carbon atoms are preferred, and -CF2- is more preferred.

[0038] Rf 22Examples of fluorinated alkylene groups include -CHF-, -CF2-, -CH2-CF2-, -CHF-CF2-, -CF2-CF2-, -CF(CF3)-, -CH2-CF2-CF2-, -CHF-CF2-CF2-, -CF2-CF2-CF2-, -CF(CF3)-CF2-, -CF2-CF(CF3)-, -C(CF3)2-, -CH2-CF2-CF2-CF2-, -CHF-CF2-CF Examples include 2-CF2-, -CF2-CF2-CF2-CF2-, -CH(CF3)-CF2-CF2-, -CF(CF3)-CF2-CF2-, and -C(CF3)2-CF2-, among which perfluorinated alkylene groups having 1 to 3 carbon atoms are preferred, and -CF2-, -CF2CF2-, -CF2-CF2-CF2-, -CF(CF3)-CF2- or -CF2-CF(CF3)- are more preferred.

[0039] Rf 23 The fluorinated alkyl group may be a partially fluorinated alkyl group in which some of the hydrogen atoms bonded to the carbon atom are replaced by fluorine atoms, or a perfluorinated alkyl group in which all of the hydrogen atoms bonded to the carbon atom are replaced by fluorine atoms. Also, Rf 23 The fluorinated alkyl group may have hydrogen atoms substituted with substituents other than fluorine atoms, but it is preferable that it does not contain substituents other than fluorine atoms (for example, -CN, -CH2I, -CH2Br, etc.).

[0040] Rf 23Examples of fluorinated alkyl groups include -CH2F, -CHF2, -CF3, -CH2-CH2F, -CH2-CHF2, -CH2-CF3, -CHF-CH2F, -CHF-CHF2, -CHF-CF3, -CF2-CH2F, -CF2-CHF2, -CF2-CF3, -CH2-CF2-CH2F, -CHF-CF2-CH2F, -CF2-CF2-CH2F, -CF(CF3)-CH2F, -CH2-CF2-CHF2, -CHF-CF2-CHF2, -CF2-CF2-CHF2, -CF(CF3)-CHF2, -CH2-CF2-CF3, -CHF-CF2-CF 3. Examples include -CF2-CF2-CF3, -CF(CF3)-CF3, -CH2-CF2-CF2-CF3, -CHF-CF2-CF2-CF3, -CF2-CF2-CF2-CF3, -CH(CF3)-CF2-CF3, -CF(CF3)-CF2-CF3, -C(CF3)2-CF3, and among these, -CF3, -CHF-CF3, -CF2-CHF2, -CF2-CF3, -CF2-CF2-CF3, -CF(CF3)-CF3, -CF2-CF2-CF2-CF3, -CH(CF3)-CF2-CF3 or -CF(CF3)-CF2-CF3 are preferred.

[0041] For p, 0 is preferable.

[0042] m is preferably an integer between 0 and 2, more preferably 0 or 1, and even more preferably 0. Furthermore, when p is 0, it is preferable that m is also 0.

[0043] n is preferably an integer between 0 and 2, more preferably 0 or 1, and even more preferably 0.

[0044] As a repeating unit, -CHF-CH[-CF3]-, -CHF-CH[-CF2CF3]-, -CHF-CH[-CF2CF2CF3]-, or -CHF-CH[-CF2CF2CF2CF3]-, This is preferable. -CHF-CH[-CF3]- This is preferable.

[0045] In the general formula (3) above, Y represents an inorganic cation and / or an organic cation. Examples of inorganic cations include H, Li, Na, K, Mg, Ca, Al, and Fe. Examples of organic cations include NH4 and NH3R. 15 NH2R 15 2. NHR 15 3. NR 15 4(R 15 Each of these independently represents an alkyl group having 1 to 4 carbon atoms. Examples of cations include ( ). Preferred Y is H, Li, Na, K, Mg, Ca, Al, NH4, more preferred H, Li, Na, K, Mg, Al, NH4, even more preferred H, Li, Al, NH4, and particularly preferred H. For convenience, specific examples of inorganic and organic cations are listed without symbols and valencies.

[0046] In general formula (3), R 1 ~R 3 Each of these independently represents a hydrogen atom, a chlorine atom, or an alkyl group having 1 to 5 carbon atoms. The alkyl group is a monovalent alkyl group. The number of carbon atoms in the alkyl group is preferably 4 or less. The alkyl group is preferably a methyl group or an ethyl group. 1 and R 2 R is preferably independently a hydrogen atom, a methyl group, or an ethyl group. 3 It is preferable that this is a hydrogen atom or a methyl group.

[0047] In general formula (3), X is an atomic group having a molecular weight of 500 or less, composed of single bonds or a main chain with 1 to 20 atoms. The atomic group is a divalent atomic group. The atomic group is preferably a hydrocarbon group having 4 or fewer carbon atoms. Examples of the hydrocarbon group include alkylene groups and alkenylene groups with the aforementioned number of carbon atoms, and among these, at least one selected from the group consisting of methylene, ethylene, ethylidene, propylidene, and isopropylidene groups is preferred, with methylene being more preferred.

[0048] The monomer represented by general formula (3) is preferably at least one selected from the group consisting of acryloyloxypropyl succinic acid and its salts, (meth)acrylic acid and its salts, vinylacetic acid (3-butenic acid) and its salts, 3-pentenoic acid and its salts, 4-pentenoic acid and its salts, 3-hexenoic acid and its salts, 4-heptenoic acid and its salts, and 5-hexenoic acid and its salts.

[0049] The aforementioned constituent unit (B) is preferably a constituent unit derived from at least one monomer selected from the group consisting of trifluoroethylene, hexafluoropropylene, chlorotrifluoroethylene, 2,3,3,3-tetrafluoropropene, perfluoro-(2,9,9-trihaloid-5-trifluoromethyl-3,6-dioxa-8-heptene) and acrylic acid, and more preferably a constituent unit derived from 2,3,3,3-tetrafluoropropene.

[0050] In the copolymer, the content of VdF units (A) is preferably 60 to 99.5 mol% relative to the total monomer units in the copolymer. If the VdF unit is less than 60 mol%, the peel strength as an electrode will be weak, and if it is more than 99.5 mol%, uniform coating of the slurry will be difficult.

[0051] The copolymer preferably contains 65 mol% or more of VdF units relative to the total polymerization units. When it contains 65 mol% or more, the cycle characteristics of a battery using an electrode obtained from the electrode mixture of this disclosure tend to be better. The composition of the copolymer can be measured using a 19F-NMR analyzer.

[0052] The content of the aforementioned constituent unit (B) is preferably less than 99.5 mol% relative to the total polymerization units of the copolymer. If it is 99.5 mol% or more, the crystallinity of the copolymer generally decreases significantly, and as a result, the non-aqueous electrolyte swelling property tends to decrease. The content of the aforementioned constituent unit (B) is more preferably 99 mol% or less.

[0053] When the copolymer contains monomer units other than VdF, such as monomer units represented by the general formula (3), the content of such monomer units is preferably 0.0001 to 50.0 mol%, more preferably 0.01 mol% or more, even more preferably 0.10 mol% or more, even more preferably 5.0 mol% or less, even more preferably 3.0 mol% or less, and particularly preferably 1.5 mol% or less, relative to the total monomer units. Furthermore, if the copolymer contains monomer units represented by the general formula (3) as other constituent units, their content can be measured by acid-base titration of the carboxylic acid group.

[0054] When the copolymer contains monomer units represented by the general formula (3), the VdF unit content of the copolymer is preferably 50.0 to 99.999 mol%, more preferably 95.0 mol% or more, even more preferably 97.0 mol% or more, particularly preferably 98.5 mol% or more, more preferably 99.99 mol% or less, and even more preferably 99.90 mol% or less, relative to the total monomer units.

[0055] The copolymer may, in addition to the monomer, contain constituent units (C) based on fluorinated monomers such as vinyl fluoride and fluoroalkyl vinyl ethers, or non-fluorinated monomers such as ethylene and propylene, to the extent that they do not impede the purpose of this disclosure.

[0056] As the fluoroalkyl vinyl ether, a fluoroalkyl vinyl ether having a fluoroalkyl group with 1 to 5 carbon atoms is preferred, and at least one selected from the group consisting of perfluoro(methyl vinyl ether) (PMVE), perfluoro(ethyl vinyl ether), and perfluoro(propyl vinyl ether) is more preferred.

[0057] In the copolymer, the content of constituent units (C) is preferably 0.0001 to 50.0 mol%, more preferably 0.01 mol% or more, even more preferably 0.10 mol% or more, even more preferably 45.0 mol% or less, even more preferably 40.0 mol% or less, and particularly preferably 35.0 mol% or less, relative to the total monomer units.

[0058] The weight-average molecular weight (in polystyrene terms) of the copolymer is preferably 10,000 to 3,000,000, more preferably 30,000 or more, even more preferably 50,000 or more, particularly preferably 200,000 or more, more preferably 2,400,000 or less, even more preferably 2,200,000 or less, and particularly preferably 2,000,000 or less. The weight-average molecular weight can be measured by gel permeation chromatography (GPC) using dimethylformamide as the solvent.

[0059] The number-average molecular weight (in terms of polystyrene) of the copolymer is preferably 7,000 to 1,500,000, more preferably 21,000 or more, even more preferably 35,000 or more, more preferably 1,400,000 or less, even more preferably 1,200,000 or less, and particularly preferably 1,100,000 or less. The number-average molecular weight can be measured by gel permeation chromatography (GPC) using dimethylformamide as the solvent.

[0060] The copolymer containing monomer units represented by the general formula (3) is preferably, for example, a VdF / acrylic acid copolymer, a VdF / methacrylic acid copolymer, or a VdF / methyl methacrylate copolymer.

[0061] The VdF / (meth)acrylic acid copolymer contains VdF units and (meth)acrylic acid units. By using the VdF / (meth)acrylic acid copolymer as the copolymer, an electrode-forming composition can be formed very easily, and a coating layer that adheres extremely firmly to the metal foil can be formed. The content of (meth)acrylic acid units is preferably 0.0001 to 5.0 mol%, more preferably 0.01 to 3.0 mol%, and even more preferably 0.10 to 1.5 mol%, relative to the total monomer units.

[0062] The VdF unit content of the VdF / (meth)acrylic acid copolymer is preferably 95.0 to 99.9999 mol%, more preferably 97.0 to 99.99 mol%, and even more preferably 98.5 to 99.90 mol%, relative to the total monomer units.

