Acrylate polymers as additives in battery electrodes

Abstract

The present invention relates to an electrode composition comprising certain branched (meth)acrylic polymers, to a method for its preparation and to its use for the manufacture of electrochemical cell components.

Description

ACRYLATE POLYMERS AS ADDITIVES IN BATTERY ELECTRODESCross reference to previous applications

[0001] This application claims priority to European application No. 24315077.8 filed on 1 March 2024, the whole content of this application being incorporated herein by reference for all purposes.Technical Field

[0001] The present invention relates to an electrode composition comprising certain branched (meth)acrylic polymers, to a method for its preparation and to its use for the manufacture of electrochemical cell components.Background Art

[0001] To date, the electrodes of a lithium secondary battery are mainly manufactured by a wet process that comprises preparing a slurry in which an electrode active material, additives and a binder are dispersed in a solvent or an aqueous medium, and processing the slurry in a way that forms an electrode film.

[0002] Dry electrode processes have been developed to reduce the time-consuming and costly drying procedures required by the aforementioned wet processes.

[0003] Typical dry processes use the fibrillation properties of certain polymers to provide a matrix for embedded conductive material. Some of the polymers in the family of fluoropolymers, such as polytetrafluoroethylene (PTFE), are particularly inert and stable in the common electrolyte solvents used in secondary batteries, even those using organic solvent at high working or storage temperatures. Thus, the stability of an electrode made using PTFE can be higher than those made with other binders.

[0004] For example, dry electrode preparation processes can include combining a PTFE binder with active electrode material in powder form, and calendering to form an electrode film. However, although PTFE has good adhesiveness to the electrode active material, it has difficulty in adhesiveness to the current collector.

[0005] Known in the art are methods to improve the adhesiveness to the current collector and electrode active material of PTFE, by using PTFE and tetrafluoroethylene / hexafluoropropylene copolymer (FEP) in combination as a binder, achieving a material having the melting point of FEP (240 to 270 °C) or higher (JP2000149954A). However, in order to heat to a temperature equal to or higher than the melting point of FEP, specifically, 280 °C or higher, a special heat treatment device is required to provide an electrode film, and it is disadvantageous in terms of energy.

[0006] WO 2023 / 094623 teaches that the addition of certain VDF-based fluororesin improves the adhesion of PTFE to current collectors, while at the same time keeping good ionic and electric conductivity properties of the PTFE.

[0007] The need for polymers that guarantee improved adhesion of PTFE to current collectors, is still felt both in research and from industrial perspectives.

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

[0009] The Applicant has now found that the addition of certain branched (meth)acrylic polymers is particularly effective in ensuring improved adhesion to PTFE when used as binders for electrodes for secondary batteries.

[0010] It is thus an object of the invention a binder composition [binder (B)] for use in the preparation of electrodes for electrochemical devices, characterized by comprising: a. a polytetrafluoroethylene (PTFE); and b. at least one branched (meth)acrylic polymer [polymer (A)] comprising: b1. recurring units derived from at least one molecule comprising at least two vinyl groups [monomer (BM)]; and b2. recurring units derived from at least one (meth)acryloyl monomer [monomer (MAM)], wherein monomer (MAM) is selected from the group consisting of- (meth)acrylamide acid esters of formula CH2=C(R)-C(=O)-NH-Rh, wherein R means hydrogen or an alkyl group with 1 to 3 carbon atoms and Rh means a linear or branched alkyl residue with 1 to 30 carbon atoms, preferably with 1 to 15 carbons, more preferably with 1 to 5 carbons, and- methylmethacrylate, wherein polymer (A) contains at least 70% by moles of monomer (MAM).

[0011] In another aspect the present invention provides an electrode-forming composition [composition (C)] for use in the preparation of electrodes for electrochemical devices, characterized by comprising: a) at least one electrode active material (AM); b) a binder (B) as above defined; and c) optionally, at least one conductive agent.

[0012] The applicant has surprisingly found that the processability of the binder (B) make it suitable for the preparation of electrodes by dry processes or extrusion at low temperatures, thus providing electrodes by a very efficient process.

[0013] In another aspect the present invention thus provides a process for manufacturing an electrode [electrode (E)] for electrochemical cell, said process comprising: -A) combining a polytetrafluoroethylene (PTFE) and at least one branched (meth)acrylic polymer (A) as above defined to provide a binder (B); -B) dry mixing the at least one electrode active material (AM), the binder (B) as above defined, and optionally, at least one conductive agent in the absence of solvent;-C) feeding the powdered dry mixture obtained in step B) to a compactor to form a self-supporting dry film; and-D) applying the dry film to an electrically conductive substrate to form the electrode.

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

[0015] In the context of the present invention, the use of parentheses “(... )” before and after symbols or numbers identifying formulae or parts of formulae has the mere purpose of better distinguishing that symbol or number with respect to the rest of the text; thus, said parentheses could also be omitted.

[0016] In the context of the present invention, the term “weight percent” (wt %) indicates the content of a specific component in a mixture, calculated as the ratio between the weight of the component and the total weight of the mixture. When referred to the recurring units derived from a certain monomer in a polymer / copolymer, weight percent (wt %) indicates the ratio between the weight of the recurring units of such monomer over the total weight of the polymer / copolymer. When referred to the total solid content of a liquid composition, weight percent (wt %) indicates the ratio between the weight of all non-volatile ingredients in the liquid.

[0017] As used herein, the terms “adheres” and “adhesion” indicate that two layers are permanently attached to each other via their surfaces of contact.

[0018] By the term "electrochemical device", it is hereby intended to denote an electrochemical cell / assembly comprising a positive electrode, a negative electrode and a liquid electrolyte, wherein a monolayer or multilayer separator is in contact to at least one surface of one of the said electrodes. Non-limitativeexamples of suitable electrochemical devices include, notably, secondary batteries, especially, alkaline or an alkaline- earth secondary batteries such as lithium ion batteries, lead-acid batteries, and capacitors, especially lithium ionbased capacitors and electric double layer capacitors (supercapacitors). Non- limitative examples of electrochemical cells include, notably, batteries, preferably secondary batteries, and electric double layer capacitors.

