Binders for secondary battery electrodes

Abstract

The present invention pertains to vinylidene fluoride polymers used as binder for electrodes in secondary batteries.

Description

Binders for secondary battery electrodesCross reference to previous applications

[0001] This application claims priority to European application No. 23210253.3 filed on 16November 2023, the whole content of this application being incorporated herein by reference for all purposes.Technical Field

[0002] The present invention pertains to vinylidene fluoride copolymers comprising recurring units derived from polar monomers and to their use as binder for electrodes in Li-ion batteries.Background Art

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

[0004] In particular, WO 2008 / 129041 (SOLVAY SPECIALTY POLYMERS ITALY S.P.A.) discloses linear semi-crystalline vinylidene fluoride (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.

[0005] In general, increasing the fluoropolymers molecular weight is known to increase the performances of articles made from these materials, in particular in terms of mechanical properties and in terms of adhesion of the electrodes to the current collector

[0006] However, increasing the fluoropolymers molecular weight will increase the viscosity of the electrode-forming formulation including the same, also called electrode slurry, making much more difficult the handling and the coating process in the fabrication of electrodes

[0007] In the technical field of batteries, notably of lithium batteries, the problem of providing electrode binders characterized by very good adhesion that at the same time do not impact negatively on the fabrication process of the electrodes, such as by an increase of the slurry viscosity to produce the same, is felt.

[0008] This invention provides a strong improvement to the current technology by combining easiness in the electrode fabrication process by dealing with electrodeforming formulations having low viscosity at low shear rates, which allow the provision of electrodes having a very high adhesion towards the current collector.Summary of invention

[0009] It has been found that certain vinylidene fluoride copolymers randomly including certain vinyl monomers comprising oxygenated functional groups are endowedwith very good adhesion to metal substrates and can be used in the preparation of electrode-forming compositions having low viscosity at low shear rates.

[0010] It is thus an object of the invention a VDF-based polymer [polymer (F)] characterized by comprising:- (i) recurring units derived from vinylidene fluoride (VDF) monomer;- (ii) recurring units derived from at least one polar vinyl monomer (MA), wherein monomer (MA) is present in an amount comprised between 0.01 % and 5.0 % by moles with respect to the total moles of recurring units of polymer (F); and wherein of at least 50% of monomer (MA) is randomly distributed into said polymer (F) and, wherein the polymer (F) is characterized by containing chain end groups of formula (I):CH3-O-CO-C(CH3)2- (I)

[0011] The chain end-groups of formula (I) are present in polymer (F) in an amount of at least 0.1 / 10000 VDF units, more preferably of at least 0.5 / 10000 VDF units, still more preferably of at least 1.0 / 10000 VDF units.

[0012] A second object of the present invention pertains to an electrode-forming composition (C) comprising: a) at least one electrode active material (AM); b) at least one binder (B), wherein binder (B) comprises at least one polymer (F) as above defined; and c) at least one solvent (S).

[0013] In another object, the present invention pertains to the use of said electrodeforming composition (C) in a process for the manufacture of an electrode [electrode (E)], said process comprising:(A) providing a metal substrate having at least one surface;(B) providing an electrode-forming composition (C) as above defined;(C) applying the composition (C) 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 (C) onto the at least one surface;(D) drying the assembly provided in step (C);(E) submitting the dried assembly obtained in step (D) to a compression step to obtain the electrode (E) of the invention.

[0014] In a further object, the present invention pertains to the electrode (E) obtainable by the process of the invention.

[0015] In still a further object, the present invention pertains to an electrochemical device comprising at least one electrode (E) of the present invention.Detailed description

[0016] By the term “VDF-based polymer” it is intended to denote a VDF homopolymer (PVDF) and VDF-based copolymers including recurring units derived from VDF and recurring units derived from at least one fluorinated comonomer (CF), different from VDF.

[0017] By the term “recurring unit derived from vinylidene fluoride” (also generally indicated as VDF or VDF unit), it is intended to denote a recurring unit of formula - CF2-CH2-.

[0018] The chain end groups of formula (I) in polymer (F) are linked to recurring units derived from VDF, thus to units of formula -CF2-CH2-. The chain end groups of formula (I) can be linked to the recurring units of formula -CF2-CH2- either through the -CF2or the -CH2group (namely, normal or reverse recurring VDF units).

[0019] Suitable polar vinyl monomers (MA) are compounds of formula (II):R?,R3R .R;(H) wherein:- Ri, R2and R3, equal to or different from each other, are independently selected from a hydrogen atom, a Ci-C3hydrocarbon group and Rx, and- Rx is a C1-C20 hydrocarbon moiety comprising at least one functional group selected from a hydroxyl, a carboxyl, an epoxy, an ester, a per-carbonate, a phosphate and an ether group.

[0020] The monomer (MA) is preferably present in an amount of from 0.05 to 5 % by moles of with respect to the total moles of recurring units of polymer (F), more preferably at most 1 .5% by moles.

[0021] The term "polar vinyl monomer" as employed herein may comprise recurring units derived from one or more than one polar vinyl monomer (MA) as above described. In the rest of the text, the expressions "polar vinyl monomer (MA)" is to be intended, both in the plural and the singular, that is to say that they denote both one or more than one polar vinyl monomer (MA).

[0022] In one embodiment, the polar vinyl monomer (MA) preferably complies with formula (HI):R, RaR{ O-R(HI) wherein each of Ri and R2have the meanings as above defined, R3is hydrogen, and ROH is a hydrogen or a C1-C5 hydrocarbon moiety comprising at least one hydroxyl group and / or at least a carboxylic group; more preferably, each of Ri, R2, R3are hydrogen, while ROH has the same meaning as above detailed.

[0023] Non-limitative examples of monomers (MA) of formula (II) are, notably:- 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,- hydroxyethyl (meth)acrylate,- hydroxypropyl(meth)acrylate,- hydroxyethylhexyl(meth)acrylates, and mixtures thereof.

