Blends of fluoropolymers and functionalized acrylic polymers as binders for electrochemical devices

Abstract

The present invention relates to polymer blends consisting of fluoropolymers and functional acrylic copolymers. The fluoropolymer constitutes the majority of the blend, at least 80% by weight. The fluoropolymer can be polyvinylidene fluoride (PVDF) and its copolymers. The VDF copolymers can contain fluorinated comonomers such as hexafluoropropylene (HFP), tetrafluoroethylene (TFE), or functionalized monomers such as vinyl carboxylic acids, phosphoric acids, sulfonic acids and their salts. The functional acrylic copolymers are poly(methyl)methacrylate copolymers with functional group-containing monomers.

Description

Technical Field

[0001] The present invention relates to blends of fluoropolymers and acrylic polymers for use as binders for electrodes or coatings for separators in electrochemical devices.

Background Art

[0002] VDF-based fluoropolymers, PVDF and its copolymers are used as binders for electrodes or coatings for separators in electrochemical devices such as lithium-ion batteries. The most common use of PVDF in lithium-ion batteries is as a binder for the positive electrode, but in some cases it is also used as a binder for the negative electrode. Another use of PVDF in lithium-ion batteries is as a coating layer for the separator. When applying a binder, one of the important properties is the adhesion / cohesion of the composite electrode structure. A typical positive electrode of a lithium-ion battery is a composite porous structure composed of an active material coated on an aluminum foil, a conductive carbon additive, and a binder. The adhesion / cohesion of the composite electrode can be characterized by a 180° peel test.

[0003] Examples of active materials for the positive electrode of a lithium-ion battery include lithium cobalt oxide, lithium iron phosphate, lithium manganese oxide, lithium nickel manganese cobalt oxide, lithium nickel cobalt aluminum oxide, etc. The active material usually occupies more than 90% of the weight of the positive electrode composite. Examples of conductive carbon additives include carbon black, carbon fiber, carbon nanotube, graphite, graphene, etc. Its main function is to provide an electron conduction network, and its proportion in the cathode composite material is usually 0.5 to 5% by weight.

[0004] In typical lithium-ion battery cathodes, PVDF or its copolymers are the main binders used industrially. The binder composition in the cathode is typically 0.5–5% by weight. One important requirement for the binder is to provide sufficient adhesion / cohesive force to the composite electrode structure.

[0005] International Publication No. 9732347 describes an electrode for a battery, comprising an electrode-forming substrate containing a binder used for fixation to the surface of a battery current collector. This binder comprises a PVDF (polyvinylidene fluoride) homopolymer or copolymer grafted with at least one acrylic polymer containing acrylic acid and / or methacrylic acid ester groups, wherein the weight content of the grafted acrylic polymer ranges from 0.1% to 20% of the binder.

[0006] International Publication No. 9749777 describes binders that can be used for metal fixation, comprising polyvinylidene fluoride polymers, acrylic or methacrylic polymers containing metal-fixable functional groups, and acrylic or methacrylic type elastomers. This type of binder cannot be used for electrodes in lithium-ion batteries because the elastomer expands upon contact with the electrolyte, damaging the electrodes.

[0007] U.S. Patent Application Publication No. 2013 / 252077 describes an electrode for a lithium-ion battery operating with a non-aqueous electrolyte. This electrode comprises an active material and a binder containing vinylidene fluoride polymer and acrylic polymer. The weight content of the acrylic polymer varies from 40% to 90%, and therefore, when in permanent contact with the electrolyte, a problem of electrode degradation arises as a result of swelling of the acrylic polymer, especially when the temperature inside the electrode is higher than the ambient temperature.

[0008] European Patent Application Publication No. 2953193 describes a binder for lithium-ion batteries comprising a fluoropolymer and an acrylic polymer containing nitrile groups.

[0009] International Publication No. 97 / 27260 describes an electrode comprising a metallic current collector coated with a layer containing an active material and a binder. This binder comprises at least two of three components: vinylidene fluoride polymer, acrylic or methacrylic polymer containing metal-fixable functional groups, and vinylidene fluoride copolymer. When the binder contains only vinylidene fluoride polymer and acrylic or methacrylic polymer, the latter is present in a proportion ranging from 0.5% to 20% by weight of the total weight of the binder. In practice, it has been found that the weight of the binder used must be considerably large to obtain good cohesive force of the active layer and good adhesion of the active layer to the metal collector. Furthermore, swelling of the cathode upon contact with the electrolyte is also observed. The vinylidene fluoride copolymers considered in this document are Kynar® 500 and Kynar® 301F. These copolymers exhibit melt flow index (MFR) measured at 1.2 g / 10 min under 12.5 kg or 4 g / 10 min under 21.6 kg. Solutions of each of these copolymers dissolved in N-methyl-2-pyrrolidone at a concentration of 5 wt% exhibit a viscosity of 75 mPa·s at 23°C. Such viscosity values ​​proved unsuitable for use as binders in lithium-ion batteries at a content of less than 5 wt% in the substrate layer covering the metal collector of the battery.

[0010] Furthermore, it is known that high molecular weight vinylidene fluoride homopolymer is used as a binder for lithium battery electrodes. When measured at a controlled shear rate of 30 revolutions / minute, a 5% solution in N-methyl-2-pyrrolidone exhibits a viscosity exceeding 100 mPa. This binder has the advantage of limited swelling and a low extract content in the electrolyte; that is, a small amount of product originating from the binder migrates into the electrolyte during electrode use. Due to the high molecular weight of the aforementioned PVDF homopolymer, good adhesion is obtained. On the other hand, because this polymer has a high molecular weight, it is viscous, making it difficult to spread a paste consisting of a mixture of this binder and electrode active material onto a metal current collector.

