Polyvinylidene fluoride, binder, electrode mixture, electrode and secondary battery

Abstract

Provided is a polyvinylidene fluoride containing a vinylidene fluoride unit and a pentenoic acid unit represented by formula (1): CH2=CH-(CH)2-COOY (In the formula, Y represents at least one selected from the group consisting of inorganic cations and organic cations.), wherein the content of vinylidene fluoride units is 95.0-99.90 mol% relative to the total monomer units of the polyvinylidene fluoride, and the content of pentenoic acid units is 0.01-5.0 mol% relative to the total monomer units of the polyvinylidene fluoride.

Description

[Technical Field]

[0001] The present disclosure relates to polyvinylidene fluoride, a binder, an electrode mixture, an electrode, and a secondary battery. [Background technology]

[0002] Patent Document 1 proposes the use of a linear semi-crystalline copolymer [polymer (A)] containing repeating units derived from vinylidene fluoride (VDF) monomer and a hydrophilic (meth)acrylic monomer (MA) such as acrylic acid as a binder for forming electrodes of lithium batteries and / or electric double layer capacitors, the copolymer containing 0.05 to 10 mol % of repeating units derived from the hydrophilic (meth)acrylic monomer (MA) and characterized by a fraction of randomly distributed units (MA) of at least 40%.

[0003] Patent Document 2 describes a composition (C): - repeating units deriving from vinylidene fluoride (VDF) in an amount of at least 50 mol %, relative to the total moles of repeating units of polymer (F1), and repeating units of formula (I) in an amount of at least 0.1 mol %, preferably at least 0.3 mol %, even more preferably at least 0.5 mol %, and not more than 5 mol %, relative to the total moles of repeating units of polymer (F1): [ka] (In the formula: R1, R2 and R3, which are equal to or different from each other, are independently selected from a hydrogen atom and a C1-C3 hydrocarbon group; -R OH is a C1-C5 hydrocarbon moiety containing a hydrogen atom or at least one hydroxyl group) at least one semi-crystalline fluoropolymer [polymer (F1)] comprising repeating units derived from at least one functional hydrogen-containing monomer comprising at least one hydrophilic (meth)acrylic monomer (MA), wherein said polymer (F1) has an intrinsic viscosity measured in dimethylformamide at 25°C of higher than 1.4 dl / g, preferably higher than 2 dl / g, even more preferably higher than 2.5 dl / g and lower than 5 dl / g; - at least one fluoropolymer [polymer (F2)] different from (F1), comprising repeating units derived from vinylidene fluoride (VDF) in an amount of at least 50 mol % relative to the total moles of repeating units of polymer (F2), and repeating units derived from at least one fluorinated monomer (FM) different from vinylidene fluoride in an amount of at least 2.5 mol %, preferably at least 4.0 mol %, and even more preferably at least 6 mol %, relative to the total moles of repeating units of polymer (F2), wherein polymer (F1) forms at least 10% by weight relative to the total weight of composition (C) and polymer (F2) forms at most 90% by weight relative to the total weight of composition (C).

[0004] Patent Document 3 proposes a binder composition used to bind an electrode active material to a current collector to which the electrode active material is applied, the binder composition comprising a first vinylidene fluoride polymer having an intrinsic viscosity of 1.7 dL / g or more and a second vinylidene fluoride polymer containing acrylic acid or methacrylic acid as a monomer unit. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2008 / 129041 [Patent Document 2] International Publication No. 2018 / 073277 [Patent Document 3] International Publication No. 2017 / 056974 Summary of the Invention [Problem to be solved by the invention]

[0006] An object of the present disclosure is to provide polyvinylidene fluoride and a binder that are excellent in resistance to electrolyte swelling and adhesion to metal foil, and that can form an electrode with excellent flexibility, as well as an electrode mixture, an electrode, and a secondary battery that use polyvinylidene fluoride and a binder. [Means for solving the problem]

[0007] According to the present disclosure, there is provided polyvinylidene fluoride containing vinylidene fluoride units and pentenoic acid units represented by formula (1): CH2=CH-(CH)2-COOY (wherein Y represents at least one selected from the group consisting of inorganic cations and organic cations), wherein the content of the vinylidene fluoride units is 95.0 to 99.99 mol % based on all monomer units of the polyvinylidene fluoride, and the content of the pentenoic acid units is 0.01 to 5.0 mol % based on all monomer units of the polyvinylidene fluoride.

[0008] It is preferable that Y in formula (1) represents at least one selected from the group consisting of H, Li, Na, K, Mg, Ca, Al and NH4. The weight average molecular weight is preferably 50,000 to 2,000,000.

[0009] The present disclosure also provides a binder containing the above-described polyvinylidene fluoride.

[0010] The present disclosure also provides an electrode mixture containing the polyvinylidene fluoride or the binder, a powdered electrode material, and water or a non-aqueous solvent.

[0011] The present disclosure also provides an electrode containing the above polyvinylidene fluoride or the above binder.

[0012] The present disclosure also provides a secondary battery including the above electrode. [Effects of the Invention]

[0013] According to the present disclosure, it is possible to provide polyvinylidene fluoride and a binder that are excellent in resistance to electrolyte swelling and adhesion to metal foil and that can form an electrode with excellent flexibility, as well as an electrode mixture, an electrode, and a secondary battery that use polyvinylidene fluoride and a binder. DETAILED DESCRIPTION OF THE INVENTION

[0014] Specific embodiments of the present disclosure will be described in detail below, but the present disclosure is not limited to the following embodiments.

[0015] The polyvinylidene fluoride (PVdF) of the present disclosure contains vinylidene fluoride (VdF) units and pentenoic acid units.

[0016] The pentenoic acid units contained in PVdF are units based on a monomer represented by the formula (1): CH═CH—(CH)—COOY (wherein Y represents at least one selected from the group consisting of inorganic cations and organic cations).

[0017] In formula (1), Y represents an inorganic cation and / or an organic cation. Examples of inorganic cations include cations such as H, Li, Na, K, Mg, Ca, Al, and Fe. Examples of organic cations include NH4, NHR. 5 , NH2R 5 2. NHR 5 3.NR 5 4(R 5independently represent an alkyl group having 1 to 4 carbon atoms.) As Y, at least one selected from the group consisting of H, Li, Na, K, Mg, Ca, Al and NH4 is preferred, at least one selected from the group consisting of H, Li, Na, K, Mg, Al and NH4 is more preferred, at least one selected from the group consisting of H, Li, Al and NH4 is even more preferred, and H is particularly preferred. For convenience, specific examples of inorganic cations and organic cations are shown without the symbols and valences.

[0018] The PVdF of the present disclosure contains 95.0 to 99.99 mol % of VdF units and 0.01 to 5.0 mol % of pentenoic acid units, relative to the total monomer units of PVdF. The PVdF of the present disclosure contains VdF units and pentenoic acid units, and the VdF unit contents and pentenoic acid unit contents are appropriately adjusted, so that the PVdF has excellent resistance to electrolyte swelling and adhesion to metal foil, and can be used to form an electrode with excellent flexibility.

[0019] Conventionally, a technique for improving the adhesion of PVdF to metal foil (current collector) by introducing acrylic acid units or the like into PVdF, which is used as a binder for forming electrodes of secondary batteries, capacitors, etc. However, there is a demand for PVdF that has excellent resistance to electrolyte swelling and flexibility while ensuring sufficient adhesion to metal foil (current collector).

[0020] It has been discovered that by selecting pentenoic acid units as the monomer units to be introduced into PVdF and further adjusting the VdF unit content and the pentenoic acid unit content within specific ranges, the electrolyte swelling resistance of PVdF is significantly improved, sufficient adhesion of PVdF to metal foil (current collector) can be ensured, and an electrode with excellent flexibility can be formed. The PVdF of the present disclosure is a novel polymer developed based on these findings.

[0021] Furthermore, since the PVdF of the present disclosure has an appropriately adjusted content of pentenoic acid units, the use of the PVdF of the present disclosure enables the formation of an electrode in which the electrode material layer and the metal foil (current collector) are sufficiently adhered to each other and which has sufficient retention capacity for the powder electrode material. In addition, a positive electrode mixture having an appropriate viscosity can be prepared despite containing a high concentration of PVdF, allowing electrodes with excellent properties to be produced with high productivity.

[0022] Furthermore, in secondary batteries using conventional binders, when the secondary battery is stored at high temperatures, the polar groups of the binder may decompose, resulting in an increase in resistance. The PVdF of the present disclosure has an appropriately adjusted content of pentenoic acid units, thereby suppressing the effects of decomposition of polar groups in high-temperature environments. Therefore, by using the PVdF of the present disclosure, it is possible to form an electrode in which the electrode material layer and the metal foil (current collector) are sufficiently adhered to each other, and to fabricate a secondary battery in which the resistance value is unlikely to increase even when stored at high temperatures.

[0023] The rate of increase in the resistance value of a secondary battery can be determined, for example, by the following method. A secondary battery (cell) placed in a 25°C thermostatic chamber is charged to 4.4 V using a constant current-constant voltage method at a rate of 0.5C-0.05C, and then the initial resistance value is measured using an AC impedance measuring device. The cell is then stored in a 40°C thermostatic chamber for one week, and then placed in a 25°C thermostatic chamber for three hours to allow the cell temperature to drop to 25°C, after which the resistance value of the cell after the durability test is measured. The average value of five cells is used as the measured value, and the rate of increase (%) in the resistance value after the durability test relative to the initial resistance value is determined ((resistance value after durability test - initial resistance value) / initial resistance value x 100).

[0024] Furthermore, in secondary batteries using conventional binders, repeated charge and discharge of the secondary battery can cause the powder electrode material to peel off from the electrode material layer, resulting in a decrease in discharge capacity. Since the PVdF of the present disclosure has an appropriately adjusted content of pentenoic acid units, the use of the PVdF of the present disclosure makes it possible to fabricate a secondary battery in which the powder electrode material is less likely to peel off from the electrode material layer even after repeated charge and discharge, and in which a sufficient discharge capacity is maintained (a secondary battery with a high capacity retention rate).

[0025] The capacity retention rate of a secondary battery can be evaluated, for example, by the following method. The secondary battery is sandwiched between plates and pressurized. At 25°C, the battery is subjected to constant-current / constant-voltage charging (hereinafter referred to as CC / CV charging) at a current equivalent to 0.5 C up to 4.2 V (0.1 C cutoff), followed by discharging to 3.0 V at a constant current of 0.5 C. This cycle constitutes one cycle, and the initial discharge capacity is determined from the discharge capacity at the third cycle. Here, 1 C represents the current value required to discharge the battery's reference capacity in one hour; for example, 0.5 C represents half that current value. Under the same conditions as above, a 300-cycle cycle test is conducted at an operating voltage of 3.0-4.2 V. The discharge capacity at the 300th cycle is defined as the capacity retention rate, with the initial discharge capacity at the third cycle being taken as 100%.

[0026] The content of VdF units in PVdF is 95.0 to 99.99 mol %, preferably greater than 95.0 mol %, more preferably 97.0 mol % or more, even more preferably 98.0 mol % or more, particularly preferably 98.5 mol % or more, and most preferably 99.0 mol % or more, and preferably 99.95 mol % or less, based on the total monomer units of PVdF. When the VdF unit content is within the above range, resistance to electrolyte swelling and adhesion to metal foil can be further improved, and an electrode with even greater flexibility can be formed.

