Polymer composition as binder for electrodes in electrochemical devices

Abstract

The invention aims to provide a polymer composition useful as a binder for electrodes of electrochemical devices, in particular positive electrodes of secondary batteries, more particularly sodium or lithium ion batteries, which achieves a good balance of (i) ease of the electrode fabrication process by providing electrode-forming formulations having low viscosity at low shear rates, (ii) high adhesion between the electrode and the current collector, and (iii) a long battery life, i.e. retention of high charging / discharging capacity after many charge / discharge cycles. To this end, the invention provides a polymer composition comprising: (1) at least one first polymer comprising (i) recurring units derived from vinylidene fluoride and (ii) recurring units derived from at least one vinyl monomer bearing a carboxy acid group, wherein the units derived from the at least one vinyl monomer are contained in an amount of 0.01 mol% to 5.0 mol% with respect to the total molar amount of recurring units in the first polymer; and (2) at least one second polymer comprising (iii) recurring units derived from vinylidene fluoride and (iv) recurring units bearing moieties of –SO3X, where X is hydrogen or an alkali metal, wherein the recurring units bearing moieties of –SO3X are present in an amount of 0.01 % by moles to 20% by moles, preferably from 0.01% to 1.0% by moles, with respect to the total molar amount of recurring units in the second polymer.

Description

SSPI 2024 / 017 POLYMER COMPOSITION AS BINDER FOR ELECTRODES IN ELECTROCHEMICAL DEVICES Cross reference to previous applications

[0001] This application claims priority to European application No. 24315305.3 filed on 21June 2024, the whole content of this application being incorporated herein by reference for all purposes. Technical field

[0002] The invention relates to a polymer composition which is useful as a binder forelectrodes of electrochemical devices, in particular positive electrodes of secondary batteries, more particularly sodium and lithium ion batteries. The invention also relates to the use of such a polymer composition as a binder in the manufacture of an electrode of an electrochemical cell; to a composition for forming an electrode for an electrochemical cell; to a process for the manufacture of an electrode for an electrochemical cell; to an electrode obtainable by that process; and to an electrochemical device comprising at least one such electrode. Background of the invention

[0003] Fluoropolymers are known in the art to be suitable as binders for the manufacture ofelectrodes for use in electrochemical devices such as secondary batteries.

[0004] In particular, WO 2008 / 129041 A1 describes linear semi-crystalline vinylidenefluoride (VDF) copolymers comprising from 0.05% to 10% by moles of recurring units derived from (meth)acrylic monomers and uses thereof as binder in electrodes for lithium-ion batteries. Further, WO 2022 / 258551 A1 describes high performance binders for lithium battery electrodes and teaches that certain vinylidene fluoride copolymers randomly including certain vinyl monomers comprising oxygenated functional groups together with certain carboxy group-containing vinyl monomers are endowed with very good adhesion to metal substrates and can be used in the preparation of electrode-forming compositions having low viscosity at low shear rates.

[0005] In general, increasing the fluoropolymers molecular weight is known to increase theperformances of electrodes made from these materials, in particular in terms of mechanical properties and in terms of adhesion of the electrodes to the current collector. However, increasing the fluoropolymers molecular weight will increase the viscosity of the electrode-forming formulation including the polymer, also called electrode slurry, making the coating process in the fabrication of electrodes much more difficult.

[0006] In the technical field of batteries, notably of lithium batteries, there is a need forelectrode binders characterized by very good adhesion (to the current collector) that,at the same time, do not impact negatively on the fabrication process of the electrodes, such as by an increase of the viscosity of the slurry used to produce the same.

[0007] Furthermore, a decay of the performance of the battery with time is known andbattery life is an important aspect of secondary batteries; that is, the battery should be able to retain as much as possible of its charging / discharging capacity even after many (e.g.800) charge / discharge cycles.

[0008] Thus, there is a need for electrode binders capable of providing good adhesion tothe current collector, the lowest possible slurry viscosity for easy fabrication of the electrodes, and, at the same time, ensuring a long life of the battery.

[0009] However, it is very difficult to improve all the aforementioned properties at the sametime. For example, a lower slurry viscosity would lead to a lower adhesion because the intrinsic viscosity of the polymer binder would be low, and it is known that a higher molecular weight of the polymer binder leads to better adhesion. Higher adhesion also helps to maintain a longer life of the battery.

[0010] The object of the present invention is to provide a solution to the above problems,i.e. to provide a polymer composition useful as a binder for electrodes of electrochemical devices, in particular positive electrodes of secondary batteries, more particularly sodium and lithium ion batteries, which achieves a good balance of (i) ease of the electrode fabrication process by providing electrode-forming formulations having low viscosity at low shear rates, (ii) high adhesion between the electrode and the current collector, and (iii) a long battery life, i.e. retention of high charging / discharging capacity after many charge / discharge cycles.

[0011] It has been found that this object can be achieved by a polymer composition thatcomprises at least two specific polymers, wherein one polymer improves the adhesion between the electrodes manufactured with use thereof and the current collector and the other polymer improves the long life of an electrochemical device, especially of a secondary battery, manufactured with use thereof (capacity retention) without deteriorating the adhesion.

[0012] Thus, the invention provides a polymer composition comprising:(1) at least one first polymer comprising recurring units derived from vinylidene fluoride; and (2) at least one second polymer, different from the first polymer, comprising (i) recurring units derived from vinylidene fluoride and (ii) recurring units bearing moieties of –SO3X, where X is hydrogen or an alkali metal,wherein the recurring units bearing moieties of –SO3X are present in an amount of 0.01 % by moles to 20% by moles, with respect to the total molar amount of recurring units in the second polymer.

[0013] The invention further provides the use of the above polymer composition as a binderin the manufacture of an electrode of an electrochemical cell, preferably a secondary battery, more preferably a Li-ion battery or a Na-ion battery, particularly preferably a positive electrode of such an electrochemical cell.

[0014] The invention also provides a composition for forming an electrode for anelectrochemical cell, wherein the composition comprises (a) at least one electro-active material; (b) the polymer composition defined above as a binder; and (c) at least one solvent.

[0015] The invention also provides a process for the manufacture of an electrode for anelectrochemical cell, wherein the process comprises: (A) providing a metal substrate having at least one surface; (B) providing the composition for forming an electrode for an electrochemical cell defined above; (C) applying the composition provided in step (B) onto the at least one surface of the metal substrate provided in step (A), thereby providing an assembly comprising a metal substrate coated with said composition onto the at least one surface; (D) drying the assembly provided in step (C).

[0016] The invention also provides an electrode obtainable by the above process.

[0017] Finally, the invention provided an electrochemical device comprising at least oneelectrode as defined above. Description of embodiments

[0018] The polymer composition according to the present invention comprises (1) at leastone first polymer comprising (i) recurring units derived from vinylidene fluoride, and (2) at least one second polymer comprising (i) recurring units derived from vinylidene fluoride and (ii) recurring units bearing moieties of –SO3X, where X is hydrogen or an alkali metal.