[0011] One example is the use of a functionalized acrylic additive, as disclosed in U.S. Patent Application No. 2018 / 0355206. U.S. Patent Application Publication No. 2018 / 0355206 teaches that the acrylic copolymer has 10 mol% of an acid-containing monomer, namely methacrylic acid, in order to obtain better peelability. [Overview of the Initiative] [Problems that the invention aims to solve]

[0012] These standard acrylic copolymers do not possess the necessary stability for battery applications.

[0013] In order to reduce the amount of binder added and increase the overall energy density of the battery, there is a continuing need in the battery industry to improve the adhesion / aggregation properties of binders. [Means for solving the problem]

[0014] Surprisingly, the present invention demonstrates that acrylic copolymers with a functional value of less than 10 mol% can significantly enhance the adhesion / cohesion of PVDF binders. The present invention also discloses that comonomers other than MAA may have a similar effect in enhancing adhesion / cohesion. The present invention provides a blend of acrylic and PVDF with high Tg that provides a binder with excellent bonding adhesion for use in batteries.

[0015] The present invention relates to a polymer blend comprising a fluoropolymer and at least one functional acrylic copolymer. The fluoropolymer constitutes the majority of the blend, accounting for 80% by weight or more, preferably 90% by weight or more.

[0016] The fluoropolymer is preferably a polyvinylidene fluoride (PVDF) homopolymer or copolymer.

[0017] The functional acrylic copolymer is a poly(methyl methacrylate) copolymer containing more than 0.5 mol% and less than 10 mol% of functional acrylic monomer units, preferably 0.5 to 8 mol%, and more preferably 1 to 8 mol%, and optionally contains hydrophobic monomers.

[0018] Functionalized acrylic polymers constitute 1% or more by weight in the polymer blend.

[0019] Embodiments of the Invention

[0020] Embodiment 1: A binder for lithium-ion batteries comprising at least one vinylidene fluoride polymer and at least one acrylic copolymer, wherein the acrylic copolymer comprises a monomer having a functional group that exhibits affinity for or can be immobilized on a metal, the acrylic copolymer has a Tg greater than 110°C and comprises a functional monomer having at least one functional group selected from the group consisting of carboxyl, hydroxyl, carboxylic acid anhydride and epoxy, the vinylidene fluoride polymer has a viscosity of 125 millipascal seconds or more and less than 2000 millipascal seconds when measured at 23°C at a controlled shear rate of 30 revolutions / min in a 5 wt% N-methyl-2-pyrrolidone solution thereof, the acrylic copolymer comprises less than 10 mol percent, preferably 8 mol percent or less, most preferably 7 mol percent or less of the functional monomer, and the fluoropolymer comprises more than 80 wt% of the total weight of the polymers in the polymer blend.

[0021] Embodiment 2: The binder according to Embodiment 1, wherein the binder contains 2% by weight or more and 15% by weight or less of an acrylic copolymer based on the total weight of the polymers in the polymer blend.

[0022] Embodiment 3: The binder according to Embodiment 2, wherein the binder contains 10% by weight or less of an acrylic copolymer.

[0023] Aspect 4: The viscosity of the solution of 5% by weight of PVDF is 300 millipascal seconds or more and less than 1500 millipascal seconds, and the binder according to any one of Aspects 1 to 3.

[0024] Aspect 5: The vinylidene fluoride polymer is a copolymer containing at least one monomer selected from the group consisting of functional monomers such as hexafluoropropylene (HFP), tetrafluoroethylene (TFE), vinyl carboxylic acid, phosphoric acid, sulfonic acid, and salts, and the binder according to any one of Aspects 1 to 4.

[0025] Aspect 6: The acrylic copolymer has a molecular weight exceeding 80,000 g / mol, preferably exceeding 100,000 g / mol, and the binder according to any one of Aspects 1 to 5.

[0026] Aspect 7: The acrylic copolymer has high heat resistance measured as having a Tg higher than 110°C, preferably higher than 115°C, more preferably higher than 120°C, and the binder according to any one of Aspects 1 to 6.

[0027] Aspect 8: The acrylic copolymer contains a functional monomer containing at least one functional group selected from a carboxyl group and a hydroxyl group, and the binder according to any one of Aspects 1 to 7.

[0028] Aspect 9: The acrylic copolymer contains a poly(methyl methacrylate) copolymer containing a functional monomer having a carboxyl functional group, and the binder according to any one of Aspects 1 to 7.

[0029] Aspect 10: The acrylic copolymer contains methyl methacrylate units and methacrylic acid units, and the binder according to any one of Aspects 1 to 7.

[0030] Embodiment 11: The binder according to any one of Embodiments 1 to 7, wherein the acrylic copolymer comprises methyl methacrylate units and carboxylalkyl acrylate units or carboxylalkyl methacrylate units.

[0031] Embodiment 12: The binder according to any one of Embodiments 1 to 11, wherein the acrylic copolymer further comprises a hydrophobic monomer.

[0032] Embodiment 13: The binder according to Embodiment 12, wherein the hydrophobic monomer is an acrylic monomer having a substituted cycloalkane group.

