GELLED POLYMER MEMBRANE FOR LI-ION BATTERY

Abstract

The invention relates to a fluoropolymer film which makes it possible to manufacture a gelled dense membrane which offers a very good compromise between ionic conductivity and mechanical strength after swelling. This membrane is intended for use as a separator for Li-ion batteries.

Claims

1. A nonporous fluoropolymer film comprising one or more layers, wherein at least one layer (layer A) consists 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 of greater than or equal to 3% by weight, and b. fluoropolymer B which comprises a VDF homopolymer and/or at least one VDF-HFP copolymer, said fluoropolymer B having a weight content of HFP which is at least 3% by weight lower than the weight content of HFP of the polymer A.

2. The film as claimed in claim 1, wherein the HFP content in said at least one VDF-HFP copolymer forming part of the composition of said fluoropolymer A is greater than or equal to 8% and less than or equal to 55%.

3. The film as claimed in claim 1, wherein the fluoropolymer A consists of a VDF-HFP copolymer having an HFP content of greater than or equal to 3%.

4. The film as claimed in claim 1, wherein 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%.

5. The film as claimed in claim 1, wherein the fluoropolymer B is a homopolymer of vinylidene fluoride or a mixture of homopolymers of vinylidene fluoride.

6. The film as claimed in claim 1, wherein the fluoropolymer B consists of a VDF-HFP copolymer having an HFP content of between 1% and 10%.

7. The film as claimed in claim 1, wherein said mixture comprises: i. a weight content of polymer A of greater than or equal to 10% and less than or equal to 99%, and ii. a weight content of polymer B of less than or equal to 90% and greater than 1%.

8. The film as claimed in claim 1, said film consisting of a single layer having a thickness of 1 to 1000 μm.

9. The film as claimed in claim 1, said film having more than one layer, at least one of the layers consists of a mixture of said polymers A and B, the overall thickness of the film being between 2 μm and 1000 μm.

10. The film as claimed in claim 9, wherein at least one layer other than layer A is chosen from the following polymeric compositions: a composition consisting of a fluoropolymer chosen from vinylidene fluoride homopolymers and VDF-HFP copolymers containing at least 90% by weight of VDF; a composition consisting of a mixture of fluoropolymer and methyl methacrylate polymer, wherein the fluoropolymer is chosen from vinylidene fluoride homopolymers and VDF-HFP copolymers containing at least 85% by weight of VDF, and wherein the methyl methacrylate (MMA) polymer is chosen from MMA homopolymer and MMA copolymers containing at least 50% by weight of MMA and at least one other monomer copolymerizable with MMA chosen from: alkyl (meth)acrylates, acrylonitrile, butadiene, styrene and isoprene.

11. A gelled polymer membrane comprising the fluoropolymer film as claimed in claim 1, and an electrolyte comprising at least one solvent and at least one lithium salt.

12. The membrane as claimed in claim 11, wherein said solvent is chosen from cyclic and acyclic alkyl carbonates, ethers, glymes, formates, esters and lactones.

13. The membrane as claimed in claim 11, wherein said lithium salt is chosen from: LiPF.sub.6 (lithium hexafluorophosphate), LiFSI (lithium bis(fluorosulfonyl)imide), LiTDI (lithium 2-trifluoromethyl-4, 5-dicyanoimidazolate), LiPOF.sub.2, LiB(C.sub.2O.sub.4).sub.2, LiF.sub.2B(C.sub.2O.sub.4).sub.2, LiBF.sub.4, LiNO.sub.3, LiClO.sub.4.

14. The membrane as claimed in claim 11, wherein said electrolyte has a salt concentration of 0.05 to 5 mol/liter in the solvent.

15. The membrane as claimed in claim 11, wherein the electrolyte/fluoropolymers ratio is from 0.05 to 20.

16. A separator for a rechargeable Li-ion battery, consisting of the gelled polymer membrane as claimed in claim 11.

17. A Li-ion storage battery comprising an anode, a cathode and a separator, wherein said separator comprises a gelled polymer membrane as claimed in claim 11.

Description

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0034] The invention is now described in more detail and in a nonlimiting way in the description which follows.