[0024] Preferably, the at least one monomer (MA) according to this embodiment is acrylic acid (AA)

[0025] In another embodiment, the polar vinyl monomer (MA) is a compound of formula (II) wherein- Ri, R2and R3, equal to or different from each other, are independently selected from a hydrogen atom and a Ci-C3hydrocarbon group, and- Rxis a C3-C2o linear or branched hydrocarbon chain moiety comprising at least two functional groups independently selected from the group consisting of ether, ketone, epoxy, per-carbonate and ester.

[0026] According to this embodiment, Rxin formula (II) is more preferably a C4-C15 linear or branched hydrocarbon chain moiety comprising at least three functional groups independently selected from the group consisting of ether, ketone, epoxy, per-carbonate and ester. Preferably Ri, R2and R3are H atoms.

[0027] Non-limitative examples of monomers (MA) of formula (II) according to this embodiment include, notably:- allyl glycidyl ether (AGE),- ethylene glycol alkyl ether acrylates of formula (IV)such as di(ethylene glycol) ethyl ether acrylate (DEGEEA);- (meth) acryloyloxyalkyl succinic acid, such as (meth) acryloyloxyethyl succinic acid and (meth) acryloyloxypropyl succinic acid; and mixtures thereof.

[0028] Preferably, the at least one monomer (MA) according to this embodiment is selected from allyl glycidyl ether (AGE) and di(ethylene glycol) ethyl ether acrylate (DEGEEA).

[0029] It is essential that in polymer (F) at least 50% of monomer (MA) be randomly distributed into said polymer (F).

[0030] 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.

[0031] Thus, when polymer (F) is prepared by a polymerization reaction that comprises continuously feeding monomer (MA) during VDF polymerization, a random distribution of monomer (MA) in the polymer chains is present, with sequences VDF-(MA)-VDF being obtained.

[0032] More preferably, in polymer (F) at least 70% of monomer (MA) is randomly distributed into said polymer (F).

[0033] The expression “randomly distributed monomer (MA)” is intended to denote the presence of sequences VDF-(MA)-VDF, and the amount of randomly distributed monomer (MA) is determined as the percent ratio between the average number of said VDF-(MA)-VDF sequences and the total average number of (MA) monomer recurring units.

[0034] When each of the (MA) recurring units is isolated, that is to say comprised between two recurring units of VDF monomer, the average number of (MA) sequences equals the average total number of (MA) recurring units, so the fraction of randomly distributed units (MA) is 100%: this value corresponds to a perfectly random distribution of (MA) recurring units. Thus, the larger is the number of isolated (MA) units with respect to the total number of (MA) units, the higher will be the percentage value of the fraction of randomly distributed units (MA), as above described.

[0035] The analytical determination of the total amount of randomly distributed monomer (MA) may be carried out by measuring the sequences VDF-(MA)-VDF by19F-NMR and the total amount of monomer in the polymer by one or more of these techniques,19F-NMR ,1H-NMR, titration of carboxyl groups, FT-IR or others.

[0036] Excellent results have been obtained using a polymer (F) comprising at least 70% by moles of recurring units derived from VDF.

[0037] The polymer (F) can be an elastomer or a semi-crystalline polymer, preferably being a semi-crystalline polymer.

[0038] 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 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.

[0039] 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.

[0040] 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.

[0041] Preferably, the intrinsic viscosity of polymer (F), measured in dimethylformamide at 25 °C, is between 0.15 l / g and 0.80 l / g, even more preferably between 0.20 l / g and 0.60 l / g.

[0042] The polymer (F) of the present invention usually has a melting temperature (Tm) comprised in the range from 120 to 200°C.

[0043] The polymer (F) of the present invention possesses a quasi-linear structure, with a very low amount of branching, which results in the insoluble fraction due to long branched chains being substantially decreased.

[0044] The polymer (F) of the present invention has in fact preferably a low fraction of insoluble components in standard polar aprotic solvents for VDF polymers, such as NMP. More preferably, solutions of polymer (F) in said standard polar aprotic solvents remain homogeneous and stable for several weeks, with substantially no insoluble residue.

[0045] Thanks to the low amount of insoluble components, the GPC and NMR analyses of polymer (F) are not affected, and there are no problems of reliability and reproducibility.

[0046] The melting temperature may be determined from a DSC curve obtained by differential 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 largest peak area.

[0047] The polymer (F) may further comprise recurring units derived from one or more fluorinated comonomers (CF) different from VDF.

[0048] By the term “fluorinated comonomer (CF)", it is hereby intended to denote an ethylenically unsaturated comonomer comprising at least one fluorine atoms.

[0049] Non-limitative examples of suitable fluorinated comonomers (CF) include, notably, the followings:(a) C2-C8fluoro- and / or perfluoroolefins such as tetrafluoroethylene (TFE), hexafluoropropylene (HFP), pentafluoropropylene and hexafluoroisobutylene;(b) C2-C8hydrogenated monofluoroolefins, such as vinyl fluoride; 1 ,2- difluoroethylene and trifluoroethylene;(c) perfluoroalkylethylenes of formula CH2=CH-Rf0, wherein Rf0is a Ci-C6perfluoroalkyl group;(d) chloro- and / or bromo- and / or iodo-C2-C6fluoroolefins such as chlorotrifluoroethylene (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); perfluoro(2,2-dimethyl-1 ,3-dioxole) (PDD).

[0050] The fluorinated comonomer (CF) is preferably HFP.

[0051] In one preferred embodiment, polymer (F) is semi-crystalline and comprises 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).

[0052] It is understood that chain ends different from those above defined, defects or other impurity-type moieties might be comprised in the polymer (F) without these impairing its properties.

[0053] The polymer (F) 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 0.01% to 1 .5% by moles, preferably from 0.01% to 1 .0% by moles of at least one acrylic monomer (MA);- optionally from 0.5 to 3.0% by moles of recurring units derived from at least one fluorinated comonomer (CF).

[0054] Polymer (F) may be obtained by a process that comprises:- polymerizing vinylidene fluoride (VDF) monomer, an initial charge of monomer (MA) and optionally comonomer (CF), in an aqueous medium in the presence of a radical initiator system that introduces in the polymer chain end groups of formula (I),- continuously feeding an aqueous solution comprising monomer (MA); and- maintaining the pressure in the reactor vessel exceeding the critical pressure of the vinylidene fluoride.