[0033] Embodiment 14: The binder according to Embodiment 12, wherein the hydrophobic monomer is selected from the group consisting of carboxylalkyl acrylate monomers or oligomers, for example, tert-butylcyclohexyl (meth)acrylate, 3,3,5-trimethylcyclohexyl (meth)acrylate, isobornyl methyl acrylate (IBOMA), and isobornyl acrylate (IBOA).

[0034] Embodiment 15: The binder according to any one of Embodiments 1 to 14, wherein the mol% of the hydrophobic monomer units is 0 to 15 mol%, preferably 0.5 to 10 mol%.

[0035] Embodiment 16: An electrode for a lithium-ion battery in which at least one side of a metal current collector is covered with a layer of a substrate containing an active material and a binder according to any one of Embodiments 1 to 15.

[0036] Embodiment 17: The electrode according to Embodiment 16, wherein the substrate contains 0.5% or more and 5% or less of the binder by weight.

[0037] Embodiment 18: The electrode according to Embodiment 16, wherein the substrate contains 1% or more and 3% or less of the binder by weight.

[0038] Embodiment 19: The electrode according to any one of Embodiments 16 to 18, wherein the active material comprises a lithium metal oxide and optionally carbon black.

[0039] Embodiment 20: The electrode according to any one of Embodiments 16 to 18, wherein the active material comprises at least one component selected from coke, carbon black, graphite, activated carbon, and carbon fiber.

[0040] Embodiment 21: The electrode according to any one of Embodiments 16 to 20, wherein the viscosity of the solution of 5% by weight PVDF is 300 millipascal seconds or more and less than 1200 millipascal seconds. [Modes for carrying out the invention]

[0041] The references cited in this application are incorporated herein by reference.

[0042] Percentages used herein are weight percentages (W%) unless otherwise specified, and molecular weights are weight-average molecular weights (Mw) unless otherwise specified. Molecular weight is measured by gel permeation chromatography (GPC) using a PMMA (polymethyl methacrylate) standard. Melt viscosity (MV) is measured at 230°C and 100 sec⁻¹. Glass transition temperature is measured using differential scanning calorimetry (DSC) according to ASTM 3418. The glass transition temperature of acrylic polymers was measured during the second heating at a heating rate of 10°C / min in N₂. In the first heating, the sample was heated to 170°C at a heating rate of 10°C / min, and then cooled to 0°C at a cooling rate of 10°C / min. Melt viscosity is measured by capillary rheometry at 230°C and 100 sec⁻¹ according to ASTM D3835.

[0043] The term "copolymer" is used to mean a polymer having two or more different monomer units, including terpolymers and higher-order polymers. The term "polymer" is used to mean both homopolymers and copolymers. For example, as used herein, "PVDF" and "polyvinylidene fluoride" are used to mean both homopolymers and copolymers unless otherwise specified. The polymer may be uniform, heterogeneous, or random, and may have a gradient distribution of comonomer units.

[0044] As used herein, "(meth)acrylic" or "(meth)acrylate" means both acrylate and methacrylate. "(meth)acrylate" is used to mean both acrylate and methacrylate, and mixtures thereof. Polymers may be linear, branched, star-shaped, comb-shaped, block-shaped, or otherwise.

[0045] "Amphiphilic polymers" are long-chain molecules that contain both hydrophobic and hydrophilic components.

[0046] The present invention relates to a binder and related electrodes comprising a functionalized acrylic polymer and PVDF, which can be used in lithium-ion batteries. The functionalized acrylic polymer preferably contains less than 10% by weight of functionalized monomer units, more preferably 8 mol% or less of functionalized monomer units.

[0047] The present invention relates to a binder that can be used in lithium-ion batteries and comprises at least one vinylidene fluoride polymer and at least one acrylic copolymer having a monomer that exhibits affinity for metals or can be immobilized on metals. The acrylic copolymer is a copolymer of methyl methacrylate and a functional acrylic monomer.

[0048] The object of the present invention is to provide the above-mentioned binder that provides good adhesion between a metal and a layer of PVDF-containing material.

[0049] Another object of the present invention is to provide a binder that enables the active material to be easily spread on a metal current collector, thereby facilitating the manufacture of electrodes for lithium-ion batteries.

[0050] Another object of the present invention is to provide a binder that reduces the use of organic solvents during the electrode processing step.

[0051] Another object of the present invention is to provide electrodes for lithium-ion batteries.

[0052] Another object of the present invention is to provide an electrode containing a binder with a relatively low weight content so as to allow for an increase in the content of the active filler in the cathode to maximize the capacity of the battery.

[0053] binder

[0054] The present invention relates to a polymer blend comprising a fluoropolymer and at least one functional acrylic copolymer for use as a battery binder. The fluoropolymer constitutes the majority of the polymer blend, accounting for 80% by weight or more, preferably 80-98% by weight, and more preferably 90-98% by weight. Preferably, the binder contains 2% by weight or more and 20% by weight or less, particularly 15% by weight or less or 10% by weight or less, of the acrylic copolymer based on the total polymer in the binder.

[0055] Blending functional acrylic polymers with fluoropolymers improves mechanical properties, such as increased adhesion, due to dipole interactions between PVDF and acrylic copolymers. The fluoropolymer / acrylic blend of the present invention is suitable for electrode binder or separator coating applications in lithium-ion batteries where improved adhesion / aggregation or bond strength is desired. The functional acrylic copolymer is contained in the acrylic copolymer at a concentration of less than 10 mol%, preferably less than 8 mol%, of monomer units having functional groups. The mol% of functional monomers in the acrylic polymer is 0.5 to less than 10 mol%, preferably 1 to 8 mol%. Preferably, the functional group is a carboxylic acid functional group.