[0063] The weight-average molecular weight (in polystyrene equivalent) of the VdF / (meth)acrylic acid copolymer is preferably 50,000 to 3,000,000, more preferably 80,000 or more, even more preferably 100,000 or more, particularly preferably 200,000 or more, more preferably 2,400,000 or less, even more preferably 2,200,000 or less, and particularly preferably 2,000,000 or less.

[0064] The number-average molecular weight (polystyrene equivalent) of the VdF / (meth)acrylic acid copolymer is preferably 20,000 to 1,500,000, more preferably 40,000 or more, even more preferably 70,000 or more, particularly preferably 140,000 or more, more preferably 1,400,000 or less, even more preferably 1,200,000 or less, and particularly preferably 1,100,000 or less.

[0065] The copolymer preferably has a Mooney viscosity (ML1 + 10 (121°C)) of 2 or more at 121°C, more preferably 5 or more, even more preferably 10 or more, and particularly preferably 30 or more. Mooney viscosity is a value measured in accordance with ASTM-D1646-15 and JIS K6300-1:2013.

[0066] The copolymer can be produced by a general radical polymerization method using vinylidene fluoride (VdF), the monomer that constitutes the constituent unit (B), and, if necessary, other monomers other than tetrafluoroethylene. The polymerization method may be bulk polymerization, solution polymerization, suspension polymerization, or emulsion polymerization, but emulsion polymerization is preferred because it is easy to implement industrially.

[0067] In polymerization, polymerization initiators, chain transfer agents, surfactants, and solvents can be used, and conventionally known ones can be used. In copolymer polymerization, oil-soluble radical polymerization initiators or water-soluble radical polymerization initiators can be used as polymerization initiators.

[0068] The oil-soluble radical polymerization initiator may be a known oil-soluble peroxide, such as dialkyl peroxycarbonates such as diisopropyl peroxydicarbonate and disec-butyl peroxydicarbonate, peroxyesters such as t-butyl peroxyisobutyrate and t-butyl peroxypivalate, and dialkyl peroxides such as dit-butyl peroxide. Also, di(ω-hydro-dodecafluoroheptanoyl) peroxide, di(ω-hydro-tetradecafluoroheptanoyl) peroxide, di(ω-hydro-hexadecafluorononanoyl) peroxide, di(perfluorobutyryl) peroxide, di(perflupareryl) peroxide, di(perfluorohexanoyl) peroxide, di(perfluoroheptanoyl) peroxide, di(perfluorooctanoyl) peroxide, di(perfluorononanoyl) peroxide, and di(ω-chloro Representative examples include di[perfluoro(or fluorochloro)acyl]peroxides such as -hexafluorobutyryl)peroxide, di(ω-chloro-decafluorohexanoyl)peroxide, di(ω-chloro-tetradecafluorooctanoyl)peroxide, ω-hydro-dodecafluoroheptanoyl-ω-hydrohexadecafluorononanoyl-peroxide, ω-chloro-hexafluorobutyryl-ω-chloro-decafluorohexanoyl-peroxide, ω-hydrododecafluoroheptanoyl-perfluorobutyryl-peroxide, di(dichloropentafluorobutanoyl)peroxide, di(trichlorooctafluorohexanoyl)peroxide, di(tetrachloroundafluorooctanoyl)peroxide, di(pentachlorotetradecafluorodecanoyl)peroxide, and di(undachlorodotriacontafluorodocosanoyl)peroxide.

[0069] The water-soluble radical polymerization initiator may be a known water-soluble peroxide, such as ammonium salts, potassium salts, or sodium salts of persulfuric acid, perboric acid, perchloric acid, superphosphate, or percarbonate, as well as t-butyl permalate and t-butyl hydroperoxide. Reducing agents such as sulfites and sulfites may also be included, and their amount may be 0.1 to 20 times the amount of the peroxide.

[0070] There are no particular limitations on the amount of radical polymerization initiator added, but it is sufficient to add at least an amount that does not significantly reduce the polymerization rate (for example, a few ppm relative to water concentration) in a lump sum at the beginning of polymerization, or sequentially or continuously. The upper limit is the range in which the heat of the polymerization reaction can be removed from the apparatus surface.

[0071] Nonionic surfactants, anionic surfactants, cationic surfactants, etc., can be used as surfactants. The amount added (relative to the polymerization water) is preferably 10 to 5000 ppm. More preferably 50 to 5000 ppm. In addition, reactive emulsifiers can be used as surfactants. The reactive emulsifier is not particularly limited as long as it is a compound having one or more unsaturated bonds and one or more hydrophilic groups.

[0072] The solvent is preferably one that does not exhibit chain transfer properties. In the case of solution polymerization, dichloropentafluoropropane (R-225) is an example, while in the case of emulsion polymerization and suspension polymerization, water, a mixture of water and a water-soluble organic solvent, or a mixture of water and a water-insoluble organic solvent are examples.

[0073] In the polymerization described above, examples of chain transfer agents include esters such as dimethyl malonate, diethyl malonate, methyl acetate, ethyl acetate, butyl acetate, and dimethyl succinate, as well as isopentane, methane, ethane, propane, isopropanol, acetone, various mercaptans, carbon tetrachloride, and cyclohexane.

[0074] Bromine compounds or iodine compounds may be used as chain transfer agents. A polymerization method using bromine compounds or iodine compounds includes, for example, emulsion polymerization in an aqueous medium under pressurized conditions in the presence of a bromine compound or iodine compound, in a substantially oxygen-free environment (iodine transfer polymerization). A typical example of a bromine compound or iodine compound used is, for example, one with the general formula: R 2 I x Br y (In the formula, x and y are integers from 0 to 2, and satisfy 1 ≤ x + y ≤ 2, R 2 Examples of compounds are those represented by a saturated or unsaturated fluorohydrocarbon group or chlorofluorohydrocarbon group having 1 to 16 carbon atoms, or a hydrocarbon group having 1 to 3 carbon atoms (which may contain an oxygen atom).

[0075] Examples of iodine compounds include 1,3-diiodoperfluoropropane, 2-iodoperfluoropropane, 1,3-diiodo-2-chloroperfluoropropane, 1,4-diiodoperfluorobutane, 1,5-diiodo-2,4-dichloroperfluoropentane, 1,6-diiodoperfluorohexane, 1,8-diiodoperfluorooctane, 1,12-diiodoperfluorododecane, 1,16-diiodoperfluorohexadecane, diiodomethane, 1,2-diiodoethane, 1,3-diiodo-n-propane, CF2Br2, BrCF2CF2Br, CF3CFBrCF2Br, CFClBr2, and BrCF2CFC. Examples include lBr, CFBrClCFClBr, BrCF2CF2CF2Br, BrCF2CFBrOCF3, 1-bromo-2-iodoperfluoroethane, 1-bromo-3-iodoperfluoropropane, 1-bromo-4-iodoperfluorobutane, 2-bromo-3-iodoperfluorobutane, 3-bromo-4-iodoperfluorobutene-1, 2-bromo-4-iodoperfluorobutene-1, monoiodomonobromo substituted derivatives of benzene, diiodomonobromo substituted derivatives, and (2-iodoethyl) and (2-bromoethyl) substituted derivatives. These compounds may be used individually or in combination with each other.

[0076] Among these, 1,4-diiodoperfluorobutane, 1,6-diiodoperfluorohexane, and 2-iodoperfluoropropane are preferred in terms of polymerization reactivity, crosslinking reactivity, and availability.

[0077] In the case of emulsion polymerization, the copolymer obtained by the above-described method can be obtained in powder form by coagulating the dispersion after polymerization, washing with water, dehydrating, and drying. Coagulation can be achieved by adding an inorganic salt such as aluminum sulfate or an inorganic acid, applying mechanical shear force, or freezing the dispersion. In the case of suspension polymerization, the copolymer can be obtained in powder form by recovering it from the dispersion after polymerization and drying it. In the case of solution polymerization, the copolymer can be obtained by drying the solution containing the polymer as is, or by purifying it by adding a poor solvent dropwise.

[0078] In the positive electrode binder, one copolymer may be used, or two or more copolymers may be used. In particular, a combination of two copolymers with different molecular structures may be used. The present disclosure also relates to a positive electrode binder for a sodium-ion battery that contains two or more copolymers.

[0079] The combination of copolymers is not particularly limited and can be selected as appropriate. In particular, a combination of a copolymer having a constituent unit (B) derived from the monomer represented by the general formula (3) and other copolymers is preferred.

[0080] When combining copolymers as described above, the ratio (mass ratio) of copolymers having constituent units derived from the monomer represented by the general formula (3) to other copolymers is preferably 80:20 to 99.9:0.1. Within this range, the effects of the present invention can be realized. The ratio (mass ratio) of copolymers having constituent units derived from the monomer represented by the general formula (3) to other copolymers is more preferably 90:10 or higher, and even more preferably 95:5 or higher. The ratio (mass ratio) of copolymers having constituent units derived from the monomer represented by the general formula (3) to other copolymers is more preferably 99.5:0.5 or lower, and even more preferably 99:1 or lower.

[0081] The positive electrode binder of this disclosure may contain a blended polymer obtained by combining one or more copolymers and one or more fluorine-containing polymers other than the copolymers. This disclosure is also a positive electrode binder for a sodium-ion battery containing the copolymers and fluorine-containing polymers other than the copolymers.

[0082] The fluorine-containing polymer other than the copolymer mentioned above is not particularly limited, but PVdF, modified PVdF, and the like are preferred. Examples of modified PVdF include acrylic acid-modified PVDF.

[0083] When the copolymer is combined with other fluorine-containing polymers, the ratio (mass ratio) of the copolymer to the other fluorine-containing polymer is preferably 0.5:99.5 to 40:60. Within this range, the peel strength as an electrode can be maintained. The ratio (mass ratio) of the copolymer to the other fluorine-containing polymer is more preferably 1:90 or higher. The ratio (mass ratio) of the copolymer to the other fluorine-containing polymer is more preferably 35:65 or lower.

[0084] The sodium-ion battery positive electrode binder of this disclosure is suitably used in an electrode mixture for the positive electrode of a sodium-ion battery. Specifically, it can constitute an electrode mixture together with a positive electrode active material and a solvent. This disclosure also provides an electrode mixture containing a sodium composite oxide as the positive electrode active material and the positive electrode binder.