[0035] According to a first aspect, the invention relates to a nonporous 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 of hexafluoropropylene (HFP) having an HFP content of 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 weight content of HFP which is at least 3% by weight lower than the weight content of HFP of the polymer A.

[0036] According to various implementations, said film comprises the following features, if appropriate combined. The contents indicated are expressed by weight, unless otherwise indicated.

[0037] According to a first embodiment, said film consists of a single layer.

[0038] The fluoropolymer A comprises at least one VDF-HFP copolymer having an HFP content of 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 of less than or equal to 55%, preferably less than or equal to 50%.

[0039] This very low-crystallinity copolymer swells readily in electrolyte solvents, thereby allowing the film to be given a good ionic conductivity. The swelling can be quantified by the gain in mass of the film in electrolyte. The gain in mass of this copolymer is advantageously at least greater than or equal to 5% by weight.

[0040] According to one embodiment, the fluoropolymer A consists of a single VDF-HFP copolymer having an HFP content of greater than or equal to 3%. According to one embodiment, the HFP content of this VDF-HFP copolymer is between 13% and 55%, endpoints included, preferably between 15% and 50%, endpoints included.

[0041] 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%, endpoints included, preferably between 15% and 50%, endpoints included.

[0042] The molar composition of the units in the fluoropolymers may be determined by various means such as infrared spectroscopy or Raman spectroscopy. Conventional methods of elemental analysis of carbon, fluorine and chlorine or bromine or iodine elements, such as X-ray fluorescence spectroscopy, make it possible to calculate unambiguously the weight composition of the polymers, from which the molar composition is deduced.

[0043] Use may also be made of multinuclear NMR techniques, notably proton (1H) and fluorine (19F) NMR techniques, by analysis of a solution of the polymer in a suitable deuterated solvent. The NMR spectrum is recorded on an FT-NMR spectrometer equipped with a multinuclear probe. The specific signals given by the various monomers in the spectra produced according to one or another nucleus are then identified.

[0044] The fluoropolymer B comprises at least one VDF-HFP copolymer having a weight content of HFP which is at least 3% lower than the weight content of HFP of the polymer A.

[0045] According to one embodiment, the fluoropolymer B is a vinylidene fluoride (VDF) homopolymer or a mixture of vinylidene fluoride homopolymers.

[0046] According to one embodiment, the fluoropolymer B consists of a single VDF-HFP copolymer. According to one embodiment, the HFP content of this VDF-HFP copolymer is between 1% and 5%, endpoints included. According to another embodiment, the HFP content of this VDF-HFP copolymer is between 1% and 10%, endpoints included.

[0047] According to one embodiment, the fluoropolymer B is a mixture of PVDF homopolymer with a VDF-HFP copolymer or else a mixture of two or more VDF-HFP copolymers.

[0048] According to one embodiment, said mixture comprises: [0049] i. a weight content of polymer A of 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 [0050] ii. a weight content of polymer B of less than or equal to 90% and greater than 1%, preferably less than 50% and greater than 5%.

[0051] According to one embodiment, said monolayer fluoropolymer film has a thickness of 1 to 1000 μm, preferably of 1 μm to 500 μm, and more preferentially still between 5 μm and 100 μm.

[0052] Advantageously, in the fluoropolymer film according to the invention, the fluoropolymers A and B are mixed so as to obtain an intimate mixture of these polymers; in this mixture, each of the polymers A and B is in the molten state.

[0053] According to one embodiment, when the film is a monolayer film, said fluoropolymer film may be manufactured by a solvent-mediated process. Polymers A and B are dissolved in a known solvent for polyvinylidene fluoride or its copolymers. Non-exhaustive examples of the solvent include N-methyl-2-pyrrolidone, dimethyl sulfoxide, dimethylformamide, methyl ethyl ketone and acetone. The film is obtained after the solution is applied to a flat substrate and the solvent evaporated.

[0054] According to one embodiment, said fluoropolymer film is a multilayer film in 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, with the thickness of the fluoropolymer layer according to the invention being between 1 μm and 999 μm.

