GELIFIED POLYMER MEMBRANE for LI-ION BATTERY

Abstract

The invention relates to a fluoropolymer film which allows the manufacture of a dense gelled membrane which presents a very good compromise between ionic conductivity and mechanical strength after swelling. This membrane is intended for a separator application for Li-ion batteries.

Description

Title of the invention: GELIFIED POLYMER MEMBRANE for LI-ION BATTERY FIELD OF THE INVENTION

[0001] The present invention relates generally to the field of electrical energy storage in rechargeable secondary batteries of the Li-ion type. More specifically, the invention relates to a fluoropolymer film which allows the manufacture of a dense gelled membrane which has a very good compromise between ionic conductivity and mechanical strength after swelling. This membrane is intended for a separator application for Li-ion batteries. TECHNICAL BACKGROUND

[0002] A Li-ion battery comprises at least one negative electrode or anode coupled to a copper current collector, one positive electrode or cathode coupled to an aluminum current collector, a separator, and an electrolyte. The electrolyte consists of a lithium salt, generally lithium hexafluorophosphate, mixed with a solvent which is a mixture of organic carbonates, chosen to optimize the transport and dissociation of ions. A high dielectric constant promotes the dissociation of ions, and therefore, the number of ions available in a given volume, while a low viscosity is favorable to ionic diffusion which plays an essential role, among other parameters, in the charging and discharging rates of the electrochemical system.

[0003] Rechargeable or secondary batteries are more advantageous than primary (non-rechargeable) batteries because the associated chemical reactions that take place at the positive and negative electrodes of the battery are reversible. The electrodes of secondary cells can be regenerated several times by the application of an electrical charge. Many advanced electrode systems have been developed to store the electrical charge. At the same time, much effort has been devoted to the development of electrolytes capable of improving the capacities of electrochemical cells.

[0004] Located between the two electrodes, the separator acts as a mechanical and electronic barrier and an ionic conductor. There are several categories of separators: dry polymer membranes, gelled polymer membranes and micro- or macroporous separators soaked in liquid electrolyte.

[0005] The separator market is dominated by the use of polyolefins (Celgard® or Solupor®) produced by extrusion and / or stretching. The separators must have low thicknesses, optimal affinity for the electrolyte and resistance sufficient mechanical strength. Among the most interesting alternatives to polyolefins, polymers with better affinity towards standard electrolytes have been proposed, in order to reduce the internal resistances of the system, such as poly(methylmethacrylate) (PMMA), poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoride-hexafluoropropene) (P(VDF-co-HFP)).

[0006] Dry polymer membranes, without liquid solvents, avoid the use of flammable liquid components as in conventional Li-ion batteries and allow the production of thinner and more flexible batteries. However, they have much inferior properties to those of liquid electrolytes, particularly for ionic conductivity. Good conductivity is necessary for the high-speed operation required, for example, by cell phones and satellites.

[0007] Dense gelled membranes are another alternative to separators soaked in liquid electrolyte. Dense membranes are membranes that no longer have any free porosity. They are swollen by the solvent, but the solvent, being strongly chemically bound to the membrane material, has lost all its solvation properties; the solvent then passes through the membrane without carrying any solute. In the case of these membranes, the free spaces correspond to those left between them by the polymer chains and have the size of simple organic molecules or hydrated ions.

[0008] The advantage of dense gelled membranes is that they offer a better guarantee of safety compared to a separator based on liquid electrolyte. In addition, this type of separator may be of interest for cell technologies using a lithium sheet as an anode and thus limit the formation of dendrites on the surface of the anode.

[0009] The difficulty with gelled membranes is to reconcile high ionic conductivity while maintaining sufficient mechanical strength after swelling to allow easy handling of the separator for cell manufacturing and to withstand mechanical stresses during battery charge / discharge cycles. Gelled electrolytes contain a significant fraction of solvents (or plasticizers), in addition to the salt and polymer of the electrolyte itself.

[0010] Poly(vinylidene fluoride) (PVDF) and its derivatives are of interest as the main constituent material of the separator for their electrochemical stability, and for their high dielectric constant which promotes the dissociation of ions and therefore conductivity. The copolymer P(VDF-co-HFP) (copolymer of vinylidene fluoride (VDF) and hexafluoropropylene (HFP)) has been studied as a gelled membrane because it has a lower crystallinity than PVDF. Therefore, the interest of these P(VDF-co-HFP) copolymers is that they allow greater swellings to be achieved and thus promote conductivity.

