Binder for secondary battery, electrode mixture agent, electrode, and non-aqueous electrolyte secondary battery

Abstract

To provide a binder for a secondary battery which can suppress thickening of a slurry while keeping the adhesion with a collector.SOLUTION: The binder for a secondary battery includes a vinylidene fluoride-based polymer containing a constitutional unit derived from vinylidene fluoride and a constitutional unit derived from a chemical compound with an atomic group represented by RX. RX is an atomic group including an -SO2-Ra group and Ra is an atomic group including at least one carbon atom.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a binder for secondary batteries, an electrode binder, an electrode, and a non-aqueous electrolyte secondary battery.

Background Art

[0002] As a battery having a high energy density, non-aqueous electrolyte secondary batteries typified by lithium ion secondary batteries are widely used.

[0003] The electrodes of lithium ion secondary batteries are usually manufactured by applying a slurry-like electrode binder in which an active material, a conductive aid, and a binder (adhesive) are dispersed in a solvent onto a current collector and then drying. As the binder for the positive electrode, a binder containing a vinylidene fluoride-based polymer is used.

[0004] For example, Patent Document 1 discloses a vinylidene fluoride-based polymer containing a structural unit derived from a vinylidene fluoride monomer and a structural unit derived from CF2=CF(CF2CF2)nSO2 as an ion conductive material used in lithium batteries and the like.

[0005] Further, Patent Document 2 discloses a binder containing a vinylidene fluoride-based polymer in which a hydrogen atom in a structural unit derived from vinylidene fluoride is substituted with a sulfonic acid group (-SO3H group).

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] By the way, as the positive electrode active material of the lithium secondary battery, a cobalt-based positive electrode active material is mainly used. However, from the viewpoint of further increasing the capacity, a high-capacity active material such as a high-nickel-based positive electrode active material containing a large amount of nickel may be used. However, a slurry containing such a high-capacity active material and a binder containing a conventional vinylidene fluoride-based polymer has a problem that the viscosity increases (gels) with time in the electrode manufacturing process, and the coatability is likely to deteriorate.

[0008] On the other hand, the present inventors have tried to suppress the thickening of the slurry by changing the type of the copolymer monomer of the vinylidene fluoride-based polymer as the binder. As a result, it has been newly found that even if the thickening of the slurry can be suppressed, the adhesiveness of the electrode mixture layer to the current collector may decrease. Therefore, there is a need for a binder that can suppress the thickening of the slurry while maintaining the adhesiveness to the current collector.

[0009] An object of the present invention is to provide a binder for a secondary battery, an electrode mixture, an electrode, and a non-aqueous electrolyte secondary battery that can suppress the thickening of a slurry while maintaining the adhesiveness to a current collector.

Means for Solving the Problems

[0010] [1] A binder for a secondary battery containing a vinylidene fluoride-based polymer containing a structural unit derived from vinylidene fluoride and a structural unit derived from a compound having an atomic group represented by RX, wherein RX is an atomic group containing -SO2-Ra, and Ra is an atomic group containing at least one carbon atom. [2] The binder for a secondary battery according to [1], wherein the compound having an atomic group represented by RX is a compound represented by the following formula (1).

Chemical formula

[10] A non-aqueous electrolyte secondary battery comprising the electrode according to [9].

Advantages of the Invention

[0011] According to the present invention, it is possible to provide a binder for a secondary battery, an electrode mixture, an electrode, and a non-aqueous electrolyte secondary battery that can suppress thickening of a slurry while maintaining adhesiveness to a current collector.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Modes for Carrying Out the Invention

[0013] As described above, there has been a problem that a slurry in which a vinylidene fluoride-based polymer is mixed with a high-capacity active material, particularly a cathode active material containing a large amount of nickel, is likely to gel.

[0014] The mechanism is considered as follows. The cathode active material contains a base, and the cathode active material having a high nickel ratio contains particularly a large amount of the base. Therefore, when the vinylidene fluoride-based polymer comes into contact with the cathode active material, the degradation of the polymer is promoted by the base. Specifically, vinylidene fluoride in the polymer undergoes dehydrofluorination to form a conjugated double bond. The vinylidene fluoride-based polymer having a conjugated double bond is likely to crosslink in a slurry-like electrode mixture (hereinafter also referred to as a slurry), and the slurry is likely to gel.

[0015] In contrast, the inventors of the present invention have found that a vinylidene fluoride-based polymer containing a structural unit derived from a compound containing -SO2-Ra (Ra is a group containing at least one carbon atom) can suppress gelation without reducing adhesiveness.

[0016] The reason is not clear, but it is considered as follows. Since Ra of -SO2-Ra contains a carbon atom, the electron density of the sulfonyl group (-SO2-) portion does not decrease, and it becomes easy to coordinate with the conductive aid, and the dispersibility of the polymer tends to be high. In addition, the compound containing -SO2-Ra is easily introduced into the low molecular weight band having high dispersibility among vinylidene fluoride-based polymers. Therefore, the combination of the coordination ability of this sulfonyl group and the introduction of the compound containing -SO2-Ra into the low molecular weight band results in an increase in the dispersibility of the vinylidene fluoride-based polymer into which the compound containing the sulfonyl group is introduced, making it difficult for the polymers to crosslink with each other, and it is considered that thickening suppression has been achieved. Also in the studies by the inventors, the above effects have been significantly confirmed in -SO2-Ra rather than -SO2-F. This is presumably because the electronegativity of the group adjacent to the sulfur atom in -SO2-Ra is lower than that in -SO2-F, so the electron density of the -SO2- portion is less likely to decrease and the coordination ability is higher.

