PVDF Acrylate Latex-based separator coating for lithium-ion batteries

Abstract

The present invention relates to a coating based on a fluoroacrylate polymer latex containing inorganic particles, which coating exhibits a very good compromise between dry adhesion and adhesion in the wet state on the one hand, and adhesion and ionic conductivity on the other hand. This coating is intended for separator applications, in particular for Li-ion batteries. The present invention also relates to a Li-ion battery comprising a separator coated with such a coating.

Description

Technical Field

[0001] The present invention generally relates to the field of electrical energy storage in Li-ion type rechargeable secondary batteries. More precisely, the present invention relates to a coating based on a fluoroacrylate polymer latex containing inorganic particles, which exhibits a very good compromise between dry adhesiveness and adhesiveness in the wet state, and on the other hand, between adhesiveness and ion conductivity. This coating is intended for separator applications, particularly for separators for Li-ion batteries. The present invention also relates to a Li-ion battery comprising a separator coated with such a coating.

Background Art

[0002] In the market for separators for electrochemical devices, the use of polyolefins (e.g., Celgard(R) or Hipore(R)) produced by extrusion and / or stretching by dry or wet processes is dominant. The separator must simultaneously exhibit a thin thickness, an optimal affinity for the electrolyte, and satisfactory mechanical strength and temperature resistance. Among the most advantageous alternatives to polyolefins, polymers that exhibit a better affinity for standard electrolytes, such as poly(methyl methacrylate) (PMMA), poly(vinylidene fluoride) (PVDF), and poly(vinylidene fluoride-co-hexafluoropropene) (P(VDF-co-HFP)), have been proposed to reduce the internal resistance of the system. Another option is to deposit a coating on one or both sides of the polyolefin separator.

[0003] The main evaluation criteria for separator coatings are as follows. That is, dry adhesiveness, adhesiveness in the wet state, ion conductivity, and thermal stability.

[0004] Dry adhesion is measured after assembly of the electrode and coated separator by press molding or lamination. This adhesion increases with the temperature and pressure applied after coating. However, it is preferable to use mild press molding / lamination conditions; that is, reduced pressure to avoid / limit pore closure and thus minimize the impact on ionic conductivity, and a moderate temperature to limit energy consumption and maintain high line speed / productivity.

[0005] The adhesion of the coating to the separator in a wet state is measured after impregnation with the electrolyte. This adhesion decreases as the coating softens with the electrolyte solvent, leading to swelling of the polymers present in the coating, and in some cases even dissolution of the coating. The degree of swelling or even dissolution, or loss of integrity, is used as the primary indicator of the adhesive performance in a wet state.

[0006] Ionic conductivity represents the movement of Li ions through the separator and its coating due to porosity. In coatings via the aqueous pathway, this porosity corresponds to the gaps between the solid particles constituting the coating, i.e., polymer particles and / or ceramic particles (from latex or powder redispersed in water). In coatings via the solvent pathway, this porosity is created by a phase inversion required before or during drying (e.g., exposure of an acetone-based coating to moisture), and without phase inversion, simple evaporation of the solvent forms a continuous non-porous coating. Gurley permeability is used as the primary indicator of ionic conductivity. In addition to the permeability of the initial coated separator, other aspects, namely interaction with the electrolyte (preferred when slight swelling of the polymer allows for improved wettability / affinity to the electrolyte, but unpreferred when excessive swelling of the polymer leads to a reduction / clogging of pore size), and the effects of press molding or lamination (reducing / clogging pore size) can affect ionic conductivity.

[0007] The thermal stability of polyolefin separators alone (made from PE, PP, or PP / PE / PP multilayers) is low, and they exhibit significant thermal shrinkage. The thermal stability can be significantly improved by coating them with inorganic particles.

[0008] Poly(vinylidene fluoride) (PVDF) and its derivatives are advantageous as main constituent materials for separators and as polyolefin separator coatings due to their electrochemical stability and their high dielectric constant, which promotes ion dissociation and thus conductivity. The crystallinity of P(VDF-co-HFP) copolymers (copolymers of vinylidene fluoride (VDF) and hexafluoropropylene (HFP)) is lower than that of PVDF. Therefore, the advantage of these P(VDF-co-HFP) copolymers is that they promote conductivity.

[0009] Mixtures of PVDF latex and acrylic latex are known for use as separator coatings. Reference US2018 / 0233727 describes a battery separator comprising a porous substrate and a porous adhesive layer provided on one or both sides of the porous substrate, comprising a mixture of a styrene-containing acrylic resin and a polyvinylidene fluoride-type resin, wherein the content of the acrylic resin in the porous adhesive layer is 2% to 40% by mass relative to the total mass of the acrylic resin and the polyvinylidene fluoride-type resin. This separator exhibits good adhesion to electrodes by dry hot pressing. However, the preparation of the coating requires a preliminary step of dissolving the PVDF and acrylic polymer in a common solvent (dimethylacetamide and tripropylene glycol), which makes the process more laborious and more difficult to apply on an industrial scale, imposing significant environmental constraints.