[0019] The first polymer is a vinylidene fluoride (VDF)-based polymer comprising recurringunits derived from VDF and optionally comprising recurring units derived from at least one vinyl monomer other than VDF.

[0020] The term “recurring unit derived from vinylidene fluoride” (also generally indicated asvinylidene difluoride 1,1-difluoroethylene or VDF) denotes a recurring unit of formula -CF2-CH2-.

[0021] When present, the total amount of the at least one vinyl monomer in the firstpolymer is preferably at most 5.0% by moles with respect to the total moles of recurring units of the first polymer. Preferably, at least 40% of the at least one vinyl monomer is randomly distributed into the first polymer.

[0022] The at least one vinyl monomer in the first polymer from which the recurring units inthe first polymer are derived may be a vinyl monomer of the following formula (I):wherein: -R1, R2 and R3, equal to or different from each other, are independently selectedfrom a hydrogen atom, a halogen atom, a C1-C5hydrocarbon group and RX, and -RX is a C1-C20 linear or branched hydrocarbon moiety comprising at least onefunctional group selected from a hydroxyl, a carboxyl, an epoxide, an ester, a phosphate and an ether group.

[0023] In one preferred embodiment, in the vinyl monomer the vinyl monomer is a vinylmonomer bearing at least one carboxylic group of formula (I), wherein RX is a C1-C20 linear or branched hydrocarbon moiety comprising at least one carboxylic acid functional group.

[0024] In a more preferred embodiment, the vinyl monomer bearing at least one carboxylicgroup preferably complies with formula (Ia):wherein each of R1and R2have the meanings as above defined, R3is hydrogen, and ROH is a hydrogen or a C1-C5 hydrocarbon moiety comprising at least one carboxylic group; more preferably, each of R1, R2, R3are hydrogen.

[0025] Non-limiting examples of the monomer of formula (I) include acrylic acid (AA),(meth)acrylic acid, 2-carboxyethyl (meth)acrylate, 3-butenoic acid, (meth)acryloyloxyethyl succinic acid, (meth)acryloyloxypropyl succinic acid, 3- (allyloxy)propanoic acid, (meth)acryloyloxyalkyl succinic acids, such as (meth)acryloyloxyethyl succinic acid and (meth)acryloyloxypropyl succinic acid, hydroxyethyl (meth)acrylate, hydroxypropyl(meth)acrylate, hydroxyethylhexyl(meth)acrylates; and mixtures thereof.

[0026] It is preferred that in the first polymer at least 40%, more preferably at least 60%, ofrecurring units derived from vinyl monomers of formula (I) be randomly distributed into the first polymer.

[0027] It is known in the art that a continuous feeding of a comonomer of VDF during VDFpolymerization 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. Thus, when the first polymer is prepared by a polymerization reaction that comprises continuously feeding the at least on vinyl monomer during VDF polymerization, a random distribution of vinyl monomer in the polymer chains is present, with sequences VDF-(vinyl monomer)-VDF being obtained.

[0028] The expression “randomly distributed comonomer” denotes the presence ofsequences VDF-(comonomer)-VDF, and the amount of randomly distributed comonomer is determined as the percent ratio between the average number of said VDF-(comonomer)-VDF sequences and the total average number of monomer recurring units.

[0029] When each of the recurring units derived from the at least one vinyl monomer isisolated, that is to say comprised between two recurring units of VDF monomer, the average number of comonomer sequences equals the average total number of comonomer recurring units, so the fraction of randomly distributed comonomer units is 100%: this value corresponds to a perfectly random distribution of comonomer recurring units. Thus, the larger is the number of isolated comonomer units with respect to the total number of comonomer units, the higher will be the percentage value of fraction of randomly distributed comonomer units, as above described.

[0030] When more than one monomer of the above formula (I) is fed continuously duringVDF polymerization, a random distribution of the comonomers in the polymer chains is present. When two comonomers of formula (I) are fed continuously during VDF polymerization, sequences VDF-(first comonomer)-VDF and VDF-(second comonomer)-VDF are obtained. The percentage amount of total randomly distributed first and second monomers is determined as the percent ratio between the average number of said VDF-(first comonomer)-VDF plus VDF-(second comonomer)-VDF sequences, and the total average number of first comonomer plus second comonomer recurring units.

[0031] The analytical determination of the total amount of randomly distributed vinylmonomers 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.

[0032] The first polymer comprises preferably at least 0.001%, more preferably at least0.01% moles of recurring units derived from the vinyl monomer of formula (I).

[0033] The first polymer comprises preferably at most 5.0%, more preferably at most 3.0%moles, even more preferably at most 2.0% moles of recurring units derived from the vinyl monomer of formula (I).

[0034] The first polymer comprising preferably at least 60% by moles of recurring unitsderived from VDF.

[0035] The first polymer can be an elastomer or a semi-crystalline polymer, preferably it isa semi-crystalline polymer.

[0036] As used herein, the term “semi-crystalline” means a fluoropolymer that has, besidesthe glass transition temperature Tg, at least one crystalline melting point on DSC analysis. For the purposes of the present invention a semi-crystalline fluoropolymer denotes a fluoropolymer having a heat of fusion of from 10 to 90 J / g, preferably of from 30 to 80 J / g, more preferably of from 35 to 75 J / g, as measured according to ASTM D3418-08. For the purpose of the invention, the term “elastomer” designates a true elastomer or a polymer resin serving as a base constituent for obtaining a true elastomer. 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.

[0037] The first polymer preferably has a melting temperature (Tm) comprised in the rangefrom 120 to 200°C.

[0038] The melting temperature may be determined from a DSC curve obtained bydifferential scanning calorimetry (hereinafter, also referred to as DSC). In the case where the DSC curve shows a plurality of melting peaks (endothermic peaks), the melting temperature (Tm) is determined on the basis of the peak having the largestpeak area.

[0039] The first polymer possesses a quasi-linear structure, with a very low amount ofbranching, which results in the insoluble fraction due to long branched chains being substantially low.

[0040] The first polymer preferably has a low fraction of insoluble components in standardpolar aprotic solvents for VDF polymers, such as NMP. More preferably, solutions of the first polymer in said standard polar aprotic solvents remain homogeneous and stable for several weeks, with substantially no insoluble residue. Due to the low amount of insoluble components, the GPC and NMR analyses of the first polymer are not affected, and there are no problems of reliability and reproducibility.

[0041] The first polymer may further include structural units derived from additionalmonomers copolymerizable with VDF and with the at least one vinyl monomer bearing at least one carboxylic acid group.

[0042] Examples of the aforementioned additional monomers include fluorinatedcomonomer and hydrocarbon comonomers.

[0043] For the purposes of the present invention, the term “fluorinated comonomer”denotes an ethylenically unsaturated comonomer comprising at least one fluorine atom.