[0085] (Cathode active material) The positive electrode active material used in this disclosure is not particularly limited as long as it is capable of electrochemically intercepting and releasing sodium ions. The positive electrode active material is not particularly limited as long as it is electrochemically capable of intercalating and releasing alkali metal ions, but for example, a material containing an alkali metal and at least one transition metal is preferred. Specific examples include alkali metal-containing transition metal composite oxides and alkali metal-containing transition metal phosphate compounds. Among these, alkali metal-containing transition metal composite oxides that produce high voltage are particularly preferred as the positive electrode active material. Examples of alkali metal ions include lithium ions, sodium ions, potassium ions, etc. In a preferred embodiment, the alkali metal ions may be lithium ions or sodium ions. That is, in this embodiment, the alkali metal ion secondary battery is a sodium ion secondary battery.

[0086] Examples of the alkali metal-containing transition metal composite oxide include: Formula (3-1): MaMn 2-b M 1 b O4 (In the formula, M is Na; 0.9 ≤ a; 0 ≤ b ≤ 1.5; M) 1 Sodium manganese spinel composite oxide represented by at least one metal selected from the group consisting of Fe, Co, Ni, Cu, Zn, Al, Sn, Cr, V, Ti, Mg, Ca, Sr, B, Ga, In, Si, and Ge. Formula (3-2): MNi 1-c M 2 c O2 (In the formula, M is Na; 0 ≤ c ≤ 0.5; M) 2 (This refers to a sodium-nickel composite oxide represented by at least one metal selected from the group consisting of Fe, Co, Mn, Cu, Zn, Al, Sn, Cr, V, Ti, Mg, Ca, Sr, B, Ga, In, Si, and Ge), or Formula (3-3):MCo 1-d M 3 d O2 (In the formula, M is Na; 0 ≤ d ≤ 0.5; M) 3 (This refers to at least one metal selected from the group consisting of Fe, Ni, Mn, Cu, Zn, Al, Sn, Cr, V, Ti, Mg, Ca, Sr, B, Ga, In, Si, and Ge.) Examples include sodium-cobalt composite oxides represented by [formula]. In particular, MCoO2, MMnO2, MNiO2, MMn2O4, and MNi are used because they offer high energy density and can provide high-output secondary batteries. 0.8 Co 0.15 Al 0.05 O2, or MNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 is preferred, and it is preferable that the compound is represented by the following formula (3-4). MNi h Co i Mn j M 5 k O2(3-4) (In the formula, M is Na, M 5 (This represents at least one element selected from the group consisting of Fe, Cu, Zn, Al, Sn, Cr, V, Ti, Mg, Ca, Sr, B, Ga, In, Si, and Ge, where (h+i+j+k)=1.0, 0≦h≦1.0, 0≦i≦1.0, 0≦j≦1.5, and 0≦k≦0.2.)

[0087] Examples of the alkali metal-containing transition metal phosphate compound include the following formula (70): M e M 4 f (PO4) g (70) (In the formula, M is Na, M 4 (where e represents at least one element selected from the group consisting of V, Ti, Cr, Mn, Fe, Co, Ni, and Cu, and is represented by the formula 0.5≦e≦3, 1≦f≦2, 1≦g≦3) As the transition metal of the sodium-containing transition metal phosphate compound, V, Ti, Cr, Mn, Fe, Co, Ni, Cu, etc. are preferable. Specific examples include, for example, iron phosphates such as NaFePO4, Na3Fe2(PO4)3, NaFeP2O7, cobalt phosphates such as NaCoPO4, and those in which a part of the transition metal atoms that are the main components of these sodium transition metal phosphate compounds are substituted with other elements such as Al, Ti, V, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga, Zr, Nb, Si, etc. As the sodium-containing transition metal phosphate compound, those having an olivine-type structure are preferable.

[0088] As other positive electrode active materials, MFePO4, MNi 0.8 Co 0.2 O2, M 1.2 Fe 0.4 Mn 0.4 O2, MNi 0.5 Mn 1.5 O4, MV3O6, M2MnO3, etc. (where M is Na.) can be mentioned. In particular, positive electrode active materials such as MNi 0.5 Mn 1.5 O4, etc. are preferable in that the crystal structure does not collapse even when the secondary battery is operated at a voltage exceeding 4.4 V or a voltage of 4.6 V or higher. Therefore, an electrochemical device such as a secondary battery using a positive electrode material containing the positive electrode active material exemplified above is preferable because the residual capacity hardly decreases even when stored at a high temperature, the resistance increase rate hardly changes, and the battery performance does not deteriorate even when operated at a high voltage.

[0089] As other positive electrode active materials, solid solution materials such as M2MnO3 and MM 6 O2 (where M is at least one metal selected from the group consisting of Li, Na, and K, and M 6 is a transition metal such as Co, Ni, Mn, Fe, etc.) can also be mentioned. As the solid solution material, for example, it is an alkali metal manganate represented by the general formula Mx[Mn(1-y)M 7 y]Oz. Here, M in the formula is Na, and M 7It consists of at least one metal element other than M and Mn, and includes one or more elements selected from the group consisting of, for example, Co, Ni, Fe, Ti, Mo, W, Cr, Zr, and Sn. Also, the values of x, y, and z in the formula are in the range of 1 < x < 2, 0 ≤ y < 1, and 1.5 < z < 3.

[0090] The average particle size of the positive electrode active material is preferably, for example, 1 to 50 μm, more preferably 1 to 20 μm, and particularly preferably 3 to 7 μm. If the average particle size of the positive electrode active material is too small, the handling property may deteriorate. If the average particle size of the positive electrode active material is too large, it may be difficult to obtain a flat positive electrode active material layer. The average particle size of the positive electrode active material can be obtained, for example, by measuring and averaging the particle size of the active material carrier observed by a scanning electron microscope (SEM).

[0091] Also, a material with a different composition adhered to the surface of the positive electrode active material may be used. Examples of the surface-adhering substance include oxides such as aluminum oxide, silicon oxide, titanium oxide, zirconium oxide, magnesium oxide, calcium oxide, boron oxide, antimony oxide, and bismuth oxide; sulfates such as lithium sulfate, sodium sulfate, potassium sulfate, magnesium sulfate, calcium sulfate, and aluminum sulfate; carbonates such as lithium carbonate, calcium carbonate, and magnesium carbonate; and carbon.

[0092] These surface-adhering substances can be adhered to the surface of the positive electrode active material by, for example, dissolving or suspending them in a solvent, impregnating and adding them to the positive electrode active material, and then drying; dissolving or suspending the surface-adhering substance precursor in a solvent, impregnating and adding it to the positive electrode active material, and then reacting it by heating or the like; or adding it to the positive electrode active material precursor and firing them simultaneously. When adhering carbon, a method of mechanically adhering the carbonaceous material in the form of, for example, activated carbon can also be used.

[0093] The amount of surface-adhered material is preferably 0.1 ppm or more, more preferably 1 ppm or more, and even more preferably 10 ppm or more, relative to the positive electrode active material by mass, with an upper limit of preferably 20% or less, more preferably 10% or less, and even more preferably 5% or less. The surface-adhered material can suppress the oxidation reaction of the electrolyte on the surface of the positive electrode active material, thereby improving battery life. However, if the amount of adhesion is too small, the effect will not be fully realized, and if it is too large, it may inhibit the movement of sodium ions, which may increase resistance.

[0094] The particle shapes of the positive electrode active material can include conventionally used shapes such as lumpy, polyhedral, spherical, ellipsoidal, plate-like, needle-like, and columnar. Furthermore, primary particles may aggregate to form secondary particles.

[0095] The tap density of the positive electrode active material is preferably 0.5 g / cm³. 3 More preferably 0.8 g / cm³ 3 More preferably 1.0 g / cm³ 3 The above is the case. If the tap density of the positive electrode active material falls below the lower limit, the amount of dispersion medium required during the formation of the positive electrode active material layer increases, as does the amount of conductive material and binder required, which may restrict the filling rate of the positive electrode active material into the positive electrode active material layer and thus limit the battery capacity. By using a composite oxide powder with a high tap density, a high-density positive electrode active material layer can be formed. Generally, a higher tap density is preferable, and there is no particular upper limit, but if it is too high, the diffusion of sodium ions using the electrolyte as a medium within the positive electrode active material layer becomes the rate-limiting factor, and the load characteristics may tend to deteriorate. Therefore, the upper limit is preferably 4.0 g / cm³. 3 More preferably, 3.7 g / cm³ 3 More preferably, 3.5 g / cm³ 3 The following applies: In this disclosure, the tap density is defined as the powder packing density (tap density) g / cm³ obtained when 5-10 g of positive electrode active material powder is placed in a 10 ml glass graduated cylinder and tapped 200 times with a stroke of approximately 20 mm. 3 We will seek it as follows.

[0096] The median diameter d50 of the positive electrode active material particles (or secondary particle diameter if primary particles aggregate to form secondary particles) is preferably 0.3 μm or more, more preferably 0.5 μm or more, even more preferably 0.8 μm or more, and most preferably 1.0 μm or more. It is also preferably 30 μm or less, more preferably 27 μm or less, even more preferably 25 μm or less, and most preferably 22 μm or less. If it falls below the lower limit, it may not be possible to obtain a high tap density product, and if it exceeds the upper limit, the diffusion of lithium within the particles will take a long time, which may lead to a decrease in battery performance or cause problems such as streaking when manufacturing the positive electrode of the battery, i.e., when the active material and conductive material or binder are slurryed with a solvent and applied as a thin film. Here, by mixing two or more types of positive electrode active materials having different median diameters d50, the packing performance during positive electrode manufacturing can be further improved.

[0097] In this disclosure, the median diameter d50 is measured using a known laser diffraction / scattering particle size distribution analyzer. When using the HORIBA LA-920 as the particle size distribution analyzer, a 0.1% by mass aqueous solution of sodium hexametaphosphate is used as the dispersion medium during measurement, and the measurement is performed after ultrasonic dispersion for 5 minutes with the measurement refractive index set to 1.24.

[0098] When primary particles aggregate to form secondary particles, the average primary particle diameter of the positive electrode active material is preferably 0.05 μm or more, more preferably 0.1 μm or more, and even more preferably 0.2 μm or more. The upper limit is preferably 5 μm or less, more preferably 4 μm or less, even more preferably 3 μm or less, and most preferably 2 μm or less. If the upper limit is exceeded, it becomes difficult to form spherical secondary particles, which can adversely affect powder packing properties and may lead to a significant decrease in specific surface area, potentially reducing battery performance such as output characteristics. Conversely, if the lower limit is exceeded, problems such as poor reversibility of charge and discharge may occur, usually due to underdeveloped crystals.

[0099] In this disclosure, the primary particle diameter is measured by observation using a scanning electron microscope (SEM). Specifically, it is determined by taking a photograph at 10,000x magnification, finding the longest value of the intercept between the left and right boundaries of the primary particle relative to a horizontal line for any 50 primary particles, and taking the average value.

