Vinylidene fluoride copolymers for lithium battery electrodes

Abstract

The present invention relates to vinylidene fluoride copolymers comprising repeating units derived from hydrophilic monomers comprising carboxyl groups, whereby these polymers have ethyl carbonate end groups; and to the use thereof as a binder for electrodes in Li-ion batteries.

Description

[0001] Cross-reference to prior applications

[0002] This application claims priority to European Application No. 22202256.8, filed on October 18, 2022, the entire content of which is incorporated herein by reference for all purposes. Technical field

[0003] The present invention relates to vinylidene fluoride copolymers comprising repeating units derived from hydrophilic monomers and to their use as binders for electrodes in Li-ion batteries. Background art

[0004] It is known in the art that fluoropolymers are suitable as binders for manufacturing electrodes for use in electrochemical devices such as secondary batteries.

[0005] In particular, WO 2008 / 129041 (SOLVAY SPECIALTY POLYMERS ITALY S.P.A.) discloses linear semi-crystalline vinylidene fluoride (VDF) copolymers and the use of such copolymers as binders in electrodes of lithium ion batteries, the copolymers comprising from 0.05% to 10% by mole of repeating units derived from (meth)acrylic acid monomers.

[0006] Generally, it is known to increase the molecular weight of fluoropolymers to improve the properties of articles made from these materials, particularly in terms of mechanical properties and adhesion of the electrode to the current collector.

[0007] However, increasing the molecular weight of the fluoropolymer will increase the viscosity of the electrode-forming formulation (also known as the electrode slurry) containing it, making the processing and coating processes in electrode manufacturing more difficult.

[0008] In the technical field of batteries, especially lithium batteries, there is a problem of providing an electrode binder characterized by very good adhesion without negatively affecting the electrode manufacturing process, such as negatively affecting electrode production due to an increase in slurry viscosity.

[0009] The present invention provides a solution to this problem by combining the ease in the electrode manufacturing process (by processing an electrode-forming formulation having a low viscosity at low shear rates) with providing an electrode having a very high adhesion to the current collector. Summary of the invention

[0010] It has been found that certain vinylidene fluoride copolymers randomly containing certain carboxyl group-containing vinyl monomers are endowed with very good adhesion to metal substrates and can be used to prepare electrode-forming compositions having a low viscosity at low shear rates.

[0011] Accordingly, an object of the present invention is a fluoropolymer [Polymer (F)], characterized in that it consists of:

[0012] (i) repeating units derived from vinylidene fluoride (VDF) monomers; and

[0013] (ii) repeating units derived from at least one carboxyl group-containing vinyl monomer (CA) having the formula (I):

[0014]

[0015] wherein:

[0016] R 1 、R 2 and R 3 , which are the same as or different from each other, are independently selected from a hydrogen atom and a C 1 -C 3 hydrocarbyl group, and R H is a C 2 -C 10 hydrocarbon moiety containing at least one carboxyl group and no aliphatic hydroxyl group,

[0017] wherein the monomer (CA) in the polymer (F) is at most 5.0 mol% based on the total number of moles of the repeating units of the polymer (F); and

[0018] wherein at least 50% of the monomer (CA) is randomly distributed in the polymer (F), and,

[0019] wherein the polymer (F) is characterized by containing end groups having the formula (I):

[0020] -(Ra) x -O-CO-O-CH 2 -CH 3 (I)

[0021] wherein R a is a C 1 -C 5 linear or branched hydrocarbyl group, and x is an integer selected from 1 and 0, and

[0022] the end groups having the formula (I) are present in an amount of at least 20% based on the total amount of the end groups of the polymer (F).

[0023] A second object of the present invention relates to an electrode-forming composition (C), which composition comprises:

[0024] a) at least one electrode active material (AM);

[0025] b) at least one binder (B), wherein the binder (B) comprises at least one polymer (F) as defined above; and

[0026] c) at least one solvent (S).

[0027] In another object, the present invention relates to the use of the electrode-forming composition (C) in a method for manufacturing an electrode [electrode (E)], the method comprising:

[0028] (I) providing a metal substrate having at least one surface;

[0029] (II) providing the electrode-forming composition (C) as defined above;

[0030] (III) applying the composition (C) provided in step (II) to the at least one surface of the metal substrate provided in step (I), thereby providing an assembly comprising the metal substrate coated with the composition (C) on the at least one surface;

[0031] (IV) drying the assembly provided in step (III);

[0032] (V) subjecting the dried assembly obtained in step (IV) to a compression step to obtain the electrode (E) of the present invention.

[0033] In a further object, the present invention relates to an electrode (E) obtainable by the method of the present invention.

[0034] In yet another object, the present invention relates to an electrochemical device comprising at least one electrode (E) of the present invention. Detailed Description

[0035] The term "repeating unit derived from vinylidene fluoride (usually also denoted as vinylidene difluoride, 1,1-difluoroethylene, VDF)" is intended to denote a repeating unit having the formula CF 2 =CH 2 of.

