Cathode composition for lithium ion batteries

Abstract

The present invention relates generally to the field of electrical energy storage in rechargeable accumulators of the Li-ion type. More specifically, the present invention relates to a cathode composition for Li-ion batteries. The present invention also relates to a process for the manufacture of such a cathode composition, and to a Li-ion accumulator comprising such a cathode.

Description

[Technical field]

[0001] The present invention relates generally to the field of electrical energy storage in rechargeable accumulators of the Li-ion type. More specifically, the present invention relates to a cathode composition for Li-ion batteries. The present invention also relates to a process for the manufacture of such a cathode composition, and to a Li-ion accumulator comprising such a cathode. [Background technology]

[0002] Lithium rechargeable batteries can be used as power sources for a variety of electronic devices, from mobile phones, laptops, and small home electronic devices to vehicles and large-capacity energy storage devices, and the demand for lithium rechargeable batteries is constantly increasing.

[0003] For the large-scale development of electric vehicles, it is essential to reduce the manufacturing costs of batteries, and one of the options envisaged is to use low-cost active materials that do not contain cobalt. Furthermore, increasing the gravimetric energy density of electric vehicle batteries remains a major challenge for the mass adoption of this technology. Increasing the thickness of the electrodes and decreasing the size of the particles of the active material would make it possible to achieve these objectives in terms of cost and energy density.

[0004] Increasing the thickness of the electrodes is accompanied by improving the electrode / current collector adhesion. Decreasing the size of the particles is accompanied by an increase in the viscosity of the ink, which precludes the use of the ink in conventional deposition processes for electrode preparation.

[0005] The publication by M. Singh et al. in Journal of The Electrochemical Society, 162(7), A1196-A1201 (2015) showed that a thicker electrode (320 μm) layer for lithium-ion cells shows a favorable electrode / current collector ratio per volume of the battery, making it possible to reduce the manufacturing costs of the cells. However, this approach of thick electrodes, although it may be sufficient for some stationary energy storage applications, would not be suitable for the manufacture of electric vehicles.

[0006] Current industrial equipment for cathode coating imposes a processability window on the viscosity of the ink deposited on the current collector, which is 2000-8000 mPa.s@10s -1 This is because an ink with a viscosity of 2000 mPa.s is an ink that is easy to apply to the current collector. Below 2000 mPa.s, relaxation of the ink is observed during application and drying of the coating, which leads to large variations in thickness and deposition weight. Above 8000 mPa.s, the ink is no longer deposited uniformly.

[0007] Furthermore, in the battery industry, the rheological stability of the ink during storage is a key parameter for optimizing productivity, since the ink must retain its above-mentioned rheological properties during storage for up to 72 hours.

[0008] Furthermore, the roll-to-roll process for the industrial manufacture of electrodes entails a minimum value of adhesion of the deposited dry material onto the current collector. For good mechanical strength of the electrode, this adhesion value, obtained by a 180° peel test, must be greater than 20 N / m. [Prior art documents] [Non-patent literature]

[0009] [Non-Patent Document 1] M. Singh et al., Journal of The Electrochemical Society, 162(7), A1196-A1201 (2015) Summary of the Invention [Problem to be solved by the invention]

[0010] There remains a need to develop binder compositions for cathodes that allow for increased gravimetric energy density in Li-ion batteries for application in electric vehicles while maintaining good mechanical and adhesive properties. [Means for solving the problem]

[0011] Surprisingly, the inventors have discovered that the physical parameters of the polymer binder used in the fabrication of the cathode are critical to the rheological and mechanical performance qualities of the resulting cathode, as on carbon-coated cathode active materials, the present invention demonstrates that an initial haze of the fluoropolymer binder between 45 and 390 NTU allows for optimal processability and mechanical strength of the final electrode.

[0012] The technical solution proposed by the present invention is to provide a cathode composition for a battery, which comprises a fluoropolymer binder, an electrode active material and a conductive material.

[0013] Characteristically, the fluoropolymer binder exhibits a polymer binder initial haze of 45 to 390 NTU, preferably 100 to 300 NTU.

[0014] Characteristically, the electrode active material is coated with a carbon layer.