[0017] That is, the binder according to one embodiment of the present invention includes a vinylidene fluoride-based polymer including a structural unit derived from vinylidene fluoride and a structural unit derived from a compound having a group represented by -SO2-Ra (Ra is a group containing at least one carbon atom).

[0018] Hereinafter, the binder according to one embodiment of the present invention will be described.

[0019] 1. Binder The binder according to the present embodiment includes a vinylidene fluoride-based polymer.

[0020] 1-1. Vinylidene fluoride-based polymer The vinylidene fluoride-based polymer contains a structural unit derived from vinylidene fluoride and a structural unit derived from a compound having an atomic group represented by RX.

[0021] The compound having an atomic group represented by RX is a compound copolymerizable with vinylidene fluoride and having an atomic group represented by RX.

[0022] For example, the compound having an atomic group represented by RX is preferably a compound represented by the following formula (1).

Chemical formula

[0023] R1 to R3 in formula (1) are each a hydrogen atom, a chlorine atom, a fluorine atom, an alkyl group having 1 to 6 carbon atoms, or a fluorine-substituted alkyl group having 1 to 6 carbon atoms. Examples of the alkyl group having 1 to 6 carbon atoms include a methyl group and an ethyl group, and preferably a methyl group. The fluorine-substituted alkyl group having 1 to 6 carbon atoms is preferably a fluorine-substituted alkyl group having 1 to 3 carbon atoms, more preferably a fluorine-substituted alkyl group having 1 carbon atom, and even more preferably a trifluoromethyl group. Among them, R1 to R3 are each preferably a hydrogen atom or an alkyl group having 1 to 6 carbon atoms, and more preferably a hydrogen atom.

[0024] RX in formula (1) is an atomic group containing -SO2-Ra. Ra is an atomic group containing at least one carbon atom. Examples of the atomic group represented by Ra include an alkyl group and an aryl group. Among them, Ra is preferably an alkyl group. These groups may be substituted with substituents such as a halogen atom, a sulfur atom, a nitrogen atom, and an oxygen atom as long as the effects of the present invention are not impaired.

[0025] RX is an atomic group preferably containing -(CR5R6)n-SO2-Ra (R5 and R6 are each a hydrogen atom or a fluorine atom), more preferably an atomic group containing -(CH2)n-SO2-Ra, and may be, for example, -L-(CH2)n-SO2-Ra. n is an integer of 1 or more, and may preferably be 1 or 2. Further, the carbon atom of Ra and -(CR5R6)n- or -(CH2)n- may be bonded to each other to form a ring. The ring is preferably a 5- or 6-membered aliphatic ring. L is a single bond or a linking group composed of one or more selected from the group consisting of -COO-, -CO-, and -O-. In these cases, since the atomic group represented by RX can contain a -C-SO2-C- bond, the sulfonyl group can act more easily, and the crosslinking of the polymer can be more suppressed.

[0026] The molecular weight of the atomic group represented by RX is preferably, for example, 76 or more and 200 or less, and more preferably 79 or more and 150 or less. When the molecular weight of the atomic group represented by RX is 200 or less, the synthesis of the compound represented by formula (1) is more likely to be easy. Further, when the molecular weight of the atomic group represented by RX is 76 or more, since it has an appropriate bulkiness, the thickening of the slurry can be more suppressed. The molecular weight can be determined by calculation from the chemical structure.

[0027] For example, the compound having an atomic group represented by RX is more preferably a compound represented by the following formula (2).

Chemical formula

[0028] R1 to R3 in formula (2) have the same meanings as R1 to R3 in formula (1).

[0029] R4 in formula (2) is a group containing -SO2-Ra, preferably a group containing -(CR5R6)n-SO2-Ra, and more preferably a group containing -(CH2)n-SO2-Ra. Ra has the same meaning as Ra described above and is preferably an alkyl group. The carbon atoms of the alkyl group and -(CH2)n- may be bonded to each other to form a ring. R5 and R6 also have the same meaning as R5 and R6 described above and are preferably hydrogen atoms.

[0030] Specific examples of the compound having an atomic group represented by RX include the following.

Chemical formula

[0031] The content of the structural unit derived from the compound having an atomic group represented by RX is not particularly limited, but is preferably 0.5 parts by mass or more and 2 parts by mass or less with respect to 100 parts by mass of the total amount of the structural unit derived from vinylidene fluoride and the structural unit derived from the compound having an atomic group represented by RX. When the above content is 0.5 parts by mass or more, thickening of the electrode binder can be further suppressed without reducing the adhesiveness. When the above content is 2.0 parts by mass or less, the adhesiveness is less likely to be impaired. From the same viewpoint, the above content of the structural unit derived from the compound having an atomic group represented by RX is more preferably 0.7 parts by mass or more and 1.5 parts by mass or less.

[0032] Also, from the same viewpoint, the content of the structural unit derived from the compound having an atomic group represented by RX is preferably 0.15 mol% or more and 1.0 mol% or less, and more preferably 0.2 mol% or more and 0.5 mol% or less with respect to the structural unit derived from vinylidene fluoride.

