Battery electrode and method of making the same

Abstract

The present invention relates to an electrode composition comprising PTFE and a cross- linkable fluororesin composition, to a method for its preparation and to its use for the manufacture of electrochemical cell components.

Description

BATTERY ELECTRODE AND METHOD OF MAKING THE SAMECross reference to previous applications

[0001] This application claims priority to European application No. 23181469.0 filed on 26 June 2023, the whole content of this application being incorporated herein by reference for all purposes.Technical Field

[0002] The present invention relates to an electrode composition comprising PTFE and a cross-linkable fluororesin composition, to a method for its preparation and to its use for the manufacture of electrochemical cell components.Background Art

[0003] 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.

[0004] Dry electrode processes have been developed to reduce the timeconsuming and costly drying procedures required by the aforementioned wet processes.

[0005] 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.

[0006] 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.

[0007] Known in the art are methods to improve the adhesiveness to the current collector and electrode active material of PTFE, by using PTFE and either PVDF or VDF / HFP copolymer in combination as a binder. For example, US 11 ,276,846 discloses a dry process for preparing electrodes comprising rolling a granular powder comprising the combination of PTFE and either PVDF or VDF / HFP copolymer on the metal current collector by a roller heated to a predetermined temperature, and enabling the rolled granular powder to be attached to the current collector by the pressure and temperature of the roller. In the process described in US 11 ,276,846 the attaching may be performed as at least one of the first binder and the second binder contained in the granular powder is melted.

[0008] WO 2021 / 023709 discloses that certain blends of a first VDF-based polymer having side chains including hydroxyl groups and a second VDF- based polymer including recurring units derived from acrylic acid can be thermally crosslinked once the electrode forming composition is casted onto the current collector in a process for preparing electrodes via solvent based slurry, thus providing an electrode having improved performances, in particular in terms of adhesion to metals.

[0009] The Applicant has surprisingly found that when the electrode is prepared by an efficient dry process comprising a step of calendering the dry material onto a substrate, the selection of a specific blend of crosslinkable VDF-based copolymers in combination with PTFE highly improves the adhesion of the electrode film to the current collector.Summary of invention

[0010] It is thus hereby provided a binder composition [binder (B)] for use in the preparation of electrodes for electrochemical devices, characterized by comprising:I. a polytetrafluoroethylene (PTFE); andII. a fluoropolymer composition [composition (F)] comprising:(a) at least one first fluoropolymer [polymer (FA)] comprising:- (ai) recurring units derived from vinylidene fluoride (VDF) monomer;- (aii) recurring units derived from at least one hydroxyl group- containing vinyl monomer (HA) of formula (I) RI R2C=CR3-RX (I) wherein Ri , R2 and R3, equal to or different from each other, are independently selected from a hydrogen atom, a halogen atom, and a C1- Cs hydrocarbon group, and wherein Rx is a C2-C20 linear or branched , optionally substituted, hydrocarbon chain moiety comprising at least one aliphatic hydroxyl group, wherein the total amount of recurring units derived from monomer (HA) in said polymer (FA) is of at most 10.0 % by moles, preferably at most 5.0% by moles, more preferably at most 1 .5% by moles, with respect to the total moles of recurring units of polymer (FA); said polymer (FA) being characterized by a fraction of randomly distributed monomer (HA) of at least 40 %; and(b) at least one second fluoropolymer [polymer (FB)], different from polymer (FA), comprising:- (bi) recurring units derived from vinylidene fluoride (VDF) monomer;- (bii) recurring units derived from at least one carboxyl group- containing acrylic monomer (CA) of formula (II):wherein:R1, R2 and R3, equal to or different from each other, are independently selected from a hydrogen atom and a C1-C3 hydrocarbon group and RH is a C1-C10 hydrocarbon chain moiety comprising at least one carboxyl group, wherein monomer (CA) is different from monomer (HA), and wherein the total amount of recurring units derived from monomer (CA) in said polymer (FB) is of at most 10.0 % by moles, preferably atmost 5.0% by moles, more preferably at most 1 .5% by moles, with respect to the total moles of recurring units of polymer (FB); said polymer (FB) being characterized by a fraction of randomly distributed monomer (CA) of at least 40 %.

[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 in a calender apparatus with very low contact time, thus providing electrodes endowed by very high adhesion to current collector 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 a fluoropolymer [polymer (F)] as above defined to provide a binder (B);-B) dry milling 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) laminating the dry film onto an electrically conductive substrate in a calender 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.