[0055] Suitable radical initiator systems include radical initiators comprising azo compound initiators such as dimethyl 2,2'-azobis(2-methylpropionate) (AIBME).

[0056] The quantity of an initiator required for a polymerization is related to its activity and to 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.

[0057] The initiator may be added in pure form, in solution, in suspension, or in emulsion, depending upon the initiator chosen.

[0058] In one embodiment, when the initiator is added in solution, it can be dissolved in the optional chain transfer agent used in the polymerization.

[0059] In one embodiment, when the initiator is added in solution, it can be dissolved in a branched chain aliphatic alcohol such as isopropanol, terbutanol, pinacol, 2,4- dimethyl-3-pentanol, 2,4,4-trimethyl-1 ,3-pentandiol, or mixtures thereof.

[0060] In one embodiment, when the initiator is added in solution, it can be dissolved in a mixture of a chain transfer agent used in the polymerization and a branched chain aliphatic alcohol.

[0061] A chain transfer agents (CT A) can be added to the radical initiator system for 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 maleate, 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 of the polymer.

[0062] Preferably, CTA are those that leads to polar oxygen containing chain end-groups, such as diethyl-carbonate.

[0063] In the 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.

[0064] Preferably, the process for preparing polymer (F) is carried out at a temperature superior to the critical temperature of the VDF monomer, i.e. of at least 31 °C.

[0065] It is essential that a continuous feeding of an aqueous solution containing monomer (MA) is carried out during the whole duration of polymerization run.

[0066] It is thus possible to obtain a nearly statistical distribution of monomer (MA) within the VDF monomer polymer backbone of polymer (F).

[0067] The expressions "continuous feeding" or "continuously feeding" means that slow, small, incremental additions the aqueous solution of monomer (MA) take place during the polymerization.

[0068] The aqueous solution of monomer (MA) continuously fed during polymerization amounts for at least 50 % wt of the total amount of monomer (MA) 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 monomer (MA) is continuously fed during polymerization. An incremental addition of VDF monomer can be carried out during polymerization, even if this requirement is not mandatory.

[0069] In one preferred embodiment of the invention, the process for preparing polymer (F) as above defined comprises:- polymerizing vinylidene fluoride (VDF) monomer, an initial charge of monomer (MA) and optionally comonomer (CF), in an aqueous medium in the presence of a radical initiator system that introduces in the polymer chain end groups of formula (I),- continuously feeding an aqueous solution comprising monomer (MA); and- maintaining the pressure in the reactor vessel exceeding the critical pressure of the vinylidene fluoride wherein the initiator system includes an azo compound initiator and a chain transfer agent.

[0070] In a more preferred embodiment of the invention, the process for preparing polymer (F) as above defined comprises:- polymerizing vinylidene fluoride (VDF) monomer, an initial charge of monomer (MA) and optionally comonomer (CF), in an aqueous medium in the presence ofa radical initiator system that introduces in the polymer chain end groups of formula (I),- continuously feeding an aqueous solution comprising monomer (MA); and- maintaining the pressure in the reactor vessel exceeding the critical pressure of the vinylidene fluoride wherein the initiator system includes dimethyl 2,2'-azobis(2-methylpropionate (AIBME) and diethylcarbonate.

[0071] The polymer (F) is typically provided in form of powder according to the process described above.

[0072] Polymer (F) in the form of powder may be optionally further extruded to provide polymer (F) in the form of pellets.

[0073] The polymer (F) as detailed above may be used as a binder for electrodes in secondary batteries,

[0074] A second object of the present invention pertains to an electrode-forming composition (C) comprising: a) at least one electrode active material (AM); b) at least one binder (B), wherein binder (B) comprises at least one polymer (F) as above defined; and c) at least one solvent (S).

[0075] For the purpose of the present invention, the term “electro-active material (AM)” is intended to denote a compound 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. The compound (AM) is preferably able to incorporate or insert and release lithium or sodium ions.

[0076] The nature of the compound (AM) in composition (C) depends on whether said composition is used in the manufacture of a positive electrode [electrode (Ep)] or a negative electrode [electrode (En)].

[0077] In the case of forming a positive electrode (Ep) for a sodium-ion secondary battery, active material is generally selected from Na-based layered transition-metal oxides, Prussian blue analogs and polyanion-type materials.

[0078] In some embodiments the active materials are Na-based layered transition-metal oxides classified as 03- , 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.

[0079] In some embodiments the active materials are Prussian blue analogs (PBA) of general formula AxP[R(CN)6]i-yny.mH2O with A and alkali metal ion, P a N-coordinated transition metal ion, R a C-coordinated transition metal ion, □ a [R(CN)6] vacancy, with 0 < x < 2 and 0 < y < 1 such as Nao8iFe[Fe(CN)6]o79Do2i ,

[0080] In some other embodiments the active materials are polyanion-type materials of general formula NaxMy(XO4)n (where X = S, P, Si, As, Mo and W and M is transition metal), which possess a series of tetrahedron anion units (XO4)n" and their derivatives (XmO3m+i)n'. Among them, phosphates NaMPO4such as NaFePO4, Nao7FeP04or NaMnPO4; natrium (sodium) superionic conductor of NASICON- type structures of general formula NaxM2(XO4)3(where 1 < x < 4 andM = V, Fe, Ni, Mn, Ti, Cr, Zr.„; X = P, S, Si, Se, Mo ...) - with single transition metal type such as Na3V2(PO4)3(NVP), Na3Cr2(PO4)3, Na3Fe2(PO4)3; - with 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, Na2CoP207, Na4.xFe2+x / 2(P2O7)2 with 2 / 3 < x < 7 / 8 e.g. Na3i2Fe3 44(P3O7)2 oorr Na332Fe234(P2O7)2, Na2(VO)P2O7, Na7V3(P2O7)4; fluorophosphates NaVPO4F, Na2CoP04F, Na2FePO4F, Na2MnPO4F, Na3(VOi. XPO4)2FI+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 M representing transition metals) such as Na4Mn3(PO4)2(P2O7), Na4Co3(P04)2(P207), 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(S04)3, Na2+2xMn2-x(SO4)3(where 0 < x < 1) ; silicates of general formula Na2MSiO4(with M = Mn, Fe, Co and Ni).

