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

Abstract

Provided are: a binder in which the interfacial resistance between a current collector and an electrode material layer in a positive electrode of a sodium ion battery is reduced; and an electrode mixture, an electrode, and a secondary battery using the binder. A binder for the positive electrode of a sodium ion battery, which contains a copolymer (A) having a vinylidene fluoride unit and a tetrafluoroethylene unit, and which is characterized in that the content of the vinylidene fluoride unit is 30-99.5 mol% with respect to the total monomer units in the copolymer (A).

Description

Technical Field

[0001] The present disclosure relates to a binder for a positive electrode, an electrode mixture, an electrode, and a secondary battery. Background Art

[0002] With the recent trend of lightweight and miniaturization of electrical products, the demand for electrochemical devices such as secondary batteries has increased rapidly. Furthermore, as electrical products become more high-performance and are given functions that were not available before, the demand for electrochemical devices that can withstand longer use under harsher conditions has increased.

[0003] In addition, research has been conducted on sodium-ion secondary batteries that use sodium ions as charge carriers. Sodium is more abundant than lithium and can be obtained at a lower cost, so it is attracting attention as a low-cost and large-scale secondary battery. Patent Documents 1 and 2 describe sodium-ion batteries that use a fluorine compound as a binder.

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 2012-79687

[0007] Patent Document 2: Japanese Patent Application Laid-Open No. 2014-26818 Summary of the Invention

[0008] Problems to be Solved by the Invention

[0009] An object of the present disclosure is 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, as well as an electrode mixture, an electrode, and a secondary battery that use the binder.

[0010] Means for Solving the Problems

[0011] The present disclosure relates to a binder for a positive electrode of a sodium-ion battery, which is a binder for a positive electrode of a sodium-ion battery containing a copolymer (A) having a vinylidene fluoride unit and a tetrafluoroethylene unit, and is characterized in that the content of the vinylidene fluoride unit is 30 mol% to 99.5 mol% with respect to all monomer units in the copolymer (A).

[0012] Preferably, the content of the vinylidene fluoride unit is 70 mol% to 99.5 mol% with respect to all monomer units.

[0013] The copolymer (A) may further have a structural unit derived from at least one monomer selected from the group consisting of hexafluoropropylene, trifluoroethylene, chlorotrifluoroethylene, a monomer represented by the following general formula (1), a monomer represented by the following general formula (2), and a monomer represented by the following general formula (3).

[0014] [Chemical Formula 1]

[0015]

[0016] (In the formula, Rf 1 is a linear or branched fluoroalkyl or fluoroalkoxy group having 1 to 12 carbon atoms. When both the fluoroalkyl and fluoroalkoxy groups have 2 or more carbon atoms, they can contain an oxygen atom (-O-) between carbon-carbon atoms.)

[0017] [Chemical Formula 2]

[0018]

[0019] (In the formula, Rf 2 is a linear or branched fluoroalkyl or fluoroalkoxy group having 1 to 12 carbon atoms. When both the fluoroalkyl and fluoroalkoxy groups have 2 or more carbon atoms, they can contain an oxygen atom (-O-) between carbon-carbon atoms.)

[0020] [Chemical Formula 3]

[0021]

[0022] (In the formula, R 1 , R 2 and R 3 are each independently a hydrogen atom, a chlorine atom, or an alkyl group having 1 to 5 carbon atoms. X is a single bond or a group having a main chain composed of 1 to 20 atoms and a molecular weight of 500 or less. Y represents an inorganic cation and / or an organic cation.)

[0023] The binder for the positive electrode may further contain one or more fluoropolymers other than the above copolymer (A).

[0024] The fluoropolymer is preferably a fluoropolymer (PVdF) composed only of VdF units, or a fluoropolymer (B) containing VdF units and other monomer units other than VdF.

[0025] In addition, the fluoropolymer is preferably a fluoropolymer (PVdF) composed only of VdF units.

[0026] The ratio (mass ratio) of the copolymer (A) to the fluoropolymer is preferably 1:15 to 9:7.

[0027] The present disclosure also relates to an electrode mixture, wherein the electrode active material contains a sodium composite oxide, and the electrode mixture contains the binder for the positive electrode of the above sodium ion battery.

[0028] The present disclosure also relates to an electrode, which includes a current collector and an electrode material layer formed of the above electrode mixture provided on one or both sides of the current collector.

[0029] The present disclosure also relates to a sodium ion secondary battery including the above electrodes.

[0030] Effects of the Invention

[0031] The present disclosure provides a binder for reducing the interfacial resistance between a current collector and an electrode material layer in a positive electrode of a sodium ion battery. In addition, an electrode and a secondary battery using the binder of the present disclosure can reduce the interfacial resistance between the current collector and the electrode material layer in the positive electrode. Detailed Description of the Invention

[0032] The present disclosure will be described in detail below.

[0033] The present disclosure is a binder for forming a positive electrode of a sodium ion battery.

[0034] In recent years, the development of sodium ion batteries (SIBs) has been advanced. Since the ionic radius of sodium ions is larger than that of lithium ions, in SIBs, compared with lithium ion batteries (LIBs), the expansion and contraction of the interlayer distance and the change in structure in the positive electrode active material caused by charge and discharge become more drastic. In the electrode material layer (except for the interface with the current collector foil), although the conductive path can be maintained by the binder even during contraction, at the interface with the current collector, since there is no binder between the active material and the current collector, it is difficult to maintain the conductive path compared with the electrode material layer (except for the interface with the current collector). Therefore, in SIBs, a lower interfacial resistance is required at the time of electrode fabrication.

[0035] The present inventors have found that, unlike LIBs, in SIBs where a lower interfacial resistance is required for the electrodes, by using a specific fluorine-based polymer having flexibility as a binder component during electrode fabrication, the conductive agent can be further pressed against the current collector during electrode pressing, and the interfacial resistance value between the current collector and the electrode material layer (conductive agent / Na active material / binder) of the electrode can be reduced.

[0036] In addition, by using the binder of the present disclosure, the resistivity of the electrode mixture in a slurry state can also be reduced.

[0037] The binder for a positive electrode of a sodium ion battery according to the present disclosure contains a copolymer (A) having vinylidene fluoride (VdF) units and tetrafluoroethylene (TFE) units, and the content of the VdF units is 30 mol% to 99.5 mol% relative to all monomer units in the copolymer (A).

[0038] If the VdF units are less than 30 mol%, the peel strength as an electrode becomes weak; if more than 99.5 mol%, uniform coating of the slurry becomes difficult.

[0039] The above copolymer (A) preferably contains 70 mol% or more of VdF units relative to all the polymer units. When the content is 70 mol% or more, the peel strength of the electrode tends to increase.

[0040] The above copolymer (A) more preferably contains 72 mol% or more of VdF units relative to all the polymer units, still more preferably contains 75 mol% or more, and particularly preferably contains 80 mol% or more. In addition, it preferably contains 99 mol% or less, more preferably contains 90 mol% or less, and still more preferably contains 85 mol% or less.

[0041] The composition of the above copolymer (A) can be measured using a 19F-NMR analyzer.

[0042] The above copolymer (A) preferably contains 0.5 mol% to 70 mol% of TFE units relative to all the monomer units in the copolymer (A). When the TFE units are less than 0.5 mol%, the uniform coating of the electrode tends to be difficult; when they are more than 70 mol%, the peel strength of the electrode tends to decrease. The above copolymer (A) more preferably contains 10 mol% or more of TFE units relative to all the polymer units, still more preferably contains 15 mol% or more. In addition, it is more preferably 30 mol% or less, still more preferably 25 mol% or less, and particularly preferably 20 mol% or less.

[0043] In addition to the VdF units and TFE units, the above copolymer (A) may further contain polymer units based on monomers that can copolymerize with VdF and TFE.

[0044] The copolymer of VdF and TFE is sufficient to achieve the effects of the present disclosure. However, copolymerizing monomers that can copolymerize with them to an extent that does not impair the excellent non-aqueous electrolyte swelling property of the copolymer can further improve the adhesiveness.

[0045] The polymer units based on other monomers that can copolymerize with the above VdF and TFE are not particularly limited. For example, they are preferably structural units derived from at least one monomer selected from the group consisting of hexafluoropropylene, trifluoroethylene, chlorotrifluoroethylene (CTFE), the monomer represented by the following general formula (1), the monomer represented by the following general formula (2), and the monomer represented by the following general formula (3). More preferably, they are structural units derived from at least one monomer selected from the group consisting of trifluoroethylene, chlorotrifluoroethylene (CTFE), the monomer represented by the following general formula (1), the monomer represented by the following general formula (2), and the monomer represented by the following general formula (3).

[0046] [Chemical formula 4]

[0047]

[0048] (In the formula, Rf1 is a linear or branched fluoroalkyl or fluoroalkoxy group having 1 to 12 carbon atoms. When both the fluoroalkyl and fluoroalkoxy groups have 2 or more carbon atoms, they can contain an oxygen atom (-O-) between carbon-carbon atoms.

[0049] [Chemical formula 5]

[0050]

[0051] (In the formula, Rf 2 is a linear or branched fluoroalkyl or fluoroalkoxy group having 1 to 12 carbon atoms. When both the fluoroalkyl and fluoroalkoxy groups have 2 or more carbon atoms, they can contain an oxygen atom (-O-) between carbon-carbon atoms.

[0052] [Chemical formula 6]

[0053]

[0054] (In the formula, R 1 、R 2 and R 3 are each independently a hydrogen atom, a chlorine atom, or an alkyl group having 1 to 5 carbon atoms. X is a single bond or a group having a main chain composed of 1 to 20 atoms and a molecular weight of 500 or less. Y represents an inorganic cation and / or an organic cation.)

[0055] By having a polymerization unit based on the above monomer, a more excellent positive electrode material layer can be formed by utilizing flexibility and adhesion to the current collector, and a secondary battery having more excellent battery characteristics can be formed.

[0056] In the fluorine-containing monomer represented by the above general formula (1), Rf 1 is a linear or branched fluoroalkyl group having 1 to 12 carbon atoms, or a linear or branched fluoroalkoxy group having 1 to 12 carbon atoms. When both the fluoroalkyl and fluoroalkoxy groups have 2 or more carbon atoms, they can contain an oxygen atom (-O-) between carbon-carbon atoms.

[0057] Rf 1 The fluoroalkyl group of can be a partially fluorinated alkyl group in which a part of the hydrogen atoms bonded to carbon atoms is substituted by fluorine atoms, or a perfluorinated alkyl group in which all of the hydrogen atoms bonded to carbon atoms are substituted by fluorine atoms. In addition, the hydrogen atoms of the fluoroalkyl group of Rf 1 can be substituted by substituents other than fluorine atoms, but preferably do not contain substituents other than fluorine atoms.

[0058] In addition, Rf 1The fluoroalkoxy group can be a partially fluorinated alkoxy group in which a part of the hydrogen atoms bonded to the carbon atom is substituted by fluorine atoms, or a perfluorinated alkoxy group in which all of the hydrogen atoms bonded to the carbon atom are substituted by fluorine atoms. Additionally, Rf 1 The hydrogen atoms of the fluoroalkoxy group can be substituted by substituents other than fluorine atoms, but preferably do not contain substituents other than fluorine atoms.

[0059] As Rf 1 the number of carbon atoms, it is preferably 1 to 10, more preferably 1 to 6, still more preferably 1 to 4, and particularly preferably 1.

[0060] As Rf 1 , preferably the general formula:

[0061] -(Rf 11 )m-(O)p-(Rf 12 -O)n-Rf 13

[0062] (In the formula, Rf 11 and Rf 12 are independently a linear or branched fluoroalkylene group having 1 to 4 carbon atoms, Rf 13 is a linear or branched fluoroalkyl group having 1 to 4 carbon atoms, p is 0 or 1, m is an integer from 0 to 4, and n is an integer from 0 to 4) represented group.

[0063] Rf 11 and Rf 12 The fluoroalkylene group can be a partially fluorinated alkylene group in which a part of the hydrogen atoms bonded to the carbon atom is substituted by fluorine atoms, or a perfluorinated alkylene group in which all of the hydrogen atoms bonded to the carbon atom are substituted by fluorine atoms. Additionally, Rf 11 and Rf 12 The hydrogen atoms of the fluoroalkylene group can be substituted by substituents other than fluorine atoms, but preferably do not contain substituents other than fluorine atoms. Rf 11 and Rf 12 can be the same or different each time they appear.

[0064] As the fluoroalkylene group of Rf 11 , examples include -CHF-, -CF 2 -, -CH 2 -CF 2 -, -CHF-CF 2 -, -CF 2 -CF 2 -, -CF(CF 3 )-, -CH 2 -CF 2 -CF 2 -, -CHF-CF2 -CF 2 -、-CF 2 -CF 2 -CF 2 -、-CF(CF 3 )-CF 2 -、-CF 2 -CF(CF 3 )-、-C(CF 3 ) 2 -、-CH 2 -CF 2 -CF 2 -CF 2 -、-CHF-CF 2 -CF 2 -CF 2 -、-CF 2 -CF 2 -CF 2 -CF 2 -、-CH(CF 3 )-CF 2 -CF 2 -、-CF(CF 3 )-CF 2 -CF 2 -、-C(CF 3 ) 2 -CF 2 - etc., among which, a perfluoroalkylene group having 1 or 2 carbon atoms is preferred, and -CF 2 - is more preferred.