[0027] The content of pentenoic acid units in PVdF is 0.01 to 5.0 mol %, preferably less than 5.0 mol %, more preferably 3.0 mol % or less, even more preferably 2.0 mol % or less, particularly preferably 1.5 mol % or less, most preferably 1.0 mol % or less, and preferably 0.05 mol % or more, based on the total monomer units of PVdF. When the content of pentenoic acid units is within the above range, resistance to electrolyte swelling and adhesion to metal foil can be further improved, and an electrode with even greater flexibility can be formed.

[0028] In the present disclosure, the composition of PVdF is, for example, 19 The content of pentenoic acid units in PVdF can be measured by F-NMR measurement. In addition, the carboxyl group (-COOY) of the pentenoic acid units is esterified, and then the PVdF after esterification is used. 1 It can be measured by H-NMR measurement.

[0029] PVdF may further contain a fluorinated monomer unit (excluding VdF units). When PVdF further contains a fluorinated monomer unit, an electrode with even greater flexibility can be formed.

[0030] Examples of fluorinated monomers include tetrafluoroethylene (TFE), vinyl fluoride, trifluoroethylene, chlorotrifluoroethylene (CTFE), fluoroalkyl vinyl ether, hexafluoropropylene (HFP), (perfluoroalkyl)ethylene, 2,3,3,3-tetrafluoropropene, and trans-1,3,3,3-tetrafluoropropene.

[0031] As the fluorinated monomer, at least one selected from the group consisting of CTFE, HFP, a fluoroalkyl vinyl ether, and 2,3,3,3-tetrafluoropropene is preferred, as it allows the formation of an electrode with even greater flexibility, at least one selected from the group consisting of CTFE, HFP, and a fluoroalkyl vinyl ether is more preferred, and at least one selected from the group consisting of HFP and a fluoroalkyl vinyl ether is even more preferred.

[0032] The fluorovinyl ether is preferably a fluoroalkyl vinyl ether having a fluoroalkyl group having 1 to 5 carbon atoms, and more preferably at least one selected from the group consisting of perfluoro(methyl vinyl ether), perfluoro(ethyl vinyl ether), and perfluoro(propyl vinyl ether).

[0033] From the viewpoint of further improving resistance to swelling in an electrolyte and adhesion to a metal foil, it is preferable that PVdF does not contain a TFE unit.

[0034] The content of the fluorinated monomer units in PVdF is preferably 0 to 4.99 mol%, more preferably 0.01 mol% or more, even more preferably 0.05 mol% or more, more preferably 1.95 mol% or less, and even more preferably 0.95 mol% or less, based on the total monomer units of PVdF.

[0035] PVdF may further contain non-fluorinated monomer units (excluding the pentenoic acid units represented by formula (1)). Examples of non-fluorinated monomers include ethylene and propylene.

[0036] The weight-average molecular weight (polystyrene equivalent) of PVdF is preferably 50,000 to 2,000,000, more preferably 80,000 or more, even more preferably 100,000 or more, particularly preferably 200,000 or more, more preferably 1,900,000 or less, even more preferably 1,700,000 or less, and particularly preferably 1,500,000 or less, because this allows the preparation of an electrode mixture with adequate viscosity and excellent coatability while ensuring sufficient adhesion to the metal foil. The weight-average molecular weight can be measured by gel permeation chromatography (GPC) using N,N-dimethylformamide as a solvent.

[0037] The number average molecular weight (polystyrene equivalent) of PVdF is preferably 20,000 to 1,500,000, more preferably 40,000 or more, even more preferably 70,000 or more, particularly preferably 140,000 or more, more preferably 1,400,000 or less, even more preferably 1,200,000 or less, and particularly preferably 1,100,000 or less, because it has a moderate viscosity while ensuring sufficient adhesion to the metal foil and allows the preparation of an electrode mixture with excellent coatability. The number average molecular weight can be measured by gel permeation chromatography (GPC) using dimethylformamide as a solvent.

[0038] When conventional PVdF is used as a binder, the higher the molecular weight of the PVdF, the better the adhesion of the electrode material layer to the metal foil. On the other hand, with conventional PVdF, the higher the molecular weight, the higher the viscosity of the electrode mixture, and the lower the coatability of the electrode mixture. The PVdF of the present disclosure can form an electrode material layer that exhibits sufficient adhesion to the metal foil, and has a moderate viscosity, allowing the preparation of an electrode mixture with excellent coatability. Furthermore, when the PVdF of the present disclosure has a weight-average molecular weight or number-average molecular weight within the above range, it is possible to achieve both high adhesion of the electrode material layer to the metal foil and excellent coatability of the electrode mixture at an even higher level.

[0039] For example, by using the PVdF of the present disclosure as a binder, it is possible to increase the contents of the powder electrode material and binder in the electrode mix compared to conventional electrode mixes, without changing the ratio of the amount of powder electrode material to the amount of binder, and even when the amount of nonaqueous solvent is reduced, it is possible to prepare an electrode mix having a viscosity equivalent to that of conventional electrode mixes, and to form an electrode material layer that exhibits sufficient adhesion to metal foil. Therefore, by using the PVdF of the present disclosure as a binder, it is possible to form electrodes that exhibit excellent properties, improve electrode productivity, and reduce the cost of the nonaqueous solvent.

[0040] The solution viscosity of PVdF is preferably 10 to 4000 mPa·s, more preferably 50 mPa·s or higher, even more preferably 100 mPa·s or higher, particularly preferably 150 mPa·s or higher, more preferably 3000 mPa·s or lower, even more preferably 2000 mPa·s or lower, and particularly preferably 1500 mPa·s or lower, because this can further improve adhesion to metal foil and allow the preparation of an electrode mixture with a moderate viscosity and excellent coatability. The solution viscosity of PVdF is the viscosity of an N-methyl-2-pyrrolidone (NMP) solution containing 5% by mass of PVdF. The viscosity of the NMP solution can be measured at 25°C using a Brookfield viscometer.

[0041] An electrode mixture with a suitable viscosity not only has excellent coatability, but also facilitates easy transfer and good dispersibility of the powder electrode material in the electrode mixture. Therefore, an electrode mixture with a suitable viscosity can shorten the time required for transfer, the powder electrode material is less likely to aggregate during transfer or storage, and the viscosity can be easily readjusted after transfer or storage. Furthermore, an electrode mixture with a suitable viscosity can easily apply a suitable shear force to the electrode mixture by stirring, thereby easily dispersing the powder electrode material in a non-aqueous solvent. When the PVdF of the present disclosure has a solution viscosity within the above range, an electrode mixture with a suitable viscosity and excellent coatability can be more easily prepared.

[0042] The melting point of PVdF is preferably 100 to 240° C. The melting point can be determined as the temperature corresponding to the maximum value on the heat of fusion curve when the temperature is increased at a rate of 10° C. / min using a differential scanning calorimetry (DSC) device.

[0043] The storage modulus of PVdF at 30°C is preferably 2000 MPa or less, more preferably 1800 MPa or less. The storage modulus of PVdF at 60°C is preferably 1500 MPa or less, more preferably 1300 MPa or less. The storage modulus of PVdF at 30°C is preferably 1000 MPa or more, more preferably 1100 MPa or more. The storage modulus of PVdF at 60°C is preferably 600 MPa or more, and more preferably 700 MPa or more. When the storage modulus of PVdF at 30° C. or 60° C. is within the above range, the flexibility of PVdF is improved, and when used as a binder, an electrode that is less likely to crack can be easily formed.

[0044] The storage modulus is the measured value at 30°C and 60°C for a sample with a length of 30 mm, width of 5 mm, and thickness of 50 to 100 μm, measured using a dynamic viscoelasticity measurement device DVA220 manufactured by IT Measurement & Control Co., Ltd. under the following conditions: tensile mode, grip width 20 mm, measurement temperature -30°C to 160°C, heating rate 2°C / min, and frequency 1 Hz.

[0045] A measurement sample can be prepared, for example, by dissolving PVdF in N-methyl-2-pyrrolidone (NMP) to a concentration of 10 to 20% by mass, casting the resulting solution onto a glass plate, drying it at 100°C for 12 hours, and further drying it under vacuum at 100°C for 12 hours, and cutting the resulting film, 50 to 100 μm thick, into a length of 30 mm and a width of 5 mm.

[0046] The distribution of pentenoic acid units in the main chain of PVdF is not particularly limited, but it is preferable that the pentenoic acid units are distributed as randomly as possible, as this further improves adhesion to metal foil, resistance to electrolyte swelling, and flexibility, as well as heat resistance. The "fraction of randomly distributed pentenoic acid units," which indicates the proportion of randomly distributed pentenoic acid units to the total number of pentenoic acid units in PVdF, is preferably 5% or more, more preferably 10% or more, and even more preferably 15% or more.

[0047] The fraction of randomly distributed pentenoic acid units can be calculated according to the following formula: (Fraction (%)) = (average number of pentenoic acid unit sequences (%)) / (average total number of pentenoic acid units (%)) × 100 In the formula, the pentenoic acid unit sequence is, for example, 19 F-NMR measurement and 1 It can be measured by H-NMR measurement. A pentenoic acid unit sequence is an isolated pentenoic acid unit between two VdF units, and the greater the number of pentenoic acid unit sequences, the higher the fraction of randomly distributed pentenoic acid units. If the pentenoic acid units are completely randomly distributed, the average number of pentenoic acid unit sequences is equal to the average total number of pentenoic acid units, and therefore the fraction of randomly distributed pentenoic acid units is 100%.

[0048] The PVdF of the present disclosure can be produced by polymerizing a monomer mixture containing at least VdF and pentenoic acid represented by formula (1). As the polymerization method, methods such as suspension polymerization, emulsion polymerization, and solution polymerization can be used, but suspension polymerization and emulsion polymerization are preferred from the viewpoint of ease of post-treatment, etc.

[0049] In the polymerization, a polymerization initiator, a surfactant, a chain transfer agent, and a solvent can be used, and conventionally known polymerization initiators can be used. As the polymerization initiator, an oil-soluble radical polymerization initiator or a water-soluble radical polymerization initiator can be used.

[0050] The oil-soluble radical polymerization initiator may be a known oil-soluble peroxide, for example: dialkyl peroxycarbonates such as di-normal propyl peroxydicarbonate, diisopropyl peroxydicarbonate, and di-sec-butyl peroxydicarbonate; peroxyesters such as t-butyl peroxyisobutyrate, t-butyl peroxypivalate, t-hexyl peroxy 2-ethylhexanoate, t-butyl peroxy-2-ethylhexanoate, and 1,1,3,3-tetramethylbutyl peroxy-2-ethylhexanoate; Dialkyl peroxides such as di-t-butyl peroxide; Di[fluoro(or fluorochloro)acyl]peroxides; are some of the most representative examples.

[0051] Di[fluoro(or fluorochloro)acyl]peroxides include diacyl peroxides represented by [(RfCOO)-]2 (Rf is a perfluoroalkyl group, an ω-hydroperfluoroalkyl group, or a fluorochloroalkyl group).

