Composition for lithium battery electrodes

Cross-reference to related applications

[0001] This application claims priority to European application number

21169090.4 filed on April 19, 2021 , 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 hydrophilic 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 does 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 solution to this problem by combining easiness in the electrode fabrication process by dealing with electrode-forming formulation having low viscosity at low shear rates, whit 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 backbone randomly including certain hydroxyl group-containing vinyl monomers and carboxy group-containing vinyl monomers and having certain chain ends 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 share rates.

[0010] It is thus an object of the invention a fluoropolymer [polymer (F)] comprising:

- (i) recurring units derived from vinylidene fluoride (VDF) monomer; - (ii) recurring units derived from at least one hydroxyl group-containing vinyl monomer (HA);

- (iii) recurring units derived from at least one carboxyl group-containing vinyl monomer (CA), wherein monomer (CA) is different from monomer (HA); wherein the total amount of monomer (HA) and monomer (CA) in said polymer (F) is of at least 0.01 % by moles, more preferably at least 0.02 % moles, more preferably at least 0.02 % by moles and at most 5.0 % by moles, preferably at most 1.5% by moles, with respect to the total moles of recurring units of polymer (F); and wherein a fraction of at least 40% of monomer (HA) and a fraction of at least 40% of monomer (CA) are randomly distributed into said polymer (F); said polymer (F) comprising end groups of formula(l):

-(Ra)x-RO-Rb (I) wherein RO is a divalent radical containing at least one oxygen atom, R _a is a C1-C5 linear or branched hydrocarbon groupand Rb is hydrogen or a Ci- C5 linear or branched hydrocarbon groupand x is an integer selected from 1 and zero, wherein said end groups are present in an amount of at least 1/10000 VDF units, preferably higher than 1.5/10000 VDF units, more preferably higher than 2/10000 VDF units.

[0011] 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).

[0012] In another object, the present invention pertains to the use of the electrode-forming composition (C) in a process for the manufacture of an electrode [electrode (E)j, said process comprising:

(A) providing a metal substrate having at least one surface;

(B) providing an electrode-forming composition [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.

[0013] In a further object, the present invention pertains to the electrode [electrode (E)] obtainable by the process of the invention.

[0014] 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

[0015] By the term “recurring unit derived from vinylidene fluoride” (also generally indicated as vinylidene difluoride 1 ,1-difluoroethylene, VDF), it is intended to denote a recurring unit of formula CF2=CH2. [0016] Suitable hydroxyl group-containing vinyl monomers (HA) are compounds of formula (I): wherein:

Ri , R2 and R3, equal to or different from each other, are independently selected from a hydrogen atom, a halogen atom, and a C1-C3 hydrocarbon group and ROH is a C2-C10 hydrocarbon chain moiety comprising at least one hydroxyl group and possibly containing in the chain one or more oxygen atoms, carbonyl groups or carboxy groups.

[0017] In a preferred embodiment, monomers (HA) are compounds of formula (la): wherein:

Ri, R2 and R3, equal to or different from each other, are independently selected from a hydrogen atom and a C1-C3 hydrocarbon group and R’OH is a C1-C5 hydrocarbon moiety comprising at least one hydroxyl group.

[0018] Non-limitative examples of monomers (HA) of formula (la) include, notably:

- hydroxyethyl(meth)acrylate (HEA),

- 2-hydroxypropyl acrylate (HPA),

- hydroxyethylhexyl(meth)acrylate, and mixtures thereof.

[0019] Preferably, the at least one monomer (HA) is hydroxyethyl(meth)acrylate (HEA) or 2-hydroxypropyl acrylate (HPA).

[0020] Suitable carboxyl group-containing vinyl monomers (CA) are compounds of formula (II): wherein:

Ri, R2 and R3, equal to or different from each other, are independently selected from a hydrogen atom and a C1-C3 hydrocarbon group and RH is a C2-C10 hydrocarbon moiety comprising at least one carboxyl group and comprising no hydroxyl groups.

[0021] In a preferred embodiment, monomers (CA) are compounds of formula (lla):

(I la) wherein

Ri, R2 and R3, equal to or different from each other, are independently selected from a hydrogen atom and a C1-C3 hydrocarbon group and R’H is a hydrogen or a C1-C5 hydrocarbon moiety comprising at least one carboxyl group and comprising no hydroxyl groups.