[0015] The present invention also aims to provide a process for making a cathode composition using a carbon coated active material and a fluoropolymer binder that exhibits an initial haze of 45-390 NTU at a concentration of 7% in NMP.

[0016] Another subject of the invention is a Li-ion accumulator comprising a negative electrode, a positive electrode and a (liquid or solid) electrolyte, the negative electrode being as defined above.

[0017] The present invention makes it possible to overcome the drawbacks of the prior art: it provides a cathode composition for batteries which makes it possible to meet all the specific characteristics necessary to manufacture and obtain high performance cathodes, in terms of mechanical properties and adhesion properties to the current collector, regardless of the nature of the fluoropolymer binder used, as long as its haze is between 45 and 390 NTU at a concentration of 7% in NMP. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The invention will now be explained in more detail and in a non-limiting manner in the following description.

[0019] According to a first aspect, the present invention provides a method for producing a medicament for the treatment of a pulmonary arthritis, comprising: - a fluoropolymer binder (component A), - an electrode active material (component B) and - Conductive material (component C) wherein the polymer binder exhibits an initial haze of 45 to 390 NTU, preferably 100 to 300 NTU, and the electrode active material is coated with a carbon layer.

[0020] According to various embodiments, the electrode composition includes the following features, combined where appropriate: The contents given are expressed by weight unless otherwise indicated.

[0021] <Component A> The polymeric binders used in the present invention are vinylidene difluoride based polymers, generally referred to by the abbreviation PVDF.

[0022] According to one embodiment, the PVDF is a poly(vinylidene fluoride) homopolymer or a mixture of vinylidene fluoride homopolymers.

[0023] According to one embodiment, the PVDF is a poly(vinylidene fluoride) homopolymer or a copolymer of vinylidene difluoride and at least one comonomer that is compatible with vinylidene difluoride.

[0024] According to one embodiment, the PVDF is semi-crystalline.

[0025] Comonomers compatible with vinylidene difluoride may be halogenated (fluorinated, chlorinated or brominated) or non-halogenated.

[0026] Examples of suitable fluorinated comonomers are vinyl fluoride, tetrafluoroethylene, hexafluoropropylene, trifluoropropene, in particular 3,3,3-trifluoropropene, tetrafluoropropene, in particular 2,3,3,3-tetrafluoropropene or 1,3,3,3-tetrafluoropropene, hexafluoroisobutylene, perfluorobutylethylene, pentafluoropropene, in particular 1,1,3,3,3-pentafluoropropene or 1,2,3,3,3-pentafluoropropene, perfluoroalkyl vinyl ethers, in particular those of the general formula Rf-O-CF-CF2, where Rf is an alkyl group, preferably a C1-C4 alkyl group (preferred examples are perfluoropropyl vinyl ether and perfluoromethyl vinyl ether).

[0027] The fluorinated comonomer may contain chlorine or bromine atoms. It may be selected in particular from bromotrifluoroethylene, chlorofluoroethylene, chlorotrifluoroethylene and chlorotrifluoropropene. Chlorofluoroethylene may represent either 1-chloro-1-fluoroethylene or 1-chloro-2-fluoroethylene. The 1-chloro-1-fluoroethylene isomer is preferred. The chlorotrifluoropropene is preferably 1-chloro-3,3,3-trifluoropropene or 2-chloro-3,3,3-trifluoropropene.

[0028] The VDF copolymer may also contain non-halogenated monomers, such as ethylene, and / or acrylic or methacrylic comonomers.

[0029] The fluoropolymer preferably contains at least 50 mole percent vinylidene difluoride.

[0030] According to one embodiment, the PVDF is a copolymer of vinylidene fluoride (VDF) and hexafluoropropylene (HFP) (P(VDF-HFP)), with the weight percentage of hexafluoropropylene monomer units being between 2% and 23% by weight, preferably between 4% and 15% by weight, based on the weight of the copolymer.

[0031] According to one embodiment, the PVDF is a mixture of poly(vinylidene fluoride) homopolymer and VDF-HFP copolymer.

[0032] According to one embodiment, the PVDF is a copolymer of vinylidene fluoride and tetrafluoroethylene (TFE).