[0010] Therefore, there remains a need to develop novel coatings for separators that are easy to implement and offer a good compromise between dry adhesion, wet adhesion, ionic conductivity, and thermal stability. [Prior art documents] [Patent Documents]

[0011] [Patent Document 1] U.S. Patent Application Publication No. 2018 / 0233727 Specification [Overview of the Initiative] [Problems that the invention aims to solve]

[0012] Therefore, an object of the present invention is to overcome at least one of the drawbacks of the prior art, namely, to propose a polymer coating for separators that can prevent swelling or dissolution in an electrolyte solvent (one or more) while maintaining good adhesive properties and good ionic conductivity.

[0013] The present invention also aims to provide a process for producing this polymer coating via an aqueous route.

[0014] Another subject of the present invention is a separator for electrochemical devices such as batteries, capacitors, electric double-layer capacitors, and membrane electrode assemblies (MEAs) for fuel cells, including the coating, and in particular a separator for lithium-ion secondary batteries.

[0015] Finally, the present invention aims to provide electrochemical devices such as rechargeable lithium-ion secondary batteries, capacitors, electric double-layer capacitors, and membrane electrode assemblies (MEAs) for fuel cells, which are equipped with such separators. [Means for solving the problem]

[0016] The present invention aims to provide a material having improved adhesion for separator coatings when used in electronic device applications, particularly in lithium-ion batteries. This material is used as a polymer binder or adhesive component on a separator.

[0017] Surprisingly, hybrid latexes containing both fluoropolymers and acrylic polymers, mixed with inorganic particles, were found to offer a better compromise in properties for use as a single-layer coating via an aqueous route compared to known coatings.

[0018] Firstly, the present invention relates to a single-layer coating for separators, wherein the coating contains a hybrid fluoro-acrylic polymer resin and inorganic particles, and the fluoropolymer portion of the resin is based on vinylidene difluoride.

[0019] Hybrid fluoroacrylic polymer resins are defined as latex-like colloidal dispersions of polymers dispersed in a continuous (generally aqueous) phase. The latex particles exhibit an interpenetrating network (IPN) type morphology in which the chains of the fluoropolymer and acrylic polymer are thoroughly intermingled. Hybrid fluoroacrylic polymer resins include fluoropolymers modified with acrylic polymers. The polyvinylidene fluoride-based fluoropolymers are selected from the group of polyvinylidene fluoride homopolymers and copolymers based on polyvinylidene fluoride and at least one comonomer compatible with polyvinylidene fluoride, particularly hexafluoropropylene. The acrylic phase of the resin may contain monomer residues having functional groups that crosslink the acrylic phase.

[0020] The present invention also relates to a separator for an electrochemical device selected from the following group, namely Li-ion batteries, capacitors, electric double-layer capacitors, and membrane electrode assemblies for fuel cells, wherein the separator comprises a porous support and at least one monolayer coating as defined above.

[0021] According to one embodiment, the separator is suitable for use in rechargeable lithium-ion batteries.

[0022] Another subject of the present invention is an electrochemical device selected from the following group, namely, a lithium-ion battery, a capacitor, an electric double layer capacitor, and a membrane electrode assembly (MEA) for a fuel cell, which includes the separator.

[0023] Finally, the present invention relates to a lithium-ion battery including a negative electrode, a positive electrode, and a separator, wherein the separator includes a porous support and at least one single-layer coating defined above.

[0024] The present invention makes it possible to overcome the drawbacks of the prior art. More specifically, the present invention provides a single-layer adhesive coating for a separator that can prevent excessive swelling or dissolution in an electrolyte solvent(s) while maintaining good adhesion to the support and electrodes of the separator, good air permeability, and good ion conductivity.

Embodiments for Carrying Out the Invention

[0025] Here, the present invention will be described in more detail in a non-limiting manner in the following description.

[0026] According to a first aspect, the present invention relates to a single-layer coating for a separator containing a hybrid fluoro-acrylic polymer resin and inorganic particles.

[0027] According to various embodiments, the coating includes the following features in combination where appropriate. The indicated contents are expressed by weight unless otherwise indicated. For all the ranges shown, the limit values are included unless otherwise indicated.

[0028] The hybrid fluoro-acrylic polymer resin consists of a fluoroacrylate polymer.

[0029] The fluoropolymer used in the present invention as a seed for acrylic polymerization is based on vinylidene difluoride and is generically represented by the abbreviation PVDF.

[0030] According to one embodiment, PVDF is a homopolymer poly(vinylidene fluoride).

[0031] According to one embodiment, PVDF is a copolymer of vinylidene difluoride and at least one comonomer compatible with vinylidene difluoride.

[0032] The comonomer compatible with vinylidene difluoride may be halogenated (fluorinated, chlorinated, or brominated) or not.