[0036] In a preferred embodiment, the carboxyl-containing vinyl monomer (CA) is a compound having the formula (Ia):

[0037]

[0038] wherein

[0039] R 1 、R 2 and R 3 , which are the same or different from each other, are independently selected from a hydrogen atom and C 1-C 3 is a hydrocarbyl group, and R’ H is hydrogen or a C 1 -C 15 hydrocarbon moiety containing at least one carboxyl group and no aliphatic hydroxyl groups.

[0040] R’ H may further contain one or more oxygen atoms, carbonyl groups or ester groups in the chain.

[0041] The term “aliphatic hydroxyl” is intended to mean a hydroxyl group bonded directly to an aliphatic carbon.

[0042] Non-limiting examples of the monomer (CA) having the formula (I) include, inter alia:

[0043] -acrylic acid (AA),

[0044] -(meth)acrylic acid,

[0045] -2-carboxyethyl (meth)acrylate,

[0046] -3-butenoic acid,

[0047] -(meth)acryloyloxyethyl succinic acid,

[0048] -(meth)acryloyloxypropyl succinic acid,

[0049] -3-(allyloxy)propionic acid,

[0050] and mixtures thereof.

[0051] Preferably, the at least one monomer (CA) is acrylic acid (AA).

[0052] It is essential that in the polymer (F), at least 50% of the monomer (CA) is randomly distributed in the polymer (F).

[0053] It is known in the art that continuously feeding a comonomer of VDF during the polymerization of VDF will result in a random distribution of the comonomer in the polymer chain, where the sequence VDF-(comonomer)-VDF generally predominates.

[0054] Thus, when the polymer (F) is prepared by a polymerization reaction including continuously feeding the monomer (CA) during the polymerization of VDF, there is a random distribution of the monomer (CA) in the polymer chain, where the sequence VDF-(CA)-VDF is obtained.

[0055] More preferably, in the polymer (F), at least 70% of the monomer (CA) is randomly distributed in the polymer (F).

[0056] The expression "randomly distributed monomer (CA)" is intended to mean the presence of the sequence VDF-(CA)-VDF, and the amount of randomly distributed monomer (CA) is determined as the percentage ratio between the average number of said VDF-(CA)-VDF sequences and the total average number of (CA) monomer repeating units.

[0057] When each (CA) repeating unit is separate, i.e., is included between two repeating units of VDF monomer, the average number of (CA) sequences is equal to the total average number of (CA) repeating units, so the fraction of randomly distributed units (CA) is 100%: this value corresponds to a completely random distribution of (CA) repeating units. Thus, as described above, the greater the number of separate (CA) units relative to the total number of (CA) units, the higher the percentage value of the fraction of randomly distributed units (CA).

[0058] Analysis for determining the total amount of randomly distributed monomer (CA) can be carried out by measuring the sequence VDF-(CA)-VDF (by 19 19F-NMR) and the total amount of monomers in the polymer (by one or more of these techniques ( 19 19F-NMR, 1 1H-NMR, titration of carboxyl groups, FT-IR or others)).

[0059] Polymer (F) preferably contains, by moles, at least 0.01%, more preferably at least 0.02%, of repeating units derived from said monomer (CA).

[0060] Polymer (F) contains, relative to the total moles of repeating units of polymer (F), preferably, by moles, at most 5.0%, more preferably at most 3.0%, even more preferably, by moles, at most 2.0%, still more preferably, by moles, at most 1.5%, of repeating units derived from monomer (CA).

[0061] Excellent results have been obtained using polymer (F) containing, by moles, at least 70% of repeating units derived from VDF.

[0062] Polymer (F) can be an elastomer or a semi-crystalline polymer, preferably a semi-crystalline polymer.

[0063] As used herein, the term "semi-crystalline" means a fluoropolymer that also has at least one crystalline melting point in addition to the glass transition temperature Tg in DSC analysis. For the purposes of the present invention, a semi-crystalline fluoropolymer is herein intended to mean a fluoropolymer having a heat of fusion of 10 to 90 J / g, preferably 30 to 80 J / g, more preferably 35 to 75 J / g, as measured according to ASTM D3418-08.

[0064] For the purposes of the present invention, the term "elastomer" is intended to designate a true elastomer or a polymer resin that serves as a base component for obtaining a true elastomer.

[0065] A true elastomer is defined by ASTM, Special Technical Bulletin, No. 184 as a material that can be stretched at room temperature to twice its natural length and, after being held under tension for 5 minutes, once released, returns within the same time to within 10% of its initial length.

[0066] 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, and even more preferably between 0.20 l / g and 0.50 l / g.

[0067] The polymer (F) of the present invention generally has a melting temperature (T m ) included in the range of 120 °C to 200 °C.

[0068] The polymer (F) of the present invention has a quasi-linear structure with a very low degree of branching, which results in a substantially negligible amount of insoluble portion due to long branched chains.

[0069] The polymer (F) of the present invention preferably has a low fraction of insoluble components in standard polar aprotic solvents (such as NMP) for VDF polymers. More preferably, the solution of polymer (F) in the standard polar aprotic solvent remains homogeneous and stable for several weeks with substantially no insoluble residue.

[0070] Due to the low amount of insoluble components, the GPC and NMR analyses of polymer (F) are not affected, and there are no reliability and reproducibility issues.