[0019] For the purpose of the present invention, by "secondary battery" it is intended to denote a rechargeable battery. Non-limitative examples of secondary batteries include, notably, alkaline or alkaline-earth secondary batteries.

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

[0021] It is understood, however, that the PTFE polymer may also comprise minor amounts of one or more co-monomers such as, but not limited to, hexafluoropropylene, perfluoro(methyl vinyl ether), perfluoro(propyl vinyl ether), perfluoro-(2,2- dimethyl-l,3-dioxole), and the like, provided, however that the latter do not significantly adversely affect the unique properties of the tetrafluoroethylene homopolymer, such as thermal and chemical stability. Preferably, the amount of such co-monomer does not exceed about 3 % by moles, and more preferably less than about 1% by moles; particularly preferred is a co-monomer content of less than 0.5 % by moles. In the case that the overall co-monomer content is greater than 0.5 % by moles, it is preferred that the amount of the perfluoro(alkyl vinylether) co-monomer is less than about 0.5 % by moles. Most preferred are PTFE homopolymers.

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

[0023] PTFE in the form of powder may be obtained by coagulating PTFE lattices by means of cryogenic coagulation or by electrolytic coagulation with the addition of an electrolyte. See, for example, US 6790932. Preferred examples of electrolytes are:-Aluminum sulphate (Ah(SO4)3), in concentration of 2g / l calculated on amount of water in the coagulation vessel,-Ammonium carbonate ((NF hCCh), in concentration of 8g / l calculated on amount of water in the coagulation vessel, or-Nitric acid (HNO3), 25ml of a solution at 65% calculated on amount of water in the coagulation vessel.

[0024] Alternatively, the powder of PTFE may be obtained from PTFE lattices in the form of gels by means of coagulation with the electrolytes mentioned above. The gels may be obtained according to patents US 6790932 and US 6780966.

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

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

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

[0028] The term “fluororesin” is hereby intended to a resin in which at least one hydrogen atom bonded to a carbon atom constituting a repeating unit of a polymer chain is substituted with a fluorine atom or an organic group having a fluorine atom.

[0029] The at least one branched (meth)acrylic polymer (A) is a polymer comprising recurring units derived from at least one (meth)acryloyl monomer [monomer (MAM)] with at least one branching monomer, which is a molecule comprising at least two vinyl groups [monomer (BM)].

[0030] Polymer (A) is a copolymer. By "copolymer" as used herein it is intended to denote a polymer having two or more different monomer units. The copolymer could be a terpolymer with three or more different monomer units, or have four or more different monomer units. The copolymer may be a random copolymer, a gradient copolymer, or a block copolymer formed by a controlled polymerization process. Preferably, the copolymer is formed by a free radical polymerization process or an anionic polymerization process, and the process can be any polymerization method known in the art, including but not limited to emulsion, solution, suspension polymerization, and can be done in bulk, and semi-bulk.

[0031] The terms “acrylic” and “acrylate” are used interchangeably (unless to do so would alter the intended meaning) and include acrylic acids and derivatives thereof. The terms “(meth)acrylic” or “(meth)acrylate” are intended to cover both the acrylic / acrylate and methacryl ic / methacrylate forms of the indicated material, e.g., a (meth)acrylate monomer.

[0032] The term (meth)acryloyl monomer (MAM) refers to the monomer having a (meth)acryloyl group in the molecule.

[0033] Suitable (meth)acryloyl monomers (MAM) are hydrophobic (meth)acryloyl monomers that may for example, be chosen from (meth)acrylamide acid esters offormula CH2=C(R)-C(=O)-NH-Rh wherein R means hydrogen or an alkyl group with 1 to 3 carbon atoms and Rh means a linear or branched alkyl residue with 1 to 30 carbon atoms, preferably with 1 to 15 carbons, more preferably with 1 to 5 carbons or methylmethacrylate.

[0034] The polymer (A) contains at least 70% by moles of monomer (MAM).

[0035] Polymer (A) may further contain recurring units derived from at least one (meth) acrylic acid ester, different from methylmethacrylate, wherein said at least one (meth) acrylic acid ester is selected from the group consisting of methyl, ethyl (meth)acrylate, n-propyl(meth)acrylate, isopropyl(meth)acrylate, n-butyl (meth)acrylate, methoxy ethyl (meth)acrylate, 2-ethoxy ethyl (meth)acrylate, tertbutyl (meth)acrylate, pentyl (meth)acrylate, heptyl (meth)acrylate, 2-tert- butylheptyl (meth)acrylate, octyl (meth)acrylate), iso-octyl (meth)acrylate, nonyl (meth)acrylate, decyl (meth)acrylate, undecyl (meth)acrylate, 5-methylundecyl (meth)acrylate, dodecyl (meth)acrylate, isobornyl (meth)acrylate, norbornyl (meth)acrylate, cycloalkyl (meth)acrylate, like cyclohexyl(meth)acrylate, phenyl (meth)acrylate.

[0036] Polymer (A) may also include recurring units derived from other alpha, beta- ethylenically unsaturated monomers bearing functionalities such as carboxyl groups or substituted alkyl esters.

[0037] Suitable alpha, beta-ethylenically unsaturated monomers bearing functionalities can be selected from hydrophilic (meth)acryloyl monomer, such as monoethylenically unsaturated monocarboxylic acid and derivatives. This include, among others, acrylic acid, methacrylic acid (MAA), hydroxyethyl(meth)acrylate, hydroxypropyl(meth)acrylate, hydroxyethylhexyl(meth)acrylate, crotonic acid, 2- carboxyethyl acrylate oligomers such as Sipomer®B-CEA.

[0038] The expression “methylmethacrylate polymer” is used within the frame of the present invention for designating a polymer made of recurring units, wherein more than 70 % by moles of said recurring units being derived from methylmethacrylate (MMA).

[0039] Preferred (meth)acrylic polymers (A) for use in composition (C) of the present invention are methylmethacrylate polymers.

