Crosslinked ionic conductive membrane

Abstract

The present invention pertains to ionic conductive membranes comprising silane-modified fluorinated copolymers, to the process for the production thereof, and to the use of the same in electrochemical devices and in photo-electrochemical devices, such as solar panels and dye-sensitized solar cells (DSSC).

Description

Crosslinked ionic conductive membraneCROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to European patent application No.24151955.2 filed on January 15, 2024, the whole content of this application being incorporated herein by reference for all purposes.Technical Field

[0001] The present invention pertains to ionic conductive membranes comprising silane-modified fluorinated copolymers, to the process for the production thereof, and to the use of the same in electrochemical devices, such as batteries, and in photo-electrochemical devices, such as solar panels and dye-sensitized solar cells (DSSC).Background Art

[0002] Conductive polymers are being studied increasingly as additives into lithium battery components, supercapacitors and other electrochemical devices due to their ability to conduct electrons and ions.

[0003] Companies in the domain are focusing on enhancing the conductivity and stability parameters of the existing polymers by adding materials such as carbonate-based organic solvents or ionic liquids. They also aim to develop novel polymers to achieve good ionic conductivity, electrochemical stability, and thermal stability.

[0004] Some of the key areas in focus include lithium-ion conductivity, electrochemical stability, thermal stability, adhesion property.

[0005] Fluoropolymers, and in particular vinylidene fluoride polymers, have been successfully used in a wide variety of applications including electrochemical applications.

[0006] In particular, fluoropolymers are used as raw materials for polymer films and polymer electrolyte membranes used in electrochemical devices due to their chemical resistance and thermal aging resistance.

[0007] Composite materials based on ionic liquids (ILs) I poly(vinylidene fluoride) (PVDF) and their copolymers have emerged as an interesting approach to develop high ionic conductivity polymers, suitable for use in lithium-ion battery application.

[0008] However, IL / PVDF composites still show a low ionic conductivity value below 10-4 S / cm and other fillers are added to increase ionic conductivity.

[0009] Moreover, the polymer films and polymer electrolyte membranes used in electrochemical devices must exhibit high mechanical stability in order to provide long-term stability and to withstand dissolution in the liquids present in the devices.

[0010] Accordingly, there is a growing interest and demand for a technology for improving the mechanical integrity and the ionic conductivity of fluoropolymer membranes for use that are suitable for use in in electrochemical devices.

[0011] Accordingly, the Applicant faced the problem of providing fluoropolymer membranes endowed with improved mechanical properties and high ionic conductivity and process for the production thereof, that are improved with respect to the ones of the prior art.Summary of invention

[0012] It is thus an object of the invention a fluoropolymer film comprising at least one crosslinked ionic conductive silane-modified fluorinated copolymer [polymer (A-XL)], said polymer (A-XL) being obtainable by a process that comprises: a) contacting at least one fluoropolymer [polymer (F)] that comprises:- recurring units derived from at least one fluorinated monomer [monomer (FM)], and- recurring units derived from at least one monomer [monomer (FPM)], said monomer (FPM) comprising at least one functional group [group (FX)] selected from the group consisting of: amine, carboxylic acid, thiol and anhydride; with at least a metal compound [compound (M)] of formula (I):X4-mSiYm (I) wherein m is an integer from 1 to 3, each occurrence of Y is a hydrolysable group and each occurrence of X is a hydrocarbon group, wherein at least one X comprises at least one epoxy functional group, so that at least a fraction of the group (FX) of monomer (FPM) of polymer (F) is reacted with at least a fraction of compound (M),wherein the reaction is carried out in the presence of a liquid medium [medium (L)] and, optionally, of a solvent (S), to obtain a polymer (A); and b) self-crosslinking polymer (A) to obtain polymer (A-XL).

[0013] The fluoropolymer film of the invention is ionic conductive, which means that it allows faster migration of ions through the polymer. The medium (L) entrapped in the polymer network during polymer construction is in fact capable of providing a polar environment suitable for the dissociation of salts and the solvation of ions.

[0014] The addition of Li salts is an effective method of further increasing conductivity.

[0015] Moreover, the fluoropolymer film of the invention is crosslinked, and thus it keeps even better mechanical integrity and resistance to swelling and dissolution when contacted to solvents than non crosslinked polymers.

[0016] A further object of the present invention pertains to the use of the fluoropolymer film as above defined in electrochemical devices, such as batteries, and in photo-electrochemical devices, such as solar panels and dye-sensitized solar cells (DSSC).Detailed description

[0017] By the term “fluorinated monomer [monomer (FM)]” it is hereby intended to denote an ethylenically unsaturated monomer comprising at least one fluorine atom.

[0018] The term “at least one fluorinated monomer” is understood to mean that the polymer (F) may comprise recurring units derived from one or more than one fluorinated monomers. In the rest of the text, the expression “fluorinated monomers” is understood, for the purposes of the present invention, both in the plural and the singular, that is to say that they denote both one or more than one fluorinated monomers as defined above.