[0033] Suitable examples of fluorocomonomers include vinyl fluoride, tetrafluoroethylene, hexafluoropropylene, trifluoropropene, especially 3,3,3-trifluoropropene, tetrafluoropropene, especially 2,3,3,3-tetrafluoropropene or 1,3,3,3-tetrafluoropropene, hexafluoroisobutylene, perfluorobutylethylene, pentafluoropropene, especially 1,1,3,3,3-pentafluoropropene or 1,2,3,3,3-pentafluoropropene, and perfluoroalkyl vinyl ethers, especially those with the general formula Rf-O-CF=CF2 (where Rf is an alkyl group, preferably a C1-C4 alkyl group) (preferred examples are perfluoropropyl vinyl ether and perfluoromethyl vinyl ether).

[0034] The fluorocomonomer may contain a chlorine atom or a bromine atom. This can be selected from bromotrifluoroethylene, chlorofluoroethylene, chlorotrifluoroethylene, and chlorotrifluoropropene in particular. Chlorofluoroethylene can represent either 1-chloro-1-fluoroethylene or 1-chloro-2-fluoroethylene. The 1-chloro-1-fluoroethylene isomer is preferred. Chlorotrifluoropropene is preferably 1-chloro-3,3,3-trifluoropropene or 2-chloro-3,3,3-trifluoropropene.

[0035] VDF copolymers may also contain non-halogenated monomers such as ethylene and / or acrylic or methacrylic copolymers.

[0036] The fluoropolymer preferably contains at least 50 mol% vinylidene difluoride.

[0037] According to one embodiment, the PVDF is a copolymer (P(VDF-HFP)) of vinylidene fluoride (VDF) and hexafluoropropylene (HFP), and the weight percentage of hexafluoropropylene monomer units is 2% to 23% by weight, preferably 4% to 15% by weight, relative to the weight of the copolymer.

[0038] According to one embodiment, PVDF is a copolymer of vinylidene fluoride and tetrafluoroethylene (TFE).

[0039] According to one embodiment, PVDF is a copolymer of vinylidene fluoride and chlorotrifluoroethylene (CTFE).

[0040] According to one embodiment, PVDF is a VDF-TFE-HFP terpolymer. According to another embodiment, PVDF is a VDF-TrFE-TFE terpolymer (TrFE is trifluoroethylene). In these terpolymers, the mass content of VDF is at least 10%, and comonomers are present in various proportions.

[0041] According to one embodiment, the PVDF comprises monomer units having at least one of the following functional groups: carboxylic acid, carboxylic acid anhydride, carboxylic acid ester, epoxy group (such as glycidyl), amide, hydroxyl, carbonyl, mercapto, sulfide, oxazoline, phenol, ester, ether, siloxane, sulfonic acid, sulfuric acid, phosphoric acid, or phosphonic acid. The functional groups are introduced by a chemical reaction, which may be grafting or copolymerization, of a fluoromonomer with a vinyl functional group that can copolymerize with a monomer having at least one of the functional groups and with the fluoromonomer, according to techniques well known to those skilled in the art.

[0042] According to one embodiment, the functional group has a carboxylic acid functional group which is a (meth)acrylic acid type group selected from acrylic acid, methacrylic acid, hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, and hydroxyethylhexyl (meth)acrylate.

[0043] According to one embodiment, the unit having a carboxylic acid functional group further comprises a heteroatom selected from oxygen, sulfur, nitrogen, and phosphorus.

[0044] According to one embodiment, functional groups are introduced by a transfer agent used during the synthesis process. The transfer agent is a polymer with a molar mass of 20,000 g / mol or less and has functional groups selected from the following group: carboxylic acids, carboxylic acid anhydrides, carboxylic acid esters, epoxy groups (such as glycidyl), amides, hydroxyl, carbonyl, mercapto, sulfide, oxazoline, phenol, ester, ether, siloxane, sulfonic acid, sulfuric acid, phosphoric acid, or phosphonic acid. An example of this type of transfer agent is an oligomer of acrylic acid. According to a preferred embodiment, the transfer agent is an oligomer of acrylic acid with a molar mass of 20,000 g / mol or less.

[0045] The functional group content of PVDF is at least 0.01 mol%, preferably at least 0.1 mol%, and at most 15 mol%, preferably at most 10 mol%.

[0046] PVDF is preferably high molecular weight. As used herein, "high molecular weight" refers to a measurement taken according to the ASTM D-3835 method at 232°C and 100 sec. -1 This is understood to mean a PVDF having a melt viscosity of over 100 Pa.s, preferably over 500 Pa.s, and more preferably over 1000 Pa.s, as measured by [the specified method].

[0047] The PVDF homopolymers and VDF copolymers used in the present invention can be obtained by known polymerization methods, such as emulsion polymerization or suspension polymerization.

[0048] According to one embodiment, they are prepared by an emulsion polymerization process in the absence of a fluorinated surfactant.

[0049] Polymerization of PVDF generally yields a latex having a solid content of 10% to 60% by weight, preferably 10% to 50% by weight, and a weight-average particle size of less than 1 micrometer, preferably less than 1000 nm, preferably less than 800 nm, and more preferably less than 600 nm. The weight-average size of the particles is generally at least 20 nm, preferably at least 50 nm, and advantageously the average size is in the range of 100 to 400 nm. The polymer particles can form weak aggregates, whose weight-average size is 1 to 30 micrometers, preferably 2 to 10 micrometers. The weak aggregates can decompose into individual particles during compounding and during coating onto the substrate.