[0022] R’H may further contain in the chain one or more oxygen atoms, carbonyl groups or ester groups.

[0023] Non-limitative examples of monomers (CA) of formula (lla) include, notably:

- acrylic acid (AA) and

- (meth)acrylic acid,

- 2-carboxyethyl (meth) acrylate,

- (meth) acryloyloxyethyl succinic acid,

- (meth) acryloyloxypropyl succinic acid, and mixtures thereof.

[0024] Preferably, the at least one monomer (CA) is acrylic acid (AA).

[0025] The molar ratio between recurring units (ii) and recurring units (iii) in polymer (F) is preferably comprised in the range from 20:1 to 1 :20, preferably from 10:1 to 1 :10, more preferably from 1 :5 to 5:1.

[0026] It is essential that in polymer (F) a fraction of at least 40% of monomer (HA) and a fraction of at least 40% of monomer (CA) are randomly distributed into said polymer (F).

[0027] The expression “fraction of randomly distributed monomer (HA)” is intended to denote the percent ratio between the average number of (HA) monomer sequences (%), said sequences being comprised between two recurring units derived from VDF monomer, and the total average number of (MA) monomer recurring units (%), according to the following formula: average number of (HA) sequences {%)

Fraction of randomly distributed units (HA) ₌ _ . ^ average total number of (HA) units (%)

[0028] When each of the (HA) recurring units is isolated, that is to say comprised between two recurring units of VDF monomer, the average number of (HA) sequences equal the average total number of (HA) recurring units, so the fraction of randomly distributed units (HA) is 100%: this value corresponds to a perfectly random distribution of (HA) recurring units.

[0029] Thus, the larger is the number of isolated (HA) units with respect to the total number of (HA) units, the higher will be the percentage value of fraction of randomly distributed units (HA), as above described.

[0030] The expression “fraction of randomly distributed monomer (CA)” is intended to denote the percent ratio between the average number of (CA) monomer sequences (%), said sequences being comprised between two recurring units derived from VDF monomer, and the total average number of (CA) monomer recurring units (%), according to the following formula: average number of (CA) sequences (%)

Fraction of randomly distribute d units (CA) ₌ _ _{, 10C} average total number of (CA) units (%)

[0031] When each of the (CA) recurring units is isolated, that is to say comprised between two recurring units of VDF monomer, the average number of (CA) sequences equal the average total number of (CA) recurring units, so the fraction of randomly distributed units (HA) is 100%: this value corresponds to a perfectly random distribution of (CA) recurring units. Thus, the larger is the number of isolated (CA) units with respect to the total number of (CA) units, the higher will be the percentage value of fraction of randomly distributed units (CA), as above described.

[0032] Determination of total average number of (HA) monomer recurring units and of (CA) monomer recurring units in polymer (F) can be performed by any suitable method, NMR being preferred.

[0033] The fraction of randomly distributed units (HA) and (CA) is preferably of at least 50%, more preferably of at least 60%, most preferably of at least 70 %.

[0034] Polymer (F) comprises preferably at least 0.01 %, more preferably at least 0.02 % moles of recurring units derived from said monomer (HA).

[0035] Polymer (F) 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 monomer (HA).

[0036] Polymer (F) comprises preferably at least 0.01 %, more preferably at least 0.02 % moles of recurring units derived from said monomer (CA).

[0037] Polymer (F) 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 monomer (CA).

[0038] Excellent results have been obtained using a polymer (F) comprising at least 70% by moles of recurring units derived from VDF.

[0039] The polymer (F) can be an elastomer or a semi-crystalline polymer, preferably being a semi-crystalline polymer.

[0040] 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. [0041] 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.

[0042] 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.

[0043] Preferably, the intrinsic viscosity of polymer (F), measured in dimethylformamide at 25 ° C, is between 0.05 l/g and 0.70 l/g more preferably between 0.20 l/g and 0.50 l/g.

[0044] The polymer (F) of the present invention usually has a melting temperature (Tm) comprised in the range from 130 to 200°C.

[0045] 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.

[0046] The polymer (F) of the present invention has in fact preferably a low fraction of insoluble components in standard polar aprotic solvents for PVDF, 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.

[0047] 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.

[0048] 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.

[0049] The polymer (F) may further comprise recurring units derived from one or more fluorinated comonomers (CF) different from VDF.

