Battery electrode and method for producing same

Abstract

The present invention relates to an electrode composition comprising PTFE and a low melting point fluoroplastic, its method of preparation and its use for the manufacture of electrochemical cell components.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to European Patent Application No. 21211090.2, filed November 29, 2021, and European Patent Application No. 22186754.2, filed July 15, 2022, the entire contents of which are incorporated herein by reference for all purposes.

[0002] The present invention relates to an electrode composition comprising PTFE and a low melting point fluoroplastic, its method of preparation and its use for the manufacture of electrochemical cell components. [Background technology]

[0003] To date, electrodes for lithium secondary batteries are primarily manufactured by a wet process that involves preparing a slurry in which electrode active materials, additives and binders are dispersed in a solvent or aqueous medium, and treating the slurry to form an electrode film.

[0004] Dry electrode processes were developed to reduce the time consuming and costly drying procedures required by the wet processes described above.

[0005] Typical dry processes use the fibrillating properties of certain polymers to provide a matrix for the embedded conductive material. Some polymers in the fluoropolymer family, such as polytetrafluoroethylene (PTFE), are particularly inert and stable in common electrode solvents used in secondary batteries, even those with organic solvents at high operating or storage temperatures. Thus, the stability of electrodes made with PTFE can be higher than those made with other binders.

[0006] For example, a dry electrode fabrication process may involve combining a PTFE binder with the active electrode material in powder form and calendaring to form an electrode film. However, while PTFE has good adhesion to the electrode active material, it has difficulty adhering to the current collector.

[0007] A method for improving the adhesion of PTFE to the current collector and the electrode active material by using a combination of PTFE and tetrafluoroethylene / hexafluoropropylene copolymer (FEP) as a binder to achieve a material having the melting point of FEP (240-270°C) or higher is known in the art (JP2000149954A). However, a special heat treatment device is required to provide the electrode film in order to heat to a temperature above the melting point of FEP, specifically, above 280°C, which is inconvenient from the viewpoint of energy.

[0008] An object of the present invention is to provide an electrode that can ensure sufficient adhesive strength and can be fabricated by an efficient process. Summary of the Invention

[0009] The applicant has now discovered that the addition of certain fluorine-containing thermoplastics is particularly effective in ensuring improved adhesion to PTFE when used as binders for electrodes for secondary batteries.

[0010] The binder composition [binder (B)] for use in the preparation of an electrode for an electrochemical device comprises: a. Polytetrafluoroethylene (PTFE); b. A low melting point VDF-based fluororesin [polymer (A)] having a melting point of less than 220°C, wherein the polymer (A) is at least 50 mol % of repeating units derived from VDF, based on all repeating units of said polymer (A); and - comprising repeat units derived from at least one comonomer (CM), said comonomer (CM) being a hydrophilic (meth)acrylic comonomer [comonomer (MA)], (i) Tetrafluoroethylene (TFE); (ii) Chloro- and / or bromo- and / or iodo-C2-C6 fluoroolefins, such as chlorotrifluoroethylene (CTFE); (iii) Formula CF2=CFOR f1 (In the formula, R f1 is a C1-C6 fluoro- or perfluoroalkyl group, for example -CF3, -C2F5, -C3F7), (per)fluoroalkyl vinyl ethers; and (iv) Hexafluoropropylene (HFP) A low melting point VDF-based fluororesin [polymer (A)] selected from the group consisting of a fluorinated comonomer [comonomer (F)] selected from the group consisting of Provided herein is a binder composition [binder (B)] comprising:

[0011] In another aspect, the present invention provides an electrode-forming composition [composition (C)] for use in preparing an electrode for an electrochemical device, comprising: a) at least one electrode active material (AM); b) a binder (B) as defined above; c) optionally, at least one conductive agent; The present invention provides an electrode-forming composition (C) comprising:

[0012] The applicant has surprisingly discovered that the processability of the binder (B) makes it suitable for fabricating electrodes by dry processing or extrusion at low temperatures, thereby providing electrodes by a very efficient process.

[0013] In another aspect, the present invention relates to a method for producing an electrode for an electrochemical cell [electrode (E)], comprising the steps of: - A) providing a binder (B) by combining polytetrafluoroethylene (PTFE) and a low melting point VDF-based fluororesin as defined above [polymer (A)]; - B) dry grinding, in the absence of a solvent, at least one electrode active material (AM), a binder (B) as defined above, and, optionally, at least one conductive agent; - C) feeding the powdered dry mixture obtained in step B) into a press to form a self-supporting dry film; and - D) applying the dry film to a conductive substrate to form an electrode. The present invention provides a method comprising:

[0014] In another aspect, the present invention provides an electrode (E) for a secondary battery, obtainable by a process as defined above.

[0015] In a further aspect, the present invention relates to an electrochemical device, such as a secondary battery or a capacitor, comprising at least one electrode (E) as defined above. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] In the context of the present invention, the term "weight percent" (wt%) indicates the content of a particular component in a mixture, calculated as the ratio between the weight of the particular component and the total weight of the mixture. When referring to repeating units derived from a certain monomer in a polymer / copolymer, weight percent (wt%) indicates the ratio between the weight of the repeating units of that monomer to the total weight of the polymer / copolymer. When referring to the total solids content of a liquid composition, weight percent (wt%) indicates the ratio between the weights of all non-volatile components in the liquid.

[0017] As used herein, the terms "adhere" and "bond" refer to two layers being permanently joined to one another via their contacting surfaces.

[0018] The term "electrochemical device" is intended herein to mean an electrochemical cell / assembly comprising a positive electrode, a negative electrode and a liquid electrolyte, in which a monolayer or multilayer separator is in contact with at least one surface of one of the electrodes. Non-limiting examples of suitable electrochemical devices include, inter alia, secondary batteries, especially alkaline or alkaline earth secondary batteries such as lithium ion batteries, lead acid batteries and capacitors, especially lithium ion-based capacitors and electric double layer capacitors (supercapacitors). Non-limiting examples of electrochemical cells include, inter alia, batteries, preferably secondary batteries, electric double layer capacitors.

[0019] For the purposes of the present invention, "secondary battery" is intended to denote a rechargeable battery. Non-limiting examples of secondary batteries include, inter alia, alkaline or alkaline earth secondary batteries.