[0011] Document US 5296318 describes separators based on VDF-HFP copolymers swollen in an electrolyte consisting of a lithium salt (LiPF6) and a mixture of carbonates as solvent. The examples described use Kynar Flex® 2801 and 2750 at 12% and 15% by weight of HFP respectively. More generally, this patent claims an optimal HFP content of between 8% and 25% by weight of HFP. Below 8% HFP, the authors mention difficulties related to the implementation of the membrane. Above 25%, the mechanical strength becomes insufficient after swelling. The manufacturing process of the separator is a solvent process which uses a very volatile solvent, tetrahydrofuran. The ionic conductivity reported in Examples 1 and 2 is 0.3 mS / cm and 0.4 mS / cm, respectively.

[0012] This document describes the need to use an additional crosslinking step, for VDF-HFP copolymer-based separators having an HFP content greater than 25% by weight, in order to strengthen their mechanical strength after swelling. These copolymers give satisfactory results even after heating up to 70°C. However, the plasticized copolymer is soluble in the liquid electrolyte at temperatures above 80°C. Melting of the electrolyte film under constant stress can lead to flow of the electrolyte and an internal short circuit of the battery, which results in rapid discharge and heating.

[0013] In order to solve this problem, document US 2019 / 088916 proposes a non-porous separator containing macromolecular materials which are gellable by an organic solvent in the electrolytic solution, and form a polymer gel electrolyte upon addition of the electrolytic solution. This non-porous separator comprises at least one synthetic macromolecular compound or a natural macromolecular compound, and further comprises, as a matrix, at least one macromolecular material which cannot be gelled by an organic solvent. The examples show that the non-gellable polymer is used in the form of a porous membrane which is soaked with a solution of the gellable polymer. This approach therefore imposes a complex manufacturing step of the porous membrane of the non-gellable polymer, which makes it possible to control the porosity rate and the nature of the porosity (pore size and open porosity rate).In addition, the manufacturing process requires the use of a solvent step to soak the pores of the porous membrane, which has the disadvantage of using solvents and requires an evaporation step.

[0014] Furthermore, this approach requires that the gellable and non-gellable macromolecular materials be of very different chemical nature. This implies a weak adhesion between the gellable and non-gellable macromolecular materials, which can be detrimental to the durability of the battery performance. Finally, the The fraction of gellable polymer, responsible for ionic conductivity, represents only a fraction of the membrane, which does not allow the contact surface of the ionic conductive part of the separator with the electrodes to be optimized.

[0015] There is still a need to develop new gelled separators which present a good compromise between ionic conductivity and mechanical strength after swelling, and which are suitable for simplified implementation, without requiring prior transformation steps.

[0016] The invention therefore aims to remedy at least one of the drawbacks of the prior art, namely to propose a polymer film capable of retaining, after swelling in a solvent, good mechanical strength and good ionic conductivity.

[0017] The invention also aims to provide a method for manufacturing this polymeric film which comprises a single extrusion step from a formulation based on fluoropolymers.

[0018] The invention also relates to gelled polymer membranes comprising said polymeric film, impregnated in an electrolyte consisting of a mixture of solvent(s) and lithium salt(s).

[0019] Another object of the invention is a separator for a Li-ion secondary battery consisting, in whole or in part, of said gelled polymer membrane.

[0020] Finally, the invention aims to provide rechargeable Li-ion secondary batteries comprising such a separator. Summary of the invention

[0021] The technical solution proposed by the present invention is a fluoropolymer film based on a mixture of at least two fluoropolymers having different crystallinities.

[0022] The invention relates firstly to a fluoropolymer film comprising at least one layer, said layer consisting of a mixture of two fluoropolymers: a fluoropolymer A which comprises at least one copolymer of vinylidene fluoride (VDF) and hexafluoropropylene (HFP) having an HFP content greater than or equal to 3% by weight, and a fluoropolymer B which comprises a homopolymer of VDF and / or at least one VDF-HFP copolymer, said fluoropolymer B having a mass content of HFP at least 3% by weight lower than the mass content of HFP of polymer A.