[0033] Whether the structural unit of the vinylidene fluoride-based polymer contains a sulfonyl group can be confirmed by XPS measurement. Specifically, XPS measurement is performed under the following conditions, and in the obtained spectrum, peaks derived from sulfur of the sulfonyl group (near 165 - 170 eV and 220 - 240 eV) are observed, and sulfur is detected by elemental analysis, whereby it can be confirmed that the structural unit of the vinylidene fluoride-based polymer contains a sulfonyl group.

[0034] XPS measurement can be carried out under the following conditions. The binder powder obtained by polymerization is sandwiched between aluminum foils, and a film-like sample is obtained by performing hot pressing at 200 °C and 10 MPa. This is cut into 1 cm squares and fixed to the XPS measurement sample stage using carbon double-sided tape. As the measurement device, an XPS measurement device (for example, PS9010MC manufactured by JEOL Ltd.) can be used. As the radiation source, Al-Kα rays of 30 W (10 kV × 5 mA) are used, electrons are supplemented with a neutralization gun of 2 W (1 kV × 1 mA), and measurement can be performed at a photoelectron detection angle of 90 degrees.

[0035] Also, the content of each structural unit in the vinylidene fluoride-based polymer can be calculated from the integration ratio of the signals in the spectrum obtained by performing 1 1H-NMR measurement.

[0036] 1 1H-NMR measurement can be carried out under the following conditions. 10 mg of the binder powder obtained by polymerization is added to 0.75 mL of DMSO-d6 and heated at 50 °C for 3 hours to completely dissolve it. This solution is put into an NMR sample tube with a 5 mm diameter to prepare a sample. As the measurement device, an NMR measurement device (for example, AVANCE AC 400FT NMR spectrometer manufactured by Bruker) can be used.

[0037] The vinylidene fluoride-based polymer may further contain a structural unit derived from a compound other than the compound having a vinylidene fluoride and a group represented by RX, as long as the object and effect of the present invention are not impaired.

[0038] The vinylidene fluoride-based polymer may contain only one kind of structural unit derived from another compound, or may contain two or more kinds. However, the total amount of the structural unit derived from vinylidene fluoride and the structural unit derived from the compound having a group represented by RX with respect to all the structural units of the vinylidene fluoride-based polymer is preferably 90% by mass or more, and more preferably 95% by mass or more.

[0039] Examples of other compounds include fluorine-based monomers copolymerizable with vinylidene fluoride, hydrocarbon-based monomers such as ethylene and propylene, and monomers copolymerizable with compounds having an atomic group represented by RX. Examples of fluorine-based monomers copolymerizable with vinylidene fluoride include vinyl fluoride, trifluoroethylene, tetrafluoroethylene, chlorotrifluoroethylene, hexafluoropropylene, fluoroalkyl vinyl ether, and perfluoroalkyl vinyl ether represented by perfluoromethyl vinyl ether. Examples of monomers copolymerizable with compounds having an atomic group represented by RX include alkyl (meth)acrylate compounds represented by methyl (meth)acrylate.

[0040] The vinylidene fluoride-based polymer may be a block polymer or a random polymer.

[0041] The melting point of the vinylidene fluoride-based polymer is preferably, for example, 160 °C or higher, more preferably 165 °C or higher. When the melting point of the vinylidene fluoride-based polymer is 160 °C or higher, it is less likely to swell by the electrolyte, and the performance of the resulting lithium ion secondary battery is more likely to be improved.

[0042] The melting point of the above vinylidene fluoride-based polymer can be determined by calorimetry using a differential scanning calorimeter (DSC). Specifically, the vinylidene fluoride-based polymer is heated from 30 °C to 230 °C at a rate of 10 °C / min (first heating), cooled from 230 °C to 30 °C at a rate of 10 °C / min (first cooling), and then heated from 30 °C to 230 °C at a rate of 10 °C / min (second heating). Then, the melting peak is identified by DSC. In this specification, the maximum melting peak temperature observed in the second heating is defined as the melting point of the vinylidene fluoride-based polymer.

[0043] The inherent viscosity of the vinylidene fluoride-based polymer is preferably, for example, 0.5 dL / g or more and 5.0 dL / g or less, more preferably 1.0 dL / g or more and 4.0 dL / g or less, and most preferably 1.0 dL / g or more and 3.5 dL / g or less. When the inherent viscosity is 0.5 dL / g or more, the adhesive strength between the binder (vinylidene fluoride-based polymer) and the active material or the current collector can be made higher. On the other hand, when the inherent viscosity is 5.0 or less, when the electrode slurry is prepared, the slurry viscosity does not become too high, and the workability is excellent.

[0044] The inherent viscosity (ηi) is represented by the logarithmic viscosity. First, 80 mg of the vinylidene fluoride-based polymer is dissolved in 20 mL of N,N-dimethylformamide, and the viscosity is measured using an Ubbelohde viscometer in a constant temperature bath at 30°C. Then, it is calculated based on the following formula from the obtained value. η i =(1 / C)·ln(η / η0) In the above formula, η is the viscosity of the solution, η0 is the viscosity of N,N-dimethylformamide alone as the solvent, and C is the concentration of the vinylidene fluoride-based polymer in the solution, that is, 0.4 g / dL.

[0045] The above vinylidene fluoride-based polymer can be prepared, for example, by copolymerizing vinylidene fluoride, a compound represented by formula (1) or (2) (a compound having an atomic group represented by RX), and, if necessary, other compounds by a known method. Examples of the method for copolymerizing these include suspension polymerization, emulsion polymerization, solution polymerization, etc. From the viewpoint of easily obtaining a binder with high adhesive strength and having few impurities, suspension polymerization is preferred.