[0052] Examples of di[fluoro(or fluorochloro)acyl]peroxides include di(ω-hydro-dodecafluorohexanoyl)peroxide, di(ω-hydro-tetradecafluoroheptanoyl)peroxide, di(ω-hydro-hexadecafluorononanoyl)peroxide, di(perfluorobutyryl)peroxide, di(perfluoroparyl)peroxide, di(perfluorohexanoyl)peroxide, di(perfluoroheptanoyl)peroxide, di(perfluorooctanoyl)peroxide, di(perfluorononanoyl)peroxide, di(ω-chloro-hexafluorobutyryl)peroxide, di(ω-chloro-decafluorohexanoyl)peroxide, Examples of the peroxide include di(ω-chloro-tetradecafluorooctanoyl) peroxide, ω-hydro-dodecafluoroheptanoyl-ω-hydrohexadecafluorononanoyl peroxide, ω-chloro-hexafluorobutyryl-ω-chloro-decafluorohexanoyl peroxide, ω-hydrododecafluoroheptanoyl-perfluorobutyryl peroxide, di(dichloropentafluorobutanoyl) peroxide, di(trichlorooctafluorohexanoyl) peroxide, di(tetrachloroundecafluorooctanoyl) peroxide, di(pentachlorotetradecafluorodecanoyl) peroxide, and di(undecachlorotriacontafluorodocosanoyl) peroxide.

[0053] The water-soluble radical polymerization initiator may be a known water-soluble peroxide, such as ammonium salts, potassium salts, or sodium salts of persulfuric acid, perborate, perchloric acid, perphosphoric acid, or percarbonate; organic peroxides such as disuccinic acid peroxide or diglutaric acid peroxide; t-butyl permaleate; or t-butyl hydroperoxide. A reducing agent such as a sulfite may be used in combination with the peroxide, and the amount used may be 0.1 to 20 times the amount of the peroxide.

[0054] As the surfactant, known surfactants can be used, such as nonionic surfactants, anionic surfactants, cationic surfactants, etc. Among them, fluorine-containing anionic surfactants are preferred, and linear or branched fluorine-containing anionic surfactants having 4 to 20 carbon atoms that may contain an ether bond (i.e., oxygen atoms may be inserted between carbon atoms) are more preferred. The amount of surfactant added (vs. solvent) is preferably 50 to 5,000 ppm.

[0055] Examples of chain transfer agents include hydrocarbons such as ethane, isopentane, n-hexane, and cyclohexane; aromatics such as toluene and xylene; ketones such as acetone; acetate esters such as ethyl acetate and butyl acetate; alcohols such as methanol and ethanol; mercaptans such as methyl mercaptan; and halogenated hydrocarbons such as carbon tetrachloride, chloroform, methylene chloride, and methyl chloride. The amount of chain transfer agent added varies depending on the chain transfer constant of the chain transfer agent, but is usually in the range of 0.01 to 20% by mass relative to the solvent.

[0056] Examples of the solvent include water and a mixed solvent of water and alcohol.

[0057] In polymerization such as suspension polymerization, a fluorine-based solvent may be used in addition to water. Examples of the fluorine-based solvent include hydrochlorofluoroalkanes such as CH3CClF2, CH3CCl2F, CF3CF2CCl2H, and CF2ClCF2CFHCl; chlorofluoroalkanes such as CF2ClCFClCF2CF3 and CF3CFClCFClCF3; perfluoroalkanes such as perfluorocyclobutane, CF3CF2CF2CF3, CF3CF2CF2CF2CF3, and CF3CF2CF2CF2CF2CF3; CF2HCF2CF2CF2H, CF3CFHCF2CF2CF3, CF3CF2CF2CF2CF2H, CF3CF2CFHCF2CF3, CF3CFHCFHCF2CF3, CF2HCF2CF2CF2CF2H, CF2HCFHCF2CF2CF3, and CF3CF2CF2CF2CF2C Examples of the fluorine-based solvent include hydrofluorocarbons such as F2H, CF3CH(CF3)CF3CF2CF3, CF3CF(CF3)CFHCF2CF3, CF3CF(CF3)CFHCFHCF3, CF3CH(CF3)CFHCF2CF3, CF2HCF2CF2CF2CF2CF2H, CF3CF2CF2CF2CH2CH3, and CF3CH2CF2CH3; (perfluoroalkyl)alkyl ethers such as F(CF2)4OCH3, F(CF2)4OC2H5, (CF3)2CFOCH3, and F(CF2)3OCH3; and hydrofluoroalkyl ethers such as CF3CH2OCF2CHF2, CHF2CF2CH2OCF2CHF2, and CF3CF2CH2OCF2CHF2, with perfluoroalkanes being preferred. From the standpoints of suspensionability and economy, the amount of the fluorine-based solvent used is preferably 10 to 100% by mass relative to the solvent.

[0058] The polymerization temperature and polymerization pressure are appropriately determined depending on the type, amount, vapor pressure, and other polymerization conditions such as the polymerization temperature of the solvent used.

[0059] Because PVdF can be produced efficiently, it is also preferable to polymerize a monomer mixture containing at least VdF and the pentenoic acid represented by formula (1) under conditions in which VdF is in a supercritical state. The critical temperature and critical pressure of VdF are 30.1°C and 4.38 MPa, respectively.

[0060] Since PVdF can be produced efficiently, it is also preferable to supply the monomer mixture to the reactor so that the density of the monomer mixture in the reactor is sufficiently high. The density of the monomer mixture in the reactor at the initial polymerization temperature is preferably 0.20 g / cm. 3 More preferably, it is 0.23 g / cm or more. 3 More preferably, it is 0.25 g / cm 3 There is no particular upper limit, but if the density is too high, the pressure change in the reactor due to the temperature change in the reactor tends to become too large. Therefore, from the viewpoint of safe production, it is recommended that the density be 0.70 g / cm 3 The density of the monomer mixture in the reactor is determined by multiplying the amount (g) of the monomer mixture fed to the reactor by the internal volume (cm 3 ) to the volume of water (cm 3 ) by the subtracted value.

[0061] In suspension polymerization using water as a dispersion medium, a suspending agent such as methyl cellulose, methoxylated methyl cellulose, propoxylated methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, polyvinyl alcohol, polyethylene oxide, or gelatin can be added in an amount of 0.005 to 1.0% by mass, preferably 0.01 to 0.4% by mass, based on the amount of water.

[0062] In this case, the polymerization initiator may be diisopropyl peroxydicarbonate, di-normal propyl peroxydicarbonate, di-normal heptafluoropropyl peroxydicarbonate, isobutyryl peroxide, di(chlorofluoroacyl) peroxide, di(perfluoroacyl) peroxide, etc. The amount used is preferably 0.001 to 5% by mass based on the total amount of monomers.

[0063] The degree of polymerization of the resulting polymer may be adjusted by adding a chain transfer agent such as ethyl acetate, methyl acetate, acetone, methanol, ethanol, n-propanol, acetaldehyde, propylaldehyde, ethyl propionate, carbon tetrachloride, etc. The amount used is usually 0.001 to 5% by mass, preferably 0.005 to 3% by mass, based on the total amount of monomers.

[0064] The total amount of monomers charged is 2:1 to 1:10, preferably 1:1 to 1:5, in terms of the mass ratio of total amount of monomers to water.

[0065] The above-mentioned PVdF can be suitably used as a binder. By using an electrode mixture containing the above-mentioned PVdF as a binder, an electrode material layer having excellent adhesion to metal foil, resistance to electrolyte swelling, and flexibility can be formed.

[0066] The binder of the present disclosure contains the above-mentioned PVdF. Because the binder of the present disclosure contains the above-mentioned PVdF, it has excellent adhesion to metal foil, resistance to electrolyte swelling, and flexibility. Furthermore, by using an electrode mixture containing the binder of the present disclosure, it is possible to form an electrode material layer that has excellent adhesion to metal foil, resistance to electrolyte swelling, and flexibility.

[0067] The binder of the present disclosure may contain a polymer other than the above-mentioned PVdF. Examples of the polymer other than the above-mentioned PVdF include fluoropolymers (excluding the above-mentioned PVdF), polymethacrylate, polymethyl methacrylate, polyacrylonitrile, polyimide, polyamide, polyamideimide, polycarbonate, styrene rubber, and butadiene rubber.

[0068] The binder of the present disclosure preferably contains a VdF polymer (excluding the above-mentioned PVdF) as the polymer other than the above-mentioned PVdF. Examples of the VdF polymer include a VdF homopolymer and a polymer containing a VdF unit and a unit based on a monomer copolymerizable with VdF (excluding VdF and the pentenoic acid represented by formula (1)).

[0069] The VdF polymer preferably contains a VdF unit and a unit based on a monomer copolymerizable with VdF (excluding VdF and pentenoic acid represented by formula (1)), because this further improves the adhesion of the binder to the metal foil, the resistance to electrolyte swelling, and the flexibility, and also makes it possible to obtain an electrode mixture that is less likely to increase in viscosity.

[0070] Monomers copolymerizable with VdF include fluorinated monomers and non-fluorinated monomers.

[0071] As the fluorinated monomer (excluding VdF), at least one selected from the group consisting of tetrafluoroethylene (TFE), vinyl fluoride, trifluoroethylene, chlorotrifluoroethylene (CTFE), fluoroalkyl vinyl ether, hexafluoropropylene (HFP), (perfluoroalkyl)ethylene, 2,3,3,3-tetrafluoropropene, and trans-1,3,3,3-tetrafluoropropene is preferred, as this further improves the adhesion of the binder to the metal foil, resistance to electrolyte swelling, and flexibility, and makes it possible to obtain an electrode mixture that is less likely to increase in viscosity; at least one selected from the group consisting of TFE, CTFE, and HFP is more preferred, and at least one selected from the group consisting of TFE and HFP is even more preferred, with TFE being particularly preferred because it suppresses swelling in the electrolyte and can improve battery characteristics.

[0072] The fluorinated monomer units (except for the VdF units) may or may not have a polar group.

[0073] Examples of the non-fluorinated monomer include non-fluorinated monomers that do not have a polar group, such as ethylene and propylene, and non-fluorinated monomers that have a polar group (hereinafter, sometimes referred to as polar group-containing monomers).When a non-fluorinated monomer that has a polar group is used, the polar group is introduced into the VdF polymer, which further improves the adhesion of the binder to the metal foil.

[0074] The VdF polymer may have a polar group, which further improves the adhesion of the binder to the metal foil. The polar group is not particularly limited as long as it is a polar functional group. However, since this further improves the adhesion of the binder to the metal foil, at least one selected from the group consisting of a carbonyl group-containing group, an epoxy group, a hydroxy group, a sulfonic acid group, a sulfate group, a phosphate group, an amino group, an amide group, and an alkoxy group is preferred. At least one selected from the group consisting of a carbonyl group-containing group, an epoxy group, and a hydroxy group is more preferred, and a carbonyl group-containing group is even more preferred. The hydroxy group does not include a hydroxy group that constitutes part of the carbonyl group-containing group. The amino group is a monovalent functional group obtained by removing hydrogen from ammonia, a primary amine, or a secondary amine.

[0075] The carbonyl group-containing group is a functional group having a carbonyl group (-C(=O)-). The carbonyl group-containing group is preferably a group represented by the general formula -COOR (R represents a hydrogen atom, an alkyl group, or a hydroxyalkyl group) or a carboxylic acid anhydride group, and more preferably a group represented by the general formula -COOR, since this further improves the adhesion of the binder to the metal foil. The number of carbon atoms in the alkyl group or hydroxyalkyl group is preferably 1 to 16, more preferably 1 to 6, and even more preferably 1 to 3. Specific examples of the group represented by the general formula -COOR include -COOCH2CH2OH, -COOCH2CH(CH3)OH, -COOCH(CH3)CH2OH, -COOH, -COOCH3, and -COOC2H5. When the group represented by the general formula -COOR is -COOH or contains -COOH, the -COOH may be a carboxylate such as a metal carboxylate or an ammonium carboxylate.