[0023] Fluorinated polymer A comprises at least one VDF-HFP copolymer having an HFP content greater than or equal to 3% by weight, preferably greater than or equal to 8%, advantageously greater than or equal to 13%.

[0024] Its mass rate in the fluorinated film formulation is greater than or equal to 10% by weight and less than or equal to 99%, preferably greater than or equal to 25% and less than or equal to 95%, preferably greater than or equal to 50% and less than or equal to 95%.

[0025] The fluoropolymer B comprises at least one VDF-HFP copolymer having a mass content of HFP at least 3% lower than the mass content of HFP in the polymer A. Its mass content in the fluorinated film formulation is less than or equal to 90% and greater than 1%, preferably it is less than 75% and greater than 5%, and advantageously less than 50% and greater than 5%.

[0026] According to one embodiment, said fluoropolymer film is single-layer.

[0027] According to another embodiment, said fluoropolymer film comprises at least two layers of polymers, at least one of which consists of the mixture of two fluoropolymers described above.

[0028] The invention also relates to a gelled polymer membrane comprising the fluoropolymer film described above, and an electrolyte comprising at least one solvent and at least one lithium salt and optionally at least one additive.

[0029] According to one embodiment, said solvent is chosen from cyclic and acyclic alkyl carbonates, ethers, formates, nitriles, esters and lactones.

[0030] Another object of the invention is a separator for a Li-ion rechargeable battery, consisting, in whole or in part, of the gelled polymer membrane described above.

[0031] Another object of the invention is a Li-ion secondary battery comprising a negative electrode, a positive electrode and a separator, wherein said separator comprises a gelled polymer membrane as described above.

[0032] The present invention makes it possible to overcome the drawbacks of the state of the art. More particularly, it provides a fluoropolymer film capable, after swelling in an electrolyte and in the presence of lithium salts, of providing a separator which combines high ionic conductivity and sufficient mechanical strength to allow easy handling of the separator.

[0033] The advantage of this technology is that it offers a better guarantee of safety compared to a liquid electrolyte-based separator. In addition, this type of separator may be of interest for cell technologies using a lithium sheet as an anode and thus limiting the formation of dendrites on the surface of the anode.

[0034] DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0035] The invention is now described in more detail and in a non-limiting manner in the following description.

[0036] According to a first aspect, the invention relates to a fluoropolymer film comprising at least one layer, said layer consisting of a mixture of two fluoropolymers: a fluoropolymer A which comprises at least one copolymer of vinylidene fluoride (VDF) and hexafluoropropylene (HFP) having an HFP content greater than or equal to 3% by weight, and a fluoropolymer B which comprises a VDF homopolymer and / or at least one VDF-HFP copolymer, said fluoropolymer B having a mass content of HFP at least 3% by weight lower than the mass content of HFP of polymer A.

[0037] According to various embodiments, said film comprises the following characteristics, where appropriate combined. The contents indicated are expressed by weight, unless otherwise indicated.

[0038] According to a first embodiment, said film consists of a single layer.

[0039] Fluorinated polymer A comprises at least one VDF-HFP copolymer having an HFP content greater than or equal to 3% by weight, preferably greater than or equal to 8%, advantageously greater than or equal to 13%. Said VDF-HFP copolymer has an HFP content less than or equal to 55%, preferably 50%.

[0040] This very slightly crystalline copolymer swells easily in electrolyte solvents, which makes it possible to provide the film with good ionic conductivity. The swelling can be quantified by the solidification of the film in electrolyte. Advantageously, the solidification of this copolymer is at least greater than or equal to 5% by weight.

[0041] According to one embodiment, the fluoropolymer A consists of a single VDF-HFP copolymer with an HFP content greater than or equal to 3%. According to one embodiment, the HFP content in this VDF-HFP copolymer is between 13% and 55% inclusive, preferably between 15% and 50% inclusive.

[0042] According to one embodiment, the fluoropolymer A consists of a mixture of two or more VDF-HFP copolymers, the HFP content of each copolymer being greater than or equal to 3%. According to one embodiment, each of the copolymers has an HFP content of between 13% and 55% inclusive, preferably between 15% and 50% inclusive.