[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.Description of embodiments

[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-limitative examples 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 ion-based 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 uniqueproperties 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 (Al2(SO4)3), in concentration of 2g / l calculated on amount of water in the coagulation vessel,-Ammonium carbonate ((NF ^COs), 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] Particle size can be expressed in terms of D50, which is the corresponding particle size when the cumulative percentage reaches50%. D50 is also called as the median particle diameter or median particle size. For example, for a powder sample with D50 = 5 pm, it means 50% of particles are larger than 5 pm and 50% particles are smaller than 5 pm.

[0029] Suitable monomers (HA) are compounds of formula (I): RI R2C=CR3-RX (I) wherein Ri, R2and R3, equal to or different from each other, are independently selected from a hydrogen atom, a halogen atom, and a C1-C5 hydrocarbon group, and wherein Rx is a is a C2-C2o linear or branched hydrocarbon chain moiety comprising at least one aliphatic hydroxyl group, which can optionally be substituted with at least a ether, ketone, epoxy, per-carbonate or ester group.

[0030] In the context of the present invention, the term "aliphatic hydroxyl group" indicates a hydroxyl group attached to an aliphatic carbon atom.

[0031] In a preferred embodiment, monomers (HA) are compounds of formula (la):wherein:R1, R2and R3, equal to or different from each other, are independently selected from a hydrogen atom and a Ci-C3hydrocarbon group and R’OH is a C1-C5 hydrocarbon moiety, optionally substituted, comprising at least one aliphatic hydroxyl group.

[0032] Non-limitative examples of monomers (HA) of formula (la) include, notably:- hydroxyethyl(meth)acrylate (HEA),- 2-hydroxypropyl acrylate (HPA),- hydroxyethylhexyl(meth)acrylate, and mixtures thereof.

[0033] Preferably, the at least one monomer (HA) is hydroxyethyl(meth)acrylate (HEA) or 2-hydroxypropyl acrylate (HPA).

[0034] Suitable carboxyl group-containing vinyl monomers (CA) are compounds of formula (II):wherein:Ri , R2 and R3, equal to or different from each other, are independently selected from a hydrogen atom and a C1-C3 hydrocarbon group and RH is a C2-C10 hydrocarbon moiety comprising at least one carboxyl group and comprising no aliphatic hydroxyl groups.

[0035] In a preferred embodiment, monomers (CA) are compounds of formula (Ha):whereinR1 , R2 and R3, equal to or different from each other, are independently selected from a hydrogen atom and a C1-C3 hydrocarbon group and R’H is a hydrogen or a C1-C5 hydrocarbon moiety comprising at least one carboxyl group and comprising no aliphatic hydroxyl groups.

[0036] R’H may further contain in the chain one or more oxygen atoms, carbonyl groups or ester groups.

[0037] Non-limitative examples of monomers (CA) of formula (Ila) include, notably:- acrylic acid (AA) and- (meth)acrylic acid,- 2-carboxyethyl (meth) acrylate,- (meth) acryloyloxyethyl succinic acid,- (meth) acryloyloxypropyl succinic acid, and mixtures thereof.

[0038] Preferably, the at least one monomer (CA) is acrylic acid (AA).

[0039] In a preferred embodiment of the present invention, monomer (CA) is acrylic acid and monomer (HA) does not contain any carboxylic groups.

[0040] The weight ratio between polymer (FA) and polymer (FB) in composition (C) can be comprised between 5:95 and 95:5, preferably comprised between 20:80 and 80:20.

[0041] The weight ratio between polymer (FA) and polymer (FB) in composition (C) is preferably such to allow obtaining a molar ratio between recurring units (aii) and recurring units (bii) in composition (C) comprised in the range from 20:1 to 1 :20, preferably from 10:1 to 1 :10.

[0042] It is essential that in polymer (FA) the fraction of randomly distributed units (HA) is of at least 40 % and that in polymer (FB) the fraction of randomly distributed units (CA) is of at least 40 %.

[0043] It is known in the art that a continuous feeding of a comonomer of VDF during VDF polymerization will lead to a random distribution of said comonomer in the polymer chains where the sequences VDF- (comonomer)-VDF are present in general in majority.

[0044] Thus, when polymer (FA) is prepared by a polymerization reaction that comprises continuously feeding monomer (HA) during VDF polymerization, a random distribution of monomer (HA) in the polymer chains is present, with sequences VDF-(HA)-VDF being obtained.