[0055] The additional layer or layers are chosen from the following polymeric compositions: [0056] a composition consisting of a fluoropolymer chosen from vinylidene fluoride homopolymers and VDF-HFP copolymers containing preferably at least 90% by weight of VDF; [0057] a composition consisting of a mixture of fluoropolymer chosen from vinylidene fluoride homopolymers and VDF-HFP copolymers containing preferably at least 85% by weight of VDF, with a methyl methacrylate (MMA) homopolymer and the copolymers containing at least 50% by weight of MMA and at least one other monomer copolymerizable with MMA. Examples of a comonomer copolymerizable with MMA include alkyl (meth)acrylates, acrylonitrile, butadiene, styrene and isoprene. The MMA polymer (homopolymer or copolymer) advantageously comprises by weight 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) may be functionalized, meaning that it contains, for example, acid, acyl chloride, alcohol or anhydride functions. These functions may be introduced by grafting or by copolymerization. The functionality advantageously is in particular the acid function provided by the acrylic acid comonomer. A monomer may also be used that has two vicinal acrylic acid functions able to undergo dehydration to form an anhydride. The proportion of functionality may be from 0 to 15% by weight of the MMA polymer, for example from 0 to 10% by weight.

[0058] According to one embodiment, said fluoropolymer film is manufactured by a melt-state polymer conversion process such as flat film extrusion, blown film extrusion, calendering or compression molding.

[0059] According to one embodiment, before the step of manufacturing the film, the fluoropolymers A and B are intimately mixed in the molten state, by extrusion using a twin-screw extruder or a co-kneader.

[0060] 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.

[0061] According to one embodiment, the membrane further comprises inorganic fillers such as silicon oxides, titanium dioxide, aluminum oxides or zirconium oxide. The weight content of the inorganic fillers is less than or equal to 25% relative to the weight of the fluoropolymers A and B.

[0062] According to one embodiment, the membrane also comprises solid electrolytes such as lithium superionic conductors [Lithium superionic conductor (LISICON)] and derivatives, thio-LISICONs, structures of Li.sub.4SiO.sub.4—Li.sub.3PO.sub.4 type, sodium superionic conductors and derivatives [Sodium superionic conductor (NASICON)], structures of Li.sub.1.3Al.sub.0.3Ti.sub.1.7(PO.sub.4).sub.3 (LATP) type, garnet structures Li.sub.7La.sub.3Zr.sub.2O.sub.12 (LLZO) and derivatives, perovskite structures Li.sub.3xLa.sub.2/3-2x□1/3-2xTiO.sub.3 (0<x<0.16) (LLTO) and amorphous, crystalline or semicrystalline sulfides. The weight content of the solid electrolytes is less than or equal to 10% relative to the weight of the fluoropolymers A and B.

[0063] According to one embodiment, said solvent is chosen from cyclic and acyclic alkyl carbonates, ethers, glymes, formates, esters and lactones.

[0064] Mention may be made, among the ethers, 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.

[0065] Mention may be made, among the esters, of phosphoric acid esters and sulfite esters. Mention may be made, for example, of methyl formate, methyl acetate, methyl propionate, ethyl acetate, butyl acetate or mixtures thereof.

[0066] The glymes used are of general formula R.sub.1—O—R.sub.2—O—R.sub.3 wherein R.sub.1 and R.sub.3 are linear alkyls of 1 to 5 carbons and R.sub.2 is a linear or branched alkyl chain of 3 to 10 carbons.

[0067] Mention may in particular be made, among the lactones, of gamma-butyrolactone.

[0068] Mention may be made, among the nitriles, for example, of acetonitrile, pyruvonitrile, propionitrile, methoxypropionitrile, dimethylaminopropionitrile, butyronitrile, isobutyronitrile, valeronitrile, pivalonitrile, isovaleronitrile, glutaronitrile, methoxyglutaronitrile, 2-methylglutaronitrile, 3-methylglutaronitrile, adiponitrile, malononitrile and mixtures thereof.

[0069] Mention may be made, among the carbonates, 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), ethyl methyl 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 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.