[0020] In the context of the present invention, the term "PTFE" means a polymer resulting from the polymerization of tetrafluoroethylene (TFE).

[0021] However, it is understood that the PTFE polymer may also contain small amounts of one or more comonomers, such as, but not limited to, hexafluoropropylene, perfluoro(methyl vinyl ether), perfluoro(propyl vinyl ether), perfluoro-(2,2-dimethyl-1,3-dioxole), provided that the latter does not significantly adversely affect the unique properties, such as the thermal and chemical stability, of the tetrafluoroethylene homopolymer. Preferably, the amount of such comonomers does not exceed about 3 mol%, more preferably less than about 1 mol%; a comonomer content of less than 0.5 mol% is especially preferred. When the total comonomer content is greater than 0.5 mol%, it is preferred that the amount of perfluoro(alkyl vinyl ether) comonomer is less than about 0.5 mol%. PTFE homopolymer is most preferred.

[0022] PTFE suitable for use in preparing the binder (B) of the present invention can be in the form of a powder or in the form of a latex.

[0023] PTFE in the form of a powder can be obtained by solidifying a PTFE lattice by cryogenic coagulation or by electrolytic coagulation with the addition of an electrolyte, see for example US Patent No. 6,790,932. Preferred examples of electrolytes are: - Aluminum sulfate (Al2(SO4)3) with a concentration of 2 g / l calculated based on the amount of water in the coagulation vessel, - Ammonium carbonate ((NH4)2CO3) at a concentration of 8 g / l calculated based on the amount of water in the coagulation vessel, or -Nitric acid (HNO3), 25ml of a 65% solution calculated based on the amount of water in the coagulation vessel.

[0024] Alternatively, the PTFE powder may be obtained from a PTFE lattice in the form of a gel by coagulation with an electrolyte as described above. The gel may be obtained according to US Pat. No. 6,790,932 and US Pat. No. 6,780,966.

[0025] After coagulation has taken place, the polymer is washed with demineralized water at room temperature. After coagulation and washing, the PTFE powder obtained thereon is then dried.

[0026] PTFE lattices are generally obtained by dispersion or emulsion polymerization.

[0027] PTFE in powder form generally has a particle size of 1 to 1600 microns, preferably 100 to 800 microns, and more preferably 400 to 700 microns.

[0028] Particle size can be expressed relative to D50, the corresponding particle size when the cumulative percentage reaches 50%. D50 is also called median particle size or median diameter. For example, for a powder sample with D50=5 μm, it means that 50% of the particles are larger than 5 μm and 50% of the particles are smaller than 5 μm.

[0029] The term "fluororesin" is intended herein to mean a resin in which at least one hydrogen atom bonded to a carbon atom constituting a repeating unit of the polymer chain is replaced with a fluorine atom or an organic group having a fluorine atom.

[0030] The polymer (A) is preferably a semi-crystalline fluororesin.

[0031] The term "semi-crystalline" is intended herein to mean a polymer (A) having a heat of fusion of more than 1 J / g, more preferably at least 8 J / g, as measured by differential scanning calorimetry (DSC) at a heating rate of 10°C / min according to ASTM D 3418.

[0032] The polymer (A) contains at least 50 mol %, preferably at least 60 mol %, more preferably at least 70 mol % of repeating units derived from VDF, based on all repeating units of the polymer (A).

[0033] The polymer (A) may comprise repeat units derived from at least one hydrophilic (meth)acrylic monomer (MA), where the comonomer (MA) preferably has the following formula: [ka] (In the formula, R1, R2, and R3 are equal to or different from each other and independently represent a hydrogen atom or a C1 to C3 hydrocarbon group; R OH is a hydroxyl group or a C1-C5 hydrocarbon moiety containing at least one hydroxyl group.

[0034] The term "at least one hydrophilic (meth)acrylic comonomer (MA)" is understood to mean that the polymer (A) may comprise repeat units derived from one or more hydrophilic (meth)acrylic comonomers (MA) as described above. In the remainder of the specification, the expressions "hydrophilic (meth)acrylic comonomer (MA)" and "comonomer (MA)" are understood for the purposes of the present invention both in the plural and in the singular, i.e. they mean one or more both hydrophilic (meth)acrylic comonomers (MA).

[0035] The hydrophilic (meth)acrylic comonomer (MA) preferably has the formula: [ka] (In the formula, R1, R2, R OH each of R1, R2, R3 has the meaning as defined above, and R3 is hydrogen; more preferably, each of R1, R2, R3 is hydrogen, while R OH have the same meaning as set out above).

[0036] Non-limiting examples of hydrophilic (meth)acrylic comonomers (MA) are, inter alia, acrylic acid, methacrylic acid, hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate; hydroxyethylhexyl (meth)acrylate.

[0037] The comonomer (MA) is more preferably selected from the group consisting of: -Hydroxyethyl acrylate (HEA) of the formula: [ka] -2-hydroxypropyl acrylate (HPA) of any of the following formulas: [ka] -acrylic acid (AA) of the formula: [ka] - and mixtures thereof.

[0038] More preferably, the comonomer (MA) is AA and / or HEA, even more preferably AA.

[0039] The determination of the amount of comonomer repeat units in the polymer (A) can be carried out by any suitable method, such as the NMR method. With regard to the monomers (MA), mention may be made in particular of the acid-base titration method, which is well suited for determining the acrylic acid content, the NMR method, which is suitable for the quantification of (MA) comonomers containing aliphatic hydrogen in the side chain (e.g. HPA, HEA), the weight difference based on the sum of the supplied (MA) comonomer and the unreacted residual (MA) comonomer during the preparation of the polymer (A), and the IR method.

[0040] When at least one hydrophilic (meth)acrylic comonomer (MA) is present, polymer (A) typically contains 0.05 to 10.0 mol % based on the total moles of repeat units of polymer (A).

[0041] The polymer (A) comprising at least one hydrophilic (meth)acrylic comonomer (MA) may be prepared according to WO 2008 / 129041.