[0100] The BET specific surface area of ​​the positive electrode active material is preferably 0.1 m². 2 / g or more, more preferably 0.2m 2 / g or more, more preferably 0.3m 2 The value is 1 / g or more, and the upper limit is preferably 50m 2 / g or less, more preferably 40m 2 / g or less, more preferably 30m 2 It is less than / g. If the BET specific surface area is smaller than this range, battery performance tends to decrease, and if it is larger, it becomes difficult to increase the tap density, which can cause problems with coating when forming the positive electrode active material layer.

[0101] In this disclosure, the BET specific surface area is defined as the value measured by a nitrogen adsorption BET single-point method using a gas flow method, after pre-drying the sample at 150°C for 30 minutes under nitrogen flow using a surface area meter (for example, a fully automatic surface area measuring device manufactured by Okura Riken Co., Ltd.), and then using a nitrogen-helium mixed gas that has been precisely adjusted so that the relative pressure of nitrogen to atmospheric pressure is 0.3.

[0102] When the secondary battery of this disclosure is used as a large sodium-ion secondary battery for hybrid vehicles or distributed power sources, high output is required, so it is preferable that the particles of the positive electrode active material consist mainly of secondary particles. The positive electrode active material particles preferably contain 0.5 to 7.0 volume percent of fine particles with an average secondary particle diameter of 40 μm or less and an average primary particle diameter of 1 μm or less. By including fine particles with an average primary particle diameter of 1 μm or less, the contact area with the electrolyte is increased, which allows for faster diffusion of lithium ions between the electrode and the electrolyte, and as a result, the output performance of the battery can be improved.

[0103] For the production of positive electrode active materials, general methods for producing inorganic compounds are used. In particular, various methods can be considered for producing spherical or ellipsoidal active materials. For example, a method can be used in which transition metal raw materials are dissolved or pulverized and dispersed in a solvent such as water, the pH is adjusted while stirring to produce and recover spherical precursors, these are dried as needed, and then an Na source such as sodium hydroxide is added and calcined at a high temperature to obtain the active material.

[0104] For the manufacture of the positive electrode, the positive electrode active material may be used alone, or two or more materials with different compositions may be used in any combination or ratio.

[0105] The solvent used in the electrode mixture is not particularly limited in type, as long as it is capable of dissolving or dispersing the positive electrode active material, binder, and conductive agents and thickeners used as needed. Either aqueous or organic solvents may be used. Examples of aqueous solvents include water and mixtures of alcohol and water. Examples of organic solvents include aliphatic hydrocarbons such as hexane; aromatic hydrocarbons such as benzene, toluene, xylene, and methylnaphthalene; heterocyclic compounds such as quinoline and pyridine; ketones such as acetone, methyl ethyl ketone, and cyclohexanone; esters such as methyl acetate and methyl acrylate; amines such as diethylenetriamine and N,N-dimethylaminopropylamine; ethers such as diethyl ether, propylene oxide, and tetrahydrofuran (THF); amides such as N-methylpyrrolidone (NMP), dimethylformamide, and dimethylacetamide; and aprotic polar solvents such as hexamethylphosphoramide and dimethyl sulfoxide.

[0106] As an organic solvent, a solvent represented by general formula (4) can also be used.

[0107] General formula (4): [ka] (In the formula, R 1 , R 2and R 3 R is independently H or an organic group, however R 1 , R 2 and R 3 The total number of carbon atoms is 6 or more, R 1 , R 2 and R 3 At least one of them is an organic group having a carbonyl group. 1 , R 2 and R 3 (Two of these may be joined together to form a ring.)

[0108] R 1 , R 2 and R 3 The total number of carbon atoms is 6 or more. That is, R 1 , R 2 and R 3 The types of groups are selected such that the total number of carbon atoms is 6 or more. 1 , R 2 and R 3 There is no upper limit on the total number of carbon atoms, but it may be 16 or less, 14 or less, or 12 or less.

[0109] R 1 , R 2 and R 3 The group is independently either H or an organic group. Preferred organic groups include alkyl groups, alkoxyalkyl groups, acylalkyl groups, alkenyl groups, amino groups, aminoalkyl groups, or cycloalkyl groups.

[0110] R 1 , R 2 and R 3 At least one of them is an organic group having a carbonyl group. An acyl group is preferred as the organic group having a carbonyl group. An acyl group has the general formula: -C OR 4 (In the formula, R 4 A group represented by (where is an alkyl group having 1 to 6 carbon atoms) is preferred. If the alkyl group has 3 or more carbon atoms, it may be linear or branched. 4If the alkyl group has two or more carbon atoms, it may contain a heteroatom such as an oxygen atom or a nitrogen atom, or a carbonyl group between the carbon atoms.

[0111] R 1 , R 2 and R 3 Any two of these atoms may bond to form a ring. The ring may also contain heteroatoms such as oxygen atoms. The ring is preferably a saturated ring. The number of members in the ring is not particularly limited, but 5-membered or 6-membered rings are preferred. Preferred rings are pyrrolidine rings, oxazoline rings, piperidine rings, or morpholine rings.

[0112] As the solvent, a solvent represented by general formula (4a) is preferred.

[0113] General formula (4a): [ka] (In the formula, R 1a R is an organic group, 2a and R 3a R is independently H or an organic group, however R 1a , R 2a and R 3a The total number of carbon atoms is 5 or more. 1a , R 2a and R 3a (Two of these may be joined together to form a ring.)

[0114] In general formula (4a), R 1a , R 2a and R 3a The total number of carbon atoms is 5 or more. That is, R 1a , R 2a and R 3a The types of groups are selected such that the total number of carbon atoms is 5 or more. 1a , R 2a and R 3a There is no upper limit on the total number of carbon atoms, but it may be 15 or less, 13 or less, or 11 or less.

[0115] In general formula (4a), R 1a is an organic group. Preferred organic groups are alkyl groups, alkoxyalkyl groups, acylalkyl groups, alkenyl groups, amino groups, aminoalkyl groups, or cycloalkyl groups, and more preferably alkyl groups, alkoxyalkyl groups, acylalkyl groups, alkenyl groups, amino groups, or aminoalkyl groups.

[0116] In general formula (4a), R 2a and R 3a R is independently either H or an organic group. 2a and R 3a As for the group, an organic group is preferred independently. Preferred organic groups are alkyl groups, alkoxyalkyl groups, acylalkyl groups, alkenyl groups, amino groups, aminoalkyl groups, or cycloalkyl groups, and more preferably alkyl groups, alkoxyalkyl groups, cycloalkyl groups, or alkenyl groups.

[0117] R 1a , R 2a and R 3a Any two of them may be joined together to form a ring. In particular, R 2a and R 3a They combine, R 2a and R 3a It is preferable that a ring is formed together with the nitrogen atom to which it is bonded. The ring may also contain heteroatoms such as oxygen atoms. The ring is preferably a saturated ring. The number of members in the ring is not particularly limited, but 5-membered or 6-membered rings are preferred. Preferred rings are pyrrolidine rings, oxazoline rings, piperidine rings, or morpholine rings.

[0118] As the solvent, at least one selected from the group consisting of solvents represented by general formula (4b-1) and solvents represented by general formula (4b-2) is more preferable.

[0119] General formula (4b-1): [ka] (In the formula, R 1bR is an alkyl group, alkoxyalkyl group, acylalkyl group, alkenyl group, amino group, or aminoalkyl group. 2b and R 3b R is independently an alkyl group or an alkoxyalkyl group. 1b , R 2b and R 3b The total number of carbon atoms is 5 or more. 2b and R 3b They combine with each other, R 2b and R 3b It may form a ring with the nitrogen atom to which it is bonded, and the ring may contain an oxygen atom as a constituent atom.

[0120] General formula (4b-2): [ka] (In the formula, ring A is a 5-membered or 6-membered amide ring, R 4b is an alkyl group, a cycloalkyl group, or an alkenyl group, and rings A and R 4b The total number of carbon atoms is 5 or more.

[0121] In general formula (4b-1), R 1b , R 2b and R 3b The total number of carbon atoms is 5 or more. That is, R 1b , R 2b and R 3b The types of groups are selected such that the total number of carbon atoms is 5 or more. 1b , R 2b and R 3b There is no upper limit on the total number of carbon atoms, but it may be 15 or less, 13 or less, or 11 or less.

[0122] In general formula (4b-1), R 1b This is an alkyl group, alkoxyalkyl group, acylalkyl group, alkenyl group, amino group, or aminoalkyl group.

[0123] R 1bThe alkyl group is preferably an alkyl group having 1 to 10 carbon atoms. If the alkyl group has 3 or more carbon atoms, it may be linear or branched.

[0124] R 1b As for alkoxyalkyl groups, the general formula is: -R 1b1 -OR 1b2 (In the formula, R 1b1 R is an alkylene group having 1 to 5 carbon atoms. 1b2 A group represented by (where is an alkyl group having 1 to 5 carbon atoms) is preferred. If the alkyl group and alkylene group have 3 or more carbon atoms, they may be linear or branched.

[0125] R 1b The acylalkyl group is of the general formula: -R 1b3 -CO-R 1b4 (In the formula, R 1b3 R is an alkylene group having 1 to 5 carbon atoms. 1b4 A group represented by (where is an alkyl group having 1 to 5 carbon atoms) is preferred. If the alkyl group and alkylene group have 3 or more carbon atoms, they may be linear or branched.

[0126] R 1b The alkenyl group is, in general formula: -R 1b5 -CR 1b6 =CR 1b7 (In the formula, R 1b5 R is a single bond or an alkylene group having 1 to 5 carbon atoms. 1b6 and R 1b7 The group is preferably one that is independently represented by H or an alkyl group having 1 to 5 carbon atoms. If the alkyl group and alkylene group have 3 or more carbon atoms, they may be linear or branched. A vinyl group is preferred as the alkenyl group.

[0127] R 1b The amino groups and aminoalkyl groups are monovalent functional groups obtained by removing hydrogen from ammonia, primary or secondary amines. 1b If it is an amino group, R 1bIt can form an amide bond with the carbonyl group to which it is bonded.

[0128] R 1b The amino group is of the general formula: -N-(R 1b8 )2(wherein, R 1b8 A group represented by H or an alkyl group having 1 to 5 carbon atoms is preferred. If the alkyl group has 3 or more carbon atoms, it may be linear or branched. As the amino group, -N-(CH3)2 or -N-(C2H5)2 is preferred.

[0129] R 1b The aminoalkyl group is, in general formula: -R 1b9 -N-(R 1b8 )2(wherein, R 1b9 R is an alkylene group having 1 to 5 carbon atoms. 1b8 A group represented by (where is H or an alkyl group having 1 to 5 carbon atoms) is preferred. If the alkyl group and alkylene group have 3 or more carbon atoms, they may be linear or branched.