[0019] Non limitative examples of suitable monomers (FM) include, notably, the followings:- C2-C8 perfluoroolefins, such as tetrafluoroethylene and hexafluoropropylene (HFP);- C2-C8 hydrogenated fluoroolefins, such as vinyl fluoride, vinylidenefluoride (VDF), 1 ,2-difluoroethylene and trifluoroethylene;- perfluoroalkylethylenes of formula CH2=CH-Rro wherein Rro is a Ci-Ce perfluoroalkyl;- chloro- and / or bromo- and / or iodo-C2-Ce fluoroolefins, such as chlorotrifluoroethylene;- (per)fluoroalkylvinylethers of formula CF2=CFORfi wherein Rn is a Ci-Ce fluoro- or perfluoroalkyl, e.g. CF3, C2F5, C3F7 ;- CF2=CFOXO (per)fluoro-oxyalkylvinylethers wherein Xo is a C1-C12 alkyl group, a C1-C12 oxyalkyl group or a C1-C12 (per)fluorooxyalkyl group having one or more ether groups, such as perfluoro-2-propoxy-propyl group;- (per)fluoroalkylvinylethers of formula CF2=CFOCF2ORf2 wherein Rf2 is a Ci-Ce fluoro- or perfluoroalkyl group, e.g. CF3, C2F5, C3F7 or a Ci-Ce (per)fluorooxyalkyl group having one or more ether groups, such as -C2F5- O-CF3;- functional (per)fluoro-oxyalkylvinylethers of formula CF2=CFOYo wherein Yo is a C1-C12 alkyl group or (per)fluoroalkyl group, a C1-C12 oxyalkyl group or a C1-C12 (per)fluorooxyalkyl group having one or more ether groups and Yo comprising a carboxylic or sulfonic acid group, in its acid, acid halide or salt form;- fluorodioxoles, preferably perfluorodioxoles.

[0020] Preferred monomer (FM) are selected from the group consisting of vinylidene fluoride (VDF), vinyl fluoride (VF1 ), chlorotrifluoroethylene (CTFE), hexafluoropropylene (HFP), tetrafluoroethylene (TFE), trifluoroethylene (TrFE), perfluoromethylvinylether (PMVE).The polymer (F) is typically obtainable by a process comprising the polymerization of at least one monomer (FM) and at least one monomer (FPM) comprising at least one functional group (FX).

[0021] The monomer (FPM) comprises at least one functional group [group (FX)] selected from the group consisting of: amine, carboxylic, thiol and anhydride.

[0022] Monomer (FPM) can be selected from (per)fluorinated monomers and hydrogenated monomers comprising at least one functional group [group (FX)]. Hydrogenated monomers are preferred.

[0023] Suitable hydrogenated monomers (FPM) are monomers of formula (I):wherein:Ri , R2 and R3, equal to or different from each other, are independently selected from a hydrogen atom, a C1-C3 hydrocarbon group and Rx, and Rx is a C1-C20 hydrocarbon moiety comprising at least one functional group [group (FX)] selected from the group consisting of: amine, carboxylic acid group, thiol group and anhydride.

[0024] Rx may contain other functional groups different from group (FX) and may include heteroatoms.

[0025] The monomer (FPM) is notably selected from the group consisting of (meth)acrylic monomers of formula (II):wherein R1, R2 and R3, are as above defined, RH is a C1-C20 hydrocarbon moiety comprising at least one functional group [group (FXH)] selected from the group consisting of: amine, carboxylic group, thiol group and anhydride. More preferably, said functional group (FXH) is a carboxylic group.

[0026] When the functional group (FX) in monomer (FPM) is an amine, it may be suitably selected from primary and secondary amines. Said amines may be both aliphatic and aromatic amines.

[0027] The polymer (F) typically comprises from 0.01 % by moles to 10.0 % by moles of recurring units derived from at least one monomer [monomer (FPM)] 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, andRx is a C1-C20 hydrocarbon moiety comprising at least one functional group [group (FX)] selected from the group consisting of: amine, carboxylic group, thiol group and anhydride, the aforementioned percentages by moles being referred to the total moles of recurring units of polymer (F).

[0028] Determination of average mole percentage of monomer (FPM) recurring units in polymer (F) can be performed by any suitable method. Mention can be notably made of acid-base titration methods, well suited e.g. for the determination of the carboxylic groups content, of NMR methods, adequate for the quantification of monomers (FPM) comprising aliphatic hydrogen atoms in side chains, of weight balance based on total fed monomer (FPM) and unreacted residual monomer (FPM) during polymer (F) manufacture.

[0029] In certain preferred embodiments, monomer (FPM) is randomly distributed in polymer (F). In said embodiments, a fraction of at least 40% of monomer (FPM) is randomly distributed into said polymer (F).

[0030] The expression “randomly distributed in polymer (F)” is intended to denote the percent ratio between the average number of monomer (FPM) sequences (%), said sequences being comprised between two recurring units derived from monomer (FM), and the total average number of monomer (FPM) recurring units (%), according to the following formula: awra je number of (FPM) sequences (%)Fraction of randomly distributed units (FPM)= - 100 awrage total number of (FPM) unis (%)

[0031] When each of the (FPM) recurring units is isolated, that is to say comprised between two recurring units of monomer (FM), the average number of (FPM) sequences equal the average total number of (FPM) recurring units, so the fraction of randomly distributed units (FPM) is 100%: this value corresponds to a perfectly random distribution of (FPM) recurring units.