[0065] As the fluoroalkylene group of Rf 12 , -CHF-, -CF 2 -, -CH 2 -CF 2 -, -CHF-CF 2 -, -CF 2 -CF 2 -, -CF(CF 3 )-, -CH 2 -CF 2 -CF 2 -, -CHF-CF 2 -CF 2 -, -CF 2 -CF 2 -CF 2 -, -CF(CF 3 )-CF 2 -, -CF 2 -CF(CF 3 )-, -C(CF 3 )2 -,-CH 2 -CF 2 -CF 2 -CF 2 -,-CHF-CF 2 -CF 2 -CF 2 -,-CF 2 -CF 2 -CF 2 -CF 2 -,-CH(CF 3 )-CF 2 -CF 2 -,-CF(CF 3 )-CF 2 -CF 2 -,-C(CF 3 ) 2 -CF 2 -etc., among which, perfluoroalkylene with 1-3 carbon atoms is preferred, and -CF 2 -,-CF 2 CF 2 -,-CF 2 -CF 2 -CF 2 -,-CF(CF 3 )-CF 2 -or -CF 2 -CF(CF 3 )-.

[0066] As the fluoroalkyl group of Rf 13 , it can be a partially fluorinated alkyl group in which a part of the hydrogen atoms bonded to carbon atoms are replaced by fluorine atoms, or a perfluorinated alkyl group in which all the hydrogen atoms bonded to carbon atoms are replaced by fluorine atoms. In addition, the hydrogen atoms of the fluoroalkyl group of Rf 13 can be replaced by substituents other than fluorine atoms, but preferably do not contain substituents other than fluorine atoms (such as -CN, -CH 2 I, -CH 2 Br, etc.).

[0067] As the fluoroalkyl group of Rf 13 , examples include -CH 2 F, -CHF 2 , -CF 3 , -CH 2 -CH 2 F, -CH 2 -CHF 2 , -CH 2 -CF 3 , -CHF-CH 2F, -CHF-CHF 2 , -CHF-CF 3 , -CF 2 -CH 2 F, -CF 2 -CHF 2 , -CF 2 -CF 3 , -CH 2 -CF 2 -CH 2 F, -CHF-CF 2 -CH 2 F, -CF 2 -CF 2 -CH 2 F, -CF(CF 3 )-CH 2 F, -CH 2 -CF 2 -CHF 2 , -CHF-CF 2 -CHF 2 , -CF 2 -CF 2 -CHF 2 , -CF(CF 3 )-CHF 2 , -CH 2 -CF 2 -CF 3 , -CHF-CF 2 -CF 3 , -CF 2 -CF 2 -CF 3 , -CF(CF 3 )-CF 3 , -CH 2 -CF 2 -CF 2 -CF 3 , -CHF-CF 2 -CF 2 -CF 3 , -CF 2 -CF 2 -CF 2 -CF 3 , -CH(CF 3 )-CF 2 -CF 3 , -CF(CF 3 )-CF 2 -CF 3 , -C(CF 3 )2-CF 3etc., among which, preferably -CF 3 , -CHF-CF 3 , -CF 2 -CHF 2 , -CF 2 -CF 3 , -CF 2 -CF 2 -CF 3 , -CF(CF 3 )-CF 3 , -CF 2 -CF 2 -CF 2 -CF 3 , -CH(CF 3 )-CF 2 -CF 3 or -CF(CF 3 )-CF 2 -CF 3 。

[0068] As p, preferably 0.

[0069] As m, it is preferably an integer from 0 to 2, more preferably 0 or 1, and still more preferably 0. Additionally, when p is 0, m is also preferably 0.

[0070] As n, it is preferably an integer from 0 to 2, more preferably 0 or 1, and still more preferably 0.

[0071] As the repeating unit, preferably

[0072] -CH 2 -CF[-CF 3 -、

[0073] -CH 2 -CF[-CF 2 CF 3 -、

[0074] -CH 2 -CF[-CF 2 CF 2 CF 3 -、

[0075] -CH 2 -CF[-CF 2 CF 2 CF 2 CF 3 -、

[0076] -CH 2 -CF[-CF 2 -O-CF(CF 3)-CF 2 -O-CHF-CF 3 -、

[0077] -CH 2 -CF[-CF 2 -O-CF(CF 3 )-CF 2 -O-CF 2 -CF 3 -、

[0078] -CH 2 -CF[-CF 2 -O-CF(CF 3 )-CF 2 -O-CF(CF 3 )-CF 3 -、

[0079] -CH 2 -CF[-CF 2 -O-CF(CF 3 )-CF 2 -O-CH(CF 3 )-CF 2 -CF 3 -、

[0080] -CH 2 -CF[-CF 2 -O-CF(CF 3 )-CF 2 -O-CF(CF 3 )-CF 2 -CF 3 -、

[0081] -CH 2 -CF[-OCF 2 OCF 3 -、

[0082] -CH 2 -CF[-OCF 2 CF 2 CF 2 2OCF 3 -、

[0083] -CH 2 -CF[-CF 2 OCFOCF 3 -、

[0084] -CH 2 -CF[-CF 2 OCF 2 CF2 CF 2 OCF 3 - or

[0085] -CH 2 -CF[-O-CF 2 -CF 3 -,

[0086] More preferably -CH 2 -CF[-CF 3 -.

[0087] In the fluorine-containing monomer (2) represented by the above general formula (2), Rf 2 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. When both the fluorinated alkyl group and the fluorinated alkoxy group have 2 or more carbon atoms, an oxygen atom (-O-) can be included between carbon-carbon atoms.

[0088] Rf 2 's fluorinated alkyl group can be a partially fluorinated alkyl group in which a part of the hydrogen atoms bonded to carbon atoms are substituted by fluorine atoms, or a perfluorinated alkyl group in which all the hydrogen atoms bonded to carbon atoms are substituted by fluorine atoms. In addition, the hydrogen atoms of Rf 2 's fluorinated alkyl group can be substituted by substituents other than fluorine atoms, but preferably do not include substituents other than fluorine atoms.

[0089] In addition, Rf 2 's fluorinated alkoxy group can be a partially fluorinated alkoxy group in which a part of the hydrogen atoms bonded to carbon atoms are substituted by fluorine atoms, or a perfluorinated alkoxy group in which all the hydrogen atoms bonded to carbon atoms are substituted by fluorine atoms. In addition, the hydrogen atoms of Rf 2 's fluorinated alkoxy group can be substituted by substituents other than fluorine atoms, but preferably do not include substituents other than fluorine atoms.

[0090] As the number of carbon atoms of Rf 2 , it is preferably 1 to 10, more preferably 1 to 6, further preferably 1 to 4, and particularly preferably 1.

[0091] As Rf 2 , the general formula is preferably:

[0092] -(Rf 21 )m-(O)p-(Rf 22 -O)n-Rf 23

[0093] (In the formula, Rf 21 and Rf 22 are independently linear or branched fluoroalkylene groups having 1 to 4 carbon atoms, Rf 23a fluorinated alkyl group having 1 to 4 carbon atoms in a straight-chain or branched-chain form, p is 0 or 1, m is an integer from 0 to 4, and n is an integer from 0 to 4).

[0094] Rf 21 and Rf 22 The fluorinated alkylene group can be a partially fluorinated alkylene group in which a part of the hydrogen atoms bonded to carbon atoms is substituted by fluorine atoms, or a perfluorinated alkylene group in which all of the hydrogen atoms bonded to carbon atoms are substituted by fluorine atoms. Additionally, for Rf 21 and Rf 22 the hydrogen atoms of the fluorinated alkylene group can be substituted by substituents other than fluorine atoms, but preferably do not contain substituents other than fluorine atoms. Rf 21 and Rf 22 can be the same or different each time they appear.

[0095] Examples of the fluorinated alkylene group for Rf 21 include -CHF-, -CF 2 -, -CH 2 -CF 2 -, -CHF-CF 2 -, -CF 2 -CF 2 -, -CF(CF 3 )-, -CH 2 -CF 2 -CF 2 -, -CHF-CF 2 -CF 2 -, -CF 2 -CF 2 -CF 2 -, -CF(CF 3 )-CF 2 -, -CF 2 -CF(CF 3 )-, -C(CF 3 ) 2 -, -CH 2 -CF 2 -CF 2 -CF 2 -, -CHF-CF 2 -CF 2 -CF 2 -, -CF 2 -CF 2 -CF 2 -CF 2 -, -CH(CF 3 )-CF 2 -CF 2 -, -CF(CF 3 )-CF2 -CF 2 -、-C(CF 3 ) 2 -CF 2 - etc., among which, a perfluorinated alkylene group having 1 or 2 carbon atoms is preferred, and -CF 2 - is more preferred.

[0096] As the fluorinated alkylene group of Rf 22 , -CHF-, -CF 2 - can be cited, -CH 2 -CF 2 -、-CHF-CF 2 -、-CF 2 -CF 2 -、-CF(CF 3 )-、-CH 2 -CF 2 -CF 2 -、-CHF-CF 2 -CF 2 -、-CF 2 -CF 2 -CF 2 -、-CF(CF 3 )-CF 2 -、-CF 2 -CF(CF 3 )-、-C(CF 3 ) 2 -、-CH 2 -CF 2 -CF 2 -CF 2 -、-CHF-CF 2 -CF 2 -CF 2 -、-CF 2 -CF 2 -CF 2 -CF 2 -、-CH(CF 3 )-CF 2 -CF 2 -、-CF(CF 3 )-CF 2 -CF 2 -、-C(CF 3 ) 2 -CF 2 - etc., among which, a perfluorinated alkylene group having 1 to 3 carbon atoms is preferred, and -CF 2 -、-CF 2 CF 2 -、-CF 2 -CF2 -CF 2 -,-CF(CF 3 )-CF 2 -or -CF 2 -CF(CF 3 )-.

[0097] As Rf 23 the fluoroalkyl group can be a partially fluorinated alkyl group in which a part of the hydrogen atoms bonded to carbon atoms are substituted by fluorine atoms, or a perfluorinated alkyl group in which all of the hydrogen atoms bonded to carbon atoms are substituted by fluorine atoms. Additionally, the hydrogen atoms of the fluoroalkyl group of Rf 23 can be substituted by substituents other than fluorine atoms, but preferably do not contain substituents other than fluorine atoms (such as -CN, -CH 2 I, -CH 2 Br, etc.).

[0098] As Rf 23 the fluoroalkyl group, examples include -CH 2 F, -CHF 2 , -CF 3 , -CH 2 -CH 2 F, -CH 2 -CHF 2 , -CH 2 -CF 3 , -CHF-CH 2 F, -CHF-CHF 2 , -CHF-CF 3 , -CF 2 -CH 2 F, -CF 2 -CHF 2 , -CF 2 -CF 3 , -CH 2 -CF 2 -CH 2 F, -CHF-CF 2 -CH 2 F, -CF 2 -CF 2 -CH 2 F, -CF(CF 3 )-CH 2 F, -CH 2 -CF 2 -CHF 2 , -CHF-CF 2 -CHF 2 , -CF 2 -CF 2 -CHF 2, -CF(CF 3 )-CHF 2 , -CH 2 -CF 2 -CF 3 , -CHF-CF 2 -CF 3 , -CF 2 -CF 2 -CF 3 , -CF(CF 3 )-CF 3 , -CH 2 -CF 2 -CF 2 -CF 3 , -CHF-CF 2 -CF 2 -CF 3 , -CF 2 -CF 2 -CF 2 -CF 3 , -CH(CF 3 )-CF 2 -CF 3 , -CF(CF 3 )-CF 2 -CF 3 , -C(CF 3 )2-CF 3 etc., among which, preferably -CF 3 , -CHF-CF 3 , -CF 2 -CHF 2 , -CF 2 -CF 3 , -CF 2 -CF 2 -CF 3 , -CF(CF 3 )-CF 3 , -CF 2 -CF 2 -CF 2 -CF 3 , -CH(CF 3 )-CF 2 -CF 3 or -CF(CF 3 )-CF 2 -CF 3 .

[0099] As p, preferably 0.

[0100] As m, it is preferably an integer of 0 to 2, more preferably 0 or 1, and still more preferably 0. Additionally, when p is 0, m is also preferably 0.

[0101] As n, it is preferably an integer of 0 to 2, more preferably 0 or 1, and still more preferably 0.

[0102] As the repeating unit, preferably

[0103] -CHF-CH[-CF 3 -、

[0104] -CHF-CH[-CF 2 CF 3 -、

[0105] -CHF-CH[-CF 2 CF 2 CF 3 - or

[0106] -CHF-CH[-CF 2 CF 2 CF 2 CF 3 -,

[0107] More preferably -CHF-CH[-CF 3 -。

[0108] In the above general formula (3), Y represents an inorganic cation and / or an organic cation. As the inorganic cation, cations such as H, Li, Na, K, Mg, Ca, Al, Fe, etc. can be cited. As the organic cation, cations such as NH 4 、NH 3 R 15 、NH 2 R 15 2 、NHR 15 3 、NR 15 4 (R 15 independently represents an alkyl group having 1 to 4 carbon atoms) etc. can be cited. As Y, H, Li, Na, K, Mg, Ca, Al, NH 4 are preferred, more preferably H, Li, Na, K, Mg, Al, NH 4 are preferred, still more preferably H, Li, Al, NH 4 are preferred, and particularly preferably H. It should be noted that for the sake of convenience, the specific examples of inorganic cations and organic cations are described omitting symbols and valences.