[0043] The molar composition of the units in fluorinated polymers can be determined by various means such as infrared spectroscopy or RAMAN spectroscopy. Conventional methods of elemental analysis in carbon, fluorine and chlorine or bromine or iodine elements, such as X-ray fluorescence spectroscopy, make it possible to unambiguously calculate the mass composition of the polymers, from which the molar composition is deduced.

[0044] Multi-nuclear NMR techniques, in particular proton (1H) and fluorine (19F), can also be implemented by analyzing a solution of the polymer in a suitable deuterated solvent. The NMR spectrum is recorded on an FT-NMR spectrometer equipped with a multi-nuclear probe. The specific signals given by the different monomers are then identified in the spectra produced according to one or the other nucleus.

[0045] Fluorinated polymer B comprises at least one VDF-HFP copolymer having a mass content of HFP at least 3% lower than the mass content of HFP in polymer A. This makes it possible to provide the polymer film with sufficient mechanical strength after swelling. Sufficient mechanical strength means that the elastic modulus of the film measured at 23°C after swelling by dynamic mechanical analysis at 1Hz is greater than 0.01 MPa, preferably greater than 0.1 MPa.

[0046] According to one embodiment, the fluoropolymer B is a vinylidene fluoride homopolymer (VDF) or a mixture of vinylidene fluoride homopolymers.

[0047] According to one embodiment, the fluoropolymer B consists of a single VDF-HFP copolymer. According to one embodiment, the HFP content in this VDF-HFP copolymer is between 1% and 5% inclusive. According to another embodiment, the HFP content in this VDF-HFP copolymer is between 1% and 10% inclusive.

[0048] According to one embodiment, the fluoropolymer B is a mixture of PVDF homopolymer with a VDF-HFP copolymer or a mixture of two or more VDF-HFP copolymers.

[0049] According to one embodiment, said mixture comprises: i. a mass content of polymer A greater than or equal to 10% and less than or equal to 99%, preferably greater than or equal to 50% and less than or equal to 95%, advantageously greater than or equal to 25% and less than or equal to 95%, and ii. a mass content of polymer B less than or equal to 90% and greater than 1%, preferably less than 50% and greater than 5%.

[0050] According to one embodiment, said single-layer fluoropolymer film has a thickness of 1 to 1000 μm, preferably of 1 μm to 500 μm, and even more preferably between 5 μm and 100 μm.

[0051] According to one embodiment, when the film is single-layer, said fluoropolymer film can be manufactured by a solvent process. Polymers A and B are solubilized in a known solvent for polyvinylidene fluoride or its copolymers. By way of non-exhaustive examples, mention may be made, as solvent, of n-methyl-2-pyrrolidone, dimethyl sulfoxide, dimethyl formamide, methyl ethyl ketone, acetone. The film is obtained after depositing the solution on a flat substrate and evaporating the solvent.

[0052] According to one embodiment, said fluoropolymer film is a multilayer film of which at least one of the layers is composed of a mixture of polymers A and B according to the invention. The overall thickness of the multilayer film is between 2 μm and 1000 μm, the thickness of the fluoropolymer layer according to the invention being between 1 μm and 999 μm.

[0053] The additional layer(s) are chosen from the following polymeric compositions: - composition consisting of a fluorinated polymer chosen from vinylidene fluoride homopolymers and VDF-HFP copolymers preferably containing at least 90% by mass of VDF; - composition consisting of a mixture of fluoropolymer, chosen from vinylidene fluoride homopolymers and VDF-HFP copolymers preferably containing at least 85% by mass of VDF, with a methyl methacrylate (MMA) homopolymer and copolymers containing at least 50% by mass of MMA and at least one other monomer copolymerizable with MMA. As an example of a comonomer copolymerizable with MMA, mention may be made of alkyl (meth)acrylates, acrylonitrile, butadiene, styrene, isoprene. Advantageously, the MMA polymer (homopolymer or copolymer) comprises by mass from 0 to 20% and preferably 5 to 15% of a C1-C8 alkyl (meth)acrylate, which is preferably methyl acrylate and / or ethyl acrylate. The MMA polymer (homopolymer or copolymer) can be functionalized, that is to say it contains, for example, acid, acid chloride, alcohol, anhydride functions.These functions can be introduced by grafting or by copolymerization. Advantageously, the functionality is in particular the acid function provided by the acrylic acid comonomer. It is also possible to use a monomer with two neighboring acrylic acid functions which can dehydrate to form an anhydride. The proportion of functionality can be from 0 to 15% by mass of the MMA polymer, for example from 0 to 10% by mass.