[0081] In some preferred embodiments the active materials are fluorophosphates preferably selected from the list consisting of NaVPO4F, Na2CoP04F, Na2FePO4F, Na2MnPO4F, Na3(VOi.xPO4)2Fi+2x (with 0 < x < 1) e.g. Na3(VOPO4)2F or Na3V2(PO4)2F3(NVPF).

[0082] The conventional active materials (AM) at the positive electrode of lithium-ion batteries may comprise 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 may include LiCo02, LiNiO2, LiNixCoi. XO2(0 < x < 1) and spinel-structured LiMn2O4.

[0083] According to another preferred embodiment, the at least one positive electrode active material (AM) is selected 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 , wherein y+w+z = 1-x, andQ is the same as defined above.

[0084] As an alternative, still, 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 representing less than 20% of the Mi metals, 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 M2metals, including 0, JO4is 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 JO4oxyanion, generally comprised between 0.75 and 1.

[0085] The MiM2(JO4)fEi.f electro-active material as defined above is preferably phosphate-based and may have an ordered or modified olivine structure.

[0086] More preferably, the electrode active material has formula Li3.xM’yM”2.y(JO4)3wherein 0<x<3, 0<y<2, M’ and M” are the same or different metals, at least one of which being 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. Still more preferably, the electrode active material (AM) is a phosphate-based electro-active material 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.

[0087] The A component is preferably Fe, Mn, and Ni, and particularly preferably Fe.

[0088] The D component is preferably Mg or Ca.

[0089] Examples of the compound having an olivine structure include lithium iron phosphate (LFP), lithium iron manganese phosphate (LMFP) and lithium manganese phosphate.

[0090] Further, as the positive electrode active material (AM), it is possible to use a material whose surface is partially or wholly covered with carbon in order to supplement the conductivity.

[0091] 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 electrode active material.

[0092] In the case of forming a negative composite electrode (En) for a secondary battery, the compound (AM) may preferably comprise a carbon-based material and / or a silicon-based material.

[0093] In some embodiments, the carbon-based material may be, for example, graphite, such as natural or artificial graphite, graphene, or carbon black.

[0094] These materials may be used alone or as a mixture of two or more thereof.

[0095] The carbon-based material is preferably graphite.

[0096] 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. More particularly, the silicon- based compound may be silicon oxide or silicon carbide.

[0097] When present in compound (AM), the at least one silicon-based compound is comprised in the compound (AM) 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 compound (AM).

[0098] The solvent (S) may preferably be an organic polar one, examples of which may 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.

[0099] An optional conductive agent may be added to the electrode-forming composition (C) in order to improve the conductivity of a resulting electrode.

[0100] When present, the conductive agent is different from the carbon-based material described above.

[0101] 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. Carbon black is available, for example, under the brand names, Super P® or Ketjenblack®.

[0102] In a preferred embodiment of the present invention, an electrode-forming composition (C) for use in the preparation of a positive electrode (Ep) is provided, said composition comprising:a) at least one positive active material (AM); b) at least one binder (B), wherein binder (B) comprises at least one polymer (F) as above defined; c) at least one solvent (S); and d) at least one conductive agent, preferably selected from carbon black or graphite fine powder carbon nanotubes.

[0103] As said above, the polymer (F) of the present invention possesses a quasi-linear structure, and very low amount of insoluble fraction when dissolved in standard polar aprotic solvents such as NMP.

[0104] Thanks to the low amount of insoluble components, polymer (F) provides solutions in organic solvents, which are not detrimentally affected by the presence of insoluble residues, which are generally referred as “gels”, and are hence more adapted for use in formulating electrodes-forming compositions.

[0105] In another object, the present invention pertains to the use of the electrode-forming composition (C) for the manufacture of an electrode (E), said process comprising:(A) providing a metal substrate having at least one surface;(B) providing an electrode-forming composition (C) as above defined;(C) applying the composition (C) 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 (C) onto the at least one surface;(D) drying the assembly provided in step (C);(E) submitting the dried assembly obtained in step (iv) to a compression step to obtain the electrode (E) of the invention.

[0106] In a further object, the present invention pertains to the electrode (E) obtainable by the process of the invention.

[0107] The Applicant has surprisingly found that the electrode (E) of the present invention shows outstanding adhesion of the binder to current collector.

[0108] The electrode (E) of the invention is thus particularly suitable for use in electrochemical devices, in particular in secondary batteries.

[0109] For the purpose of the present invention, the term “secondary battery” is intended to denote a rechargeable battery.

[0110] The secondary battery of the invention is preferably an alkaline or an alkaline-earth metal secondary battery.

[0111] The secondary battery of the invention is more preferably a Sodium-ion or a Lithium-ion secondary battery.

[0112] In still a further object, the present invention pertains to an electrochemical device comprising at least one electrode (E) of the present invention.

[0113] The electrochemical device according to the present invention, being preferably a secondary battery, comprises:- a positive electrode and a negative electrode, wherein at least one of the positive electrode and the negative electrode is the electrode (E) of the present invention.

[0114] In one preferred embodiment of the present invention it is provided an electrochemical device, which is preferably a secondary battery, which comprises:- a positive electrode and a negative electrode, wherein the positive electrode is the electrode (E) according to the present invention.

[0115] An electrochemical device according to the present invention can be prepared by standard methods known to a person skilled in the art.

[0116] 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.

[0117] The invention will be now described with reference to the following examples, whose purpose is merely illustrative and not intended to limit the scope of the invention.

[0118] EXPERIMENTAL PART

[0119] Determination of intrinsic viscosity of polymer (F1-F4; A-C)

[0120] Intrinsic viscosity (q) [dl / g] was measured using the following equation on the basis of dropping time, at 25°C, of a solution obtained by dissolving the polymer (F1-F4; A-C) in N,N-dimethylformamide at a concentration of about 0.2 g / dl using a Ubbelhode viscometer:where c is polymer concentration [g / dl], qr is the relative viscosity, i.e. the ratio between the dropping time of sample solution and the dropping time of solvent, qsp is the specific viscosity, i.e. qr -1 , and F is an experimental factor, which for polymer (F1-F4; A-C) corresponds to 3.