[0044] Non-limiting examples of suitable fluorinated comonomers include:(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-Rf0, wherein Rf0 is a C1-C6perfluoroalkyl group; (d) chloro- and / or bromo- and / or iodo-C2-C6 fluoroolefins such aschlorotrifluoroethylene (CTFE). (e) perfluoro(alkyl)vinyl ethers such as perfluoro(methyl)vinyl ether (PMVE),perfluoro(ethyl) vinyl ether (PEVE) and perfluoro(propyl)vinyl ether (PPVE); (f) perfluoro(1,3-dioxole) and perfluoro(2,2-dimethyl-1,3-dioxole) (PDD).

[0045] The fluorinated comonomer is preferably HFP.

[0046] In one preferred embodiment, the first polymer is semi-crystalline and comprisesfrom 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.

[0047] Suitable hydrocarbon comonomers are compounds of formula (II), different from thevinyl monomers of formula (I), R1R2C=CR3-Ry (II) wherein R1, 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 Ry is a C3-C20, preferably C4-C15 linear or branched hydrocarbon chainmoiety comprising at least two functional groups independently selected from the group consisting of ether (-O-), ketone (–C=O-), epoxy, per-carbonate (–O-CO-O-) and ester (-OCO-).

[0048] Preferably, the hydrocarbon comonomer of formula (II) is allyl glycidyl ether (AGE)or di(ethylene glycol) ethyl ether acrylate (DEGEEA).

[0049] It is understood that chain ends not derived from VDF or the aforementionedcomonomers, defects or other impurity-type moieties might be comprised in the first polymer without these impairing its properties.

[0050] The first polymer more preferably comprises recurring units derived from:– at least 70% by moles, preferably at least 75% by moles, more preferably atleast 85% by moles of vinylidene fluoride (VDF), –from 0.005% to 1.5% by moles, preferably from 0.01% to 1.0% by moles of atleast one vinyl monomer of the above formula (I); –optionally, from 0.01% to 1.5% by moles, preferably from 0.01% to 1.0% bymoles of at least one vinyl monomer of the above formula (II); –optionally from 0.5 to 3.0% by moles of recurring units derived from at leastone fluorinated comonomer.

[0051] The first polymer preferably comprises end groups of formula (III):-(Ra)x-RO-Rb (III) wherein RO is a divalent radical containing at least one oxygen atom, Ra is 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 per 10000 VDF units, preferably higher than 1.5 per 10000 VDF units, more preferably higher than 2 per 10000 VDF units.

[0052] Non-limiting examples of divalent radical RO include: ether (-O-), ester (-O-CO-),ketone (-CO-), epoxide, and per-carbonate (-O-CO-O-) groups. In a 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.

[0053] Preferably, Ra and Rb are both C2-C3 linear or branched alkyl radicals, morepreferably C3 linear or branched alkyl radicals.

[0054] Preferably, x is zero.

[0055] The first polymer may be obtained by polymerization of a VDF monomer, at leastone monomer of formula (I), at least one monomer of formula (II) and optionally at least one fluorinated comonomer, 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).

[0056] The preferred process for preparing the first polymer comprises polymerizing thevinylidene fluoride (VDF) monomer, the at least one vinyl monomer, and optionally further comonomers, in an aqueous medium in the presence of a radical initiator, wherein the process comprises: continuously feeding an aqueous solutioncomprising the comonomers; and maintaining the pressure in said reactor vessel exceeding the critical pressure of the vinylidene fluoride.

[0057] Suitable initiators known for the polymerization of fluorinated monomers are organicperoxides, such as those selected from the group consisting of dialkyl peroxides, diacyl-peroxides, peroxyesters, and peroxydicarbonates. An exemplary dialkyl peroxide is di-t-butyl peroxide; exemplary peroxyesters are t-butyl peroxypivalate and t-amyl peroxypivalate; and exemplary peroxydicarbonates are di(ethyl) peroxydicarbonate, di(n-propyl) peroxydicarbonate, diisopropyl peroxydicarbonate, di(sec-butyl) peroxydicarbonate, di(2-ethylhexyl) peroxydicarbonate and di(4-tert- butylcyclohexyl) peroxydicarbonate.

[0058] Preferably, the initiator used for preparing the first polymer is an organic peroxide,more preferably it is selected from di(ethyl) peroxydicarbonate, di(n-propyl) peroxydicarbonate, di(iso-propyl) peroxydicarbonate and di(4-tert-butylcyclohexyl) peroxydicarbonate.

[0059] The quantity of an initiator required for a polymerization is related to its activity andthe 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. The initiator may be added in pure form, in solution, in suspension, or in emulsion, depending upon the initiator chosen.

[0060] A chain transfer agent (CTA) can be added to the polymerization. Suitable CTAs forthis polymerization are known in the art and are typically short hydrocarbon chains like ethane and propane, esters such as ethyl acetate or diethyl maleate, diethyl carbonate 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. Theamount of CTA and its mode of addition depend on the desired properties.

[0061] In the preferred preparation process, pressure is maintained above critical pressureof 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.

[0062] It is essential that a continuous feeding of an aqueous solution containingcomonomers is carried out mostly during the whole duration of polymerization run. It is thus possible to obtain a nearly statistic distribution of the comonomers within the VDF monomer polymer backbone of the first polymer. The expressions “continuous feeding” or “continuously feeding” mean that slow, small, incremental additions the aqueous solution of comonomers take place during the polymerization.

[0063] The aqueous solution of comonomers continuously fed during polymerizationamounts to at least 50% wt of the total amount of comonomers supplied during the reaction (i.e. initial charge plus continuous feed). Preferably at least 60% wt, more preferably at least 70% wt, most preferably at least 80% wt of the total amount of comonomers is continuously fed during polymerization. An incremental addition of VDF monomer can be effected during polymerization, even if this requirement is not mandatory.

[0064] When the polymerization is carried out in suspension, the first polymer is typicallyprovided in the form of a powder. When the polymerization to obtain the first polymer is carried out in emulsion, the first polymer is typically provided in the form of an aqueous dispersion, which may be used as directly obtained by the emulsionpolymerization or after coagulation and separation step.

[0065] The first polymer obtained by emulsion polymerization can be isolated from theaqueous dispersion by concentration and / or coagulation of the dispersion and obtained in powder form by subsequent drying. The first polymer in the form of powder may be optionally further extruded to provide the first polymer in the form of pellets.

[0066] The first polymer preferably has an intrinsic viscosity of 0.15 l / g or greater, morepreferably of 0.20 l / g or greater. A sufficiently high intrinsic viscosity (which correlates with the molecular weight) ensures that the polymer composition, when used asbinder in the manufacture of an electrode of an electrochemical cell provides high adhesion between the electrode and a current collector.