[0040] According to said preferred embodiment, polymer (A) may contain from 1 to 45, preferably 3 to 30, and more preferably 5 to 20 % by moles of at least one comonomer copolymerizable with methylmethacrylate, including but not limited to monomers (MAM) as above defined, or other alpha, beta-ethylenically unsaturated monomers bearing functionalities such as carboxyl groups or substituted alkyl esters.

[0041] Polymer (A) contains at least one branching monomer (BM).

[0042] According to the present invention, a branching monomer is a monomer that during polymerization can react at least in two different positions, resulting in branched chain growth.

[0043] During the preparation of polymer (A), the (meth)acryloyl monomer (MAM) may grow in the polymerization reaction in two directions, reacting with another (meth)acryloyl monomer or with the branching monomer.

[0044] According to the present invention, the branching monomer (BM) is a molecule comprising at least two vinyl groups. The branching monomer (BM) may also comprise more than two vinyl groups. These vinyl groups are suitable for polymerization in an addition polymerization reaction. Many of such molecules are readily available, or may be prepared by reacting any di- or multifunctional molecule with a suitably reactive vinylic reactant. Examples include di- or multivinyl esters, di- or multivinyl amides, di- or multivinyl aryl compounds (including those with heterocyclic aryl groups), and di- or multivinyl alkyl / aryl ethers.

[0045] Branching monomers include, but are not limited to, divinyl aryl monomers such as divinyl benzene; (meth)acrylate diesters such as alkylene di(meth)acrylates such as ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, 1,4- butylene glycol di(meth)acrylate; oligo alkylene glycol di(meth)acrylates such as e.g. tetraethyleneglycol di(meth)acrylate, poly(ethyleneglycol) di(meth)acrylate, poly (propyleneglycol) di(meth)acrylate; divinyl (meth)acrylamides such as methylene bisacrylamide; divinyl ethers such as poly(ethyleneglycol)divinyl ether; and tetra- or tri-(meth)acrylate esters such as pentaerythritol tetra (meth) acrylate, trimethylolpropane tri(meth)acrylate or glucose di- to penta (meth)acrylate.

[0046] Preferred branching monomers are divinyl benzene, a,co-alkylene di(meth)acrylates or divinyl (meth)acrylamides, most preferred may be a, coalkylene di(meth)acrylates such as ethylene glycol di(meth)acrylate and 1 ,4- butylene glycol di(meth)acrylate or divinyl (meth)acrylamides, such as methylene bisacrylamide.

[0047] More preferably, the branching monomer (BM) is divinyl benzene (DVB).

[0048] According to said embodiment, polymer (A) may contain from 0.1 to 5, preferably from 0.2 to 1% by moles of at least one branching monomer (BM).

[0049] The branched (meth)acrylic polymer (A) is prepared by polymerizing a mixture of at least one hydrophilic (meth)acryloyl monomer (MAM) with at least one monomer (BM), optionally in the presence of other alpha, beta-ethylenicallyunsaturated monomers bearing functionalities such as carboxyl groups or substituted alkyl esters.

[0050] When the branched (meth)acrylic polymer (A) includes hydrophilic (meth)acryloyl monomer such as monoethylenically unsaturated monocarboxylic acid, said polymer (A) may further be at least partially salified to obtain at least a fraction of the acidic moieties in the form of a salt.

[0051] In an embodiment of the present invention, it is thus provided a branched (meth)acrylic polymer (A) that is at least partially salified.

[0052] The preparation of branched (meth)acrylic polymer (A) may thus further include a step of neutralization of at least a fraction of acid groups with a salt [salt (SA)] including a monovalent cation in a suitable solvent.

[0053] The salt (SA) can be any salt capable of neutralizing the acid groups, and it is preferably selected from a salt capable of providing an alkali metal cation, a tertiary or quaternary ammonium cation, more preferably Na+, K+, Li+and or quaternary ammonium cation.

[0054] The polymer (A) for use in the composition (C) of the present invention preferably has a number average molecular weight (Mn) of at least 1 kDa, for example between 1 and 150 kDa. More preferably, the polymer (A) has a number average molecular weight (Mn) between 15 and 100 kDa.

[0055] The polymer (A) for use in the composition (C) of the present invention preferably has a weight average molecular weight (Mw) of about 1 kDa to 150 kDa, preferably from 5 kDa to 100 kDa.

[0056] According to another preferred embodiment, polymer (A) is a methylmethacrylate copolymer comprising at least 70% by moles of methylmethacrylate monomer units, up to 20% by moles of methacrylic acid monomer units and up to 1% by moles of branching monomer (BM).

[0057] Binder (B) may be obtained by mixing the PTFE and the polymer (A) both in the powder form or through mixing of a PTFE latex with a polymer (A) latex, followed by co-coagulation by cryogenic or electrolytic procedure and isolation.

[0058] In order to obtain the desired polymer ratio in the blend, the dry content of the PTFE latex and / or the polymer (A) latex may be evaluated by drying in a thermobalance 50 grams of polymeric latex at 200°C.

[0059] Generally the weight ratio PTFE / polymer (A) will be comprised between 95 / 5 wt / wt to 10 / 90 wt / wt. The skilled in the art will select most appropriate weight ratio in view of target final properties of the binder (B).

[0060] The applicant has surprisingly found that an amount of polymer (A) added to PTFE does not affect the ability to fibrillate PTFE.

[0061] In another aspect the present invention provides an electrode-forming composition [composition (C)] for use in the preparation of electrodes for electrochemical devices, characterized by comprising: a) at least one electrode active material (AM); b) a binder (B) as above defined; and c) optionally, at least one conductive agent.

[0062] The amount of binder (B) which may be used in the electrode-forming composition (C) is subject to various factors. One such factor is the surface area and amount of the active material, and the surface area and amount of any electroconductivity-imparting additive which are added to the electrode-forming composition. These factors are believed to be important because the binder particles provide bridges between the conductor particles and conductive material particles, keeping them in contact.

[0063] The electrode forming composition [composition (C)] of the present invention includes one or more electrode active material (AM). For the purpose of the present invention, the term “electrode active material” is intended to denote a compound that is able to incorporate or insert into its structure, and substantially release therefrom, alkaline or alkaline-earth metal ions during the charging phase and the discharging phase of an electrochemical cell. The electrode active material is preferably able to incorporate or insert and release lithium ions or sodium ions.