[0046] Note that the compound (2-1), which is a compound having an atomic group represented by RX, can be obtained, for example, by reacting 3-hydroxy sultone and acryloyl chloride in dichloromethane. Also, the compound (2-4) can be obtained, for example, by reacting 2-hydroxyethyl methyl sulfone and acryloyl chloride in the presence of dichloromethane.

[0047] 1-2. Non-aqueous solvent The binder may contain a non-aqueous solvent as necessary. By containing a non-aqueous solvent, the vinylidene fluoride-based polymer can be dissolved or dispersed, and the binder can be made into a liquid state.

[0048] Examples of the non-aqueous solvent include polar solvents (polar solvents). Examples of polar solvents include amide compounds such as dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone; alcohols such as methanol, ethanol, isopropyl alcohol, 2-ethyl-1-hexanol, 1-nonanol, lauryl alcohol, and tripropylene glycol; amine compounds such as o-toluidine, m-toluidine, and p-toluidine; 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide; lactones such as γ-butyrolactone and δ-butyrolactone; and sulfoxide-sulfone compounds such as dimethyl sulfoxide and sulfolane. The binder may contain only one type of non-aqueous solvent or two or more types.

[0049] The amount of the non-aqueous solvent in the binder is preferably 400 parts by mass or more and 5000 parts by mass or less, more preferably 500 parts by mass or more and 5000 parts by mass or less, based on 100 parts by mass of the vinylidene fluoride-based polymer. When the amount of the non-aqueous solvent in the binder is within this range, the vinylidene fluoride-based polymer can be uniformly dispersed or dissolved by the non-aqueous solvent.

[0050] 1-3. Other Components The binder may further contain other resins such as acrylic resins, fillers such as inorganic fillers, and various additives, etc., as long as the object and effect of the present invention are not impaired.

[0051] 2. Electrode Binder The electrode binder contains an active material, a conductive auxiliary agent, and the above-described binder. The electrode binder can be preferably used as an electrode binder for a non-aqueous electrolyte secondary battery described later. The electrode binder may further contain a conductive auxiliary agent, a solvent, and other additives, etc.

[0052] 2-1. Active material The active material may be a positive electrode active material or a negative electrode active material. Among them, the active material is preferably a positive electrode active material.

[0053] The positive electrode active material is not particularly limited, and a general lithium-based positive electrode active material can be used. Examples of the lithium-based positive electrode active material include lithium metal oxides represented by the following formula (3). LiMxO2···(3)

[0054] In formula (3), M represents at least one metal element containing Ni. The metal element other than Ni is preferably selected from the group consisting of Co, Al, Fe, Mn, Cr, and V. More preferably, in addition to Ni, M further contains one or more selected from the group consisting of Co, Mn, and Al. Further, in the lithium metal oxide represented by formula (3), when the total of the metal elements constituting M is 100 mol%, the content ratio of Ni is preferably 55 mol% or more, and more preferably 70% or more.

[0055] In formula (3), 0.5 ≦ x ≦ 1.5, and more preferably 0.7 ≦ x ≦ 1.3.

[0056] The positive electrode active material may be one obtained by coating the surface of the above compound. Also, the positive electrode active material may be a commercially available product.

[0057] Examples of lithium-based positive electrode active materials including the lithium-based positive electrode active material represented by formula (3) include Li 1.0 Ni 0.8 Co 0.2 O2, Li 1.0 Ni 0.5 Mn 0.5 O2, Li 1.00 Ni 0.35 Co 0.34 Mn 0.34 O2 (NCM111), Li 1.00 Ni 0.52 Co 0.20 Mn 0.30O2(NCM523), Li 1.00 Ni 0.50 Co 0.30 Mn 0.20 O2(NCM532), Li 1.00 Ni 0.6 Co 0.2 Mn 0.2 O2(NCM622), Li 1.00 Ni 0.83 Co 0.12 Mn 0.05 O2(NCM811), Li 1.00 Ni 0.85 Co 0.15 Al 0.05 O2(NCA811), LiCoO2 (LCO), and LiFePO4 (LFP) etc. are included.

[0058] In addition, the positive electrode active material may contain a plurality of types of lithium-based positive electrode active materials. For example, from the above lithium-based positive electrode active materials, a plurality of LiNixCoyMnzO2 with different compositions (x, y, and z are the same as the above specific examples) may be included, or LiNixCoyMnzO2 (x, y, and z are as shown in the above specific examples) and LiNixCoyAlzO2 (x, y, and z are as shown in the above specific examples) may be included.

[0059] Examples of the negative electrode active material include carbon materials, silicon materials, metal oxides, and lithium alloys.

[0060] The content of the active material in the electrode binder is appropriately selected according to the use of the electrode binder, etc. It is preferably 40% by mass or more and 99.7% by mass or less with respect to the total amount of the active material, the conductive assistant, and the solid content derived from the above binder. When the content of the active material is within this range, for example, a more sufficient charge-discharge capacity can be obtained, and the battery performance is more likely to be better.

[0061] 2-2. Conductive Assistant The conductive aid is not particularly limited as long as it is a compound that can enhance the conductivity between active materials or between an active material and a current collector. Examples of the conductive aid include acetylene black, ketjen black, carbon black, graphite powder, graphene, carbon nanofiber, carbon nanotube, carbon fiber, and the like.