[0050] By the term “fluorinated comonomer (CF)”, it is hereby intended to denote an ethylenically unsaturated comonomer comprising at least one fluorine atoms.

[0051] Non-limitative examples of suitable fluorinated comonomers (CF) include, notably, the followings:

(a) C2-C8 fluoro- and/or perfluoroolefins such as tetrafluoroethylene (TFE), hexafluoropropylene (HFP), pentafluoropropylene and hexafluoroisobutylene;

(b) C2-C8 hydrogenated monofluoroolefins, such as vinyl fluoride; 1 ,2- difluoroethylene and trifluoroethylene;

(c) perfluoroalkylethylenes of formula CH2=CH-Rfo, wherein Rfo is a C1-C6 perfluoroalkyl group;

(d) chloro- and/or bromo- and/or iodo-C2-C6 fluoroolefins such as chlorotrifluoroethylene (CTFE).

[0052] The fluorinated comonomer (CF) is preferably HFP.

[0053] In one preferred embodiment, polymer (F) is semi-crystalline and comprises from 0.1 to 15.0% 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).

[0054] 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.

[0055] The polymer (F) more preferably comprises recurring units derived from:

- at least 80% by moles, preferably at least 85% by moles of vinylidene fluoride (VDF),

- from 0.01% to 1.5% by moles, preferably from 0.05% to 1.0% by moles of at least one hydroxyl group-containing vinyl monomer (HA);

- from 0.01% to 1.5% by moles, preferably from 0.05% to 1.0% by moles of at least one carboxyl group-containing vinyl monomer (CA);

- optionally from 0.5 to 3.0% by moles of recurring units derived from at least one fluorinated comonomer (CF).

[0056] The polymer (F) of the present invention comprises end groups of formula (I):

-(Ra)x-RO-Rb (I) wherein:

- RO is a divalent radical containing at least one oxygen atom,

- R _a is a C1-C5 linear or branched hydrocarbon group,

- Rb is hydrogen or a C1-C5 linear or branched hydrocarbon group, and

- x is an integer selected from 1 and zero, said end groups are present in an amount of at least 1/10000 VDF units, preferably higher than 2/10000 VDF units.

[0057] Non limitative examples of divalent radical RO include, notably: ether (-0-), ester (-O-CO-), ketone (-CO-), epoxide, and per-carbonate (-O-CO-O-) groups.

[0058] 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.

[0059] Preferably, R _a and Rb are both C2-C3 linear or branched hydrocarbon groups, more preferably C3 linear or branched hydrocarbon groups.

[0060] Preferably, x is zero.

[0061] The polymer (F) of the present invention may be obtained by polymerization of a VDF monomer, at least one monomer (HA), at least one monomer (CA) and optionally at least one comonomer (CF), 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).

[0062] The preferred process for preparing the polymer (F) comprises polymerizing the vinylidene fluoride (VDF) monomer, monomer (HA) and monomer (CA), and optionally comonomer (CF), in an aqueous medium in the presence of a radical initiator, said process comprising

- continuously feeding an aqueous solution comprising monomer (HA) and monomer (CA); and - maintaining the pressure in said reactor vessel exceeding the critical pressure of the vinylidene fluoride.

[0063] Suitable initiator known for the polymerization of fluorinated monomers are organic peroxides, such as those selected from the group consisting of: dialkyl peroxides, diacyl-peroxides, peroxyesters, and peroxydicarbonates.

[0064] Exemplary dialkyl peroxides is di-t-butyl peroxide, of peroxyesters are t- butyl peroxypivalate and t-amyl peroxypivalate, and of peroxydicarbonate, are di(ethyl) peroxydicarbonate, di(n-propyl) peroxydicarbonate, diisopropyl peroxydicarbonate, di(sec-butyl) peroxydicarbonate, di(2- ethylhexyl) peroxydicarbonate and di(4-tert-butylcyclohexyl) peroxydicarbonate.

[0065] Preferably, the initiator used for preparing polymer (F) of the present invention is an organic peroxide, more preferably is selected from di(ethyl) peroxydicarbonate, di(n-propyl) peroxydicarbonate, di(iso-propyl) peroxydicarbonate and di(4-tert-butylcyclohexyl) peroxydicarbonate.

[0066] The quantity of an initiator required for a polymerization is related to its activity and the temperature used for the polymerization. The total amount of initiator used is generally between 100 to 30000 ppm by weight on the total monomer weight used.