[0109] In the general formula (3), R 1 ~R 3Each independently represents a hydrogen atom, a chlorine atom, or an alkyl group having 1 to 5 carbon atoms. The above alkyl group is a monovalent alkyl group. The number of carbon atoms in the above alkyl group is preferably 4 or less. As the above alkyl group, methyl or ethyl is preferred. R 1 and R 2 are independently preferably a hydrogen atom, methyl, or ethyl, and R 3 is preferably a hydrogen atom or methyl.

[0110] In the general formula (3), X is a single bond or a group having a molecular weight of 500 or less and a main chain composed of 1 to 20 atoms. The above group is a divalent group. The above group is preferably a hydrocarbon group having 4 or less carbon atoms. As the above hydrocarbon group, an alkylene group, an alkenylene group, etc. having the above number of carbon atoms can be mentioned. Among them, at least one selected from the group consisting of methylene, ethylene, ethylidene, propylidene, and isopropylidene is preferred, and methylene is more preferred.

[0111] As the monomer represented by the general formula (3), at least one selected from the group consisting of acryloyloxypropyl succinic acid and its salts, (meth)acrylic acid and its salts, vinylacetic acid (3-butenoic 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 is preferred.

[0112] In addition to the above monomers, the copolymer (A) may also contain polymerization units based on fluorinated monomers such as vinylidene fluoride, hexafluoropropylene (HFP), and fluoroalkyl vinyl ethers, or non-fluorinated monomers such as ethylene and propylene within the scope that does not hinder the object of the present disclosure.

[0113] As the fluoroalkyl vinyl ether, a fluoroalkyl vinyl ether having a fluoroalkyl group having 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.

[0114] The content of the polymerization unit based on the monomer copolymerizable with the above VdF and TFE is preferably less than 5.0 mol% with respect to all the polymerization units of the copolymer (A). If it is 5.0 mol% or more, the crystallinity of the copolymer of VdF and TFE usually decreases significantly, and as a result, the swelling property of the non-aqueous electrolyte tends to decrease.

[0115] The content of the polymerization unit based on the monomer copolymerizable with the above VdF and TFE is more preferably 3.5 mol% or less.

[0116] It should be noted that when the copolymer (A) contains the monomer unit represented by the above general formula (3) as other monomer units, its content can be measured by acid-base titration of the carboxyl group.

[0117] The weight-average molecular weight (in terms of polystyrene) of the copolymer (A) is preferably from 10,000 to 3,000,000, more preferably 30,000 or more, further preferably 50,000 or more, particularly preferably 200,000 or more, more preferably 2,400,000 or less, further preferably 2,200,000 or less, and particularly preferably 2,000,000 or less.

[0118] The weight-average molecular weight can be measured by gel permeation chromatography (GPC) using dimethylformamide as a solvent.

[0119] The number-average molecular weight (in terms of polystyrene) of the copolymer (A) is preferably from 7,000 to 1,500,000, more preferably 21,000 or more, further preferably 35,000 or more, more preferably 1,400,000 or less, further preferably 1,200,000 or less, and particularly preferably 1,100,000 or less.

[0120] The number-average molecular weight can be measured by gel permeation chromatography (GPC) using dimethylformamide as a solvent.

[0121] The Mooney viscosity (ML1+10(121 °C)) of the above copolymer (A) at 121 °C is preferably 2 or more, more preferably 5 or more, further preferably 10 or more, and particularly preferably 30 or more.

[0122] The Mooney viscosity is a value measured in accordance with ASTM-D1646-15 and JIS K6300-1:2013.

[0123] The above copolymer (A) can be produced by a general radical polymerization method. The polymerization method can be any one of bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization. From the aspect of being easily implemented industrially, emulsion polymerization is preferred.

[0124] In the polymerization, a polymerization initiator, a chain transfer agent, a surfactant, and a solvent can be used, and known substances can be used respectively. In the polymerization of the copolymer, an oil-soluble radical polymerization initiator or a water-soluble radical initiator can be used as the polymerization initiator.

[0125] As an oil-soluble radical polymerization initiator, it can be a known oil-soluble peroxide. Representative examples include the following substances: dialkyl peroxydicarbonates such as diisopropyl peroxydicarbonate and di-sec-butyl peroxydicarbonate; peroxyesters such as tert-butyl peroxyisobutyrate and tert-butyl peroxypivalate; dialkyl peroxides such as di-tert-butyl peroxide; and diperoxides of bis(ω-hydroperfluorooctanoyl), bis(ω-hydroperfluorononanoyl), bis(ω-hydroperfluorodecanoyl), bis(perfluorobutanoyl), bis(perfluoropentanoyl), bis(perfluorohexanoyl), bis(perfluoroheptanoyl), bis(perfluorooctanoyl), bis(perfluorononanoyl), bis(ω-chloroperfluorohexanoyl), bis(ω-chloroperfluorooctanoyl), ω-hydroperfluorooctanoyl-ω-hydroperfluorodecanoyl peroxide, ω-chloroperfluorohexanoyl-ω-chloroperfluorooctanoyl peroxide, ω-hydroperfluorooctanoyl-perfluorobutanoyl peroxide, bis(dichloroperfluorobutanoyl) peroxide, bis(trichloroperfluorooctanoyl) peroxide, bis(tetrachloroperfluorodecanoyl) peroxide, bis(pentachloroperfluorododecanoyl) peroxide, bis(undecachlorotriacontadifluorodocosanoyl) peroxide, etc.

[0126] As a water-soluble radical polymerization initiator, it can be a known water-soluble peroxide. Examples include ammonium salts, potassium salts, sodium salts of persulfuric acid, perboric acid, perchloric acid, perphosphoric acid, percarbonic acid, etc., tert-butyl peroxymaleate, tert-butyl hydroperoxide, etc. Reducing agents such as sulfites and bisulfites can also be included simultaneously, and their usage amounts can be 0.1 to 20 times that of the peroxide.

[0127] The addition amount of the radical polymerization initiator is not particularly limited, and it can be added all at once, sequentially, or continuously at the beginning of polymerization in an amount such that the polymerization rate does not decrease significantly (for example, several ppm with respect to the water concentration) or more. The upper limit is within the range where the heat of the polymerization reaction can be removed from the equipment surface.

[0128] As the surfactant, nonionic surfactants, anionic surfactants, cationic surfactants, etc. can be used. The addition amount (with respect to the polymerization water) is preferably 10 ppm to 5000 ppm, more preferably 50 ppm to 5000 ppm. In addition, as the surfactant, a reactive emulsifier can be used. The reactive emulsifier is not particularly limited as long as it is a compound having one or more unsaturated bonds and hydrophilic groups.

[0129] As the solvent, a solvent having no chain transferability is preferred. In the case of solution polymerization, dichloropentafluoropropane (R-225) can be cited. 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 can be cited.

[0130] In the above polymerization, as the chain transfer agent, for example, in addition to esters such as dimethyl malonate, diethyl malonate, methyl acetate, ethyl acetate, butyl acetate, dimethyl succinate, etc., isopentane, methane, ethane, propane, isopropyl alcohol, acetone, various thiols, carbon tetrachloride, cyclohexane, etc. can also be cited.

[0131] A bromine compound or an iodine compound can also be used as the chain transfer agent. As a polymerization method using a bromine compound or an iodine compound, for example, a method of carrying out emulsion polymerization in a water medium while pressurizing in the presence of a bromine compound or an iodine compound in a substantially oxygen-free state (iodine transfer polymerization method) can be cited. As a representative example of the bromine compound or iodine compound used, for example, the general formula:

[0132] R 2 I x Br y

[0133] (In the formula, x and y are each an integer of 0 to 2 and satisfy 1 ≤ x + y ≤ 2, and R 2 is 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, and this R 2 contains or does not contain an oxygen atom) the compound shown.

[0134] As the iodine compound, for example, 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-diiodon-propane, CF 2 Br 2 , BrCF 2 CF 2 Br, CF 3 CFBrCF 2 Br, CFClBr 2 , BrCF 2 CFClBr, CFBrClCFClBr, BrCF 2 CF 2 CF 2 Br, BrCF 2 CFBrOCF3 , 1-bromo-2-iodoperfluoroethane, 1-bromo-3-iodoperfluoropropane, 1-bromo-4-iodoperfluorobutane, 2-bromo-3-iodoperfluorobutane, 3-bromo-4-iodoperfluoro-1-butene, 2-bromo-4-iodoperfluoro-1-butene, monoiodomonobromo substituents of benzene, diiodomonobromo substituents, and (2-iodoethyl) and (2-bromoethyl) substituents, etc. These compounds can be used alone or in combination with each other.

[0135] Among these, from the viewpoints of polymerization reactivity, crosslinking reactivity, ease of obtaining, etc., 1,4-diiodoperfluorobutane, 1,6-diiodoperfluorohexane, and 2-iodoperfluoropropane are preferably used.

[0136] Regarding the copolymer obtained by the above method, in the case of emulsion polymerization, the copolymer in powder form can be obtained by precipitating the completed polymerization dispersion liquid, followed by washing with water, dehydration, and drying. The precipitation can be carried out by adding inorganic salts or inorganic acids such as aluminum sulfate, or by applying mechanical shear force, or by freezing the dispersion liquid. In the case of suspension polymerization, the copolymer in powder form can be obtained by recovering and drying from the completed polymerization dispersion liquid. In the case of solution polymerization, it can be obtained by directly drying the solution containing the polymer, or by dropping a poor solvent and refining.

[0137] As the copolymer (A), one kind can be used, or two or more kinds can be used. In particular, it can also be in a form of using two copolymers with different molecular structures in combination.

[0138] The binder for the positive electrode of the present disclosure may contain a blend polymer formed by combining one or more of the above copolymer (A) with a fluoropolymer other than the above copolymer (A). The present disclosure also relates to a binder for the positive electrode further containing one or more fluoropolymers other than the above copolymer (A).

[0139] As the fluoropolymer other than the above copolymer (A), there is no particular limitation. For example, a fluoropolymer (PVdF) composed only of VdF units, or a fluoropolymer (B) containing VdF units and other monomer units other than VdF is preferred. The other monomers other than the above VdF can be fluorinated monomers or non-fluorinated monomers.

[0140] As the above fluorinated monomers, the same substances as those used in the above copolymer (A) can be cited.

[0141] As the above non-fluorinated monomers, non-fluorinated monomers having no polar group such as ethylene and propylene, and the monomer (3) represented by the above general formula (3), etc. can be cited.

[0142] As the above monomer (3), at least one selected from the group consisting of (meth)acrylic acid and its salts, vinylacetic acid (3-butenoic 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 is preferably selected.

[0143] The above fluoropolymer (B) particularly preferably contains a VdF unit and a structural unit derived from at least one monomer selected from the group consisting of acrylic acid, methacrylic acid, methyl methacrylate, 2-(methylthio)ethyl acrylate, pyridin-3-yl acrylate, and 2-(thiophen-2-yl)ethyl acrylate.

[0144] In the fluoropolymer (B), the content of monomer units other than VdF is preferably 0.0001 mol% to 50.0 mol%, more preferably 0.01 mol% or more, further preferably 0.10 mol% or more, more preferably 45.0 mol% or less, further preferably 40.0 mol% or less, and particularly preferably 35.0 mol% or less, relative to all monomer units.

[0145] The content of the VdF unit in the fluoropolymer (B) is 50.0 mol% to 99.9999 mol%, more preferably 55.0 mol% or more, further preferably 60.0 mol% or more, particularly preferably 65.0 mol% or more, more preferably 99.99 mol% or less, and further preferably 99.90 mol% or less, relative to all monomer units.

[0146] When the fluoropolymer (B) contains the monomer unit represented by the above general formula (3) as a monomer unit other than VdF, the content of the monomer unit is preferably 0.0001 mol% to 50.0 mol%, more preferably 0.01 mol% or more, further preferably 0.10 mol% or more, more preferably 5.0 mol% or less, further preferably 3.0 mol% or less, and particularly preferably 1.5 mol% or less, relative to all monomer units.

[0147] When the fluoropolymer (B) contains the monomer unit represented by the above general formula (3) as other monomer units, the content of the VdF unit in the fluoropolymer (B) is preferably 50.0 mol% to 99.999 mol%, more preferably 95.0 mol% or more, further preferably 97.0 mol% or more, particularly preferably 98.5 mol% or more, more preferably 99.99 mol% or less, and further preferably 99.90 mol% or less, relative to all monomer units.

[0148] The weight-average molecular weight (in terms of polystyrene) of the fluoropolymer (B) is preferably from 10,000 to 3,000,000, more preferably 30,000 or more, still more preferably 50,000 or more, particularly preferably 200,000 or more, more preferably 2,400,000 or less, still 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 a solvent.

[0149] The number-average molecular weight (in terms of polystyrene) of the fluoropolymer (B) is preferably from 7,000 to 1,500,000, more preferably 21,000 or more, still more preferably 35,000 or more, more preferably 1,400,000 or less, still 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 a solvent.