[0076] The carbonyl group-containing group may also be a group represented by the general formula: -X-COOR (X represents an atomic group having a main chain composed of 1 to 20 atoms and a molecular weight of 500 or less; R represents a hydrogen atom, an alkyl group, or a hydroxyalkyl group). The alkyl group and the hydroxyalkyl group preferably have 1 to 16 carbon atoms, more preferably 1 to 6, and even more preferably 1 to 3 carbon atoms.

[0077] The amide group is preferably a group represented by the general formula: -CO-NRR' (R and R' independently represent a hydrogen atom or a substituted or unsubstituted alkyl group), or a bond represented by the general formula: -CO-NR"- (R" represents a hydrogen atom, a substituted or unsubstituted alkyl group, or a substituted or unsubstituted phenyl group).

[0078] The polar group can be introduced into a VdF polymer by polymerizing VdF with a polar group-containing monomer, or by reacting a VdF polymer with a compound having the polar group. However, from the viewpoint of productivity, it is preferable to polymerize VdF with a polar group-containing monomer.

[0079] Examples of the polar group-containing monomer include hydroxyalkyl (meth)acrylates such as hydroxyethyl acrylate and 2-hydroxypropyl acrylate; alkylidene malonic acid esters such as dimethyl methylidene malonate; vinyl carboxyalkyl ethers such as vinyl carboxymethyl ether and vinyl carboxyethyl ether; carboxyalkyl (meth)acrylates such as 2-carboxyethyl acrylate and 2-carboxyethyl methacrylate; (meth)acryloyloxyalkyl dicarboxylic acid esters such as acryloyloxyethyl succinate, acryloyloxypropyl succinate, methacryloyloxyethyl succinate, acryloyloxyethyl phthalate, and methacryloyloxyethyl phthalate; monoesters of unsaturated dibasic acids such as maleic acid monomethyl ester, maleic acid monoethyl ester, citraconic acid monomethyl ester, and citraconic acid monoethyl ester; [ka] (In the formula, R 1 ~R 3 R independently represents a hydrogen atom or a hydrocarbon group having 1 to 8 carbon atoms. 4 represents a single bond or a hydrocarbon group having 1 to 8 carbon atoms. 1 represents an inorganic cation and / or an organic cation.) (excluding pentenoic acid represented by formula (1));

[0080] The VdF polymer preferably contains a unit based on the monomer (2) represented by formula (2) as the polar group-containing monomer.

[0081] In equation (2), Y 1 represents an inorganic cation and / or an organic cation. Examples of inorganic cations include cations such as H, Li, Na, K, Mg, Ca, Al, and Fe. Examples of organic cations include NH4, NHR, and the like. 5 , NH2R 5 2. NHR 5 3.NR 5 4(R 5 and each independently represent an alkyl group having 1 to 4 carbon atoms. 1 As the cation, H, Li, Na, K, Mg, Ca, Al, and NH4 are preferred, H, Li, Na, K, Mg, Al, and NH4 are more preferred, H, Li, Al, and NH4 are still more preferred, and H is particularly preferred. For convenience, the symbols and valences of specific examples of inorganic cations and organic cations are omitted.

[0082] In equation (2), R 1 ~R 3 R independently represent a hydrogen atom or a hydrocarbon group having 1 to 8 carbon atoms. The hydrocarbon group is a monovalent hydrocarbon group. The hydrocarbon group preferably has 4 or less carbon atoms. Examples of the hydrocarbon group include alkyl groups, alkenyl groups, alkynyl groups, etc. having the above carbon numbers, with a methyl group or an ethyl group being preferred. 1 and R 2are preferably independently a hydrogen atom, a methyl group, or an ethyl group, and R 3 is preferably a hydrogen atom or a methyl group.

[0083] In equation (2), R 4 represents a single bond or a hydrocarbon group having 1 to 8 carbon atoms. The hydrocarbon group is a divalent hydrocarbon group. The hydrocarbon group preferably has 4 or less carbon atoms. Examples of the hydrocarbon group include alkylene groups and alkenylene groups having the above carbon numbers, and among these, at least one selected from the group consisting of a methylene group, an ethylene group, an ethylidene group, a propylidene group, and an isopropylidene group is preferred, with a methylene group being more preferred.

[0084] Monomer (2) is preferably at least one selected from the group consisting of (meth)acrylic acid and salts thereof, vinylacetic acid (3-butenoic acid) and salts thereof, 3-pentenoic acid and salts thereof, 3-hexenoic acid and salts thereof, 4-heptenoic acid and salts thereof, and 5-hexenoic acid and salts thereof.

[0085] When the polar group is introduced into the VdF polymer by reacting the VdF polymer with the compound having the polar group, the compound having the polar group can be the polar group-containing monomer, or a silane-based coupling agent or titanate-based coupling agent having a group reactive with the VdF polymer and a hydrolyzable group. The hydrolyzable group is preferably an alkoxy group. When a coupling agent is used, it can be added to the VdF polymer by reacting it with the VdF polymer dissolved or swollen in a solvent.

[0086] The VdF polymer may also be one obtained by partially dehydrofluorinating a VdF polymer with a base and then reacting the partially dehydrofluorinated VdF polymer with an oxidizing agent, such as hydrogen peroxide, hypochlorite, palladium halide, chromium halide, alkali metal permanganate, peroxy compounds, alkyl peroxides, or alkyl persulfates.

[0087] The content of the polar group-containing monomer units in the VdF polymer is preferably 0.001 to 8.0 mol %, more preferably 0.01 to 5.0 mol %, and even more preferably 0.30 to 3.0 mol %, based on the total monomer units of the VdF polymer, since this further improves the adhesion of the binder to the metal foil.

[0088] In the present disclosure, the composition of the VdF polymer may be, for example, 19 The content of polar group-containing monomer units in the VdF polymer can be measured by F-NMR measurement. When the polar group is an acid group such as a carboxylic acid, the content of the polar group-containing monomer units in the VdF polymer can be measured by acid-base titration of the acid group.

[0089] The VdF polymer is preferably a polymer containing VdF units and fluorinated monomer units (excluding VdF units). In addition to these monomer units, the VdF polymer may also contain non-fluorinated monomer units such as polar group-containing monomer units.

[0090] The content of VdF units in the VdF polymer is preferably 57.0 to 99.9 mol %, more preferably 60.0 mol % or more, even more preferably 63.0 mol % or more, and more preferably 99.5 mol % or less, based on the total monomer units of the VdF polymer.

[0091] The content of the fluorinated monomer units in the VdF polymer is preferably 0.1 to 43.0 mol %, more preferably 0.5 mol % or more, more preferably 40.0 mol % or less, and even more preferably 37.0 mol % or less, based on the total monomer units of the VdF polymer.

[0092] It is also preferred that the VdF polymer contains a relatively small amount of VdF units and a relatively large amount of fluorinated monomer units. For example, the content of VdF units is preferably 57.0 mol% or more, more preferably 60.0 mol% or more, even more preferably 63.0 mol% or more, and preferably 95.0 mol% or less, more preferably 90.0 mol% or less, and even more preferably 85.0 mol% or less, based on the total monomer units of the VdF polymer. The content of fluorinated monomer units is preferably 5.0 mol% or more, more preferably 8.0 mol% or more, particularly preferably 10.0 mol% or more, most preferably 15.0 mol% or more, and preferably 43.0 mol% or less, more preferably 40.0 mol% or less, even more preferably 38.0 mol% or less, and especially preferably 37.0 mol% or less, based on the total monomer units of the VdF polymer.

[0093] A VdF polymer containing a relatively small amount of VdF units and a relatively large amount of fluorinated monomer units also preferably contains polar group-containing monomer units. The content of the polar group-containing monomer units is preferably 0.01 to 2.0 mol %, more preferably 0.05 mol % or more, and more preferably 1.0 mol % or less, based on the total monomer units of the VdF polymer.

[0094] It is also preferred that the VdF polymer contains a relatively large amount of VdF units and a relatively small amount of monomer units copolymerizable with VdF. For example, the content of VdF units is preferably 92.0 to 99.9 mol%, more preferably 95.0 mol% or more, and more preferably 99.5 mol% or less, based on the total monomer units of the VdF polymer. The content of monomer units copolymerizable with VdF is preferably 0.10 to 8.0 mol%, more preferably 0.50 mol% or more, and more preferably 5.0 mol% or less, based on the total monomer units of the VdF polymer.

[0095] The weight-average molecular weight (polystyrene equivalent) of the VdF polymer is preferably 50,000 to 3,000,000, more preferably 80,000 or more, even more preferably 100,000 or more, particularly preferably 200,000 or more, more preferably 2,400,000 or less, even more preferably 2,200,000 or less, and particularly preferably 2,000,000 or less. The weight-average molecular weight can be measured by gel permeation chromatography (GPC) at 50°C using dimethylformamide as a solvent.

[0096] The number average molecular weight (polystyrene equivalent) of the VdF polymer is preferably 20,000 to 1,500,000, more preferably 40,000 or more, even more preferably 70,000 or more, particularly preferably 140,000 or more, more preferably 1,400,000 or less, even more preferably 1,200,000 or less, and particularly preferably 1,100,000 or less. The number average molecular weight can be measured by gel permeation chromatography (GPC) at 50°C using dimethylformamide as a solvent.

[0097] The melting point of the VdF polymer is preferably 100 to 240° C. The melting point can be determined as the temperature corresponding to the maximum value on the heat of fusion curve when the temperature is increased at a rate of 10° C. / min using a differential scanning calorimetry (DSC) device.

[0098] The VdF polymer preferably has a storage modulus of 1100 MPa or less at 30° C. and a storage modulus of 500 MPa or less at 60° C. When the VdF polymer has a storage modulus of 1100 MPa or less at 30° C. and a storage modulus of 500 MPa or less at 60° C., flexibility is further improved. The storage modulus of the VdF polymer at 30° C. is more preferably 800 MPa or less, and even more preferably 600 MPa or less. The storage modulus of the VdF polymer at 60°C is more preferably 350 MPa or less. The storage modulus of the VdF polymer at 30° C. is preferably 100 MPa or more, more preferably 150 MPa or more, and even more preferably 200 MPa or more. The storage modulus of the VdF polymer at 60° C. is preferably 50 MPa or more, more preferably 80 MPa or more, and even more preferably 130 MPa or more. The storage modulus of the VdF polymer can be measured in the same manner as the storage modulus of PVdF.

[0099] Examples of the VdF polymer include VdF / TFE copolymer, VdF / HFP copolymer, VdF / 2,3,3,3-tetrafluoropropene copolymer, VdF / TFE / HFP copolymer, VdF / TFE / 2,3,3,3-tetrafluoropropene copolymer, VdF / TFE / (meth)acrylic acid copolymer, VdF / HFP / (meth)acrylic acid copolymer, VdF / CTFE copolymer, VdF / CTFE / TFE copolymer, VdF / Examples of the copolymer include TFE / 3-butenoic acid copolymer, VdF / TFE / HFP / (meth)acrylic acid copolymer, VdF / TFE / HFP / 3-butenoic acid copolymer, VdF / TFE / 2-carboxyethyl acrylate copolymer, VdF / TFE / HFP / 2-carboxyethyl acrylate copolymer, VdF / TFE / acryloyloxyethyl succinic acid copolymer, and VdF / TFE / HFP / acryloyloxyethyl succinic acid copolymer.

[0100] Among them, the VdF polymer is preferably at least one selected from the group consisting of VdF / TFE copolymer, VdF / HFP copolymer, VdF / 2,3,3,3-tetrafluoropropene copolymer, VdF / TFE / HFP copolymer, VdF / TFE / 2,3,3,3-tetrafluoropropene copolymer, VdF / TFE / (meth)acrylic acid copolymer, VdF / HFP / (meth)acrylic acid copolymer, VdF / CTFE copolymer, and VdF / CTFE / TFE copolymer.