[0121] DSC analysis

[0122] DSC analyses were carried out according to ASTM D 3418 standard; the melting point (Tf2) was determined at a heating rate of 10°C / min.

[0123] Determination of the polar end-groups

[0124] The amount of polar end groups of the polymers (F3, F4; C) arising from the chain transfer diethyl carbonate (DCE) used in the polymerization process, was determined by1H-NMR, measuring the intensity of the H atoms of the CH2groups for linear chain end and CH groups for branched one (in bold in following formulas) with respect to the total intensity of CH2moieties of the polymer (F) backbone VDF monomer units:

[0125] CH3-COO-CH2-CH2- and CH3-COO-CH-(CH3)-

[0126] About 20 mg of polymer were dissolved in 0.7 ml of hexadeuteroacetone. The1H- NMR spectrum, recorded at 60°C, revealed the aforementioned CH2at 4.05 ppm and CH at 5.1 ppm whereas CH2signals from VDF recurring normal and reverse units resonated as broad peaks centered at 2.93 and 2.36 ppm respectively.

[0127] The amount of polar end groups of the polymers (F1-F4) arising from the initiator dimethyl 2,2'-azobis(2-methylpropionate) used in the polymerization process, was determined by1H-NMR, measuring the intensity of the H atoms of the CH3groups (in bold in following formula) with respect to the total intensity of CH2moieties of the polymer (F1-F4) backbone VDF monomer units:CH3-O-CO-C(CH3)2-

[0128] About 20 mg of polymer were dissolved in 0.7 ml of hexadeuteroacetone. The1H- NMR spectrum, recorded at 60°C, revealed the aforementioned CH3at 3.66 ppm whereas CH2signals from VDF recurring normal and reverse units resonated as broad peaks centered at 2.93 and 2.36 ppm respectively

[0129] The amount of polar end groups of the polymers (A-C) arising from the initiator tert amylperoxypivalate (TAPPI) used in the polymerization process, was determined by1H-NMR, measuring the intensity of the H atoms of the (CH3)3groups (in bold in following formula) with respect to the total intensity of CH2moieties of the polymer (F) backbone VDF monomer units:(CH3)3-C-

[0130] About 20 mg of polymer were dissolved in 0.7 ml of hexadeuteroacetone. The 1 H- NMR spectrum, recorded at 60°C, revealed the aforementioned (CH3)3at 1.08 whereas CH2signals from VDF recurring normal and reverse units resonated as broad peaks centered at 2.93 and 2.36 ppm respectively.

[0131] The content of chain end groups was calculated by applying the following formula: [EG] = (IEG / IVDF) x 10000 wherein:- [EG] is the content of the generic end-groups expressed as moles per 10000 VDF units,- IEG is the intensity, normalized to one hydrogen, of the integral of the end-group [EG]- IVDF is the intensity, normalized to one hydrogen, of the integrals of normal and reverse VDF recurring units.

[0132] Determination of the amount of monomers acrylic Acid (AA) and difethylene glycol) ethyl ether acrylate (DEGEEA) in the polymers (F1-F4; A-C) by NMR

[0133] Alternated AA and DEGEEA contents in the polymers were determined by19F- NMR and1H-NMR spectroscopy. Signals related to CF2moieties of VDF units (in bold in following formula) adjacent to isolated hydrogenated comonomers have been found to resonate at about -94 ppm in the19F-NMR:-CH2CF2-CH2CH(COOH)-CH2CF2-CH2-CH2CF2-CH2CH[COO(CH2CH2O)2CH2CH3]-CH2CF2-CH2

[0134] Additionally, signals related to CH2moieties of DEGEEA units (in bold in following formula) have been found to resonate at about -3.5 ppm in the1H-NMR:-CH2CF2-CH2CH[COO(CH2CH2O)2CH2CH3]-CH2CF2-CH

[0135] From the ratio between the normalized intensities of this signal and those of all the VDF peaks in the spectrum it is possible to determine the average number of comonomer statistically inserted between two VDF units.

[0136] For the polymers below (F 1 , F2, F3, F4, A, B, C) due to the way in which they were produced - with continuous feeding of both monomers - a very high alternation of monomer sequences was estimated, making the theoretical composition very close to the real one.

[0137] Example 1: Preparation of Polymer F1

[0138] In a 4L reactor equipped with an impeller running at a speed of 650 rpm were introduced in sequence and per kg of total monomers: 1843 g of demineralized water and 1.33 g of Polyvinyl alcohol (Alcotex™ 80 from Synthomer). The oxygen 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.

[0139] Then, a solution obtained by dissolving 3.37 g of the initiator dimethyl 2,2'-azobis(2- methylpropionate) (from Fujifilm) in 2.37 g of tert-Butanol (from ThermoFisher Scientific) was introduced in the reactor.

[0140] Then, 0.57 g of acrylic acid (AA) were introduced in the reactor at a stirring speed of 880 rpm. Immediately after, 1332 g of VDF were added to the mixture. The reactor was then gradually heated until the set-point temperature of 62°C was reached.

[0141] The pressure was kept constantly equal to 120 bars during the whole polymerization run by feeding an aqueous solution comprising 13.90 g of AA perliter of solution. A total of 1015 g of the solution was loaded into the reactor. After 1676 minutes the polymerization was stopped by degassing the suspension until reaching atmospheric pressure.

[0142] The polymer was then collected by filtration and suspended against clean water in a stirred tank. After the washing treatment, the polymer was dried in an oven at 65°C overnight. 938 g of dry powder were collected.

[0143] A polymer comprising VDF-AA (0.9% by moles), having an intrinsic viscosity of 0.314 l / g in DMF at 25°C and a T2f of 161 ,5°C was obtained.

[0144] In addition, the presence of 8.0 / 10000 VDF units of -CF2H and 3.7 / 10000 VDF units of -CF2CH3end-groups was determined.