[0045] More preferably, in polymer (FA) at least 70% of monomer (HA) is randomly distributed into said polymer (FA).

[0046] The expression “randomly distributed monomer (HA)” is intended to denote the presence of sequences VDF-(HA)-VDF, and the amount of randomly distributed monomer (HA) is determined as the percent ratio between the average number of said VDF-(HA)-VDF sequences and the total average number of (HA) monomer recurring units.

[0047] When each of the (HA) recurring units is isolated, that is to say comprised between two recurring units of VDF monomer, the average number of (HA) sequences equals the average total number of (HA) recurring units, so the fraction of randomly distributed units (HA) is 100%: this value corresponds to a perfectly random distribution of (HA) recurring units. Thus, the larger is the number of isolated (HA) units with respect to the total number of (HA) units, the higher will be the percentage value of fraction of randomly distributed units (HA), as above described.

[0048] Similarly, when polymer (FB) is prepared by a polymerization reaction that comprises continuously feeding monomer (CA) during VDF polymerization, a random distribution of monomer (CA) in the polymer chains is present, with sequences VDF-(CA)-VDF being obtained.

[0049] More preferably, in polymer (FB) at least 70% of monomer (CA) is randomly distributed into said polymer (FB).

[0050] The expression “randomly distributed monomer (CA)” is intended to denote the presence of sequences VDF-(CA)-VDF, and the amount of randomly distributed monomer (CA) is determined as the percent ratio between the average number of said VDF-(CA)-VDF sequences and the total average number of (CA) monomer recurring units.

[0051] When each of the (CA) recurring units is isolated, that is to say comprised between two recurring units of VDF monomer, the average number of (CA) sequences equals the average total number of (CA) recurring units, so the fraction of randomly distributed units (CA) is 100%: this value corresponds to a perfectly random distribution of (CA) recurring units. Thus, the larger is the number of isolated (CA) units with respect to the total number of (CA) units, the higher will be the percentage value of fraction of randomly distributed units (CA), as above described.

[0052] The analytical determination of the total amount of randomly distributed monomers (HA) in polymer (FA) and of monomers (CA) in polymer (FB) may be carried out by measuring the sequences VDF-(comonomer)-VDF by19F-NMR and the total amount of monomers in the polymer by one or more of these techniques,19F-NMR ,1H-NMR, titration of carboxyl groups, FT-IR or others.

[0053] The fraction of randomly distributed units (HA) and (CA) in polymer (FA) and in polymer (FB), respectively, is preferably of at least 50%, more preferably of at least 60%.

[0054] Polymer (FA) comprises preferably at least 0.05%, more preferably at least 0.2 % by moles of recurring units derived from said monomer (HA).

[0055] Polymer (FA) comprises preferably at most 5.0%, more preferably at most 5.0 % by moles, even more preferably at most 3.0 % by moles of recurring units derived from monomer (HA).

[0056] Polymer (FB) comprises preferably at least 0.05%, more preferably at least 0.2 % by moles of recurring units derived from said monomer (CA).

[0057] Polymer (FB) comprises preferably at most 7.0%, more preferably at most 5.0 % by moles, even more preferably at most 3.0 % by moles of recurring units derived from monomer (CA).

[0058] Both polymers (FA) and (FB) can independently be elastomers or semicrystalline polymers.

[0059] Preferably, both polymer (FA) and polymer (FB) are semi-crystalline polymers.

[0060] As used herein, the term “semi-crystalline” means a fluoropolymer that has, besides the glass transition temperature Tg, at least one crystalline melting point on DSC analysis. For the purposes of the present invention a semi-crystalline fluoropolymer is hereby intended to denote a fluoropolymer having a heat of fusion determined according to ASTM D 3418 of advantageously at least 0.4 J / g, preferably of at least 0.5 J / g, more preferably of at least 1 J / g.

[0061] To the purpose of the invention, the term "elastomer" is intended to designate a true elastomer or a polymer resin serving as a base constituent for obtaining a true elastomer.

[0062] True elastomers are defined by the ASTM, Special Technical Bulletin, No. 184 standard as materials capable of being stretched, at room temperature, to twice their intrinsic length and which, once they have been released after holding them under tension for 5 minutes, return to within 10 % of their initial length in the same time.