[0050] According to some embodiments, PVDF homopolymers and VDF copolymers are composed of bio-based VDF. The term "bio-based" means "derived from biomass." This can improve the ecological footprint of the membrane. Bio-based VDF is prepared according to standard NF EN 16640. 14VDF may be characterized by a content of at least 1 atomic percent of renewable carbon, i.e., carbon of natural origin, derived from biomaterials or biomass, as determined by the C content. The term “renewable carbon” means that the carbon is of natural origin and derived from biomaterials (or biomass), as shown below. According to some embodiments, the biocarbon content of VDF may be greater than 5%, preferably greater than 10%, preferably greater than 25%, preferably greater than 33%, preferably greater than 50%, preferably greater than 66%, preferably greater than 75%, preferably greater than 90%, preferably greater than 95%, preferably greater than 98%, preferably greater than 99%, and advantageously equal to 100%.

[0051] Hybrid fluoro-acrylic polymer resins are synthesized by emulsion polymerization of acrylate / methacrylate monomers using the latex of the fluoropolymer as a seed, thereby obtaining hybrid fluoro-acrylic polymer compositions. The acrylic portion of the acrylic-modified fluoropolymer can be optionally crosslinked (depending on the selection of acrylic monomers used).

[0052] According to one embodiment, alkyl acrylates having an alkyl group having 1 to 18 carbon atoms, used as monomers for emulsion polymerization in the presence of PVDF polymer particles, include methyl acrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate, isobutyl acrylate, t-butyl acrylate, n-dodecyl acrylate, amyl acrylate, isoamyl acrylate, hexyl acrylate, 2-ethylhexyl acrylate, diacetone acrylamide, lauryl acrylate, and n-octyl acrylate. Among these, alkyl acrylates having an alkyl group having 1 to 8 carbon atoms are preferred, and alkyl acrylates having an alkyl group having 1 to 5 carbon atoms are more preferred. These compounds may be used individually or as a mixture of two or more.

[0053] In this specification, the term "acrylate" encompasses both acrylates and methacrylates.

[0054] Any ethylenically unsaturated compounds copolymerizable with alkyl acrylates and alkyl methacrylates include the following: - (A) Alkenyl compounds containing functional groups, and - (B) Alkenyl compounds that do not have functional groups.

[0055] Examples of alkenyl compounds (A) containing functional groups include α,β-unsaturated carboxylic acids such as acrylic acid, methacrylic acid, fumaric acid, crotonic acid, and itaconic acid; vinyl ester compounds such as vinyl acetate and vinyl neodecanoate; and amide compounds such as acrylamide, methacrylamide, N-methylacrylamide, N-methylmethacrylamide, N-methylolacrylamide, N-methylolmethacrylamide, N-alkylacrylamide, N-alkylmethacrylamide, N,N-dialkylacrylamide, N,N-dialkylmethacrylamide, and diacetoneacrylamide. Examples of suitable compounds include acrylic acid esters, such as 2-hydroxyethyl acrylate, N-dialkylaminoethyl acrylate, glycidyl acrylate, n-dodecyl acrylate, and fluoroalkyl acrylate; methacrylic acid esters, such as dialkylaminoethyl methacrylate, fluoroalkyl methacrylate, 2-hydroxyethyl methacrylate, n-octyl methacrylate, t-butyl methacrylate, glycidyl methacrylate, and ethylene glycol dimethacrylate; maleic anhydride; and alkenyl glycidyl ether compounds, such as allyl glycidyl ether. Among these, acrylic acid, methacrylic acid, itaconic acid, fumaric acid, N-methylolacrylamide, N-methylolmethacrylamide, diacetoneacrylamide, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, and allyl glycidyl ether are preferred. These compounds may be used individually or as a mixture of two or more.

[0056] Examples of alkenyl compounds (B) that do not have functional groups include conjugated dienes, such as 1,3-butadiene and isoprene; divinyl hydrocarbon compounds, such as divinylbenzene; and alkenyl cyanides, such as acrylonitrile and methacrylonitrile. Among these, 1,3-butadiene and acrylonitrile are preferred. These may be used individually or as a mixture of two or more.

[0057] It is preferable that the functionalized alkenyl compound (A) is used in a proportion of less than 50% by weight relative to the weight of the monomer mixture, and the non-functional alkenyl compound (B) is used in a proportion of less than 30% by weight relative to the weight of the monomer mixture.