[0067] The initiator may be added in pure form, in solution, in suspension, or in emulsion, depending upon the initiator chosen.

[0068] A chain transfer agent, CTA, can be added to the polymerization. Suitable CTA for this polymerization are known in the art and are typically short hydrocarbon chains like ethane and propane, esters such as ethyl acetate or diethyl maliate, diethylcarbonate and others. When an organic peroxide is used as the initiator, it could act also as effective CTA during the course of free radical polymerization. The additional CTA however, may be added all at once at the beginning of the reaction, or it may be added in portions, or continuously throughout the course of the reaction. The amount of CTA and its mode of addition depend on the desired properties.

[0069] 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.

[0070] It is essential that a continuous feeding of an aqueous solution containing monomer (HA) and monomer (CA) is carried out mostly during the whole duration of polymerization run.

[0071] It is thus possible to obtain a nearly statistic distribution of both the monomer (HA) and monomer (CA) within the VDF monomer polymer backbone of polymer (F).

[0072] The expressions "continuous feeding" or "continuously feeding" means that slow, small, incremental additions the aqueous solution of monomer (HA) and monomer (CA) take place until polymerization has concluded.

[0073] The aqueous solution of monomer (HA) and monomer (CA) continuously fed during polymerization amounts for at least 50 % wt of the total amount of monomer (HA) and monomer (CA) 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 (HA) and monomer (CA) is continuously fed during polymerization. An incremental addition of VDF monomer can be effected during polymerization, even if this requirement is not mandatory.

Generally, the process of the invention is carried out at a temperature of at least 35° C, preferably of at least 40° C, more preferably of at least 45°

C.

[0074] When the polymerization is carried out in suspension, polymer (F) is typically provided in form of powder.

[0075] When the polymerization to obtain polymer (F) is carried out in emulsion, polymer (F) is typically provided in the form of an aqueous dispersion (D), which may be used as directly obtained by the emulsion polymerization or after a concentration step.

[0076] Polymer (F) obtained by emulsion polymerization can be isolated from the aqueous dispersion (D) by concentration and/or coagulation of the dispersion and obtained in powder form by subsequent drying.

[0077] Polymer (F) in the form of powder may be optionally further extruded to provide polymer (F) in the form of pellets.

[0078] The polymer (F) as above detailed may be used as binder for electrodes in Li-ion batteries.

[0079] 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; c) at least one solvent (S); and d) optionally, at least one conductive agent.

[0080] 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 ions.

[0081] 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)].

[0082] In the case of forming a positive electrode (Ep) for a Lithium-ion secondary battery, the compound (AM) may comprise a composite metal chalcogenide of formula L1MQ2, wherein M is at least one metal selected from transition metals such as Co, Ni, Fe, Mn, Cr and V or a metal such as Al and a mixture of thereof 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 UMO2, wherein M is the same as defined above. Preferred examples thereof may include UC0O2, LiNiC>2, LiNixCd-xC (0 < x < 1), LiNiaCobAlc02 (a+b+c=1) and spinel-structured LiM C .As an alternative, still in the case of forming a positive electrode (Ep) for a Lithium-ion secondary battery, the compound (AM) may comprise a lithiated or partially lithiated transition metal oxyanion-based electro-active material of formula MiM2(JC )fEi-f, wherein Mi is lithium, which may be partially substituted by another alkali metal representing less than 20% of the Mi metals, M2 is a transition metal at the oxidation level of +2 selected from Fe, Mn, Ni or mixtures thereof, which may be partially substituted by one or more additional metals at oxidation levels between +1 and +5 and representing less than 35% of the M2 metals, including 0, JCU 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 JCU oxyanion, generally comprised between 0.75 and 1.

[0083] The MiM2(JCU)fEi-f electro-active material as defined above is preferably phosphate-based and may have an ordered or modified olivine structure.

[0084] More preferably, the compound (AM) in the case of forming a positive electrode (Ep) has formula Lb-xM’ _y M’ ’ 2- _y (JCU)3 wherein 0<x<3, 0<y<2, M’ and M’ ’ are the same or different metals, at least one of which being a transition metal, JCU is preferably PCU which may be partially substituted with another oxyanion, wherein J is either S, V, Si, Nb, Mo or a combination thereof. Still more preferably, the compound (AM) is a phosphate-based electro-active material of formula Li(FexMni-x)PCU wherein 0<x<1, wherein x is preferably 1 (that is to say, lithium iron phosphate of formula LiFePCU).