[0062] The content of the conductive aid in the electrode binder is appropriately selected according to its type and the like. From the viewpoint of further enhancing the conductivity and the dispersibility of the conductive aid, it is preferably 0.1% by mass or more and 15% by mass or less, more preferably 0.1% by mass or more and 7% by mass or less, and still more preferably 0.1% by mass or more and 5% by mass or less with respect to the total amount of the solid content derived from the active material, the conductive aid, and the above binder.

[0063] 2-3. Binder Even when the above-mentioned binder is mixed with the above active material, particularly the positive electrode active material containing a relatively large amount of Ni, it is difficult to cause gelation while maintaining the adhesiveness of the electrode binder to the current collector.

[0064] The content of the binder is not particularly limited, but from the viewpoints of battery performance and adhesiveness, it is preferably 0.2 parts by mass or more and 15 parts by mass or less, more preferably 0.5 parts by mass or more and 10 parts by mass or less, and still more preferably 0.5 parts by mass or more and 4 parts by mass or less with respect to 100 parts by mass of the active material.

[0065] 2-4. Other components The electrode binder may further contain a solvent different from the non-aqueous solvent contained in the above binder. As the solvent, it can be selected from the above-mentioned non-aqueous solvents that can be contained in the binder.

[0066] The total amount of the solvent in the electrode binder (including the amount of the non-aqueous solvent in the binder) is not particularly limited, but usually, it is preferably 20 parts by mass or more and 150 parts by mass or less with respect to 100 parts by mass of the above-mentioned active material.

[0067] The electrode mixture may further contain a dispersant, an adhesion aid, a thickener, etc. Known compounds can be used for these. The amounts of these are not particularly limited as long as the objects and effects of the present invention are not impaired, but are preferably 15% by mass or less based on the total amount of the solid content derived from the binder and the active material.

[0068] In addition, the electrode mixture may further contain nitrogen compounds such as phosphorus compounds, sulfur compounds, organic acids, amine compounds, and ammonium compounds; organic esters, various silane-based, titanium-based, and aluminum-based coupling agents; fluorinated vinylidene-based polymers other than the above-mentioned vinylidene fluoride-based polymers, resins such as polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), and polyacrylonitrile (PAN); and other additives. The amounts of these are not particularly limited as long as the objects and effects of the present invention are not impaired, but are preferably 15% by mass or less based on the total amount of the solid content derived from the binder and the positive electrode active material.

[0069] The viscosity of the electrode mixture is not particularly limited as long as it can further suppress dripping, coating unevenness, or drying delay after coating when forming the electrode mixture layer by coating the electrode mixture, and has a viscosity that provides good workability and coatability during the production of the electrode mixture layer. Usually, the viscosity (slurry viscosity) measured at 20 °C and a rotation speed of 12 rpm with a B-type viscometer is preferably 100 mPa·s or more and 100000 mPa·s or less, more preferably 1000 mPa·s or more and 80000 mPa·s or less, and particularly preferably 2000 mPa·s or more and 70000 mPa·s or less. The viscosity (slurry viscosity) of the electrode mixture in this specification is the value measured 2 minutes after the start of rotation with the above-mentioned B-type rotational viscometer.

[0070] The electrode mixture may be prepared by mixing all the components at once, or some components may be mixed first and the remaining components may be mixed later.

[0071] The above-mentioned binder, electrode mixture, and electrode can be used as the electrodes of a non-aqueous electrolyte secondary battery, preferably as the positive electrode.

[0072] 3. Electrode The electrode may be any one that includes an electrode mixture layer containing the above-described electrode mixture. In the present embodiment, the electrode includes a current collector and an electrode mixture layer disposed on the current collector.

[0073] 3-1. Current Collector The current collector is a terminal for extracting electricity. The current collector is not particularly limited, and a metal foil or metal mesh such as aluminum, copper, iron, stainless steel, steel, nickel, or titanium can be used. Among them, an aluminum foil is preferable as the current collector for the positive electrode.

[0074] 3-2. Electrode Mixture Layer The electrode mixture layer is a layer formed by applying a composition (for example, the above-described electrode mixture) containing an active material, a conductive assistant, and the above-described binder onto a current collector and drying it. The electrode mixture layer may be disposed on only one surface of the current collector or on both surfaces.

[0075] The electrode mixture layer contains at least an active material, a conductive assistant, and a solid content (vinylidene fluoride-based polymer) derived from the above binder, and may further contain various additives such as a dispersant, an adhesion aid, and a thickener as necessary. These are the same as those described for the electrode mixture.

[0076] The thickness of the electrode mixture layer is not particularly limited, but in one example, it is preferably 1 μm or more and 1000 μm or less. Also, the basis weight of the electrode mixture layer formed on one surface of the current collector is not particularly limited, but in one example, it is preferably 50 g / m 2 or more and 1000 g / m 2 or less, more preferably 100 g / m 2 or more and 500 g / m 2 or less.

[0077] The electrode mixture layer can be formed by performing a step of applying the above-described electrode mixture onto a current collector and a step of drying it.

[0078] The method for applying the electrode mixture is not particularly limited, and methods such as the doctor blade method, reverse roll method, comma bar method, gravure method, air knife method, die coat method, and dip coat method can be applied.