[0070] According to one embodiment, said lithium salt is chosen from: LiPF.sub.6 (lithium hexafluorophosphate), LiFSI (lithium bis(fluorosulfonyl)imide), LiTDI (lithium 2-trifluoromethyl-4,5-dicyanoimidazolate), LiPOF.sub.2, LiB(C.sub.2O.sub.4).sub.2, LiF.sub.2B(C.sub.2O.sub.4).sub.2, LiBF.sub.4, LiNO.sub.3, LiClO.sub.4.

[0071] According to one embodiment, the electrolyte comprises at least one additive in addition to the solvent and the lithium salt. The additive may be chosen from the group consisting of fluoroethylene carbonate (FEC), vinylene carbonate, 4-vinyl-1,3-dioxolan-2-one, pyridazine, vinylpyridazine, quinoline, vinylquinoline, 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, propane sultone, acrylonitrile, 2-vinylpyridine, maleic anhydride, methyl cinnamate, phosphonates, silane compounds containing a vinyl, and 2-cyanofuran.

[0072] 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 solely of cations and anions.

[0073] Examples of organic cations include in particular the following cations ammonium, sulfonium, pyridinium, pyrrolidinium, imidazolium, imidazolinium, phosphonium, lithium, guanidinium, piperidinium, thiazolium, triazolium, oxazolium, pyrazolium, and mixtures thereof.

[0074] Examples of anions include in particular the imides, especially bis(trifluoromethanesulfonyl)imide (abbreviated NTf2.sup.−); borates, especially tetrafluoroborate (abbreviated BF.sub.4.sup.−); phosphates, especially hexafluorophosphate (abbreviated PF.sub.6.sup.−); phosphinates and phosphonates, especially alkyl-phosphonates; amides, especially dicyanamide (abbreviated DCA.sup.−); aluminates, especially tetrachloroaluminate (AlCl.sub.4.sup.−), halides (such as bromide, chloride or iodide anions), cyanates, acetates (CH.sub.3COO.sup.−), especially trifluoroacetate; sulfonates, especially methanesulfonate (CH.sub.3SO.sub.3.sup.−), trifluoromethanesulfonate; sulfates, especially hydrogen sulfate.

[0075] According to one embodiment, said electrolyte has a salt concentration of 0.05 mol/liter to 5 mol/liter in the solvent.

[0076] According to one embodiment, the electrolyte/fluoropolymers ratio in the membrane according to the invention is from 0.05 to 20, preferentially from 0.1 to 10.

[0077] 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.

[0078] According to one embodiment, the membrane according to the invention exhibits a mechanical strength characterized by an elastic modulus, measured at 1 Hz and 23° C. by dynamic mechanical analysis, of greater than 0.01 MPa, preferentially of greater than 0.1 MPa.

[0079] A conductivity cell is then immersed in each of the solutions and three impedance spectroscopy determinations were carried out. These spectroscopy determinations are carried out between 500 mHz and 100 kHz with an amplitude of 10 mV. The constant of the cell used is 1.12 and the ionic conductivity is calculated according to the following formula:

[00001]$\sigma =\frac{1}{R}⨯1.12$

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

[0080] According to one embodiment, said film in the membrane according to the invention exhibits a gain in mass at least greater than or equal to 5% by weight, preferably ranging from 10% to 1000%.

[0081] The separator according to the invention is advantageously nonporous, meaning 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 cm.sup.2, the difference in pressure of gas on either side is 1 atm, and the time is 10 minutes).

[0082] According to one embodiment, the gelled polymer membrane is obtained from the succession of the following steps: [0083] Production of a mixture of the fluoropolymers A and B by a melt-mixing process such as twin-screw extrusion. [0084] Manufacture of a film by extruding the mixture using a blow-molding or flat extrusion process. [0085] Impregnation of the film obtained by immersion in an electrolyte consisting of a solvent and of 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 into the membrane taking place in-situ in the cell.

[0086] Another subject of the invention is a separator for a Li-ion storage battery consisting, in all or 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 wherein each layer has the composition of the film according to the invention. In the separator according to the invention, the membrane is advantageously not supported by a support.

[0087] Another subject of the invention is a Li-ion storage battery comprising a negative electrode, a positive electrode and a separator, wherein said separator comprises a gelled polymer membrane as described above.