[0042] The polymer (A) may comprise repeat units derived from at least one [comonomer (F)] different from VDF and the comonomer (MA), the [comonomer (F)] being selected from the group consisting of: (i) Tetrafluoroethylene (TFE); (ii) -Chloro- and / or bromo- and / or iodo-C2-C6 fluoroolefins, such as chlorotrifluoroethylene (CTFE) and 1,2-dichloro-1,2-difluoroethylene; (iii) Formula CF2=CFOR f1 (In the formula, R f1is a C1-C6 fluoro- or perfluoroalkyl group, for example -CF3, -C2F5, -C3F7), (per)fluoroalkyl vinyl ethers; and (iv) Hexafluoropropylene (HFP).

[0043] The most preferred fluorinated comonomers (F) are tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE), perfluoromethylvinylether (PMVE) hexafluoropropylene (HFP).

[0044] At least one comonomer (F) may be present in polymer (A) typically in an amount of from 0.05 mol % to 30.0 mol %, based on the total moles of repeat units of polymer (A).

[0045] According to a particular embodiment, the polymer (A) consists essentially of repeat units derived from VDF and monomer (MA), which monomer (MA) is preferably AA and the polymer (A) is a VDF-AA copolymer.

[0046] According to another embodiment, the polymer (A) consists essentially of repeat units derived from VDF and at least one comonomer (F). According to said embodiment, the comonomer (F) is preferably selected from CTFE, TFE and MVE. Preferred polymers (A) according to said embodiment are VDF-CTFE copolymers, VDF-TFE copolymers, VDF-HFP copolymers or VDF / TFE / PMVE terpolymers.

[0047] The polymer (A) may further comprise other moieties such as defects, end groups, etc., which do not affect or impair its physicochemical properties.

[0048] The polymer (A) is obtained by polymerizing the VDF monomer and optionally at least one comonomer (F), in suspension in an organic medium or in aqueous emulsion, according to procedures known in the literature.

[0049] The procedure for preparing the polymer (A) comprises polymerization in an aqueous medium in the presence of the radical initiator vinylidene fluoride (VDF) and, optionally, at least one comonomer (F), optionally in the presence of a chain transfer agent and a dispersing agent in the reaction vessel.

[0050] Generally, the process of the present invention is carried out at a temperature of at least 40°C, preferably at least 50°C, more preferably at least 60°C.

[0051] When the polymerization is carried out in suspension, the polymer (A) is typically provided in the form of a powder.

[0052] When the polymerization to obtain the polymer (A) is carried out in emulsion, the polymer (A) is typically provided in the form of an aqueous dispersion (D), which can be used as obtained directly by emulsion polymerization or after a concentration step. Preferably, the solids content of the polymer (A) in the dispersion (D) is in the range comprised between 20% and 50% by weight.

[0053] The polymer (A) obtained by emulsion polymerization can be isolated from the aqueous dispersion (D) by concentration and / or coagulation of the dispersion and can be obtained in powder form by subsequent drying.

[0054] The polymer (A) obtained by emulsion polymerization can also be isolated from the aqueous dispersion (D) by spray drying, a process in which a liquid material (solution or dispersion) is evaporated and a dry powder is produced by spraying the liquid feed into a hot drying medium (dry hot air) to obtain a small powder particle size as measured by PSD (particle size distribution).

[0055] The polymer (A) of the present invention is preferably obtained by emulsion polymerization in an aqueous polymerization medium, for example according to the procedures described in WO 2018 / 189090, in WO 2018 / 189091 and in WO 2018 / 189092.

[0056] In one embodiment of the present invention, the polymer (A) is advantageously obtained by emulsion polymerization, by a process carried out in the absence of any fluorosurfactant, for example according to the procedure described in WO 2019 / 076901.

[0057] The melting temperature of polymer (A) can be determined from the DSC curve obtained by differential scanning calorimetry (hereinafter also referred to as DSC). When the DSC curve shows multiple melting peaks (endothermic peaks), the melting temperature (Tm) is determined based on the peak with the largest peak area.

[0058] It is essential that the melting temperature of polymer (A) is below 220°C.

[0059] By using a VDF-based fluororesin having a higher melting temperature, adhesion of the binder (B) to the metal collector cannot be obtained and there is a high risk of delamination.

[0060] The binder (B) is obtained by mixing PTFE and polymer (A), both in powder form, or by mixing a PTFE latex with a polymer (A) latex, followed by co-coagulation and isolation by cryogenic or electrolytic procedures.

[0061] To obtain the desired polymer ratio in the blend, the dry content of the PTFE latex and / or polymer (A) latex can be estimated by drying 50 grams of the polymer latex at 200° C. in a thermobalance.

[0062] Generally, the weight ratio PTFE / polymer (A) will be comprised between 95 / 5 wt / wt and 30 / 70 wt / wt. The skilled person will select the most suitable weight ratio taking into account the target final properties of the binder (B).

[0063] Applicants have surprisingly discovered that the amount of polymer (A) added to PTFE does not affect its ability to fibrillate.

[0064] In another aspect, the present invention provides an electrode-forming composition [composition (C)] for use in preparing an electrode for an electrochemical device, comprising: a) at least one electrode active material (AM); b) a binder (B) as defined above; c) optionally, at least one conductive agent; The present invention provides an electrode-forming composition (C) comprising:

[0065] The amount of binder (B) that can be used in the electrode-forming composition (C) is influenced by a variety of factors. One such factor is the surface area and amount of the active material, as well as the surface area and amount of the conductivity-imparting additive that is added to the electrode-forming composition. These factors are believed to be important because the binder particles provide a bridge between the conductive material particles, keeping them in contact.

[0066] The electrode-forming composition of the invention [composition (C)] comprises one or more electrode active materials (AM). For the purposes of the present invention, the term "electrode active material" is intended to mean a compound capable of incorporating or intercalating into its structure and subsequently releasing therefrom alkali or alkaline earth metal ions during the charging and discharging stages of an electrochemical cell. The electrode active material is preferably capable of incorporating or intercalating and releasing lithium ions.

[0067] The nature of the electrode active material in the electrode-forming composition (C) of the present invention varies depending on whether the composition is used to prepare a negative electrode (anode) or a positive electrode (cathode).