[0071] The melting temperature can be determined from the DSC curve obtained by differential scanning calorimetry (hereinafter also referred to as DSC). In the case where the DSC curve shows multiple melting peaks (endothermic peaks), the melting temperature (Tm) is determined based on the peak with the largest peak area.

[0072] Polymer (F) may further comprise repeating units derived from one or more fluorinated comonomers (CF) different from VDF.

[0073] The term "fluorinated comonomer (CF)" is herein intended to denote an ethylenically unsaturated comonomer containing at least one fluorine atom.

[0074] Non-limiting examples of suitable fluorinated comonomers (CF) include, inter alia, the following:

[0075] (a) C 2 -C 8 Fluoroolefins and / or perfluoroolefins such as tetrafluoroethylene (TFE), hexafluoropropylene (HFP), pentafluoropropylene and hexafluoroisobutene;

[0076] (b) C 2 -C 8 Hydrogenated monofluoroolefins such as vinyl fluoride; 1,2-difluoroethylene and trifluoroethylene;

[0077] (c) Perfluoroalkyl vinylenes having the formula CH 2 =CH-R f0 wherein R f0 is C 1 -C 6 perfluoroalkyl;

[0078] (d) Chloro- and / or bromo- and / or iodo-C 2 -C 6 fluoroolefins such as chlorotrifluoroethylene (CTFE);

[0079] (e) Perfluoro(alkyl) vinyl ethers such as perfluoro(methyl) vinyl ether (PMVE), perfluoro(ethyl) vinyl ether (PEVE) and perfluoro(propyl) vinyl ether (PPVE);

[0080] (f) Perfluoro(1,3-dioxolene); perfluoro(2,2-dimethyl-1,3-dioxolene) (PDD).

[0081] In a preferred embodiment, the polymer (F) is semi-crystalline and comprises, relative to the total molar amount of the repeating units of the polymer (F), from 0.1% to 10.0% by mole, preferably from 0.3% to 5.0% by mole, more preferably from 0.5% to 3.0% by mole of repeating units derived from said fluorinated comonomer (CF).

[0082] It should be understood that parts of different chain ends, defects or other impurity types than those defined above may be included in the polymer (F) without impairing its properties.

[0083] The polymer (F) more preferably consists of:

[0084] - at least 70% by mole, preferably at least 75% by mole, more preferably at least 85% by mole of repeating units derived from vinylidene fluoride (VDF);

[0085] - from 0.01% to 2% by mole, preferably from 0.05% to 1.5% by mole of repeating units derived from at least one vinyl monomer (CA);

[0086] - Optionally, repeating units derived from at least one fluorinated comonomer (CF) in an amount of 0.5% to 10% by mole;

[0087] All these amounts are relative to the total number of moles of the repeating units of the polymer (F).

[0088] According to certain embodiments of the present invention, the polymer (F) is characterized by containing end groups of formula (I) as defined above, wherein x is 0.

[0089] According to other embodiments of the present invention, the polymer (F) is characterized by containing end groups of formula (I) as defined above, wherein x is 1, and R a is C 2 -C 3 a straight-chain or branched alkyl group.

[0090] According to other embodiments of the present invention, the polymer (F) is characterized by containing end groups of formula (I) as defined above wherein x is 0 and containing end groups of formula (I) wherein x is 1 and R a is C 2 -C 3 is a straight-chain or branched alkyl group.

[0091] The polymer (F) can be obtained by a process comprising the following steps:

[0092] - polymerizing vinylidene fluoride (VDF) monomer, an initial charge of monomer (CA), and optionally comonomer (CF) in an aqueous medium in the presence of a free radical initiator system that introduces end groups of formula (I) into the polymer chain,

[0093] - continuously feeding an aqueous solution containing monomer (CA); and

[0094] - maintaining the pressure in the reactor vessel above the critical pressure of vinylidene fluoride.

[0095] Suitable free radical initiator systems comprise free radical initiators such as bis(ethyl) peroxydicarbonate and hydro-ethyl peroxydicarbonate.

[0096] The amount of free radical initiator required for polymerization is related to its activity and the temperature at which the polymerization is carried out. The total amount of free radical initiator used is typically between 100 and 30,000 ppm by weight based on the total weight of the monomers used.

[0097] The free radical initiator can be added in pure form, as a solution, as a suspension, or as an emulsion, depending on the initiator selected.

[0098] The free radical initiator system can comprise a chain transfer agent (CTA).

[0099] CTAs suitable 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 (such as ethane and propane), esters (such as ethyl acetate or diethyl maleate, diethyl carbonate). When using an organic peroxide as an initiator, it can also act as an effective CTA during the free radical polymerization process.

[0100] When used, the CTA can be added all at once at the start of the reaction, or can be added in batches, or continuously throughout the reaction. The amount of CTA and its mode of addition depend on the desired properties of the polymer (F) to be obtained.

[0101] A preferred CTA for use in the process of the present invention is diethyl carbonate.

[0102] In the process for preparing the polymer (F), the pressure is maintained above the critical pressure of vinylidene fluoride. Generally, the pressure is maintained at a value greater than 50 bar, preferably greater than 75 bar, even more preferably greater than 100 bar.