[0056] The presence of the acrylic copolymer is expected to reduce the viscosity of the binder / active material mixture, thereby facilitating the application of the active material during electrode manufacturing. Furthermore, the reduced viscosity of the binder, and consequently the binder / active material mixture, reduces the amount of organic solvent that must be used in electrode manufacturing. Therefore, the use of the binder according to the present invention is more environmentally friendly.

[0057] The binder of the present invention does not contain an elastomeric polymer or an elastomeric copolymer, particularly an acrylic-based elastomeric (co)polymer, and acrylic-based polymers are not elastomers.

[0058] The fluoropolymer / acrylic blend of the present invention is not an acrylic-modified fluoropolymer as described in U.S. Patent No. 6,680,357 or U.S. Patent No. 6,635,714. In the present invention, the polymers are polymerized in separate polymerization processes and then mixed or blended together by physical means.

[0059] Fluoropolymer

[0060] The PVDF of the present invention is a homopolymer of vinylidene fluoride, or a copolymer having more than 50% by weight of vinylidene fluoride monomer units, wherein the vinylidene fluoride monomer preferably exceeds 65% by weight, more preferably exceeds 75% by weight, and most preferably exceeds 90% by weight.

[0061] The vinylidene fluoride polymer copolymer includes those containing at least 50% by weight, preferably at least 75% by weight, more preferably at least 80% by weight, and even more preferably at least 90% by weight of vinylidene fluoride copolymerized with one or more comonomers. Exemplary comonomers can be selected from the group consisting of: tetrafluoroethylene (TFE), trifluoroethylene (TrFE), chlorotrifluoroethylene (CTFE), 1,2-difluoroethylene, perfluorobutylethylene (PFBE), hexafluoropropene (HFP), vinyl fluoride (VF), pentafluoropropene, tetrafluoropropene, trifluoropropene, fluorinated (alkyl) vinyl ethers (e.g., perfluoroethyl vinyl ether (PEVE), perfluoro-2-propoxypropyl vinyl ether, perfluoromethyl vinyl ether (PMVE), perfluoropropyl vinyl ether (PPVE), perfluorobutyl vinyl ether (PBVE), long-chain perfluorovinyl ethers), and other monomers that readily copolymerize with vinylidene fluoride, one or more partially or fully fluorinated alphaolefins (e.g., 3,3,3-trifluoro-1-propene, 2-trifluoromethyl -3,3,3-trifluoropropene, 1,2,3,3,3-pentafluoropropene, 3,3,3,4,4-pentafluoro-1-butene, hexafluoroisobutylene (HFIB)), fluorinated dioxoles (e.g., perfluoro(1,3-dioxole) and perfluoro(2,2-dimethyl-1,3-dioxole) (PDD), partially or perfluorinated alphaolefins of C4 or higher, partially or perfluorinated cyclic alkenes of C3 or higher, allyl monomers, partially fluorinated allyl monomers, or fluorinated allyl monomers (e.g., 2-hydroxyethyl allyl ether and 3-allyloxypropanediol), and ethene or propene, and combinations thereof. Other monomer units in these polymers may include any monomer containing a polymerizable C=C double bond. Additional monomers include 2-hydroxyethyl allyl ether, 3-allyloxypropanediol, allyl monomers, ethene or propene, acrylic acid, and methacrylic acid.

[0062] In one preferred embodiment, the fluoropolymer is an acid-functionalized fluoropolymer, preferably an acid-functionalized PVDF.

[0063] Methods for producing acid-functionalized fluoropolymers are known in the art. International Publication No. 2019 / 199753, International Publication No. 2016149238, and U.S. Patent No. 8,337,725 provide several known methods for producing acid-functionalized fluoropolymers, the contents of which are incorporated herein by reference.

[0064] In one embodiment, up to 30% by weight, preferably up to 25% by weight, and more preferably up to 15% by weight, of hexafluoropropene (HFP) units and 70% or more by weight, preferably 75% or more by weight, and more preferably 85% or more by weight, or more of VDF units are present in the vinylidene fluoride polymer.

[0065] Preferably, the vinylidene fluoride polymer is such that, when measured at 23°C with a controlled shear rate of 30 revolutions / min, the viscosity of a solution of N-methyl-2-pyrrolidone containing 5% by weight of the vinylidene fluoride polymer is 125 mPa·s or more, preferably 300 mPa·s or more, preferably 300 mPa·s or more but less than 2000 mPa·s, less than 1500 mPa·s, and preferably less than 1200 mPa·s.

[0066] The vinylidene fluoride polymer of the type described above has a molecular weight of about 1 million grams and is already used as a binder in lithium-ion batteries. Mixing it with an acrylic copolymer can lower the viscosity of the binder, and therefore the viscosity of the paste used in the manufacture of lithium-ion battery electrodes can be lowered. Thus, the manufacture of electrodes becomes easier. However, it was not clear that adding a functionalized acrylic polymer having a much lower molar mass than PVDF in an amount of less than 10 weight percent, preferably less than 8 weight percent, significantly increased the adhesive strength. This is because it is known to those skilled in the art that the higher the molar mass of the binder, the more satisfactory the adhesion of the binder to the metal plate becomes, and the cohesive force of the electrode containing this binder also improves.