[0032] Thus, the larger is the number of isolated (FPM) units with respect to the total number of (FPM) units, the higher will be the percentage value of fraction of randomly distributed units (FPM), as above described.

[0033] The polymer (F) may be amorphous or semi-crystalline.

[0034] The term “amorphous” is hereby intended to denote a polymer (F) having a heat of fusion of less than 5 J / g, preferably of less than 3 J / g, more preferably of less than 2 J / g, as measured according to ASTM D-3418-08.

[0035] The term “semi-crystalline” is hereby intended to denote a polymer (F) having a heat of fusion of from 10 to 90 J / g, preferably of from 30 to 60 J / g, more preferably of from 35 to 55 J / g, as measured according to ASTM D3418-08.

[0036] The polymer (F) is preferably semi-crystalline.

[0037] Preferably, the intrinsic viscosity of polymer (F), measured in dimethylformamide at 25 °C, is comprised between 0.05 l / g and 0.80 l / g, more preferably between 0.10 l / g and 0.50 l / g even more preferably between 0.2 l / g and 0.4 l / g.

[0038] The polymer (F) preferably comprises recurring units derived from vinylidene fluoride (VDF), at least one monomer (FPM) as defined above and, optionally, at least one further monomer (FM) different from VDF. The further monomer (FM) in polymer (F) is preferably HFP.

[0039] The polymer (F) preferably comprises:(a) at least 60% by moles, preferably at least 75% by moles, more preferably at least 85% by moles of vinylidene fluoride (VDF);(b) optionally, from 0.1 % to 15% by moles, preferably from 0.5% to 10% by moles, more preferably from 1 % to 5% by moles of at least one monomer (FM) selected from vinyl fluoride (VF1 ), chlorotrifluoroethylene (CTFE), hexafluoropropylene (HFP), tetrafluoroethylene (TFE), trifluoroethylene (TrFE), perfluoromethylvinylether (PMVE); and(c) from 0.01 % to 10% by moles, preferably from 0.05% to 5% by moles, more preferably from 0.1 % to 2% by moles of at least one monomer (FPM) of formula (I) as defined above.

[0040] The polymer (F) is typically obtainable by emulsion polymerization or by suspension polymerization.

[0041] According to a first variant of the present invention, the reaction of at least a portion of the polymer (F) with a compound (M) in step a) comprises reacting a polymer (F) with at least a compound (M) as above defined in the presence of a liquid medium [medium (L)] and of a solvent (S), to provide a mixture comprising polymer (A).

[0042] According to this embodiment, the reaction of at least a portion of the polymer (F) with a compound (M) in step a) bearing at least a silane functional group comprises:- i) a step of providing a solution of at least one polymer (F) as above defined in a solvent (S),- ii) a step of contacting the solution obtained in step i) with at least a compound (M) as above defined, and- iii) a step of adding a liquid medium [medium (L)], to obtain a mixture comprising polymer (A).

[0043] Within the present invention, solvent (S) is intended to denote a solvent suitable for dissolving polymer (A) as defined above. To this aim, solvent (S) is typically selected from the group consisting of: N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide, N,N-dimethylacetamide, dimethylsulfoxide, hexamethylphosphamide, dioxane, tetrahydrofuran, tetramethylurea, triethyl phosphate, and trimethyl phosphate, aliphatic ketones, cycloaliphatic ketones, cycloaliphatic esters. These solvents may be used singly or in mixture of two or more species.

[0044] For the purpose of the present invention, by the term “liquid medium [medium (L)]” it is hereby intended to denote one or more substances in the liquid state at 20°C under atmospheric pressure.

[0045] According to some embodiments of the present invention, said medium (L) is preferably selected from organic carbonates, ionic liquids (IL) and mixtures thereof.

[0046] Non-limiting examples of suitable organic carbonates include, notably, ethylene carbonate, propylene carbonate, mixtures of ethylene carbonate and propylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, fluoropropylene carbonate and mixtures thereof.

[0047] By the term “ionic liquid (IL)”, it is hereby intended to denote a compound formed by the combination of positively charged cations and negatively charged anions which exists in the liquid state at temperatures below 100°C under atmospheric pressure.

[0048] The ionic liquid (IL) can be selected from protic ionic liquids (ILP), aprotic ionic liquids (ILa) and mixtures thereof.

[0049] By the term “protic ionic liquid (ILP)", it is hereby intended to denote an ionic liquid wherein the cation comprises one or more H+hydrogen ions.

[0050] Non-limitative examples of cations comprising one or more H+hydrogen ions include, notably, imidazolium, pyridinium, pyrrolidinium or piperidinium rings, wherein the nitrogen atom carrying the positive charge is bound to a H+hydrogen ion.

[0051] By the term “aprotic ionic liquid (ILa)", it is hereby intended to denote an ionic liquid wherein the cation is free of H+hydrogen ions.

[0052] The ionic liquid (IL) is typically selected from those comprising as cation a sulfonium ion or an imidazolium, pyridinium, pyrrolidinium or piperidinium ring, said ring being optionally substituted on the nitrogen atom, in particular by one or more alkyl groups with 1 to 8 carbon atoms, and on the carbon atoms, in particular by one or more alkyl groups with 1 to 30 carbon atoms.