[0101] The VdF / TFE copolymer contains VdF units and TFE units. The content of VdF units is preferably 50 to 95 mol%, more preferably 55 mol% or more, even more preferably 60 mol% or more, more preferably 92 mol% or less, and even more preferably 89 mol% or less, based on the total monomer units of the VdF / TFE copolymer. The content of TFE units is preferably 50 to 5 mol%, more preferably 45 mol% or less, even more preferably 40 mol% or less, more preferably 8 mol% or more, and even more preferably 11 mol% or more, based on the total monomer units of the VdF / TFE copolymer.

[0102] The VdF / TFE copolymer may contain, in addition to VdF units and TFE units, units based on a monomer copolymerizable with VdF and TFE (excluding VdF, TFE, and pentenoic acid represented by formula (1)). From the viewpoint of electrolyte swelling resistance, the content of units based on a monomer copolymerizable with VdF and TFE is preferably 3.0 mol % or less based on all monomer units of the VdF / TFE copolymer.

[0103] Examples of the monomer copolymerizable with VdF and TFE include the above-mentioned fluorinated monomers and non-fluorinated monomers. Among them, the monomer copolymerizable with VdF and TFE is preferably at least one selected from the group consisting of fluorinated monomers and polar group-containing monomers, and more preferably at least one selected from the group consisting of HFP, 2,3,3,3-tetrafluoropropene, and monomer (2) (excluding pentenoic acid represented by formula (1)).

[0104] The weight average molecular weight (in terms of polystyrene) of the VdF / TFE copolymer is preferably 50,000 to 2,000,000, more preferably 80,000 to 1,700,000, and even more preferably 100,000 to 1,500,000.

[0105] The number average molecular weight (in terms of polystyrene) of the VdF / TFE copolymer is 35,000 to 1,400,000, more preferably 40,000 to 1,300,000, and even more preferably 50,000 to 1,200,000.

[0106] The VdF / HFP copolymer contains VdF units and HFP units. The content of VdF units is preferably 80 to 98 mol%, more preferably 83 mol% or more, even more preferably 85 mol% or more, more preferably 97 mol% or less, and even more preferably 96 mol% or less, based on the total monomer units of the VdF / HFP copolymer. The content of HFP units is preferably 20 to 2 mol%, more preferably 17 mol% or less, even more preferably 15 mol% or less, more preferably 3 mol% or more, and even more preferably 4 mol% or more, based on the total monomer units of the VdF / HFP copolymer.

[0107] The VdF / HFP copolymer may contain, in addition to VdF units and HFP units, units based on a monomer copolymerizable with VdF and HFP (excluding VdF, HFP, and pentenoic acid represented by formula (1)). From the viewpoint of electrolyte swelling resistance, the content of units based on a monomer copolymerizable with VdF and HFP is preferably 3.0 mol % or less based on all monomer units of the VdF / HFP copolymer.

[0108] Examples of the monomer copolymerizable with VdF and HFP include the above-mentioned fluorinated monomers and non-fluorinated monomers. Among them, the monomer copolymerizable with VdF and HFP is preferably at least one selected from the group consisting of fluorinated monomers and polar group-containing monomers, more preferably at least one selected from the group consisting of TFE, 2,3,3,3-tetrafluoropropene, and monomer (2) (excluding pentenoic acid represented by formula (1)), and even more preferably monomer (2) (excluding pentenoic acid represented by formula (1)).

[0109] The weight average molecular weight (in terms of polystyrene) of the VdF / HFP copolymer is preferably 50,000 to 2,000,000, more preferably 80,000 to 1,700,000, and even more preferably 100,000 to 1,500,000.

[0110] The number average molecular weight (in terms of polystyrene) of the VdF / HFP copolymer is preferably 35,000 to 1,400,000, more preferably 40,000 to 1,300,000, and even more preferably 50,000 to 1,200,000.

[0111] The VdF / CTFE copolymer contains VdF units and CTFE units. The content of VdF units is preferably 80 to 98 mol%, more preferably 85 mol% or more, even more preferably 90 mol% or more, more preferably 97.5 mol% or less, and even more preferably 97 mol% or less, based on the total monomer units of the VdF / CTFE copolymer. The content of CTFE units is preferably 20 to 2 mol%, more preferably 15 mol% or less, even more preferably 10 mol% or less, more preferably 2.5 mol% or more, and even more preferably 3 mol% or more, based on the total monomer units of the VdF / CTFE copolymer.

[0112] The VdF / CTFE copolymer may contain, in addition to VdF units and CTFE units, units based on a monomer copolymerizable with VdF and CTFE (excluding VdF, CTFE, and pentenoic acid represented by formula (1)). From the viewpoint of electrolyte swelling resistance, the content of units based on a monomer copolymerizable with VdF and CTFE is preferably 3.0 mol % or less based on all monomer units of the VdF / CTFE copolymer.

[0113] Examples of the monomer copolymerizable with VdF and CTFE include the above-mentioned fluorinated monomers and non-fluorinated monomers. Among them, the monomer copolymerizable with VdF and CTFE is preferably at least one selected from the group consisting of fluorinated monomers and polar group-containing monomers, more preferably at least one selected from the group consisting of TFE, HFP, 2,3,3,3-tetrafluoropropene, and monomer (2) (excluding pentenoic acid represented by formula (1)), and even more preferably TFE.

[0114] The weight average molecular weight (in terms of polystyrene) of the VdF / CTFE copolymer is preferably 50,000 to 2,000,000, more preferably 80,000 to 1,700,000, and even more preferably 100,000 to 1,500,000.

[0115] The number average molecular weight (in terms of polystyrene) of the VdF / CTFE copolymer is preferably 35,000 to 1,400,000, more preferably 40,000 to 1,300,000, and even more preferably 50,000 to 1,200,000.

[0116] The VdF / 2,3,3,3-tetrafluoropropene copolymer contains VdF units and 2,3,3,3-tetrafluoropropene units. The content of VdF units is preferably 80 to 98 mol%, more preferably 85 mol% or more, even more preferably 90 mol% or more, more preferably 97.5 mol% or less, and even more preferably 97 mol% or less, based on the total monomer units of the VdF / 2,3,3,3-tetrafluoropropene copolymer. The content of 2,3,3,3-tetrafluoropropene units is preferably 20 to 2 mol%, more preferably 15 mol% or less, even more preferably 10 mol% or less, more preferably 2.5 mol% or more, and even more preferably 3 mol% or more, based on the total monomer units of the VdF / 2,3,3,3-tetrafluoropropene copolymer.

[0117] The VdF / 2,3,3,3-tetrafluoropropene copolymer may contain, in addition to VdF units and 2,3,3,3-tetrafluoropropene units, units based on a monomer copolymerizable with VdF and 2,3,3,3-tetrafluoropropene (excluding VdF, 2,3,3,3-tetrafluoropropene, and pentenoic acid represented by formula (1)). From the viewpoint of electrolyte swelling resistance, the content of units based on a monomer copolymerizable with VdF and 2,3,3,3-tetrafluoropropene is preferably 3.0 mol % or less based on all monomer units of the VdF / 2,3,3,3-tetrafluoropropene copolymer.

[0118] Examples of the monomer copolymerizable with VdF and 2,3,3,3-tetrafluoropropene include the above-mentioned fluorinated monomers and non-fluorinated monomers. Among them, the monomer copolymerizable with VdF and 2,3,3,3-tetrafluoropropene is preferably at least one selected from the group consisting of fluorinated monomers and polar group-containing monomers, and more preferably at least one selected from the group consisting of TFE, HFP, 2,3,3,3-tetrafluoropropene, and monomer (2) (excluding pentenoic acid represented by formula (1)).

[0119] The weight average molecular weight (polystyrene equivalent) of the VdF / 2,3,3,3-tetrafluoropropene copolymer is preferably 50,000 to 2,000,000, more preferably 80,000 to 1,700,000, and even more preferably 100,000 to 1,500,000.

[0120] The number average molecular weight (polystyrene equivalent) of the VdF / 2,3,3,3-tetrafluoropropene copolymer is preferably 35,000 to 1,400,000, more preferably 40,000 to 1,300,000, and even more preferably 50,000 to 1,200,000.

[0121] The mass ratio ((A) / (B)) of PVdF (A) to VdF polymer (B) in the binder is preferably 95 / 5 to 10 / 90, more preferably 90 / 10 or less, more preferably 40 / 60 or more, even more preferably 45 / 55 or more, and particularly preferably 50 / 50 or more, because this allows for the production of an electrode mixture that has improved flexibility and is less likely to increase in viscosity.

[0122] The PVdF and binder of the present disclosure can be suitably used as materials for forming secondary batteries. The PVdF and binder of the present disclosure have excellent resistance to electrolyte swelling and adhesion to metal foil, and can form electrodes with excellent flexibility, making them suitable as binders for use in secondary battery electrodes. The PVdF and binder of the present disclosure can also be used as binders for separator coatings in secondary batteries. By using the PVdF and binder of the present disclosure, it is possible to fabricate secondary batteries that are less likely to increase in resistance even when stored at high temperatures and that maintain sufficient discharge capacity even after repeated charge and discharge.

[0123] The PVdF of the present disclosure may be a PVdF for secondary batteries. In the present disclosure, the PVdF for secondary batteries includes PVdF used in the positive electrode, negative electrode, and separator of secondary batteries. The binder of the present disclosure may be a binder for a secondary battery. In the present disclosure, the binder for a secondary battery includes a binder used in a positive electrode, a negative electrode, and a separator of a secondary battery. The secondary battery is preferably a lithium-ion secondary battery.

[0124] The PVdF or binder of the present disclosure can also constitute an electrode mixture together with a powder electrode material and water or a non-aqueous solvent. A secondary battery to which the PVdF or binder of the present disclosure can be applied includes a positive electrode in which a positive electrode mixture is supported on a positive electrode current collector, a negative electrode in which a negative electrode mixture is supported on a negative electrode current collector, and an electrolyte.

[0125] The electrode mixture of the present disclosure contains the above-described PVdF or binder, a powder electrode material, and water or a nonaqueous solvent. The electrode mixture of the present disclosure may be an electrode mixture for a secondary battery or an electrode mixture for a lithium-ion secondary battery. Because the electrode mixture of the present disclosure contains the above-described PVdF or binder, even when the PVdF or binder is contained at a high concentration, the viscosity can be easily adjusted to a level suitable for application to a current collector. The electrode mixture also has excellent resistance to electrolyte swelling and adhesion to metal foil, allowing for the formation of an electrode with excellent flexibility. Furthermore, because the electrode mixture of the present disclosure contains the above-described PVdF or binder, even when adjusted to a moderate viscosity and with excellent application properties, sufficient adhesion to metal foil (current collector) and sufficient retention of the powder electrode material can be ensured. Furthermore, the electrode mixture of the present disclosure can be used to fabricate a secondary battery that is less likely to increase in resistance even when stored at high temperatures and maintains sufficient discharge capacity even after repeated charge and discharge.

[0126] The electrode mixture may be a positive electrode mixture used in preparing a positive electrode or a negative electrode mixture used in preparing a negative electrode, but is preferably a positive electrode mixture. The electrode material layer formed from the electrode mixture of the present disclosure may be a positive electrode material layer or a negative electrode material layer, as long as it contains the above-mentioned PVdF or binder and a powder electrode material.