[0067] It is possible to manufacture electrodes for lithium-ion batteries with a reduced amount of binder, which allows for an increase in the content of active fillers in the cathode, and therefore an increase in the charging capacity of the cathode.

[0068] Acrylic polymer

[0069] The acrylic polymer of the present invention contains a majority of polymethyl acrylate monomer units (more than 50%, preferably more than 80 mol%). The acrylic polymer contains less than 10 mol%, preferably 8 mol% or less, or 7% or less, of acrylic monomer units having a functional group ("functional monomer"). Functional groups that can be fixed to a metal, or functional groups that exhibit affinity for a metal, are well known to those skilled in the art. These may contain, for example, at least one group selected from the group consisting of carboxylic acids, hydroxyls, carboxylic acid anhydrides, and epoxys. Preferably, the acrylic copolymer contains monomers containing a carboxylic acid functional group, most preferably containing a carboxylic acid functional group.

[0070] In some embodiments, the acrylic copolymer comprises carboxylalkyl acrylate units or carboxylalkyl methacrylate units.

[0071] Non-limiting examples of monomers having functional groups include 2-carboxyethyl acrylate (CEA), acrylic acid, and (meth)acrylic acid such as methacrylic acid.

[0072] In some embodiments, the acrylic copolymer includes hydrophobic monomer units in addition to monomers containing functional groups, resulting in an amphiphilic acrylic copolymer. The mole percentage of hydrophobic monomer units is 0 to 15 mol%, or 0.5 to 10 mol%, or 0.5 to 8 mol%. An example of a hydrophobic monomer is an acrylic monomer having a substituted cycloalkane group.

[0073] In some embodiments, the hydrophobic acrylic copolymer comprises carboxylalkyl acrylate monomer units or oligomers, such as tert-butylcyclohexyl (meth)acrylate, 3,3,5-trimethylcyclohexyl (meth)acrylate, isobornyl methyl acrylate (IBOMA), and isobornyl acrylate (IBOA).

[0074] The acrylic copolymer may optionally contain additional acrylate and methacrylate monomers, or other ethylenically unsaturated monomers, including but not limited to styrene, alpha-methylstyrene, and acrylonitrile. Suitable acrylate and methacrylate comonomers include, but are not limited to, methyl acrylate, ethyl acrylate and ethyl methacrylate, butyl acrylate and butyl methacrylate, isooctyl methacrylate and isooctyl acrylate, lauryl acrylate and lauryl methacrylate, stearyl acrylate and stearyl methacrylate, isobornyl acrylate and isobornyl methacrylate, methoxyethyl acrylate and methoxy methacrylate, 2-ethoxyethyl acrylate and 2-ethoxyethyl methacrylate, and dimethylaminoethyl acrylate and dimethylaminoethyl methacrylate monomers.

[0075] In some embodiments, the acrylic polymer is PMMA / hydrophilic 2-carboxyethyl acrylate (CEA) with a Tg greater than 100°C and a MW greater than 100,000 g / mol. In some embodiments, the acrylic polymer is a PMMA / hydrophilic 2-carboxyethyl acrylate (CEA) polymer, further comprising hydrophobic monomer units selected from the group consisting of tert-butylcyclohexyl methacrylate, 3,3,5-trimethylcyclohexyl (meth)acrylate, isobornyl methyl acrylate (IBOMA), and isobornyl acrylate (IBOA), and having a Tg greater than 100°C and a MW greater than 100,000 g / mol. High heat-resistant acrylic copolymers containing functional CEA maintain a high Tg and improve adhesion.

[0076] Acrylic copolymers have high heat resistance, measured as having a Tg of over 110°C, preferably over 115°C, and more preferably over 120°C, with the Tg generally being in the range of 110°C to 140°C.

[0077] The weight-average molecular weight of the acrylic copolymer is higher than 65,000 g / mol, preferably higher than 80,000 g / mol, and more preferably higher than 100,000 g / mol.

[0078] Acrylic copolymers or mixtures of acrylic(co)polymers are not elastomers; that is, they do not exhibit a glass transition temperature below 20°C.

[0079] Advantageously, the acrylic copolymer contains less than 10 mol% of monomers having functional groups, preferably 8 mol% or less of groups that exhibit affinity for metals or can be immobilized on metals, preferably acidic functional groups, and optionally contains hydrophobic monomers. The applicant company has demonstrated that such copolymers provide good adhesion to materials containing them that are deposited on metal sheets.

[0080] electrode

[0081] The present invention also relates to an electrode for a lithium-ion battery of the type that includes a metal collector, at least one side of which is covered with a layer of substrate. The layer of substrate contains an active material and a binder characterized by the binder according to the present invention, or comprising the binder according to the present invention.

[0082] Active materials that can be used to form anodes or cathodes are well known to those skilled in the art.

[0083] The electrode may be a cathode, in which case the substrate may contain lithium metal oxide and optionally carbon black as the active material.

[0084] The electrode may be an anode, in which case the substrate may contain at least one component selected from coke, carbon black, graphite, activated carbon, and carbon fiber as an active material.

[0085] Solution viscosity is measured using a Brookfield rotational viscometer with an SC4-34 type spindle.

[0086] The present invention, its features, and the various advantages it offers will become clearer upon reading the following examples, which are provided as descriptive and non-limiting examples. [Examples]

[0087] PVDF1 is a VDF homopolymer having a melt viscosity of 4450 Pa·s to 5450 Pa·s at 230°C and 100 s⁻¹, according to ASTM D3835.