[0150] Examples of the fluoropolymer (B) preferably include VdF / acrylic acid copolymer, VdF / methacrylic acid copolymer, VdF / methyl methacrylate copolymer, VdF / acrylic acid / 2-(methylthio)ethyl acrylate copolymer, VdF / acrylic acid / pyridin-3-yl acrylate copolymer, and VdF / acrylic acid / 2-(thiophen-2-yl)ethyl acrylate copolymer.

[0151] 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, it is possible to very easily form an electrode binder, and a coating layer that is extremely firmly adhered to the metal foil can be formed. The content of the (meth)acrylic acid units is preferably from 0.0001 mol% to 5.0 mol%, more preferably from 0.01 mol% to 3.0 mol%, still more preferably from 0.10 mol% to 1.5 mol% relative to all monomer units.

[0152] The content of the VdF units in the VdF / (meth)acrylic acid copolymer is preferably from 95.0 mol% to 99.9999 mol%, more preferably from 97.0 mol% to 99.99 mol%, still more preferably from 98.5 mol% to 99.90 mol% relative to all monomer units.

[0153] The weight-average molecular weight (in terms of polystyrene) of the VdF / (meth)acrylic acid copolymer is preferably from 50,000 to 3,000,000, more preferably 80,000 or more, still more preferably 100,000 or more, particularly preferably 200,000 or more, more preferably 2,400,000 or less, still more preferably 2,200,000 or less, and particularly preferably 2,000,000 or less.

[0154] The number average molecular weight (in terms of polystyrene) of the VdF / (meth)acrylic acid copolymer is preferably from 20,000 to 1,500,000, more preferably 40,000 or more, still more preferably 70,000 or more, particularly preferably 140,000 or more, more preferably 1,400,000 or less, still more preferably 1,200,000 or less, and particularly preferably 1,100,000 or less.

[0155] The fluorine-containing polymer (B) can be produced by the same method as the method for producing the above copolymer (A).

[0156] When the above copolymer (A) is combined with PVdF and / or the fluorine-containing polymer (B), the ratio (mass ratio) of the above copolymer (A) to PVdF and / or the fluorine-containing polymer (B) is preferably 1:15 to 9:7. If it is within such a range, the peel strength as an electrode can be maintained.

[0157] The ratio (mass ratio) of the above copolymer (A) to the fluorine-containing polymer (B) is more preferably 1:14 or more. The ratio (mass ratio) of the above copolymer (A) to the fluorine-containing polymer (B) is more preferably 9:7 or less, still more preferably 8:7 or less.

[0158] The binder for the positive electrode for a sodium ion battery of the present disclosure is suitable for the electrode mixture for the positive electrode of a sodium ion battery. Specifically, it can form an electrode mixture together with the positive electrode active material and the solvent. The present disclosure also relates to an electrode mixture containing a sodium composite oxide as the positive electrode active material and including the above positive electrode binder.

[0159] As the positive electrode active material used in the present disclosure, there is no particular limitation as long as it can electrochemically occlude / release sodium ions.

[0160] As the positive electrode active material, there is no particular limitation as long as it can electrochemically occlude / release alkali metal ions. For example, a substance containing an alkali metal and at least one transition metal is preferred. As a specific example, a transition metal composite oxide containing an alkali metal and a transition metal phosphate compound containing an alkali metal can be cited. Among them, as the positive electrode active material, a transition metal composite oxide containing an alkali metal that can generate a high voltage is particularly preferred. As the above alkali metal ions, lithium ions, sodium ions, potassium ions, etc. can be cited. In a preferred mode, the alkali metal ions can be lithium ions or sodium ions. That is, in this mode, the alkali metal ion secondary battery is a sodium ion secondary battery.

[0161] As the above transition metal composite oxide containing an alkali metal, for example,

[0162] Formula (3-1): MaMn 2-b M 1 b O 4

[0163] (wherein, M is Na; 0.9 ≤ a; 0 ≤ b ≤ 1.5; M 1 is 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) the sodium-manganese spinel composite oxide,

[0164] Formula (3-2): MNi 1-c M 2 c O 2

[0165] (wherein, M is Na; 0 ≤ c ≤ 0.5; M 2 is 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) the sodium-nickel composite oxide, or

[0166] Formula (3-3): MCo 1-d M 3 d O 2

[0167] (wherein, M is Na; 0 ≤ d ≤ 0.5; M 3 is 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) the sodium-cobalt composite oxide.

[0168] Among them, from the aspect of being able to provide a secondary battery with high energy density and high output, MCoO 2 , MMnO 2 , MNiO 2 , MMn 2 O 4 , MNi 0.8 Co 0.15 Al 0.05 O 2 , or MNi 1 / 3 Co 1 / 3 Mn 1 / 3 O 2 etc. are preferred, and a compound represented by the following general formula (3-4) is preferred.

[0169] MNi h Co i Mn j M 5 k O 2(3-4)

[0170] (where M is Na, M 5 is at least one selected from the group consisting of Fe, Cu, Zn, Al, Sn, Cr, V, Ti, Mg, Ca, Sr, B, Ga, In, Si and Ge, (h+i+j+k)=1.0, 0≤h≤1.0, 0≤i≤1.0, 0≤j≤1.5, 0≤k≤0.2.)

[0171] Examples of the alkali metal-containing transition metal phosphate compound include the following formula (70):

[0172] M e M 4 f (PO 4 ) g (70)

[0173] (where M is Na, M 4 It represents at least one selected from the group consisting of V, Ti, Cr, Mn, Fe, Co, Ni and Cu, and a compound represented by 0.5≤e≤3, 1≤f≤2, and 1≤g≤3).

[0174] As the transition metal of the sodium-containing transition metal phosphate compound, V, Ti, Cr, Mn, Fe, Co, Ni, Cu, etc. are preferred. As specific examples, NaFePO 4 、Na 3 Fe 2 (PO 4 ) 3 、NaFeP 2 O 7 etc. iron phosphates; NaCoPO 4 Cobalt phosphates, etc.; substances obtained by replacing part of the transition metal atoms serving as the main body of these sodium transition metal phosphate compounds with other elements such as Al, Ti, V, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga, Zr, Nb, Si, etc.

[0175] The sodium-containing transition metal phosphate compound preferably has an olivine structure.

[0176] As other positive electrode active materials, MFePO can be cited. 4 、MNi 0.8 Co 0.2 O 2 、M 1.2 Fe 0.4 Mn 0.4 O 2 、MNi 0.5 Mn1.5 O 4 、 MV 3 O 6 、 M 2 MnO 3 etc. (where M is Na). In particular, MNi 0.5 Mn 1.5 O 4 etc. When the positive electrode active material operates a secondary battery at a voltage exceeding 4.4 V or a voltage of 4.6 V or higher, the crystal structure will not collapse, and is preferred from this aspect. Therefore, an electrochemical device such as a secondary battery using a positive electrode material containing the above-exemplified positive electrode active material is preferred because even when stored at a high temperature, the remaining capacity is not easily reduced, the resistance increase rate is not easily changed, and even when operating at a high voltage, the battery performance does not deteriorate.

[0177] As other positive electrode active materials, M 2 MnO 3 and MM 6 O 2 (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.) solid solution materials, etc.

[0178] As the above solid solution material, for example, it is an alkali metal manganese oxide represented by the general formula Mx[Mn(1 - y)M 7 y]Oz. Here, M in the formula is Na, and M 7 is composed of at least one metal element other than M and Mn, and for example, contains one or more elements selected from the group consisting of Co, Ni, Fe, Ti, Mo, W, Cr, Zr, and Sn. In addition, 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.

[0179] The average particle size of the above positive electrode active material is, for example, 1 μm to 50 μm, preferably 1 μm to 20 μm, and particularly preferably 3 μm to 7 μm. This is because if the average particle size of the positive electrode active material is too small, the operability may deteriorate, and 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. It should be noted that the average particle size of the positive electrode active material can be obtained, for example, by measuring the particle size of the active material carrier observed by a scanning electron microscope (SEM) and averaging.

[0180] In addition, a material in which a substance having a composition different from that of the above positive electrode active material is attached to the surface of the positive electrode active material can also be used. Examples of the surface-attached substance include oxides such as alumina, silica, 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; carbon; and the like.

[0181] These surface-attached substances can be attached to the surface of the positive electrode active material, for example, by the following methods: a method of dissolving or suspending them in a solvent and impregnating them into the positive electrode active material and then drying; a method of dissolving or suspending a precursor of the surface-attached substance in a solvent and impregnating it into the positive electrode active material and then reacting it by heating or the like; a method of adding it to a precursor of the positive electrode active material and firing it simultaneously; and the like. It should be noted that in the case of attaching carbon, a method of mechanically attaching carbonaceous materials in the form of activated carbon or the like later can also be used.

[0182] As the amount of the surface-attached substance, it is used in the following amounts based on the mass of the above positive electrode active material: preferably 0.1 ppm or more, more preferably 1 ppm or more, and further preferably 10 ppm or more as the lower limit, and preferably 20% or less, more preferably 10% or less, and further preferably 5% or less as the upper limit. By the surface-attached substance, the oxidation reaction of the electrolytic solution on the surface of the positive electrode active material can be suppressed, and the battery life can be improved. However, if the attached amount is too small, this effect cannot be fully exhibited; if it is too large, the entry and exit of sodium ions will be hindered, and thus the resistance may increase.

[0183] Examples of the shape of the particles of the positive electrode active material include the block shape, polyhedron shape, spherical shape, ellipsoidal shape, plate shape, needle shape, columnar shape, etc. used in the past. In addition, primary particles can also aggregate to form secondary particles.

[0184] The tapped density of the positive electrode active material is preferably 0.5 g / cm 3 or more, more preferably 0.8 g / cm 3 or more, and further preferably 1.0 g / cm 3 or more. If the tapped density of the positive electrode active material is lower than the above lower limit, the amount of the dispersion medium required for forming the positive electrode active material layer increases, and the required amounts of the conductive material and the binder increase, and the filling rate of the positive electrode active material in the positive electrode active material layer is limited, and the battery capacity may be limited. By using a composite oxide powder with a high tapped density, a high-density positive electrode active material layer can be formed. Generally, the larger the tapped density, the more preferable, and there is no particular upper limit. However, if it is too large, the diffusion of sodium ions using the electrolytic solution in the positive electrode active material layer as a medium becomes rate-determining, and sometimes the load characteristics are likely to decrease. Therefore, the upper limit is preferably 4.0 g / cm3 Hereinafter, it is more preferably 3.7 g / cm 3 Hereinafter, it is further preferably 3.5 g / cm 3 Hereinafter.

[0185] It should be noted that in the present disclosure, regarding the tapped density, 5 to 10 g of the positive electrode active material powder is filled into a 10 ml glass graduated cylinder, and the powder packing density (tapped density) g / cm when oscillating 200 times with a stroke of about 20 mm is 3 obtained.

[0186] The median diameter d50 of the particles of the positive electrode active material (the secondary particle diameter in the case where primary particles aggregate to form secondary particles) is preferably 0.3 μm or more, more preferably 0.5 μm or more, further preferably 0.8 μm or more, most preferably 1.0 μm or more, and preferably 30 μm or less, more preferably 27 μm or less, further preferably 25 μm or less, most preferably 22 μm or less. If it is less than the above lower limit, a high tapped density product may not be obtained; if it exceeds the upper limit, the diffusion of lithium in the particles takes time, so that the battery performance may sometimes be reduced, or problems such as streaks may occur when the positive electrode of the battery is manufactured, that is, when the active material, the conductive material, the binder, etc. are slurried with a solvent and coated into a thin film. Here, by mixing two or more kinds of the above positive electrode active materials having different median diameters d50, the filling property during the production of the positive electrode can be further improved.

[0187] It should be noted that in the present disclosure, the median diameter d50 is measured by a known laser diffraction / scattering type particle size distribution measuring device. When using LA-920 manufactured by HORIBA Ltd. as the particle size distribution meter, 0.1 mass% sodium hexametaphosphate aqueous solution is used as the dispersion medium during measurement, and the refractive index for measurement is set to 1.24 after ultrasonic dispersion for 5 minutes for measurement.

[0188] In the case where primary particles aggregate to form secondary particles, the average primary particle diameter of the above positive electrode active material is preferably 0.05 μm or more, more preferably 0.1 μm or more, further preferably 0.2 μm or more, and the upper limit is preferably 5 μm or less, more preferably 4 μm or less, further preferably 3 μm or less, most preferably 2 μm or less. If it exceeds the above upper limit, it is difficult to form spherical secondary particles, which has an adverse effect on the powder filling property, or the specific surface area is greatly reduced, so the possibility of reducing battery performance such as output characteristics sometimes increases. On the contrary, if it is lower than the above lower limit, the crystallization is usually underdeveloped, so problems such as poor charge-discharge reversibility may occur.

[0189] Note that in the present disclosure, the primary particle size is measured by observation using a scanning electron microscope (SEM). Specifically, in a photograph at a magnification of 10,000 times, for any 50 primary particles, the maximum value of the section intercepted by the left and right boundary lines of the primary particles on a horizontal straight line is obtained, and the average value is taken, thereby obtaining the primary particle size.

