VINYLIDENE FLUORIDE COPOLYMERS FOR LITHIUM BATTERY ELECTRODES

Cross reference to previous applications

[0001] This application claims priority to European application No. 22202256.8 filed on 18 October 2022, 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, with 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 carboxy group-containing vinyl monomers are endowed with very good adhesion to metal substrates and can be used in the preparation of electrodeforming compositions having low viscosity at low shear rates.

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

(i) recurring units derived from vinylidene fluoride (VDF) monomer; and

(ii) recurring units derived from at least one carboxyl group-containing vinyl monomer (CA) of formula (I): 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 is a C2-C10 hydrocarbon moiety comprising at least one carboxyl group and comprising no aliphatic hydroxyl group, wherein monomer (CA) in said polymer (F) is of at most 5.0 % by moles, with respect to the total moles of recurring units of polymer (F); and wherein of at least 50% of monomer (CA) is randomly distributed into said polymer (F) and, where the polymer (F) is characterized by containing end groups of formula (I): -(Ra)x-O-CO-O-CH ₂ -CH ₃ (I) wherein R _a is a Ci -C5 linear or branched hydrocarbon group and x is an integer selected from 1 and zero, and the end-groups of formula (I) are present in an amount of at least 20% with respect to the total amount of end groups of polymer (F).

[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)], said process comprising:

(I) providing a metal substrate having at least one surface; (II) providing an electrode-forming composition (C) as above defined;

(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.

[0013] In a further object, the present invention pertains to the 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 CF ₂ =CH2.

[0016] In preferred embodiments, the carboxyl group-containing vinyl monomers (CA) 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’H is a hydrogen or a C1-C15 hydrocarbon moiety comprising at least one carboxyl group and comprising no aliphatic hydroxyl group,.

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

[0017] By the wording “aliphatic hydroxyl group” it is intended to mean a hydroxyl group directly bonded to an aliphatic carbon.

[0018] Non-limitative examples of monomers (CA) of formula (I) include, 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, and mixtures thereof.

[0019] Preferably, the at least one monomer (CA) is acrylic acid (AA).

[0020] It is essential that in polymer (F) at least 50% of monomer (CA) be randomly distributed into said polymer (F).

[0021] 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.

[0022] Thus, when polymer (F) is prepared by a polymerization reaction that comprises continuously feeding monomer (CA) during VDF polymerization, a random distribution of monomer (CA) in the polymer chains is present, with sequences VDF-(CA)-VDF being obtained.

[0023] More preferably, in polymer (F) at least 70% of monomer (CA) is randomly distributed into said polymer (F).

[0024] The expression “randomly distributed monomer (CA)” is intended to denote the presence of sequences VDF-(CA)-VDF, and the amount of randomly distributed monomer (CA) is determined as the percent ratio between the average number of said VDF-(CA)-VDF sequences and the total average number of (CA) monomer recurring units.

[0025] When each of the (CA) recurring units is isolated, that is to say comprised between two recurring units of VDF monomer, the average number of (CA) sequences equals the average total number of (CA) recurring units, so the fraction of randomly distributed units (CA) is 100%: this value corresponds to a perfectly random distribution of (CA) recurring units. Thus, the larger is the number of isolated (CA) units with respect to the total number of (CA) units, the higher will be the percentage value of fraction of randomly distributed units (CA), as above described.

[0026] The analytical determination of the total amount of randomly distributed monomer (CA) may be carried out by measuring the sequences VDF-(CA)-VDF by ¹⁹ F-NMR and the total amount of monomer in the polymer by one or more of these techniques, ¹⁹ F-NMR , ¹ H-NMR, titration of carboxyl groups, FT-IR or others.

[0027] Polymer (F) comprises preferably at least 0.01 %, more preferably at least 0.02 % moles of recurring units derived from said monomer (CA). [0028] Polymer (F) comprises preferably at most 5.0 %, more preferably at most 3.0 % moles, even more preferably at most 2.0 % moles, still more preferably at most 1.5% by moles of recurring units derived from monomer (CA) with respect to the total moles of recurring units of polymer (F).

[0029] Excellent results have been obtained using a polymer (F) comprising at least 70% by moles of recurring units derived from VDF.

[0030] The polymer (F) can be an elastomer or a semi-crystalline polymer, preferably being a semi-crystalline polymer.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] Preferably, the intrinsic viscosity of polymer (F), measured in dimethylformamide (DMF) at 25 °C, is between 0.05 l/g and 1.0 l/g, more preferably between 0.10 l/g and 0.70 l/g, even more preferably between 0.20 l/g and 0.50 l/g

[0035] The polymer (F) of the present invention usually has a melting temperature (T _m ) comprised in the range from 120 to 200°C.