[0085] In the case of forming a negative composite electrode (En) for a Lithium- ion secondary battery, the compound (AM) may preferably comprise a carbon-based material and/or a silicon-based material.

[0086] In some embodiments, the carbon-based material may be, for example, graphite, such as natural or artificial graphite, graphene, or carbon black.

[0087] These materials may be used alone or as a mixture of two or more thereof.

[0088] The carbon-based material is preferably graphite.

[0089] 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.

[0090] 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).

[0091] 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.

[0092] An optional conductive agent may be added in order to improve the conductivity of a resulting electrode (AM).

[0093] Examples thereof may include: carbonaceous materials, such as carbon black, graphite fine powder carbon nanotubes (CNT), 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 ^® .

[0094] When present, the conductive agent is different from the carbon-based material described above.

[0095] 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 electrode 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.

[0096] 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.

[0097] 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.

[0098] In another object, the present invention pertains to the use of the electrode-forming composition (C) 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 [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.

[0099] In a further object, the present invention pertains to the electrode [electrode (E)] obtainable by the process of the invention.

[00100] The Applicant has surprisingly found that the electrode (E) of the present invention shows outstanding adhesion of the binder to current collector. [00101] The electrode (E) of the invention is thus particularly suitable for use in electrochemical devices, in particular in secondary batteries.

[00102] For the purpose of the present invention, the term “secondary battery” is intended to denote a rechargeable battery.

[00103] The secondary battery of the invention is preferably an alkaline or an alkaline-earth metal secondary battery.

[00104] The secondary battery of the invention is more preferably a Lithium-ion secondary battery.

[00105] In still a further object, the present invention pertains to an electrochemical device comprising at least one electrode (E) of the present invention. [00106] 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.

[00107] In one preferred embodiment of the present invention it is provided an electrochemical device is a secondary battery comprising:

- a positive electrode and a negative electrode, wherein the negative electrode is the electrode (E) according to the present invention.

[00108] An electrochemical device according to the present invention can be prepared by standard methods known to a person skilled in the art.

[00109] 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.

[00110] 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.

[00111] EXPERIMENTAL PART

[00112] Determination of intrinsic viscosity of polymer (F)

Intrinsic viscosity ( h ) [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 (F) in N,N-dimethylformamide at a concentration of about 0.2 g/dl using a Ubbelhode viscosimeter: where c is polymer concentration [g/dl], h _r is the relative viscosity, i.e. the ratio between the dropping time of sample solution and the dropping time of solvent, h _sp is the specific viscosity, i.e. h _r -1 , and G is an experimental factor, which for polymer (F) corresponds to 3.

[00113] DSC analysis

[00114] DSC analyses were carried out according to ASTM D 3418 standard; the melting point (TK) was determined at a heating rate of 10°C/min.

[00115] Determination of the polar end-groups

[00116] The amount of polar end groups arising from the initiator diethyl peroxydicarbonate used in the polymerization process, was determined by 1H-NMR, measuring the intensity of the H atoms of the CH2 group (in bold in following formula) with respect to the total intensity of CH2 moieties of the polymer (F) backbone VDF monomer units:

CH3-CH2-OCOO-VF2-

[00117] The content of 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 mols per 10000 VDF units,

- 1 EG is the intensity, normalized to one hydrogen, of the integral of the end-group [EG]

- 1 VDF is the intensity, normalized to one hydrogen, of the integrals of normal and reverse VDF recurring units.

[00118] About 20 mg of polymer were dissolved in 0.7 ml of hexadeuteroacetone. The ¹ H-NMR spectrum revealed the aforementioned CH2 at 4.24 ppm whereas CH2 signals from VDF recurring normal and reverse units resonated as broad peaks centred at 3.2 and 2.5 ppm respectively.

[00119] Example 1 : Preparation of Polymer F-1 [00120] In a 4L reactor equipped with an impeller running at a speed of 650 rpm were introduced in sequence: 2,014 g of demineralized water and 0.4 g of PEO (Alkox® -E45 from Alkorox) per kg of total VDF monomers and 0.5 g of hydroxypropyl methylcellulose (Methocel®-K100 from Dow) per kg of total VDF monomers and 59.91 g of a solution of trisodium phosphate. 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.