[0190] The BET specific surface area of the positive electrode active material is preferably 0.1 m 2 / g or more, more preferably 0.2 m 2 / g or more, further preferably 0.3 m 2 / g or more, and the upper limit is preferably 50 m 2 / g or less, more preferably 40 m 2 / g or less, further preferably 30 m 2 / g or less. If the BET specific surface area is less than this range, the battery performance is likely to decrease. If it is greater than this range, it is difficult to improve the tap density, and sometimes problems are likely to occur in the coating property when forming the positive electrode active material layer.

[0191] Note that in the present disclosure, the BET specific surface area is defined as follows: After pre-drying a sample at 150 °C for 30 minutes under a nitrogen flow using a surface area meter (for example, a fully automatic surface area measuring device manufactured by Okura Riken Co., Ltd.), a nitrogen-helium mixed gas accurately adjusted so that the relative pressure value of nitrogen with respect to the atmospheric pressure becomes 0.3 is used, and the value measured by the BET single-point method of nitrogen adsorption based on the gas flow method is used.

[0192] When the secondary battery of the present disclosure is used as a large sodium ion secondary battery for hybrid vehicles or distributed power sources, high output is required. Therefore, the particles of the positive electrode active material preferably have secondary particles as the main body.

[0193] The particles of the positive electrode active material preferably contain fine particles with an average secondary particle size of 40 μm or less and an average primary particle size of 1 μm or less in an amount of 0.5 vol% to 7.0 vol%. By containing fine particles with an average primary particle size of 1 μm or less, the contact area with the electrolyte becomes larger, and the diffusion of lithium ions between the electrode and the electrolyte can be made faster. As a result, the output performance of the battery can be improved.

[0194] As a method for manufacturing the positive electrode active material, common methods used for manufacturing inorganic compounds are used. In particular, various methods are considered for producing spherical or ellipsoidal active materials. For example, the following methods can be cited: Dissolving or pulverizing and dispersing a raw material substance of a transition metal in a solvent such as water, adjusting the pH under stirring to produce a spherical precursor and recovering it, drying it as needed, and then adding a Na source such as sodium hydroxide and firing it at a high temperature to obtain the active material.

[0195] To manufacture the positive electrode, the above positive electrode active material may be used alone, or two or more of different compositions may be used in any combination or ratio.

[0196] The solvent for the electrode mixture is not particularly limited as long as it can dissolve or disperse the positive electrode active material, the binder, and the conductive agent and thickener used as needed, and either an aqueous solvent or an organic solvent can be used. Examples of the aqueous solvent include water, a mixed solvent of alcohol and water, etc. Examples of the organic solvent 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; aprotic polar solvents such as hexamethylphosphoramide and dimethyl sulfoxide, etc.

[0197] As the organic solvent, the solvent represented by the general formula (4) can also be used.

[0198] General formula (4):

[0199] [Chemical formula 7]

[0200]

[0201] (In the formula, R 1 , R 2 and R 3 are independently H or an organic group, where the total number of carbon atoms of R 1 , R 2 and R 3 is 6 or more, and at least one of R 1 , R 2 and R 3 is an organic group having a carbonyl group. Any two of R 1 , R 2 and R 3 can be bonded to form a ring.)

[0202] R 1 , R 2 and R 3 have a total number of carbon atoms of 6 or more. That is, R 1 , R 2 and R 3 respectively select the types of groups in such a way that the total number of carbon atoms is 6 or more. R 1 , R 2 and R 3The upper limit of the total number of carbon atoms is not limited, and may be 16 or less, 14 or less, or 12 or less.

[0203] R 1 , R 2 and R 3 and are independently H or an organic group. As the organic group, an alkyl group, an alkoxyalkyl group, an acylalkyl group, an alkenyl group, an amino group, an aminoalkyl group or a cycloalkyl group is preferred.

[0204] R 1 , R 2 and R 3 At least one of them is an organic group having a carbonyl group. As the organic group having a carbonyl group, an acyl group is preferred. As the acyl group, a general formula of -CO-R is preferred. 4 (Where R 4 is an alkyl group having 1 to 6 carbon atoms). When the alkyl group has 3 or more carbon atoms, it may be linear or branched. 4 When the number of carbon atoms of the alkyl group is 2 or more, a hetero atom such as an oxygen atom or a nitrogen atom or a carbonyl group may be contained between carbon atoms.

[0205] R 1 , R 2 and R 3 Any two of them may be bonded to form a ring. In addition, as the constituent atoms of the ring, heteroatoms such as oxygen atoms may be included. The ring is preferably a saturated ring. The number of ring members is not particularly limited, preferably 5 or 6. As the ring, a pyrrolidine ring, an oxazoline ring, a piperidine ring or a morpholine ring is preferred.

[0206] As the solvent, a solvent represented by the general formula (4a) is preferred.

[0207] General formula (4a):

[0208] [Chemistry 8]

[0209]

[0210] (Where R 1a is an organic group, R 2a and R 3a are independently H or an organic group, wherein R 1a , R 2a and R 3a The total number of carbon atoms is 5 or more. 1a , R 2a and R 3a Any two of them can be bonded to form a ring. )

[0211] In the general formula (4a), R 1a , R 2a and R 3aThe total number of carbon atoms is 5 or more. That is, R 1a , R 2a and R 3a select the types of groups such that the total number of carbon atoms is 5 or more. R 1a , R 2a and R 3a There is no limitation on the upper limit of the total number of carbon atoms and it may be 15 or less, 13 or less, or 11 or less.

[0212] In general formula (4a), R 1a is an organic group. As the organic group, an alkyl group, an alkoxyalkyl group, an acylalkyl group, an alkenyl group, an amino group, an aminoalkyl group, or a cycloalkyl group is preferable, and an alkyl group, an alkoxyalkyl group, an acylalkyl group, an alkenyl group, an amino group, or an aminoalkyl group is more preferable.

[0213] In general formula (4a), R 2a and R 3a are independently H or an organic group. As R 2a and R 3a , an organic group is independently preferable. As the organic group, an alkyl group, an alkoxyalkyl group, an acylalkyl group, an alkenyl group, an amino group, an aminoalkyl group, or a cycloalkyl group is preferable, and an alkyl group, an alkoxyalkyl group, a cycloalkyl group, or an alkenyl group is more preferable.

[0214] Any two of R 1a , R 2a and R 3a may be bonded to form a ring. It is particularly preferable that R 2a and R 3a are bonded and together with the nitrogen atom to which R 2a and R 3a are bonded, form a ring. In addition, as the atoms constituting the ring, heteroatoms such as an oxygen atom may be included. The ring is preferably a saturated ring. The number of members of the ring is not particularly limited and 5-membered or 6-membered is preferable. As the ring, a pyrrolidine ring, an oxazoline ring, a piperidine ring, or a morpholine ring is preferable.

[0215] As the solvent, at least one selected from the group consisting of the solvent represented by general formula (4b-1) and the solvent represented by general formula (4b-2) is more preferable.

[0216] General formula (4b-1):

[0217] [Chemical formula 9]

[0218]

[0219] (In the formula, R 1b is an alkyl group, an alkoxyalkyl group, an acylalkyl group, an alkenyl group, an amino group, or an aminoalkyl group, R 2b and R 3b are independently an alkyl group or an alkoxyalkyl group, R 1b , R2b and R 3b have a total carbon atom number of 5 or more. R 2b and R 3b may be bonded to each other and together with the nitrogen atom to which R 2b and R 3b is bonded form a ring, and may also contain an oxygen atom as a ring-constituting atom.)

[0220] General formula (4b-2):

[0221] [Chemical formula 10]

[0222]

[0223] (In the formula, ring A is a 5- or 6-membered amide ring, R 4b is an alkyl group, a cycloalkyl group or an alkenyl group, and the total carbon atom number of ring A and R 4b is 5 or more.)

[0224] In general formula (4b-1), the total carbon atom number of R 1b , R 2b and R 3b is 5 or more. That is, R 1b , R 2b and R 3b select the types of the groups such that the total carbon atom number is 5 or more. The upper limit of the total carbon atom number of R 1b , R 2b and R 3b is not limited, and may be 15 or less, 13 or less, or 11 or less.)

[0225] In general formula (4b-1), R 1b is an alkyl group, an alkoxyalkyl group, an acylalkyl group, an alkenyl group, an amino group or an aminoalkyl group.)

[0226] As the alkyl group of R 1b , an alkyl group having 1 to 10 carbon atoms is preferred. When the number of carbon atoms of the alkyl group is 3 or more, it may be linear or branched.)

[0227] As the alkoxyalkyl group of R 1b , a group represented by the general formula: -R 1b1 -O-R 1b2 (In the formula, R 1b1 is an alkylene group having 1 to 5 carbon atoms, and R 1b2 is an alkyl group having 1 to 5 carbon atoms) is preferred. When the number of carbon atoms of the alkyl group and the alkylene group is 3 or more, it may be linear or branched.)

[0228] As the acylalkyl group of R 1b , a group represented by the general formula: -R 1b3-CO-R 1b4 (wherein, R 1b3 is an alkylene group having 1 to 5 carbon atoms, and R 1b4 is an alkyl group having 1 to 5 carbon atoms). When the number of carbon atoms of the alkyl group and the alkylene group is 3 or more, it may be linear or branched.

[0229] As the alkenyl group of R 1b , the general formula: -R 1b5 -CR 1b6 =CR 1b7 (wherein, R 1b5 is a single bond or an alkylene group having 1 to 5 carbon atoms, and R 1b6 and R 1b7 are independently H or an alkyl group having 1 to 5 carbon atoms) is preferred. When the number of carbon atoms of the alkyl group and the alkylene group is 3 or more, it may be linear or branched. As the alkenyl group, vinyl is preferred.

[0230] The amino group of R 1b and the amino group of the aminoalkyl group are monovalent functional groups obtained by removing hydrogen from ammonia, primary amine or secondary amine. When R 1b is an amino group, it can form an amide bond together with the carbonyl group to which R 1b is bonded.

[0231] As the amino group of R 1b , the general formula: -N-(R 1b8 ) 2 (wherein, R 1b8 is H or an alkyl group having 1 to 5 carbon atoms) is preferred. When the number of carbon atoms of the alkyl group is 3 or more, it may be linear or branched. As the amino group, -N-(CH 3 ) 2 or -N-(C 2 H 5 ) 2 is preferred.

[0232] As the aminoalkyl group of R 1b , the general formula: -R 1b9 -N-(R 1b8 ) 2 (wherein, R 1b9 is an alkylene group having 1 to 5 carbon atoms, and R 1b8 is H or an alkyl group having 1 to 5 carbon atoms) is preferred. When the number of carbon atoms of the alkyl group and the alkylene group is 3 or more, it may be linear or branched.

[0233] In the general formula (1b-1), R 2b and R 3b are independently an alkyl group or an alkoxyalkyl group.

[0234] As R 2b and R 3b is an alkyl group, preferably an alkyl group having 1 to 10 carbon atoms. When the number of carbon atoms of the alkyl group is 3 or more, it may be linear or branched.

[0235] As R 2b and R 3b is an alkoxyalkyl group, preferably a group represented by the general formula: -R2 b1 -O-R 2b2 (In the formula, R 2b1 is an alkylene group having 1 to 5 carbon atoms, and R 2b2 is an alkyl group having 1 to 5 carbon atoms). When the number of carbon atoms of the alkyl group and the alkylene group is 3 or more, it may be linear or branched.

[0236] R 2b and R 3b can be bonded to each other to form a ring together with the nitrogen atom to which R 2b and R 3b are bonded, and may contain an oxygen atom as a ring-constituting atom. The ring is preferably a saturated ring. The number of members of the ring is not particularly limited, and 5-membered or 6-membered rings are preferred. As the ring, a pyrrolidine ring, an oxazoline ring, a piperidine ring, or a morpholine ring is preferred.

[0237] In the general formula (1b-2), the total number of carbon atoms of the ring A and R 4b is 5 or more. That is, the types of the ring and the group are selected such that the total number of carbon atoms of the ring A and R 4b is 5 or more. The upper limit of the total number of carbon atoms of the ring A and R 4b is not limited, and may be 15 or less, 13 or less, or 11 or less.

[0238] The ring A is a 5-membered or 6-membered amide ring. Therefore, the ring A is composed of carbon atoms, nitrogen atoms, and an alkylene group having 3 to 4 carbon atoms. The hydrogen atom bonded to the carbon atom of the alkylene group constituting the ring A may or may not be substituted by a substituent, and is preferably not substituted by a substituent. Examples of the substituent include an alkyl group such as a methyl group.

[0239] R 4b is an alkyl group, a cycloalkyl group, or an alkenyl group.

[0240] As the alkyl group of R 4b is preferably an alkyl group having 1 to 10 carbon atoms. When the number of carbon atoms of the alkyl group is 3 or more, it may be linear or branched.

[0241] As the cycloalkyl group of R 4b is preferably a cycloalkyl group having 3 to 10 carbon atoms.

[0242] As the cycloalkyl group, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl or cyclooctyl is preferred.

[0243] As R 4b 's alkenyl group, the general formula: -R 4b1 -CR 4b2 =CR 4b3 (wherein, R 4b1 is a single bond or an alkylene group having 1 to 5 carbon atoms, and R 4b2 and R 4b3 are independently H or an alkyl group having 1 to 5 carbon atoms) is preferred. When the number of carbon atoms of the alkyl group and the alkylene group is 3 or more, it can be linear or branched. As the alkenyl group, vinyl is preferred.