[0036] 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 negligible.

[0037] The polymer (F) of the present invention has 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. [0038] 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.

[0039] 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.

[0040] The polymer (F) may further comprise recurring units derived from one or more fluorinated comonomers (CF) different from VDF.

[0041] By the term “fluorinated comonomer (CF)”, it is hereby intended to denote an ethylenically unsaturated comonomer comprising at least one fluorine atoms.

[0042] 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 Rro is a Ci-Ce perfluoroalkyl group;

(d) chloro- and/or bromo- and/or iodo-C2-Ce fluoroolefins 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).

[0043] 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) with respect to the total moles of recurring units of polymer (F).

[0044] 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.

[0045] The polymer (F) more preferably consists of:

- at least 70% by moles, preferably at least 75% by moles, more preferably at least 85% by moles of recurring units derived from vinylidene fluoride (VDF);

- from 0.01 % to 2% by moles, preferably from 0.05% to 1.5% by moles of recurring units derived from at least one vinyl monomer (CA); - optionally from 0.5 to 10% by moles of recurring units derived from at least one fluorinated comonomer (CF); all the amounts being with respect to the total moles of recurring units of polymer (F).

[0046] According to certain embodiments of the present invention, the polymer (F) is characterized by containing end groups of formula (I) as above defined, wherein x is zero.

[0047] According to other embodiments of the present invention, the polymer (F) is characterized by containing end groups of formula (I) as above defined, wherein x is 1 , and R _a is a C2-C3 linear or branched alkyl radical.

[0048] According to other embodiments of the present invention, the polymer (F) is characterized by containing end groups of formula (I) as above defined, wherein x is zero and containing end groups of formula (I) wherein x is 1 , and R _a is a C2-C3 linear or branched alkyl radical.

[0049] Polymer (F) may be obtained by a process that comprises:

- polymerizing vinylidene fluoride (VDF) monomer, an initial charge of monomer (CA) 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 (CA); and

- maintaining the pressure in the reactor vessel exceeding the critical pressure of the vinylidene fluoride.

[0050] Suitable radical initiator systems include radical initiators such as di(ethyl) peroxydicarbonate and hydro-ethyl peroxydicarbonate.

[0051] The amount of radical initiator required for a polymerization is related to its activity and the temperature used for the polymerization. The total amount of radical initiator used is generally between 100 to 30000 ppm by weight on the total monomers weight used.

[0052] The radical initiator may be added in pure form, in solution, in suspension, or in emulsion, depending upon the initiator chosen.

[0053] The radical initiator systems may include a chain transfer agent (CT A).

[0054] Suitable CTA for the polymerization process for preparing the polymer (F) according to the present invention are those known in the art and are typically selected from the group consisting of short hydrocarbon chains like ethane and propane, esters such as ethyl acetate or diethyl maleate, diethylcarbonate. When an organic peroxide is used as the initiator, it could act also as effective CTA during the course of free radical polymerization. [0055] When used, the CTA 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 polymer (F) to be obtained.

[0056] Preferred CTA for use in the process of the present invention is diethylcarbonate.

[0057] In the process for preparing the polymer (F), 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.

[0058] Monomer (CA) is suitably added to the reaction vessel as an aqueous solution.

[0059] It is essential that a continuous feeding of the aqueous solution containing monomer (CA) is carried out during the whole duration of polymerization run.

[0060] It is thus possible to obtain a nearly statistic distribution of monomer (CA) within the backbone of polymer (F).

[0061] The expressions "continuous feeding" or "continuously feeding" means that slow, small, incremental additions the aqueous solution of monomer (CA) take place during the polymerization.

[0062] The aqueous solution of monomer (CA) continuously fed during polymerization amounts for at least 50 % wt of the total amount of 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 (CA) is continuously fed during polymerization. An incremental addition of VDF monomer can be effected during polymerization.

[0063] Preferably, the process of the invention is carried out at a temperature superior to the critical temperature of the VDF monomer, i.e. of at least 31°C.

[0064] The polymer (F) is typically provided in form of powder according to the process described above.

[0065] Polymer (F) in the form of powder may be optionally further extruded to provide polymer (F) in the form of pellets.

[0066] The polymer (F) as above detailed may be used as binder for electrodes in Li-ion batteries.