[0130] In general formula (4b-1), R 2b and R 3b These are independently alkyl groups or alkoxyalkyl groups.

[0131] R 2b and R 3b The alkyl group is preferably an alkyl group having 1 to 10 carbon atoms. If the alkyl group has 3 or more carbon atoms, it may be linear or branched.

[0132] R 2b and R 3b As for alkoxyalkyl groups, the general formula is: -R 2b1 -OR 2b2 (In the formula, R 2b1 R is an alkylene group having 1 to 5 carbon atoms. 2b2 A group represented by (where is an alkyl group having 1 to 5 carbon atoms) is preferred. If the alkyl group and alkylene group have 3 or more carbon atoms, they may be linear or branched.

[0133] R 2b and R 3b They combine with each other, R 2b and R 3b The nitrogen atom to which the compound is bonded may form a ring, and the ring may contain an oxygen atom as a constituent atom. The ring is preferably a saturated ring. The number of members in the ring is not particularly limited, but 5-membered or 6-membered rings are preferred. Preferred rings are pyrrolidine rings, oxazoline rings, piperidine rings, or morpholine rings.

[0134] In general formula (4b-2), rings A and R 4b The total number of carbon atoms is 5 or more. That is, rings A and R 4b The types of rings and groups are selected such that the total number of carbon atoms is 5 or more. Rings A and R 4b There is no upper limit on the total number of carbon atoms, but it may be 15 or less, 13 or less, or 11 or less.

[0135] Ring A is a five- or six-membered amide ring. Therefore, ring A is composed of a carbon atom, a nitrogen atom, and a carbon-3 to carbon-4 alkylene group. The hydrogen atoms bonded to the carbon atoms of the alkylene group constituting ring A may or may not be substituted with substituents, but it is preferable that they are not substituted with substituents. Examples of substituents include alkyl groups such as methyl groups.

[0136] R 4b This is an alkyl group, a cycloalkyl group, or an alkenyl group.

[0137] R 4b The alkyl group is preferably an alkyl group having 1 to 10 carbon atoms. If the alkyl group has 3 or more carbon atoms, it may be linear or branched.

[0138] R 4b As the cycloalkyl group, a cycloalkyl group having 3 to 10 carbon atoms is preferred. Preferred cycloalkyl groups are cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, or cyclooctyl groups.

[0139] R 4b The alkenyl group is, in general formula: -R 4b1 -CR 4b2 =CR 4b3 (In the formula, R 4b1 R is a single bond or an alkylene group having 1 to 5 carbon atoms. 4b2 and R 4b3 The group is preferably one that is independently represented by H or an alkyl group having 1 to 5 carbon atoms. If the alkyl group and alkylene group have 3 or more carbon atoms, they may be linear or branched. A vinyl group is preferred as the alkenyl group.

[0140] As a solvent, at least one selected from the group consisting of 3-methoxy-N,N-dimethylpropanamide, N-ethyl-2-pyrrolidone (NEP), N-butyl-2-pyrrolidone (NBP), acryloylmorpholine, N-cyclohexyl-2-pyrrolidone, N-vinyl-2-pyrrolidone, 3-butoxy-N,N-dimethylpropanamide, N,N,N',N'-tetraethylurea, N,N-dimethylacetacetamide, N-octyl-2-pyrrolidone, and N,N-diethylacetamide is preferred.

[0141] As a solvent, at least one selected from the group consisting of 3-methoxy-N,N-dimethylpropanamide, N-ethyl-2-pyrrolidone, and N-butyl-2-pyrrolidone is more preferred. In particular, when an electrode mixture containing 3-methoxy-N,N-dimethylpropanamide is used as the solvent, the amount of gas generated in the resulting battery tends to be suppressed. In particular, when an electrode mixture containing N-ethyl-2-pyrrolidone (NEP) is used as the solvent, the high-temperature storage capacity retention rate of the resulting battery tends to be higher. In particular, when an electrode mixture containing N-butyl-2-pyrrolidone (NBP) is used as the solvent, the resistance of the resulting battery tends to increase less.

[0142] The amount of solvent in the electrode mixture is determined considering factors such as its applicability to the current collector and its ability to form a thin film after drying. Typically, the ratio of copolymer to solvent is preferably 0.5:99.5 to 20:80 by mass ratio.

[0143] The electrode mixture of this disclosure may further contain a conductive agent as needed. Any known conductive agent can be used as the conductive agent. Specific examples include metal materials such as copper and nickel, graphite such as natural graphite and artificial graphite, carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black, and carbon materials such as needle coke, carbon nanotubes, fullerenes, and amorphous carbon such as VGCF. These may be used individually or in any combination and ratio of two or more.

[0144] The conductive agent is typically contained in the electrode material layer in an amount of 0.01% by mass or more, preferably 0.1% by mass or more, more preferably 1% by mass or more, and typically 50% by mass or less, preferably 30% by mass or less, more preferably 15% by mass or less. If the content is lower than this range, the conductivity may be insufficient. Conversely, if the content is higher than this range, the battery capacity may decrease.

[0145] The electrode mixture of this disclosure may further contain a thickening agent as needed. Examples of the thickening agent include carboxymethylcellulose, methylcellulose, hydroxymethylcellulose, ethylcellulose, polyvinyl alcohol, starch oxide, phosphorylated starch, casein, polyvinylpyrrolidone, and salts thereof. One of these may be used alone, or two or more may be used in any combination and ratio.

[0146] The ratio of the thickener to the active material is usually 0.1% by mass or more, preferably 0.2% by mass or more, more preferably 0.3% by mass or more, and usually within the range of 5% by mass or less, preferably 3% by mass or less, and more preferably 2% by mass or less. If it is below this range, the coating properties may be significantly reduced. If it is above this range, the proportion of active material in the electrode material layer will decrease, which may lead to problems such as a decrease in battery capacity or an increase in resistance between positive electrode active materials.

[0147] The electrode mixture may contain materials other than those mentioned above. However, the content of such materials is preferably 8% by mass or less, and more preferably 4% by mass or less, when the entire electrode material layer is considered to be 100% by mass.

[0148] In the process of preparing the electrode mixture according to this disclosure, the above-mentioned components are mixed to form a slurry. The mixing order of the components is not particularly limited; each component can be added to the solvent and mixed.

[0149] The solid content concentration in the slurry is preferably 70 to 95% by mass. Within this range, interfacial resistance during coating is suppressed. The lower limit is preferably 73% by mass or more, more preferably 73.5% by mass or more. The upper limit is preferably 85% by mass or less.

[0150] The electrode mixture of this disclosure can be suitably used as an electrode material layer in the positive electrode of a sodium-ion secondary battery. An electrode comprising a current collector and an electrode material layer formed from the electrode mixture of the present disclosure, provided on one or both sides of the current collector, is also part of the present disclosure. The sodium-ion secondary battery described above can adopt a known structure and typically comprises a positive electrode and a negative electrode capable of intercalating and releasing sodium ions, and an electrolyte. A sodium-ion secondary battery comprising the electrodes of this disclosure is also one of the disclosures.

[0151] <Positive electrode> The positive electrode is preferably composed of an electrode material layer containing the positive electrode active material (hereinafter sometimes referred to as the positive electrode active material layer) and a current collector.

[0152] Suitable materials for the positive electrode current collector include metals such as aluminum, titanium, tantalum, stainless steel, and nickel, or their alloys; and carbon materials such as carbon cloth and carbon paper. Among these, metal materials, particularly aluminum or its alloys, are preferred.

[0153] Examples of current collector shapes include metal foil, metal cylinders, metal coils, metal plates, expanded metal, punched metal, and foamed metal in the case of metal materials, and carbon plates, carbon thin films, and carbon cylinders in the case of carbon materials. Of these, metal foil is preferred. The thin film may be formed in a mesh shape as appropriate. The thickness of the thin film is arbitrary, but is usually 1 μm or more, preferably 3 μm or more, more preferably 5 μm or more, and usually 1 mm or less, preferably 100 μm or less, more preferably 50 μm or less. If the thin film is thinner than this range, it may lack the necessary strength as a current collector. Conversely, if the thin film is thicker than this range, its handling may be impaired.

[0154] Furthermore, it is preferable that a conductive additive is applied to the surface of the current collector, from the viewpoint of reducing the electrical contact resistance between the current collector and the positive electrode active material layer. Examples of conductive additives include carbon and precious metals such as gold, platinum, and silver.

[0155] The ratio of the thickness of the current collector to the thickness of the positive electrode active material layer is not particularly limited, but the value of (thickness of the positive electrode active material layer on one side immediately before electrolyte injection) / (thickness of the current collector) is preferably 20 or less, more preferably 15 or less, most preferably 10 or less, and also preferably 0.5 or more, more preferably 0.8 or more, most preferably 1 or more. If it exceeds this range, the current collector may generate heat due to Joule heating during high current density charging and discharging. If it falls below this range, the volume ratio of the current collector to the positive electrode active material increases, which may reduce the battery capacity.

[0156] The positive electrode can be manufactured by conventional methods. For example, the positive electrode active material can be mixed with the aforementioned binder, thickener, conductive material, solvent, etc., to form a slurry-like electrode mixture, which can then be applied to a current collector, dried, and pressed to increase its density.

[0157] The aforementioned densification can be achieved by hand pressing, roller pressing, or the like. The density of the positive electrode active material layer is preferably 1.5 g / cm³. 3 More preferably 2 g / cm³ 3 More preferably 2.2 g / cm³ 3 The above is true, and preferably 5 g / cm³ 3 More preferably, 4.5 g / cm³ 3 More preferably, 4 g / cm³ 3 The range is as follows. If the range is exceeded, the penetration of the electrolyte near the current collector / active material interface decreases, which can reduce charge / discharge characteristics, especially at high current densities, and may prevent high output from being obtained. Conversely, if the range is below this, the conductivity between the active materials decreases, increasing battery resistance and potentially preventing high output from being obtained.

[0158] Furthermore, from the viewpoint of increasing high output and stability at high temperatures, it is preferable that the area of ​​the positive electrode active material layer be large relative to the outer surface area of ​​the battery casing. Specifically, it is preferable that the sum of the positive electrode areas relative to the surface area of ​​the secondary battery casing be 15 times or more in area ratio, and more preferably 40 times or more. The outer surface area of ​​the battery casing refers to the total area calculated from the length, width, and thickness of the case portion filled with the power generation elements, excluding the terminal protrusions, in the case of a bottomed rectangular shape. In the case of a bottomed cylindrical shape, it is the geometric surface area approximating the case portion filled with the power generation elements, excluding the terminal protrusions, as a cylinder. The sum of the positive electrode areas refers to the geometric surface area of ​​the positive electrode mixture layer facing the mixture layer containing the negative electrode active material, and in a structure in which positive electrode mixture layers are formed on both sides via a current collector foil, it refers to the sum of the areas calculated separately for each surface.