[0244] As the 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), acrylylmorpholine, N-cyclohexyl-2-pyrrolidone, N-vinyl-2-pyrrolidone, 3-butoxy-N,N-dimethylpropanamide, N,N,N',N'-tetraethylurea, N,N-dimethylacetylethylamide, N-octyl-2-pyrrolidone and N,N-diethylethylamide is preferred.

[0245] As the solvent, among them, 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 using an electrode binder containing 3-methoxy-N,N-dimethylpropanamide as the solvent, the gas generation amount of the obtained battery tends to be suppressed. In particular, when using an electrode binder containing N-ethyl-2-pyrrolidone (NEP) as the solvent, the high-temperature storage capacity retention rate of the obtained battery tends to be high. In particular, when using an electrode binder containing N-butyl-2-pyrrolidone (NBP) as the solvent, the resistance of the obtained battery hardly increases.

[0246] The amount of the solvent in the electrode binder is determined in consideration of the coatability on the current collector, the film-forming property after drying, etc. Usually, the ratio of the copolymer (A) to the solvent is preferably 0.5:99.5 to 20:80 by mass ratio.

[0247] The electrode binder of the present disclosure preferably further contains a conductive agent.

[0248] As the above conductive agent, any well-known conductive material can be used. As specific examples, 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, thermal cracking carbon black, acicular coke, carbon nanotubes, fullerenes, amorphous carbon such as VGCF, and other carbon materials can be cited. It should be noted that they can be used alone, or two or more of them can be used in any combination and proportion.

[0249] The content of the conductive agent in the electrode material layer is usually 0.01% by mass or more, preferably 0.1% by mass or more, more preferably 1% by mass or more. In addition, it is used in a manner that usually contains 50% by mass or less, preferably contains 30% by mass or less, and more preferably contains 15% by mass or less. If the content is lower than this range, the conductivity may sometimes be insufficient. On the contrary, if the content is higher than this range, the battery capacity may sometimes decrease.

[0250] The electrode mixture of the present disclosure may further contain a thickener as needed. As the above thickener, carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, casein, polyvinylpyrrolidone, and their salts can be cited. One kind can be used alone, or two or more kinds can be used in any combination and proportion.

[0251] 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. In addition, it is usually in 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 lower than this range, the coatability may be significantly reduced. If it is higher than this range, the proportion of the active material in the positive electrode active material layer decreases, which may cause problems such as a decrease in the battery capacity or an increase in the resistance between the positive electrode active materials.

[0252] The electrode mixture may also contain materials other than the above materials. However, when the entire electrode material layer is set to 100% by mass, the content ratio of this material is preferably 8% by mass or less, and more preferably 4% by mass or less.

[0253] In the process of preparing the electrode mixture of the present disclosure, the above components are mixed to form a slurry. The mixing order of the components is not particularly limited, and the components can be added to the solvent and mixed.

[0254] The solid component concentration in the above slurry is preferably 70% by mass to 95% by mass. If it is within this range, the interfacial resistance during coating can be suppressed. In addition, the lower limit is preferably 73% by mass or more, more preferably 73.5% by mass or more, and the upper limit is preferably 85% by mass or less.

[0255] The electrode mixture of the present disclosure can be suitably used as an electrode material layer in the positive electrode of a sodium-ion secondary battery.

[0256] An electrode having a current collector and an electrode material layer formed of the electrode mixture of the present disclosure provided on one or both sides of the current collector is also one of the present disclosures.

[0257] As described above, in the positive electrode of a sodium-ion secondary battery, by providing an electrode material layer formed of an electrode mixture containing the binder for positive electrode of the present disclosure, the interfacial resistance between the electrode material layer and the current collector becomes low. By reducing the interfacial resistance of the electrode, the initial impedance in the sodium-ion secondary battery can be reduced, and the battery characteristics become good.

[0258] The above-mentioned sodium-ion secondary battery can adopt a known structure. Typically, it includes a positive electrode and a negative electrode capable of occluding / releasing sodium ions, and an electrolytic solution. A sodium-ion secondary battery having the electrode of the present disclosure is also one of the present disclosures.

[0259] <Positive Electrode>

[0260] The positive electrode preferably includes an electrode material layer containing the above-mentioned positive electrode active material (hereinafter sometimes referred to as a positive electrode active material layer) and a current collector.

[0261] As the material of the current collector for the positive electrode, metal materials such as aluminum, titanium, tantalum, stainless steel, nickel, or their alloys can be mentioned; carbon materials such as carbon cloth and carbon paper. Among them, metal materials, particularly aluminum or its alloys, are preferred.

[0262] As the shape of the current collector, in the case of metal materials, metal foils, metal cylinders, metal coils, metal plates, porous metal meshes, stamped metals, foamed metals, etc. can be mentioned, and in the case of carbon materials, carbon plates, carbon films, carbon cylinders, etc. can be mentioned. Among these, metal foils are preferred. It should be noted that the film can be appropriately formed into a mesh shape. The thickness of the film is arbitrary, 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 film is thinner than this range, the strength required as a current collector may sometimes be insufficient. On the contrary, if the film is thicker than this range, the processability may sometimes be impaired.

[0263] In addition, from the aspect of reducing the electrical contact resistance between the current collector and the positive electrode active material layer, it is also preferred that a conductive aid is coated on the surface of the current collector. As the conductive aid, carbon or noble metals such as gold, platinum, and silver can be mentioned.

[0264] The ratio of the thickness of the current collector to that of the positive electrode active material layer is not particularly limited. The value of (the thickness of the positive electrode active material layer on one side before injecting the electrolyte) / (the thickness of the current collector) is preferably 20 or less, more preferably 15 or less, and most preferably 10 or less. Additionally, it is preferably in the range of 0.5 or more, more preferably 0.8 or more, and most preferably 1 or more. If it exceeds this range, heat generation caused by Joule heat may occur in the current collector during charge and discharge at high current densities. If it is less than this range, the volume ratio of the current collector to the positive electrode active material increases, and the capacity of the battery may sometimes decrease.

[0265] The positive electrode can be manufactured according to a conventional method. For example, a method can be cited in which the above-mentioned binder, thickener, conductive material, solvent, etc. are added to the above-mentioned positive electrode active material to form a paste-like positive electrode mixture, which is coated on the current collector and pressed after drying to achieve high density.

[0266] The above-mentioned high densification can be carried out by hand pressing, roll pressing, etc. The density of the positive electrode active material layer is preferably 1.5 g / cm 3 or more, more preferably 2 g / cm 3 or more, further preferably 2.2 g / cm 3 or more. Additionally, it is preferably 5 g / cm 3 or less, more preferably 4.5 g / cm 3 or less, further preferably 4 g / cm 3 or less. If it exceeds this range, the permeability of the electrolyte to the vicinity of the current collector / active material interface decreases, especially the charge and discharge characteristics at high current densities decrease, and sometimes high output cannot be obtained. Additionally, if it is less than this range, the conductivity between the active materials decreases, and sometimes the battery resistance increases and high output cannot be obtained.

[0267] In addition, from the aspect of high output and improved stability at high temperatures, the area of the positive electrode active material layer is preferably larger than the outer surface area of the battery case. Specifically, the total area of the positive electrode areas, in terms of area ratio, is preferably 15 times or more, and more preferably 40 times or more, relative to the outer surface area of the secondary battery. Regarding the outer surface area of the battery case, in the case of a bottomed square shape, it refers to the total area calculated from the length, width, and thickness dimensions of the case part filled with the power generation element excluding the protruding parts of the terminals. In the case of a bottomed cylindrical shape, it refers to the geometric surface area of approximating the case part filled with the power generation element excluding the protruding parts of the terminals as a cylinder. The total area of the positive electrode mixture areas is the geometric surface area of the positive electrode mixture layer facing the mixture layer containing the negative electrode active material. In a structure where the positive electrode mixture layers are formed on both sides by a current collector foil, it refers to the sum of the areas calculated for each side.

[0268] The content of the copolymer (A) in the electrode material layer is preferably 0.1% by mass to 5% by mass, more preferably 0.5% by mass to 3% by mass. Additionally, 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 (A) in the electrode material layer is low, the flexibility may sometimes decrease. On the contrary, if the content is too high, the electrode performance may sometimes decrease.

[0269] When using a blend polymer formed by combining one or more of the above copolymer (A) and a fluoropolymer other than the above copolymer (A), the content of the blend polymer in the electrode material layer is preferably 0.1% by mass to 5% by mass, more preferably 0.5% by mass to 3% by mass. Additionally, 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 (A) in the electrode material layer is low, the flexibility of the electrode may sometimes decrease. On the contrary, if the content is too high, the battery performance may sometimes decrease.

[0270] From the perspective of high battery capacity, the content of the above positive electrode active material is preferably 50% by mass to 99.5% by mass of the electrode material layer, more preferably 80% by mass to 99% by mass. Additionally, 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. Additionally, 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 capacitance may sometimes become insufficient. On the contrary, if the content is too high, the strength of the positive electrode may sometimes be insufficient.

[0271] The thickness of the positive electrode plate is not particularly limited. From the perspective of high capacity and high output, the thickness of the binder layer after subtracting the thickness of the metal foil of the core material is preferably 10 μm or more, more preferably 20 μm or more, and preferably 500 μm or less, more preferably 450 μm or less, with respect to one side of the current collector.

[0272] Additionally, a positive electrode plate having a substance different in composition from it attached to the surface of the above positive electrode plate may also be used. Examples of the surface-attached substance include oxides such as alumina, silica, 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; carbon; and so on.

[0273] <Negative electrode>

[0274] The negative electrode preferably consists 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.

[0275] (Negative electrode active material)

[0276] The negative electrode active material is not particularly limited, and examples thereof include substances containing carbonaceous materials such as those selected from lithium metal, artificial graphite, graphite carbon fiber, resin-fired carbon, thermally decomposed vapor-grown carbon, coke, mesocarbon microbeads (MCMB), furfuryl resin-fired carbon, polyacene, pitch-based carbon fiber, vapor-grown carbon fiber, natural graphite, and hardly graphitizable carbon, silicon and silicon-containing compounds such as silicon alloys, Li 4 Ti 5 O 12 and any one or a mixture of two or more thereof. Among them, substances containing carbonaceous materials or silicon-containing compounds in at least part can be particularly preferably used.

[0277] The negative electrode active material used in the present disclosure preferably contains silicon in its constituent elements. By containing silicon in the constituent elements, a high-capacity battery can be fabricated.

[0278] 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 preferred. By using these substances, 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.

[0279] The silicon oxide in the present disclosure refers to a general term for amorphous silicon oxides, and the silicon oxide before disproportionation is represented by the general formula SiOx (0.5 ≤ x ≤ 1.6). x is preferably 0.8 ≤ x < 1.6, 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 and cooling and precipitating the generated silicon monoxide gas.

[0280] Particles having a structure in which fine particles of silicon are dispersed in a silicon-based compound can be obtained, for example, by a method of mixing fine particles of silicon and a silicon-based compound and firing the mixture; 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°C to 1,100°C, to carry out a disproportionation reaction. In particular, the material obtained by the latter method is preferred because the silicon microcrystals are uniformly dispersed. By the above disproportionation reaction, the size of the silicon nanoparticles can be made 1 nm to 100 nm. It should be noted that, regarding the silicon oxide in the particles having a structure in which silicon nanoparticles are dispersed in silicon oxide, silicon dioxide is preferred. It should be noted that the dispersion of the silicon nanoparticles (crystals) in the amorphous silicon oxide can be confirmed by transmission electron microscopy.

[0281] The physical properties of the silicon-containing particles can be appropriately selected according to the target composite particles. For example, the average particle diameter is preferably 0.1 μm to 50 μm, more preferably 0.2 μm or more, and still more preferably 0.5 μm or more. The upper limit is more preferably 30 μm or less, and still more preferably 20 μm or less. It should be noted that the average particle diameter in the present disclosure is represented by the weight average particle diameter in the particle size distribution measurement based on the laser diffraction method.

[0282] The BET specific surface area is preferably 0.5 m 2 / g to 100 m 2 / g, more preferably 1 m 2 / g to 20 m 2 / g. If the BET specific surface area is 0.5 m 2 / g or more, there is no possibility of reducing the adhesiveness when coated on the electrode or reducing the battery characteristics. In addition, if it is 100 m 2 / g or less, the proportion of silica on the particle surface becomes large, and the battery capacity will not decrease when used as the negative electrode material for a sodium ion secondary battery.

[0283] By carbon-coating the above-mentioned silicon-containing particles to impart conductivity, an improvement in battery characteristics was observed. As a method for imparting conductivity, a method of mixing with conductive particles such as graphite, a method of coating the surface of the above-mentioned silicon-containing particles with a carbon coating film, and a method of combining the two can be cited. The method of coating with a carbon coating film is preferred, and the method of chemical vapor deposition (CVD) is more preferred.

[0284] In order to increase the capacity of the obtained electrode mixture, the content of the negative electrode active material in the electrode mixture is preferably 40% by mass or more, more preferably 50% by mass or more, and particularly preferably 60% by mass or more. In addition, the upper limit is preferably 99% by mass or less, and more preferably 98% by mass or less.

[0285] (Conductive agent)

[0286] The negative electrode active material layer may further contain a conductive agent as needed.