[0033] According to one embodiment, the PVDF is a copolymer of vinylidene fluoride and chlorotrifluoroethylene (CTFE).

[0034] According to one embodiment, the PVDF is a VDF-TFE-HFP terpolymer. According to one embodiment, the PVDF is a VDF-TrFE-TFE terpolymer (TrFE is trifluoroethylene). In these terpolymers, the weight content of VDF is at least 10% and the comonomers are present in various proportions.

[0035] According to one embodiment, the PVDF comprises monomer units bearing at least one of the following functional groups: carboxylic acid, carboxylic anhydride, carboxylic ester, epoxy (such as glycidyl), amide, hydroxyl, carbonyl, mercapto, sulfide, oxazoline, phenol, ester, ether, siloxane, sulfonic acid, sulfuric acid, phosphoric acid or phosphonic acid groups. The functional groups are introduced by chemical reaction, which may be grafting or copolymerization, of a fluorinated monomer with a monomer bearing at least one of said functional groups and a vinyl functional group capable of copolymerizing with the fluorinated monomer, according to techniques well known to those skilled in the art.

[0036] According to one embodiment, the functional groups have carboxylic acid functional groups which are (meth)acrylic acid type groups selected from acrylic acid, methacrylic acid, hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate and hydroxyethylhexyl (meth)acrylate.

[0037] According to one embodiment, the unit having a carboxylic acid functionality further comprises a heteroatom selected from oxygen, sulfur, nitrogen and phosphorus.

[0038] According to one embodiment, the functional groups are introduced by a transfer agent used during the synthesis process.The transfer agent is a polymer with a molar mass of less than or equal to 20000 g / mol, with functional groups selected from the following groups: carboxylic acid, carboxylic anhydride, carboxylic ester, epoxy (such as glycidyl), amide, hydroxyl, carbonyl, mercapto, sulfide, oxazoline, phenol, ester, ether, siloxane, sulfonic acid, sulfuric acid, phosphoric acid or phosphonic acid groups.An example of this type of transfer agent is an acrylic acid oligomer.

[0039] The content of functional groups in PVDF is 0.01 mol % or more, preferably 0.1 mol % or more, and 15 mol % or less, preferably 10 mol % or less.

[0040] The PVDF is preferably of high molecular weight. The term "high molecular weight" as used herein is understood to mean a PVDF having a melt viscosity of more than 100 Pa.s, preferably more than 500 Pa.s, more preferably more than 1000 Pa.s and advantageously more than 2000 Pa.s. The viscosity is measured according to standard ASTM D3825 using a capillary or parallel plate rheometer at 232°C for 100 s. -1 The shear gradient is measured at 100 nm. The two methods give similar results.

[0041] The PVDF homopolymers and VDF copolymers used in the present invention can be obtained by known polymerization methods, for example emulsion polymerization.

[0042] According to one embodiment, they are prepared by an emulsion polymerization process in the absence of fluorinated surfactants.

[0043] The polymerization of PVDF generally results in a latex having a solids content of 10% to 60% by weight, preferably 10% to 50% by weight, and a weight average particle size of less than 1 micrometer, preferably less than 1000 nm, preferably less than 800 nm, more preferably less than 600 nm. The weight average size of the particles is generally at least 10 nm, preferably at least 50 nm, advantageously with an average size in the range of 100 to 400 nm. The polymer particles can form weak agglomerates, called secondary particles, whose weight average size is less than 5000 μm, preferably less than 1000 μm, advantageously between 1 and 80 micrometers, preferably between 2 and 50 micrometers. The weak agglomerates can be broken down into separate particles during formulation and application to a substrate.

[0044] According to some embodiments, the PVDF homopolymer and VDF copolymer are composed of bio-based VDF. The term "bio-based" means "obtained from biomass". This allows improving the ecological footprint of the membrane. Bio-based VDF is according to the standard NF EN 16640 14It can be characterized by a content of at least 1 atomic % of renewable carbon, i.e. carbon of natural origin derived from biological material or biomass, as determined by the content of C. The term "renewable carbon" indicates that the carbon is of natural origin and derived from biological material (or biomass), as shown below. According to some embodiments, the biocarbon content of the VDF can be greater than 5%, preferably greater than 10%, preferably greater than 25%, preferably greater than 33%, preferably greater than 50%, preferably greater than 66%, preferably greater than 75%, preferably greater than 90%, preferably greater than 95%, preferably greater than 98%, preferably greater than 99%, advantageously equal to 100%.