[0054] According to one embodiment, said fluoropolymer film is manufactured by a process for transforming polymers in the molten state such as flat extrusion, sheath blown extrusion, calendering, thermocompression.

[0055] The invention also relates to a gelled polymer membrane comprising the fluoropolymer film described above, and an electrolyte comprising at least one solvent and at least one lithium salt.

[0056] According to one embodiment, the membrane further comprises inorganic fillers such as silicon oxides, titanium dioxide, aluminum oxides, zirconia.

[0057] According to one embodiment, the membrane further comprises solid electrolytes such as lithium superionic conductors [Lithium superionic conductor (LISICON)] and derivatives, thio-LISICON, structures of the Li4SiO4-Li3PO4 type, sodium superionic conductors and derivatives [Sodium superionic conductor (NASICON)], structures of the Lii 3Al0.3Tii 7(PO4)3 (LATP) type, garnet structures (garnet) Li7La3Zr20i2 (LLZO) and derivatives, perovskite structures Li3xLa2 / 3-2xΠ/ 3 2xTiO3 (0 <x<0,16) (LLTO) et les sulfures amorphes, cristallins ou semi-cristallins.

[0058] According to one embodiment, said solvent is chosen from cyclic and acyclic alkyl carbonates, ethers, glymes, formates, esters and lactones.

[0059] Among the ethers, mention may be made of linear or cyclic ethers, such as dimethoxyethane (DME), methyl ethers of oligoethylene glycols of 2 to 100 oxyethylene units, dioxolane, dioxane, dibutyl ether, tetrahydrofuran, and mixtures thereof.

[0060] Among the esters, mention may be made of phosphoric acid esters and sulfite esters. For example, mention may be made of methyl formate, methyl acetate, methyl propionate, ethyl acetate, butyl acetate or mixtures thereof.

[0061] The glymes used are of general formula RrO-R2-O-R3 where R1 and R3 are linear alkyls of 1 to 5 carbons and R2 a linear or branched alkyl chain of 3 to 10 carbons.

[0062] Among the lactones, we can notably cite gamma-butyrolactone

[0063] Among the nitriles, mention may be made, for example, of acetonitrile, pyruvonitrile, propionitrile, methoxypropionitrile, dimethylaminopropionitrile, butyronitrile, isobutyronitrile, valeronitrile, pivalonitrile, isovaleronitrile, glutaronitrile, methoxyglutaronitrile, 2-methylglutaronitrile, 3-methylglutaronitrile, adiponitrile, malononitrile, and mixtures thereof.

[0064] Among the carbonates, mention may be made, for example, of cyclic carbonates such as, for example, ethylene carbonate (EC) (CAS: 96-49-1), propylene carbonate (PC) (CAS: 108-32-7), butylene carbonate (BC) (CAS: 4437-85-8), dimethyl carbonate (DMC) (CAS: 616-38-6), diethyl carbonate (DEC) (CAS: 105-58-8), methyl ethyl carbonate (EMC) (CAS: 623-53-0), diphenyl carbonate (CAS 102-09-0), methyl phenyl carbonate (CAS: 13509-27-8), dipropyl carbonate (DPC) (CAS: 623-96-1), methyl carbonate and propyl carbonate (MPC) (CAS: 1333-41-1), ethyl propyl carbonate (EPC), vinylene carbonate (VC) (CAS: 872-36-6), fluoroethylene carbonate (FEC) (CAS: 114435-02-8), trifluoropropylene carbonate (CAS: 167951-80-6) or mixtures thereof.

[0065] According to one embodiment, said lithium salt is chosen from: LiPF6 (lithium hexafluorophosphate), LiFSI (lithium bis(fluorosulfonyl)imide), LiTDI (lithium 2-trifluoromethyl-4,5-dicyanoimidazolate), LiPOF2, LiB(C2O4)2, LiF2B(C2O4)2, LiBF4, LiNO3, LiC104.