[0127] The powder electrode material is a powder electrode material used in batteries, and preferably contains an electrode active material. Electrode active materials are divided into positive electrode active materials and negative electrode active materials. In the case of lithium ion secondary batteries, the positive electrode active material is not particularly limited as long as it is capable of electrochemically absorbing and desorbing lithium ions, but lithium composite oxides are preferred, and lithium transition metal composite oxides are more preferred. The positive electrode active material is also preferably a lithium-containing transition metal phosphate compound. It is also preferred that the positive electrode active material be a substance containing lithium and at least one transition metal, such as a lithium transition metal composite oxide or a lithium-containing transition metal phosphate compound.

[0128] The transition metal of the lithium transition metal composite oxide is preferably V, Ti, Cr, Mn, Fe, Co, Ni, Cu, etc. Specific examples of the lithium transition metal composite oxide include lithium-cobalt composite oxides such as LiCoO2, lithium-nickel composite oxides such as LiNiO2, lithium-manganese composite oxides such as LiMnO2, LiMn2O4, and Li2MnO3, and those in which some of the transition metal atoms that make up the main components of these lithium transition metal composite oxides have been substituted with other metals such as Al, Ti, V, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga, Zr, and Si. Examples of the substituted oxides include lithium-nickel-manganese composite oxide, lithium-nickel-cobalt-aluminum composite oxide, lithium-nickel-cobalt-manganese composite oxide, lithium-manganese-aluminum composite oxide, and lithium-titanium composite oxide. More specifically, LiNi 0.5 Mn 0.5 O2, LiNi 0.85 Co 0.10 Al 0.05 O2, LiNi 0.33 Co 0.33 Mn 0.33 O2, LiNi 0.5 Mn 0.3 Co 0.2 O2, LiNi 0.6 Mn 0.2 Co 0.2 O2, LiNi 0.8 Mn 0.1 Co 0.1 O2, LiMn 1.8 Al 0.2 O4, LiMn 1.5 Ni 0.5 O4, Li4Ti5O 12 , LiNi 0.82 Co 0.15 Al 0.03 Examples include O2.

[0129] The transition metal of the lithium-containing transition metal phosphate compound is preferably V, Ti, Cr, Mn, Fe, Co, Ni, Cu, etc., and specific examples of the lithium-containing transition metal phosphate compound include iron phosphates such as LiFePO4, Li3Fe2(PO4)3, and LiFeP2O7, cobalt phosphates such as LiCoPO4, and lithium transition metal phosphate compounds in which a portion of the transition metal atoms that constitute the main components of these lithium transition metal phosphate compounds has been substituted with other metals such as Al, Ti, V, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga, Zr, Nb, and Si.

[0130] In particular, from the viewpoint of high voltage, high energy density, charge / discharge cycle characteristics, etc., LiCoO2, LiNiO2, LiMn2O4, LiNi 0.82 Co 0.15 Al 0.03 O2, LiNi 0.33 Mn 0.33 Co 0.33 O2, LiNi 0.5 Mn 0.3 Co 0.2 O2, LiNi 0.6 Mn 0.2 Co 0.2 O2, LiNi 0.8 Mn 0.1 Co 0.1 O2 and LiFePO4 are preferred.

[0131] Furthermore, a substance having a different composition from the substance constituting the main positive electrode active material may be attached to the surface of the positive electrode active material. Examples of the surface-attached substance include oxides such as aluminum oxide, silicon oxide, titanium oxide, zirconium oxide, magnesium oxide, calcium oxide, boron oxide, antimony oxide, and bismuth oxide, sulfates such as lithium sulfate, sodium sulfate, potassium sulfate, magnesium sulfate, calcium sulfate, and aluminum sulfate, and carbonates such as lithium carbonate, calcium carbonate, and magnesium carbonate.

[0132] These surface-attaching substances can be attached to the surface of the positive electrode active material by, for example, a method of dissolving or suspending the surface-attaching substance precursor in a solvent, adding the substance to the positive electrode active material by impregnation, and drying the solvent; a method of dissolving or suspending the surface-attaching substance precursor in a solvent, adding the substance to the positive electrode active material by impregnation, and then reacting the substance by heating or the like; a method of adding the substance to the positive electrode active material precursor and simultaneously baking the substance; or the like.

[0133] The amount of the surface-attached substance is preferably 0.1 ppm or more, more preferably 1 ppm or more, and even more preferably 10 ppm or more by mass relative to the positive electrode active material, and is preferably 20% or less, more preferably 10% or less, and even more preferably 5% or less by mass relative to the positive electrode active material. The surface-attached substance can suppress the oxidation reaction of the nonaqueous electrolyte on the surface of the positive electrode active material and improve the battery life, but if the amount of attachment is too small, the effect will not be fully exerted, and if it is too large, the movement of lithium ions into and out of the positive electrode active material will be hindered, which may increase the resistance.

[0134] The particle shape of the positive electrode active material may be a block, polyhedron, sphere, ellipsoid, plate, needle, column, or the like, as conventionally used. However, a spherical or ellipsoidal secondary particle is preferred, in which primary particles aggregate to form secondary particles. Typically, electrochemical devices experience stress-induced expansion and contraction of the active material in the electrode during charging and discharging, which can lead to deterioration such as destruction of the active material and disconnection of the conductive path. Therefore, a material in which primary particles aggregate to form secondary particles is preferred over a single-particle active material consisting of only primary particles, as this relieves the stress of expansion and contraction and prevents deterioration. Furthermore, spherical or ellipsoidal particles are preferred over plate-like equiaxially oriented particles because they are less oriented during electrode molding, resulting in less expansion and contraction of the electrode during charging and discharging, and are also more easily mixed uniformly with the conductive agent when preparing the electrode.

[0135] The tap density of the positive electrode active material is typically 1.3 g / cm 3 or more, preferably 1.5 g / cm 3 More preferably, 1.6 g / cm 3 or more, most preferably 1.7 g / cm 3That is all. If the tap density of the positive electrode active material is below the above lower limit, the amount of dispersion medium required when forming the positive electrode material layer increases, and the amounts of conductive agent and binder required also increase, which may restrict the filling rate of the positive electrode active material in the positive electrode material layer and restrict the battery capacity. By using a metal composite oxide powder with a high tap density, a high-density positive electrode material layer can be formed. Generally, the higher the tap density, the more preferable it is, and there is no particular upper limit. However, if it is too high, the diffusion of lithium ions in the positive electrode material layer using the non-aqueous electrolyte solution as a medium becomes rate-limiting, which may lead to a decrease in load characteristics. Therefore, the tap density is usually set to 2.5 g / cm. 3 or less, preferably 2.4 g / cm 3 The following is the result.

[0136] The tap density of the positive electrode active material is measured by passing it through a sieve with a mesh size of 300 μm and measuring it in 20 cm 3 After dropping the sample into the tapping cell to fill the cell volume, tapping is performed 1000 times with a stroke length of 10 mm using a powder density measuring device (for example, Tap Denser manufactured by Seishin Enterprise Co., Ltd.), and the density calculated from the volume and weight of the sample at that time is defined as the tap density.

[0137] The median particle diameter d50 of the positive electrode active material (the secondary particle diameter when primary particles aggregate to form secondary particles) is typically 0.1 μm or larger, preferably 0.5 μm or larger, more preferably 1 μm or larger, and most preferably 3 μm or larger, and typically 20 μm or smaller, preferably 18 μm or smaller, more preferably 16 μm or smaller, and most preferably 15 μm or smaller. Below the lower limit, high bulk density products may not be obtained. Above the upper limit, lithium diffusion within the particles takes too long, resulting in poor battery performance and problems such as streaking during battery positive electrode preparation, i.e., when the active material, conductive agent, binder, etc. are slurried with a solvent and applied as a thin film. Mixing two or more positive electrode active materials with different median diameters d50 can further improve the packing properties during positive electrode preparation.

[0138] The median diameter d50 in this disclosure is measured using a known laser diffraction / scattering particle size distribution analyzer. When using a HORIBA LA-920 as the particle size distribution analyzer, the measurement is performed using a 0.1% by mass aqueous solution of sodium hexametaphosphate as the dispersion medium, and after ultrasonic dispersion for 5 minutes, the measurement is performed with a refractive index set to 1.24.

[0139] When primary particles aggregate to form secondary particles, the average primary particle diameter of the positive electrode active material is typically 0.01 μm or more, preferably 0.05 μm or more, more preferably 0.08 μm or more, and most preferably 0.1 μm or more, and typically 3 μm or less, preferably 2 μm or less, more preferably 1 μm or less, and most preferably 0.6 μm or less. If the diameter exceeds the upper limit, it becomes difficult to form spherical secondary particles, adversely affecting powder packing, and the specific surface area is significantly reduced, potentially resulting in a decrease in battery performance, such as output characteristics. Conversely, if the diameter is below the lower limit, problems such as poor charge / discharge reversibility due to underdeveloped crystals may occur. The primary particle diameter is measured by observation using a scanning electron microscope (SEM). Specifically, the diameter is determined by taking a 10,000x magnification photograph of 50 primary particles and averaging the longest intercepts of a horizontal line at the left and right boundaries of the primary particles.

[0140] The BET specific surface area of ​​the positive electrode active material is 0.2 m 2 / g or more, preferably 0.3m 2 / g or more, more preferably 0.4m 2 / g or more, 4.0m 2 / g or less, preferably 2.5m 2 / g or less, more preferably 1.5m 2 If the BET specific surface area is smaller than this range, the battery performance is likely to decrease, whereas if it is larger, it becomes difficult to increase the tap density, which may easily cause problems with the coating properties when forming the positive electrode material layer.

[0141] The BET specific surface area is defined as the value measured by a surface area meter (for example, an automatic surface area measuring device manufactured by Okura Riken) using a nitrogen-helium mixed gas precisely adjusted so that the relative pressure of nitrogen to atmospheric pressure is 0.3, after which the sample is pre-dried at 150°C for 30 minutes under a nitrogen flow, by the nitrogen adsorption BET single-point method using a gas flow method.

[0142] The positive electrode active material is produced by a method generally used for producing inorganic compounds. In particular, various methods can be considered for producing spherical or oval-spherical active materials, such as a method of dissolving or pulverizing and dispersing transition metal raw materials such as transition metal nitrates and sulfates, and if necessary, raw materials of other elements, in a solvent such as water, adjusting the pH while stirring to produce and recover spherical precursors, which are then dried as needed, and then adding a Li source such as LiOH, Li2CO3, or LiNO3, and calcining at a high temperature to obtain an active material; a method of dissolving or pulverizing transition metal raw materials such as transition metal nitrates, sulfates, hydroxides, or oxides, and if necessary, raw materials of other elements, in a solvent such as water, Examples of such methods include dispersing a precursor in the form of a sphere or ellipsoid, drying and molding it using a spray dryer or the like to form a spherical or ellipsoidal precursor, adding a Li source such as LiOH, Li2CO3, or LiNO3 to the precursor, and firing it at a high temperature to obtain an active material; and dissolving or pulverizing and dispersing a transition metal raw material such as a transition metal nitrate, sulfate, hydroxide, or oxide, a Li source such as LiOH, Li2CO3, or LiNO3, and, if necessary, raw materials of other elements, in a solvent such as water, drying and molding it using a spray dryer or the like to form a spherical or ellipsoidal precursor, and firing this at a high temperature to obtain an active material.

[0143] In the present disclosure, one type of positive electrode active material powder may be used alone, or two or more types having different compositions or different powder properties may be used in any combination and ratio.