[0063] Preferably, the intrinsic viscosity of both polymers (FA) and polymer (FB), measured in dimethylformamide at 25 °C. Preferably, polymer (FA) hasintrinsic viscosity between 0.05 l / g and 0.50 l / g more preferably between 0.10 l / g and 0.40 l / g and for polymer (FB) preferably between 0.15 l / g and 0.55 l / g even more preferably between 0.20 l / g and 0.50 l / g.

[0064] Both polymers (FA) and polymer (FB) may further comprise recurring units derived from one or more fluorinated comonomers (CF) different from VDF.

[0065] By the term “fluorinated comonomer (CF)”, it is hereby intended to denote an ethylenically unsaturated comonomer comprising at least one fluorine atoms.

[0066] Non-limitative examples of suitable fluorinated comonomers (CF) include, notably, the followings:(a) C2-C8 fluoro- and / or perfluoroolefins such as tetrafluoroethylene (TFE), hexafluoropropylene (HFP), pentafluoropropylene and hexafluoroisobutylene;(b) C2-C8 hydrogenated monofluoroolefins, such as vinyl fluoride; 1 ,2- difluoroethylene and trifluoroethylene;(c) perfluoroalkylethylenes of formula CH2=CH-RfO, wherein RfO is a C1 - C6 perfluoroalkyl group;(d) chloro- and / or bromo- and / or iodo-C2-C6 fluoroolefins such as chlorotrifluoroethylene (CTFE).

[0067] The fluorinated comonomer (CF) in polymer (FA) and in polymer (FB) is preferably HFP.

[0068] In one preferred embodiment, polymer (FA) and polymer (FB) are semicrystalline.

[0069] In an embodiment according to the present invention, at least one of polymer (FA) or polymer (FB) may comprise from 0.1 to 10.0% by moles, preferably from 0.3 to 5.0% by moles, more preferably from 0.5 to 3.0% by moles of recurring units derived from said fluorinated comonomer (CF).

[0070] It is understood that chain ends, defects or other impurity-type moieties might be comprised in polymer (FA) or in polymer (FB) without these impairing their properties.

[0071] The polymer (FA) more preferably comprises recurring units derived from:- at least 70% by moles, preferably at least 75% by moles, more preferably at least 85% by moles of vinylidene fluoride (VDF),- from 0.05% to 3.0% by moles, preferably from 0.05% to 1 .5% by moles, more preferably from 0.15% to 1 .0% by moles of at least one hydroxyl group-containing vinyl monomer (HA);- optionally from 0.5 to 3.0% by moles of recurring units derived from at least one fluorinated comonomer (CF).

[0072] The polymer (FB) more preferably comprises recurring units derived from:- at least 70% by moles, preferably at least 75% by moles, more preferably at least 85% by moles of vinylidene fluoride (VDF),- from 0.05% to 3.0% by moles, preferably from 0.05% to 1 .5% by moles, more preferably from 0.15% to 1 .0% by moles of at least one hydroxyl group-containing vinyl monomer (CA);- optionally from 0.5 to 3.0% by moles of recurring units derived from at least one fluorinated comonomer (CF).

[0073] Polymer (FA) and polymer (FB) may be obtained by polymerization of a VDF monomer, optionally at least one comonomer (CF), and, respectively, at least one monomer (HA) and at least one monomer (CA) either in suspension in organic medium, according to the procedures described, for example, in WO 2008129041 , or in aqueous emulsion, typically carried out as described in the art (see e.g. US 4,016,345, US 4,725,644 and US 6,479,591 ).

[0074] The procedure for preparing polymer (FA) comprises polymerizing in an aqueous medium in the presence of a radical initiator the vinylidene fluoride (VDF) monomer, monomer (HA) and optionally comonomer (CF) in a reaction vessel, said process comprising continuously feeding an aqueous solution comprising monomer (HA).

[0075] The procedure for preparing polymer (FB) comprises polymerizing in an aqueous medium in the presence of a radical initiator the vinylidene fluoride (VDF) monomer, monomer (CA) and optionally comonomer (CF) in a reaction vessel, said process comprising continuously feeding an aqueous solution comprising monomer (CA).

[0076] Suitable initiator known for the polymerization of fluorinated monomers are organic peroxides, such as those selected from the group consistingof: dialkyl peroxides, diacyl-peroxides, peroxyesters, and peroxydicarbonates.

[0077] Exemplary dialkyl peroxides is di-t-butyl peroxide, of peroxyesters are t- butyl peroxypivalate and t-amyl peroxypivalate, and of peroxydicarbonate, are di(ethyl) peroxydicarbonate, di(n-propyl) peroxydicarbonate, diisopropyl peroxydicarbonate, di(sec-butyl) peroxydicarbonate, di(2- ethylhexyl) peroxydicarbonate and di(4-tert-butylcyclohexyl) peroxydicarbonate.