[0064] The nature of the electrode active material (AM) in the electrode forming composition (C) of the invention depends on whether said composition is used in the manufacture of a negative electrode (anode) or a positive electrode (cathode).

[0065] The conventional active materials (AM) at the positive electrode of sodium-ion batteries are generally selected from Na-based layered transition-metal oxides, Prussian blue analogs and polyanion-type materials.

[0066] In some embodiments the active materials are Na-based layered transition-metal oxides classified as O3-, P2-, and P3-types depending on the stacking sequence of oxygen layers. P2-type structures generally respond to the general formula NaxMCh wherein M stands for a transition metal ion such as Co, Mn and x is 2 / 3.

[0067] In some embodiments the active materials are Prussian blue analogs (PBA) of general formula AxP[R(CN)6]i-ymH20, with A being and alkali metal ion, P being a N-coordinated transition metal ion, R being a C-coordinated transition metal ion, y being a [R(CN)6] vacancy, with 0 < x < 2 and 0 < y < 1 , such as Nao.8iFe[Fe(CN)6]o.79, NaFe2(CN)e, Nai.63Fei.89(CN)e, Nai.72MnFe(CN)e,Nai.76Nio.i2Mno.88[Fe(CN)6]o.98, Na2NixCoi.xFe(CN)6with 0 < x < 1 e.g. Na2CoFe(CN)6.

[0068] In some other embodiments the active materials are polyanion-type materials of general formula NaxMy(XO4)n (where X = S, P, Si, As, Mo and W, and M is transition metal), which possess a series of tetrahedron anion units (XCL)"' and their derivatives (Xm03m+i)n'. Among them, phosphates NaMPCL such as NaFePC NaojFePCU or NaMnPCL; natrium (sodium) superionic conductor of NASICON-type structures of general formula NaxM2(XC>4)3 (where 1 < x < 4, M = V, Fe, Ni, Mn, Ti, Cr, Zr ; X = P, S, Si, Se, Mo - with single transition metal type such as Na3V2(PO4)3 (NVP), NasC^PCUh, Na3Fe2(PO4)3; - with binary transition metal type such as Na2VTi(PO4)3, Na3FeV(PC>4)3, Na4MnV(PC>4)3, NasMnZr(PO4)3, NasMnTi(PO4)3, Na4Fe3(PO4)2(P2O?) (NFPP); pyrophosphates Na2FeP2O7, Na2MnP2O7, Na2CoP2Oy, Na4-xFe2+x / 2(P2C>7)2 with 2 / 3 < x < 7 / 8 e.g. Na3.i2Fe2.44(P2O?)2 or Na3.32Fe2.34(P2Oy)2, Na2(VO)P2O?, NayV3(P20y)4i fluorophosphates NaVPCLF, Na2CoPC>4F, Na2FePC>4F, Na2MnPC>4F, Na3(VOi. XPO4)2FI+2X(with 0 < x < 1) e.g. Na3(VOPO4)2F or Na3V2(PO4)2F3 (NVPF); fluoro sulfates such as NaMSCLF (with M = Fe, Co, Ni); mixed phosphates / pyrophosphates of general formula Na4M3(PO4)2(P2Oy) (with M representing transition metals) such as Na4Mn3(PO4)2(P2Oy), Na4Co3(PO4)2(P2O7), Na4Ni3(PO4)2(P2O7), Na4Fe3(PO4)2(P2O7) (NFPP), Na7V4(P2Oy)4(PO4); sulfates such as Na2Fe2(SO4)3, Na2+2XFe2.x(SO4)3, Na2+2XCo2- X(SC>4)3, Na2+2XMn2-x(SC>4)3 (where 0 < x < 1) ; silicates of general formula Na2MSiO4 (with M = Mn, Fe, Co and Ni).

[0069] In some preferred embodiments the active materials are fluorophosphates preferably selected from the list consisting of NaVPCLF, Na2CoPC>4F, Na2FePC>4F, Na2MnPO4F, Na3(VOi-xPO4)2Fi+2X(with 0 < x < 1) e.g. Na3(VOPO4)2F or Na3V2(PO4)2F3 (NVPF).

[0070] The conventional active materials (AM) at the positive electrode of lithium-ion batteries may comprise a composite metal chalcogenide of formula LiMCh, wherein 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 preferred to use a lithium-based composite metal oxide of formula LiMCh, wherein M is the same as defined above. Preferred examples thereof may include LiCoO2, LiNiO2, LiNixCoi.xO2 (0 < x < 1) and spinel-structured LiMn2O4.

[0071] As an alternative, still in the case of forming a positive electrode for a Lithium-ion secondary battery, the electrode active material may comprise a lithiated or partially lithiated transition metal oxyanion-based electro-active material offormula MiM2(JC>4)fEi-f, wherein Mi is lithium, which may be partially substituted by another alkali metal representing less than 20% of the Mi metals, M2 is a transition metal at the oxidation level of +2 selected from Fe, Mn, Ni or mixtures thereof, which may be partially substituted by one or more additional metals at oxidation levels between +1 and +5 and representing less than 35% of the M2 metals, including 0, JO4 is any oxyanion wherein J is either P, S, V, Si, Nb, Mo or a combination thereof, E is a fluoride, hydroxide or chloride anion, f is the molar fraction of the JO4 oxyanion, generally comprised between 0.75 and 1 .

[0072] The MiM2(JC>4)fEi-f electro-active material as defined above is preferably phosphate-based and may have an ordered or modified olivine structure.

[0073] More preferably, the electrode active material in the case of forming a positive electrode has formula Li3-xM’yM”2-y(JO4)3 wherein 0<x<3, 0<y<2, M’ and M” are the same or different metals, at least one of which being a transition metal, JO4 is preferably PO4 which may be partially substituted with another oxyanion, wherein J is either S, V, Si, Nb, Mo or a combination thereof. Still more preferably, the electrode active material is a phosphate-based electro-active material of formula Li(FexMni-x)PO4 wherein 0<x<1 , wherein x is preferably 1 (that is to say, lithium iron phosphate of formula LiFePC ).