[0067] In the context of the present invention, intrinsic viscosities of the first and secondpolymers are measured on the basis of dropping time, at 25°C, of a solution obtained by dissolving the polymer in N,N-dimethylformamide (DMF) at a concentration of about 0.2 g / dl using a Ubbelhode viscosimeter (as described moreparticularly below).

[0068] The second polymer is a VDF-based polymer comprising recurring units derivedfrom VDF and recurring units bearing moieties of –SO3X where X is hydrogen or an alkaline metal.

[0069] The second polymer may further include structural units derived from additionalmonomers copolymerizable with VDF and with recurring units bearing moieties of – SO3X as above defined.

[0070] Examples of the aforementioned additional monomers include vinyl monomersbearing at least one carboxylic acid group, fluorinated comonomer and hydrocarbon comonomers as above defined in relation to the first polymer.

[0071] In the second polymer, the moieties of –SO3X are present in an amount of 0.01% bymoles to 20% by moles, preferably in an amount of 0.1% by moles to 10% by moles, more preferably in an amount from 0.2% by moles to 5.0% by moles, with respect to the total molar amount of recurring units in the second polymer.

[0072] The second polymer more preferably comprises:– from at least 70% by moles, preferably at least 75% by moles, more preferablyat least 85% by moles of recurring units derived vinylidene fluoride (VDF), –from 0.01 % by moles to 20% by moles, preferably from 0.01% to 5.0% bymoles of recurring units bearing moieties of –SO3X; –optionally, from 0.01% to 1.5% by moles, preferably from 0.01% to 5.0% bymoles of at least one vinyl monomer of the above formula (I); –optionally from 0.5 to 3.0% by moles of recurring units derived from at leastone fluorinated comonomer.

[0073] The second polymer preferably has an intrinsic viscosity of 0.01 l / g to 0.50 l / g, morepreferably of 0.05 l / g to 0.30 l / g. Preferably, the intrinsic viscosity of the second polymer is lower than the one for the first polymer.

[0074] The second polymer may be prepared by various methods such as: (i) co-polymerization of VDF monomer with a monomer that contains –SO3X groups; or (ii) grafting moieties bearing an –SO3X group to a VDF polymer (for example, the polymerization of sodium 4-vinylbenzene sulfonate monomer may be performed directly on the surface of PVDF as described by Ghahramani et al., Journal of Electroanalytical Chemistry, 936 (2023), 117342; lithium sulfonate-grafted poly(vinylidenefluoride-hexafluoro propylene) (P(VDF-HFP)) polymers may be synthesized through covalent attachment (grafting) of taurine onto P(VDF-HFP) polymer chains as described by Wang et al., RSC Advances, 2018, 8, 20025- 20031), or (iii) sulfonation of a VDF polymer with a sulfonating agent like chlorosulfonic acid (Martin et al. ACS Appl. Polym. Mater.2022, 4, 9463−9471) or oleum (Bottino et al. International Journal of Hydrogen Energy, 40 (2015), 14690 – 14698).

[0075] The –SO3X moieties present in the second polymer can thus derive from themonomer that contains –SO3X groups used in the co-polymerization with VDF orthey can derive from the grafting of moieties bearing an –SO3X group to a VDF polymer. The determination of the composition can be carried out by liquid NMR or IEC technique as shown in the examples.

[0076] The polymer composition according to the present invention is useful as a binder inthe manufacture of an electrode of an electrochemical cell, preferably a secondary battery, more preferably a Li-ion battery or a Na-ion battery, particularly preferably a positive electrode of such an electrochemical cell.

[0077] Advantages provided by the polymer composition when used as a binder for anelectrode of an electrochemical device, especially a secondary battery, relate to a combination of three critical aspects in a battery: good adhesion of the electrodes (to current collectors), low slurry viscosity of the electrodes raw material and good cycling stability, i.e. retention of a high capacity

[0078] The polymer composition according to the invention preferably comprises the firstpolymer in an amount of 5% to 95% by weight, more preferably in an amount of 25% to 75% by weight, even more preferably in an amount of 50% to 70% by weight.

[0079] The polymer composition according to the invention may consist or may consistessentially of the first polymer and the second polymer. The polymer composition according to the invention “consists of” the first polymer and the second polymer if, apart from unavoidable impurities, it contains no components other than the first polymer and the second polymer. The polymer composition according to the invention “consists essentially of” the first polymer and the second polymer if it contains no components other than the first polymer and the second polymer that would materially affect the essential characteristics of the polymer composition.

[0080] The composition for forming an electrode for an electrochemical cell according tothe present invention comprises (a) at least one electro-active material; (b) the polymer composition defined in claim 1 as a binder; and (c) at least one solvent. This composition for forming an electrode may herein also be referred to as “electrode-forming composition”.

[0081] In the context of the present invention, the term “electro-active material” denotes acompound or material which 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 device. (Such materials or compounds may also be referred to as “electrode-active material” or “electrode-active compound”). The electro-active material is preferably able to incorporate or insert and release lithium or sodium ions.

[0082] The nature of the electro-active material in the electrode-forming compositiondepends on whether said composition is used in the manufacture of a positive electrode or a negative electrode.

[0083] In the case of forming a positive electrode for a sodium-ion secondary battery, theelectro-active material is generally selected from Na-based layered transition-metal oxides, Prussian blue analogs and polyanion-type materials.

[0084] In some embodiments the electro-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 NaxMO2wherein M stands for a transition metal ion such as Co, Mn and x is 2 / 3.

[0085] In some embodiments the electro-active materials are Prussian blue analogs (PBA)of general formula AxP[R(CN)6]1-y□y · m H2O, wherein A is an alkali metal ion, P is aN-coordinated transition metal ion, R is a C-coordinated transition metal ion,is a [R(CN)6] vacancy, 0 ≤ x ≤ 2 and 0 ≤ y < 1, such as Na0.81Fe[Fe(CN)6]0.79□0.21, NaFe2(CN)6, Na1.63Fe1.89(CN)6, Na1.72MnFe(CN)6, Na1.76Ni0.12Mn0.88[Fe(CN)6]0.98, Na2NixCo1-xFe(CN)6with 0 ≤ x ≤ 1, e.g. Na2CoFe(CN)6.