[0078] The quantity of an initiator required for a polymerization is related to its activity and the temperature used for the polymerization. The total amount of initiator used is generally between 100 to 30000 ppm by weight on the total monomer weight used.

[0079] The initiator may be added in pure form, in solution, in suspension, or in emulsion, depending upon the initiator chosen.

[0080] A chain transfer agents, CTA, can be added to the polymerization. Suitable CTA for this polymerization are known in the art and are typically short hydrocarbon chains like ethane and propane, esters such as ethyl acetate or diethyl maliate, diethylcarbonate and others. When an organic peroxide is used as the initiator, it could act also as effective CTA during the course of free radical polymerization. The additional CTA however, may be added all at once at the beginning of the reaction, or it may be added in portions, or continuously throughout the course of the reaction. The amount of CTA and its mode of addition depend on the desired properties.

[0081] In a preferred preparation process, pressure is maintained above critical pressure of vinylidene fluoride. Generally, the pressure is maintained at a value of more than 50 bars, preferably of more than 75 bars, even more preferably of more than 100 bars.

[0082] It is essential that a continuous feeding of an aqueous solution containing either monomer (HA) or monomer (CA) is continued during the polymerization run for preparing polymer (FA) or polymer (FB), respectively.

[0083] In this way, it is possible to obtain a nearly statistic distribution of the monomer (HA) in polymer (FA) and of monomer (CA) in polymer (FB) within the VDF monomer polymer backbone of polymer.

[0084] The expressions "continuous feeding" or "continuously feeding" means that slow, small, incremental additions of the aqueous solution of monomer (HA) or of the aqueous solution of monomer (CA) take place for most of the polymerization duration, at least until the conversion of 70% by moles of the VDF monomer.

[0085] The aqueous solutions of monomer (HA) or of monomer (CA) continuously fed during polymerization amounts for at least 50 % by weight of the total amount of said monomers supplied during the reaction (i.e. initial charge plus continuous feed) for the preparation of polymer (FA) or polymer (FB), respectively. Preferably at least 60 % by weight, more preferably at least 70 % by weight, most preferably at least 80 % by weight of the total amount of either monomer (HA) or monomer (CA) is continuously fed during polymerization. An incremental addition of VDF monomer can be effected during polymerization, even if this requirement is not mandatory.

[0086] Generally, the process for preparing polymer (FA) and polymer (FB) of the invention is carried out at a temperature of at least 35°C, preferably of at least 40°C, more preferably of at least 45°C.

[0087] When the polymerization to obtain either polymer (FA) or polymer (FB) is carried out in suspension, polymers (FA) and polymers (FB) are typically provided in form of powder.

[0088] When the polymerization to obtain either polymer (FA) or polymer (FB) is carried out in emulsion, polymer (FA) and polymer (FB), typically provided in the form of an aqueous dispersion, aqueous dispersion (DA) and aqueous dispersion (DB), respectively, which may be used as directly obtained by the emulsion polymerization or after a concentration step. Preferably, the solid content of polymer (FA) and of polymer (FB) in dispersion (DA) and in dispersion (DB), respectively, is in the range comprised between 20 and 50% by weight.

[0089] Polymer (FA) and polymer (FB) obtained by emulsion polymerization can be isolated from the aqueous dispersion (DA) and dispersion (DB),respectively, by concentration and / or coagulation of the dispersion and obtained in powder form by subsequent drying.

[0090] Polymer (FA) and polymer (FB) may be optionally further extruded to provide polymer (FA) and polymer (FB) in the form of pellets.

[0091] Extrusion is suitably carried out in an extruder. Duration of extrusion suitably ranges from few seconds to 3 minutes.

[0092] According to a preferred embodiment of the present invention, at least one of polymer (FA) or polymer (FB) comprises end groups of formula (HI):-(Ra)x-RO-Rb (III) wherein RO is a divalent radical containing at least one oxygen atom, Rais a C1-C5 linear or branched hydrocarbon group and Rb is hydrogen or a C1-C5 linear or branched hydrocarbon group and x is an integer selected from 1 and zero, wherein said end groups are present in an amount of at least 1 / 10000 VDF units, preferably higher than 1.5 / 10000 VDF units, more preferably higher than 2 / 10000 VDF units.