[0067] 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). [0068] For the purpose of the present invention, the term “electro-active material (AM)” is intended to denote a compound that is able to incorporate or insert into its structure and substantially release therefrom alkaline or alkaline-earth metal ions during the charging phase and the discharging phase of an electrochemical device. The compound (AM) is preferably able to incorporate or insert and release lithium ions. [0069] 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)].

[0070] 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 LiMQ ₂ , 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 LiMO ₂ , wherein M is the same as defined above. Preferred examples thereof may include LiCoO ₂ , LiNiO ₂ , LiNi _x Coi. _x O ₂ (0 < x < 1), LiNi _a CobAl _c O ₂ (a+b+c=1) and spinel-structured LiMn ₂ C>4.

[0071] 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 MiM ₂ (JC>4)fEi.f, wherein Mi is lithium, which may be partially substituted by another alkali metal representing less than 20% of the Mi metals, M ₂ 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 M ₂ metals, including 0, JO4 is any oxyanion wherein J is either P, S, V, Si, Nb, Mo or a combination thereof, E is a fluoride, hydroxide or chloride anion, f is the molar fraction of the JO4 oxyanion, generally comprised between 0.75 and 1.

[0072] The MiM ₂ (JC>4)fEi.f electro-active material as defined above is preferably phosphate-based and may have an ordered or modified olivine structure.

[0073] More preferably, the compound (AM) in the case of forming a positive electrode (Ep) has formula Li _3-x M’yM” ₂ .y(JO4)3 wherein 0<x<3, 0<y<2, M’ and M” are the same or different metals, at least one of which being a transition metal, JO4 is preferably PO4 which may be partially substituted with another oxyanion, wherein J is either S, V, Si, Nb, Mo or a combination thereof. Still more preferably, the compound (AM) is a phosphate-based electro-active material of formula Li(Fe _x Mni. _x )PC>4 wherein 0<x<1 , wherein x is preferably 1 (that is to say, lithium iron phosphate of formula LiFePC (LFP)). LFP active materials suitable for use in the electrodes of the present invention may have nanometric particle size, which means that the size is less than 1 micrometer, or micrometric particle size, which means particles means with size between 1 micrometer and 1 millimeter.

[0074] 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.

[0075] In some embodiments, the carbon-based material may be, for example, graphite, such as natural or artificial graphite, graphene, or carbon black.

[0076] These materials may be used alone or as a mixture of two or more thereof.

[0077] The carbon-based material is preferably graphite.

[0078] 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.

[0079] 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).

[0080] 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.

[0081] An optional conductive agent may be added in order to improve the conductivity of a resulting electrode (AM).

[0082] Examples thereof may include: carbonaceous materials, such as carbon black, graphite fine powder carbon nanotubes, graphene, or fiber, or fine powder or fibers of metals such as nickel or aluminum. The optional conductive agent is preferably carbon black. Carbon black is available, for example, under the brand names, Super P® or Ketjenblack®.

[0083] The electro-forming composition (C) of the invention may further optionally include at least one conductive agent.

[0084] When present, the conductive agent is different from the carbon-based material described above.

[0085] 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.

[0086] 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.

[0087] 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.

[0088] 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:

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

(II) providing an electrode-forming composition (C) as above defined;

(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 (III) to a compression step to obtain the electrode (E) of the invention.

[0089] In a further object, the present invention pertains to the electrode (E) obtainable by the process of the invention.

[0090] The Applicant has surprisingly found that the electrode (E) of the present invention shows outstanding adhesion of the binder to current collector.

[0091] The electrode (E) of the invention is thus particularly suitable for use in electrochemical devices, in particular in secondary batteries.

[0092] For the purpose of the present invention, the term “secondary battery” is intended to denote a rechargeable battery.

[0093] The secondary battery of the invention is preferably an alkaline or an alkaline-earth metal secondary battery.

[0094] The secondary battery of the invention is more preferably a Lithium-ion secondary battery. [0095] In still a further object, the present invention pertains to an electrochemical device comprising at least one electrode (E) of the present invention.

[0096] 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.

[0097] 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.

[0098] An electrochemical device according to the present invention can be prepared by standard methods known to a person skilled in the art.

[0099] 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.

[00100] EXPERIMENTAL PART

[00101] Determination of intrinsic viscosity of polymer (F)

[00102] 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 (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], r| _r is the relative viscosity, i.e. the ratio between the dropping time of sample solution and the dropping time of solvent, r| _sp is the specific viscosity, i.e. q _r -1 , and F is an experimental factor, which for polymer (F) corresponds to 3.

[00103] DSC analysis

[00104] DSC analyses were carried out according to ASTM D 3418 standard; the melting point (T ₂ f) was determined at a heating rate of 10°C/min.