[0145] In addition, the presence of 5.8 / 10000 VDF units of end-groups from Dimethyl 2,2'- azobis(2-methylpropionate) was determined.

[0146] Example 2: Preparation of Polymer F2

[0147] In a 4L reactor equipped with an impeller running at a speed of 650 rpm were introduced in sequence and per kg of total monomers: 1716 g of demineralized water and 1.32 g of polyvinyl alcohol (Alcotex™ 80 from Synthomer). The oxygen 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.

[0148] Then, a solution obtained by dissolving 4.71 g of the initiator dimethyl 2,2'-azobis(2- methylpropionate) (from Fujifilm) in 3.33 g of tert-Butanol (from ThermoFisher Scientific) was introduced in the reactor.

[0149] Then, 0.26 g of acrylic acid (AA) were introduced in the reactor at a stirring speed of 880 rpm. Immediately after, 1340 g of VDF were added to the mixture. The reactor was then gradually heated until the set-point temperature of 70°C was reached.

[0150] The pressure was kept constantly equal to 120 bars during the whole polymerization run by feeding an aqueous solution comprising 5.38 g of AA per liter of solution. A total of 1202 g of the solution was loaded into the reactor. After 545 minutes the polymerization was stopped by degassing the suspension until reaching atmospheric pressure.

[0151] The polymer was then collected by filtration and suspended against clean water in a stirred tank. After the washing treatment, the polymer was dried in an oven at 65°C overnight. 1056 g of dry powder were collected.

[0152] A polymer comprising VDF-AA (0.4% by moles), having an intrinsic viscosity of 0.280 l / g in DMF at 25°C and a T2f of 164.2°C was obtained.

[0153] In addition, the presence of 5.6 / 10000 VDF units of -CF2H and 3.9 / 10000 VDF units of -CF2CH3end-groups was determined.

[0154] In addition, 6.0 / 10000 VDF units of end-groups from dimethyl 2,2'-azobis(2- methylpropionate).

[0155] Example 3: Preparation of Polymer F3

[0156] In a 4L reactor equipped with an impeller running at a speed of 650 rpm were introduced in sequence and per kg of total monomers: 1765 g of demineralized water and 1.34 g of polyvinyl alcohol (Alcotex™ 80 from Synthomer). The oxygen 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.

[0157] Then, a solution obtained by dissolving 4.71 g of the initiator dimethyl 2,2'-azobis(2- methylpropionate) (from Fujifilm) in 4.04 g of diethyl carbonate was introduced in the reactor.

[0158] Then, 0.13 g of acrylic acid (AA) were introduced in the reactor at a stirring speed of 880 rpm. Immediately after, 1346 g of VDF were added to the mixture. The reactor was then gradually heated until the set-point temperature of 65°C was reached.

[0159] The pressure was kept constantly equal to 120 bars during the whole polymerization run by feeding an aqueous solution comprising 2.76 g of AA per liter of solution. A total of 1171 g of the solution was loaded into the reactor. After 522 minutes the polymerization was stopped by degassing the suspension until reaching atmospheric pressure.

[0160] The polymer was then collected by filtration and suspended against clean water in a stirred tank. After the washing treatment, the polymer was dried in an oven at 65°C overnight. 1055 g of dry powder were collected.

[0161] A polymer comprising VDF-AA (0.2% by moles), having an intrinsic viscosity of 0.263 l / g in DMF at 25°C and a T2f of 168.4°C was obtained.

[0162] In addition, the presence of 4.5 / 10000 VDF units of -CF2H and 2.8 / 10000 VDF units of -CF2CH3end-groups was determined.

[0163] In addition, 4.0 / 10000 VDF units of end-groups from Dimethyl 2,2'-azobis(2- methylpropionate) and 0.6 / 10000 VDF units of end-groups from diethyl carbonate was determined.

[0164] Example 4 comparative: Preparation of Polymer A

[0165] The polymer A has been synthesized according to the teaching ofW02008129041. The characteristics of the polymer are the following:Composition: VDF-AA (0.9% by moles), polymer having an intrinsic viscosity of 0.288 l / g in DMF at 25°C and a T2f of 162.6°C.- The polymer contained: 2.3 / 10000 VDF units of the end-group from TAPPI addition, 6.8 / 10000 VDF units of -CF2H and 3.1 / 10000 VDF units of -CF2CH3end-groups.No end groups CH3CH2-OCOO- were present.

[0166] Example 5 comparative: Preparation of Polymer B

[0167] In a 4L reactor equipped with an impeller running at a speed of 650 rpm were introduced in sequence and per kg of total monomers: 2192 g of demineralized water; 0.49 g of PEO (Alkox®-E45 from Alroko); 0.59 g of hydroxypropyl methylcellulose (Methocel®-K100 from DuPont Nutrition Biosciences SAS). The oxygen 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.

[0168] Then, 8.47 g of a solution of the initiator t-amylperpivalate (TAPPI, from United Initiators) in isododecane (75%) were introduced in the reactor.

[0169] Then, 0.35 g of acrylic acid (AA) were introduced in the reactor at a stirring speed of 880 rpm. Immediately after, 1170 g of VDF were added to the mixture. The reactor was then gradually heated until the set-point temperature of 50°C was reached.

[0170] The pressure was kept constantly equal to 120 bars during the whole polymerization run by feeding an aqueous solution comprising 6.55 g of AA per liter of solution. A total of 833 g of the solution was loaded into the reactor. After 400 minutes the polymerization was stopped by degassing the suspension until reaching atmospheric pressure.

[0171] The polymer was then collected by filtration and suspended against clean water in a stirred tank. After the washing treatment, the polymer was dried in an oven at 65°C overnight. 971 g of dry powder were collected.

[0172] A polymer comprising VDF-AA (0.4% by moles), having an intrinsic viscosity of 0.270 l / g in DMF at 25°C and a T2f of 167.1 °C was obtained.

[0173] In addition, the presence of 3.9 / 10000 VDF units of -CF2H and 2.3 / 10000 VDF units of -CF2CH3end-groups was determined.

[0174] In addition, 2.1 / 10000 VDF units of end-groups from TAPPI were determined.