[0287] As the above-mentioned conductive agent, known conductive materials can be arbitrarily used. As specific examples, metal materials such as copper and nickel, graphite such as natural graphite and artificial graphite, carbon blacks such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking carbon black, acicular coke, carbon nanotubes, fullerenes, amorphous carbon materials such as VGCF, etc. can be cited. It should be noted that they can be used alone, or two or more of them can be used in any combination and ratio.

[0288] The conductive agent is usually 0.01% by mass or more, preferably 0.1% by mass or more, more preferably 1% by mass or more in the negative electrode active material layer, and is used in a manner such that it usually contains 50% by mass or less, preferably 30% by mass or less, more preferably 15% by mass or less. If the content is below this range, the conductivity may sometimes be insufficient. On the contrary, if the content is above this range, the battery capacity may sometimes decrease.

[0289] (Binder)

[0290] The negative electrode active material layer preferably contains a binder.

[0291] The binder as described above is not particularly limited, and examples thereof include the same binders as those that can be used for the positive electrode 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. In addition, it is preferably 20% by mass or less, more preferably 15% by mass or less, further preferably 10% by mass or less, particularly preferably 8% by mass or less. If the ratio of the binder to the negative electrode active material is higher than the above range, the proportion of the binder that does not contribute to the battery capacity increases, sometimes resulting in a decrease in the battery capacity. In addition, if it is below the above range, the strength of the negative electrode may sometimes decrease.

[0292] Especially when the main component contains a rubber-like polymer represented by SBR, 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. In addition, it is usually 5% by mass or less, preferably 3% by mass or less, more preferably 2% by mass or less. In addition, when the main component contains a fluorine-based polymer represented by polyvinylidene fluoride, 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 is usually 15% by mass or less, preferably 10% by mass or less, more preferably 8% by mass or less.

[0293] The negative electrode active material layer may further contain a thermoplastic resin. Examples of the thermoplastic resin include vinylidene fluoride, polypropylene, polyethylene, polystyrene, polyethylene terephthalate, polyethylene oxide, etc. One kind can be used alone, or two or more kinds can be used in any combination and ratio.

[0294] The proportion of the thermoplastic resin relative to the negative electrode active material is generally 0.01% by mass or more, preferably 0.05% by mass or more, more preferably 0.10% by mass or more, and further, it is generally 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. In addition, if it exceeds this range, the proportion of the electrode active material in the electrode mixture decreases, and sometimes problems such as a decrease in the capacity of the battery or an increase in the resistance between the active materials may occur.

[0295] The negative electrode active material layer may contain a thickener.

[0296] As the above-mentioned thickener, the same substances as those that can be used in the positive electrode can be cited. The proportion of the thickener relative to the negative electrode active material is generally 0.1% by mass or more, preferably 0.5% by mass or more, more preferably 0.6% by mass or more, and generally 5% by mass or less, preferably 3% by mass or less, more preferably 2% by mass or less. If the proportion of the thickener relative to the negative electrode active material is lower than the above range, sometimes the coatability is significantly reduced. In addition, if it is greater than the above range, the proportion of the negative electrode active material in the negative electrode active material layer decreases, there is a problem of a decrease in the capacity of the battery, and sometimes the resistance between the negative electrode active materials increases.

[0297] (Other components)

[0298] The negative electrode mixture of the present disclosure may further contain other components such as a leveling agent and a reinforcing material.

[0299] 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 mixture, and can be provided on one side of the current collector or on both sides.

[0300] As the current collector included in the negative electrode, for example, metal foils or metal meshes such as iron, stainless steel, copper, aluminum, nickel, and titanium, carbon materials such as carbon cloth and carbon paper, etc. can be cited, and among them, copper foil is preferred.

[0301] As the shape of the current collector, in the case of a metal material, metal foils, metal cylinders, metal coils, metal plates, porous metal meshes, stamped metals, foamed metals, etc. can be cited, and in the case of a carbon material, carbon plates, carbon films, carbon cylinders, etc. can be cited. Among these, metal foils are preferred. It should be noted that the film can be appropriately formed into a mesh shape. The thickness of the film is arbitrary, generally 1 μm or more, preferably 3 μm or more, more preferably 5 μm or more, and generally 1 mm or less, preferably 100 μm or less, more preferably 50 μm or less. If the film is thinner than this range, sometimes the strength required as a current collector is insufficient. On the contrary, if the film is thicker than this range, sometimes the processability is impaired.

[0302] The negative electrode can be manufactured according to a conventional method. For example, after mixing a binder and a solvent, a negative electrode active material or the like is added to the resulting mixture and further mixed to prepare a slurry-like negative electrode mixture. Then, the obtained negative electrode mixture is uniformly coated on a current collector such as a metal foil or a metal mesh, and the coating film is dried, optionally heat-treated, and, if necessary, the obtained dried coating film is pressed to form a thin negative electrode material layer on the current collector, thereby producing a thin-film electrode. Alternatively, the negative electrode active material and the binder or the like may be first mixed, and then a solvent may be added to prepare the negative electrode mixture.

[0303] The above negative electrode mixture preferably contains a solvent. Examples of the solvent include water or an organic solvent. When the negative electrode material layer is formed using the negative electrode mixture, an organic solvent is preferred in terms of significantly reducing the possibility of moisture remaining in the negative electrode material layer.

[0304] Examples of the organic solvent 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-butoxy-N,N-dimethylpropionamide, and β-hexyloxy-N,N-dimethylpropionamide; and mixed solvents thereof and other low-boiling general organic solvents.

[0305] As the organic solvent, the solvent represented by the above general formula (4) can be used.

[0306] The amount of the solvent in the negative electrode mixture is determined in consideration of the coatability on the current collector, the film-forming property after drying, and the like. Usually, the ratio of the binder to the solvent is 0.5:99.5 to 20:80 by mass.

[0307] The thickness of the negative electrode plate is designed according to the positive electrode plate used and is not particularly limited. The thickness of the mixture layer after subtracting the thickness of the metal foil of the core material is usually 15 μm or more, preferably 20 μm or more, more preferably 30 μm or more, and is preferably 300 μm or less, more preferably 280 μm or less, and still more preferably 250 μm or less.

[0308] (Electrolyte solution)

[0309] As the non-aqueous electrolyte solution, an electrolyte solution obtained by dissolving a known electrolyte salt in a known organic solvent for dissolving the electrolyte salt can be used.

[0310] As the organic solvent for dissolving the electrolyte salt, there is no particular limitation, and known hydrocarbon solvents such as propylene carbonate, ethylene carbonate, butylene carbonate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate can be used; one or more of fluorine-based solvents such as fluoroethylene carbonate, fluorinated ethers, and fluorinated carbonates.

[0311] As the electrolyte salt, for example, NaPF 6 , NaBF 4 , NaClO 4 , NaAlF 4 , NaSbF 6 , NaTaF 6 , NaWF 7 , NaAsF 6 , NaAlCl 4 , NaI, NaBr, NaCl, NaB 10 Cl 10 , Na 2 , NaSiF 6 , Na 2 , NaPFO 3 , NaPO 2 F 2 and other inorganic sodium salts;

[0312] NaWOF 5 and other sodium tungstates;

[0313] HCO 2 Na, CH 3 CO 2 Na, CH 2 FCO 2 Na, CHF 2 CO 2 Na, CF 3 CO 2 Na, CF 3 CH 2 CO 2 Na, CF 3 CF 2 CO 2 Na, CF 3 CF 2 CF 2 CO 2 Na, CF 3 CF 2 CF 2 CF 2 CO 2 Na and other sodium carboxylates;

[0314] FSO 3Na, CH 3 SO 3 Na, CH 2 FSO 3 Na, CHF 2 SO 3 Na, CF 3 SO 3 Na, CF 3 CF 2 SO 3 Na, CF 3 CF 2 CF 2 SO 3 Na, CF 3 CF 2 CF 2 CF 2 SO 3 Na, sodium methyl sulfate, sodium ethyl sulfate (C 2 H 5 OSO 3 Na), sodium salts with S=O groups such as sodium 2,2,2-trifluoroethyl sulfate;

[0315] NaN(FCO) 2 , NaN(FCO)(FSO 2 ), NaN(FSO 2 ) 2 , NaN(FSO 2 )(CF 3 SO 2 ), NaN(CF 3 SO 2 ) 2 , NaN(C 2 F 5 SO 2 ) 2 , sodium bis(perfluoroethanesulfonyl)imide, cyclic sodium 1,2-perfluorobutanesulfonylimide, cyclic sodium 1,3-perfluoropropanesulfonylimide, cyclic sodium 1,2-ethanedisulfonylimide, cyclic sodium 1,3-propanedisulfonylimide, cyclic sodium 1,4-perfluorobutanesulfonylimide, NaN(CF 3 SO 2 )(FSO 2 ), NaN(CF 3 SO 2 )(C 3 F 7 SO 2 ), NaN(CF 3 SO 2 )(C 4 F 9 SO 2)、NaN(POF 2 ) 2 and sodium imide salts such as;

[0316] NaC(FSO 2 ) 3 、NaC(CF 3 SO 2 ) 3 、NaC(C 2 F 5 SO 2 ) 3 and sodium methylide salts such as;

[0317] and salts of the formula: NaPF a (C n F 2n+1 ) 6-a (wherein a is an integer from 0 to 5 and n is an integer from 1 to 6), such as NaPF 3 (C 2 F 5 ) 3 、NaPF 3 (CF 3 ) 3 、NaPF 3 (iso-C 3 F 7 ) 3 、NaPF 5 (iso-C 3 F 7 )、NaPF 4 (CF 3 ) 2 、NaPF 4 (C 2 F 5 ) 2 )、NaPF 4 (CF 3 SO 2 ) 2 、NaPF 4 (C 2 F 5 SO 2 ) 2 、NaBF 3 CF 3 、NaBF 3 C 2 F 5 、NaBF 3 C 3 F 7 、NaBF 2 (CF 3 ) 2 、NaBF2 (C 2 F 5 ) 2 、NaBF 2 (CF 3 SO 2 ) 2 、NaBF 2 (C 2 F 5 SO 2 ) 2 and other sodium fluorinated organic salts, NaSCN, LiB(CN) 4 、NaB(C 6 H 5 ) 4 、Na 2 (C 2 O 4 )、NaP(C 2 O 4 ) 3 、Na 2 B 12 F b H 12-b (where b is an integer from 0 to 3), etc.

[0318] Among them, from the aspect of having effects such as improving output characteristics, high-rate charge and discharge characteristics, high-temperature storage characteristics, and cycle characteristics, NaPF 6 、NaBF 4 、NaSbF 6 、NaTaF 6 、NaPO 2 F 2 、FSO 3 Na、CF 3 SO 3 Na、NaN(FSO 2 ) 2 、NaN(FSO 2 )(CF 3 SO 2 )、NaN(CF 3 SO 2 ) 2 、NaN(C 2 F 5 SO 2 ) 2 、sodium cyclic 1,2-perfluoroethane disulfonimide, sodium cyclic 1,3-perfluoropropane disulfonimide, NaC(FSO 2 ) 3 、NaC(CF 3 SO 2 ) 3 、NaC(C2 F 5 SO 2 ) 3 , NaBF 3 CF 3 , NaBF 3 C 2 F 5 , NaPF 3 (CF 3 ) 3 , NaPF 3 (C 2 F 5 ) 3 , etc., and most preferably selected from NaPF 6 , NaN(FSO 2 ) 2 and NaBF 4 and at least one lithium salt selected from the group consisting of.

[0319] The concentration of the electrolyte salt needs to be 0.8 mol / L or more, and further preferably 1.0 mol / L or more. The upper limit also depends on the organic solvent used for dissolving the electrolyte salt and is usually 1.5 mol / L.

[0320] <Separator>

[0321] The secondary battery of the present disclosure preferably further includes a separator.

[0322] Regarding the material and shape of the above-mentioned separator, there is no particular limitation as long as the electrolyte is stable and the liquid retention property is excellent, and a known separator can be used. Among them, a porous sheet or non-woven fabric-like material formed of a material stable to the electrolyte of the present disclosure, using resin, glass fiber, inorganic substances, etc., and having excellent liquid retention property is preferably used.

[0323] As materials for resin and glass fiber separators, for example, polyolefins such as polyethylene and polypropylene, aromatic polyamides, polytetrafluoroethylene, polyethersulfone, glass filters, etc. can be used. Polypropylene / polyethylene two-layer films, polypropylene / polyethylene / polypropylene three-layer films, etc. These materials can be used alone or in combination of two or more in any combination and ratio. Among them, from the aspect of good electrolyte permeability and shut-off effect, the above-mentioned separator is preferably a porous sheet or non-woven fabric made of polyolefins such as polyethylene and polypropylene.

[0324] The thickness of the separator is arbitrary, 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, more preferably 30 μm or less. If the separator is too thin compared to the above range, the insulation and mechanical strength may sometimes be reduced. In addition, if it is too thick compared to the above range, the battery performance such as rate characteristics may sometimes be reduced, and the energy density of the electrolyte battery as a whole may sometimes be reduced.