[00121] Then, 30 g of demineralized water, 11.28 g of hydrogen peroxide (from Brenntag) and 3.53 g of ethyl chloroformiate (from Framochem) were introduced in the reactor.

[00122] After 15 minutes, 0.28 g of acrylic acid (AA) and 0.07g of hydroxyethyl acrylate (HEA) were introduced in the reactor at a stirring speed of 880 rpm. Immediately after, 1.166 g of VDF were added to the mixture. The reactor was then gradually heated until the set-point temperature of 45°C was reached, corresponding to a pressure of the reactor of 120 bars.

[00123] The pressure was kept constantly equal to 120 bars during the whole polymerization run by feeding an aqueous solution comprising 9.40 g of AA per liter of solution and 4.70 g of HEA per liter of solution. After 365 minutes the polymerization was stopped by degassing the suspension until reaching atmospheric pressure. A total of 721 g of the AA and HEA solution was charged to the reactor.

[00124] 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 for twelve hours. 919 g of dry powder were collected.

[00125] A polymer comprising VDF-AA (0.45% by moles)-HEA (0.15% by moles), having an intrinsic viscosity of 0.28 l/g in DMF at 25°C and a T2f of 165.5°C was obtained

[00126] The polymer contained 3.0 /10000 VDF units of the end-group CH3CH2- OCOO-.

[00127] In addition, the presence of 4.5/10000 VDF units of -CF ₂ H and 2.5 /10000 VDF units of -CF2CH3 end-groups was determined.

[00128] Example 2: Preparation of Polymer F-2

[00129] In a 4L reactor equipped with an impeller running at a speed of 650 rpm were introduced in sequence: 1 ,983 g of demineralized water and 0.4 g of PEO (Alkox® -E45 from Alkorox) per kg of total VDF monomers and 0.5 g of hydroxypropyl methylcellulose (Methocel®-K100 from Dow) per kg of total VDF monomers and 87.89 g of a solution of trisodium phosphate. 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.

[00130] Then, 100 g of demineralized water, 15.66 g of hydrogen peroxide (from Brenntag) and 5 g of ethyl chloroformiate (from Framochem) were introduced in the reactor.

[00131] After 15 minutes, 0.39 g of acrylic acid (AA) and 0.047 g of hydroxypropyl acrylate (HPA) were introduced in the reactor at a stirring speed of 880 rpm. Immediately after, 1.165 g of VDF were added to the mixture. The reactor was then gradually heated until the set-point temperature of 40°C was reached, corresponding to a pressure of the reactor of 120 bars. [00132] The pressure was kept constantly equal to 120 bars during the whole polymerization run by feeding an aqueous solution comprising 13.75 g of AA per liter of solution and 1.66 g of HPA per liter of solution. After 429 minutes the polymerization was stopped by degassing the suspension until reaching atmospheric pressure. A total of 681 g of the AA and HPA solution was charged to the reactor.

[00133] 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 for twelve hours. 872 g of dry powder were collected.

[00134] A polymer VDF-AA (0.7% by moles)-HPA (0.05% by moles), having an intrinsic viscosity of 0.249 l/g in DMF at 25°C and a T2f of 164.3°C was obtained.

[00135] The polymer contained 2.8/10000 VDF units of the end-group CH3CH2- OCOO-.

[00136] In addition, the presence of 4.8 /10000 VDF units of -CF2H and 2.1 /10000 VDF units of -CF2CH3 end-groups was determined.

[00137] Example 3: Preparation of Polymer F-3

[00138] In a 4L reactor equipped with an impeller running at a speed of 650 rpm were introduced in sequence: 1 ,932 g of demineralized water and 0.4 g of PEO (Alkox® -E45 from Alkorox) per kg of total VDF monomers and 0.5 g of hydroxypropyl methylcellulose (Methocel®-K100 from Dow) per kg of total VDF monomers and 124.08 g of a solution of trisodium phosphate. 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.

[00139] Then, 100 g of demineralized water, 22.11 g of hydrogen peroxide (from Brenntag) and 7.06 g of ethyl chloroformiate (from Framochem) were introduced in the reactor.