[0175] Example 6 comparative: Preparation of Polymer C

[0176] In a 4L reactor equipped with an impeller running at a speed of 650 rpm were introduced in sequence and per kg of total monomers: 2206 g of demineralized water; 0.47 g of PEO (Alkox®-E45 from Alroko); 0.59 g of hydroxypropyl methylcellulose (Methocel®-K100 from DuPont Nutrition Biosciences SAS). The oxygen 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.

[0177] Then, 4.62 g of a solution of the initiator TAPPI in isododecane (75%) and 6.17 g of diethyl carbonate were introduced in the reactor.

[0178] Then, 0.18 g of acrylic acid (AA) were introduced in the reactor at a stirring speed of 880 rpm. Immediately after, 1176 g of VDF were added to the mixture. The reactor was then gradually heated until the set-point temperature of 50°C was reached.

[0179] The pressure was kept constantly equal to 120 bars during the whole polymerization run by feeding an aqueous solution comprising 3.33 g of AA per liter of solution. A total of 831 g of the solution was loaded into the reactor. After 354 minutes the polymerization was stopped by degassing the suspension until reaching atmospheric pressure.

[0180] The polymer was then collected by filtration and suspended against clean water in a stirred tank. After the washing treatment, the polymer was dried in an oven at 65°C overnight. 987 g of dry powder were collected.

[0181] A polymer comprising VDF-AA (0.2% by moles), having an intrinsic viscosity of 0.286 l / g in DMF at 25°C and a T2f of 169.6°C was obtained.

[0182] In addition, the polymer contained 1.1 / 10000 VDF units of the end-group from TAPPI addition, and the presence of 3.2 / 10000 VDF units of -CF2H and 2.1 / 10000 VDF units of -CF2CH3end-groups was determined.

[0183] In addition, the presence of 1.1 / 10000 VDF units of end-groups coming from diethyl carbonate was determined.

[0184] Example 7: Preparation of Polymer F4

[0185] In a 4L reactor equipped with an impeller running at a speed of 650 rpm were introduced in sequence and per kg of total monomers: 1768 g of demineralized water and 1.34 g of polyvinyl alcohol (Alcotex™ 80 from Synthomer). The oxygen 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.

[0186] Then, a solution obtained by dissolving 3.37 g of the initiator dimethyl 2,2'-azobis(2- methylpropionate) (from Fujifilm) in 2.02 g of Diethyl carbonate was introduced in the reactor.

[0187] Then, 0.13 g of acrylic acid (AA) and 0.06 g of Di(ethylene glycol) ethyl ether acrylate (DEGEEA) were introduced in the reactor at a stirring speed of 880 rpm. Immediately after, 1342 g of VDF were added to the mixture. The reactor was then gradually heated until the set-point temperature of 65°C was reached.

[0188] The pressure was kept constantly equal to 120 bars during the whole polymerization run by feeding an aqueous solution comprising 2.96 g of AA per liter of solution and 1 .36 g of DEGEEA per liter of solution. A total of 1094 g of thesolution was loaded into the reactor. After 692 minutes the polymerization was stopped by degassing the suspension until reaching atmospheric pressure.

[0189] The polymer was then collected by filtration and suspended against clean water in a stirred tank. After the washing treatment, the polymer was dried in an oven at 65°C overnight. 969 g of dry powder were collected.

[0190] A polymer comprising VDF-AA (0.21% by moles) and DEGEEA (0.02% by moles), having an intrinsic viscosity of 0.299 l / g in DMF at 25°C and a T2f of 165.5°C was obtained.

[0191] In addition, the presence of 6.8 / 10000 VDF units of -CF2H and 4.1 / 10000 VDF units of -CF2CH3end-groups was determined.

[0192] In addition, 3.5 / 10000 VDF units of end-groups from Dimethyl 2,2'-azobis(2- methylpropionate) and 1 .9 / 10000 VDF units of end-groups from diethyl carbonate was determined.

[0193] General Preparation of the Electrodes with NMC active material

[0194] The positive electrodes having final composition of 98% by weight of NMC811 , 1.1% by weight of polymer, 0.9% by weight of conductive additive were prepared as follows.

[0195] First the slurry components were added to the mixing cup as follows: 33.6 g of multi-walled carbon nanotubes dispersion at solid content of 4.1% by weight, 21.1 g of a 8% by weight solution of a polymer in NMP, 150 g of NMC, 7.9 g of NMP.

[0196] The mixture was then mixed using a high speed disk impeller at 500 rpm for 5 minutes, followed by 75 minutes at 1900 rpm to obtain the electrode-forming slurry dispersion.

[0197] Positive electrodes were obtained by casting the obtained slurry dispersion on 15 pm thick Al foil with doctor blade and drying the as coated layers in a vacuum oven at temperature of 90°C for about 50 minutes. The thickness of the dried coating layers was about 110 pm.

[0198] Slurry Viscosity Measurement

[0199] The slurry viscosity of the slurry dispersions obtained as above described was measured with an AntonPaar Rheolab QC using a Concentric cylinder setup (Measuring Cup: C-CC27 / QC-LTD Bob: CC27 / P6) with peltier temperature control at 25°C. Steady state viscosities were measured from shear rate of 0.1 to 1000 1 / s.

[0200] The results are shown in Table 1.

[0201] Adhesion Measurement

[0202] Adhesion Peeling Force between Aluminium foil and the electrode obtained as above described was measured as follows:180° peeling tests were performed following the setup described in the standard ASTM D903 at a speed of 300 mm / min at 20°C in order to evaluate the adhesion of the dried coating layer to the Al foil.

[0203] The results are shown in Table 1.Table 1Polymers Initiator Comonomer Electrode Slurry adhesion Viscosity at 2.5 type type (N / m) s1(mPa.s)(mol% by NMR)F1 AIBME AA (0.9) 29 6810AA (0.9) 27 7153AA (0.2) 19 3590AA (0.2) 20 4266AA (0.2) 22 4004DEGEEA (0.02)

[0204] The results show that the polymer of the present invention can be used to prepare electrode slurry compositions for electrodes endowed with a better compromise between adhesion to current collector and viscosity of the slurry in comparison with the electrodes of the prior art.