[0144] The negative electrode active material is not particularly limited as long as it can electrochemically absorb and release lithium ions, and examples thereof include carbonaceous materials, metal oxides such as tin oxide and silicon oxide, metal composite oxides, lithium alone, lithium alloys such as lithium-aluminum alloys, and metals capable of forming alloys with lithium, such as Sn and Si. These may be used alone or in any combination and ratio of two or more. Of these, carbonaceous materials or lithium composite oxides are preferably used from the standpoint of safety.

[0145] The metal composite oxide is not particularly limited as long as it is capable of absorbing and releasing lithium, but it is preferable that the metal composite oxide contains titanium and / or lithium as a constituent component from the viewpoint of high current density charge / discharge characteristics.

[0146] Carbonaceous materials include: (1) Natural graphite, (2) Artificial carbonaceous materials and artificial graphitic materials; carbonaceous materials {for example, natural graphite, coal-based coke, petroleum-based coke, coal-based pitch, petroleum-based pitch, or oxidized versions of these pitches, needle coke, pitch coke, and partially graphitized carbon materials, furnace black, acetylene black, pitch-based carbon fiber, and other organic pyrolysis products; carbonizable organic materials (for example, coal tar pitch ranging from soft pitch to hard pitch, coal-based heavy oils such as carbonized liquefied oil, atmospheric residue, straight-run heavy oils such as vacuum residue, cracked petroleum heavy oils such as ethylene tar produced as a by-product during the thermal decomposition of crude oil, naphtha, etc.); aromatic hydrocarbons such as acenaphthylene, decacyclene, anthracene, and phenanthrene; N-ring compounds such as phenazine and acridine; S-ring compounds such as thiophene and bithiophene; biphenyls; terphenyls; carbonaceous materials obtained by heat-treating the following at least once in the range of 400 to 3200°C: (polyphenylenes such as phenylene, polyvinyl chloride, polyvinyl alcohol, polyvinyl butyral, insolubilized products of these, nitrogen-containing organic polymers such as polyacrylonitrile and polypyrrole, sulfur-containing organic polymers such as polythiophene and polystyrene, natural polymers such as cellulose, lignin, mannan, polygalacturonic acid, chitosan, and polysaccharides represented by saccharose, thermoplastic resins such as polyphenylene sulfide and polyphenylene oxide, and thermosetting resins such as furfuryl alcohol resin, phenol-formaldehyde resin, and imide resin), and charcoals thereof, or solutions of carbonizable organic substances dissolved in low-molecular organic solvents such as benzene, toluene, xylene, quinoline, and n-hexane, and charcoals thereof; (3) A carbonaceous material in which the negative electrode material layer is made of carbonaceous materials having at least two or more different crystallinities and / or has an interface where the carbonaceous materials having different crystallinities are in contact with each other; (4) A carbonaceous material in which the negative electrode layer is made of carbonaceous materials having at least two or more different orientations and / or has an interface where the carbonaceous materials having different orientations are in contact with each other; The material selected from the above is preferable because it has a good balance between initial irreversible capacity and high current density charge / discharge characteristics.

[0147] The content of the electrode active material (positive electrode active material or negative electrode active material) is preferably 40 mass % or more in the electrode mixture in order to increase the capacity of the resulting electrode.

[0148] The powder electrode material may further contain a conductive agent, such as carbon blacks such as acetylene black and ketjen black, carbon materials such as graphite, carbon fiber, carbon nanotubes, and carbon nanohorns.

[0149] The ratio of the powder components (active material and conductive agent) to the above-mentioned PVdF or binder in the electrode mixture is usually about 80:20 to 99.5:0.5 by mass, and is determined taking into consideration the retention of the powder components, adhesion to the current collector, and the conductivity of the electrode.

[0150] With the blending ratios described above, the PVdF or binder described above cannot completely fill the voids between the powder components in the electrode material layer formed on the current collector. However, if a liquid that well dissolves or disperses the PVdF or binder is used as a solvent, the PVdF or binder will be uniformly dispersed and form a mesh-like structure in the electrode material layer after drying, which is preferable as it will hold the powder components well.

[0151] The liquid may be water or a non-aqueous solvent, such as nitrogen-containing organic solvents like N-methyl-2-pyrrolidone, N,N-dimethylacetamide, dimethylformamide, etc.; ketone solvents like acetone, methyl ethyl ketone, cyclohexanone, methyl isobutyl ketone, etc.; ester solvents like ethyl acetate, butyl acetate, etc.; ether solvents like tetrahydrofuran, dioxane, etc.; and mixed solvents thereof, etc., all of which are low-boiling point general-purpose organic solvents. Among these, the liquid is preferably at least one selected from the group consisting of N-methyl-2-pyrrolidone and N,N-dimethylacetamide, in view of the excellent stability and coatability of the electrode mixture.

[0152] The amount of the liquid in the electrode mixture is determined in consideration of the coating properties on the current collector, the thin film formability after drying, etc. Usually, the ratio of PVdF or binder to the liquid is preferably 0.5:99.5 to 20:80 by mass.

[0153] Furthermore, the above-mentioned PVdF or binder is preferably used with a small particle size of 1000 μm or less, particularly 50 to 350 μm, so as to enable rapid dissolution or dispersion in the liquid.

[0154] To further improve adhesion to the current collector, the electrode mixture may further contain, for example, an acrylic resin such as polymethacrylate or polymethyl methacrylate, or a polyimide, polyamide, or polyamideimide resin. A crosslinking agent may be added, and a crosslinked structure may be formed by irradiating with radiation such as gamma rays or electron beams. The crosslinking method is not limited to radiation irradiation, and other crosslinking methods may also be used, such as adding a thermally crosslinkable amine group-containing compound or cyanurate group-containing compound to perform thermal crosslinking.

[0155] To the electrode mixture, a dispersant such as a resin having a surface active effect, a cationic surfactant, or a nonionic surfactant may be added in order to improve the dispersion stability of the electrode slurry.

[0156] The content of PVdF or binder in the electrode mixture is preferably 0.1 to 20 mass %, more preferably 1 to 10 mass %, relative to the mass of the electrode mixture.

[0157] The electrode mixture can be prepared by dispersing and mixing the powder electrode material in a solution or dispersion prepared by dissolving or dispersing PVdF or a binder in the liquid. The resulting electrode mixture is then uniformly applied to a current collector such as a metal foil or a metal mesh, dried, and pressed as necessary to form a thin layer of electrode material on the current collector, thereby forming a thin-film electrode.

[0158] Alternatively, the PVdF or binder powder and the electrode material powder may be mixed first, and then the liquid may be added to prepare the electrode mixture. Alternatively, the PVdF or binder powder and the electrode material powder may be heated and melted, and extruded using an extruder to prepare a thin film of the electrode mixture, which may then be attached to a current collector coated with a conductive adhesive or a general-purpose organic solvent to prepare an electrode sheet. Furthermore, a solution or dispersion of the PVdF or binder powder and the electrode material powder may be applied to a preformed electrode material. Thus, the method of application of the PVdF or binder is not particularly limited.

[0159] The electrode of the present disclosure contains the above-mentioned PVdF or binder. Because the electrode of the present disclosure contains the above-mentioned PVdF or binder, the electrode does not crack even when a thick coating of powder electrode material is applied and then wound and pressed to increase density, and the powder electrode material does not fall off or peel from the current collector. Furthermore, the electrode of the present disclosure also has excellent resistance to electrolyte swelling. Furthermore, by using the above-mentioned electrode, a secondary battery can be fabricated that is resistant to increases in resistance even when stored at high temperatures and maintains sufficient discharge capacity even after repeated charge and discharge.

[0160] The electrode preferably comprises a current collector and an electrode material layer formed on the current collector, the electrode material layer containing the powder electrode material and the above-mentioned PVdF or binder. The electrode may be a positive electrode or a negative electrode, but is preferably a positive electrode.

[0161] Examples of the current collectors (positive electrode current collector and negative electrode current collector) include metal foils or metal meshes made of iron, stainless steel, copper, aluminum, nickel, titanium, etc. Among these, aluminum foil is preferred as the positive electrode current collector, and copper foil is preferred as the negative electrode current collector.

[0162] The electrode of the present disclosure can be manufactured by, for example, the method described above. Because the electrode mixture has excellent coatability, by manufacturing the electrode of the present disclosure using the electrode mixture, it is possible to easily manufacture an electrode having a smooth, uniform, and thick electrode material layer.

[0163] The secondary battery of the present disclosure includes the above-described electrode. In the secondary battery of the present disclosure, at least one of the positive electrode and the negative electrode may be the above-described electrode, and the positive electrode is preferably the above-described electrode. The secondary battery is preferably a lithium-ion secondary battery. The secondary battery of the present disclosure exhibits a low resistance increase rate and a high capacity retention rate.

[0164] The secondary battery of the present disclosure preferably further comprises a nonaqueous electrolyte. The nonaqueous electrolyte is not particularly limited, but may be one or more of known hydrocarbon solvents such as propylene carbonate, ethylene carbonate, butylene carbonate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; or fluorine-containing solvents such as fluoroethylene carbonate, fluoroethers, and fluorinated carbonates. Any of the conventionally known electrolytes may be used, such as LiClO4, LiAsF6, LiPF6, LiBF4, LiCl, LiBr, CH3SO3Li, CF3SO3Li, and cesium carbonate.

[0165] A separator may be interposed between the positive electrode and the negative electrode. As the separator, a conventionally known separator may be used, or a separator coated with the above-mentioned PVdF or binder may be used.

[0166] It is also preferable to use the above-mentioned PVdF or binder in at least one of the positive electrode, negative electrode, and separator of a secondary battery (preferably a lithium ion secondary battery).

[0167] A film for a secondary battery made of the above-mentioned PVdF or binder is also one of the preferred embodiments of the present disclosure.

[0168] A preferred embodiment of the present disclosure is a laminate for a secondary battery having a substrate and a layer made of PVdF or a binder formed on the substrate. Examples of the substrate include those exemplified as the current collector and known substrates (such as porous membranes) used in separators for secondary batteries.

[0169] Although the embodiments have been described above, it will be understood that various changes in form and details can be made without departing from the spirit and scope of the claims. [Example]

[0170] Next, embodiments of the present disclosure will be described with reference to examples, but the present disclosure is not limited to these examples.

[0171] The values ​​in the examples were measured by the following methods.

[0172] (polymer composition) The content of pentenoic acid units in PVdF is determined by esterifying the carboxyl groups of the pentenoic acid units to convert them into ester groups, and then 1 The esterified PVdF was analyzed using H-NMR. Specifically, 400 mg of PVdF, 10 mg of trimethylsilyldiazomethane, and 3 mg of methanol were reacted at 25°C for 12 hours, and the resulting polymer was washed with methanol and dried in vacuum at 70°C for 24 hours. 1 The dried polymer was analyzed using H-NMR. 1 It was determined from the H-NMR spectrum at 3.7 ppm.

[0173] (solution viscosity) A 5% by mass solution of PVdF (binder) in NMP was prepared. The viscosity of the NMP solution was measured 10 minutes after the start of measurement using a Brookfield viscometer (TV-10M, manufactured by Toki Sangyo Co., Ltd.) at 25°C, rotor No. M4, and a rotation speed of 6 rpm.

[0174] (Weight average molecular weight) Measurements were made by gel permeation chromatography (GPC). Calculations were made using data measured using Tosoh AS-8010, CO-8020, and columns (three GMHHR-H columns connected in series) and Shimadzu RID-10A, with dimethylformamide (DMF) as the solvent at a flow rate of 1.0 ml / min (reference: polystyrene).