[0093] Non limitative examples of divalent radical RO include, notably: ether (-O-), ester (-O-CO-), ketone (-CO-), epoxide, and per-carbonate (-O-CO-O-) groups.

[0094] In a further preferred embodiment of the present invention, RO is a divalent radical containing at least two oxygen atoms. More preferably, RO is a per-carbonate group.

[0095] Preferably, Raand Rb are both C2-C3 linear or branched alkyl radicals, more preferably C3 linear or branched alkyl radicals.

[0096] Preferably, x is zero.

[0097] When the polymerization is carried out in suspension, polymer (FA) and polymer (FB) are typically provided in form of powder.

[0098] The powder particles of polymer (FA) and polymer (FB), when prepared through a suspension polymerization process, have a size distribution with a D50 value of from 10 to 1000 pm, preferably from 10 to 500 pm.

[0099] When the polymerization to obtain any of polymer (FA) or polymer (FB) is carried out in emulsion, the polymer is typically provided in the form of an aqueous dispersion (D), which may be used as directly obtained by the emulsion polymerization or after a concentration step.

[0100] Any of polymer (FA) or polymer (FB) obtained by emulsion polymerization can be isolated in solid form from the aqueous dispersion (D) by means known in the art, such as, but not limited to, spray drying, freeze- drying, coagulating, and drum drying.

[0101] The powder particles of polymer (FA) and polymer (FB), when prepared through a suspension polymerization process, have a size distribution with a D50 value of from 0.25 to 200 microns, preferably from 0.3 to 20 microns, more preferably from 0.5 to 1 microns.

[0102] Polymer (FA) and polymer (FB) in the form of powder may be optionally further extruded to provide polymer (FA) and polymer (FB) in the form of pellets.

[0103] Polymer (FA) and polymer (FB) are typically provided in the form of powder according to the processes described above.

[0104] Binder (B) may be obtained by mixing the PTFE and the composition (F) both in the powder form or through mixing of a PTFE latex with a latex of composition (F), followed by co-coagulation by cryogenic or electrolytic procedure and isolation.

[0105] In order to obtain the desired polymer ratio in the blend, the dry content of the PTFE latex and / or the latex of composition (F) may be evaluated by drying in a thermobalance 50 grams of polymeric latex at 200°C.

[0106] Generally the weight ratio PTFE / composition (F) will be comprised between 95 / 5 wt / wt to 30 / 70 wt / wt. The skilled in the art will select most appropriate weight ratio in view of target final properties of the binder (B).

[0107] The applicant has surprisingly found that an amount of composition (F) added to PTFE does not affect the ability to fibril late PTFE.

[0108] 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; andc) optionally, at least one conductive agent.

[0109] 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.

[0110] 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.

[0111] The nature of the electrode active material 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).

[0112] In the case of forming a positive electrode for a Lithium-ion secondary battery, the electrode active material may comprise a composite metal chalcogenide of formula LiMCte, 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 LiMC , wherein M is the same as defined above. Preferred examples thereof may include LiCoC , LiNiC>2, LiNixCoi-xO2 (0 < x < 1 ) and spinel-structured LiMn2O4.

[0113] 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 of formula MiM2(JO4)fEi-f, wherein Mi is lithium, which may be partially substituted by another alkali metal representingless 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.

[0114] The MiM2(JO4)fEi-f electro-active material as defined above is preferably phosphate-based and may have an ordered or modified olivine structure.

[0115] 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 PCM 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(FexMm-x)PO4 wherein 0<x<1 , wherein x is preferably 1 (that is to say, lithium iron phosphate of formula LiFePCk).

[0116] In the case of forming a negative electrode for a Lithium-ion secondary battery, the electrode active material may preferably comprise one or more carbon-based materials and / or one or more silicon-based materials.

[0117] 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.

[0118] The carbon-based material is preferably graphite.

[0119] 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.

[0120] More particularly, the silicon-based compound may be silicon oxide or silicon carbide.

[0121] When present in the electrode active material, the silicon-based compounds are comprised in an amount ranging from 1 to 60 % byweight, preferably from 5 to 30 % by weight with respect to the total weight of the electro active compounds.

[0122] 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.

[0123] 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®.

[0124] When present, the conductive agent is different from the carbon-based material described above.

[0125] 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.

[0126] For anode 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.

[0127] 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.

[0128] Mixing with high shear forces involves the f i bri 11 izat ion 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 ofdesired thickness and density. The high shear forces can be provided by subjecting the mixture to an extruder.