Claims

ClaimsClaim 1 . A VDF-based polymer [polymer (F)] consisting of :- (i) recurring units derived from vinylidene fluoride (VDF) monomer;- (ii) recurring units derived from at least one polar vinyl monomer (MA), wherein monomer (MA) is present in an amount comprised between 0.01 % and 5.0 % by moles with respect to the total moles of recurring units of polymer (F); and wherein of at least 50% of monomer (MA) is randomly distributed into said polymer (F) and, wherein the polymer (F) is characterized by containing chain end-groups of formula (I):CH3-O-CO-C(CH3)2- (I)Claim 2. The polymer (F) according to claim 1 , which further includes recurring units derived from at least one fluorinated comonomer (CF), different from VDF.Claim 3. The polymer (F) according to any one of claims 1 to 3, wherein the fluorinated comonomer (CF) is selected from the group consisting of:(a) C2-C8fluoro- and / or perfluoroolefins such as tetrafluoroethylene (TFE), hexafluoropropylene (HFP), pentafluoropropylene and hexafluoroisobutylene;(b) C2-C8hydrogenated monofluoroolefins, such as vinyl fluoride; 1 ,2- difluoroethylene and trifluoroethylene;(c) perfluoroalkylethylenes of formula CH2=CH-Rf0, wherein Rf0is a Ci-C6perfluoroalkyl group;(d) chloro- and / or bromo- and / or iodo-C2-C6fluoroolefins such as chlorotrifluoroethylene (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); perfluoro(2,2-dimethyl-1 ,3-dioxole) (PDD).Claim 4. The polymer (F) according to any one of the preceding claims, wherein the end- groups of formula (I) are present in an amount of at least 0.1 / 10000 VDF units, more preferably of at least 0.5 / 10000 VDF units, still more preferably of at least 1.0 / 10000 VDF units.Claim 5. The polymer (F) according to any one of the preceding claims, wherein the polar vinyl monomer (MA) is a compound of formula (II):R ■< , f R1 H.,(II) wherein:- Ri, R2and R3, equal to or different from each other, are independently selected from a hydrogen atom and a Ci-C3hydrocarbon group, and- Rx is a C1-C20 hydrocarbon moiety comprising at least one functional group selected from a hydroxyl, a carboxyl, an epoxy, an ester, a per-carbonate, a phosphate and an ether group.Claim 6. The polymer (F) according to any one of the preceding claims, wherein the polar vinyl monomer (MA) is a compound of formula (III):R R.R( -O-R(III) wherein each of R1 and R2have the meanings as above defined, R3is hydrogen, and ROH is a hydrogen or a C1-C5 hydrocarbon moiety comprising at least one hydroxyl group and / or at least a carboxylic group; more preferably, each of R1, R2, R3are hydrogen, while ROH has the same meaning as above detailed.Claim 7. The polymer (F) according to claim 6, wherein monomer (MA) is selected from the group consisting of:- 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,- hydroxyethyl (meth)acrylate,- hydroxypropyl(meth)acrylate,- hydroxyethylhexyl(meth)acrylates, and mixtures thereof.Claim 8. The polymer (F) according to claim 5, wherein the polar vinyl monomer (MA) is a compound of formula (II) wherein- Ri, R2and R3, equal to or different from each other, are independently selected from aa hydrogen atom and a C1-C3 hydrocarbon group, and- Rx is a C3-C20 linear or branched hydrocarbon chain moiety comprising at least two functional groups independently selected from the group consisting of ether, ketone, epoxy, per-carbonate and ester.Claim 9. The polymer (F) according to claim 8, wherein monomer (MA) is selected from the group consisting of:- allyl glycidyl ether (AGE),- ethylene glycol alkyl ether acrylates of formula n such as di(ethylene glycol) ethyl ether acrylate (DEGEEA);- (meth) acryloyloxyalkyl succinic acid, such as (meth) acryloyloxyethyl succinic acid and (meth) acryloyloxypropyl succinic acid; and mixtures thereof.Claim 10. A process for preparing the polymer (F) according to any one of claims 1 to9, said process comprising:- polymerizing vinylidene fluoride (VDF) monomer, an initial charge of monomer (MA) and optionally comonomer (CF), in an aqueous medium in the presence of a radical initiator system that introduces in the polymer chain end groups of formula (I),- continuously feeding an aqueous solution comprising monomer (MA); and- maintaining the pressure in the reactor vessel exceeding the critical pressure of the vinylidene fluoride wherein the initiator system includes an azo compound initiator and a chain transfer agent.Claim 11. The process according to claim 10, wherein the radical initiator system includes dimethyl 2,2'-azobis(2-methylpropionate) (AIBME) and diethylcarbonate.Claim 12. An electrode-forming composition (C) comprising: a) at least one electrode active material (AM); b) at least one binder (B), wherein binder (B) comprises at least one polymer (F) according to any one of claims 1 to 9; and c) at least one solvent (S).Claim 13. The electrode-forming composition (C) according to claim 12, for use in the preparation of a positive electrode (Ep), said composition comprising: a) at least one positive active material (AM);b) at least one binder (B), wherein binder (B) comprises at least one polymer (F) according to any one of claims 1 to 9; c) at least one solvent (S); and d) at least one conductive agent, preferably selected from carbon black or graphite fine powder carbon nanotubes.Claim 14. A process for the manufacture of an electrode [electrode (E)], said process comprising:(I) providing aa mmeettaall substrate having aatt least one surface;(II) providing an electrode-forming composition (C) according to any one of claim 12 or claim 13;(III) applying the composition (C) provided in step (II) onto the at least one surface of the metal substrate provided in step (I), thereby providing an assembly comprising a metal substrate coated with said composition (C) onto the at least one surface;(IV) drying the assembly provided in step (III);(V) submitting the dried assembly obtained in step (IV) to a compression step to obtain the electrode (E) of the invention.Claim 15. An electrode (E) obtainable by the process of claim 14.Claim 16. An electrochemical device, which is preferably a secondary battery such as a lithium-ion or a sodium-ion secondary battery, comprising at least one electrode (E) according to claim 15.

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