[0058] According to one embodiment, the acrylic-modified fluoropolymer resin used in connection with the present invention can be crosslinked by self-condensation of its functional groups, or by reactions using catalysts and / or crosslinking agents, such as melamine resins, epoxy resins, and known low molecular weight crosslinking agents, such as diisocyanates or higher polyisocyanates, polyaziridines, polycarbodiimides, polyoxazolines, dialdehydes, such as glyoxal, acetacetate, malonate, acetal, bifunctional and trifunctional acrylates and thiols, alicyclic epoxy molecules, organosilanes, such as epoxysilanes and aminosilanes, carbamates, diamines and triamines, and inorganic chelating agents, such as certain zinc and zirconium salts, titanium, glycoryl, and other aminoplasts. In certain cases, functional groups derived from other polymerization components, such as surfactants, initiators, and seed particles, may be involved in the crosslinking reaction. When two or more functional groups are involved in the crosslinking process, complementary reactive group pairs include, for example, hydroxyl-isocyanate, acid-epoxy, amine-epoxy, hydroxyl-melamine, and acetoacetate-acid.

[0059] Acrylate and / or methacrylate monomers that do not contain functional groups that can enter a crosslinking reaction after polymerization should preferably account for 70% by weight or more of the total monomer mixture, and more preferably more than 90% by weight.

[0060] According to one embodiment, the fluoroacrylic polymer resin contains a crosslinking agent selected from the group consisting of isocyanates, diamines, adipic acid, dihydrazides, and combinations thereof.

[0061] According to one embodiment, the fluoroacrylic polymer resin is not crosslinked and is provided in a non-crosslinked form in the separator coating of the present invention.

[0062] The hybrid fluoroacrylic polymer resin is an aqueous dispersion obtained by emulsion polymerization of a mixture of monomers in an aqueous medium, in the presence of 100 parts by weight of the vinylidene fluoride polymer particles defined above, in 5 to 100 parts by weight, preferably 5 to 95 parts by weight, of at least one monomer selected from the group consisting of alkyl acrylates having 1 to 18 carbon atoms in an alkyl group and alkyl methacrylates having 1 to 18 carbon atoms in an alkyl group, and optionally an ethylenically unsaturated compound copolymerizable with alkyl acrylates and alkyl methacrylates. The PVDF particles function as seeds for the polymerization of the acrylic monomers.

[0063] PVDF particles can be added to the polymerization system in any form, as long as they are dispersed in an aqueous medium in particle form. Since vinylidene fluoride polymers are generally manufactured in the form of aqueous dispersions, it is practical to use such manufactured aqueous dispersions as seed particles. The diameter of the vinylidene fluoride particles is preferably in the range of 0.04 to 2.9 micrometers. In a preferred embodiment, the diameter of the polymer particles is preferably 50 nm to 700 nm.

[0064] The polymerization product is latex, which can usually be used in this form after the solid by-products of the polymerization process have been removed by filtration. For use in latex form, the latex can be stabilized by the addition of a surfactant, which may be the same as or different from the surfactant present during polymerization (where appropriate). This surfactant added later may be, for example, an ionic or nonionic surfactant.

[0065] PVDF particles used as seeds may have homogeneous or heterogeneous properties, or a gradient between the core and surface of the particles, with respect to their composition (e.g., HFP comonomer content) and / or molecular weight.

[0066] In hybrid fluoroacrylic polymer resins, the mass ratio of PVDF to acrylic polymer varies, ranging from 95 / 5 to 5 / 95, preferably 75 / 25 to 25 / 75, and more favorably 60 / 40 to 40 / 60.

[0067] In the hybrid fluoroacrylic polymer resin, the average particle diameter is 0.05 to 3 μm, preferably 0.05 to 1 μm, and more preferably 0.1 to 1 μm.

[0068] Hybrid fluoroacrylic polymer resins are characterized by thorough mixing between fluoropolymer chains and acrylic polymer chains.

[0069] The separator coating according to the present invention contains, in addition to the above-mentioned hybrid fluoroacrylic polymer resin, inorganic particles that help form micropores (gaps between inorganic particles) in the coating. These aggregates of inorganic particles also contribute to heat resistance.

[0070] According to one embodiment, the coating contains inorganic particles in an amount of 50 to 99 weight percent relative to the weight of the coating.

[0071] These inorganic particles must be electrochemically stable (not oxidized and / or reduced within the voltage range used). Furthermore, powdered inorganic materials preferably have high ionic conductivity. Low-density materials are preferred over high-density materials because they can reduce the weight of the manufactured battery. Their dielectric constant is preferably 5 or higher.

[0072] According to one embodiment, the inorganic particles are from the group consisting of the following: BaTiO3, Pb(Zr,Ti)O3, Pb 1-x La x Zr y O3(0 <x<1、0<y<1)、PBMg3Nb 2 / 3)3 Selected from PbTiO3, hafnia (HfO(HfO2), SrTiO3, SnO2, CeO2, MgO, NiO, CaO, ZnO, Y2O3, boehmite (y-AlO(OH)), Al2O3, TiO2, SiC, ZrO2, boron silicate, BaSO4, nanoclay, or mixtures thereof.