[0086] In some other embodiments the electro-active materials are polyanion-typematerials 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 tetrahedral anion units (XO4)n-and their derivatives (XmO3m+1)n-. Among them, there may be used: phosphates NaMPO4such as NaFePO4, Na0.7FePO4or NaMnPO4; sodium superionic conductors with NASICON-type structures of general formula NaxM2(XO4)3 (where 1 ≤ x ≤ 4 and M = V, Fe, Ni, Mn, Ti, Cr, Zr...; X = P, S, Si, Se, Mo …) of the single transition metal type such as Na3V2(PO4)3(NVP), Na3Cr2(PO4)3, Na3Fe2(PO4)3or of the binary transition metal type such as Na2VTi(PO4)3, Na3FeV(PO4)3, Na4MnV(PO4)3, Na3MnZr(PO4)3, Na3MnTi(PO4)3, Na4Fe3(PO4)2(P2O7) (NFPP); pyrophosphates Na2FeP2O7, Na2MnP2O7, Na2CoP2O7, Na4-xFe2+x / 2(P2O7)2with 2 / 3 ≤ x ≤ 7 / 8, e.g. Na3.12Fe2.44(P2O7)2 or Na3.32Fe2.34(P2O7)2, Na2(VO)P2O7, Na7V3(P2O7)4; fluorophosphates NaVPO4F, Na2CoPO4F, Na2FePO4F, Na2MnPO4F, Na3(VO1- xPO4)2F1+2x(with 0 ≤ x ≤ 1), e.g. Na3(VOPO4)2F or Na3V2(PO4)2F3(NVPF); fluoro sulfates such as NaMSO4F (with M = Fe, Co, Ni); mixed phosphates / pyrophosphates of general formula Na4M3(PO4)2(P2O7) (with Mrepresenting transition metals), such as Na4Mn3(PO4)2(P2O7), Na4Co3(PO4)2(P2O7), Na4Ni3(PO4)2(P2O7), Na4Fe3(PO4)2(P2O7) (NFPP), Na7V4(P2O7)4(PO4); sulfates such as Na2Fe2(SO4)3, Na2+2xFe2-x(SO4)3, Na2+2xCo2-x(SO4)3, Na2+2xMn2-x(SO4)3 (where 0 ≤ x ≤ 1); and silicates of general formula Na2MSiO4 (with M = Mn, Fe, Co and Ni).

[0087] In some preferred embodiments the electro-active materials are fluorophosphatespreferably selected from the group consisting of NaVPO4F, Na2CoPO4F, Na2FePO4F, Na2MnPO4F and Na3(VO1-xPO4)2F1+2x(with 0 ≤ x ≤ 1), e.g. Na3(VOPO4)2F or Na3V2(PO4)2F3 (NVPF).

[0088] The electro-active material at the positive electrode of lithium-ion batteries maycomprise a composite metal chalcogenide of formula LiMQ2, 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 LiMO2, wherein M is the same as defined above. Preferred examples thereof include LiCoO2, LiNiO2, LiNixCo1-xO2 (0 < x < 1) and spinel-structured LiMn2O4.

[0089] According to another embodiment, the at least one positive electro-active material isselected from lithium-containing complex metal oxides of general formula (II) LiNixM1yM2wM3zQ2 (II) wherein M1, M2and M3are the same or different from each other and are transition metals selected from Al, Co, Fe, Mn, Cr and V, 0.5 ≤ x ≤ 1, y + w + z = 1 – x, and Qis a chalcogen such as O or S.

[0090] Alternatively, the electro-active material may comprise a lithiated or partially lithiatedtransition metal oxyanion-based electro-active material of formula M1M2(JO4)fE1-f, wherein M1is lithium, which may be partially substituted by another alkali metal representing less than 20% of the M1metals, M2is 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.

[0091] The M1M2(JO4)fE1-f electro-active material as defined above is preferably phosphate-based and may have an ordered or modified olivine structure.

[0092] More preferably, the electro-active material 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 is a transition metal, JO4is preferably PO4which may be partially substituted with another oxyanion, wherein J is either S, V, Si, Nb, Mo or a combination thereof.

[0093] Still more preferably, the electro-active material is a phosphate-based electro-activematerial of formula LixAyDzPO4, wherein A is selected from the group consisting of Mn, Fe, Co, Ni and Cu; D is selected from the group consisting of Mg, Ca, Sr, Ba; x, y and z are numbers that satisfy the following relationships: 0 < x < 2, 0 < y < 1.5, 0≤ z < 1.5. A is preferably Fe, Mn, and Ni, and particularly preferably Fe. D ispreferably Mg or Ca.

[0094] Examples of the compound having an olivine structure include lithium ironphosphate (LFP), lithium iron manganese phosphate (LMFP) and lithium manganese phosphate.

[0095] Further, as the positive electro-active material, it is possible to use a material whosesurface is partially or wholly coated with carbon in order to enhance the conductivity. The amount of carbon coated is preferably 20 parts by weight or less, more preferably 10 parts by weight or less, still more preferably 5 parts by weight or less, based on 100 parts by weight of the positive electro-active material.

[0096] In the case of forming a negative electrode for a secondary battery, the electro-active material may preferably comprise a carbon-based material and / or a silicon- based material.

[0097] In some embodiments, the carbon-based material may be, for example, 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. The carbon-based material is preferably graphite.

[0098] The silicon-based compound may be one or more selected from the groupconsisting of chlorosilane, alkoxysilane, aminosilane, fluoroalkylsilane, silicon, silicon chloride, silicon carbide and silicon oxide. More particularly, the silicon-based compound may be silicon oxide or silicon carbide.

[0099] When present in the electro-active material, the at least one silicon-basedcompound is contained in the electro-active material in an amount ranging from 1 to 30% by weight, preferably from 5 to 20% by weight with respect to the total weight of the electro-active material.

[0100] The solvent may preferably be an organic polar solvent, examples of which include:N-methyl-2-pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethylsulfoxide, hexamethylphosphamide, dioxane, tetrahydrofuran, tetramethylurea, triethyl phosphate, and trimethyl phosphate. These solvents may be used singly or in mixture of two or more species.

[0101] An optional conductive agent may be added in order to improve the conductivity of aresulting electrode.

[0102] 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®.

[0103] The electro-forming composition of the invention may further optionally include atleast one conductive agent.

[0104] In a preferred embodiment of the present invention, an electrode-formingcomposition for use in the preparation of a positive electrode is provided, said composition comprising: a) at least one electrode-active material; b) at least one polymer composition as above defined as a binder; c) at least one solvent; and d) at least one conductive agent, preferably selected from carbon black or graphite fine powder carbon nanotubes.

[0105] The electrode of the invention is thus particularly suitable for use in electrochemicaldevices, in particular in secondary batteries. For the purpose of the present invention, the term “secondary battery” denotes a rechargeable battery.

[0106] The secondary battery of the invention is preferably an alkaline or an alkaline-earthmetal secondary battery.

[0107] The secondary battery of the invention is more preferably a Lithium-ion secondarybattery.

[0108] The electrochemical device according to the present invention, being preferably asecondary battery, comprises: a positive electrode and a negative electrode, wherein at least one of the positive electrode and the negative electrode is the electrode of the present invention.

[0109] In one preferred embodiment, the present invention provides an electrochemicaldevice, which is a secondary battery comprising: a positive electrode and a negative electrode, wherein the negative electrode is the electrode according to the present invention.

[0110] An electrochemical device according to the present invention can be prepared bystandard methods known to a person skilled in the art.

[0111] Should the disclosure of any patents, patent applications, and publications which areincorporated 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.