[0066] According to one embodiment, the electrolyte comprises, in addition to the solvent and the lithium salt, at least one additive. The additive may be selected from the group consisting of fluoroethylene carbonate (FEC), vinylene carbonate, 4-vinyl-1,3-dioxolan-2-one, pyridazine, vinyl pyridazine, quinoline, vinyl quinoline, butadiene, sebaconitrile, alkyl disulfides, fluorotoluene, 1,4-dimethoxytetrafluorotoluene, t-butylphenol, di-t-butylphenol, tris(pentafluorophenyl)borane, oximes, aliphatic epoxides, halogenated biphenyls, methacrylic acids, allyl ethyl carbonate, vinyl acetate, divinyl adipate, propanesultone, acrylonitrile, 2-vinylpyridine, maleic anhydride, methyl cinnamate, phosphonates, vinyl-containing silane compounds, 2-cyanofuran.

[0067] The additive may also be chosen from salts having a melting temperature of less than 100°C such as ionic liquids, which form liquids consisting only of cations and anions.

[0068] As examples of organic cations, mention may be made in particular of the cations: ammonium, sulfonium, pyridinium, pyrrolidinium, imidazolium, imidazolinium, phosphonium, lithium, guanidinium, piperidinium, thiazolium, triazolium, oxazolium, pyrazolium, and mixtures thereof.

[0069] As examples of anions, mention may be made in particular of imides, in particular bis(trifluoromethanesulfonyl)imide (abbreviated NTf2-); borates, in particular tetrafluoroborate (abbreviated BF4-); phosphates, in particular hexafluorophosphate (abbreviated PF6-); phosphinates and phosphonates, in particular alkylphosphonates; amides, in particular dicyanamide (abbreviated DCA-); aluminates, in particular tetrachloroaluminate (A1C14-), halides (such as bromide, chloride, iodide anions, etc.), cyanates, acetates (CH3COO-), in particular trifluoroacetate; sulfonates, in particular methanesulfonate (CH3SO3-), trifluoromethanesulfonate; sulfates, in particular hydrogen sulfate, etc.

[0070] According to one embodiment, said electrolyte has a salt concentration of 0.05 moles / liter to 5 moles / liter in the solvent.

[0071] According to one embodiment, in the membrane according to the invention, the electrolyte / fluorinated polymer ratio is from 0.05 to 20, preferably from 0.1 to 10.

[0072] According to one embodiment, the membrane according to the invention has an ionic conductivity ranging from 0.01 to 5 mS / cm. The conductivity is measured by impedance spectroscopy.

[0073] A conductivity cell is then immersed in each of the solutions and three impedance spectroscopies were carried out. These spectroscopies are carried out between 500 mHz and 100 kHz with an amplitude of 10 mV. The cell constant used is 1.12 and the ionic conductivity is calculated according to the following formula: 1 a = ~X 1.12 Jri

[0074] where R represents the resistance which is obtained by linear regression of the curve Im(Z) = f (Re(Z)). In the particular case of Im(Z) = 0, R is equal to the opposite of the ordinate at the origin divided by the slope coefficient of the linear regression equation.

[0075] According to one embodiment, in the membrane according to the invention, said film has a mass gain at least greater than or equal to 5% by weight, preferably ranging from 10% to 1000%.

[0076] Advantageously, the separator according to the invention is non-porous, which means that the gas permeability of the separator is 0 ml / min, as detected by the gas permeability test (when the surface area of the separator is 10 cm2, the gas pressure difference between the two sides is 1 atm, and the time is 10 minutes).

[0077] According to one embodiment, the gelled polymer membrane is obtained from the succession of the following steps: - Production of a mixture of fluorinated polymers A and B by a melt mixing process such as twin-screw extrusion. - Production of a film by extruding the mixture using a blown film extrusion or flat extrusion process. - Impregnation of the film obtained by immersion in an electrolyte consisting of a solvent and a lithium salt until the film is saturated. The film thus obtained constitutes the gelled membrane intended to be incorporated into a lithium-ion battery cell. A variant of the impregnation step is possible. The film can be placed in the cell in the dry state and the electrolyte added in a second step, the impregnation of the electrolyte in the membrane taking place in-situ in the cell.

[0078] Another subject of the invention is a separator for a Li-ion secondary battery consisting, in whole or in part, of said gelled polymer membrane. According to one embodiment, said separator contains a single gelled polymer membrane according to the invention. According to another embodiment, said separator consists of a multilayer film, each layer of which has the composition of the film according to the invention. Advantageously, in the separator according to the invention, the membrane is not supported by a support.