[0079] Further, after applying the electrode mixture, it is heated at an arbitrary temperature to dry the non-aqueous solvent. The drying temperature is preferably 60°C or higher and 500°C or lower, more preferably 80°C or higher and 200°C or lower in one example. The heating may be performed multiple times at different temperatures. Note that the solvent in the mixture may be dried under atmospheric pressure, under pressure, or under reduced pressure. Further heat treatment may be performed after drying.

[0080] After applying and drying the electrode mixture, a pressing treatment may be further performed. By performing the pressing treatment, the electrode density can be improved. The pressing pressure is preferably 1 kPa or higher and 10 GPa or lower in one example.

[0081] 4. Non-aqueous electrolyte secondary battery The non-aqueous electrolyte secondary battery according to this embodiment includes the above electrode. In this embodiment, it is preferable that the non-aqueous electrolyte secondary battery includes the above electrode as a positive electrode. Examples of the non-aqueous electrolyte secondary battery include lithium-ion secondary batteries.

[0082] Note that in the above embodiment, an example of using a vinylidene fluoride-based polymer as a binder for a non-aqueous electrolyte secondary battery is shown, but the present invention is not limited thereto, and it may be used as a binder for other secondary batteries such as all-solid-state batteries.

Examples

[0083] Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited thereto.

[0084] 1. Preparation of vinylidene fluoride-based polymer [Preparation of vinylidene fluoride-based polymer 1 (VDF / SA)] Into an autoclave with an internal volume of 2 liters, 1213 g of ion-exchanged water, 27.6 g of a 1.45 mass% SM-100 (manufactured by Shin-Etsu Chemical Co., Ltd.) aqueous solution, 2.2 g of a 50 mass% diisopropyl peroxydicarbonate-HFE-347pc-f solution, 400 g of vinylidene fluoride, and 4 g of a compound represented by the following formula (a compound having atomic groups represented by SA and RX) were charged, and the mixture was heated to 45 °C and reacted. The obtained polymer slurry was dehydrated and dried to obtain a vinylidene fluoride-based polymer 1 (VDF / SA = 100 / 0.34 molar ratio, 99 / 1 mass ratio). [Chemical formula]

[0085] [Preparation of vinylidene fluoride-based polymer 2 (VDF / MSEA)] Into an autoclave with an internal volume of 2 liters, 1213 g of ion-exchanged water, 27.6 g of a 1.45 mass% SM-100 (manufactured by Shin-Etsu Chemical Co., Ltd.) aqueous solution, 2.4 g of a 50 mass% diisopropyl peroxydicarbonate-HFE-347pc-f solution, 400 g of vinylidene fluoride, and 4 g of a compound represented by the following formula (a compound having atomic groups represented by MSEA and RX) were charged, and the mixture was heated to 45 °C and reacted. The obtained polymer slurry was dehydrated and dried to obtain a vinylidene fluoride-based polymer 3 (VDF / MSEA = 100 / 0.36 molar ratio, 99 / 1 mass ratio). [Chemical formula]

[0086] [Preparation of vinylidene fluoride-based polymer 3 (VDF / SA)] In the preparation of the vinylidene fluoride-based polymer 1, except that the blending amount of SA was changed, a vinylidene fluoride-based polymer 3 (100 / 0.24 molar ratio, 99.3 / 0.7 mass ratio) was obtained in the same manner.

[0087] [Preparation of vinylidene fluoride-based polymer 4 (VDF / APS)] Into an autoclave with an internal volume of 2 liters, 1096 g of ion-exchanged water, 0.2 g of Metholose 90SH-100 (manufactured by Shin-Etsu Chemical Co., Ltd.), 2.2 g of a 50 wt% diisopropyl peroxydicarbonate-HFE-347pc-f solution, 426 g of vinylidene fluoride, and an initial addition amount of 0.2 g of acryloyloxypropyl succinic acid (APS) were charged, and the temperature was raised to 26°C in 1 hour. Then, 26°C was maintained, and a 6 wt% aqueous solution of acryloyloxypropyl succinic acid was gradually added at a rate of 0.5 g / min. The obtained polymer slurry was dehydrated and dried to obtain a vinylidene fluoride-based polymer 4 containing polar groups (VDF / APS = 100 / 0.28 molar ratio, 99 / 1 mass ratio). A total of 4.0 g of acryloyloxypropyl succinic acid was added, including the amount initially added.

[0088] [Vinylidene fluoride-based polymer 5 (PVDF)] Polyvinylidene fluoride (manufactured by Kuraray Co., Ltd., KF#7300) was used.

[0089] [Preparation of vinylidene fluoride-based polymer 6 (VDF / CTFE)] Into an autoclave with an internal volume of 2 liters, 1036 g of ion-exchanged water, 0.4 g of Metholose SM-100 (manufactured by Shin-Etsu Chemical Co., Ltd.), 8 g of a 50 wt% diisopropyl peroxydicarbonate-HFE-347pc-f solution, 6.0 g of ethyl acetate, 20 g of a 2 wt% aqueous solution of sodium acid pyrophosphate, 20 g of a 2 wt% aqueous solution of sodium pyrophosphate, 360 g of vinylidene fluoride, and 40 g of chlorotrifluoroethylene (CTFE) were charged, and it was heated to 28°C. The obtained polymer slurry was dehydrated and dried to obtain a vinylidene fluoride-based polymer 6 (VDF / CTFE = 100 / 2.3 molar ratio, 96 / 4 mass ratio).