[0112] Examples

[0113] Hereinafter, the invention will be described by reference to specific illustrative, non-limiting examples.

[0114] Measurement methods

[0115] Certain properties of the materials made or tested in these examples weremeasured by the following methods:

[0116] Determination of intrinsic viscosity of polymers

[0117] Intrinsic viscosity (η) [dl / g] was measured on the basis of dropping time, at 25°C, ofa solution obtained by dissolving the polymer in N,N-dimethylformamide (DMF) at a concentration of about 0.2 g / dl using a Ubbelhode viscosimeter and the followingequation:where c is the polymer concentration [g / dl], ηris the relative viscosity, i.e. the ratio between the dropping time of the sample solution and the dropping time of the solvent (DMF), ηsp is the specific viscosity, i.e. ηr – 1, and Γ is an experimental factor, which,for the first and second polymer of the invention, corresponds to 3.

[0118] DSC analysis

[0119] DSC analyses were carried out according to the ASTM D 3418 standard; the meltingpoint (T2f) was determined at a heating rate of 10°C / min.

[0120] Synthesis examples

[0121] Synthesis of Polymer (F-1)

[0122] In a 4L reactor equipped with an impeller running at a speed of 650 rpm wereintroduced in sequence: 2.434 g of demineralized water and 0.4 g of PEO(polyethylene oxide – Alkox®-45 from Alroko) per kg of total monomers and 0.5 g ofhydroxypropyl methylcellulose (Methocel®-K100 from Dow) per kg of total monomers, 86.53 g of a solution of trisodium phosphate at 506.1 meq / kg. Theoxygen present in the reactor was removed with a sequence of vacuum and purge of nitrogen at a fixed temperature of 14°C. This sequence was repeated 3 times.

[0123] Then, 14.60 g of hydrogen peroxide solution at 5.1 wt% (from Brenntag) and 4.75 gof ethyl chloroformate (from Framochem) were introduced in the reactor.

[0124] After 15 minutes, 0.18 g of acrylic acid (AA) and 0.08 g of di(ethylene glycol) ethylether acrylate (DEGEEA) were introduced in the reactor at a stirring speed of 880 rpm. Immediately thereafter, 1.184 g of vinylidene fluoride (VDF) were added to themixture. The reactor was then gradually heated until the set-point temperature of 35°C was reached.

[0125] The pressure was kept constantly equal to 120 bars during the whole polymerizationrun by feeding an aqueous solution comprising 4.16 g of AA per liter of solution and1.91 g of DEGEEA per liter of solution. A total of 672 g of the solution was chargedto the reactor. After 333 minutes the polymerization was stopped by degassing the suspension until reaching atmospheric pressure.

[0126] The polymer was then collected by filtration and suspended in clean water in astirred tank. After the washing treatment, the polymer was dried in an oven at 65°C overnight.896 g of dry powder was collected.

[0127] A polymer (F-1) consisting of recurring units derived from VDF, AA (0.22 mol-%) andDEGEEA (0.02 mol-%) and having an intrinsic viscosity of 0.30 l / g in DMF at 25°C and a T2f of 169.9°C was obtained. The molar composition of the polymer was measured by NMR in the liquid phase.

[0128] Synthesis of Polymer (FS-1)

[0129] In a 4L reactor equipped with an impeller running at a speed of 650 rpm wereintroduced in sequence: 1.911 g of demineralized water and 1.49 g of PVOH(polyvinyl alcohol – AlcotexTM 80 from Synthomer) per kg of total monomers. Theoxygen present in the reactor was removed with a sequence of vacuum and purge of nitrogen at a fixed temperature of 11°C. This sequence was repeated 3 times.

[0130] Then, a solution obtained by the dissolution of 4.71 g of dimethyl 2,2'-azobis(2-methylpropionate) (Wako V601 from Fujifilm) in 3.37 g of diethyl carbonate (fromFramochem) was introduced in the reactor.

[0131] After 15 minutes, 7.54 g of 3-sulfopropyl acrylate (3-SPAS) and 101.47 g of asolution of hydrochloric acid 2N were introduced in the reactor at a stirring speed of 880 rpm. Immediately thereafter, 1.252 g of VDF were added to the mixture. The reactor was then gradually heated until the set-point temperature of 67°C was reached.

[0132] The pressure was kept constantly equal to 120 bars during the whole polymerizationrun by feeding an aqueous solution comprising 81.86 g of 3-SPAS per liter ofsolution. A total of 1.058 g of the solution was charged to the reactor. After 623minutes the polymerization was stopped by degassing the suspension until reaching atmospheric pressure.

[0133] The polymer was then collected by filtration and suspended in clean water in astirred tank. After the washing treatment, the polymer was dried in an oven at 65°C overnight.946 g of dry powder was collected.

[0134] A polymer (FS-1) consisting of recurring units derived from VDF and 3-SPAS (0.2mol-%) and having an intrinsic viscosity of 0.231 l / g in DMF at 25°C and a T2fof 168.2°C was obtained. The molar composition of the polymer was measured by NMR in the liquid phase.

[0135] Synthesis of Polymer (FS-2)

[0136] In a 1L round bottom flask under nitrogen with magnetic stirring (480 rpm) andcondenser, were introduced in sequence 792 mL of 11% oleum and 120 g of PVDF powder (a homopolymer of VDF having intrinsic viscosity of 0.30 l / g, second melting temperature of 171°C and about 100 micron as average of particle size). The slurry was stirred 2 hours at 60°C. After cooling, the slurry was slowly poured in 1L of demineralized water cooled by an ice bath. After filtration and washing until neutral pH (4x 500 mL of demineralized water), the solid was heated 30 min in 600 mL of 30% aqueous ethanol at reflux. After cooling, the solid was filtrated and washed again with demineralized water until neutral (3x 500 mL). The polymer was then dried at 90°C under vacuum for 16 h, giving 118.5 g of a pale yellow powder comprising recurring units derived from VDF and presumably of –C(SO3H)=CF- units. The polymer (FS-2) is characterized by a second melting temperature of 167°C and an intrinsic viscosity of 0.134 l / g. The lower second melting temperature of the polymer after grafting is a confirmation that the grafting occurred.

[0137] Determination of the percentage amount of –SO3H groups (DS) in Polymer FS-2.

[0138] The percentage of sulfonation in polymer FS-2 was calculated by applying thefollowing formula: 100*A*64 / [1-A*(124-64)] wherein A is IEC in mol / g 64 is the molecular mass of the VDF monomer; 124 is the molecular mass of the unit –C(SO3H)=CF- .

[0139] Determination of Ion Exchange Capacity (IEC) of Polymer FS-2.

[0140] 0.3 of polymer FS-2 were stirred in 50 ml of 1 M NaCl at 60°C for 48 hours. Theslurry was then titrated by 0.1 M NaOH. Equivalence was obtained after addition of 0.48 mL, indicating an IEC of 0.00016 mol / g for polymer FS-2.