[00105] Determination of the polar end-groups

[00106] The amount of polar end groups of the polymers (F) arising from the ethyl chloroformate initiator precursor used in the polymerization process, was determined by ¹ H-NMR, measuring the intensity of the H atoms of the CH ₂ group (in bold in following formula) with respect to the total intensity of CH ₂ moieties of the polymer (F) backbone VDF monomer units: CH3-CH2-OCOO-CH2-CF2-

[00107] 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,

- 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.

[00108] 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 CHa at 4.47 ppm, whereas CH ₂ signals from VDF recurring normal and reverse units resonated as broad peaks centered at 2.93 and 2.36 ppm respectively.

[00109] Similar NMR methods were applied to the determination of end groups deriving from the use of diethylcarbonate chain transfer agent (CH3-CH2-OCOO-CH2-CH2- , CH3-CH2-OCOO-CH(CH3)-) and for the determination of -CF2H and -CF2CH3 end groups, as known to people skilled in the art.

[00110] Determination of the amount of monomers AA in the polymer (F) by NMR

[00111] Alternated AA content in the polymer (F) was determined by ¹⁹ F-NMR spectroscopy.

Signals related to CF2 moieties of VDF units (in bold in following formula) adjacent to isolated hydrogenated comonomers have been found to resonate at about -94 ppm in the ¹⁹ F-NMR.

-CH ₂ CF2-CH2CH(COOH)-CH ₂ CF2-CH2

[00112] 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.

[00113] Example 1 : Preparation of Polymer F-1

[00114] In a 4L reactor equipped with an impeller running at a speed of 650 rpm were introduced in sequence: 2370 g of demineralized water and 0.4 g of PEG (Alkox® -E45 from Alroko) per kg of total monomers and 0.5 g of hydroxypropyl methylcellulose (Methocel®-K100 from Dow) per kg of total monomers and 20.6 g of trisodium phosphate dodecahydrated. 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. [00115] Then, 16.93 g of hydrogen peroxide solution (from Brenntag), 5.9 g of ethyl chloroformate (from Framochem) and 5.9 g of diethylcarbonate were introduced in the reactor.

[00116] After 15 minutes, 0.18 g of acrylic acid (AA) were introduced in the reactor at a stirring speed of 880 rpm. Immediately after, 1174 g of VDF were added to the mixture. The reactor was then gradually heated until the set-point temperature of 35°C was reached.

[00117] The pressure was kept constantly equal to 120 bars during the whole polymerization run by feeding an aqueous solution of 4.15 g of AA per liter of solution. A total of 666 g of the solution was charged to the reactor. After 269 minutes the polymerization was stopped by degassing the suspension until reaching atmospheric pressure.

[00118] The obtained 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. 837 g of dry powder were collected.

[00119] A polymer comprising VDF-AA (0.2% by moles), having an intrinsic viscosity of 0.299 l/g in DMF at 25°C and a T ₂ f of 170.3°C was obtained.

[00120] The polymer contained 2.0/10000 VDF units of end-group CH3CH2-OCOO-: 1.3 /10000 VDF units derived from the ethyl chloroformate initiator precursor and 0.7/10000 VDF units derived from diethylcarbonate.

[00121] In addition, the presence of 2.7/10000 VDF units of -CF ₂ H and 1.5 /10000 VDF units of -CF2CH3 end-groups was determined.

[00122] The amount of end groups CH3CH2-OCOO- is 32.3% with respect to the overall amount of end groups of polymer (F)

[00123] Example 2: Preparation of Polymer F-2

[00124] In a 4L reactor equipped with an impeller running at a speed of 650 rpm were introduced in sequence: 2383 g of demineralized water and 0.4 g of PEO (Alkox® -E45 from Alroko) per kg of total monomers and 0.5 g of hydroxypropyl methylcellulose (Methocel®-K100 from Dow) per kg of total monomers and 20.6 g of trisodium phosphate dodecahydrated. 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.

[00125] Then, 16.93 g of hydrogen peroxide solution (from Brenntag) and 5.9 g of ethyl chloroformate (from Framochem) were introduced in the reactor.

[00126] After 15 minutes, 0.77 g of acrylic acid (AA) were introduced in the reactor at a stirring speed of 880 rpm. Immediately after, 1162 g of VDF were added to the mixture. The reactor was then gradually heated until the set-point temperature of 35°C was reached.

[00127] The pressure was kept constantly equal to 120 bars during the whole polymerization run by feeding an aqueous solution of 18.27 g of AA per liter of solution. A total of 659 g of the solution was charged to the reactor. After 624 minutes the polymerization was stopped by degassing the suspension until reaching atmospheric pressure.