[0079] Another object of the invention is a Li-ion secondary battery comprising a negative electrode, a positive electrode and a separator, wherein said separator comprises a gelled polymer membrane as described above. EXAMPLES

[0080] The following examples illustrate in a non-limiting manner the scope of the invention.

[0081] Products:

[0082] PVDF 1: Copolymer of vinylidene fluoride (VDF) and vinylidene hexafluoride (HFP) with

[0083] 25% by weight of HFP, characterized by a melt viscosity of 1000 Pa.s at 100 s 1 and 230°C.

[0084] PVDF 2: Copolymer of vinylidene fluoride (VDF) and vinylidene hexafluoride (HFP) with 18% by weight of HFP, characterized by a melt viscosity of 1200 Pa.s at 100 s 1 and 230°C.

[0085] PVDF 3: Homopolymer of vinylidene fluoride characterized by a melt viscosity of 1000 Pa.s at 100 s 1 and 230°C.

[0086] PVDF 4 (Kynarflex 2750-10): Copolymer of vinylidene fluoride (VDF) and vinylidene hexafluoride (HFP) with 15% by weight of HFP characterized by a melt viscosity of 900 Pa.s at 100 s 1 and 230°C

[0087] PVDF 5 (Kynarflex 2801): Copolymer of vinylidene fluoride (VDF) and vinylidene hexafluoride (HFP) with 12% by weight of HFP characterized by a melt viscosity of 2500 Pa.s at 100 s 1 and 230°C

[0088] Lithium salt: Lithium bis(fluorosulfonyl)imide (LiFSI) marketed by Arkema

[0089] Preparation of fluoropolymer blends at different HFP levels and manufacture of films:

[0090] The fluoropolymer blends were made with a Haake® 2 laboratory twin-screw extruder. The films were obtained using a flat extrusion method with a Randcastle laboratory single-screw extruder equipped with a flat die. The thickness obtained is approximately 50 μm for each film.

[0091] Table 1 illustrates the composition of the films prepared according to the invention and of the films for the comparative examples.

[0092] [Tables 1] Example 1 Example 2 Comparative example 1 Comparative example 2 Comparative example 3 Comparative example 4 PVDF 1 75 100 PVDF 2 90 100 PVDF 3 25 10 PVDF 4 100 PVDF 5 100 Impregnation of films in the electrolyte:

[0093] The films were impregnated in an electrolyte consisting of a mixture of ethyl methyl carbonate (EMC) and LiFSI at a concentration of one mole per liter. To do this, a 16 mm diameter disc is cut from the film and then immersed for one hour at 30°C in the electrolyte. The mass gain of the film is measured by the difference between the masses before and after immersion in the electrolyte.

[0094] Measurement of the ionic conductivity of the membrane after swelling in the electrolyte:

[0095] The conductivity is measured by impedance spectroscopy by placing the swollen fluoropolymer film between two electrodes made of lithium sheets. Table 2 illustrates the ionic conductivity and mass setting values of the films after immersion in the EMC +1M LiFSI electrolyte.

[0096] [Tables2] Example 1 Example 2 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4% by weight of HFP in the formulation 18.75 16.2 25 18 15 12 Mass gain (% by weight) 190 200 Dissolved film Dissolved film 86 30 Conductivity (mS / cm) 0.4 0.6 Not measurable Not measurable 0.11 0.04

[0097] Examples 1 and 2 show that a higher conductivity is obtained than the polymers of comparative examples 3 and 4 described in the literature (US5296318). This better conductivity comes in particular from the swelling which is greater.

[0098] In addition, the films of Examples 1 and 2 after swelling retain good mechanical strength unlike Comparative Examples 1 and 2, for which the films dissolve in the electrolyte and are not usable. The good mechanical strength of the swollen films is characterized by the fact that the film remains in the form of an integral and manipulable film, unlike the films of Comparative Examples 1 and 2 which dissolve in the electrolyte.

[0099] Finally, the comparison of Example 1 with Comparative Example 2 shows that it is possible to obtain better properties (mechanical, ionic conductivity) with a formulation according to the invention compared to a fluorinated polymer used in the prior art at the same overall HFP content. Thus, the copolymer of Comparative Example 2 at 18% by weight of HFP is dissolved in the electrolyte, which makes the film unmanipable, and its ionic conductivity unmeasurable.