[0068] When forming a positive electrode for a lithium-ion secondary battery, the electrode active material can include a composite metal chalcogenide of the formula LiMQ2 (where M is at least one metal selected from transition metals such as Co, Ni, Fe, Mn, Cr, and V, and Q is a chalcogen such as O or S). Among these, it is preferable to use a lithium-based composite metal oxide of the formula LiMO2 (where M is the same as defined above). Preferred examples of these include LiCoO2, LiNiO2, LiNi x Co 1-x O2 (0 < x < 1) and spinel-structured LiMn2O4 can be mentioned.

[0069] As an alternative form, when forming a positive electrode for a lithium-ion secondary battery, further, the electrode active material is of the formula M1M2(JO4) f E 1-f (where M1 is lithium and can be partially substituted by another alkali metal corresponding to less than 20% of the M1 metal, M2 is a transition metal of +2 oxidation level selected from Fe, Mn, Ni, or a mixture thereof, and can be partially substituted by one or more additional metals of +1 to +5 oxidation levels corresponding to less than 35% of the M2 metal including 0, JO4 is any oxyanion, J is any of P, S, V, Si, Nb, Mo, or a combination thereof, E is a fluoride, hydroxide, or chloride anion, and f is usually the mole fraction of the JO4 oxyanion included in the range of 0.75 to 1) of a lithiated or partially lithiated transition metal oxyanion-based electroactive material.

[0070] M1M2(JO4) as defined above f E 1-f The electroactive material preferably is phosphate-based and can have an ordered or modified olivine structure.

[0071] More preferably, the electrode active material when forming the positive electrode is of the formula Li 3-x M’ y M’’ 2-y(JO4)3, where 0≦x≦3 and 0≦y≦2, M′ and M″ are the same or different metals, at least one of which is a transition metal, JO4 is preferably PO4, which may be partially substituted with another oxyanion, and J is any of S, V, Si, Nb, Mo or a combination thereof. Even more preferably, the electrode active material has the formula Li(Fe x Mn 1-x LiFePO4 is a phosphate-based electroactive material (i.e., lithium iron phosphate of formula LiFePO4), where 0≦x≦1, and x is preferably 1.

[0072] When forming a negative electrode for a lithium ion secondary battery, the electrode active material may preferably include one or more carbon-based materials and / or one or more silicon-based materials.

[0073] In some embodiments, the carbon-based material may be selected from graphite, such as natural or artificial graphite, graphene, or carbon black. These materials may be used alone or as a mixture of two or more thereof.

[0074] The carbon-based material is preferably graphite.

[0075] The silicon-based compound may be one or more selected from the group consisting of chlorosilanes, alkoxysilanes, aminosilanes, fluoroalkylsilanes, silicon, silicon chloride, silicon carbide, and silicon oxide.

[0076] More specifically, the silicon-based compound may be silicon oxide or silicon carbide.

[0077] When present in the electrode active material, the silicon-based compound is included in an amount ranging from 1 to 60% by weight, preferably from 5 to 30% by weight, based on the total weight of the electroactive compound.

[0078] One or more optional conductivity-imparting additives may be added to improve the electrical conductivity of the resulting electrodes made from the compositions of the present invention. Conductive agents for batteries are known in the art.

[0079] Examples of these may include carbon-based materials such as carbon black, graphite fine powder, carbon nanotubes, graphene, or fibers, or metal fine powders or fibers such as nickel or aluminum. The optional conductive agent is preferably carbon black. Carbon black is available, for example, under the trademarks Super P® or Ketjenblack®.

[0080] When present, the conductive agent is different from the carbon-based material described above.

[0081] The amount of the optional conductive agent is preferably 0 to 30% by weight of the total solids in the electrode-forming composition. In particular, for the cathode-forming composition, the optional conductive agent is typically 0 to 10% by weight, more preferably 0 to 5% by weight of the total solids in the composition.

[0082] For anode-forming compositions that do not include a silicon based electroactive compound, the optional conductive agent is typically present in an amount of from 0% to 5% by weight, more preferably from 0% to 2% by weight, of the total amount of solids in the composition, while for anode-forming compositions that include a silicon based electroactive compound, it has been found beneficial to incorporate a larger amount of the optional conductive agent, typically from 0.5 to 30% by weight of the total amount of solids in the composition.

[0083] The electrode-forming composition (C) may be prepared by thoroughly mixing at least one electrode active material (AM), a binder (B) and, optionally, at least one conductive agent.

[0084] The high shear mixing induces fibrillation of the binder particles to produce fibrils that ultimately form a matrix or lattice to support the resulting composition. The resulting dough-like material can be calendered multiple times to produce a conductive film of desired thickness and density. The high shear can be provided by subjecting the mixture to an extruder.

[0085] The electrode-forming composition (C) of the present invention can be used in a method for producing an electrode [electrode (E)], the method comprising the steps of: - A) providing a binder (B) by combining polytetrafluoroethylene (PTFE) and a low melting point VDF-based fluororesin as defined above [polymer (A)]; - B) dry mixing, in the absence of a solvent, at least one electrode active material (AM), a binder (B) as defined above, and, optionally, at least one conductive agent; - C) feeding the powdered dry mixture obtained in step B) into a press to form a self-supporting dry film; and - D) applying the dry film to a conductive substrate to form an electrode. Includes.

[0086] In step B), mixing the electrode active material (AM), the binder (B) as defined above, and optionally at least one conductive agent is carried out by dry mixing these components without adding any solvent, liquid, processing aid, etc. to the particle mixture. Dry mixing can be carried out, for example, in a mill, mixer or blender (such as a V-blender equipped with a high-strength stirring bar or other alternative equipment as further described below) until a homogeneous dry mixture is formed. Those skilled in the art will recognize after reading this specification that the mixing time may vary based on batch size, materials, particle size, density, and other characteristics and still remain within the scope of the present invention.

[0087] In step C) of the process of the present invention, the powdery dry mixture obtained in step B) is subjected to a physical compression step to obtain a self-supporting dry film.

[0088] The compaction of the dry mixture obtained in step B) can be carried out as a physical compaction, for example by means of a roller compactor or a tablet press, but it may also be carried out as a rolling, build-up or by any other technique suitable for this purpose.

[0089] The mechanical compression step may be associated with a thermal consolidation step. The combination of pressure and heat treatment allows for thermal consolidation at lower temperatures than if it were carried out alone.