[0159] The content in the electrode material layer of the copolymer is preferably 0.1 to 5% by mass, more preferably 0.5 to 3% by mass. Also, the lower limit is preferably 0.6% by mass or more, more preferably 1% by mass or more, and particularly preferably 1.1% by mass or more. The upper limit is preferably 2% by mass or less, more preferably 1.7% by mass or less. If the content of the copolymer in the electrode material layer is low, the flexibility of the electrode may decrease. Conversely, if the content is too high, the battery performance may decrease.

[0160] When using a blend polymer in which the copolymer and one or more fluorine-containing polymers other than the copolymer are combined, the content of the blend polymer in the electrode material layer is preferably 0.1 to 5% by mass, more preferably 0.5 to 3% by mass. Also, the lower limit is preferably 0.6% by mass or more, more preferably 1% by mass or more, and particularly preferably 1.1% by mass or more. The upper limit is preferably 2% by mass or less, more preferably 1.7% by mass or less. If the content of the blend polymer in the electrode material layer is low, the flexibility of the electrode may decrease. Conversely, if the content is too high, the battery performance may decrease.

[0161] The content of the positive electrode active material is preferably 50 to 99.5% by mass of the electrode material layer, more preferably 80 to 99% by mass, in terms of high battery capacity. Also, the content of the positive electrode active material in the positive electrode active material layer is preferably 80% by mass or more, more preferably 82% by mass or more, and particularly preferably 84% by mass or more. The upper limit is preferably 99% by mass or less, more preferably 98% by mass or less. If the content of the positive electrode active material in the electrode material layer is low, the electric capacity may be insufficient. Conversely, if the content is too high, the strength of the positive electrode may be insufficient.

[0162] The thickness of the positive electrode plate is not particularly limited, but from the viewpoints of high capacity and high output, the thickness of the mixture layer obtained by subtracting the thickness of the metal foil of the core material is preferably 10 μm or more, more preferably 20 μm or more, as the lower limit, with respect to one side of the current collector, and is preferably 500 μm or less, more preferably 450 μm or less.

[0163] Alternatively, a material with a composition different from that of the positive electrode plate may be used on the surface of the positive electrode plate. Examples of surface adherent substances include oxides such as aluminum oxide, silicon oxide, titanium oxide, zirconium oxide, magnesium oxide, calcium oxide, boron oxide, antimony oxide, bismuth oxide, sulfates such as lithium sulfate, sodium sulfate, potassium sulfate, magnesium sulfate, calcium sulfate, aluminum sulfate, carbonates such as lithium carbonate, calcium carbonate, magnesium carbonate, and carbon.

[0164] <Negative electrode> The negative electrode is preferably composed of an electrode material layer containing a negative electrode active material (hereinafter sometimes referred to as a negative electrode active material layer) and a current collector.

[0165] (Negative electrode active material) The negative electrode active material is not particularly limited. For example, lithium metal, artificial graphite, graphite carbon fiber, resin-fired carbon, thermally decomposed vapor-phase grown carbon, coke, mesocarbon microbeads (MCMB), furfuryl alcohol resin-fired carbon, polyacene, pitch-based carbon fiber, vapor-phase grown carbon fiber, natural graphite, and those containing carbonaceous materials such as non-graphitizable carbon, silicon-containing compounds such as silicon and silicon alloys, Li4Ti5O 12 Any one selected from the above, or a mixture of two or more types, etc. can be mentioned. Among them, those containing at least a part of carbonaceous materials and silicon-containing compounds can be particularly preferably used.

[0166] In the present disclosure, it is preferable that the negative electrode active material used contains silicon as a constituent element. By making it contain silicon as a constituent element, a high-capacity battery can be produced.

[0167] As the silicon-containing material, silicon particles, particles having a structure in which fine particles of silicon are dispersed in a silicon-based compound, silicon oxide particles represented by the general formula SiOx (0.5 ≦ x ≦ 1.6), or a mixture thereof are preferable. By using these, a negative electrode binder for a lithium ion secondary battery with higher initial charge-discharge efficiency, high capacity, and excellent cycle characteristics can be obtained.

[0168] In this invention, silicon oxide refers to a general term for amorphous silicon oxides, and silicon oxide before disproportionation is represented by the general formula SiOx (0.5 ≤ x ≤ 1.6). x is preferably 0.8 ≤ x < 1.6, and more preferably 0.8 ≤ x < 1.3. This silicon oxide can be obtained, for example, by heating a mixture of silicon dioxide and metallic silicon to produce silicon monoxide gas, which is then cooled and precipitated.

[0169] Particles having a structure in which silicon nanoparticles are dispersed in a silicon-based compound can be obtained, for example, by calcining a mixture of silicon nanoparticles and a silicon-based compound, or by heat-treating silicon oxide particles before disproportionation, represented by the general formula SiOx, in an inert, non-oxidizing atmosphere such as argon at a temperature of 400°C or higher, preferably 800-1,100°C, to carry out a disproportionation reaction. The material obtained by the latter method is particularly preferable because the silicon microcrystals are uniformly dispersed. The size of the silicon nanoparticles can be made to 1-100 nm by the disproportionation reaction described above. It is desirable that the silicon oxide in the particles having a structure in which silicon nanoparticles are dispersed in silicon oxide is silicon dioxide. It can be confirmed by transmission electron microscopy that silicon nanoparticles (crystals) are dispersed in amorphous silicon oxide.

[0170] The physical properties of silicon-containing particles can be appropriately selected depending on the target composite particle. For example, the average particle size is preferably 0.1 to 50 μm, the lower limit is more preferably 0.2 μm or more, and even more preferably 0.5 μm or more. The upper limit is more preferably 30 μm or less, and even more preferably 20 μm or less. In this invention, the average particle size is expressed as the weight-average particle size measured by the particle size distribution method using laser diffraction.

[0171] BET specific surface area is 0.5-100m 2 / g is preferred, 1 to 20m 2 / g is more preferable. BET specific surface area is 0.5m² 2 If the value is 1 / g or higher, there is no risk of reduced adhesion to the electrodes and a decrease in battery performance. Also, 100m 2If the value is less than / g, the proportion of silicon dioxide on the particle surface will be high, and there will be no risk of a decrease in battery capacity when used as a negative electrode material for sodium-ion secondary batteries.

[0172] By carbon coating the silicon-containing particles, conductivity is imparted, resulting in improved battery characteristics. Methods for imparting conductivity include mixing with conductive particles such as graphite, coating the surface of the silicon-containing particles with a carbon film, and combining both methods. However, coating with a carbon film is preferred, and chemical vapor deposition (CVD) is more preferred.

[0173] The content of the negative electrode active material is preferably 40% by mass or more, more preferably 50% by mass or more, and particularly preferably 60% by mass or more, in order to increase the volume of the resulting electrode mixture. The upper limit is preferably 99% by mass or less, and more preferably 98% by mass or less.

[0174] (Conductive agent) The negative electrode active material layer may further contain a conductive agent as needed. Any known conductive agent can be used as the conductive agent. Specific examples include metal materials such as copper and nickel, graphite such as natural graphite and artificial graphite, carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black, and carbon materials such as needle coke, carbon nanotubes, fullerenes, and amorphous carbon such as VGCF. These may be used individually or in any combination and ratio of two or more.

[0175] The conductive agent is typically contained in the negative electrode active material layer in an amount of 0.01% by mass or more, preferably 0.1% by mass or more, more preferably 1% by mass or more, and typically 50% by mass or less, preferably 30% by mass or less, more preferably 15% by mass or less. If the content is lower than this range, the conductivity may be insufficient. Conversely, if the content is higher than this range, the battery capacity may decrease.

[0176] (binder) The negative electrode active material layer preferably contains a binder. The binder is not particularly limited, and examples include those similar to the binder that can be used for the positive electrode as described above. The ratio of the binder to the negative electrode active material is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, particularly preferably 0.6% by mass or more, preferably 20% by mass or less, more preferably 15% by mass or less, even more preferably 10% by mass or less, and particularly preferably 8% by mass or less. If the ratio of the binder to the negative electrode active material exceeds the above range, the proportion of binder that does not contribute to the battery capacity increases, which may lead to a decrease in battery capacity. Also, if it falls below the above range, it may lead to a decrease in the strength of the negative electrode.

[0177] In particular, when a rubbery polymer such as SBR is included as the main component, the ratio of the binder to the negative electrode active material is usually 0.1% by mass or more, preferably 0.5% by mass or more, more preferably 0.6% by mass or more, and usually 5% by mass or less, preferably 3% by mass or less, and more preferably 2% by mass or less. Furthermore, when a fluorine-based polymer such as polyvinylidene fluoride is included as the main component, the ratio to the negative electrode active material is usually 1% by mass or more, preferably 2% by mass or more, more preferably 3% by mass or more, and usually 15% by mass or less, preferably 10% by mass or less, and more preferably 8% by mass or less.

[0178] The negative electrode active material layer may further contain a thermoplastic resin. Examples of thermoplastic resins include vinylidene fluoride, polypropylene, polyethylene, polystyrene, polyethylene terephthalate, and polyethylene oxide. One type may be used alone, or two or more types may be used in any combination and ratio.

[0179] The proportion of the thermoplastic resin with respect to the electrode active material is usually 0.01% by mass or more, preferably 0.05% by mass or more, more preferably 0.10% by mass or more, and is usually in the range of 3.0% by mass or less, preferably 2.5% by mass or less, more preferably 2.0% by mass or less. By adding the thermoplastic resin, the mechanical strength of the electrode can be improved. Also, if it exceeds this range, problems such as a decrease in the proportion of the electrode active material in the electrode binder and a decrease in the capacity of the battery, or an increase in the resistance between the active materials may occur.

[0180] The negative electrode active material layer may contain a thickener. Examples of the thickener include the same ones as those that can be used for the positive electrode described above. The proportion of the thickener with respect to the negative electrode active material is usually 0.1% by mass or more, preferably 0.5% by mass or more, more preferably 0.6% by mass or more, and is usually 5% by mass or less, preferably 3% by mass or less, more preferably 2% by mass or less. If the proportion of the thickener with respect to the negative electrode active material is below the above range, the coating property may be significantly reduced. Also, if it exceeds the above range, problems such as a decrease in the proportion of the negative electrode active material in the negative electrode active material layer and a decrease in the capacity of the battery, or an increase in the resistance between the negative electrode active materials may occur.