[0175] (Melting Point) Using a differential scanning calorimetry (DSC) device, the temperature was raised from 30°C to 220°C at a rate of 10°C / min, then lowered to 30°C at 10°C / min, and then raised again to 220°C at a rate of 10°C / min. The temperature at the maximum value on the heat of fusion curve was determined as the melting point.

[0176] (Electrolyte swelling resistance) A 200 μm thick film was produced by casting an NMP solution (8 mass%) of PVdF (binder) onto a glass petri dish and vacuum drying it for 6 hours at 100° C. The obtained film was cut into a size of 10 mm diameter and placed in a sample bottle containing an electrolyte (a solution of LiPF6 dissolved at a concentration of 1 M in a solvent of ethylene carbonate and ethyl methyl carbonate at a volume ratio of 3:7). After leaving it at 60° C. for 1 week, the weight gain rate was calculated using the following formula to evaluate the resistance to electrolyte swelling. Weight increase rate (%) = (film weight after immersion in electrolyte / film weight before immersion in electrolyte) × 100

[0177] (Viscosity of positive electrode mixture) After preparing the positive electrode mixture, the viscosity of the positive electrode mixture was immediately measured. Using a B-type viscometer (TV-10M, manufactured by Toki Sangyo Co., Ltd.), the viscosity of the positive electrode mixture was measured 10 minutes after the start of measurement under the conditions of 25°C, rotor No. M4, and a rotation speed of 6 rpm.

[0178] (Positive electrode material layer density) The density of the positive electrode layer was calculated from the area, thickness and weight of the positive electrode layer.

[0179] (Peel strength between the positive electrode material layer and the positive electrode current collector) A 1.2 cm x 7.0 cm test piece was prepared by cutting out the positive electrode. The positive electrode material layer side of the test piece was fixed to a movable jig with double-sided tape, and then the tape was attached to the surface of the positive electrode current collector. The stress (N / cm) when the tape was pulled at a 90-degree angle at a speed of 100 mm / min was measured using an autograph. A 1 N load cell was used for the autograph.

[0180] (Positive electrode flexibility) A test piece of 2 cm x 10 cm was prepared by cutting out the positive electrode, and was wrapped around a round rod of 3.0 mm in diameter. The positive electrode was visually inspected and evaluated according to the following criteria. ◯: No cracks or breaks were observed. Δ: Cracks were observed in the positive electrode material layer, but no breakage was observed in the positive electrode material layer or the current collector. ×: The positive electrode material layer and the current collector were broken.

[0181] (Cycle capacity retention rate) The cycle capacity retention rate was measured using the aluminum laminate cell (secondary battery) prepared in the experimental example. The secondary battery was charged at 25°C to 4.2 V at a constant current equivalent to 0.5 C, then charged at a constant voltage of 4.2 V until the current reached 0.1 C, and then discharged at a constant current of 0.5 C to 3.0 V. This cycle was repeated three times to stabilize the battery. The secondary battery was then charged at a constant current of 0.5 C to 4.2 V, then charged at a constant voltage of 4.2 V until the current reached 0.1 C, and then discharged at a constant current of 0.5 C to 3.0 V to determine the initial discharge capacity. Charge and discharge were repeated in the same manner, and the discharge capacity after 300 cycles was measured. The ratio of the discharge capacity after 300 cycles to the initial discharge capacity was calculated using the following formula, and this was defined as the cycle capacity retention rate (%). The results are shown in Table 2. (Discharge capacity after 300 cycles) / (Initial discharge capacity) x 100 = Cycle capacity retention rate (%)

[0182] Example 1 A 2.5-liter autoclave was charged with 668 g of VdF together with 1,546 g of purified water, 1.5 g of methylcellulose, 1 ml of 4-pentenoic acid, 2 ml of methanol, and 1 g of di-n-propyl peroxydicarbonate, and the temperature was raised to 31°C over 1.5 hours, and then maintained at 31°C for 9 hours. The maximum pressure reached during this period was 7 MPaG.

[0183] The polymerization was terminated 9 hours after the completion of the temperature increase to 31° C. After the polymerization was completed, the obtained polymer slurry was recovered, dehydrated, washed with water, and further dried at 118° C. for 12 hours to obtain PVdF powder.

[0184] Example 2 A 2.5-liter autoclave was charged with 520 g of VdF together with 1,700 g of purified water, 1.7 g of methylcellulose, 2 ml of 4-pentenoic acid, 2 ml of methanol, and 1 g of di-n-propyl peroxydicarbonate, and the temperature was raised to 36°C over 1.5 hours, after which the temperature was maintained at 36°C for 30 hours. The maximum pressure reached during this period was 7 MPaG.

[0185] The polymerization was terminated 30 hours after the completion of the temperature increase to 36° C. After the polymerization was completed, the obtained polymer slurry was recovered, dehydrated, washed with water, and further dried at 118° C. for 12 hours to obtain PVdF powder.

[0186] Example 3 A 2.5-liter autoclave was charged with 520 g of VdF together with 1,700 g of purified water, 1.7 g of methylcellulose, 2 ml of 4-pentenoic acid, 2 ml of methanol, and 1 g of di-n-propyl peroxydicarbonate, and the temperature was raised to 36°C over 1.5 hours, after which the temperature was maintained at 36°C for 37 hours. The maximum pressure reached during this period was 7 MPaG.

[0187] The polymerization was terminated 37 hours after the completion of the temperature increase to 36° C. After the polymerization was completed, the obtained polymer slurry was recovered, dehydrated, washed with water, and further dried at 118° C. for 12 hours to obtain PVdF powder.

[0188] Example 4 A 2.5-liter autoclave was charged with 520 g of VdF together with 1,700 g of purified water, 1.7 g of methylcellulose, 2 ml of 4-pentenoic acid, 2 ml of methanol, and 1 g of di-n-propyl peroxydicarbonate, and the temperature was raised to 36°C over 1.5 hours, after which the temperature was maintained at 36°C for 39 hours. The maximum pressure reached during this period was 7 MPaG.

[0189] The polymerization was terminated 39 hours after the completion of the temperature increase to 36° C. After the polymerization was completed, the obtained polymer slurry was recovered, dehydrated, washed with water, and further dried at 118° C. for 12 hours to obtain PVdF powder.

[0190] Example 5 A 2-liter autoclave was charged with 700 g of purified water, 0.35 g of methylcellulose, 0.8 ml of 4-pentenoic acid, 1.0 ml of methanol, and 2.0 g of t-butylperoxy-2-ethylhexanoate, along with 357 g of VdF. The mixture was heated to 72°C over 1.5 hours and then maintained at 72°C for 18 hours. The maximum pressure reached during this period was 7.9 MPaG.

[0191] The polymerization was terminated 18 hours after the completion of the temperature increase to 72° C. After the polymerization was completed, the obtained polymer slurry was recovered, dehydrated, washed with water, and further dried at 118° C. for 12 hours to obtain PVdF powder.

[0192] Comparative Example 1 A 2.5-liter autoclave was charged with 839 g of purified water, 0.84 g of methylcellulose, 2 ml of methanol, and 1 g of di-n-propyl peroxydicarbonate, along with 1,173 g of VdF. The temperature was raised to 31°C over 1.5 hours, and the temperature was maintained at 31°C for 3 hours. The maximum pressure reached during this period was 7 MPaG.

[0193] The polymerization was terminated 3 hours after the completion of the temperature increase to 31° C. After the polymerization was completed, the obtained polymer slurry was recovered, dehydrated, washed with water, and further dried at 118° C. for 12 hours to obtain PVdF powder.

[0194] Experimental example (Preparation of Positive Electrode Mixture) The PVdF (binder) obtained in the examples and comparative examples was dissolved in N-methyl-2-pyrrolidone (NMP) to prepare an NMP solution so that the concentration of PVdF (binder) in the NMP solution was 8 mass %. The positive electrode active material (NMC(811)(LiNi 0.8 Mn 0.1 Co 0.1 O2) and a conductive agent (acetylene black (AB)) were added and thoroughly mixed with a stirrer to prepare a positive electrode mixture. The mass ratio of the positive electrode active material, conductive agent, and PVdF (binder) in the positive electrode mixture was 96 / 2 / 2. The solid content of the positive electrode mixture was as shown in Table 1.

[0195] (Preparation of positive electrode) The obtained positive electrode mixture was applied to one side of a positive electrode current collector (aluminum foil with a thickness of 20 μm) in an amount of 22.5 mg / cm 2 After completely volatilizing the NMP, the cathode was pressed using a roll press at a pressure of 10 tons to produce a cathode comprising a cathode material layer and a cathode current collector. The density of the cathode material layer was 2.75 g / cc.

[0196] (Preparation of Electrolyte) Ethylene carbonate, a high-dielectric constant solvent, and ethyl methyl carbonate, a low-viscosity solvent, were mixed in a volume ratio of 30:70, and LiPF6 was added to this to a concentration of 1.0 mol / L. 2% by mass of vinylene carbonate was then added to this to obtain a nonaqueous electrolyte solution.

[0197] (Fabrication of lithium-ion secondary batteries) The positive electrode obtained above was cut into a shape having a coated portion (positive electrode material layer) 50 mm wide and 30 mm long, and an uncoated portion 5 mm wide and 9 mm long.

[0198] To 98 parts by mass of artificial graphite, 1 part by mass of an aqueous dispersion of sodium carboxymethylcellulose (concentration of sodium carboxymethylcellulose: 1% by mass) and 1 part by mass of an aqueous dispersion of styrene-butadiene rubber (concentration of styrene-butadiene rubber: 50% by mass) were added as thickeners and binders, and the mixture was mixed in a disperser to form a slurry. The resulting slurry was applied to a 20 μm thick copper foil, dried, and rolled in a press. The foil was then cut into a shape with a coated portion (negative electrode material layer) 52 mm wide and 32 mm long, and an uncoated portion 5 mm wide and 9 mm long to form a negative electrode.

[0199] The positive electrode and the negative electrode were placed opposite each other with a 20 μm-thick microporous polyethylene film (separator) interposed therebetween, and the nonaqueous electrolyte solution obtained above was poured into the cell. After the nonaqueous electrolyte solution had sufficiently permeated the separator and the like, the cell was sealed, pre-charged, and aged to produce an aluminum laminate cell (lithium ion secondary battery).

[0200] The evaluation results are shown in Table 1.

[0201] [Table 1]

Claims

1. vinylidene fluoride units and a compound represented by the formula (1): CH ２ =CH-(CH2) ２ -COOY (wherein Y represents at least one selected from the group consisting of inorganic cations and organic cations), the content of vinylidene fluoride units is 98.5 to 99.99 mol% based on the total monomer units of the polyvinylidene fluoride; The content of the pentenoic acid unit is 0.01 to 1.5 mol % based on the total monomer units of the polyvinylidene fluoride. Polyvinylidene fluoride.

2. Y is H, Li, Na, K, Mg, Ca, Al and NH ４ The polyvinylidene fluoride according to claim 1, which represents at least one selected from the group consisting of:

3. 3. The polyvinylidene fluoride according to claim 1, which has a weight average molecular weight of 50,000 to 2,000,000.

4. A binder containing the polyvinylidene fluoride according to any one of claims 1 to 3.

5. 10. An electrode mixture comprising the polyvinylidene fluoride according to claim 1 or the binder according to claim 4, a powder electrode material, and water or a non-aqueous solvent.

6. An electrode comprising the polyvinylidene fluoride according to any one of claims 1 to 3 or the binder according to claim 4.

7. A secondary battery comprising the electrode according to claim 6.

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