[0073] In the separator coating according to the present invention, the ratio of polymer solids to inorganic particles is 0.5 to 30 parts by weight of hybrid fluoro-acrylic polymer resin solids per 70 to 99.5 parts by weight of inorganic particles, preferably 0.5 to 25 parts by weight of polymer solids per 85 to 99.5 parts by weight of inorganic particles, then 0.5 to 20 parts by weight, then 0.5 to 15 parts by weight, more preferably 1 to 10 parts by weight of polymer solids per 90 to 99 parts by weight of inorganic particles, and in one embodiment, 0.5 to 8 parts by weight of polymer solids per 92 to 99.5 parts by weight of inorganic particles.

[0074] The separator coating of the present invention may optionally contain, based on the polymer, 0 to 15 weight percent, preferably 0.1 to 10 weight percent, of additives selected from thickeners, pH adjusters, anti-settling agents, surfactants, foaming agents, fillers, defoaming agents, and temporary or non-temporary adhesion promoters.

[0075] The separator coating of the present invention exhibits an excellent compromise between the properties of a separator coating for application as a single layer with inorganic particles via an aqueous route, namely, good dry adhesion, good storage integrity, good resistance to electrolyte solvents with moderate swelling, and good Gurley permeability. Methods that may be used to characterize these properties are described in the examples.

[0076] The above coating is used to coat at least one side of the separator support in a single-layer form.

[0077] Advantageously, the coating according to the present invention is applied via an aqueous route.

[0078] The porous separator is coated with the coating composition according to the present invention on at least one side. The selection of the separator substrate to be coated with the aqueous coating composition of the present invention is not particularly limited as long as it is a porous substrate having pores.

[0079] The porous substrate may take the form of a membrane or a fibrous fabric. If the porous substrate is fibrous, it may be a nonwoven web that forms a porous web, for example, a web obtained by direct spinning or melt-blown (spunbond or melt-blown type).

[0080] Examples of porous substrates used as separators in the present invention include, but are not limited to, polyolefins, polyethylene terephthalate, polybutylene terephthalate, polyesters, polyacetals, polyamides, polycarbonates, polyimides, polyetheretherketones, polyethersulfones, poly(phenylene oxide), poly(phenylene sulfide), polyethylene naphthalate, or mixtures thereof. However, other heat-resistant engineering plastics may be used without particular limitation. Nonwoven materials made from natural or synthetic materials may also be used as substrates for separators.

[0081] Porous substrates generally have a thickness of 1 to 50 μm and are typically films or cast nonwoven fabrics obtained by extrusion and stretching (wet or dry processes). Porous substrates preferably have a porosity between 5% and 95%. The average size (diameter) of the pores is preferably between 0.001 and 50 μm, more preferably between 0.01 and 10 μm.

[0082] According to one embodiment, a process for preparing a coated separator according to the present invention includes the following steps: a) A step of coating at least one side of the separator with the above-mentioned single-layer coating via an aqueous route by dip coating, spray coating, gravure coating, or slot die coating. b) A step of drying the coated separator at a temperature of 25 to 85°C to form a dry adhesive layer on the separator.

[0083] The application of coating via an aqueous pathway allows for the acquisition of a porous / discontinuous coating that is permeable to Li ions. The pores correspond to the gaps left between the particles. The selection of particles, i.e., inorganic particles that can improve temperature resistance and polymer particles that can provide adhesion while resisting the electrolyte solvent(s), allows for the adjustment of desired property compromises through guidance.

[0084] According to one embodiment, the thickness of the coating on at least one side of the separator is 0.5 to 10 micrometers.

[0085] The present invention also relates to a separator for an electrochemical device selected from the group consisting of Li-ion batteries, capacitors, electric double-layer capacitors, and membrane electrode assemblies (MEAs) for fuel cells, comprising a porous support and at least one of the above-mentioned single-layer coatings.

[0086] According to one embodiment, the present invention relates to a separator for a Li-ion battery coated with the above-described adhesive single-layer coating.

[0087] The present invention also relates to an electrochemical device selected from the group consisting of Li-ion batteries, capacitors, electric double-layer capacitors, and membrane electrode assemblies (MEAs) for fuel cells, which includes a separator coated with the above-mentioned adhesive monolayer coating.

[0088] Electrochemical devices can be manufactured by any conventional method known to those skilled in the art. In one embodiment of a process for manufacturing an electrochemical device, the electrochemical device is provided by forming an electrode assembly from a porous organic / inorganic composite separator interposed between a cathode and an anode, and injecting an electrolyte into this assembly.

[0089] Another subject of the present invention is a lithium-ion secondary battery comprising a negative electrode, a positive electrode and a separator, wherein the separator is coated with the adhesive single-layer coating described above. [Examples]

[0090] The following embodiments illustrate the scope of the present invention in a non-limiting manner.

[0091] The examples and comparative examples of the present invention described below were carried out according to the same protocol, but each example used a different latex or a mixture of two latexes. Table 1 summarizes the different latexes used, their main properties, and the results obtained for each of them.