[0053] According to one embodiment of the invention, said medium (L) comprises a mixture of at least one organic carbonate as defined above.

[0054] According to another embodiment of the invention, said medium (L) comprises a mixture of at least one organic carbonate as defined above and at least one ionic liquid (IL) as defined above.

[0055] According to a further embodiment of the invention, said medium (L) comprises a mixture of at least one organic solvent (S) and at least one organic carbonate as defined above and / or at least one ionic liquid (IL) as defined above.

[0056] According to certain embodiments of the present invention, medium (L) further comprises at least one electrolytic salt (ES).

[0057] The electrolyte salt (ES) is a lithium salt, preferably lithium bistrifluoromethanesulfonimide and / or lithium bis(fluorosulfonyl)imide.

[0058] The medium (L) may further comprise one or more additives.

[0059] Should one or more additives be present in the liquid medium, non-limitative examples of suitable additives include, notably, those which are soluble in the liquid medium.

[0060] In step ii) the weight ratio between the amount of compound (M) and polymer (F) is advantageously from 0.01 to 0.5, preferably from 0.050 to 0.25.

[0061] The polymer (F) and the compound (M) are reacted at temperatures typically comprised between 20°C and 250°C.

[0062] The skilled in the art will properly select the temperature depending on the boiling point of the medium (L), the equipment and the technique used for the reactions in the process.

[0063] Under step ii) the reaction mixture advantageously further comprises at least one catalyst.

[0064] The catalyst for the grafting reaction of polymer (F) with compound (M) is preferably selected from the group consisting of organic aluminium compounds such as aluminum trifluoromethanesulfonate.

[0065] In general, the molar amount of compound (M) added in step ii) corresponds at least to the molar amount of monomer (FPM) present in the solution of polymer (F).

[0066] When the molar amount of compound (M) added in step ii) is lower than the molar amount of monomer (FPM) present in the solution of polymer (F), polymer (A) includes recurring units derived from monomer (FPM) bearing unreacted functional groups (FX) selected from the group consisting of: hydroxyl group, amine, carboxylic group, thiol group and anhydride.

[0067] The weight ratio between the amount of medium (L) and polymer (F) in the reaction mixture is advantageously between 0.1 and 10 preferably between 1 and 4.

[0068] Under step ii) of the process of the invention, the catalyst is typically added to the reaction mixture in an amount comprised between 0.1 % and 50% by moles, preferably between 0.3% and 10% by moles, more preferably between 0.5% and 5% by moles, based on the total amount by moles of compound (M).

[0069] Under step ii) and iii) at least a fraction of the group (FX) of monomer (FPM) of polymer (F) is reacted with at least a fraction of compound (M), thereby providing a composition comprising at least one polymer (A) bearing -SiYm pendant groups.

[0070] In the reaction conditions of step iii), the polymer (A) can start selfcrosslinking step b) to obtain polymer (A-XL) by the formation of bonds between the -SiYm pendant groups.

[0071] The mixture comprising polymer (A) obtained at the end of step ii) can then be suitably casted onto an inert support, and the solvent (S) be evaporated in a drying step. During the drying step, the self-crosslinking of polymer (A) to obtain polymer (A-XL) proceeds until completion, to obtain a fluoropolymer film comprising at least one crosslinked ionic conductive silane-modified fluorinated copolymer [polymer (A-XL)].

[0072] Drying can be performed either under atmospheric pressure or under vacuum. Alternatively, drying can be performed under modified atmosphere, e.g. under an inert gas, typically exempt notably from moisture (water vapour content of less than 0.001 % v / v).

[0073] According to a second variant of the present invention, the reaction of at least a portion of the polymer (F) with a compound (M) is preferably carried out in the absence of a solvent (S), in the molten state. The reaction according to this variant can be carried out by blending the polymer (F) with a compound (M) in the presence of a liquid medium [medium (L)]. The mixing may be carried out, for example, in a mill, mixer or blender (such as a V-blender equipped with a high intensity mixing bar, or other alternative equipment as described further below), until a uniform mixture is formed. Those skilled in the art will identify, after perusal of this document, that blending time can vary based on batch size, materials, particle size, densities, as well as other properties, and yet remain within the scope hereof.

[0074] The reaction of at least a portion of the polymer (F) with a compound (M) according to this variant can be suitably carried out in a closed device, such as a reactor or in a semi-closed device, such as an extruder, a twin-screw compounder or an internal mixer, wherein the reaction is carried out at high temperature, preferably at a temperature in the range of from 90 to 120°C.

[0075] The residence time in said devices depends on the equipment used and also on the reactivity of the system. The skilled in the art will select the proper timing (and temperature) for completing the reactions.

[0076] The time of the reaction shall be adapted to the device architecture and rpm. The residence time in a semi-closed is typically lower than 30 minutes, preferably lower than 15 minutes.

[0077] In said reaction conditions in the molten state polymer (A) may start undergoing self-crosslinking, providing at least a part of polymer (A-XL); the degree of self-crosslinking in this step of the process depends on the residence time in the closed device or semi-closed device on and temperature of reaction. The skilled in the art would adjust residence time and temperature in order to achieve a degree of crosslinking that is suitable for the following steps of the process.