[0325] In addition, when a porous material such as a porous sheet or nonwoven fabric is used as a diaphragm, the porosity of the diaphragm is arbitrary, usually 20% or more, preferably 35% or more, more preferably 45% or more, and usually 90% or less, preferably 85% or less, more preferably 75% or less. If the porosity is too small compared to the above range, there is a tendency for the membrane resistance to increase and the rate characteristics to deteriorate. In addition, if it is too large compared to the above range, there is a tendency for the mechanical strength of the diaphragm to decrease and the insulation to decrease.

[0326] In addition, the average pore size of the diaphragm is also arbitrary, 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 circuit is likely to occur. In addition, if it is less than the above range, the membrane resistance sometimes increases and the rate characteristics decrease.

[0327] On the other hand, as inorganic materials, oxides such as aluminum oxide and silicon dioxide, nitrides such as aluminum nitride and silicon nitride, and sulfates such as barium sulfate and calcium sulfate are used, and materials in the form of particles or fibers are used.

[0328] As the form, a non-woven fabric, a woven fabric, a microporous film or other thin film-shaped material is used. In the case of a thin film, it is preferred to use a thin film with a pore size of 0.01 μm to 1 μm and a thickness of 5 μm to 50 μm. In addition to the above-mentioned independent thin film shapes, a diaphragm can also be used in which a composite porous layer containing particles of the above-mentioned inorganic substance is formed on the surface of the positive electrode and / or the negative electrode using a resin binder. For example, a porous layer can be formed on both sides of the positive electrode using a fluororesin as a binder and alumina particles with a particle size of 90% less than 1 μm.

[0329] <Battery Design>

[0330] The electrode group may be any one of a laminated structure in which the separator is interposed between the positive electrode plate and the negative electrode plate, and an electrode group in which the positive electrode plate and the negative electrode plate are wound in a spiral shape with the separator interposed therebetween. The ratio of the volume of the electrode group to the internal volume of the battery (hereinafter referred to as the electrode group occupancy) is usually 40% or more, preferably 50% or more, and usually 90% or less, preferably 80% or less.

[0331] If the occupancy rate of the electrode group is less than the above range, the battery capacity becomes smaller. In addition, if it exceeds the above range, there is less void space, the battery temperature rises, causing the components to expand or the vapor pressure of the liquid component of the electrolyte to increase, and the internal pressure rises, which will reduce various characteristics such as the charge-discharge cycle performance and high-temperature storage of the battery. Furthermore, the gas release valve that releases the internal pressure to the outside may sometimes operate.

[0332] There is no particular limitation on the current collector structure. In order to more effectively improve the charge-discharge characteristics of high current density caused by the electrolyte of the present disclosure, a structure that reduces the resistance of the wiring part or the bonding part is preferably adopted. In the case where the internal resistance is reduced in this way, the effect of using the electrolyte of the present disclosure can be particularly well exerted.

[0333] When the electrode group has the above laminated structure, a structure formed by bundling and welding the metal core parts of each electrode layer to the terminals is preferably used. When the area of one electrode becomes large, since the internal resistance becomes large, it is also preferable to provide a plurality of terminals in the electrode to reduce the resistance. When the electrode group has the above winding structure, a plurality of lead structures can be provided on the positive electrode and the negative electrode respectively and bundled to the terminals, thereby reducing the internal resistance.

[0334] The material of the outer shell is not particularly limited as long as it is a substance stable to the electrolyte used. Specifically, metals such as nickel-plated steel sheets, stainless steels, aluminum or aluminum alloys, and magnesium alloys are used; or a laminated film (laminated film) of resin and aluminum foil. From the aspect of weight reduction, metals or laminated films of aluminum or aluminum alloys are preferably used.

[0335] Among the outer shells using metals, there are outer shells in which metals are fused to each other by laser welding, resistance welding, or ultrasonic welding to form a hermetic sealed structure; or outer shells in which the above metals are used to form a riveted structure through a resin gasket. Among the outer shells using the above laminated film, there are outer shells in which the resin layers are heat-bonded to each other to form a hermetic sealed structure, etc. In order to improve the sealing performance, a resin different from the resin used in the laminated film can also be interposed between the above resin layers. In particular, in the case where a hermetic structure is formed by heat-bonding the resin layers through a current collector terminal, since metal and resin are joined, as the interposed resin, a resin having a polar group or a modified resin into which a polar group is introduced is preferably used.

[0336] The shape of the sodium ion secondary battery of the present disclosure is arbitrary, and shapes such as cylindrical, square, laminated, button-shaped, and large-sized can be cited. It should be noted that the shapes and structures of the positive electrode, negative electrode, and separator can be changed according to the shape of each battery.

[0337] Examples

[0338] Next, examples are given to illustrate the present disclosure, but the present disclosure is not limited to the described examples.

[0339] In the following examples, unless otherwise specified, "parts" and "%" represent "parts by mass" and "mass %", respectively.

[0340] The (co)polymers used in the examples and comparative examples are shown in Table 1 below.

[0341] [Table 1]

[0342]

[0343] It should be noted that the composition and molecular weight of each polymer, etc. are measured by the following methods.

[0344] (Polymer composition)

[0345] The composition of the polymer is measured by solution NMR method.

[0346] Measuring device: VNMRS400 manufactured by Varian

[0347] Resonance frequency: 376.04 (Sfrq)

[0348] Pulse width: 30° (pw = 6.8)

[0349] (Content of monomer units containing polar groups in the polymer)

[0350] The content of monomer units containing polar groups (acrylic acid units) is measured by acid-base titration of carboxyl groups. Specifically, about 0.5 g of the copolymer is dissolved in acetone at a temperature of 70 °C to 80 °C. To avoid solidification of the copolymer, 5 ml of water is added dropwise with vigorous stirring. Titration is carried out with an aqueous NaOH solution having a concentration of 0.1 N until complete neutralization of the acidity is achieved at a neutral transfer of about -270 mV. From the measurement results, the content of the monomer units containing polar groups contained in 1 g of the copolymer is obtained, and the content of the monomer units containing polar groups is calculated.

[0351] (Weight-average molecular weight)

[0352] It is measured by gel permeation chromatography (GPC). Using AS-8010 and CO-8020 columns manufactured by Tosoh Corporation (three GMHHR-H columns are connected in series) and RID-10A manufactured by Shimadzu Corporation, dimethylformamide (DMF) as a solvent is allowed to flow at a flow rate of 1.0 ml / min, and the weight-average molecular weight is calculated from the measured data (reference: polystyrene).

[0353] Examples 1 to 19, Comparative Examples 1 to 5

[0354] (Preparation of Positive Electrode Mix)

[0355] Weigh NaFeO 2 (manufactured by High Purity Chemical Research Institute Co., Ltd.), binder, and carbon black (SUPER-P Li Imerys) in a mass ratio of 97.00:1.50:1.50.

[0356] It should be noted that as the binder, the copolymers (A), PVdF, and fluoropolymer (B) shown in Tables 1 to 3 are used.

[0357] After dissolving the binder in N-methyl-2-pyrrolidone (NMP) to a concentration of 8% by mass, add a specified amount of NaFeO 2 and SP to the resulting NMP solution, and stir at 100 rpm for 60 minutes using a stirrer (T.K.HIVIS MIX manufactured by PRIMIX Corporation). Furthermore, stir at 100 rpm for 30 minutes while performing vacuum degassing treatment. Filter the stirred slurry using a Ni mesh (200 mesh) to homogenize the particle size of the solid components, and obtain the positive electrode mix.

[0358] (Fabrication of Positive Electrode with Positive Electrode Material Layer)

[0359] Apply the obtained positive electrode mix uniformly onto one side of a positive electrode current collector (aluminum foil with a thickness of 20 μm) at a coating amount of 20 mg / cm 2 . After completely evaporating the NMP, apply a pressure of 10 t using a roll press for pressing to fabricate a positive electrode having a positive electrode material layer and a positive electrode current collector.

[0360] (Preparation of Electrolyte)

[0361] Dissolve NaPF 6 in an electrolyte (a solvent with a volume ratio of ethylene carbonate (EC) to ethyl methyl carbonate (EMC) of 3 / 7) at a concentration of 1 mol / L.

[0362] (Fabrication of Negative Electrode)

[0363] Add styrene-butadiene rubber and carboxymethyl cellulose dispersed in distilled water at 1.2% by mass based on the solid content to hard carbon, mix with a disperser to form a slurry, coat it uniformly on a negative electrode current collector (copper foil with a thickness of 10 μm), dry to form a negative electrode mix layer, and then perform compression molding using a roll press to fabricate a negative electrode.

[0364] (Fabrication of Sodium Ion Secondary Battery)

[0365] The strip-shaped positive electrode is cut into 20 mm × 30 mm (with a 5 mm × 5 mm positive terminal), and the strip-shaped negative electrode is cut into 24 mm × 34 mm (with a 5 mm × 5 mm negative terminal). Lead bodies are welded to the respective terminals. Additionally, a microporous polyethylene film with a thickness of 20 μm is cut into a size of 28 mm × 38 mm as the separator, and the positive and negative electrodes are arranged in a manner that sandwiches the separator, and they are placed into an aluminum laminated packaging material. Then, 2 ml of electrolyte is added to the packaging material and sealed to fabricate a laminated battery.

[0366] The evaluation results are shown in Tables 2 to 4.

[0367] [Measurement of Interfacial Resistivity]

[0368] For the fabricated positive electrode, at 25 °C, using an electrode resistance system (manufactured by Hioki Electric Co., Ltd., "RM2610"), the interfacial resistivity (Ω·cm 2 ) of the positive electrode material layer is measured.

[0369] [Measurement of Initial Impedance]

[0370] For the above-fabricated laminated battery, after performing 3 charge-discharge cycles of charging at a constant current (0.2C) - constant voltage (4.2V) and discharging at 0.2C to the discharge cut-off voltage of 3.0V at a temperature environment of 25 °C, the voltage drop (the voltage drop value 15 seconds after the start of discharge) during discharge at 0.5C, 1C, 2C, and 5C at a state of charge (SOC) of 100% is measured, and the initial impedance (Ω) is obtained from each current value and each voltage drop value.

[0371]

[0372]

[0373]

[0374] The results in Tables 2 to 4 show that the sodium-ion batteries of the examples using the binder of the present disclosure have low interfacial resistivity and initial impedance.

[0375] Industrial Applicability

[0376] The binder of the present disclosure is suitable for sodium-ion batteries and can be used as various power sources such as portable power sources and automotive power sources.

Claims

1. A binder for the positive electrode of a sodium-ion battery, which is a binder for the positive electrode of a sodium-ion battery containing a copolymer (A) having vinylidene fluoride units and tetrafluoroethylene units. It is characterized in that the content of vinylidene fluoride units is 30 mol% to 99.5 mol% relative to all monomer units in the copolymer (A).

2. The binder for the positive electrode of a sodium-ion battery according to claim 1, wherein the content of the above-mentioned vinylidene fluoride units is 70 mol% to 99.5 mol% relative to all monomer units.

3. The binder for the positive electrode of a sodium-ion battery according to claim 1 or 2, wherein the copolymer (A) further has a structural unit derived from at least one monomer selected from the group consisting of hexafluoropropylene, trifluoroethylene, chlorotrifluoroethylene, the monomer represented by the general formula (1), the monomer represented by the general formula (2), and the monomer represented by the general formula (3). [Chemical formula 1] In the formula, Rf 1 is a linear or branched fluoroalkyl or fluoroalkoxy group having 1 to 12 carbon atoms. When both the fluoroalkyl and fluoroalkoxy groups have 2 or more carbon atoms, they may or may not contain an oxygen atom (-O-) between carbon-carbon atoms. In the formula, Rf 2 is a linear or branched fluoroalkyl or fluoroalkoxy group having 1 to 12 carbon atoms. When both the fluoroalkyl and fluoroalkoxy groups have 2 or more carbon atoms, they may or may not contain an oxygen atom (-O-) between carbon-carbon atoms. [Chemical formula 3] In the formula, R 1 , R 2 and R 3 are each independently a hydrogen atom, a chlorine atom or an alkyl group having 1 to 5 carbon atoms; X is a single bond or a group having a molecular weight of 500 or less and a main chain composed of 1 to 20 atoms; Y represents an inorganic cation and / or an organic cation.

4. The binder for the positive electrode of a sodium-ion battery according to any one of claims 1 to 3, which further contains one or more fluorine-containing polymers other than the above-mentioned copolymer (A).

5. The binder for the positive electrode of a sodium-ion battery according to claim 4, wherein the fluorine-containing polymer is a fluorine-containing polymer (PVdF) composed only of VdF units, or a fluorine-containing polymer (B) containing VdF units and other monomer units other than VdF.

6. The binder for the positive electrode of a sodium-ion battery according to claim 4, wherein the fluorine-containing polymer is a fluorine-containing polymer (PVdF) composed only of VdF units.

7. The binder for the positive electrode of a sodium-ion battery according to any one of claims 4 to 6, wherein the ratio of the copolymer (A) to the fluorine-containing polymer by mass is 1:15 to 9:

7.

8. An electrode mixture, wherein the electrode active material contains a sodium composite oxide, and the electrode mixture contains the binder for the positive electrode of a sodium-ion battery according to any one of claims 1 to 7.

9. An electrode comprising a current collector and an electrode material layer formed of the electrode mixture according to claim 8 provided on one or both sides of the current collector.

10. A sodium-ion secondary battery comprising the electrode according to claim 9.

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