[0088] Acrylic PMMA-MAA with MAA content of 10% or less, added to a PVDF-based binder at a concentration of <0.1 to 10% by weight. An example of the base PVDF was PVDF1 for cathode binder applications. Table 1 shows the functionalized PMMA copolymers used in the examples and their main properties.

[0089] [Table 1]

[0090] Preparation of polymer blend binder solution

[0091] An 8.0 wt% PVDF solution was prepared by adding 92 g of N-methyl-2-pyrrolidone (NMP, Biograde of Alfa Aesar) to 8.0 g of PVDF1 and mixing overnight in a roll mixer heated to approximately 60°C. An 8.0 wt% acrylic solution was prepared by dissolving 1.0 g of acrylic copolymer in 11.5 g of NMP and mixing in the same manner.

[0092] One method for creating a uniform polymer blend is by solution blending. Appropriate amounts of PVDF solution and acrylic solution are added to a bail and mixed by rotation overnight. For example, to create a 95:5 PVDF / acrylic blend solution, 19 g of 8 wt% PVDF solution and 1 g of 8 wt% acrylic solution are mixed. In all cases, the total binder (PVDF + acrylic) concentration is 8.0 wt%.

[0093] Cathode formulation and manufacturing

[0094] Here, we describe two examples of laboratory-scale cathode slurry preparation procedures. Process #1 involves first mixing carbon black with a binder solution, and then mixing it with the active material. Process #2 involves mixing carbon black and active material as dry powders, and then mixing them with a binder solution. Both processes are used in the lithium-ion battery industry. The following procedure is laboratory-scale and uses a target formulation of NMC622 / SuperP / binder = 97 / 1.5 / 1.5 on a dry basis.

[0095] Slurry process #1

[0096] 0.36 g of Timcal's conductive carbon additive SuperP-Li is added to 4.5 g of an 8.0% binder solution and mixed using a Thinky AR-310 centrifugal planetary mixer at 2000 rpm for 120 seconds, followed by 1 minute of air cooling, repeated three times. Once the conductive carbon is dispersed in the binder solution, 23.28 g of the active material Celcore® NMC622 (Umicore) and a small amount of NMP (0.5 g) are added to the mixture and mixed typically at 2000 rpm for 60 seconds to form a thick, uniform paste. Then, a small amount of NMP (0.5 g) is added to the paste and mixed for 60 seconds at 2000 rpm to gradually reduce the slurry solids and viscosity. This dilution step is repeated multiple times until the slurry viscosity reaches a level suitable for coating, typically 3,000-15,000 cP at a shear rate of 1 / s. Typically, the final solids content level of a mixture containing NMC622 / SuperP / binder=97 / 1.5 / 1.5 is approximately 80% by weight.

[0097] Electrode casting and drying

[0098] Next, using the adjustable doctor blade of the automatic film applicator (Elcometer 4340), the cathode slurry is cast onto aluminum foil (current collector, 15 microns thick) at a coating speed of 0.3 m / min. The gap in the doctor blade is set so that the dry thickness is approximately 80 microns, or the mass load is approximately 200 g / m². 2 The material is empirically adjusted to achieve this. Next, the wet casting is transferred to a convection oven and dried at 120°C for 30 minutes. After drying, the electrodes are calendered using a roll mill (Hohsen HSTK-1515H). The final density of the NMC622-based electrodes is typically about 3.4 g / cm³. 3 That is the case.

[0099] Peel test

[0100] In the peel test, the sample was cut into stripes 1 inch wide and 5–8 inches long. The sample was dried overnight in a vacuum oven at approximately 85°C and stored in a drying room. The cathode peel strength was obtained by a 180° peel test with several modifications to ASTM D903. The first modification was that the elongation rate used was 50 mm / min (peeling rate 25 mm / min). The second modification was that the test sample was dried before the peel test (as described above) because variations in exposure to ambient humidity can significantly affect the peeling results, and the peel test was performed in a drying room. A 1-inch wide test strip was attached to an alignment plate via 3M 410M double-sided paper tape, and the flexible aluminum foil current collector was peeled off by the grip of the test apparatus. The mechanical testing machine was an Instron 3343 model with a 10N load cell. Peeling results are reported in N / m.

[0101] Example 1: Cathodes were prepared using slurry process 1, which involved a solution blend of PVDF1 / PMMA1 in various ratios or additive levels of 2–6% by weight relative to the total binder base (PVDF1 + acrylic). The active material used was Celcore® NMC622, the carbon additive was Super-P, and the cathode composition on a dry basis was NMC622 / SuperP / binder = 97 / 1.5 / 1.5. The mass loading of the cathode example was approximately 205 g / m². 2 The final compressed density is approximately 3.4 g / cm³. 3 That was the case.

[0102] Several factors can affect the absolute value of the peel test results. For example, ambient conditions during slurry preparation (humidity and temperature), the slurry mixing protocol, and the conditioning protocol for the peeled sample can all significantly influence the absolute value. Comparing the relative valve to a control is more meaningful. In this case, a pure PVDF1 sample is selected as the control. The ambient conditions for these examples are 26-29°C and relative humidity ~12%.

[0103] Comparison 1: The same process as in Example 1 was followed, except that the binder was neat, additive-free battery-grade PVDF1.