[00140] After 15 minutes, 0.39 g of acrylic acid (AA) and 0.046 g of hydroxypropyl acrylate (HPA) were introduced in the reactor at a stirring speed of 880 rpm. Immediately after, 1.165 g of VDF were added to the mixture. The reactor was then gradually heated until the set-point temperature of 40°C was reached, corresponding to a pressure of the reactor of 120 bars.

[00141] The pressure was kept constantly equal to 120 bars during the whole polymerization run by feeding an aqueous solution comprising 13.38 g of AA per liter of solution and 1.61 g of HPA per liter of solution. After 369 minutes the polymerization was stopped by degassing the suspension until reaching atmospheric pressure. A total of 700 g of the AA and HPA solution was charged to the reactor.

[00142] 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 for twelve hours. 864 g of dry powder were collected.

[00143] Polymer VDF-AA (0.7% by moles)-HPA (0.05% by moles), having an intrinsic viscosity of 0.207 l/g in DMF at 25°C and a T2f of 165.2°C was obtained.

[00144] The polymer contained 3.6 /10000 VDF units of the end-group CH3CH2- OCOO-.

[00145] In addition, the presence of there are 5.0 /10000 VDF units of -CF2H and 2.3 /10000 units of -CF2CH3 end-groups was determined.

[00146] Example 4 comparative: Preparation of Polymer A [00147] In a 4L reactor equipped with an impeller running at a speed of 650 rpm were introduced in sequence: 1.928 g of demineralized water and 0.4 g of PEO (Alkox® -E45 from Alkorox) per kg of total VDF monomers and 0.5 g of hydroxypropyl methylcellulose (Methocel®-K100 from Dow) per kg of total VDF monomers. The oxygen 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.

[00148] Then, 0.39 g of acrylic acid (AA) and 6.77 g of a solution of the initiator t- amylperpivalate (from United Initiators) in isododecane (75%) were introduced in the reactor. Immediately after, 1 ,164 g of VDF were added to the mixture. The reactor was then gradually heated until the set point temperature of 50°C was reached, corresponding to a pressure of 120 bars.

[00149] The pressure was kept constantly equal to 120 bars during the whole polymerization run by feeding an aqueous solution comprising 11.91 g of AA per liter of solution and 1.49 g of hydroxypropyl acrylate (HPA) per liter of solution. After 480 minutes the polymerization was stopped by degassing the suspension until reaching atmospheric pressure. A total of 787 g of the AA and HPA solution was charged to the reactor.

[00150] 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 for twelve hours. 896 g of dry powder were collected.

[00151] Polymer VDF-AA (0.7% by moles)-HPA (0.05% by moles), polymer having an intrinsic viscosity of 0.231 l/g in DMF at 25°C and a T2f of 165.2°C was obtained.

[00152] The polymer contained: 2.2 /10000 VDF units of -C(CH ₃ )3, 6.4 /10000 VDF units of -CF2H and 2.7/10000 VDF units of -CF2CH3 end-groups.

[00153] No end groups CH3CH2-OCOO- were present.

[00154] General Preparation of the Electrodes with NMC active material

[00155] The positive electrodes having final composition of 96.5% by weight of NMC, 1.5% by weight of polymer, 2% by weight of conductive additive were prepared as follows.

[00156] A first dispersion was prepared by pre-mixing for 10 minutes in a centrifugal mixer 34.7 g of a 6% by weight solution of a polymer in NMP, 133.8 g of NMC, 2.8 g of SC-65 and 8.8 g of NMP.

[00157] The mixture was then mixed using a high speed disk impeller at 2000 rpm for 50 minutes. Additional 7.2 g of NMP were subsequently added to the dispersion, which was further mixed with a butterfly type impeller for 20 minutes at lOOOrpm. Positive electrodes were obtained by casting the as obtained compositions 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.

[00158] Slurry Viscosity Measurement

[00159] The slurry viscosity 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 200 1/s.

[00160] Adhesion Measurement

[00161] Adhesion Peeling Force between Aluminium foil and Electrode: [00162] 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.

[00163] Example 5 : Adhesion and slurry viscosity

[00164] The polymers of examples 1 to 4 have been used as binders and the electrode compositions have been produced according to the procedure shown above. The values of slurry viscosity and adhesion are shown in Table 1.

Tab e 1

[00165] The results show that the polymers of the present invention are more performing and easier to be handled in the fabrication process of electrodes thanks to a low slurry viscosity and good adhesion.