[0074] In the case of forming a negative electrode for a Lithium-ion secondary battery, the negative electrode active material may preferably comprise one or more carbon-based materials and / or one or more silicon-based materials.

[0075] In some embodiments, the carbon-based materials 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.

[0076] The carbon-based material is preferably graphite.

[0077] The silicon-based compound may be one or more selected from the group consisting of chlorosilane, alkoxysilane, aminosilane, fluoroalkylsilane, silicon, silicon chloride, silicon carbide and silicon oxide.

[0078] More particularly, the silicon-based compound may be silicon oxide or silicon carbide.

[0079] When present in the electrode active material, the silicon-based compounds are comprised in an amount ranging from 1 to 60 % by weight, preferably from 5 to 30 % by weight with respect to the total weight of the electro active compounds.

[0080] One or more optional electroconductivity-imparting additives may be added in order to improve the conductivity of a resulting electrode made from the composition of the present invention. Conducting agents for batteries are known in the art.

[0081] Examples thereof may include: carbonaceous materials, such as carbon black, graphite fine powder, carbon nanotubes, graphene, or fiber, or fine powder or fibers of metals such as nickel or aluminum. The optional conductive agent is preferably carbon black. Carbon black is available, for example, under the brand names, Super P® or Ketjenblack®.

[0082] When present, the conductive agent is different from the carbon-based material described above.

[0083] The amount of optional conductive agent is preferably from 0 to 30 wt. % of the total solids in the electrode forming composition. In particular, for cathode forming compositions the optional conductive agent is typically from 0 wt. % to 10 wt. %, more preferably from 0 wt. % to 5 wt. % of the total amount of the solids within the composition.

[0084] For negative electrodes-forming compositions which are free from silicon based electro active compounds the optional conductive agent is typically from 0 wt. % to 5 wt. %, more preferably from 0 wt. % to 2 wt.% of the total amount of the solids within the composition, while for anode forming compositions comprising silicon based electro active compounds it has been found to be beneficial to introduce a larger amount of optional conductive agent, typically from 0.5 to 30 wt. % of the total amount of the solids within the composition.

[0085] The electrode-forming composition (C) may be prepared by thoroughly mixing the at least one electrode active material (AM), the binder (B) and optionally, the at least one conductive agent.

[0086] Mixing with high shear forces involves the fibrillization of the binder particles to produce fibrils that eventually form a matrix or lattice for supporting the resulting composition of matter. The resulting dough-like material may be calendared many times to produce a conductive film of desired thickness and density. The high shear forces can be provided by subjecting the mixture to an extruder.

[0087] The electrode-forming composition (C) of the invention can be used in a process for the manufacture of an electrode [electrode (E)], said process comprising: -A) combining a polytetrafluoroethylene (PTFE) and at least one branched (meth)acrylic polymer (A) as above defined to provide a binder (B);-B) dry mixing the at least one electrode active material (AM), the binder (B) as above defined, and optionally, at least one conductive agent in the absence of solvent;-C) feeding the powdered dry mixture obtained in step B) to a compactor to form a self-supporting dry film; and-D) applying the dry film to an electrically conductive substrate to form the electrode.

[0088] In step B), mixing electrode active material (AM), the binder (B) as above defined, and optionally, at least one conductive agent is performed by dry-blending these ingredients without the addition of any solvents, liquids, processing aids, or the like to the particle mixture. Dry-mixing may be carried out, for example, in a mill, mixer or blender (such as a V-blender equipped with a high intensity mixing bar, or other alternative equipment as described further below), until a uniform dry mixture is formed. Those skilled in the art will identify, after perusal of this document, that blending time can vary based on batch size, materials, particle size, densities, as well as other properties, and yet remain within the scope hereof.

[0089] In step C) of the process of the invention, the powdered dry mixture obtained in step B) is subjected to mechanical compaction step to provide a self-supporting dry film.

[0090] The compacting of the dry mixture obtained in step B) can take place as a mechanical compaction, for example by means of a roller compactor or a tablet press, but it can also take place as rolling, build-up or by any other technique suitable for this purpose.

[0091] The mechanical compaction step may be associated to a thermal consolidation step. The combination of an applied pressure and a heat treatment makes thermal consolidation possible at lower temperatures than if it were done alone.

[0092] In one embodiment, the mechanical compaction step is carried out by compression, suitably by compressing the dry mixture obtained in step B) between two metal foils. Preferably, the mechanical compaction step is done by application of a compression pressure between 5 and 50 MPa, and preferably between 10 and 30 MPa.

[0093] The compaction step is conveniently carried out at a temperature not exceeding 200 °C, preferably at a temperature lower than 180 °C.

[0094] In step D), the dry film obtained in step C) is applied onto an electrically conductive substrate to form the electrode.

[0095] The sheet of substrate material may comprise a metal foil, an aluminum foil in particular.

[0096] Thanks to the improved adhesion of the binder (B), the dry film obtained in step C) can be applied onto the electrically conductive substrate without the need for any primer or adhesive layer.

[0097] The electrode (E) of the invention is particularly suitable for use in electrochemical devices, in particular in secondary batteries.

[0098] The Applicant has surprisingly also found that polymer (A) as above defined may be suitably used alone in the preparation of negative electrodes by dry process or by extrusion at low temperatures.

[0099] In still a further object, therefore, the present application provides a binder composition [binder (B1)] for use in the preparation of negative electrodes for electrochemical devices, characterized by consisting of at least one branched (meth)acrylic polymer [polymer (A)] derived from the polymerization of at least one (meth)acryloyl monomer [monomer (MAM)] with at least one molecule comprising at least two vinyl groups [monomer (BM)].

[0100] In another aspect the present invention provides a negative electrode-forming composition [composition (C1)] for use in the preparation of negative electrodes for electrochemical devices, characterized by consisting of: a1) at least one negative electrode active material; b1) a binder (B1) as above defined; and c1) optionally, at least one conductive agent.

[0101] The features of the electrode forming composition (C) as above defined applies also to composition (C1), limited to what concerns negative electrodes and the absence of PTFE.