[0078] The process for preparing polymer (A) is suitably carried out in a semiclosed device when the medium (L) is selected from organic carbonates and ionic liquids (IL) as above defined.

[0079] When the process for preparing polymer (A) is carried out in a semi-closed device whit the medium (L) being selected from ionic liquids (IL), an amount of solvent (S) as above defined can be added to the mixture.

[0080] When the process for preparing polymer (A-XL) according to this variant is carried out in a closed device, polymer (A) may be isolated as an extruded material and then subjected to a lamination step to obtain a film. Said lamination step can be carried out, as an example, by compression molding. During the lamination step, self-crosslinking is completed to provide polymer (A-XL).

[0081] The lamination step to form a fluoropolymer film comprising at least one crosslinked ionic conductive silane-modified fluorinated copolymer [polymer (A-XL)] can be carried out, for example, by lamination between two plates or rolls.

[0082] The present invention provides a process for the preparation of a fluoropolymer film comprising at least one crosslinked ionic conductive silane-modified fluorinated copolymer [polymer (A-XL)], said process comprising:A) providing a polymer (A) by contacting at least one fluoropolymer [polymer (F)] that comprises:- recurring units derived from at least one fluorinated monomer [monomer (FM)], and- recurring units derived from at least one monomer [monomer (FPM)], said monomer (FPM) comprising at least one functional group [group(FX)] selected from the group consisting of: amine, carboxylic acid, thiol and anhydride; with at least a metal compound [compound (M)] of formula (I):X4-mSiYm (I) wherein m is an integer from 1 to 3, each occurrence of Y is a hydrolysable group and each occurrence of X is a hydrocarbon group, wherein at least one X comprises at least one epoxy functional group, so that at least a fraction of the group (FX) of monomer (FPM) of polymer (F) is reacted with at least a fraction of compound (M), wherein the reaction is carried out in the presence of a liquid medium [medium (L)] and of a solvent (S);B) processing polymer (A) in the form of a film, preferably by casting; andC) self-crosslinking the film of polymer (A) to obtain a film of polymer (A-XL).

[0083] At least a percentage of crosslinking of polymer (A) to provide polymer (A- XL) can begin to take place in either step A or step B, and be completed in step C.

[0084] In another embodiment, the present invention provides a process for the preparation of a fluoropolymer film comprising at least one crosslinked ionic conductive silane-modified fluorinated copolymer [polymer (A-XL)], said process comprising:I) providing a polymer (A) by contacting at least one fluoropolymer [polymer (F)] that comprises:- recurring units derived from at least one fluorinated monomer [monomer (FM)], and- recurring units derived from at least one monomer [monomer (FPM)], said monomer (FPM) comprising at least one functional group [group (FX)] selected from the group consisting of: amine, carboxylic acid, thiol and anhydride; with at least a metal compound [compound (M)] of formula (I):X4-mSiYm (I) wherein m is an integer from 1 to 3, each occurrence of Y is a hydrolysable group and each occurrence of X is a hydrocarbon group, wherein at least one X comprises at least one epoxy functional group,so that at least a fraction of the group (FX) of monomer (FPM) of polymer (F) is reacted with at least a fraction of compound (M), wherein the reaction is carried out in the molten state in the presence of a liquid medium [medium (L)];II) processing polymer (A) in the form of a film; andIII) self-crosslinking the film of polymer (A) to obtain a film of polymer (A- XL).

[0085] At least a percentage of crosslinking of polymer (A) to provide polymer (A- XL) can begin to take place in either step I or step II, and be completed in step III.

[0086] The fluoropolymer film obtained by the processes of the invention suitably has a thickness in the range of from 10 to 500 microns.

[0087] Polymer (A) as above defined, obtained by reaction of at least a fraction of the group (FX) of monomer (FPM) of polymer (F) with at least a fraction of compound (M), is characterized by including recurring unit bearing at least one group of formula -SiYm, wherein m is an integer from 1 to 3 and each occurrence of Y is a C1-C10 hydrolyzable group, preferably a C2-C5 hydrolyzable group.

[0088] The films of polymer (A-XL) obtained through the processes described above are endowed with ion conductivity and good mechanical properties; further, they resist swelling and dissolution when contacted with the liquid present in the devices and with organic solvents such as DMF.

[0089] It has been found that incorporating into a fluoropolymer backbone certain monomers bearing at least one silane group that can undergo crosslinking provides crosslinkable fluorinated copolymers that can be self-crosslinked to provide films of polymer (A-XL) endowed with very good mechanical properties.

[0090] Films comprising crosslinked ionic conductive silane-modified fluorinated copolymer [polymer (A-XL)] can be obtained by subjecting the reaction mixture comprising polymer (A) to a step of self-crosslinking, which can be promoted by chemical and / or thermal treatment.

[0091] In order to have self-crosslinking of polymer (A), an acid can be suitably added to the reaction mixture comprising polymer (F) and compound (M).The acid is preferably an organic acid, more preferably selected from formic acid or citric acid.

[0092] The weight ratio between the amount of polymer (A) and the acid added to the reaction mixture to promote self-crosslinking of polymer (A) is suitably comprised in the range of from 1 to 0.01 . This ratio is strictly dependent from the acid used.