[0092] Latex Preparation: P(VDF-HFP) copolymer latex was used as a seed for synthesizing a latex containing a fluoroacrylic polymer composition by emulsion polymerization in the presence of an acrylic oligomer-type transfer agent with a molar mass of less than 20,000 g / mol (Examples 1 and 2). The transfer agent allows for the incorporation of acrylic functional groups into the P(VDF-HFP) copolymer. The solids content of this latex is about 30% to 40% by weight. Acrylic latex is obtained in the same manner, except without the use of a seed. In Example 3, P(VDF-HFP) copolymer latex was used as a seed for synthesizing a latex containing a fluoroacrylic polymer composition by emulsion polymerization in the presence of propane as a transfer agent and poly(ethylene glycol) as a surfactant that does not introduce any functionalization as described in this application.

[0093] Preparation of aqueous formulation: At an ambient temperature of approximately 22°C, 10 g of alumina (Sumitomo Chemical AES-11) is added to 20 g of 0.5 wt% CMC aqueous solution (Nippon Paper FT-3), and then dispersed in a mixer (Filmix Model 40-L) at 30 m / s for 30 seconds. Latex (or two types of latex in the case of a mixture of PVDF latex and acrylic latex according to the ratios shown in the table) is added to this dispersion, and 4 g of the corresponding polymer (the amount of latex adjusted according to the solid content of each latex is in the range of 30-45%) and demineralized water are incorporated to make a total of 50 g of preparation. The mixture is then homogenized using a vertical stirrer (IKA, Euro-ST) at 600 rpm for 10 minutes. To 48 g of this mixture, 0.24 g of wetting agent (BYK349) is added, intended to facilitate the spreading of the formulation on the separator by mixing under the same conditions as the latex. The resulting dispersion is stable and does not show any visible sedimentation after standing for 30 minutes.

[0094] Preparation of coated separators: The aqueous formulation was applied to a Celgard 2400 separator sample (PP single layer, 25 μm thick, 89 mm wide, 30 cm long) at an ambient temperature of approximately 22°C using a manual applicator (bar coater, Hohsen Corp., wet deposition thickness approximately 23 μm, manual application speed approximately 100 mm / sec), and then dried on a hot plate at 65°C for 10 minutes. The dried deposition had a thickness of 5-6 μm, measured according to the sample (Mitutoyo Digimatic Indicator IDH053D micrometers). The resulting separator had a width of 89 mm and a length of 30 cm.

[0095] Gurley Permeability: The Gurley permeability of each coated separator was measured (using a Gurley 4110N densometer with a 4320EN auto-timer), and then the permeability of the support (measured at 575 seconds / 100cc) was subtracted to obtain the coating permeability values ​​shown in Table 1. A coating permeability of less than 85 seconds / 100cc is considered satisfactory.

[0096] Resistance to electrolyte solvents, assessed by swelling or even dissolution and / or loss of integrity of the coating binder: 50 × 60 mm samples of each coated separator are weighed (W0), and they are immersed in an electrolyte solvent mixture of EC / EMC = 3 / 7 vol% at an ambient temperature of approximately 22°C for 96 hours. They are then removed from the bath, both sides are wiped, and they are weighed (W1). Finally, they are placed in an oven at 120°C for 24 hours and then weighed once more (W2). The same procedure is performed using samples of uncoated separators as a reference, yielding weights expressed as W0ref, W1ref, and W2ref. Finally, since the coating contains 28.6% polymer from latex, the following values ​​are calculated. Weight increase due to polymer (%): [(W1-W1ref)-(W0-W0ref)] / (W0-W0ref)*100*0.286 Polymer swelling (%): [(W1-W1ref)-(W2-W2ref)] / (W2-W2ref)*100*0.286 Polymer extract (dissolved) (%): [(W0-W0ref)-(W2-W2ref)] / (W0-W0ref)*100*0.286

[0097] These values ​​assume that only the polymer from the latex swells or dissolves in the electrolyte solvent, and that the alumina (the main component of the coating) remains in the coating. Therefore, visual confirmation was also made of whether any solids or particles remained in the bath and / or whether the coating peeled off from the separator support or peeled off easily as a result of gentle rubbing with a finger (loss of integrity) (in which case it is considered to have insufficient resistance to the electrolyte solvent). Other signs (increased weight, swelling, polymer extract) are not reported in the table.

[0098] Dry Adhesion: A 40 x 90 mm coated separator sample is brought into contact with a cathode (NMC111 with a PVDF binder, prepared by Elexcel) on its coated surface. The assembly is then pressed between two rollers (Tester Industries, Model: SA-602) at 90°C and 1.5 kgf / cm at a speed of 2.4 m / min to bond the coated separator and cathode. The assembly is then cut to dimensions of 30 x 80 mm and fixed to a rigid metal support by double-sided adhesive tape applied across the entire surface by the rear surface of the cathode (aluminum current collector). On the other side, single-sided adhesive tape is fixed to the coating of the separator, with the adhesive tape protruding only a few centimeters. The free end of the single-sided adhesive tape and the free end of the metal support are placed in the upper and lower jaws of a tensile testing machine (Autograph AGS-X, 10N load cell), respectively. A 180° peel test is performed at ambient temperature (approximately 22°C) at a speed of 50 mm / second. The peel force (in N units) is measured at the plateau of the curve. This value is related to the width of the sample, and the values ​​are shown in Table 1 (N / m).