[0090] In one embodiment, the mechanical pressing step is carried out by pressing, suitably by pressing the dry mixture obtained in step B) between two metal foils. Preferably, the mechanical pressing step is carried out by applying a pressing pressure of 5-50 MPa, preferably 10-30 MPa.

[0091] The compression step is conveniently carried out at a temperature below 200°C, preferably below 180°C.

[0092] In step D), the dry film obtained in step C) is applied onto a conductive substrate to form an electrode.

[0093] The sheet of substrate material may comprise a metal foil, in particular an aluminium foil.

[0094] Due to the improved adhesion of the binder (B), the dry film obtained in step C) can be applied onto a conductive substrate without the need for any primer or adhesive layer.

[0095] The electrode (E) of the present invention is particularly suitable for use in electrochemical devices, in particular in secondary batteries.

[0096] In one aspect, the present invention provides an electrochemical device that is a secondary battery, the secondary battery comprising: -Positive and negative electrodes Including, Here, at least one of the positive electrode and the negative electrode is an electrode (E) according to the present invention.

[0097] Preferably, the electrochemical device comprises: -Positive and negative electrodes A secondary battery comprising: Here, the positive electrode is the electrode (E) according to the invention.

[0098] The secondary battery of the present invention is preferably an alkaline secondary battery or an alkaline earth secondary battery.

[0099] The secondary battery of the present invention is more preferably a lithium ion secondary battery.

[0100] Electrochemical devices according to the present invention can be prepared by standard methods known to those skilled in the art.

[0101] The present invention will now be described with reference to the following examples, the purposes of which are merely illustrative and are not intended to limit the scope of the invention.

[0102] raw materials Polymer (A-1): Intrinsic viscosity in DMF of 0.38 l / g at 25° C. and T 2f VDF-AA (0.6 mol %) polymer having Polymer (A-6): VDF-HFP (15 wt%) copolymer commercially available as Solef® 21510 from Solvay Specialty Polymers SpA; Polymer (A-8): VDF-CTFE copolymer commercially available as Solef® 32008 from Solvay Specialty Polymers SpA; PTFE 1: PTFE homopolymer powder having a specific gravity of 2160, measured according to ASTM D792, and a rheometric pressure of 9.50 MPa, measured according to ASTM D4895; PTFE 2: PTFE homopolymer powder having a specific gravity of 2.18, measured according to ASTM D792, and a rheometric pressure of 8.00 MPa, measured according to ASTM D4895; Lithium iron phosphate, LFP, available as Life Power manufactured by Matthey; Carbon black available as SC65 available from Imerys SA; Galden HT80, available from Solvay Materials; Electrolyte mixture of 1M LiPF6 in EC / DMC 1 / 1 v / v from Solvionic.

[0103] Polymer (A-2): Preparation of VDF / CTFE copolymer 1.3 l of demineralized water were introduced into a vertical autoclave made of AISI 316 steel, equipped with baffles with an agitator operating at 550 rpm. The temperature was then brought to the reaction temperature of 75 ° C; once this was reached, VDF was introduced, resulting in a pressure change of 4 absolute bar. A pure gas mixture of 90% VDF and 10% CTFE in molar amounts was then added via a compressor until a pressure of 20 absolute bar was reached. 45 ml of a solution of ammonium persulfate (APS) in demineralized water at a concentration of 3% by weight were then fed. The polymerization pressure was kept constant by feeding the above mixture; when 300 g of gas had been fed, the reactor was cooled at room temperature and then stripped. The polymer (A-2) containing 92% by mole of VDF and 8% by mole of CTFE was discharged in latex form and degassed.

[0104] Polymer (A-3): Preparation of VDF / CTFE copolymer The same procedure was followed as for the preparation of polymer (A-2), except that a reactor with a volume of 5 l, 3.4 l of demineralized water, 25 g of a solution of c-C6O4 ammonium salt dissolved in demineralized water at a concentration of 35%, 25 ml of a solution of ammonium persulfate (APS) in demineralized water at a concentration of 3% by weight, and 500 g of a mixture of 90% VDF and 10% CTFE in molar amounts were fed into the reactor. Polymer (A-3), containing 90.4 mol% VDF and 9.6 mol% CTFE, was discharged in latex form and degassed.

[0105] Polymer (A-4): Preparation of VDF / TFE / PMVE terpolymer The same procedure was followed as for the preparation of polymer (A-2), except for the stirrer at 650 rpm, 1.4 l of demineralized water, a temperature of 80° C., the introduction of VDF resulting in a pressure change of 8 bar, the introduction of MVE (methyl vinyl ether) resulting in a pressure change of 6 bar, 10 g of a solution of c-C6O4 ammonium salt dissolved in demineralized water at a concentration of 35%, and 15 ml of a solution of ammonium persulfate (APS) in demineralized water at a concentration of 2% by weight. A gaseous mixture of VDF / TFE in a molar ratio of 90% / 10% was then added via a compressor until a pressure of 20 absolute bar was reached. Once the polymerization had started, a mixture of 150 grams of 10% TFE and 90% VDF in molar amounts was fed into the reactor. Polymer (A-4), containing 81.1 mol % VDF, 10.3 mol % TFE and 8.6 mol % PMVE, was discharged in latex form and degassed.

[0106] Polymer (A-5): Preparation of VDF / TFE copolymer The same procedure was followed as in Example (A-2), except that a 5 liter reactor was used, 3.5 liters of demineralized water were introduced, VDF was introduced resulting in a pressure change of 7.8 bar, and 10 ml of a solution of ammonium persulfate (APS) in demineralized water at a concentration of 3% by weight, 500 grams of a mixture of 25% TFE and 75% VDF in molar amounts were fed into the reactor.

[0107] Polymer (A-7): Preparation of VDF / CTFE copolymer. Spray drying The same procedure was followed for the preparation of polymer (A-2), except that the isolation of the polymer was carried out by spray drying.