[0181] (Other components) The negative electrode active material layer of the present disclosure may further contain other components such as a leveling agent and a reinforcing material.

[0182] The negative electrode includes a negative electrode active material layer and a current collector. The negative electrode active material layer is formed using the above-mentioned negative electrode binder and may be provided on one side or both sides of the current collector.

[0183] Examples of the current collector included in the negative electrode include metal foils or metal meshes such as iron, stainless steel, copper, aluminum, nickel, titanium, etc., carbon materials such as carbon cloth and carbon paper, etc. Among them, copper foil is preferred.

[0184] Examples of current collector shapes include metal foil, metal cylinders, metal coils, metal plates, expanded metal, punched metal, and foamed metal in the case of metal materials, and carbon plates, carbon thin films, and carbon cylinders in the case of carbon materials. Of these, metal foil is preferred. The thin film may be formed in a mesh shape as appropriate. The thickness of the thin film is arbitrary, but is usually 1 μm or more, preferably 3 μm or more, more preferably 5 μm or more, and usually 1 mm or less, preferably 100 μm or less, more preferably 50 μm or less. If the thin film is thinner than this range, it may lack the necessary strength as a current collector. Conversely, if the thin film is thicker than this range, its handling may be impaired.

[0185] The negative electrode can be manufactured by conventional methods. For example, after mixing a binder and a solvent, a slurry-like negative electrode mixture can be prepared by adding a negative electrode active material and further mixing to the resulting mixture. The obtained negative electrode mixture is then uniformly applied to a current collector such as a metal foil or metal mesh, the coating is dried, and optionally heat-treated. The resulting dried coating is then pressed as needed to form a thin negative electrode material layer on the current collector, creating a thin-film electrode. Alternatively, the negative electrode active material and binder may be mixed first, and then the solvent may be added to prepare the negative electrode mixture.

[0186] Examples of the aforementioned solvent include water or an organic solvent. An organic solvent is preferred because it significantly reduces the possibility of moisture remaining in the negative electrode material layer when the negative electrode mixture is used to form the negative electrode material layer.

[0187] Examples of organic solvents include nitrogen-containing organic solvents such as N-methyl-2-pyrrolidone, N,N-dimethylacetamide, and dimethylformamide; ketone solvents such as acetone, methyl ethyl ketone, cyclohexanone, and methyl isobutyl ketone; ester solvents such as ethyl acetate and butyl acetate; ether solvents such as tetrahydrofuran and dioxane; β-alkoxypropionamides such as β-methoxy-N,N-dimethylpropionamide, β-n-butoxy-N,N-dimethylpropionamide, and β-n-hexyloxy-N,N-dimethylpropionamide; and general-purpose low-boiling point organic solvents such as mixed solvents thereof. As the organic solvent, a solvent represented by the general formula (4) may also be used.

[0188] The amount of solvent in the negative electrode mixture is determined considering factors such as its applicability to the current collector and its ability to form a thin film after drying. Typically, the ratio of binder to solvent is 0.5:99.5 to 20:80 by mass.

[0189] The thickness of the negative electrode plate is designed to match the positive electrode plate used and is not particularly limited, however, the thickness of the composite layer after subtracting the thickness of the core metal foil is usually 15 μm or more, preferably 20 μm or more, more preferably 30 μm or more, and usually 300 μm or less, preferably 280 μm or less, more preferably 250 μm or less.

[0190] (electrolyte) As a non-aqueous electrolyte, a known electrolyte salt dissolved in a known organic solvent for dissolving electrolyte salts can be used.

[0191] The organic solvent for dissolving the electrolyte salt is not particularly limited, but one or more of the following can be used: known hydrocarbon solvents such as propylene carbonate, ethylene carbonate, butylene carbonate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; and fluorinated solvents such as fluoroethylene carbonate, fluoroether, and fluorinated carbonate.

[0192] Examples of electrolyte salts include NaPF6, NaBF4, NaClO4, NaAlF4, NaSbF6, NaTaF6, NaWF7, NaAsF6, NaAlCl4, NaI, NaBr, NaCl, and NaB 10 Cl 10 Inorganic sodium salts such as Na2SiF6, Na2PFO3, and NaPO2F2; Tungsten sodium acids such as NaWOF5; Sodium carboxylate salts such as HCO2Na, CH3CO2Na, CH2FCO2Na, CHF2CO2Na, CF3CO2Na, CF3CH2CO2Na, CF3CF2CO2Na, CF3CF2CF2CO2Na, CF3CF2CF2CF2CO2Na, etc. Sodium salts containing an S=O group, such as FSO3Na, CH3SO3Na, CH2FSO3Na, CHF2SO3Na, CF3SO3Na, CF3CF2SO3Na, CF3CF2CF2SO3Na, CF3CF2CF2CF2SO3Na, sodium methyl sulfate, sodium ethyl sulfate (C2H5OSO3Na), and sodium 2,2,2-trifluoroethyl sulfate; Sodium imide salts such as NaN(FCO)2, NaN(FCO)(FSO2), NaN(FSO2)2, NaN(FSO2)(CF3SO2), NaN(CF3SO2)2, NaN(C2F5SO2)2, sodium bisperfluoroethanesulfonylimide, sodium cyclic 1,2-perfluoroethanedisulfonylimide, sodium cyclic 1,3-perfluoropropanedisulfonylimide, sodium cyclic 1,2-ethanedisulfonylimide, sodium cyclic 1,3-propanedisulfonylimide, sodium cyclic 1,4-perfluorobutanedisulfonylimide, NaN(CF3SO2)(FSO2), NaN(CF3SO2)(C3F7SO2), NaN(CF3SO2)(C4F9SO2), NaN(POF2)2, etc. Sodium methide salts such as NaC(FSO2)3, NaC(CF3SO2)3, and NaC(C2F5SO2)3; Other formulas: NaPF a (C n F 2n+1) 6-a Salts represented by (wherein the formula a is an integer from 0 to 5 and n is an integer from 1 to 6) (e.g., NaPF3(C2F5)3, NaPF3(CF3)3, NaPF3(iso-C3F7)3, NaPF5(iso-C3F7), NaPF4(CF3)2, NaPF4(C2F5)2), NaPF4(CF3SO2)2, NaPF4(C2F5SO2)2, NaBF3CF3, NaBF3C2F5, NaBF3C3F7, NaBF2(CF3)2, NaBF2(C2F5)2, NaBF2(CF3SO2)2, NaBF2(C2F5SO2)2, etc., including fluorine-containing organic sodium salts, NaSCN, LiB(CN)4, NaB(C6H5)4, Na2(C2O4), NaP(C2O4)3, Na2B 12 F b H 12-b Examples include (where b is an integer between 0 and 3). Among these, NaPF6, NaBF4, NaSbF6, NaTaF6, NaPO2F2, FSO3Na, CF3SO3Na, NaN(FSO2)2, NaN(FSO2)(CF3SO2), NaN(CF3SO2)2, NaN(C2F5SO2)2, sodium cyclic 1,2-perfluoroethanedisulfonyliimide, sodium cyclic 1,3-perfluoropropanedisulfonyliimide, NaC(FSO2)3, NaC(CF3SO2)3, NaC(C2F5SO2)3, NaBF3CF3, NaBF3C2F5, NaPF3(CF3)3, NaPF3(C2F5)3, etc. are particularly preferred because they have the effect of improving output characteristics, high-rate charge / discharge characteristics, high-temperature storage characteristics, and cycle characteristics, and at least one lithium salt selected from the group consisting of NaPF6, NaN(FSO2)2, and NaBF4 is most preferred.

[0193] The electrolyte salt concentration must be 0.8 mol / liter or higher, and even higher, 1.0 mol / liter or higher. The upper limit depends on the organic solvent used to dissolve the electrolyte salt, but is usually 1.5 mol / liter.

[0194] <Separator> The secondary battery of this disclosure preferably further comprises a separator. The material and shape of the separator are not particularly limited as long as they are stable in the electrolyte and have excellent liquid retention properties, and known materials can be used. In particular, it is preferable to use a porous sheet or nonwoven fabric made of a material that is stable in the electrolyte of this disclosure, such as a resin, glass fiber, or inorganic material, and has excellent liquid retention properties.

[0195] As materials for the resin and glass fiber separator, for example, polyolefins such as polyethylene and polypropylene, aromatic polyamides, polytetrafluoroethylene, polyethersulfone, and glass filters can be used. These materials, such as polypropylene / polyethylene two-layer films and polypropylene / polyethylene / polypropylene three-layer films, may be used individually or in any combination and ratio of two or more. In particular, the separator is preferably a porous sheet or nonwoven fabric made from polyolefins such as polyethylene and polypropylene, as it has good electrolyte permeability and shut-off effect.

[0196] The thickness of the separator is arbitrary, but is usually 1 μm or more, preferably 5 μm or more, more preferably 8 μm or more, and usually 50 μm or less, preferably 40 μm or less, and more preferably 30 μm or less. If the separator is too thin compared to the above range, the insulating properties and mechanical strength may decrease. If it is too thick compared to the above range, not only may the battery performance such as rate characteristics decrease, but the energy density of the electrolyte battery as a whole may decrease.

[0197] Furthermore, when using porous materials such as porous sheets or nonwoven fabrics as separators, the porosity of the separator is arbitrary, but is usually 20% or more, preferably 35% or more, more preferably 45% or more, and usually 90% or less, preferably 85% or less, and more preferably 75% or less. If the porosity is too small compared to the above range, the film resistance tends to increase and the rate characteristics tend to deteriorate. Also, if it is too large compared to the above range, the mechanical strength of the separator tends to decrease and the insulating properties tend to deteriorate.

[0198] Furthermore, while the average pore size of the separator is arbitrary, it is usually 0.5 μm or less, preferably 0.2 μm or less, and usually 0.05 μm or more. If the average pore size exceeds the above range, short circuits are more likely to occur. Conversely, if it falls below the above range, the film resistance increases and the rate characteristics may deteriorate.

[0199] On the other hand, inorganic materials such as oxides of alumina and silicon dioxide, nitrides of aluminum nitride and silicon nitride, and sulfates of barium sulfate and calcium sulfate are used, and these are available in particulate or fibrous form.