[0103] The monomer (CA) is suitably added to the reaction vessel in the form of an aqueous solution.

[0104] It is necessary that the continuous feed of the aqueous solution containing the monomer (CA) is carried out throughout the entire duration of the polymerization run.

[0105] Thus, an approximately statistical distribution of the monomer (CA) can be obtained within the main chain of the polymer (F).

[0106] The expression "continuous feed" or "feeding continuously" means a slow, small, stepwise addition of the aqueous solution of the monomer (CA) during the polymerization.

[0107] The aqueous solution of the monomer (CA) fed continuously during the polymerization accounts for at least 50% wt of the total amount of the monomer (CA) supplied during the reaction (i.e., the initial charge plus the continuous feed). Preferably at least 60% wt, more preferably at least 70% wt, most preferably at least 80% wt of the total amount of the monomer (CA) is fed continuously during the polymerization. The stepwise addition of the VDF monomer can be achieved during the polymerization.

[0108] Preferably, the process of the present invention is carried out at a temperature above the critical temperature of the VDF monomer (i.e., at least 31 °C).

[0109] The polymer (F) is typically provided in the form of a powder according to the method described above.

[0110] The polymer (F) in powder form can optionally be further extruded to provide the polymer (F) in granule form.

[0111] The polymer (F) as detailed above can be used as a binder for the electrodes in a Li-ion battery.

[0112] A second object of the present invention relates to an electrode-forming composition (C) comprising:

[0113] a) at least one electrode active material (AM);

[0114] b) at least one binder (B), wherein the binder (B) comprises at least one polymer (F) as defined above; and

[0115] c) at least one solvent (S).

[0116] For the purposes of the present invention, the term "electroactive material (AM)" is intended to mean a compound that is capable of incorporating or inserting alkali metal or alkaline earth metal ions into its structure and of substantially releasing alkali metal or alkaline earth metal ions therefrom during the charging and discharging phases of an electrochemical device. The compound (AM) is preferably capable of incorporating or inserting and releasing lithium ions.

[0117] The nature of the compound (AM) in the composition (C) depends on whether the composition is for manufacturing a positive electrode [electrode (Ep)] or a negative electrode [electrode (En)].

[0118] In the case of forming a positive electrode (Ep) for a lithium-ion secondary battery, the compound (AM) can comprise a complex metal chalcogenide having the formula LiMQ 2 wherein M is at least one metal selected from transition metals such as Co, Ni, Fe, Mn, Cr and V or metals such as Al and mixtures thereof, and Q is a chalcogen element such as O or S. Among these, it is preferred to use a lithium-based complex metal oxide having the formula LiMO 2 wherein M is the same as defined above. Preferred examples thereof can include LiCoO 2 , LiNiO 2 , LiNi x Co 1-x O 2 (0 < x < 1), LiNi a Co b Al c O 2 (a + b + c = 1) and spinel-structured LiMn 2 O 4 .

[0119] As an alternative, still in the case of forming a positive electrode (Ep) for a lithium ion secondary battery, the compound (AM) can comprise an electroactive material based on lithiated or partially lithiated transition metal oxyanions having the formula M 1 M 2 (JO 4 ) f E 1-f , where M 1 is lithium, which can be partially replaced by another alkali metal less than 20% of the M 1 metal; M 2 is a transition metal selected from Fe, Mn, Ni or mixtures thereof at an oxidation level of +2, which can be partially replaced by one or more additional metals at an oxidation level between +1 and +5 and accounting for less than 35% of the M 2 metal, including 0; JO 4 is any oxyanion, where J is P, S, V, Si, Nb, Mo or combinations thereof; E is a fluoride anion, a hydroxide anion or a chloride anion; f is the mole fraction of the JO 4 oxyanion, typically included between 0.75 and 1.

[0120] M as defined above 1 M 2 (JO 4 ) f E 1-f The electroactive material is preferably phosphate-based and can have an ordered or modified olivine structure.

[0121] More preferably, in the case of forming the positive electrode (Ep), the compound (AM) has the formula Li 3-x M’ y M” 2-y (JO 4 ) 3 , where 0 ≤ x ≤ 3, 0 ≤ y ≤ 2; M' and M” are the same or different metals, at least one of which is a transition metal; JO 4 is preferably PO 4 , which can be partially replaced by another oxyanion, where J is S, V, Si, Nb, Mo or combinations thereof. Even more preferably, the compound (AM) is a phosphate-based electroactive material having the formula Li(Fe x Mn 1-x )PO 4 , where 0 ≤ x ≤ 1, where x is preferably 1 (i.e., having the formula LiFePO 4Lithium iron phosphate (LFP)). The LFP active material suitable for use in the electrodes of the present invention may have a nano-scale particle size (which means a size less than 1 micron) or a micron-scale particle size (which means particles having a size between 1 micron and 1 mm).

[0122] In the case of forming the composite negative electrode (En) of the lithium-ion secondary battery, the compound (AM) may preferably contain a carbon-based material and / or a silicon-based material.

[0123] In some embodiments, the carbon-based material may be, for example, graphite (such as natural or artificial graphite), graphene, or carbon black.

[0124] These materials may be used alone or in a mixture of two or more thereof.