[0141] Polymer FS-2 has a DS of 1.0 %

[0142] Application examples

[0143] Cathode preparation

[0144] A positive electrode material powder, LiNi0.8Mn0.1Co0.1O2 (NMC811 from COSMO),and MWCNT (Multi Walled Carbon Nanotube) solution (from ANP (Advance Nano Products)) as conductive agents, and a binder as specified below were added to N- methyl-2-pyrrolidone (NMP from SAMCHUN). The mass ratio of the positive electrode material powder, conductive agent, and binder was set at 96.7 / 0.9 / 1.5, respectively. After mixing all ingredients, a cathode slurry was obtained, where the solid content was in the range 60 to 80 wt.%.

[0145] The slurry viscosity was measured at 72 wt.% of total solid content (TSC) in aBrookfield Viscometer (DV2T). The motor speed was set to 6, 12, 30, 60 rpm with a No.64 spindle used. The viscosity was read out after 60 seconds. All operations were in a dry room with a dew point of –27 ºC.

[0146] A positive electrode (cathode) was prepared by coating the cathode slurry onto bothsides of an aluminum foil (current collector) having a thickness of 15 µm using a comma roll coater (roll to roll system, CLC-020 from CIS) with a loading weight on each side is 20 mg / cm2. The coated films on the aluminum foil were dried for 2.5 minutes by a 1.5-meter conveyor hot air drying oven as a sub-module of the transfer coater operated at a roll-to-roll conveyor speed of about 0.6 meters / minute to obtain a positive electrode. The drying oven temperature was set to 120°C. Then the coated films on the aluminum foil were calendared using a roll-press machine (CLP- 2025 from CIS). The typical electrode density is 3.0 g / cm3.

[0147] Anode preparation

[0148] A negative slurry was prepared by mixing 97.2 wt. % of natural graphite (918-II fromBTR) with 10.0 wt.% carboxymethyl cellulose (CMC, MAC350HC from Nippon Paper) and 1.2 wt.% SBR (BM451B from ZEON) as a binder and 0.6 wt.% Super P (Super – P Li commercially available from IMERYS) as a conductive agent indeionized water. The solid content of the anode slurry was around 45 to 55 wt.%.

[0149] The slurry was coated onto both sides of a copper foil having a thickness of 11 µmusing a transfer coater with an area density of about 12.4 mg / cm2on each side. The coated films on the copper foil were dried at about 60°C for 2.5 minutes by a 1.5- meter conveyor hot air dryer operated at a conveyor speed of about 0.6 meters / minute to obtain a negative electrode. Then the coated films on the copper foil were calendared using a roll-press machine (CLP-2025 from CIS). The typical electrode density is 1.5 g / cm3.

[0150] Adhesion measurement

[0151] The adhesion peeling force between the electrode and the current collector wasmeasured. The cathode was prepared with the above method and then dried under a vacuum oven for 15 hours at a fixed temperature of 100°C. The cathode was cut (width 20 mm x length 200 mm) and one side was fixed to a double-sided tape (3M9070 high tack double coated tissue tape). The work was repeated 10 times with a hand roller (2 kg) to uniformly attach the electrode and the tape on the slide glass. The peel strength was measured at 90º using a Universal Testing Machine (UTM) (Material testing Machine LS-1, Load cell 10N, from LLOYD)

[0152] Battery preparation

[0153] After electrode calendaring, the resulting cathode films and anode films were usedto prepare the cathode and anode, respectively, by cutting them into individual electrode plates, which were then dried under a vacuum oven for 15 hours at a temperature of 100°C. In addition, an aluminum plate serving as a positive electrode current collector tab was ultra-sonically welded to an end portion of the cathode electrode (Ultrasonic welding, 40MA-XAE from BRANSON). A nickel plate serving as a negative electrode current collector tab was welded to an end portion of the anode electrode.

[0154] All operations were carried out in a dry room with a dew point of –27°C. A pouch cellwas assembled by a jelly roll winding machine (from CORN). The cathode and anode electrode plates were kept apart by the separators. The thickness of the separator was 20 µm (SK520H, PE separator from SK). The jelly roll winding typecell assembly was put in an aluminum laminated pouch and dried in a vacuum oven for 3 days at a fixed temperature of 55°C, then electrolyte was put in the aluminum pouch. Commercially available electrolyte (from Soulbrain) was used. The electrolyte was a solution of LiPF6 (1 M) in a mixture of ethylene carbonate (EC),and ethyl methyl carbonate (EMC) in a volume ratio of 3:7 with 2 wt.% vinylene carbonate (VC) and 0.5 wt.-% 1,3-propane sultone (PS). After electrolyte filling, the pouch cell was vacuum-sealed.

[0155] The non-aqueous electrolyte solution was impregnated for 24 hours at roomtemperature. The battery was pre-charged at 30% of its theoretical capacity and aged 1 day at room temperature and aged 1 day at a fixed temperature of 60°C. The battery was then degassed and the aluminum pouch was sealed and then mechanically pressed. The pouch-type lithium battery was prepared and the design capacity of the battery was 1000 mAh.

[0156] The battery was prepared for use as follows: the battery was charged using acurrent 1 C in CC mode (constant current) up to 4.2 V then CV mode (constantvoltage) until a cut-off current of C / 20 was reached. Then, the battery was discharged in CC mode at 1 C rate down to a cut-off voltage of 3.0 V.

[0157] Test examples

[0158] Example 1: Adhesion and slurry viscosity

[0159] The above polymers F-1, FS-1 as well as a blend A-1 of 67 wt. % of polymer F-1and 33 wt. % of polymer FS-1 were used as binders to produce cathodes as described above. The values of slurry viscosity and adhesion are shown in Table 1.

[0160] Polymer blend A-1 is a polymer composition according to the invention. Polymers F-1 and FS-1 alone are tested for comparison.

[0161] Example 2: Adhesion and slurry viscosity

[0162] A blend A-2 comprising 67 wt. % of polymer F-1 and 33 wt. % of polymer FS-2 wereused as binders to produce cathodes as described above. The values of slurry viscosity and adhesion are shown in Table 1.

[0163] Polymer blend A-2 is a polymer composition according to the inventionTable 1 Polymer IntrinsicNormalized Slurry viscosity at viscosity adhesion6 rpm[l / g] [N / m] [mPa·s] F-1 0.29 1.00 8050FS-1 0.23 0.79 5700A-1 – 1.02 6730A-2 – 1.0 5800

[0164] The results show that the binder A-1 of the present invention is more performing andeasier to be handled in the fabrication process of electrodes thanks to a low slurry viscosity and an excellent adhesion to the current collector (similar to F-1), in spite of mixing two polymers where one of them has a much lower adhesion. It is clear that a synergistic effect is shown by the binder A-1, where the adhesion is the same as in the cases with F-1, coupled with a lower viscosity of the slurry allowing an easier battery manufacturing. Very good performances in terms of adhesion andslurry viscosity are obtained also for blend A-2.