[0093] The degree of crosslinking in polymer (A-XL) is related to the amount of - SiYm pendant groups in polymer (A), and thus, accordingly, to the amount of recurring units derived from monomer (FPM) in polymer (F). The higher the amount of -SiYm pendant groups, the higher degree of crosslinking in polymer (A-XL) is reached. And with a polymer (A-XL) having a high degree of crosslinking it is possible to obtain membranes having very good mechanical properties, which can hold high amounts of liquid medium and electrolyte salts.

[0094] That is, a higher degree of crosslinking of polymer (A-XL) allows the preparation of membranes having a lower percentage amount of said polymer (A-XL).

[0095] The Applicant has also demonstrated that the percentage amount of polymer (A) in the mixture to be subjected to self-crosslinking to prepare the fluoropolymer film comprising said polymer (A-XL) is related to the amount of acid that is used to promote the crosslinking. In fact, the higher the percentage of polymer (A) in the mixture at the end of the reaction step a), the lower amount of acid is needed to obtain full self-crosslinking.

[0096] The use of the acid for promoting crosslinking is recommended when the reaction mixture at the end of step a) includes lower than 40% of polymer (A).

[0097] In order to promote self-crosslinking of polymer (A), additional treatments could be applied to the reaction mixture of step a) of the process, such as thermal treatment or exposition to microwave radiations.

[0098] A further object of the present invention pertains to the use of the fluoropolymer film as above defined in electrochemical devices, such as batteries, and in photo-electrochemical devices, such as solar panels and dye-sensitized solar cells (DSSC).

[0099] 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.

[0100] 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.

[0101] EXPERIMENTAL PART

[0102] Raw materials

[0103] Polymer (F-1 ): VDF-AA (0.9% by moles)-HFP (2.4% by moles) polymer having a viscosity of 0.30 l / g in DMF at 25°C.

[0104] Epoxy silane (EPP-1 ): [3-(2,3-epoxypropoxy)propyl]triethoxysilane.

[0105] Catalyst (ATS): Aluminum trifluoromethanesulfonate.

[0106] LiTFSI: bis(trifluoromethanesulfonyl)imide lithium salt.

[0107] Medium (EL-1 ): ethylene carbonate (EC) I propylene carbonate (PC) (1 / 1 by weight).

[0108] Medium (EL-2): solution of LiTFSI (1 mol / L) in ethylene carbonate (EC) I propylene carbonate (PC) (1 / 1 by weight).

[0109] General procedure for preparing extruded material in semi closed device:

[0110] A 15 ml twin screw compounder (DSM Xplore) (Miniextruder) was used. All tests were run at 100rpm.

[0111] General procedure for preparing films by compression molding:

[0112] Films were obtained by compression molding using a Collin P 200 T press. The material was heated at 90°C and pressed at 0 bar for 3 minutes. Then the press was degassed and a pressure of 100 bar as applied for 2 minutes, still at 90°C. After that, the press was allowed to cool down and it was open at about 30-40°C.

[0113] Determination of intrinsic viscosity of polymer (F)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], qris the relative viscosity, i.e. the ratio between the dropping time of sample solution and the dropping time of solvent, qspis the specific viscosity, i.e. qr-1 , and r is an experimental factor, which corresponds to 3 for polymer (F).

[0114] Dissolution test

[0115] A sample of about 5-10 mg of film was placed in about 10 mL of N,N- dimethylformamide (DMF) for 60 minutes at room temperature.

[0116] DMF is a very good solvent for polymer (F). The more crosslinking density of the membrane, the more swelling and less dissolution of the film in DMF is attainable. On the contrary, a poor crosslinked film will lead to a mostly dissolution of the membrane in DMF.

[0117] Example 1 - Manufacture of a film by solvent casting

[0118] The polymer (F-1 ) (0.8 g) was dissolved in 7.2 g of acetone at room temperature thereby providing a solution containing 10% by weight of the polymer (F-1 ). The solution was homogeneous. Then, to this solution were added in the following order: 0.16 g of EPP-1 and 1.6 mg of ATS. The solution was mixed for about 2 minutes and 2.8 g of EL-1 were added to the solution, wherein the weight ratio between the amount of medium (EL-1 ) and polymer (F-1 ) in the reaction mixture was about 3.5. Then, after 2 minutes, it was poured into a petri glass of 80x15 mm and let it overnight under the hood at room temperature to evaporate the acetone. Then a film of about 300 microns was obtained and tested in DMF according to the dissolution method and the result was that no dissolution was observed in DMF, though the film lost its shape due to the excessive swelling of the same.

[0119] The results are shown in Table 1 .

[0120] Comparative Example 2 - Manufacture of a film by solvent casting

[0121] The same procedure of Example 1 was followed, but with the weight ratio between the amount of medium (EL-1 ) and polymer (F-1 ) in the reaction mixture was about 6.0. The film as obtained was totally dissolved in DMF.

[0122] The results are shown in Table 1.