Claims

Claims

1. Fluorinated polymer film comprising at least one layer, said layer consisting of a mixture of two fluorinated polymers: a fluorinated polymer A which comprises at least one copolymer of vinylidene fluoride (VDF) and hexafluoropropylene (HFP) having an HFP content greater than or equal to 8% by weight, and a fluorinated polymer B which comprises a homopolymer of VDF and / or at least one VDF-HFP copolymer, said fluorinated polymer B having a mass content of HFP at least 3% by weight lower than the mass content of HFP of polymer A.

2. Film according to claim 1, wherein the HFP content in said at least one VDF-HFP copolymer included in the composition of said fluoropolymer A is greater than or equal to 13% and less than or equal to 55%.

3. Film according to claim 1, in which the fluoropolymer A consists of a single VDF-HFP copolymer with an HFP content greater than or equal to 3%.

4. The film of claim 1, wherein the fluoropolymer A consists of a mixture of two or more VDF-HFP copolymers, the HFP content of each copolymer is greater than or equal to 3%.

5. Film according to one of claims 1 to 4, in which the fluoropolymer B is a vinylidene fluoride homopolymer or a mixture of vinylidene fluoride homopolymers.

6. Film according to one of claims 1 to 4, in which the fluoropolymer B consists of a single VDF-HFP copolymer having an HFP content of between 1 and 10%.

7. Film according to one of claims 1 to 6, in which said mixture comprises: i. a mass content of polymer A greater than or equal to 10% and less than or equal to 99%, preferably greater than or equal to 50% and less than or equal to 95%, advantageously greater than or equal to 25% and less than or equal to 95%, and ii. a mass content of polymer B less than or equal to 90% and greater than 1%, preferably less than 50% and greater than 5%.

8. Film according to one of claims 1 to 7, said film consisting of a single layer having a thickness of 1 to 1000 pm, preferably of 1 to 500 pm and more preferably of 5 pm to 100 pm.

9. Film according to one of claims 1 to 7, said film having several layers, at least one of which consists of a mixture of said polymers A and B, the overall thickness of the multilayer film being between 2 μm and 1000 μm.

10. Film according to claim 9, in which the additional layer(s) are chosen from the following polymeric compositions: • composition consisting of a fluoropolymer chosen from vinylidene fluoride homopolymers and VDF-HFP copolymers preferably containing at least 90% by mass of VDF; • composition consisting of a mixture of fluoropolymer, chosen from vinylidene fluoride homopolymers and VDF-HFP copolymers preferably containing at least 85% by mass of VDF, with a methyl methacrylate (MMA) homopolymer and copolymers containing at least 50% by mass of MMA and at least one other monomer copolymerizable with MMA chosen from: alkyl (meth)acrylates, acrylonitrile, butadiene, styrene, isoprene.

11. A gelled polymer membrane comprising the fluoropolymer film according to one of claims 1 to 10, and an electrolyte comprising at least one solvent and at least one lithium salt.

12. The membrane of claim 11, wherein said solvent is selected from cyclic and acyclic alkyl carbonates, ethers, glymes, formates, esters and lactones.

13. Membrane according to one of claims 11 or 12, wherein said lithium salt is chosen from: LiPF6 (lithium hexafluorophosphate), LiFSI (lithium bis(fluorosulfonyl)imide), LiTDI (lithium 2-trifluoromethyl-4,5-dicyano-imidazolate), LiPOF2, LiB(C2O4)2, LiF2B(C2O4)2, LiBF4, LiNO3, LiClO4.

14.

15.

16.

17. Membrane according to one of claims 11 to 13, in which said electrolyte has a salt concentration of 0.05 to 5 moles / liter in the solvent. Membrane according to one of claims 11 to 14, in which the electrolyte / fluoropolymer ratio is from 0.05 to 20 and preferably from 0.1 to 10. Separator for a Li-ion rechargeable battery, consisting of the gelled polymer membrane according to one of claims 11 to 15. Li-ion secondary battery comprising an anode, a cathode and a separator, wherein said separator comprises a gelled polymer membrane according to one of claims 11 to 15.