[0104] [Table 2]

[0105] The addition level significantly affects the final peel strength of the cathode composite material. In this case, the minimum addition level is greater than 2% by weight. Adding a small amount (4-6% by weight) of PMMA1 to PVDF1 resulted in a 39-56% improvement in peel strength compared to the PVDF1 control. PMMA1 containing 4.5% by weight of MAA comonomer had a Tg of 122°C as measured by DSC. The weight-average molecular weight Mw of the resin was measured using GPC, and the Mw / Mn (polydispersity) value was 1.9, corresponding to 85,000 g / mol.

[0106] For comparative purposes, all of the following examples use a 5 wt% (PVDF1 / additive = 95 / 5) level.

[0107] Example 2 The binder used is PVDF1 / PMMA2 in a 95 / 5 blend ratio by solution blending. PMMA2 is PMMA-MAA, which has a higher molecular weight than PMMA1. The cathode is prepared using slurry procedure #1 and has a nominal composition of NMC622 / SuperP / binder = 97 / 1.5 / 1.5 on a dry basis. The mass loading of the cathode example is approximately 205 g / m². 2 The final compressed density is approximately 3.4 g / cm³. 3 That was the case.

[0108] PMMA2 copolymers containing 6 wt% MAA were prepared by bulk polymerization at 160°C, with a conversion rate of >50%. The glass transition temperature of the resin was measured at 126°C in N2 using DSC at a heating rate of 10°C / min. The weight-average molecular weight Mw of the resin was measured using GPC, and the Mw / Mn (polydispersity) value was 1.9, corresponding to 115,000 g / mol.

[0109] comparison 2 The same process as in Example 2 was followed, except that the additive was PMMA5 (ethyl acrylate) copolymer. PMMA5 containing 0.6 wt% EA had a Tg of 114°C as measured by DSC. The weight-average molecular weight Mw of the resin was measured using GPC, and the Mw / Mn (polydispersity) value was 1.9, which was 109,000 g / mol.

[0110] Example 3 The same process as in Example 2 was followed, except that the additive was PMMA3, a third monomer that is amphiphilic, has a higher molecular weight, and fine-tunes the dipole-dipole interaction with PVDF.

[0111] PMMA3 copolymers containing 4 wt% MAA and 1.5 wt% tert-butylcyclohexyl methacrylate (BCHMA, Sartomer) were prepared by bulk polymerization at 160°C when the conversion rate was >50%. The glass transition temperature of the resin was measured at 121°C in N2 using DSC at a heating rate of 10°C / min. The weight-average molecular weight Mw of the resin was measured using GPC, and the Mw / Mn (polydispersity) value was 1.9, corresponding to 105,000 g / mol.

[0112] Example 4 The same process as in Example 2 was followed, except that the additive was PMMA4, an acrylic copolymer having 2-carboxyethyl acrylate, instead of MAA.

[0113] [ka]

[0114] PMMA4 copolymer prepared by solution polymerization in toluene at 70°C: This example demonstrates the preparation of a high molecular weight copolymer of methyl methacrylate and 2-carboxyethyl acrylate. 94.80 parts methyl methacrylate and 5.20 parts 2-carboxyethyl acrylate (Aldrich 2-CEA) were charged into a reaction vessel containing 300 parts toluene at approximately 23°C with mechanical stirring at 380 rpm. AIBN (Aldrich) was used as an initiator at a level of 0.241 parts. The polymerization reaction was carried out at 65-68°C for 6 hours. When the conversion rate reached >50%, residual monomers were removed by precipitation in methanol (MeOH, ×20 times). The solid polymer powder was then dissolved in acetone at a solid content of 25% by weight, and the polymer solution was re-precipitationd in sufficient MeOH. The re-precipitationd white powder sample was dried in a vacuum oven at 180°C for 16 hours.

[0115] The glass transition temperature of PMMA4 resin was measured at 123°C in N2 using DSC at a heating rate of 10°C / min. The weight-average molecular weight Mw of the resin was measured using GPC, and the Mw / Mn (polydispersity) value was 1.8, corresponding to 130,000 g / mol.

[0116] The surrounding environmental conditions may affect the absolute value of delamination, but the relative value is not affected. Examples 2-4 and the comparative examples were performed under the same environmental conditions.

[0117] [Table 3]

[0118] Blending small amounts (5% by weight) of different acid-functionalized acrylic copolymers into a PVDF1-based resin dramatically improved the peel mechanical strength by 39-84% compared to the PVDF1 control.

[0119] Comparative Example 2 is PVDF1 blended with PMMA5, which does not have an acidic functional group. As can be seen, its effectiveness in increasing peel strength is somewhat limited compared to other acrylic copolymers containing acidic functional monomers in Examples 2, 3, and 4. Example 2 was found to have a higher molecular weight and improved peel strength. Example 3, using PMMA3, has a higher Mw and a third monomer to modulate the dipole-dipole interaction with PVDF, showing a significant improvement in peel strength. Example 4 demonstrated 2-carboxyethyl acrylate (CEA), which can provide improvements similar to those of general MAA.

[0120] The peel tests in Examples 5-7 were conducted under similar ambient environmental conditions so that the results could be compared.

[0121] Example 5 (a functional pMMA copolymer containing 2-CEA blended with PVDF1).