[0129] 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 a fluoropolymer [polymer (F)] as above defined to provide a binder (B);-B) dry milling 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) laminating the dry film onto an electrically conductive substrate in a calender to form the electrode.

[0130] 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.

[0131] 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.

[0132] 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.

[0133] The mechanical compaction step may be associated to a thermal consolidation step. The combination of an applied pressure and a heattreatment makes thermal consolidation possible at lower temperatures than if it were done alone.

[0134] 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.

[0135] The compaction step is conveniently carried out at a temperature not exceeding 120 °C, preferably at a temperature in the range from 40 to 100°C.

[0136] The compaction step C) can be suitably repeated several times, in order to reach a homogeneous film having the desired thickness.

[0137] In step D), the dry film obtained in step C) is laminated onto an electrically conductive substrate in a calender to form the electrode.

[0138] The sheet of substrate material may comprise a metal foil, an aluminum foil in particular.

[0139] 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.

[0140] It has been found that blending certain VDF copolymers that randomly incorporate their backbone certain monomers that can undergo crosslinking provides blends of crosslinkable vinylidene fluoride copolymers that can be mixed with PTFE and thermally crosslinked during the lamination step (D) of the process of the invention to provide electrodes endowed with very high adhesion to current collector compared to electrodes obtained by a dry process using PTFE and PVDF not modified to undergo crosslinking.

[0141] As used herein, "thermal treatment", “thermally crosslinked” and "occurs thermally" are understood to mean that the cross-linking process of the invention is activated by temperature alone.

[0142] The lamination step (D) is carried out in a calender, whit the dry film obtained in step (C) having a very short residence time between the calender rolls. This short residence time, combined with the high temperature of the calendering step has the effect that composition (F)has no time to fully melt, but it is submitted for a short time to a temperature that at least a level sufficient to activate cross-linking.

[0143] One skilled in the art will recognize that the time required to achieve cross-linking will in general depend on the temperature, with cross-linking occurring more rapidly as temperature increases. The residence time in the calender may conveniently vary from 10 seconds up to 30 days, depending on the temperature and on the nature of composition (F), which is enough to achieve crosslinking but not to have the complete melt of the composition (F).

[0144] Thermal crosslinking involves reaction of at least a portion of the hydroxyl groups of recurring units derived from monomer (HA) of polymer (FA) with at least a portion of the carboxyl groups of recurring units derived from monomer (CA) of polymer (FB).

[0145] The electrode (E) of the invention is particularly suitable for use in electrochemical devices, in particular in secondary batteries.

[0146] In one aspect, the present invention provides an electrochemical device being a secondary battery comprising:- a positive electrode and a negative electrode, wherein at least one of the positive electrode and the negative electrode is the electrode (E) according to the present invention.

[0147] 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.

[0148] The secondary battery of the invention is preferably an alkaline or an alkaline-earth secondary battery.

[0149] The secondary battery of the invention is more preferably a lithium-ion secondary battery.

[0150] An electrochemical device according to the present invention can be prepared by standard methods known to a person skilled in the art.

[0151] 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.