EXAMPLES

[0088] The following examples non-limitingly illustrate the scope of the invention.

[0089] Products:

[0090] PVDF 1: Copolymer of vinylidene fluoride (VDF) and of vinylidene hexafluoride (HFP) containing 25% by weight of HFP, characterized by a melt viscosity of 1000 Pa.Math.s at 100 s.sup.−1 and 230° C.

[0091] PVDF 2: Copolymer of vinylidene fluoride (VDF) and of vinylidene hexafluoride (HFP) at 18% by weight of HFP, characterized by a melt viscosity of 1200 Pa.Math.s at 100 s.sup.−1 and 230° C.

[0092] PVDF 3: Vinylidene fluoride homopolymer, characterized by a melt viscosity of 1000 Pa.Math.s at 100 s.sup.−1 and 230° C.

[0093] PVDF 4 (Kynarflex 2750-10): Copolymer of vinylidene fluoride (VDF) and of vinylidene hexafluoride (HFP) at 15% by weight of HFP, characterized by a melt viscosity of 900 Pa.Math.s at 100 s.sup.−1 and 230° C.

[0094] PVDF 5 (Kynarflex 2801): Copolymer of vinylidene fluoride (VDF) and of vinylidene hexafluoride (HFP) at 12% by weight of HFP, characterized by a melt viscosity of 2500 Pa.Math.s at 100 s.sup.−1 and 230° C.

[0095] Lithium Salt: Lithium bis(fluorosulfonyl)imide (LiFSI) sold by Arkema

[0096] Preparation of the Mixtures of Fluoropolymers with Different HFP Contents and Manufacture of the Films:

[0097] The fluoropolymer mixtures were produced with a Haake® 2-type laboratory twin-screw extruder.

[0098] 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.

[0099] Table 1 illustrates the composition of the films prepared according to the invention and of the films for the comparative examples.

TABLE-US-00001 TABLE 1 Comparative Comparative Comparative Comparative Example 1 Example 2 Example 1 Example 2 Example 3 Example 4 PVDF 1 75 100 PVDF 2 90 100 PVDF 3 25 10 PVDF 4 100 PVDF 5 100

[0100] Impregnation of the Films Into the Electrolyte:

[0101] The films were impregnated in an electrolyte consisting of a mixture of ethyl methyl carbonate (EMC) and LiFSI at a concentration of one mol per liter. To do this, a disk 16 mm in diameter is cut out of the film and then immersed for one hour at 30° C. in the electrolyte. The gain in mass of the film is measured by the difference between the masses before and after immersion in the electrolyte.

[0102] Measurement of the Ionic Conductivity of the Membrane After Swelling in the Electrolyte:

[0103] 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 gain in mass values of the films after immersion in the EMC+1M LiFSI electrolyte.

TABLE-US-00002 TABLE 2 Comparative Comparative Comparative Comparative Example 1 Example 2 Example 1 Example 2 Example 3 Example 4 % by weight of 18.75 16.2 25 18 15 12 HFP in the formulation Gain in mass 190 200 Dissolved Dissolved 86 30 (% by weight) film film Conductivity 0.4 0.6 Not Not 0.11 0.04 (mS/cm) measurable measurable

[0104] Examples 1 and 2 show that a higher conductivity than the polymers of comparative examples 3 and 4 described in the literature (U.S. Pat. No. 5,296,318) is obtained. This better conductivity comes in particular from the swelling which is greater.

[0105] In addition, the films of examples 1 and 2 after swelling retain good mechanical strength, contrary to comparative examples 1 and 2, for which the films dissolve in the electrolyte and cannot be used. The good mechanical strength of the swollen films is characterized by the fact that the film remains in the form of an intact and manipulable film, contrary to the films of comparative examples 1 and 2, which dissolve in the electrolyte.

[0106] 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 fluoropolymer used from the prior art at the same overall content of HFP. Thus, the copolymer of comparative example 2 at 18% by weight of HFP is dissolved in the electrolyte, which makes the film non-manipulable, and its ionic conductivity non-measurable.