[0122] A blended dry coating copolymer of 95 wt% PVDF1 and 5 wt% pMMA / 2-carboxyethyl acrylate (97.8 / 2.2 wt / wt) with a Tg=123°C and MW=130,000 copolymer exhibited a peel adhesion strength of 180 N / m on Al foil of a battery cathode binder, compared to 105 N / m for the PVDF1 control, using a 180°C peel adhesion strength test.

[0123] Example 6 (Functional pMMA copolymer containing 2-CEA+MAA blended with PVDF1)

[0124] A mixed-dry coating of 95 wt% PVDF1 (Arkema) and 5 wt% pMMA / 2-carboxyethyl acrylate / MAA (96.9 / 2.1 / 1.0 w / w / w) (Tg=125℃, MW=140000) copolymer showed peel adhesion on the Al foil of the battery cathode binder, compared to 105 N / m for the PVDF1 control, using a 180°C peel adhesion test.

[0125] Example 7 (Functional pMMA copolymer containing 2-CEA+SR218A blended with PVDF1)

[0126] A mixed-dry coating copolymer of 95% by weight of PVDF1 (Arkema) and 5% by weight of pMMA / 2-carboxyethyl acrylate / tert-butylcyclohexyl methacrylate (96.9 / 2.6 / 0.5 w / w / w) (Tg=124°C, MW=135000) exhibited a peel adhesion strength of 127 N / m on the Al foil of a battery cathode binder, compared to 105 N / m for the PVDF1 control, using a 180°C peel adhesion test.

Claims

1. A binder for lithium-ion batteries comprising at least one vinylidene fluoride polymer and at least one acrylic copolymer, The acrylic copolymer comprises a functional monomer containing at least one functional group selected from the group consisting of carboxyl, hydroxyl, carboxylic acid anhydride, and epoxy, wherein the at least one functional group exhibits affinity for metal or can be immobilized on metal. The aforementioned acrylic copolymer has a Tg greater than 110°C. When measured at 23°C with a controlled shear rate of 30 revolutions / min, the vinylidene fluoride polymer's 5 wt% N-methyl-2-pyrrolidone solution exhibited a viscosity of 125 millipascal seconds or more and less than 2000 millipascal seconds. The acrylic copolymer contains less than 10 mole percent of functional monomers. The vinylidene fluoride polymer is present in an amount exceeding 80% by weight of the total weight of the polymers in the polymer blend. binder.

2. The binder according to claim 1, wherein the binder contains 2% by weight or more and 15% by weight or less of an acrylic copolymer based on the total weight of the polymers in the polymer blend.

3. The binder according to claim 2, wherein the binder contains 10% by weight or less of an acrylic copolymer.

4. The binder according to claim 1, wherein the viscosity of a 5% by weight solution of the vinylidene fluoride polymer is 300 millipascal seconds or more and less than 1,500 millipascal seconds.

5. The binder according to claim 1, wherein the vinylidene fluoride polymer is a copolymer comprising at least one monomer selected from the group consisting of hexafluoropropylene (HFP), tetrafluoroethylene (TFE), vinyl carboxylic acid, phosphoric acid, sulfonic acid, and salts thereof.

6. The binder according to claim 1, wherein the acrylic copolymer has a molecular weight exceeding 80,000 g / mol.

7. The binder according to any one of claims 1 to 6, wherein the acrylic copolymer has high heat resistance, as measured by having a Tg higher than 110°C.

8. The binder according to any one of claims 1 to 6, wherein the acrylic copolymer comprises a functional monomer having at least one functional group selected from a carboxyl group and a hydroxyl group.

9. The binder according to any one of claims 1 to 6, wherein the acrylic copolymer comprises a poly(methyl methacrylate) copolymer containing a functional monomer having a carboxyl functional group.

10. The binder according to any one of claims 1 to 6, wherein the acrylic copolymer comprises methyl methacrylate units and methacrylic acid units.

11. The binder according to any one of claims 1 to 6, wherein the acrylic copolymer comprises methyl methacrylate units and carboxylalkyl acrylate units or carboxylalkyl methacrylate units.

12. The binder according to any one of claims 1 to 6, wherein the acrylic copolymer further comprises a hydrophobic monomer.

13. The binder according to claim 12, wherein the hydrophobic monomer is an acrylic monomer having a substituted cycloalkane group.

14. The binder according to claim 12, wherein the hydrophobic monomer is selected from the group consisting of carboxylalkyl acrylate monomers or oligomers.

15. The binder according to claim 12, wherein the hydrophobic monomer is selected from the group consisting of tert-butylcyclohexyl (meth)acrylate, 3,3,5-trimethylcyclohexyl (meth)acrylate, isobornyl methyl acrylate (IBOMA), and isobornyl acrylate (IBOA), or oligomers thereof.

16. The binder according to claim 12, wherein the mol% of the hydrophobic monomer units is 0 to 15 mol%.

17. An electrode for a lithium-ion battery of the type in which at least one side of a metal current collector is covered with a layer of a substrate containing an active material and a binder according to any one of claims 1 to 6.

18. The electrode according to claim 17, wherein the substrate contains 0.5% or more and 5% or less of the binder by weight.

19. The electrode according to claim 17, wherein the substrate contains 1% or more and 3% or less of the binder by weight.

20. The electrode according to claim 17, wherein the active material comprises a lithium metal oxide and optionally carbon black.

21. The electrode according to claim 17, wherein the active material comprises at least one component selected from coke, carbon black, graphite, activated carbon, and carbon fiber.

22. The electrode according to claim 17, wherein the acrylic copolymer further comprises a hydrophobic monomer.

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