[0108] Polymer (C-1): Preparation of VDF / TFE Copolymer - Comparative Example The same procedure was followed as for the preparation of polymer (A-4), except that a 5 liter reactor was used, 3.5 liters of demineralized water, the introduction of VDF resulting in a pressure change of 8 bar, 30 g of a solution of c-C6O4 ammonium salt dissolved in demineralized water at a concentration of 35%, 20 ml of a solution of ammonium persulfate (APS) in demineralized water at a concentration of 3% by weight, 500 g of a mixture of 80% TFE and 20% VDF in molar amounts, and polymer (C-1), containing 31 mol% VDF and 69 mol% TFE, was discharged in latex form and degassed.

[0109] Polymer Post-Treatment The polymers (A-2), (A-3), (A-4), (A-5) and (C-1) in powder form are - Aluminum sulfate (Al2(SO4)3) with a concentration of 2 g / l calculated based on the amount of water in the coagulation vessel, - Ammonium carbonate ((NH4)2CO3) at a concentration of 8 g / l calculated based on the amount of water in the coagulation vessel, or -Nitric acid (HNO3), 25 ml of 65% solution calculated based on the amount of water in the coagulation vessel The latexes were obtained by cryogenic or electrolytic coagulation using

[0110] After coagulation had taken place, each polymer was washed with demineralized water at room temperature. After coagulation and washing, the resulting powder was then dried in a ventilated oven at a temperature of at least 80° C. and less than 120° C. for 24 hours.

[0111] The melting temperatures and fusion enthalpies (ΔH) of polymers (A-1) to (A-7) and (C-1), characterized by DSC according to the ASTM D3418 standard, are reported in Table 1.

[0112] [Table 1]

[0113] Polymer Blend The fluororesin and PTFE were mixed in powder form or by latex mixing and then co-coagulated according to the procedure reported in the post-treatment section above. The composition details are reported in Table 2.

[0114] Metal substrate lamination Films containing compositions of either polymers (A-1) to (A-6) and (C-1) with either PTFE-1 or PTFE-2 were obtained by compressing the powder compositions between two aluminum foils. The lamination process was carried out at a temperature below 180° C. and a pressure of 160 bar. The material was left in the heating device for 6 minutes and then pressed at operating pressure with 6 degassing steps (by manually releasing the pressure) for a total of 180 seconds. The samples were then cooled at room temperature by a cold press. The samples were left in the cold press at the same pressure as in the first step for a period comprised between 4 and 6 minutes.

[0115] Adhesion evaluation and measurement Adhesion evaluation and measurements were carried out between the film obtained as above and the aluminum foil in the three-layer structure (metal foil / film / metal foil) obtained after lamination according to ASTM D 1876. The adhesion values ​​are reported in Table 2.

[0116] [Table 2]

[0117] * The dry content of the latex was evaluated by drying 50 g of the polymer latex at 200° C. in a thermobalance to obtain the desired polymer ratio in the blend.

[0118] No adhesion was observed for PTFE-1 and PTFE-2 powders when used alone in the preparation of a film by compression between two aluminum foils.

[0119] Particle size of polymers (A-2) and (A-7) The particle size distribution (PSD) of polymers (A-2) and (A-7) was measured for binding of dry samples according to ISO 13-320:2020 using a Laser Diffraction Beckman Coulter LS 13320. The results are shown in Table 3 as D50.

[0120] [Table 3]

[0121] Processability The processability of the materials was examined using a Capillary Rheometer (Goettefert Rheograph 2003) at shear rates of 10 s-1 and 100 s-1 and a temperature of 180 °C, using a die (L / D 10, diameter 1 mm, operating at an angle of 20°).

[0122] The applicant has surprisingly discovered that the binder composition according to the invention can be suitably processed at temperatures well below the melting point of PTFE, and thus binder extrudates are obtained despite the presence of amounts of low melting point VDF-based fluoroplastic additive. EXAMPLES

[0123] Example 1: Electrode preparation: composition of polymer (A-2) and PTFE-1 in a ratio of 20 / 80 A dry mixture of 1.8 g of LFP and 0.1 g of SC65 was prepared by grinding the two powders in a ceramic mortar with a pestle for 5 minutes.

[0124] 1 ml of Galden HT80 was added to the powder mixture and the complex was mixed in a Vortex mixer at 20000 rpm for 0.5 min. A homogenous paste was obtained.

[0125] 0.02 g of polymer (F-2) and 0.08 g of PTFE-1 were added to the homogenous paste and the mixture was mixed in a Vortex mixer at 16000 rpm for 0.5 min. A homogenous composite was obtained.

[0126] The composite was then mixed in a ceramic mortar with a pestle for an additional 5 minutes to fibrillate the polymer.

[0127] The composite was finally pressed in a flat press at 24 tons for 5 minutes to obtain tablets of 20 mm diameter and 0.9 mm thickness.

[0128] This procedure was repeated to prepare three tablets.

[0129] The tablets were dried in a vacuum oven (Buchi) at 90° C. under reduced pressure for 2 hours to obtain the electrode composition.

[0130] The resulting positive electrode had the following composition: 90% by weight of LFP, 1% by weight of polymer (A-2), 4% by weight of PTFE-1 and 5% by weight of carbon black. Thus, electrode EC1 was obtained.

[0131] Comparative Example 1: Electrode preparation using PTFE-1 A dry mixture of 1.8 g of LFP and 0.1 g of SC65 was prepared by grinding the two powders in a ceramic mortar with a pestle for 2 minutes.

[0132] 1 ml of Galden HT80 was added to the powder mixture and the complex was mixed in a Vortex mixer at 20000 rpm for 0.5 min. A homogenous paste was obtained.

[0133] 0.1 g of polymer PTFE-1 was added to the homogenous paste and the mixture was mixed in a Vortex mixer at 16000 rpm for 0.5 minutes. A homogenous composite was obtained.

[0134] The composite was then mixed in a ceramic mortar with a pestle for an additional 5 minutes to fibrillate the polymer.

[0135] The composite was finally pressed in a flat press at 24 tons for 5 minutes to obtain tablets of 20 mm diameter and 0.9 mm thickness.

[0136] This procedure was repeated to prepare three tablets.

[0137] The tablets were dried in a vacuum oven (Buchi) at 90° C. under reduced pressure for 2 hours to obtain the electrode composition.

[0138] The positive electrode obtained had the following composition by weight: 90% LFP, 5% polymer PTFE-1 and 5% carbon black. Electrode CE1 was thus obtained.