[0099] [Table 1]

[0100] The separator coating according to the present invention exhibits an excellent compromise in properties for the target application, namely good dry adhesion, good preserved integrity, good resistance to electrolyte solvents with moderate swelling, and good Gurley permeability.

[0101] In contrast, the comparative examples exhibit at least one highly undesirable characteristic for each of the latexes, namely, - PVDF latex alone exhibits low dry adhesion. - Acrylic latex alone has low resistance to electrolyte solvents, and - Mixtures of these two types of latex exhibit low resistance to electrolyte solvents.

Claims

1. A single-layer coating for separators, wherein the coating comprises a hybrid fluoroacrylic polymer resin and inorganic particles, the fluoropolymer being selected from the group consisting of polyvinylidene fluoride homopolymers and copolymers based on polyvinylidene fluoride and at least one comonomer compatible with polyvinylidene fluoride. The aforementioned comonomer is selected from vinyl fluoride, tetrafluoroethylene, hexafluoropropylene, trifluoropropene, tetrafluoropropene, hexafluoroisobutylene, perfluorobutylethylene, pentafluoropropene, perfluoroalkyl vinyl ether, bromotrifluoroethylene, chlorofluoroethylene, chlorotrifluoroethylene, chlorotrifluoropropene, and ethylene, and is a single-layer coating for separators.

2. The coating according to claim 1, wherein the fluoropolymer is a polyvinylidene fluoride-hexafluoropropylene copolymer having a weight percentage of 2% to 23% by weight of hexafluoropropylene monomer units relative to the weight of the copolymer.

3. The coating according to any one of claims 1 to 2, wherein the fluoropolymer comprises a monomer unit having at least one of the following functional groups: carboxylic acid, carboxylic acid anhydride, carboxylic acid ester, epoxy group, amide, hydroxyl, carbonyl, mercapto, sulfide, oxazoline, phenol, ester, ether, siloxane, sulfonic acid, sulfuric acid, phosphoric acid, or phosphonic acid.

4. The coating according to any one of claims 1 to 3, wherein the acrylic portion contains a monomer selected from the group consisting of methyl acrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate, isobutyl acrylate, t-butyl acrylate, n-dodecyl acrylate, amyl acrylate, isoamyl acrylate, hexyl acrylate, 2-ethylhexyl acrylate, diacetone acrylamide, lauryl acrylate, n-octyl acrylate, and combinations thereof.

5. The coating according to any one of claims 1 to 4, wherein the mass ratio of PVDF / acrylic polymer in the hybrid fluoroacrylic polymer resin varies from 95 / 5 to 5 / 95.

6. The inorganic particles are BaTiO ３ , ２ , ４ , ２ 、Pb(Zr,Ti)O ３ 、Pb １－ｘ La ｘ Zr ｙ O ３ (0 < x < 1, 0 < y < 1), PBMg ３ Nb ２／３ ) ３ 、PbTiO ３ 、hafnia (HfO(HfO ２ )、SrTiO ３ 、SnO ２ 、CeO ２ 、MgO、NiO、CaO、ZnO、Y ２ O ３ 、boehmite (γ - AlO(OH)), Al ２ O ３ 、TiO ２ 、SiC、ZrO ２ 、boron silicate, BaSO ４ 、nanoclay, or a coating according to any of claims 1 to 5 selected from the group consisting of mixtures thereof.

7. The coating according to any one of claims 1 to 6, comprising 50 to 99 weight percent inorganic particles.

8. The coating according to any one of claims 1 to 7, wherein the ratio of the solid content of the polymer to the inorganic particles is 0.5 to 30 parts by weight of the solid content of the hybrid fluoroacrylic polymer resin per 70 to 99.5 parts by weight of the inorganic particles.

9. The coating according to any one of claims 1 to 8, wherein the thickness of the coating on at least one side of the separator is 0.5 to 10 micrometers.

10. A separator for an electrochemical device selected from the group consisting of Li-ion batteries, capacitors, electric double-layer capacitors, and membrane electrode assemblies (MEAs) for fuel cells, comprising a porous support and at least one single-layer coating according to any one of claims 1 to 9.

11. A manufacturing process for coated separators, including the following steps: a) A step of coating at least one side of a separator with a single-layer coating according to any one of claims 1 to 9 via an aqueous medium, by dip coating, spray coating, gravure coating, or slot die coating. b) A step of drying the coated separator at a temperature of 25 to 85°C to form a dry adhesive layer on the separator.

12. An electrochemical device selected from the group consisting of Li-ion batteries, capacitors, electric double-layer capacitors, and membrane electrode assemblies (MEAs) for fuel cells, comprising the separator described in claim 10.

13. A lithium-ion secondary battery comprising an anode, a cathode, and a separator, wherein the separator is a lithium-ion secondary battery according to claim 10.

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