[0125] The carbon-based material is preferably graphite.

[0126] 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.

[0127] When present in the compound (AM), at least one silicon-based compound is included in the compound (AM) in an amount ranging from 1% to 30% by weight, preferably 5% to 20% by weight, based on the total weight of the compound (AM).

[0128] The solvent (S) may preferably be an organic polar solvent, examples of which may include: N-methyl-2-pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, hexamethylphosphoramide, dioxane, tetrahydrofuran, tetramethylurea, triethyl phosphate, and trimethyl phosphate. These solvents may be used alone or in a mixture of two or more substances.

[0129] An optional conductive agent may be added to improve the conductivity of the resulting electrode (AM).

[0130] Examples thereof may include: carbonaceous materials such as carbon black, graphite fine powder, carbon nanotubes, graphene, or fibers, or fine powders or fibers of metals such as nickel or aluminum. The optional conductive agent is preferably carbon black. Carbon black is, for example, available under the trade name or Available.

[0131] The electrode-forming composition (C) of the present invention may further optionally contain at least one conductive agent.

[0132] When present, the conductive agent is different from the carbon-based materials described above.

[0133] In a preferred embodiment of the present invention, there is provided an electrode forming composition (C) for use in the preparation of a positive electrode (Ep), said composition comprising:

[0134] a) at least one electrode active material (AM);

[0135] b) at least one binder (B), wherein the binder (B) comprises at least one polymer (F) as defined above;

[0136] c) at least one solvent (S); and

[0137] d) at least one electrically conductive agent, preferably selected from carbon black or fine graphite powder or carbon nanotubes.

[0138] As described above, the polymer (F) of the present invention has a quasi-linear structure and, when dissolved in a standard polar aprotic solvent (such as NMP), the amount of the insoluble part is very low.

[0139] Due to the low amount of the insoluble component, the polymer (F) provides a solution in an organic solvent that is not adversely affected by the presence of insoluble residues, which are commonly referred to as "gels", and is thus more suitable for formulating an electrode forming composition.

[0140] In another object, the present invention relates to the use of the electrode forming composition (C) for manufacturing an electrode (E), said method comprising:

[0141] (I) providing a metal substrate having at least one surface;

[0142] (II) providing the electrode forming composition (C) as defined above;

[0143] (III) applying the composition (C) provided in step (II) to the at least one surface of the metal substrate provided in step (I), thereby providing an assembly comprising a metal substrate having the composition (C) coated on the at least one surface;

[0144] (IV) drying the assembly provided in step (III);

[0145] (V) subjecting the dried assembly obtained in step (III) to a compression step to obtain the electrode (E) of the present invention.

[0146] In a further object, the present invention relates to an electrode (E) obtainable by the method of the present invention.

[0147] The applicant has surprisingly found that the electrode (E) of the present invention shows excellent adhesion of the binder to the current collector.

[0148] The electrode (E) of the present invention is thus particularly suitable for use in electrochemical devices, especially in secondary batteries.

[0149] For the purposes of the present invention, the term "secondary battery" is intended to denote a rechargeable battery.

[0150] The secondary battery of the present invention is preferably an alkali metal or alkaline earth metal secondary battery.

[0151] The secondary battery of the present invention is more preferably a lithium ion secondary battery.

[0152] For yet further purposes, the present invention relates to an electrochemical device comprising at least one electrode (E) of the present invention.

[0153] The electrochemical device according to the present invention (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.

[0154] In a preferred embodiment of the present invention, there is provided an electrochemical device which 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.

[0155] The electrochemical device according to the present invention can be prepared by standard methods known to those skilled in the art.

[0156] The present invention will now be described with reference to the following examples, which are for illustrative purposes only and are not intended to limit the scope of the present invention.

[0157] Experimental section

[0158] Determination of the intrinsic viscosity of polymer (F)

[0159] The intrinsic viscosity (η) [dl / g] was measured using the following equation based on the drop time of a solution at a concentration of about 0.2 g / dl obtained by dissolving polymer (F) in N,N-dimethylformamide at 25 °C using an Ubbelhode viscometer:

[0160]

[0161] where c is the polymer concentration [g / dl], η r is the relative viscosity, i.e., the ratio between the drop time of the sample solution and the drop time of the solvent, η sp is the specific viscosity, i.e., η r - 1, and Γ is an experimental factor which corresponds to 3 for polymer (F).

[0162] DSC analysis

[0163] DSC analysis was carried out according to ASTM D 3418 standard; the melting point (T 2f ) was determined at a heating rate of 10 °C / min.

[0164] Determination of polar end groups

[0165] The amount of polar end groups of the polymer (F) produced from the ethyl chloroformate initiator precursor used in the polymerization process was determined by 1 1H-NMR, measuring the intensity of the H atoms of the CH 2 group (shown in bold in the following formula) relative to the total intensity of the CH 2 portion of the VDF monomer units in the main chain of the polymer (F):

[0166] CH 3 -CH 2 -OCOO-CH 2 -CF 2 -

[0167] The content of the end groups was calculated by applying the following formula:

[0168] [EG] = (I EG / I VDF ) × 10000

[0169] Where:

[0170] -[EG] is the content of the general end group, expressed as moles / 10000 VDF units,

[0171] -I EG is the integrated intensity of the end group [EG], normalized to one hydrogen,

[0172] -I VDF is the integrated intensity of the normal and reverse VDF repeat units, normalized to one hydrogen.