[0102] In another aspect the present invention thus provides a process for manufacturing a negative electrode [electrode (EN)] for electrochemical cell, said process comprising:A1) dry mixing the at least one negative electrode active material, the binder (B1) as above defined, and optionally, at least one conductive agent in the absence of solvent;B1) feeding the powdered dry mixture obtained in step A1) to a compactor to form a self-supporting dry film; andC1) applying the dry film to an electrically conductive substrate to form the electrode.

[0103] The features of the process for manufacturing an electrode (E) as above defined applies also to process for manufacturing a negative electrode [electrode (EN)], with the exception of step A) directed to the mixing with PTFE and to the nature of the active material.

[0104] 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) and / or a negative electrode (EN) as defined above.

[0105] Preferably, the electrochemical device is a secondary battery comprising:- a positive electrode and a negative electrode, wherein the positive electrode is the electrode (E) according to the present invention.

[0106] In another preferred embodiment, the electrochemical device is a secondary battery comprising:- a positive electrode and a negative electrode, wherein the negative electrode is the electrode (EN) according to the present invention.

[0107] The secondary battery of the invention is preferably an alkaline or an alkaline- earth secondary battery.

[0108] The secondary battery of the invention is more preferably a lithium-ion secondary battery.

[0109] An electrochemical device according to the present invention can be prepared by standard methods known to a person skilled in the art.

[0110] Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

[0111] The invention will be now described with reference to the following examples, whose purpose is merely illustrative and not intended to limit scope of the invention.Experimental section

[0112] Raw materials

[0113] PTFE: PTFE homopolymer powder having specific gravity, measured according to ASTM D792, of 2160 and having rheometric pressure, measured according to ASTM D4895, of 9.50 MPa.

[0114] Carbon black, commercially available as SC45 from Imerys S.A.

[0115] Carbon black, commercially available as SC65 from Imerys S.A.

[0116] Silicon oxide, commercially available as KSC-1064 from Shin-Etsu, theoretical capacity is about 2100 mAh / g;

[0117] Graphite GHDR 15-4, commercially available from Imerys S.A.

[0118] NMC811 (Cosmo Advanced Materials & Technology, d50 = 10.28 pm).

[0119] MMA: methylmethacrylate, commercially available from Sigma-Aldrich

[0120] MAA: methacrylic acid, commercially available from Sigma-Aldrich

[0121] DVB: divinyl benzene, commercially available from Sigma-Aldrich

[0122] AMBN: 2,2'-azobis(2-methylbutyronitrile), commercially available from Sigma- Aldrich

[0123] LFP: commercially available as Life Power from Shenzhen Dynanonic Co., Ltd.

[0124] Galden HT80 commercially available from Solvay Materials.

[0125] Preparation 1 : Polymer (A-1): Poly(MMA-MAA-DVB) 79.8 / 20 / 0.2 mol% in NMPIn a 2L jacketed reactor equipped with a multi-stage lightning A320 stirring blade, counter-blades, a condenser connected to a minichiller and a cryothermostatic bath were introduced, at room temperature, MMA (24.864 g, 0.246 mol), MAA (5.358 g, 0.061 mol), DVB (0.100 g, 0.616 mmol, 80% purity), AMBN (2.821 g, 0.014 mol) and 414.583 g of NMP. The mixture was purged with nitrogen for 20 minutes at room temperature and under stirring, after the nitrogen flow was left in the sky, the temperature of the cryothermostatic bath was programmed at 75°C over a temperature ramp of 1 hour. In parallel, a solution of monomers MMA (140.89 g, 1.393 mol), MAA (30.36 g, 0.349 mol) and DVB (0.568 g, 3.492 mmol, 80% purity) was prepared. Once the temperature of 75°C was reached in the reactor, the monomer solution, previously prepared, was introduced over 1 hour. After completion of the addition, the reaction was aged at 75°C for an additional time of 6 hours. Finally, the reaction was diluted up to 25.40 wt% with NMP, cooled down to room temperature and the reactor was discharged. Conversions to monomers MMA, MAA and DVB were found to be nearly quantitative (>99%) by1H NMR (done in CDCI3).

[0126] EXAMPLE 1 : Preparation of anode

[0127] A dry mixture of 5.41 g of graphite, 1.36 g of silicon oxide and 0.072 g of SC45 was prepared by grinding for 10 minutes the powders in an electric mortar.

[0128] 5 ml of Galden HT80 was added to the powder mixture and the composite mixed in an electric mortar for 1 minute. A homogeneous paste was obtained.

[0129] 0.29 g of polymer (A-1) powder, 0.07 g of PTFE with 4 ml of Galden HT80 were added to the homogeneous paste and mixed in a mortar grinder for 5 minutes. A homogeneous composite was obtained.

[0130] The composite was then manipulated manually in order to fibrillate the polymer and obtain gross and cohese self-standing film.

[0131] Film was calendered to lower the thickness below 200 pm.

[0132] The resulting negative electrode had the following composition: 75.2 wt% of graphite, 18.8 wt% of silicon, 4 wt% of polymer (A-1), 1 wt % of PTFE and 1 wt % of carbon black.

[0133] Negative electrode NE1 was thus obtained.

[0134] A negative electrode NE1 sample was placed between two copper current collectors and calendered in a symmetric calender heated at 200°C for 5 times in order to laminate the electrode onto the collector.

[0135] EXAMPLE 2: Preparation of LFP cathode

[0136] A dry mixture of 6.48 g of LFP and 0.36 g of SC65 was prepared by grinding for 10 minutes the powders in an electric mortar.

[0137] 4 ml of Galden HT80 was added to the powder mixture and the composite mixed in an electric mortar for 1 minute. A homogeneous paste was obtained.

[0138] 0.18 g of polymer (A-1) powder, 0.18 g of PTFE with 3 ml of Galden HT80 were added to the homogeneous paste and mixed in a mortar grinder for 5 minutes. A homogeneous composite was obtained.

[0139] The composite was then manipulated manually in order to fibrillate the polymer and obtain gross and cohese self-standing film.

[0140] Film was calendered to lower the thickness below 200 pm.