[0123] Example 3 - Manufacture of a film by solvent casting

[0124] The polymer (F-1 ) (0.8 g) was dissolved in 7.2 g of acetone at room temperature thereby providing a solution containing 10% by weight of the polymer (F-1 ). The solution was homogeneous. Then, to this solution were added in the following order: 0.16 g of EPP-1 and 1.6 mg of ATS. The solution was mixed for about 2 minutes and 1 .2 g of EL-1 were added to the solution, wherein the weight ratio between the amount of medium (EL-1 ) and polymer (F-1 ) in the reaction mixture was about 1.5. Then, after 2 minutes, it was poured into a petri glass of 80x15 mm and let it overnight under the hood at room temperature to evaporate the acetone. Then a film of about 300 microns was obtained and tested in DMF according to the dissolution method and the result was that no dissolution was observed in DMF.

[0125] The results are shown in Table 1.

[0126] Example 4- Manufacture of a film by processing in the molten state

[0127] The following ingredients were fed into the miniextruder: Polymer (F-1 ) (7.09 g), EL-1 (10.64 g), ATS (14.2 mg) and EPP-1 (1.42 g) and the mixture was blended for about 4 minutes at 110°C. wherein the weight ratio between the amount of medium (EL-1 ) and polymer (F-1 ) in the reaction mixture was about 1.5. The resulting extruded material was compressed molded in the equipment and conditions described above. Then a film of about 100 microns was obtained and tested in DMF: no dissolution was observed in DMF.

[0128] The results are shown in Table 1.

[0129] Example 5- Manufacture of a film by processing in the molten state

[0130] The same procedure of Example 4 was followed, but with a lower amount of polymer (F-1 ) and with a residence time in the micro-extruder of 10 minutes at 110°C. wherein the weight ratio between the amount of medium (EL-1 ) and polymer (F-1 ) in the reaction mixture was about 2.0. Then a film of about 100 microns was obtained and tested in DMF: no dissolution was observed in DMF.

[0131] The results are shown in Table 1 .

[0132] Example 6 - Manufacture of a film by solvent casting

[0133] The polymer (F-1 ) (0.8 g) was dissolved in 7.2 g of acetone at room temperature thereby providing a solution containing 10% by weight of thepolymer (F-1 ). The solution was homogeneous, wherein the weight ratio between the amount of medium (EL-2) and polymer (F-1 ) in the reaction mixture was about 1.5. Then, to this solution were added in the following order: 0.16 g of EPP-1 and 1.6 mg of ATS. The solution was mixed for about 10 minutes and 1.2 g of EL-2 were added to the solution. Then, after 2 minutes, it was poured into a petri glass of 80x15 mm and let it overnight under the hood at room temperature to evaporate the acetone. Then a film of about 300 microns was obtained and tested in DMF according to the dissolution method and the result was that no dissolution was observed in DMF.

[0134] The results are shown in Table 1 .Table 1* Percentage by weigh of polymer (F) vs the total amount of polymer (F) and medium (L) in the reaction mixture.** Values from o to 10: 0= no dissolution, 10= completely dissolved