[0173] Approximately 20 mg of the polymer was dissolved in 0.7 ml of deuterated acetone. The 1 1H-NMR spectrum recorded at 60 °C showed the above-mentioned CH 2 at 4.47 ppm, while the CH 2 signals from the normal and reverse VDF repeat units resonated as broad peaks centered at 2.93 ppm and 2.36 ppm, respectively.

[0174] As known to those skilled in the art, a similar NMR method was applied to determine the end groups (CH 3 -CH 2 -OCOO-CH 2 -CH 2 -, CH 3-CH 2 -OCOO-CH(CH 3 )-) and for determining -CF 2 H and -CF 2 CH 3 end groups.

[0175] Determination of the amount of monomer AA in polymer (F) by NMR

[0176] The alternating AA content in polymer (F) is determined by 19 F-NMR spectroscopy.

[0177] It has been found that the signals associated with the CF 2 portion (shown in bold in the following formula) adjacent to the VDF units separating the separated hydrogenated comonomers resonate in 19 F-NMR at about -94 ppm.

[0178] -CH 2 CF 2 -CH 2 CH(COOH)-CH 2 CF 2 -CH 2

[0179] From the ratio between the normalized intensity of this signal in the spectrum and the normalized intensity of all VDF peaks, the average number of comonomers statistically inserted between two VDF units can be determined.

[0180] Example 1: Preparation of Polymer F-1

[0181] In a 4 L reactor equipped with an impeller operating at 650 rpm, the following components were introduced sequentially: 2370 g of demineralized water and 0.4 g of PEO ( -E45) from Alroko per kg of total monomers, and 0.5 g of hydroxypropyl methylcellulose ( -K100) from Dow Chemical Company per kg of total monomers, and 20.6 g of trisodium phosphate dodecahydrate. At a fixed temperature of 14 °C, the oxygen present in the reactor was removed by sequential vacuum and nitrogen purges. This sequence was repeated 3 times.

[0182] Then, 16.93 g of hydrogen peroxide solution (from Brenntag), 5.9 g of ethyl chloroformate (from Framochem), and 5.9 g of diethyl carbonate were introduced into the reactor.

[0183] After 15 minutes, 0.18 g of acrylic acid (AA) was introduced into the reactor at a stirring speed of 880 rpm. Immediately thereafter, 1174 g of VDF was added to the mixture. Then, the reactor was gradually heated until the set point temperature of 35 °C was reached.

[0184] During the entire polymerization run, the pressure was kept constantly equal to 120 bar by feeding an aqueous solution of 4.15 g AA per liter of solution. A total of 666 g of the solution was charged into the reactor. After 269 minutes, the polymerization was stopped by degassing the suspension until atmospheric pressure was reached.

[0185] Then the polymer obtained was collected by filtration and suspended in clean water in a stirred tank. After the washing treatment, the polymer was dried overnight in an oven at 65 °C. 837 g of dry powder was collected.

[0186] A polymer containing VDF-AA (0.2% by mole) was obtained, which had an intrinsic viscosity of 0.299 l / g in DMF at 25 °C and a T 2 f.

[0187] The polymer contained terminal groups CH of 2.0 / 10000 VDF units 3 CH 2 -OCOO-: 1.3 / 10000 VDF units were derived from the ethyl chloroformate initiator precursor and 0.7 / 10000 VDF units were derived from diethyl carbonate.

[0188] In addition, the presence of -CF 2 H of 2.7 / 10000 VDF units and -CF 2 CH 3 of 1.5 / 10000 VDF units was determined.

[0189] The amount of the terminal group CH 3 CH 2 -OCOO- was 32.3% relative to the total amount of the terminal groups of the polymer (F).

[0190] Example 2: Preparation of Polymer F-2

[0191] In a 4 L reactor equipped with an impeller operating at 650 rpm, the following components were introduced sequentially: 2383 g of demineralized water and 0.4 g of PEO (from Alroko -E45) per kg of total monomers, and 0.5 g of hydroxypropyl methylcellulose (from Dow Chemical -K100), and 20.6 g of trisodium phosphate dodecahydrate. At a fixed temperature of 14 °C, the oxygen present in the reactor was removed by sequential vacuum and nitrogen purging. This sequence was repeated 3 times.

[0192] Then, 16.93 g of a hydrogen peroxide solution (from Brenntag) and 5.9 g of ethyl chloroformate (from Framochem) were introduced into the reactor.

[0193] After 15 minutes, 0.77 g of acrylic acid (AA) was introduced into the reactor at a stirring speed of 880 rpm. Immediately thereafter, 1162 g of VDF was added to the mixture. Then, the reactor was gradually heated until the set point temperature of 35 °C was reached.

[0194] During the entire polymerization run, the pressure was kept constantly equal to 120 bar by feeding an aqueous solution of 18.27 g AA per liter of solution. A total of 659 g of the solution was charged into the reactor. After 624 minutes, the polymerization was stopped by degassing the suspension until atmospheric pressure was reached.