[0141] The resulting positive electrode had the following composition: 90 wt% of LFP, 2.5 wt% of polymer (A-1), 2.5 wt % of PTFE and 5 wt % of carbon black.

[0142] Positive electrode PE1 was thus obtained.

[0143] A positive electrode PE1 sample was placed between two Aluminum current collectors and calendered in a symmetric calender heated at 200°C for 5 times in order to laminate the electrode onto the collector.

[0144] EXAMPLE 3: Preparation of NMC811 cathode

[0145] A dry mixture of 6.48 g of NMC811 and 0.36 g of SC65 was prepared by grinding for 10 minutes the powders in an electric mortar.

[0146] 4 ml of Galden HT80 was added to the powder mixture and the composite mixed in an electric mortar for 1 minute. A homogeneous paste was obtained.

[0147] 0.18 g of polymer (A-1) powder, 0.18 g of PTFE with 3 ml of Galden HT80 were added to the homogeneous paste and mixed in a mortar grinder for 5 minutes. A homogeneous composite was obtained.

[0148] The composite was then manipulated manually in order to fibrillate the polymer and obtain gross and cohese self-standing film.

[0149] Film was calendered to lower the thickness below 200 pm.

[0150] The resulting positive electrode had the following composition: 90 wt% of NMC811 , 2.5 wt% of polymer (A-1), 2.5 wt % of PTFE and 5 wt % of carbon black.

[0151] Positive electrode PE2 was thus obtained.

[0152] A positive electrode PE2 sample was placed between two Aluminum current collectors and calendered in a symmetric calender heated at 200°C for 5 times in order to laminate the electrode onto the collector.

[0153] COMPARATIVE EXAMPLE 4: Preparation of anode

[0154] A dry mixture of 5.41 g of graphite, 1.36 g of silicon oxide and 0.072 g of SC45 was prepared by grinding for 10 minutes the powders in an electric mortar.

[0155] 5 ml of Galden HT80 was added to the powder mixture and the composite mixed in an electric mortar for 1 minute. A homogeneous paste was obtained.

[0156] 0.36 g of PTFE with 4 ml of Galden HT80 were added to the homogeneous paste and mixed in a mortar grinder for 5 minutes. A homogeneous composite was obtained.

[0157] The composite was then manipulated manually in order to fibrillate the polymer and obtain gross and cohese self-standing film.

[0158] Film was calendered to lower the thickness below 200 pm.

[0159] The resulting negative electrode had the following composition: 75.2 wt% of graphite, 18.8 wt% of silicon, 5 wt % of PTFE and 1 wt % of carbon black.

[0160] Negative electrode CE1 was thus obtained.

[0161] A negative electrode CE1 sample was placed between two copper current collectors and calendered in a symmetric calender heated at 200°C for 5 times in order to laminate the electrode onto the collector.

[0162] COMPARATIVE EXAMPLE 5: Preparation of LFP cathode

[0163] A dry mixture of 6.48 g of LFP and 0.36 g of SC65 was prepared by grinding for 10 minutes the powders in an electric mortar.

[0164] 4 ml of Galden HT80 was added to the powder mixture and the composite mixed in an electric mortar for 1 minute. A homogeneous paste was obtained.

[0165] 0.36 g of PTFE with 3 ml of Galden HT80 were added to the homogeneous paste and mixed in a mortar grinder for 5 minutes. A homogeneous composite was obtained.

[0166] The composite was then manipulated manually in order to fibrillate the polymer and obtain gross and cohese self-standing film.

[0167] Film was calendered to lower the thickness below 200 pm.

[0168] The resulting positive electrode had the following composition: 90 wt% of LFP, 5 wt % of PTFE and 5 wt % of carbon black.

[0169] Positive electrode CE2 was thus obtained.

[0170] A positive electrode CE2 sample was placed between two Aluminum current collectors and calendered in a symmetric calender heated at 200°C for 5 times in order to laminate the electrode onto the collector.

[0171] COMPARATIVE EXAMPLE 6: Preparation of NMC811 cathode

[0172] A dry mixture of 6.48 g of NMC811 and 0.36 g of SC65 was prepared by grinding for 10 minutes the powders in an electric mortar.

[0173] 4 ml of Galden HT80 was added to the powder mixture and the composite mixed in an electric mortar for 1 minute. A homogeneous paste was obtained.

[0174] 0.36 g of PTFE with 3 ml of Galden HT80 were added to the homogeneous paste and mixed in a mortar grinder for 5 minutes. A homogeneous composite was obtained.

[0175] The composite was then manipulated manually in order to fibrillate the polymer and obtain gross and cohese self-standing film.

[0176] Film was calendered to lower the thickness below 200 pm.

[0177] The resulting positive electrode had the following composition: 90 wt% of NMC811 , 5 wt % of PTFE and 5 wt % of carbon black.

[0178] Positive electrode CE3 was thus obtained.

[0179] A positive electrode CE3 sample was placed between two Aluminum current collectors and calendered in a symmetric calender heated at 200°C for 5 times in order to laminate the electrode onto the collector.

[0180] Adhesion assessment and measure

[0181] Adhesion assessment and measurements were carried out between the laminated samples as above specified and foil by following ASTM S1876 on the 3 layer structure obtained after the lamination (metal foil / film / metal foil). The adhesion values are shown in Table 1.

[0182] A larger value for the peel strength indicates better close adherence between the polymer and the current collector.Table 1*A=Peel strength of at least 50.0 N / mB= Peel strength of at least 5.0 N / m and less than 50.0 N / m C=Peel strength of at least 0.1 N / m and less than 5.0 N / m D=Adhesion obtained, difficult to measure due to rigidity of specimen E=No adhesion

[0183] No adhesion was observed with PTFE powders when employed alone in the preparation of films by compression between two aluminium foils or between two copper foils.