[0045] Characteristically, said fluoropolymer binders exhibit an initial haze of 45-390 NTU at 7% concentration in NMP, preferably 100-300 NTU at 7% concentration in NMP. The acronym "NTU" stands for Nephelometric Turbidity Units.

[0046] The haze value is measured using a turbidimeter pre-calibrated with several standard solutions of Stablcal® formazin in the range of 10-800 NTU. The measurement is carried out on a solution of the polymer binder dissolved in N-methylpyrrolidone (NMP) at a weight concentration of 77.5 g / l at a temperature of 25 °C, which is equivalent to a concentration of 7% in NMP (dry substance).

[0047] The polymer binder is dissolved in NMP by any method known to those skilled in the art, such as a pseudoplanetary mixer, a planetary mixer, a roll-type mixer, a disperser, and conventional stirring. The term "initial" refers to the state of the fluoropolymer dissolved in NMP.

[0048] <Component B> The expression "carbon-coated active material" is understood to mean any inorganic lithium insertion compound covered with a graphitic layer ranging from 5 nm to 1 μm, as measured by transmission electron microscopy.

[0049] The size of the elemental particles of the active material is 100 nm to 5 μm as measured by laser particle size analysis.

[0050] According to one embodiment, the active material in the positive electrode is manganese dioxide (MnO2), iron oxide, copper oxide, nickel oxide, lithium / manganese composite oxide (e.g., Li x Mn2O4 or Li x MnO2), lithium / nickel composite oxide (e.g., Li x NiO2), lithium / cobalt composite oxide (e.g., Li x CoO2), lithium / nickel / cobalt composite oxide (e.g., LiNi 1-y Co y O2), lithium / nickel / cobalt / manganese composite oxide (e.g., LiNi x Mn y Co z O2 where x + y + z = 1), lithium-rich lithium / nickel / cobalt / manganese composite oxide (e.g., Li 1+x (Ni x Mn y Co z ) 1-x O2), lithium / transition metal composite oxide, spinel-structured lithium / manganese / nickel composite oxide (e.g., Li x Mn 2-y Ni y O4), high-voltage nickel / manganese composite oxide (e.g., LiMn 1.5 Ni 0.5 -X x O4 where X is selected from Al, Fe, Cr, Co, Rh, and Nd and 0 < x < 0.1), vanadium oxide, S8-type sulfur oxide, and mixtures thereof.

[0051] According to one embodiment, the active material does not contain cobalt. This is selected from LiFePO4, LiMnPO4, LiFe x Mn y PO4, LiFePO4F, LiMnPO4F, and LiFe x Mn y PO4F (x + y = 1).

[0052] <Component C> The conductive materials are selected from carbon black, natural or synthetic graphite, carbon fibers, carbon nanotubes, metal fibers and powders, and conductive metal oxides. Preferentially, they are selected from carbon black, natural or synthetic graphite, carbon fibers and carbon nanotubes.

[0053] The composition by weight of the cathode coating according to the invention is: - Component A in a proportion of 80% to 99%; - Component B in a proportion of 1% to 10%; - Component C in a proportion of 1% to 10% and the sum of these percentages is 100%.

[0054] The present invention also relates to a process for making a cathode composition using a carbon coated active material, a conductive material and a fluoropolymer binder, the process comprising the steps of: - preparation of a polymer binder solution by dissolving component A in N-methylpyrrolidone (NMP) with a dry matter content of 2% to 20%, preferably 2% to 15%, advantageously 4% to 14%, - adding and mixing components B and C to said solution of polymer binder to obtain an electrode formulation (also called ink) that can be applied to a metal support.

[0055] The present invention also relates to a process for producing a Li-ion battery positive electrode, comprising the steps of: - providing an electrode formulation that can be applied to a metal support; - depositing said electrode formulation onto a metal substrate; - consolidating said electrode by thermal treatment (application of a temperature range up to 50°C above the melting point of the polymer without mechanical pressure) and / or by thermo-mechanical treatment such as calendering.