[0090] [Measurement of inherent viscosity] The inherent viscosity of the vinylidene fluoride-based polymer was measured as follows. First, 80 mg of the vinylidene fluoride-based polymer was dissolved in 20 mL of N,N-dimethylformamide, and the viscosity was measured using an Ubbelohde viscometer in a constant temperature bath at 30°C. Then, from the obtained value, the inherent viscosity (η i ) of the vinylidene fluoride-based polymer was calculated based on the following formula. η i =(1 / C)·ln(η / η0) In the above formula, η is the viscosity of the solution, η0 is the viscosity of N,N-dimethylformamide alone as the solvent, and C is the concentration of the vinylidene fluoride-based polymer in the solution, which is 0.4 g / dL.

[0091]

Table 1

[0092] 2. Preparation, and Evaluation of Electrode Binder and Electrode (1) [Example 1] (Preparation of Binder) As the binder, the above vinylidene fluoride-based polymer 1 was dissolved in N-methyl-2-pyrrolidone (hereinafter also referred to as "NMP") to prepare a 6 mass% vinylidene fluoride-based polymer solution.

[0093] (Preparation of Electrode Binder) To the above vinylidene fluoride-based polymer solution, NCA811 (Li 1.00 Ni 0.85 Co 0.15 Al 0.05 O2) as the active material, carbon black (SP: SuperP (registered trademark) manufactured by Timcal Japan, average particle size: 40 nm, specific surface area: 60 m 2)(g) and NMP were added and kneaded using a Shinkii Awa Taro kneader. Specifically, after first kneading the vinylidene fluoride-based polymer solution and carbon black at 2000 rpm for 1 minute, NCA811 was further added and second kneaded at 2000 rpm for 2 minutes. To this, NMP was further added to adjust the solid content concentration to 75% by mass, and third kneaded at 2000 rpm for 3 minutes to obtain an electrode binder. The mass ratio of NCA811, carbon black, and vinylidene fluoride-based polymer 1 is 100:1.5:1.5.

[0094] (Fabrication of electrode) The obtained electrode binder was coated on a 15-μm-thick aluminum foil as a current collector using a bar coater, and this was first dried at 110 °C for 30 minutes in a constant temperature bath under a nitrogen atmosphere. Next, it was second dried at 130 °C for 2 hours under a nitrogen atmosphere to obtain an electrode (electrode peel measurement sample) with a basis weight of approximately 250 g / m 2 .

[0095] [Examples 2 - 3, Comparative Examples 1 - 3] Electrode binders and electrodes were prepared in the same manner as in Example 1, except that vinylidene fluoride-based polymer 1 was changed to the vinylidene fluoride-based polymers shown in Table 1.

[0096] [Evaluation] (1) Storage test 1 of electrode binder The slurry of the above-prepared electrode binder was stored at 25 °C under a nitrogen atmosphere for a predetermined time (24 hours, 96 hours, 168 hours, 264 hours). Then, the viscosities of the slurry before and after storage were measured at 25 °C and a shear rate of 2 s -1 using an E-type viscometer. The viscosity measurement was carried out by charging the slurry into the measuring device, waiting for 60 seconds, and then rotating the rotor. Also, the value 300 seconds after the start of rotor rotation was taken as the viscosity of the slurry.

[0097] (2) Electrode peel strength test The fabricated electrode was cut into pieces with a length of 100 mm and a width of 20 mm. Then, according to JIS F6854-1, a tensile testing machine (ORIENTE CHSIA-1150 UNIVERSAL TESTING MACHINE) was used to conduct a 90° peel test at a head speed of 10 mm / min to measure the peel strength.

[0098] The evaluation results of Examples 1 to 3 and Comparative Examples 1 to 3 are shown in Table 2. Also, a graph showing the change in viscosity over time is shown in Figure 1.

[0099]

Table 2

[0100] As shown in Table 2, the slurry of Comparative Example 3 containing the vinylidene fluoride-based polymer 6 containing a structural unit derived from CTFE was suppressed from thickening over time. However, it can be seen that the peel strength is lower than that of Comparative Example 2 containing the vinylidene fluoride-based polymer 5 (homopolymer), and the adhesiveness decreases. On the other hand, it can be seen that the slurry of Comparative Example 1 containing the vinylidene fluoride-based polymer 4 containing a structural unit derived from APS thickens significantly over time.

[0101] In contrast, it can be seen that the slurries of Examples 1 to 3 containing a vinylidene fluoride-based polymer containing a structural unit derived from a compound (SA, MSEA) containing a sulfonyl group can achieve both high adhesiveness and suppression of thickening.

[0102] 3. Preparation and Evaluation of Electrode Binder (2) [Comparative Examples 4 to 7] (Preparation of Binder) The above vinylidene fluoride-based polymer 5 (#7300) was dissolved in NMP to prepare a 6 mass% vinylidene fluoride-based polymer solution.

[0103] (Preparation of Electrode Binder) To the above vinylidene fluoride-based polymer solution, NCA811 (Li 1.00 Ni 0.85 Co 0.15 Al0.05 O2), carbon black as a conductive assistant (SP: SuperP (registered trademark) manufactured by Timcal Japan, average particle size: 40 nm, specific surface area: 60 m 2 / g) and NMP were added and kneaded using a Shinchi Mixer. Specifically, after kneading the active material and carbon black at 800 rpm for 1 minute for the first time, the vinylidene fluoride-based polymer solution was further added and kneaded at 2000 rpm for 2.5 minutes for the second time. To this, the NMP solution of the additive was further added to adjust the solid content concentration to 75% by mass, and kneaded at 2000 rpm for 3 minutes for the third time to obtain an electrode mixture containing NCA811 / SP / binder / additive = 100 / 2 / 2 (mass ratio) and the additives shown in Table 2.