Claims

ClaimsClaim 1. A binder composition [binder (B)] for use in the preparation of electrodes for electrochemical devices, characterized by comprising: a. a polytetrafluoroethylene (PTFE); and b. at least one branched (meth)acrylic polymer [polymer (A)] comprising: b1. recurring units derived from at least one molecule comprising at least two vinyl groups [monomer (BM)]; and b2. recurring units derived from at least one (meth)acryloyl monomer [monomer (MAM)] , wherein monomer (MAM) is selected from the group consisting of- (meth)acrylamide acid esters of formula CH2=C(R)-C(=O)-NH-Rh, wherein R means hydrogen or an alkyl group with 1 to 3 carbon atoms and Rh means a linear or branched alkyl residue with 1 to 30 carbon atoms, preferably with 1 to 15 carbons, more preferably with 1 to 5 carbons, and- methylmethacrylate, wherein polymer (A) contains at least 70% by moles of monomer (MAM).Claim 2. The binder composition according to any one of the preceding claims, wherein polymer (A) further comprises recurring units derived from at least one (meth) acrylic acid ester selected from the group consisting of methyl acrylate, ethyl (meth)acrylate, n-propyl(meth)acrylate, isopropyl(meth)acrylate, n-butyl (meth)acrylate, methoxy ethyl (meth)acrylate, 2-ethoxy ethyl (meth)acrylate, tertbutyl (meth)acrylate, pentyl (meth)acrylate, heptyl (meth)acrylate, 2-tert- butylheptyl (meth)acrylate, octyl (meth)acrylate), iso-octyl (meth)acrylate, nonyl (meth)acrylate, decyl (meth)acrylate, undecyl (meth)acrylate, 5-methylundecyl (meth)acrylate, dodecyl (meth)acrylate, isobornyl (meth)acrylate, norbornyl (meth)acrylate, cycloalkyl (meth)acrylate, like cyclohexyl(meth)acrylate, phenyl (meth)acrylate.Claim 3. The binder composition according to any one of the preceding claims, wherein polymer (A) further comprises recurring units derived from at least one hydrophilic (meth)acryloyl monomer selected from the group consisting of acrylic acid, methacrylic acid (MAA), hydroxyethyl(meth)acrylate, hydroxypropyl(meth)acrylate, hydroxyethylhexyl(meth)acrylate, crotonic acid, 2- carboxyethyl acrylate oligomers such as Sipomer®B-CEA.Claim 4. The binder composition according to any one of the preceding claims, wherein monomer (BM) is selected from the group consisting of divinyl aryl monomers such as divinyl benzene; (meth)acrylate diesters such as alkylenedi(meth)acrylates such as ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, 1,4-butylene glycol di(meth)acrylate; oligo alkylene glycol di(meth)acrylates such as e.g. tetraethyleneglycol di(meth)acrylate, poly(ethyleneglycol) di(meth)acrylate, poly (propyleneglycol) di(meth)acrylate; divinyl (meth)acrylamides such as methylene bisacrylamide; divinyl ethers such as poly(ethyleneglycol)divinyl ether; and tetra- or tri-(meth)acrylate esters such as pentaerythritol tetra (meth) acrylate, trimethylolpropane tri(meth)acrylate or glucose di- to penta (meth)acrylate.Claim 5. The binder composition according to any one of the preceding claims, wherein polymer (A) contains at least 70% by moles of methylmethacrylate monomer units, up to 20% by moles of methacrylic acid monomer units and up to 1% by moles of monomer (BM).Claim 6. The binder (B) according to any one of the preceding claims, wherein the weight ratio PTFE / polymer (A) is comprised between 95 / 5 wt / wt to 10 / 90 wt / wt.Claim 7. An electrode-forming composition [composition (C)] for use in the preparation of electrodes for electrochemical devices, characterized by comprising: a) at least one electrode active material (AM); b) a binder (B) according to any one of claims 1 to 6; and c) optionally, at least one conductive agent.Claim 8. A process for manufacturing an electrode [electrode (E)] for electrochemical cell, said process comprising:-A) combining a polytetrafluoroethylene (PTFE) and at least one branched (meth)acrylic polymer [polymer (A)] to provide a binder (B) according to anyone of claims 1 to 6;-B) dry mixing the at least one electrode active material (AM), the binder (B) according to any one of claims 1 to 6, and optionally, at least one conductive agent in the absence of solvent;-C) feeding the powdered dry mixture obtained in step B) to a compactor to form a self-supporting dry film; and-D) applying the dry film to an electrically conductive substrate to form the electrode. Claim 9. The electrode-forming composition according to claim 7, which is a negative electrode-forming composition [composition (C1 )] for use in the preparation of negative electrodes for electrochemical devices, characterized by consisting of: a1) at least one negative electrode active material; b1) a binder (B1) consisting of at least one branched (meth)acrylic polymer [polymer (A)] comprising:b1. recurring units derived from at least one molecule comprising at least two vinyl groups [monomer (BM)]; and b2. recurring units derived from at least one (meth)acryloyl monomer [monomer (MAM)] , wherein monomer (MAM) is selected from the group consisting of- (meth)acrylamide acid esters of formula CH2=C(R)-C(=O)-NH-Rh, wherein R means hydrogen or an alkyl group with 1 to 3 carbon atoms and Rh means a linear or branched alkyl residue with 1 to 30 carbon atoms, preferably with 1 to 15 carbons, more preferably with 1 to 5 carbons, and- methylmethacrylate, wherein polymer (A) contains at least 70% by moles of monomer (MAM); and c1) optionally, at least one conductive agent.Claim 10. The process according to claim 8, which is a process for manufacturing a negative electrode [electrode (EN)] for electrochemical cell, said process comprising: A1) dry mixing the at least one negative electrode active material, a binder (B1) consisting of at least one branched (meth)acrylic polymer [polymer (A)] derived from the polymerization of at least one (meth)acryloyl monomer [monomer (MAM)] with at least one molecule comprising at least two vinyl groups [monomer (BM)], and optionally, at least one conductive agent in the absence of solvent; B1) feeding the powdered dry mixture obtained in step A1) to a compactor to form a self-supporting dry film; andC1) applying the dry film to an electrically conductive substrate to form the electrode.Claim 11. An electrode (E) for a secondary battery obtainable by the process according to claim 8 or claim 10.Claim 12. An electrochemical device, such as a secondary battery or a capacitor, comprising at least one electrode (E) according to claim .

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