Claims

ClaimsClaim 1 . A binder composition [binder (B)] for use in the preparation of electrodes for electrochemical devices, characterized by comprising:I. a polytetrafluoroethylene (PTFE); andII. a fluoropolymer composition [composition (F)] comprising:(a) at least one first fluoropolymer [polymer (FA)] comprising:- (ai) recurring units derived from vinylidene fluoride (VDF) monomer;- (aii) recurring units derived from at least one hydroxyl group- containing vinyl monomer (HA) of formula (I)RI R2C=CR3-RX (I) wherein Ri , R2 and R3, equal to or different from each other, are independently selected from a hydrogen atom, a halogen atom, and a C1- Cs hydrocarbon group, and wherein Rx is a C2-C20 linear or branched , optionally substituted, hydrocarbon chain moiety comprising at least one aliphatic hydroxyl group, wherein the total amount of recurring units derived from monomer (HA) in said polymer (FA) is of at most 10.0 % by moles, preferably at most 5.0% by moles, more preferably at most 1 .5% by moles, with respect to the total moles of recurring units of polymer (FA); said polymer (FA) being characterized by a fraction of randomly distributed monomer (HA) of at least 40 %, being determined as the percent ratio between the average number of said VDF-(HA)-VDF sequences and the total average number of (HA) monomer recurring units; and(b) at least one second fluoropolymer [polymer (FB)], different from polymer (FA), comprising:- (bi) recurring units derived from vinylidene fluoride (VDF) monomer;- (bii) recurring units derived from at least one carboxyl group- containing acrylic monomer (CA) of formula (II):wherein:Ri , R2 and R3, equal to or different from each other, are independently selected from a hydrogen atom and a C1-C3 hydrocarbon group and RH is a C1-C10 hydrocarbon chain moiety comprising at least one carboxyl group, wherein monomer (CA) is different from monomer (HA), and wherein the total amount of recurring units derived from monomer (CA) in said polymer (FB) is of at most 10.0 % by moles, preferably at most 5.0% by moles, more preferably at most 1 .5% by moles, with respect to the total moles of recurring units of polymer (FB); said polymer (FB) being characterized by a fraction of randomly distributed monomer (CA) of at least 40 %, being determined as the percent ratio between the average number of said VDF-(CA)-VDF sequences and the total average number of (CA) monomer recurring units.Claim 2. The binder (B) according to claim 1 , wherein the weight ratio PTFE / composition (F) is comprised between 95 / 5 wt / wt to 30 / 70 wt / wt.Claim 3. The binder (B) according to anyone of the preceding claims, wherein the monomer (HA) is a compound of formula (I):RI R2C=CR3-RX (I) wherein R1, R2 and R3, equal to or different from each other, are independently selected from a hydrogen atom, a halogen atom, and a C1- Cs hydrocarbon group, and wherein Rx is a is a C3-C20 linear or branched hydrocarbon chain moiety comprising at least one aliphatic hydroxyl group, and possibly containing in the chain one or more oxygen atoms, carbonyl groups or carboxy groups, preferably the monomer (HA) is selected from the group consisting of hydroxyethyl(meth)acrylate (HEA), 2-hydroxypropyl acrylate (HPA), hydroxyethylhexyl(meth)acrylate and mixtures thereof.Claim 4. The binder (B) according to anyone of the preceding claims, wherein the monomer (CA) is a compound of formula (II):wherein:Ri , R2 and R3, equal to or different from each other, are independently selected from a hydrogen atom and a C1-C3 hydrocarbon group and RH is a C2-C10 hydrocarbon moiety comprising at least one carboxyl group and comprising no aliphatic hydroxyl groups, preferably the monomer (CA) is selected from the group consisting of acrylic acid (AA), (meth)acrylic acid, 2-carboxyethyl (meth) acrylate, (meth) acryloyloxyethyl succinic acid, (meth) acryloyloxypropyl succinic acid and mixtures thereof.Claim 5. The binder (B) according to claim 4, wherein at least one of polymer (FA) or polymer (FB) comprises end groups of formula (III):-(Ra)x-RO-Rb (III) wherein RO is a divalent radical containing at least one oxygen atom, Rais a C1-C5 linear or branched hydrocarbon group and Rb is hydrogen or a C1-C5 linear or branched hydrocarbon group and x is an integer selected from 1 and zero, wherein said end groups are present in an amount of at least 1 / 10000 VDF units, preferably higher than 1 .5 / 10000 VDF units, more preferably higher than 2 / 10000 VDF units.Claim 6. The binder (B) according to anyone of the preceding claims, wherein the molar ratio between polymer (FA) and polymer (FB) in composition (F) is comprised between 5:95 and 95:5, preferably comprised between 20:80 and 80:20.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 anyone 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 a composition (F) to provide a binder (B);-B) dry milling 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) laminating the dry film onto an electrically conductive substrate in a calender to form the electrode.Claim 9. The process according to claim 8, wherein in step B) the dry mixing carried out, for example, in a mill, mixer or blender until a uniform dry mixture is formed.Claim 10. The process according to claim 9, wherein step C) is carried out at a temperature not exceeding 120 °C.Claim 11. An electrode (E) for a secondary battery obtainable by the process according to anyone of claims 8 to 10.Claim 12. An electrochemical device, such as a secondary battery, comprising at least one electrode (E) according to claim 11 .Claim 13. The electrochemical device according to claim 12, said electrochemical device being a secondary battery comprising:- a positive electrode and a negative electrode, wherein at least one of the positive electrode and the negative electrode is the electrode (E) according to claim 11 .Claim 14. The electrochemical device according to claim 13, said electrochemical device being a secondary battery comprising:- a positive electrode and a negative electrode, wherein the positive electrode is the electrode (E) according to claims 11 .

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