[0139] Electrical Resistance Measurement Electrical resistance measurements of EC1 and CE1 were carried out in ElCel between two stainless steel plates (diameter 18 mm) by applying a perturbation of 10 mV rms at frequencies between 1 Hz and 200 kHz recording 10 points per decade. The electrical resistance results were taken from the intercept of the complex impedance spectrum with the x-axis. The average resistance of the two samples, calculated as the average value of the measurements repeated in triplicate, is reported in Table 3 below. The values ​​are similar for both compositions, indicating that no negative contribution arises from the use of a polymer different from PTFE.

[0140] Ionic Resistance Measurement Electrical resistance measurements of EC1 and CE1 were carried out in ElCel between two stainless steel plates (18 mm diameter) by wetting the disk with 0.5-0.6 ml of electrolyte and waiting 10 minutes to have the disk wetted. A perturbation of 10 mV rms with frequencies between 1 Hz and 200 kHz is applied recording 10 points per decade. The ionic resistance results were taken from the intercept with the x-axis of the complex impedance spectrum. The average resistance of the two samples, calculated as the average of measurements repeated in triplicate, is reported in Table 3 below. The values ​​are similar for both compositions.

[0141] [Table 4]

[0142] The data show that the binders of the present invention have improved adhesion to current collectors while retaining good ionic and conductive properties, making them suitable for use as binders for electrodes.

[0143] Example 2: Electrode Fabrication: Composition of Polymer (A-8) and PTFE-2 in a 50 / 50 Ratio A dry mixture of 5.41 g of graphite, 1.36 g of silicon, and 0.072 g of SC45 was prepared by grinding the powders in an electric mortar for 10 minutes. 5 ml of Galden HT80 was added to the powder mixture and the composite was mixed in the electric mortar for 1 minute. A homogeneous paste was obtained. 0.18 g of polymer (A-8) powder, 0.18 g of PTFE, along with 4 ml of Galden HT80, were added to the homogeneous paste and mixed in the mortar grinder for 5 minutes. A homogeneous composite was obtained. The composite was then manually manipulated to fibrillate the polymer and obtain an overall bonded free-standing film. The film was calendered to reduce the thickness to less than 200 μm. The resulting negative electrode had the following composition: 75.2% by weight of graphite, 18.8% by weight of silicon, 2.5% by weight of polymer (A-8), 2.5% by weight of PTFE and 1% by weight of carbon black. Thus, electrode EC2 was obtained. An EC2 sample was placed between two copper current collectors and preheated in a press at 150° C. for 10 minutes. Then, 160 bar was applied for 10 minutes to laminate EC2 onto the current collectors at a temperature of 150° C. and a pressure of 160 bar. Then, the press was cooled to room temperature without releasing the pressure.

[0144] Adhesion evaluation and measurement Adhesion evaluation and measurements were performed between the samples laminated as described above and the foils in the three-layer structure (metal foil / film / metal foil) obtained after lamination according to ASTM D 1876. The adhesion levels are reported below in Table 4. No adhesion was observed for the PTFE powder, neither between two aluminum foils nor between two copper foils, when used alone in the preparation of a film by compression.

[0145] The peel strength was evaluated based on the following criteria: A higher value for peel strength indicates better intimate adhesion between the polymer and the current collector. A: Peel strength of at least 10.0N / m B: Peel strength of at least 2N / m and less than 10.0N / m C: Adhesion was achieved but difficult to measure due to sample stiffness D: No adhesion

[0146] [Table 5]

[0147] Cathode fabrication (NMC / C65 / binder) Cathodes were prepared by a two-step process using a binder composition comprising either polymer (A-2), (A-3), (A-6), (A-7) or (A-8) and PTFE-1.

[0148] The first step was dry mixing of the powders in a planetary mixer (Speedmixer) by first mixing NMC111 and C65 together at high speed for 10 minutes. Then the polymer blend PTFE-1 / polymer was added to the mixture and mixed in the same mixer at a lower speed for 2 minutes. The resulting mixture was poured into an agate mortar (heated at 90° C.) and shear was applied to the mixture for about 5 minutes. The mixture produced a “pre-film” with dimensions of about 50×20×2 mm (l×L×t).

[0149] The second step was a hot rolling step in a two-roll calender (Collin W100 T) heated at 90 ° C or 180 ° C (different fibrillation temperatures). Calender rolls with a speed ratio of 1.2 to 1 were used. The main roll was driven at a rotation speed of 2 rpm and the second roll at a speed of 2.4 rpm. The material was introduced and processed into the calender five times, decreasing the gap from 2000 μm to 250 μm, gradually applying shear forces to the membrane, thereby avoiding excessive increases in compression forces. At each step, the gap was about 75% of the previous step. Once the minimum calender gap was reached (250 μm), the membrane was folded in four, rotated 90 ° and then inserted, starting here from 1000 μm and decreasing to 250 μm by a series of about 6-7 reductions, and processed into the calender again following the same method. As a result, a free-standing membrane of about 250 μm was obtained. The thickness of the cathode produced by this process was further reduced using another calendering machine (GK300L Saueressig). This second calendering machine with two rolls rotates at the same speed but allows a gap between the rolls of the order of 50 μm. A target thickness of 90 μm was reached. The thickness reduction was carried out without the use of solvents, making the process an "all-dry-process".

[0150] The cathode was then co-laminated onto a 20 μm thick aluminum sheet in a GK300L Saueressig calender at a temperature of 180° C. and a pressure of 25 bar.

[0151] Adhesion of cathode onto Al current collector (peel test) The adhesive strength of the positive electrode to the Al current collector was evaluated using a 180° peel test. * 10 cm) was attached to a rigid Al plate (2.6 cm) using double-sided tape (width 25 mm; thickness 0.24 mm). *The electrode was fixed on a 10 cm (10 cm) plate with the electrode facing down and the current collector facing up. The Al current collector was peeled off from the electrode using a motorized tension / compression test stand (ESM303 from Mark-10 Corporation) while maintaining an angle of 180° and a constant speed of 300 mm / min. The force required to remove the Al current collector from the electrode was recorded in Table 5 as the average value of three independent strips generated from three independent electrodes using three independent slurries with the same composition. The peel test was performed in a dry room with a dew point of -40°C.