[00128] 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. 861 g of dry powder were collected.

[00129] A polymer comprising VDF-AA (0.9% by moles), having an intrinsic viscosity of 0.294 l/g in DMF at 25°C and a T ₂ f of 164.8°C was obtained.

[00130] The polymer contained 2.5 /10000 VDF units of the end-group CH ₃ CH ₂ -OCOO- derived from the ethyl chloroformate initiator precursor.

[00131] In addition, the presence of 4.6/10000 VDF units of -CF ₂ H and 2.2 /10000 VDF units of -CF ₂ CHS end-groups was determined.

[00132] The amount of end groups CH3CH ₂ -OCOO- is 26.9% with respect to the overall amount of end groups of polymer (F).

[00133] Example 3 comparative: Preparation of Polymer A

[00134] In a 4L reactor equipped with an impeller running at a speed of 650 rpm were introduced in sequence: 2205 g of demineralized water and 0.4 g of PEO (Alkox® -E45 from Alroko) per kg of total monomers and 0.5 g of hydroxypropyl methylcellulose (Methocel®-K100 from Dow) per kg of total 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.

[00135] Then, 4.62 g of a solution of the initiator t-amylperpivalate (TAPPI, from United Initiators) in isododecane (75%), 6.17 g of diethylcarbonate were introduced in the reactor.

[00136] Then the reactor was brought at a stirring speed of 880 rpm. Immediately after, 0.18 g of acrylic acid (AA) and 1176 g of VDF were added to the reactor. The reactor was then gradually heated until the set-point temperature of 50°C was reached.

[00137] The pressure was kept constantly equal to 120 bars during the whole polymerization run by feeding an aqueous solution of 3.33 g of AA per liter of solution. A total of 830 g of the solution was charged to the reactor. After 354 minutes the polymerization was stopped by degassing the suspension until reaching atmospheric pressure. [00138] 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.

[00139] 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 Taf of 169.6°C was obtained.

[00140] The polymer contained 1.1 /10000 VDF units of the end-group from TAPPI addition, the presence of 3.2/10000 VDF units of -CF2H and 2.1 /10000 VDF units of -CF2CH3 end-groups.

[00141] The polymer contained 1.1/10000 VDF units of end-group CH3CH2-OCOO- derived from diethylcarbonate.

[00142] The amount of end groups CH3CH2-OCOO- is 14.7% with respect to the overall amount of end groups of polymer (F).

[00143] Example 4 comparative: Preparation of Polymer B

[00144] The polymer B has been synthesized according to the teaching of WO 2008/129041 (SOLVAY SPECIALTY POLYMERS ITALY S.P.A.). The characteristics of the polymer are the following:

[00145] Composition: VDF-AA (0.9% by moles), polymer having an intrinsic viscosity of 0.274 l/g in DMF at 25°C and a T ₂ f of 162.6°C.

[00146] End groups: 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 -CF2CH3 end-groups.

[00147] No end groups of formula CH3CH2-OCOO- were determined.

[00148] General Preparation of the electrodes with NMC 622 active material

[00149] Positive electrodes having final composition of 96.5% by weight of NMC 622 (Umicore, d50 11.6 pm), 1.5% by weight of anyone of polymer (F-1), (F-2), A and B, 2% by weight of conductive additive were prepared as follows.

[00150] 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 the polymer in NMP, 133.8 g of NMC622, 2.8 g of SC-65 and 8.8 g of NMP.

[00151] Additional 7.2 g of NMP were added and the dispersion was mixed again in a centrifugal mixer for 10 minutes.

[00152] The final slurry was obtained by further stirring with high speed disk impeller at 1900 rpm for 70 minutes.

[00153] Positive electrodes were obtained by casting the obtained compositions on 15 pm thick Aluminium foil with doctor blade and drying the 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.

[00154] Example 5 : Adhesion and slurry viscosity [00155] The polymers of examples 1 to 3 were used as binders and the electrode compositions have been produced according to the procedure shown above.

[00156] The slurry viscosity of the compositions as above defined 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.

[00157] Adhesion Peeling Force between Aluminium foil and Electrode 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 as above defined to the Aluminium foil.

[00158] The values of slurry viscosity and adhesion are shown in Table 1.

Table 1

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 lower slurry viscosity and an higher adhesion to the current collector than with polymers of the prior art. It has been demonstrated that the presence of a certain amount of particular end-groups provides surprising effects both on the slurry viscosity of the electrode-forming compositions and the adhesion of the electrodes to the current collector.