Claims

ClaimsClaim 1 . A fluoropolymer film comprising at least one crosslinked ionic conductive silane-modified fluorinated copolymer [polymer (A-XL)], said polymer (A-XL) being obtainable by a process that comprises: a) contacting at least one fluoropolymer [polymer (F)] that comprises:- recurring units derived from at least one fluorinated monomer [monomer (FM)], and- recurring units derived from at least one monomer [monomer (FPM)], said monomer (FPM) comprising at least one functional group [group (FX)] selected from the group consisting of: amine, carboxylic acid, thiol and anhydride; with at least a metal compound [compound (M)] of formula (I):X4-mSiYm (I) wherein m is an integer from 1 to 3, each occurrence of Y is a hydrolysable group and each occurrence of X is a hydrocarbon group, wherein at least one X comprises at least one epoxy functional group, so that at least a fraction of the group (FX) of monomer (FPM) of polymer (F) is reacted with at least a fraction of compound (M), wherein the reaction is carried out in the presence of a liquid medium [medium (L)] and, optionally, of a solvent (S), to obtain a polymer (A); and b) self-crosslinking polymer (A) to obtain polymer (A-XL), wherein the weight ratio between the amount of medium (L) and polymer (F) in the reaction mixture is between 1 and 4.Claim 2. The fluoropolymer film according to claim 1 , wherein the monomer(FM) is selected from the group consisting of:- C2-C8 perfluoroolefins, such as tetrafluoroethylene and hexafluoropropylene (HFP);- C2-C8 hydrogenated fluoroolefins, such as vinyl fluoride, vinylidene fluoride (VDF), 1 ,2-difluoroethylene and trifluoroethylene;- perfluoroalkylethylenes of formula CH2=CH-Rro wherein Rro is a Ci-Ce perfluoroalkyl;- chloro- and / or bromo- and / or iodo-C2-Ce fluoroolefins, such aschlorotrifluoroethylene;- (per)fluoroalkylvinylethers of formula CF2=CFORfi wherein Rn is a Ci-Ce fluoro- or perfluoroalkyl, e.g. CF3, C2F5, C3F7 ;- CF2=CFOXO (per)fluoro-oxyalkylvinylethers wherein Xo is a C1-C12 alkyl group, a C1-C12 oxyalkyl group or a C1-C12 (per)fluorooxyalkyl group having one or more ether groups, such as perfluoro-2-propoxy-propyl group;- (per)fluoroalkylvinylethers of formula CF2=CFOCF2ORf2 wherein Rf2 is a Ci-Ce fluoro- or perfluoroalkyl group, e.g. CF3, C2F5, C3F7 or a Ci-Ce (per)fluorooxyalkyl group having one or more ether groups, such as -C2F5-O- CF3;- functional (per)fluoro-oxyalkylvinylethers of formula CF2=CFOYo wherein Yo is a C1-C12 alkyl group or (per)fluoroalkyl group, a C1-C12 oxyalkyl group or a C1-C12 (per)fluorooxyalkyl group having one or more ether groups and Yo comprising a carboxylic or sulfonic acid group, in its acid, acid halide or salt form; and- fluorodioxoles, preferably perfluorodioxoles.Claim 3. The fluoropolymer film according to anyone of the preceding claims, wherein monomer (FPM) is a monomer of formula (I):wherein:R1 , R2 and R3, equal to or different from each other, are independently selected from a hydrogen atom, a C1-C3 hydrocarbon group and Rx, and Rx is a C1-C20 hydrocarbon moiety comprising at least one functional group [group (FX)] selected from the group consisting of: amine, carboxylic acid group, thiol group and anhydride.Claim 4. The fluoropolymer film according to any one of the preceding claims, wherein the medium (L) is selected from organic carbonates, ionic liquids (IL) and mixtures thereof.Claim 5. The fluoropolymer film according to any one of the preceding claims, wherein the medium (L) further comprises at least one electrolytic salt (ES).Claim 6. The fluoropolymer film according to any one of the preceding claims, wherein the electrolyte salt (ES) is a lithium salt, preferably lithium bistrifluoromethanesulfonimide and / or lithium bis(fluorosulfonyl)imide.Claim 7. The fluoropolymer film according to any one of the preceding claims, wherein the reaction of at least a portion of the polymer (F) with a compound (M) in step a) comprises reacting a polymer (F) with at least a compound (M) in the presence of a liquid medium [medium (L)] and of a solvent (S), to provide a mixture comprising polymer (A).Claim 8. The fluoropolymer film according to claim 7, wherein step a) comprises:- i) a step of providing a solution of at least one polymer (F) as above defined in a solvent (S),- ii) a step of contacting the solution obtained in step i) with at least a compound (M) as above defined, and- iii) a step of adding a liquid medium [medium (L)], to obtain a mixture comprising polymer (A).Claim 9. The fluoropolymer film according to any one of claims 1 to 6, wherein the reaction of at least a portion of the polymer (F) with a compound (M) in step a) comprises reacting a polymer (F) with at least a compound (M) in the presence of a liquid medium [medium (L)] in the molten state, to provide a mixture comprising polymer (A).Claim 10. The fluoropolymer film according to any one of the preceding claims, wherein the fluoropolymer film has a thickness in the range of from 10 to 500 microns.Claim 11 . The fluoropolymer film according to any one of the preceding claims, wherein an acid is added to the reaction mixture comprising polymer (F) and compound (M) in step a).Claim 12. A process for the preparation of the fluoropolymer film according to any one of claims 1 to 8, said process comprising:A) providing a polymer (A) by contacting at least one fluoropolymer [polymer (F)] that comprises:- recurring units derived from at least one fluorinated monomer [monomer (FM)], and- recurring units derived from at least one monomer [monomer (FPM)], said monomer (FPM) comprising at least one functional group [group(FX)] selected from the group consisting of: amine, carboxylic acid, thiol and anhydride; with at least a metal compound [compound (M)] of formula (I):X4-mSiYm (I) wherein m is an integer from 1 to 3, each occurrence of Y is a hydrolysable group and each occurrence of X is a hydrocarbon group, wherein at least one X comprises at least one epoxy functional group, so that at least a fraction of the group (FX) of monomer (FPM) of polymer (F) is reacted with at least a fraction of compound (M), wherein the reaction is carried out in the presence of a liquid medium [medium (L)] and of a solvent (S);B) processing polymer (A) in the form of a film, preferably by casting; andC) self-crosslinking the film of polymer (A) to obtain a film of polymer (A- XL).Claim 13. A process for the preparation of the fluoropolymer film according to claim 9, said process comprising:I) providing a polymer (A) by contacting at least one fluoropolymer [polymer (F)] that comprises:- recurring units derived from at least one fluorinated monomer [monomer (FM)], and- recurring units derived from at least one monomer [monomer (FPM)], said monomer (FPM) comprising at least one functional group [group (FX)] selected from the group consisting of: amine, carboxylic acid, thiol and anhydride; with at least a metal compound [compound (M)] of formula (I):X4-mSiYm (I) wherein m is an integer from 1 to 3, each occurrence of Y is a hydrolysable group and each occurrence of X is a hydrocarbon group, wherein at least one X comprises at least one epoxy functional group, so that at least a fraction of the group (FX) of monomer (FPM) of polymer (F) is reacted with at least a fraction of compound (M), wherein the reaction is carried out in the molten state in the presence of a liquid medium [medium (L)];II) processing polymer (A) in the form of a film; andIII) self-crosslinking the film of polymer (A) to obtain a film of polymer (A- XL).Claim 14. Use of the fluoropolymer film according to anyone of claims 1 to 11 in electrochemical devices, such as batteries, and in photo-electrochemical devices, such as solar panels and dye-sensitized solar cells (DSSC).

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