[0152] [Table 6]

[0153] In all the positive electrodes of EC3 to EC7, excellent adhesion properties to the Al current collector were clearly observed, which were distinguishable from those of CE2.

[0154] Furthermore, the data in Table 5 demonstrates that additional benefits to adhesion to the current collector, including improved peel strength and reduced variability, are obtained by decreasing the particle size of the additive. Also, visually, it is apparent that the electrodes obtained with the smaller particle size additive are more uniform.

[0155] In contrast, additives with higher melt temperatures perform slightly worse with respect to adhesion than comparable additives with lower melt temperatures.

[0156] The data also show that higher fibrillation temperatures result in higher adhesion.

[0157] Capacity retention test Battery manufacturing: The Pat cell (EL-Cell GmbH) was fitted with a balanced graphite anode disk (graphite 4.5 mAh / cm purchased from NEI). 2Coin cells were fabricated by stamping small disks of the positive electrodes prepared according to CE2, EC3, EC6 and EC7 together with a 100% ethylene glycol fluoride (Nanomyte® BE-200E). The electrolyte used to fabricate the coin cells was a mixture of 1 M LiPF6 solution in EC / DMC 1 / 1 v / v from Sigma Aldrich. Polyethylene separators (commercially available from Tonen Chemical Co., Ltd.) were used as received.

[0158] Cycling stability of full cells at a C-rate of 1C (open capacity was measured in duplicate and is shown in Table 6 below):

[0159] [Table 7]

[0160] The results in Table 6 show that electrodes according to the invention have better cycling stability than electrodes containing only PTFE.

Claims

1. A binder composition [binder (B)] for use in preparing an electrode for an electrochemical device, comprising: a. Polytetrafluoroethylene (PTFE); b. A low-melting-point VDF-based fluororesin [polymer (A)] having a melting point of less than 220°C, wherein the polymer (A) is at least 50 mol % of repeating units derived from VDF, based on all repeating units of said polymer (A); and - comprising repeat units derived from at least one comonomer (CM), said comonomer (CM) being a hydrophilic (meth)acrylic comonomer [comonomer (MA)], and (i) tetrafluoroethylene (TFE); (ii) chloro- and / or bromo- and / or iodo-C, such as chlorotrifluoroethylene (CTFE) ２ ~C ６ Fluoroolefins; (iii) Formula CF ２ =CFOR ｆ１ (In the formula, R ｆ１ is C １ ~C ６ Fluoro- or perfluoroalkyl groups, such as —CF ３ , -C ２ F ５ , -C ３ F ７ (per)fluoroalkyl vinyl ethers of the formula (iv) hexafluoropropylene (HFP) a low-melting-point VDF-based fluororesin [polymer (A)] selected from the group consisting of a fluorinated comonomer [comonomer (F)] selected from the group consisting of: Including, A binder composition [binder (B)] characterized in that the at least one comonomer (F) is present in the polymer (A) in an amount of 0.05 mol % to 30.0 mol % relative to the total number of moles of repeating units of the polymer (A).

2. The binder (B) according to claim 1, wherein the polymer (A) contains at least 60 mol %, more preferably at least 70 mol %, of repeating units derived from VDF, based on all repeating units of the polymer (A).

3. A binder (B) as described in claim 1, wherein the weight ratio of PTFE / polymer (A) is between 95 / 5 wt / wt and 30 / 70 wt / wt.

4. The comonomer (MA) has the formula: 【Chemistry 1】 (Wherein, R1, R2, and R3 are equal to or different from each other and independently represent a hydrogen atom or C １ ~C ３ is a hydrocarbon group, R ＯＨ is a hydroxyl group or a C containing at least one hydroxyl group １ ~C ５ The binder (B) according to claim 1, wherein the hydrocarbon moiety is a hydrocarbon moiety.

5. The comonomer (MA) Hydroxyethyl acrylate (HEA) of the formula: 【Chemistry 2】 2-hydroxypropyl acrylate (HPA) of any of the following formulas: 【Transformation 3】 Acrylic acid (AA) of the formula: 【Chemistry 4】 The binder (B) according to claim 4, selected from the group consisting of: and mixtures thereof.

6. The binder (B) according to claim 5, wherein the polymer (A) is a VDF-AA copolymer.

7. 2. The binder (B) according to claim 1, wherein the polymer (A) is selected from VDF-CTFE copolymers, VDF-TFE copolymers, VDF-HFP copolymers or VDF / TFE / PMVE terpolymers.

8. An electrode-forming composition [composition (C)] for use in producing an electrode for an electrochemical device, comprising: a) at least one electrode active material (AM); b) a binder (B) according to any one of claims 1 to 7; c) optionally at least one conductive agent; An electrode-forming composition [composition (C)] comprising:

9. A method for producing an electrode [electrode (E)] for an electrochemical cell, comprising the steps of: -A) providing the binder (B) according to any one of claims 1 to 7 by combining polytetrafluoroethylene (PTFE) and a low-melting-point VDF-based fluororesin [polymer (A)]; - B) dry mixing, in the absence of a solvent, at least one electrode active material (AM), said binder (B) as defined above, and optionally at least one conductive agent; -C) feeding the powdered dry mixture obtained in step B) into a press to form a self-supporting dry film; and -D) applying said dry film to a conductive substrate to form said electrode. A method comprising:

10. 10. The method according to claim 9, wherein in step B) the dry mixing is carried out, for example in a mill, mixer or blender, until a homogeneous dry mixture is formed.

11. 10. The process according to claim 9, wherein step C) is carried out at a temperature of not more than 200°C, preferably at a temperature below 180°C.

12. An electrode (E) for a secondary battery obtainable by the method according to claim 9.

13. Electrochemical device, such as a secondary battery, comprising at least one electrode (E) according to claim 12.

14. The electrochemical device is - Positive and negative electrodes A secondary battery comprising:

14. The electrochemical device according to claim 13, wherein at least one of the positive electrode and the negative electrode is the electrode (E) according to claim 12.

15. The electrochemical device is - Positive and negative electrodes A secondary battery comprising:

15. The electrochemical device according to claim 14, wherein the positive electrode is the electrode (E) according to claim 12.

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