[0195] The polymer was then collected by filtration and suspended in clean water in a stirred tank. After the washing treatment, the polymer was dried overnight in an oven at 65 °C. 861 g of dry powder was collected.

[0196] A polymer containing VDF-AA (0.9 mol%) was obtained, which had an intrinsic viscosity of 0.294 l / g in DMF at 25 °C and a T 2 f of 164.8 °C.

[0197] The polymer contained end groups CH 3 CH 2 -OCOO- derived from the ethyl chloroformate initiator precursor at 2.5 / 10000 VDF units.

[0198] In addition, the presence of -CF 2 H at 4.6 / 10000 VDF units and -CF 2 CH 3 end groups at 2.2 / 10000 VDF units was determined.

[0199] The amount of the end group CH 3 CH 2 -OCOO- was 26.9% relative to the total amount of end groups of the polymer (F).

[0200] Example 3 (comparative): Preparation of Polymer A

[0201] In a 4 L reactor equipped with an impeller operating at 650 rpm, the following components were introduced sequentially: 2205 g of demineralized water and 0.4 g of PEO ( -E45) from Alroko per kg of total monomers and 0.5 g of hydroxypropyl methylcellulose ( -K100) from Dow Chemical per kg of total monomers. At a fixed temperature of 11 °C, the oxygen present in the reactor was removed by sequential vacuum and nitrogen purges. This sequence was repeated 3 times.

[0202] Then, a solution (75%) of 4.62 g of the initiator tert-amyl perneodecanoate (TAPPI, from United Initiators) in isododecane and 6.17 g of diethyl carbonate were introduced into the reactor.

[0203] Then the reactor was set at a stirring speed of 880 rpm. Immediately afterwards, 0.18 g of acrylic acid (AA) and 1176 g of VDF were added to the reactor. Then the reactor was gradually heated until a set point temperature of 50 °C was reached.

[0204] During the entire polymerization run, the pressure was kept constantly equal to 120 bar by feeding an aqueous solution of 3.33 g AA per liter of solution. A total of 830 g of the solution was loaded into the reactor. After 354 minutes, the polymerization was stopped by degassing the suspension until atmospheric pressure was reached.

[0205] Then the polymer was collected by filtration and suspended in clean water in a stirred tank. After the washing treatment, the polymer was dried overnight in an oven at 65 °C. 987 g of dry powder was collected.

[0206] A polymer containing VDF-AA (0.2 mol%) was obtained, which had an intrinsic viscosity of 0.286 l / g in DMF at 25 °C and a T 2 f of 169.6 °C.

[0207] The polymer contained end groups from TAPPI addition of 1.1 / 10000 VDF units, there were 3.2 / 10000 VDF units of -CF 2 H and 2.1 / 10000 VDF units of -CF 2 CH 3 end groups.

[0208] The polymer contained end groups CH 3 CH 2 -OCOO- derived from diethyl carbonate of 1.1 / 10000 VDF units.

[0209] End group CH 3CH 2 The amount of -OCOO- is 14.7% relative to the total amount of end groups of polymer (F).

[0210] Example 4 (comparative): Preparation of Polymer B

[0211] Polymer B has been synthesized according to the teachings of WO 2008 / 129041 (Solvay Specialty Polymers Italy S.p.A.). The characteristics of the polymer are as follows:

[0212] Composition: VDF-AA (0.9 mol %), having an intrinsic viscosity of 0.274 l / g in DMF at 25 °C and a T 2 f of the polymer.

[0213] End groups: End groups from TAPPI addition of 2.3 / 10000 VDF units, -CF 2 H of 6.8 / 10000 VDF units and -CF 2 CH 3 End groups.

[0214] No end groups of the formula CH 3 CH 2 -OCOO- were determined.

[0215] General Preparation of Electrodes with NMC 622 Active Material

[0216] A positive electrode with a final composition of 96.5% by weight of NMC 622 (Umicore, d50 11.6 μm), 1.5% by weight of any one of polymers (F-1), (F-2), A and B, and 2% by weight of a conductive additive was prepared as follows.

[0217] A first dispersion was prepared by premixing 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 in a centrifugal mixer for 10 minutes.

[0218] An additional 7.2 g of NMP was added and the dispersion was mixed again in the centrifugal mixer for 10 minutes.

[0219] The final slurry was obtained by further stirring at 1900 rpm with a high-speed disk impeller for 70 minutes.

[0220] The positive electrode was obtained by casting the resulting combined stream onto a 15-μm thick aluminum foil with a doctor blade and drying the coating in a vacuum oven at 90 °C for about 50 minutes. The thickness of the dried coating was about 110 μm.

[0221] Example 5: Adhesion and Slurry Viscosity

[0222] The polymers of Examples 1 to 3 were used as binders, and the electrode compositions were produced according to the procedures shown above.

[0223] The slurry viscosity of the composition as defined above was measured using an AntonPaar Rheolab QC with a concentric cylinder device (Measuring cup: C-CC27 / QC-LTD Bob: CC27 / P6) at 25 °C under Peltier temperature control. The steady-state viscosity was measured at shear rates from 0.1 to 200 1 / s.