[0165] Example 2: Cycling stability

[0166] A pouch cell battery having a capacity of about 1,000 mAh was prepared asdescribed above using the above polymer binders F-1 and A-1 for the cathode. The cycling stability thereof was tested in a test range of 4.2 V to 3.0 V at a temperatureof 25°C (1 C / 1 C). The results are shown in Table 2.Table 2 Electrochemical evaluation Retention of capacity [%] Number of cycles Polymer F-1 as Polymer blend A-1 as binder binder 0100 100100 98.75 98.40200 97.24 97.17300 95.46 95.86400 92.84 94.31500 88.45 92.39600 82.71 89.40700 76.93 85.53800 69.54 81.95900 – 78.72

[0167] This example shows that the battery produced with the polymer blend A-1 as binderhas a higher retention of capacity that the battery produced with only polymer F-1 as binder; after 800 cycles the capacity is still over 80% of the initial value. Thus, the polymer composition according to the invention solves the problem to provide a low viscosity slurry (i.e. is easier to handle in the fabrication process of electrodes thanks to a low slurry viscosity), high adhesion to the current collector and excellent cyclability.

Claims

CLAIMS 1. A polymer composition comprising, preferably consisting of:(1) at least one first polymer comprising recurring units derived from vinylidenefluoride, and (2) at least one second polymer, different from the first polymer, comprising(iii) recurring units derived from vinylidene fluoride and(iv) recurring units bearing moieties of –SO3X, where X is hydrogen or analkali metal, wherein the recurring units bearing moieties of –SO3X are present in an amount of 0.01 % by moles to 20% by moles, preferably from 0.1% to 10.0% by moles, more preferably in an amount from 0.2% by moles to 5.0% by moles, with respect to the total molar amount of recurring units in the second polymer.

2. The polymer composition according to claim 1, wherein the first polymer further includes recurring units derived from at least a vinyl monomer of formula (I):wherein: -R1, R2 and R3, equal to or different from each other, are independently selectedfrom a hydrogen atom, a halogen atom, a C1-C5 hydrocarbon group and RX, and -RX is a C1-C20 linear or branched hydrocarbon moiety comprising at least onefunctional group selected from a hydroxyl, a carboxyl, an epoxide, an ester, a phosphate and an ether group.

3. The polymer composition according to claim 2, wherein the at least one vinyl monomer is a vinyl monomer of the following formula (I):wherein: -R1, R2 and R3, equal to or different from each other, are independently selected froma hydrogen atom, a halogen atom, a C1-C5 hydrocarbon group and RX, and -RX is a C1-C20 linear or branched hydrocarbon moiety comprising at least onecarboxylic acid functional group.

4. The polymer composition according to claim 2 or 3, wherein the at least one vinyl monomer is a vinyl monomer of the following formula (Ia):wherein each of R1 and R2, equal to or different from each other, are independentlyselected from a hydrogen atom, a halogen atom, a C1-C5 hydrocarbon group have the meanings as above defined, R3 is hydrogen, and -ROH is a hydrogen or a C1-C5 hydrocarbon moiety comprising at least onecarboxylic group.

5. The polymer composition according to any one of claims 1 to 4, wherein the firstpolymer further includes at least one fluorinated comonomer selected from the group consisting of: (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-Rf0, wherein Rf0 is a C1-C6perfluoroalkyl group; (d) chloro- and / or bromo- and / or iodo-C2-C6 fluoroolefins such aschlorotrifluoroethylene (CTFE). (e) perfluoro(alkyl)vinyl ethers such as perfluoro(methyl)vinyl ether (PMVE),perfluoro(ethyl) vinyl ether (PEVE) and perfluoro(propyl)vinyl ether (PPVE); (f) perfluoro(1,3-dioxole) and perfluoro(2,2-dimethyl-1,3-dioxole) (PDD).

6. The polymer composition according to any one of claims 1 to 5, wherein the first polymer further includes at least one hydrocarbon comonomer of formula (II), different from the vinyl monomers of formula (I), R1R2C=CR3-Ry (II) wherein R1, R2and R3, equal to or different from each other, are independently selected from a hydrogen atom, a halogen atom, and a C1-C5hydrocarbon group, and wherein Ry is a C3-C20, preferably C4-C15 linear or branched hydrocarbon chainmoiety comprising at least two functional groups independently selected from the group consisting of ether (-O-), ketone (–C=O-), epoxy, per-carbonate (–O-CO-O-) and ester (-OCO-).

7. The polymer composition according to any one of claims 1 to 6, wherein the polymercomposition comprises the first polymer in an amount of 5% to 95% by weight, preferably 25% to 75% by weight.

8. The polymer composition according to any one of claims 1 to 7, wherein the firstpolymer has an intrinsic viscosity, determined according to the method described in the experimental part, of 0.15 l / g or greater, preferably of 0.20 l / g or greater.

9. The polymer composition according to any one of claims 1 to 8, wherein the secondpolymer has an intrinsic viscosity, determined according to the method described in the experimental part, of 0.05 l / g to 0.50 l / g, preferably of 0.15 l / g to 0.35 l / g.

10. Use of the polymer composition according to any one of claims 1 to 9 as a binder inthe manufacture of an electrode of an electrochemical cell, preferably a secondary battery, more preferably a Li-ion battery or a Na-ion battery, particularly preferably a positive electrode of such an electrochemical cell.

11. A composition for forming an electrode for an electrochemical cell, wherein thecomposition comprises (a) at least one electro-active material;(b) the polymer composition according to any one of claims 1 to 9 as a binder;and (c) at least one solvent.

12. The composition for forming an electrode for an electrochemical cell according toclaim 11, wherein the electro-active material comprises a composite metal chalcogenide of formula LiMQ2, 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, preferably a lithium-based composite metal oxide of formula LiMO2, wherein M is the same as defined above, more preferably LiCoO2, LiNiO2, LiNixCo1-xO2 (0 < x < 1) and spinel-structured LiMn2O4.

13. A process for the manufacture of an electrode for an electrochemical cell, wherein theprocess comprises: (A) providing a metal substrate having at least one surface;(B) providing the composition for forming an electrode according to any one ofclaims 11 or 12; (C) applying the composition provided in step (B) onto the at least one surface ofthe metal substrate provided in step (A), thereby providing an assembly comprising a metal substrate coated with said composition onto the at least one surface; (D) drying the assembly provided in step (C);(E) subjecting the dried assembly obtained in step (D) to a compression step toobtain the electrode.

14. An electrode obtainable by the process according to claim 13.

15. An electrochemical device comprising at least one electrode according to claim 14.

16. The electrochemical device according to claim 15, wherein the electrochemicaldevice is a Li-ion battery or a Na-ion battery.

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