[0056] According to one embodiment, the cathode was manufactured according to the following steps: A 7% by weight solution of the polymer binder in N-methyl-2-pyrrolidone is prepared until the polymer binder is completely dissolved. Super P C65 carbon black (supplied by Timcal) is then added to this solution. The solution is mixed using a mechanical stirrer. The carbon-coated active material is then added. An ink is obtained containing 94 parts by weight of the carbon-coated active material, 3 parts by weight of carbon black, and 3 parts by weight of the binder for 100 parts by weight of the carbon-coated active material / carbon black / binder mixture. The ink obtained is deposited on an aluminum sheet to a wet thickness of 200 μm. The NMP is then evaporated by heating the coated sheet at 90° C. for 15 minutes and then at 150° C. for 30 minutes. A coating having a thickness of 70±10 μm is thus obtained.

[0057] The metal support of the electrode is usually made of aluminum for the cathode. The metal support may be surface treated and may have a conductive primer with a thickness of 5 μm or more. The support may be a woven or non-woven fabric made of carbon fibers.

[0058] Another subject of the invention is a Li-ion accumulator comprising an anode, a cathode and an electrolyte, the cathode being as defined above. EXAMPLES

[0059] The following examples illustrate, but do not limit, the scope of the invention.

[0060] Homopolymer 1: Vinylidene fluoride homopolymer characterized by a viscosity in solution of 4000 mPa.s in NMP Homopolymer 2: Vinylidene fluoride homopolymer characterized by a viscosity in solution of 5000 mPa.s in NMP Homopolymer 3: Vinylidene fluoride homopolymer characterized by a viscosity in solution of 2000 mPa.s in NMP Homopolymer 4: A vinylidene fluoride homopolymer characterized by a viscosity in a solution of 11000 mPa.s in NMP. Homopolymer 5: A vinylidene fluoride homopolymer characterized by a viscosity in a solution of 10000 mPa.s in NMP. Functional copolymer 1: A copolymer of vinylidene fluoride and acrylic acid characterized by a viscosity in a solution of 6000 mPa.s in NMP. Functional copolymer 2: A copolymer of vinylidene fluoride and acrylic acid characterized by a viscosity in a solution of 2000 mPa.s in NMP.

[0061] For all of these products, the dry matter content is 8%.

[0062] <Measurement of the solution viscosity of the polymer binder dissolved in NMP> The viscosity measurement is carried out with a Brookfield DV2T viscometer equipped with an SC4 chamber and a 25 spindle. The temperature is adjusted with a Huber bath at 25 °C.

[0063] <Measurement of haze> The haze of the polymer binder is measured by dissolving the polymer in N-methylpyrrolidone (NMP) at a weight concentration of 77.5 g / l at a temperature of 25 °C.

[0064] The haze value is determined with a Hach 2100 Q turbidimeter calibrated in advance with several standard solutions of Stablcal formazine in the range of 10 - 800 NTU.

[0065] <Measurement of adhesion> The adhesion between the layer formed of the carbon-coated active material / carbon black / binder mixture and the aluminum sheet is measured. To do this, strips with a width of 25 mm are cut out. This strip is subsequently adhesively bonded to a rigid aluminum plate using a double-sided adhesive, the adhesive being deposited on the side of the carbon-coated active material / carbon black / binder coating. A peel test is carried out using an Instron dynamometer of type 34SC1 by fixing in one jaw a rigid aluminum plate and in the other jaw a flexible aluminum sheet on which the deposition has been carried out. In this configuration, the peel angle is 180°. The displacement speed of the jaws is set at 100 mm / min.

[0066] <Measurement of Viscosity of Electrode Formulations that can be Applied to Metallic Substrates> The viscosity of the electrode formulation was measured at 23° C. using a TA HR 10 rheometer. For 100 parts by weight of the carbon-coated active material / carbon black / binder mixture, an ink containing 94 parts by weight of carbon-coated active material, 3 parts by weight of carbon black, and 3 parts by weight of binder is deposited between two parallel plates (diameter 40 mm) separated by 1 mm (=gap 1 mm). -1 ~100s -1 Obtain viscosity values ​​at different shear rates in the range of 10s -1 Values ​​are used to compare between different formulations.