[0200] In terms of form, thin films such as nonwoven fabrics, woven fabrics, and microporous films are used. In the thin film form, those with a pore size of 0.01 to 1 μm and a thickness of 5 to 50 μm are preferably used. In addition to the independent thin film form described above, a separator can be used in which a composite porous layer containing the inorganic particles is formed on the surface of the positive electrode and / or negative electrode using a resin binder. For example, a porous layer can be formed on both sides of the positive electrode using alumina particles with a 90% particle size of less than 1 μm and a fluororesin as a binder.

[0201] <Battery design> The electrode group may be either a laminated structure in which the positive electrode plate and the negative electrode plate are separated by the separator, or a structure in which the positive electrode plate and the negative electrode plate are wound in a spiral shape with the separator. The ratio of the volume of the electrode group to the internal volume of the battery (hereinafter referred to as the electrode group occupancy rate) is usually 40% or more, preferably 50% or more, and usually 90% or less, preferably 80% or less.

[0202] If the electrode group occupancy rate falls below the aforementioned range, the battery capacity will decrease. Conversely, if it exceeds the aforementioned range, the void space will be small, causing the battery to become hot, which can lead to expansion of components and an increase in the vapor pressure of the electrolyte's liquid component, resulting in an increase in internal pressure. This can degrade the battery's charge / discharge cycle performance and its ability to withstand high temperatures, and may even cause the gas release valve, which releases internal pressure, to activate.

[0203] The current collection structure is not particularly limited, but in order to more effectively achieve improved high-current-density charge-discharge characteristics with the electrolyte of this disclosure, it is preferable to have a structure that reduces the resistance of the wiring and junction parts. When the internal resistance is reduced in this way, the effects of using the electrolyte of this disclosure are particularly well exhibited.

[0204] In the case of electrode groups with the aforementioned laminated structure, a structure formed by bundling the metal core portions of each electrode layer and welding them to a terminal is preferably used. When the area of ​​a single electrode is large, the internal resistance increases, so it is also preferable to reduce the resistance by providing multiple terminals within the electrode. In the case of electrode groups with the aforementioned wound structure, the internal resistance can be reduced by providing multiple lead structures for both the positive and negative electrodes and bundling them to a terminal.

[0205] The material of the outer casing is not particularly limited as long as it is a stable material for the electrolyte used. Specifically, metals such as nickel-plated steel, stainless steel, aluminum or aluminum alloy, magnesium alloy, or laminated films of resin and aluminum foil can be used. From the viewpoint of weight reduction, aluminum or aluminum alloy metals or laminated films are preferably used.

[0206] In the case of an outer casing using metals, examples include a sealed structure formed by welding the metals together using laser welding, resistance welding, or ultrasonic welding, or a crimped structure using the metals via a resin gasket. In the case of an outer casing using a laminate film, examples include a sealed structure formed by heat-fusing the resin layers together. To improve sealing performance, a resin different from the resin used in the laminate film may be interposed between the resin layers. In particular, when a sealed structure is formed by heat-fusing the resin layers via a current collector terminal, since it is a joint between metal and resin, a resin having polar groups or a modified resin with introduced polar groups is preferably used as the interposing resin.

[0207] The shape of the sodium-ion secondary battery in this disclosure is arbitrary and can be any shape, such as cylindrical, prismatic, laminated, coin-type, or large. The shape and configuration of the positive electrode, negative electrode, and separator can be changed and used according to the shape of each battery. [Examples]

[0208] Next, the present disclosure will be described with reference to examples, but the present disclosure is not limited to such examples. In the following examples, unless otherwise specified, "parts" and "%" represent "parts by mass" and "% by mass," respectively.

[0209] The (co)polymers used in the examples and comparative examples are shown in Table 1 below.

[0210] [Table 1]

[0211] The composition and molecular weight of each polymer were measured using the following method.

[0212] (Polymer composition) The polymer composition was measured by solution NMR spectroscopy. Measurement device: Varian VNMRS400; Resonance frequency: 376.04 (Sfrq); Pulse width: 30° (pw=6.8)

[0213] (Content of polar group-containing monomer units in the polymer) The content of polar group-containing monomer units (acrylic acid units) was measured by acid-base titration of the carboxyl group. Specifically, approximately 0.5 g of the copolymer was dissolved in acetone at a temperature of 70-80°C. 5 ml of water was added dropwise under vigorous stirring to avoid coagulation of the copolymer. Titration was performed with a 0.1 N aqueous NaOH solution until the acidity was completely neutralized at a neutralization transition of approximately -270 mV. From the measurement results, the amount of polar group-containing monomer units contained in 1 g of copolymer was determined, and the content of polar group-containing monomer units was calculated.

[0214] (Weight average molecular weight) Measurements were taken using gel permeation chromatography (GPC). A Tosoh AS-8010, CO-8020, and column (three GMHHR-H columns connected in series) and a Shimadzu RID-10A were used. Dimethylformamide (DMF) was used as the solvent, flowing at a rate of 1.0 ml / min. The data (reference: polystyrene) were used for calculation.

[0215] Examples 1-12, Comparative Examples 1, 2 (Preparation of positive electrode mixture) NaFeO2 (manufactured by Kojun Chemical Laboratory Co., Ltd.), binder, and carbon black (manufactured by SUPER-P Li Imerys) were weighed in a mass ratio of 97.00:1.50:1.50. The polymers shown in Table 2 were used as binders. The binder was dissolved in N-methyl-2-pyrrolidone (NMP) to a concentration of 8% by mass. Then, a predetermined amount of NaFeO2 and carbon black were added to the resulting NMP solution, and the mixture was stirred at 100 rpm for 60 minutes using a stirrer (TKHIVIS MIX, manufactured by Primix). Further stirring was performed at 100 rpm for 30 minutes while vacuum degassing was carried out. The slurry after stirring was filtered using a Ni mesh (200 mesh) to homogenize the particle size of the solids, thereby obtaining the cathode mixture.

[0216] (Fabrication of a positive electrode with a positive electrode material layer) The resulting positive electrode mixture was applied to one side of the positive electrode current collector (aluminum foil with a thickness of 20 μm) at a rate of 22.5 mg / cm². 2 The material was applied uniformly, and after the NMP was completely evaporated, a positive electrode comprising a positive electrode material layer and a positive electrode current collector was fabricated by pressing with a pressure of 10 tons using a roll press machine.

[0217] (Preparation of electrolyte solution) The electrolyte was prepared by dissolving NaPF6 at a concentration of 1 mole / liter in a solvent consisting of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3 / 7.

[0218] (Fabrication of the negative electrode) Styrene-butadiene rubber and carboxymethylcellulose, dispersed in distilled water, were added to hard carbon in a solid content of 1.2% by mass. The mixture was then mixed in a disperser to form a slurry, which was uniformly applied to a negative electrode current collector (10 μm thick copper foil). The slurry was dried to form a negative electrode mixture layer, and then compressed and molded using a roller press to produce the negative electrode.

[0219] (Fabrication of sodium-ion secondary batteries) A strip of positive electrode was cut to 40mm x 72mm (with a 10mm x 10mm positive electrode terminal), and a strip of negative electrode was cut to 43mm x 75mm (with a 10mm x 10mm negative electrode terminal). Lead bodies were welded to each terminal. A 20μm thick microporous polyethylene film was cut to 78mm x 46mm to serve as a separator. The positive and negative electrodes were set on either side of the separator, and these were placed inside an aluminum laminate packaging material. Then, 2ml of electrolyte was added to each packaging material and sealed to create a laminate cell. The evaluation results are shown in Table 2.

[0220] [Measurement of electrode water content] In the fabricated electrodes, the electrode water content (ppm) was measured by calculating the content of physically adsorbed water in the composite layer of the electrode (positive electrode) after the heating and drying process at 150°C using the Karl Fischer method.

[0221] [Measurement of interfacial resistance] Under a 25°C environment, the interface resistance (Ωcm) of the positive electrode was measured using an electrode resistance measurement system (HIOKI RM2610). 2 ) was measured.

[0222] [Measurement of initial impedance] The laminate-type batteries prepared as described above were charged at a constant current (0.2C) and constant voltage (4.1V) in a 25°C environment, and discharged at 0.2C until the discharge termination voltage was 3.2V. After performing three charge-discharge cycles, the voltage drop (voltage drop value 15 seconds after the start of discharge) was measured during discharge at 0.5C, 1C, 2C, and 5C at a state of charge (SOC) of 100%, and the initial impedance (Ω) was determined from each current value and each voltage drop value.

[0223] [Measurement of initial discharge capacity] The above battery was charged at 25°C with a current equivalent to 0.2C to 4.1V (with a 0.1C cut-off), and then discharged to 3.2V with a constant current of 0.2C. This was repeated for three cycles. The discharge capacity after the third cycle was defined as the initial discharge capacity (mAh / g).

[0224] [Measurement of cycle maintenance rate] The above cycle was performed 300 times, and the discharge capacity was measured. The ratio of the discharge capacity after 300 cycles to the initial discharge capacity was calculated and defined as the cycle maintenance rate (%).

[0225] [Table 2]

[0226] The results in Table 2 show that the sodium-ion secondary batteries in the examples using the binder of this disclosure had low interfacial resistance and initial internal resistance. [Industrial applicability]

[0227] The binder of this disclosure is suitable for sodium-ion batteries and can be used as various power sources such as portable power supplies and automotive power supplies.

Claims

1. A binder for the positive electrode of a sodium-ion battery comprising a copolymer having vinylidene fluoride units (A) and constituent units (B) derived from at least one monomer selected from the group consisting of trifluoroethylene, 2,3,3,3-tetrafluoropropene, and perfluoro-(2,9,9-trihaloid-5-trifluoromethyl-3,6-dioxa-8-heptene), and not having tetrafluoroethylene units, wherein the content of vinylidene fluoride units (A) is 60 to 99.5 mol% relative to the total monomer units in the copolymer.

2. The binder for the positive electrode of a sodium-ion battery according to claim 1, wherein the constituent unit (B) is a constituent unit derived from 2,3,3,3-tetrafluoropropene.

3. A binder for the positive electrode of a sodium-ion battery according to claim 1, comprising two or more of the copolymers.

4. Furthermore, the binder for the positive electrode of a sodium-ion battery according to claim 1 or 2, further comprising a fluorine-containing polymer other than the copolymer.

5. The binder for the positive electrode of a sodium-ion battery according to claim 4, wherein the fluorine-containing polymer is polyvinylidene fluoride or modified polyvinylidene fluoride.

6. An electrode mixture comprising an electrode active material containing a sodium composite oxide and a binder for the positive electrode of a sodium-ion battery according to claim 1 or 2.

7. An electrode comprising a current collector and an electrode material layer formed from the electrode mixture described in claim 6, provided on one or both sides of the current collector.

8. A sodium-ion secondary battery comprising the electrode described in claim 7.

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