[0224] The adhesion peel force between the aluminum foil and the electrode was measured as follows:

[0225] At 20 °C, a 180° peel test was carried out at a speed of 300 mm / min according to the setup described in Standard ASTM D903 to evaluate the adhesion of the dried coating to the aluminum foil as defined above.

[0226] The values of the slurry viscosity and adhesion are shown in Table 1.

[0227] Table 1

[0228]

[0229] The results show that the polymers of the present invention perform better and are easier to process in the electrode manufacturing process due to their lower slurry viscosity and higher adhesion to the current collector compared to the polymers of the prior art. It has been demonstrated that the presence of a certain amount of specific end groups provides unexpected effects on both the slurry viscosity of the electrode-forming composition and the adhesion of the electrode to the current collector.

Claims

1. A fluoropolymer [Polymer (F)], characterized in that it consists of: (i) repeating units derived from vinylidene fluoride (VDF) monomers; and (ii) repeating units derived from at least one carboxyl-containing vinyl monomer (CA) having the formula (I): wherein: R 1 , R 2 and R 3 , which are the same or different from each other, are independently selected from hydrogen atoms and C 1 -C 3 Hydrocarbon, and R H is a C containing at least one carboxyl group and no aliphatic hydroxyl group 2 -C 10 The hydrocarbon part, wherein the monomer (CA) in the polymer (F) is at most 5.0 mol% based on the total number of moles of the repeating units of the polymer (F); and where at least 50% of the monomer (CA) is randomly distributed in the polymer (F), and, wherein the polymer (F) is characterized by containing end groups having the formula (I): -(Ra) x -O-CO-O-CH 2 -CH 3 (I) wherein R a is C 1 -C 5 a straight-chain or branched-chain hydrocarbon group, and x is an integer selected from 1 and 0, and These end groups having the formula (I) are present in an amount of at least 20% based on the total amount of the end groups of the polymer (F).

2. The polymer (F) according to claim 1, wherein, the carboxyl-containing vinyl monomer (CA) is a compound having the formula (Ia): where R 1 , R 2 and R 3 , which are the same or different from each other, are independently selected from hydrogen atoms and C 1 -C 3 Hydrocarbyl, and R' H is hydrogen or contains at least one carboxyl group and no aliphatic hydroxyl group. 1 -C 15 Hydrocarbon part.

3. The polymer (F) according to claim 1 or claim 2, wherein, the carboxyl-containing vinyl monomer (CA) 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)propionic acid, and mixtures thereof.

4. The polymer (F) according to any one of the preceding claims, which further comprises repeating units derived from one or more fluorinated comonomers (CF) different from VDF.

5. The polymer (F) according to any one of the preceding claims, which consists of the following components: - at least 70 mol%, preferably at least 75 mol%, more preferably at least 85 mol% of repeating units derived from vinylidene fluoride (VDF); - 0.01 mol% to 1.5 mol%, preferably 0.01 mol% to 1.0 mol% of repeating units derived from at least one vinyl monomer (CA); - optionally, 0.5 mol% to 3.0 mol% of repeating units derived from at least one fluorinated comonomer (CF); All these amounts are based on the total number of moles of the repeating units of the polymer (F).

6. The polymer (F) according to any one of the preceding claims, which contains end groups of formula (I) in which x is 0, and / or end groups of formula (I) in which x is 1 and R a is C 2 -C 3 is a linear or branched alkyl group.

7. A method for preparing the polymer (F) according to any one of claims 1 to 6, the method comprising: - polymerizing vinylidene fluoride (VDF) monomers, an initial charge of monomer (CA), and optionally comonomer (CF) in an aqueous medium in the presence of a free radical initiator system that introduces end groups having the formula (I) into the polymer chain, - continuously feeding an aqueous solution containing monomer (CA); and - maintaining the pressure in the reactor vessel above the critical pressure of the vinylidene fluoride.

8. The method according to claim 7, wherein, the free radical initiator system comprises free radical initiators such as bis(ethyl) peroxydicarbonate and ethyl peroxy dicarbonate.

9. The method according to claim 8, wherein, the free radical initiator system comprises a chain transfer agent (CTA).

10. An electrode-forming composition (C) comprising: a) at least one electrode active material (AM); b) at least one binder (B), wherein the binder (B) comprises at least one polymer (F) according to any one of claims 1 to 6; and c) at least one solvent (S).

11. The electrode-forming composition (C) according to claim 10, wherein, the at least one electrode active material (AM) is LFP.

12. A method for manufacturing an electrode [electrode (E)], the method comprising: (I) providing a metal substrate having at least one surface; (II) providing the electrode-forming composition (C) according to claim 10 or claim 11; (III) applying the composition (C) provided in step (II) to the at least one surface of the metal substrate provided in step (I), thereby providing an assembly comprising a metal substrate having the composition (C) coated on the at least one surface; (IV) drying the assembly provided in step (III); (V) subjecting the dried assembly obtained in step (IV) to a compression step to obtain the electrode (E) of the present invention.

13. An electrode (E) obtainable by the method according to claim 12.

14. An electrochemical device comprising at least one electrode (E) according to claim 13.

Images