[0104] [Example 4] (Preparation of Binder) A vinylidene fluoride-based polymer solution was prepared in the same manner as in Comparative Example 4, except that vinylidene fluoride-based polymer 5 was changed to vinylidene fluoride-based polymer 1.

[0105] (Preparation of Electrode Mixture) An electrode mixture was prepared in the same manner as in Comparative Example 4, except that the obtained vinylidene fluoride-based polymer solution was used and no additive was added.

[0106] [Evaluation] (Storage Test 2 of Electrode Mixture) The slurry of the above-prepared electrode mixture was stored at 40 °C under a nitrogen atmosphere for a predetermined time (3 hours, 6 hours, 24 hours, 48 hours, 72 hours, 96 hours, 168 hours). Then, the viscosities of the slurry before and after storage were measured at 25 °C and a shear rate of 2 s -1 using an E-type viscometer. The viscosity was measured by charging the slurry into the measuring device, waiting for 60 seconds, and then rotating the rotor. Also, the value 300 seconds after the start of rotor rotation was taken as the viscosity of the slurry.

[0107] The evaluation results of Example 4 and Comparative Examples 4 to 7 are shown in Table 3. Also, a graph showing the change in viscosity with the passage of time is shown in Figure 2.

[0108]

Table 3

[0109] As shown in Table 3, it can be seen that in the electrode binders of Comparative Examples 4 to 7 in which a sulfone-containing compound was added to the binder of the vinylidene fluoride homopolymer, the thickening over time was remarkable.

[0110] On the other hand, it can be seen that the electrode binder of Example 4 using a binder of a vinylidene fluoride-based polymer containing a structural unit derived from a compound containing a sulfonyl group (SA, MSEA) has suppressed thickening over time.

[0111] From these facts, it can be understood that in order to suppress the thickening of the electrode binder, it is not sufficient that only a sulfone-containing structure (-SO2-) is present in the slurry, and it is necessary to contain a polymer having a structural unit containing sulfone in the slurry.

Industrial Applicability

[0112] The binder of the present invention can suppress the thickening of the slurry while maintaining the adhesiveness of the electrode binder to the current collector, even when mixed with a positive electrode active material containing a large amount of nickel in particular. Therefore, the binder, the electrode binder containing the same, and the electrode are very useful for manufacturing non-aqueous electrolyte secondary batteries including lithium ion secondary batteries.

Claims

1. A binder for a secondary battery, containing a vinylidene fluoride-based polymer including a structural unit derived from vinylidene fluoride and a structural unit derived from a compound having an atomic group represented by RX, RX is an atomic group containing -SO ２ -Ra, and wherein Ra is an atomic group containing at least one carbon atom. A binder for a secondary battery.

2. The compound having an atomic group represented by RX is a compound represented by the following formula (1). The binder for a secondary battery according to Claim 1. 【Chemical 1】 (In formula (1), R １ , R ２ , R ３ are each independently a hydrogen atom, a chlorine atom, a fluorine atom, an alkyl group having 1 to 6 carbon atoms, or a fluorine-substituted alkyl group having 1 to 6 carbon atoms, RX is an atomic group containing -SO ２ -Ra, and Ra is an atomic group containing at least one carbon atom.)

3. The compound having an atomic group represented by RX is a compound represented by the following formula (2). The binder for a secondary battery according to Claim 1. [Chemical Formula 2] (In formula (2), R １ 、 R ２ 、 R ３ are each independently a hydrogen atom, a chlorine atom, a fluorine atom, an alkyl group having 1 to 6 carbon atoms or a fluorine-substituted alkyl group having 1 to 6 carbon atoms, R ４ is a group containing -SO ２ -Ra, Ra is an atomic group containing at least one carbon atom.)

4. R ４ is a group containing -(CH ２ )n-SO ２ -Ra, Ra is an alkyl group, The alkyl group and -(CH ２ )n- may be bonded to the carbon atom to form a ring, n is an integer of 1 or more. The binder for a secondary battery according to Claim 3.

5. The content of the structural unit derived from the compound having an atomic group represented by RX is 0.5 part by mass or more and 2 parts by mass or less with respect to 100 parts by mass of the total amount of the structural unit derived from vinylidene fluoride and the structural unit derived from the compound having an atomic group represented by RX. The binder for a secondary battery according to Claim 1.

6. An electrode binder, including an active material, a conductive assistant, and the binder for a secondary battery according to any one of Claims 1 to 5. An electrode binder.

7. The active material is a positive electrode active material. The electrode binder according to Claim 6.

8. The positive electrode active material includes a compound represented by the following formula (3). The electrode binder according to Claim 7. LiMxO ２ ...(3) (In formula (3), M represents at least one metal element containing Ni, and when the total of the metal elements represented by M is 100 mol%, the content ratio of Ni is 55 mol% or more. 0.5 ≤ x ≤ 1.5)

9. A current collector, and an electrode binder layer including the electrode binder according to Claim 6 held on the current collector. An electrode, including an electrode.

10. A non-aqueous electrolyte secondary battery, including the electrode according to Claim 9. A non-aqueous electrolyte secondary battery.

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