[0067] The performance qualities in terms of adhesion and electrode formulation viscosity of cathodes having compositions according to the invention (Examples 1, 2 and 7) versus Comparative Examples 3, 4 and 5 are shown in Table 1.

[0068] [Table 1]

Claims

1. 1. A cathode composition for a battery, comprising: a fluoropolymer binder (component A), - an electrode active material (component B), and - Conductive material (component C) Including, A cathode composition for a battery, wherein the polymer binder exhibits a polymer binder initial haze of 45 to 390 NTU, and the electrode active material is coated with a carbon layer.

2. 10. The cathode composition of claim 1, wherein the carbon-coated active material comprises an inorganic lithium insertion compound coated with a graphitic layer ranging from 5 nm to 1 μm.

3. 10. The cathode composition of claim 1, wherein the carbon-coated active material is formed of elemental particles having a size of 100 nm to 5 μm as measured by laser particle size analysis.

4. The active material may be manganese dioxide, iron oxide, copper oxide, nickel oxide, lithium / manganese composite oxide, lithium / nickel composite oxide, lithium / cobalt composite oxide, lithium / nickel / cobalt composite oxide, lithium / nickel / cobalt / manganese composite oxide, lithium-enriched lithium / nickel / cobalt / manganese composite oxide, lithium / transition metal composite oxide, spinel-structured lithium / manganese / nickel composite oxide, high-voltage nickel / manganese composite oxide, vanadium oxide, S ８ 10. The cathode composition of claim 1, wherein the oxides of sulfur are selected from the group consisting of sulfur oxides of the types and mixtures thereof.

5. The active material is LiFePO ４ , LiMnPO ４ , LiFe ｘ Mn ｙ P.O. ４ , LiFePO ４ F, LiMnPO ４ F and LiFe ｘ Mn ｙ P.O. ４ 2. The cathode composition of claim 1, wherein x is selected from the group consisting of fluorine, ...

6. 2. The cathode composition of claim 1, wherein Component A is selected from poly(vinylidene fluoride) homopolymers and copolymers of vinylidene difluoride with at least one comonomer selected from vinyl fluoride, tetrafluoroethylene, hexafluoropropylene, 3,3,3-trifluoropropene, 2,3,3,3-tetrafluoropropene, 1,3,3,3-tetrafluoropropene, hexafluoroisobutylene, perfluorobutylethylene, 1,1,3,3,3-pentafluoropropene, 1,2,3,3,3-pentafluoropropene, perfluoropropyl vinyl ether, perfluoromethyl vinyl ether, bromotrifluoroethylene, chlorofluoroethylene, chlorotrifluoroethylene, chlorotrifluoropropene, ethylene, and mixtures thereof.

7. 7. The cathode composition of claim 6, wherein the PVDF comprises monomer units having at least one of the following functional groups: carboxylic acid, carboxylic anhydride, carboxylic ester, epoxy, amide, hydroxyl, carbonyl, mercapto, sulfide, oxazoline, phenol, ester, ether, siloxane, sulfonic acid, sulfate, phosphoric acid, or phosphonic acid group.

8. 10. The cathode composition of claim 1, wherein component C is selected from carbon black, natural or synthetic graphite, carbon fibers, carbon nanotubes, metal fibers and powders, and conductive metal oxides.

9. Composition by weight of: - component A in a proportion of 80% to 99%; component B in a proportion of 1% to 5%; - Component C in a proportion of 1% to 5% (However, the total of these percentages is 100%) 10. The cathode composition of claim 1, wherein

10. 10. A process for producing a cathode composition according to any one of claims 1 to 9, using a carbon-coated active material (component B), a conductive material (component C) and a fluoropolymer binder (component A), comprising the following steps: - preparing a polymer binder solution by dissolving component A in N-methylpyrrolidone (NMP) at a dry matter content of 2% to 20%; - adding and mixing components B and C to said solution of polymer binder to obtain an electrode formulation that can be applied to a metal support; A process involving:

11. A Li-ion storage battery comprising an anode, the cathode according to any one of claims 1 to 9, and an electrolyte.