TFE-based fluoropolymer with excellent metal adhesion properties

Abstract

The present invention relates to VDF-based fluoropolymers with excellent metal adhesion properties for use as binders for electrodes for batteries.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to European Patent Application No. 21211097.7, filed November 29, 2021, the entire contents of which are incorporated herein by reference for all purposes.

[0002] The present invention relates to VDF-based fluoropolymers with excellent metal adhesion properties for use as binders for electrodes for batteries. [Background technology]

[0003] To date, electrodes for lithium secondary batteries are primarily manufactured by a wet process that involves preparing a slurry in which electrode active materials, additives and binders are dispersed in a solvent or aqueous medium, and treating the slurry to form an electrode film.

[0004] Copolymers of vinylidene fluoride (VDF) and tetrafluoroethylene (TFE), with a proportion of VDF of 50-80 mol %, are known for use as binders for the preparation of electrodes for secondary batteries by wet processes. EP 0964464, for example, discloses the use of VDF / TFE copolymers containing repeat units derived from at least 60 mol % VDF in the preparation of an electrode-forming slurry composition comprising an active material and an organic solvent, such as NMP; said electrode-forming slurry composition is coated onto a metal foil to prepare an electrode.

[0005] Dry electrode processes were developed to reduce the time consuming and costly drying procedures required by the wet processes described above.

[0006] Typical dry processes use the fibrillating properties of certain polymers to provide a matrix for the embedded conductive material. Some polymers in the fluoropolymer family, such as polytetrafluoroethylene (PTFE), are particularly inert and stable in common electrode solvents used in secondary batteries, even those with organic solvents at high operating or storage temperatures.

[0007] For example, a dry electrode fabrication process can include combining a PTFE binder with active electrode material in powder form and calendaring to form an electrode film.

[0008] However, although PTFE has good bonding with the electrode active material and can form a free-standing film of the PTFE binder with the active electrode material, it has difficulty adhering to the current collector.

[0009] The applicant has surprisingly discovered that certain VDF-based fluoropolymers exhibit improved adhesion to metals with respect to PTFE, while at the same time having good processability, making them suitable for the fabrication of electrodes by dry processing or extrusion at low temperatures, thereby providing electrodes by a very efficient process. Summary of the Invention

[0010] The binder composition [binder (B)] for use in the preparation of an electrode for an electrochemical device comprises: - a repeating unit derived from vinylidene fluoride (VDF), -Repeating units derived from tetrafluoroethylene (TFE) A VDF-based copolymer [polymer (A)] containing Provided herein is a binder composition [binder (B)], characterized in that the polymer (A) has at least two melting points, one melting point below 220°C and one melting point above 250°C.

[0011] In another aspect, the present invention provides an electrode-forming composition [composition (C)] for use in preparing an electrode for an electrochemical device, comprising: a) at least one electrode active material (AM); b) a binder (B) as defined above; c) optionally, at least one conductive agent; The present invention provides an electrode-forming composition [composition (C)] comprising:

[0012] The applicant has surprisingly discovered that the processability of the binder (B) makes it suitable for fabricating electrodes by dry processing or extrusion at low temperatures, thereby providing electrodes by a very efficient process.

[0013] In another aspect, the present invention relates to a method for producing an electrode for an electrochemical cell [electrode (E)], comprising the steps of: A) providing a VDF-based copolymer [polymer (A)] as defined above; - B) dry mixing, in the absence of a solvent, at least one electrode active material (AM), a polymer (A) as defined above, and, optionally, at least one conductive agent, to provide a dry electrode-forming composition [composition (C)]; - C) feeding the composition (C) obtained in step B) into a press to form a self-supporting dry film; and - D) applying the dry film to a conductive substrate to form an electrode. The present invention provides a method comprising:

[0014] In another aspect, the present invention provides an electrode (E) for a secondary battery, obtainable by a process as defined above.

[0015] In a further aspect, the present invention relates to an electrochemical device, such as a secondary battery or a capacitor, comprising at least one electrode (E) as defined above. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] In the context of the present invention, the term "weight percent" (wt%) indicates the content of a particular component in a mixture, calculated as the ratio between the weight of the particular component and the total weight of the mixture. When referring to repeating units derived from a certain monomer in a polymer / copolymer, weight percent (wt%) indicates the ratio between the weight of the repeating units of that monomer to the total weight of the polymer / copolymer. When referring to the total solids content of a liquid composition, weight percent (wt%) indicates the ratio between the weights of all non-volatile components in the liquid.

[0017] As used herein, the terms "adhere" and "bond" refer to two layers being permanently joined to one another via their contacting surfaces.

[0018] The term "electrochemical device" is intended herein to mean an electrochemical cell / assembly comprising a positive electrode, a negative electrode and a liquid electrolyte, in which a monolayer or multilayer separator is in contact with at least one surface of one of the electrodes. Non-limiting examples of suitable electrochemical devices include, inter alia, secondary batteries, especially alkaline or alkaline earth secondary batteries such as lithium ion batteries, lead acid batteries and capacitors, especially lithium ion-based capacitors and electric double layer capacitors (supercapacitors). Non-limiting examples of electrochemical cells include, inter alia, batteries, preferably secondary batteries, electric double layer capacitors.

[0019] For the purposes of the present invention, "secondary battery" is intended to denote a rechargeable battery. Non-limiting examples of secondary batteries include, in particular, alkaline or alkaline earth secondary batteries.

[0020] Each of the polymers (A) described herein has at least one melting temperature below 220°C, preferably below 180°C, and one melting temperature above 250°C, or even above 300°C.

[0021] The Applicant has surprisingly discovered that when at least one melting point of the polymer (A) is below 220° C., even when at least another higher melting point is present (higher than 250° C., or even higher than 300° C.), the polymer exhibits high adhesion to metals and is therefore suitable for use as a binder for making electrodes with high adhesion to metal current collectors.

[0022] The melting temperature of the polymer (A) can be determined from a DSC curve obtained by differential scanning calorimetry (hereinafter also referred to as DSC).

[0023] The polymer (A) is preferably a semi-crystalline fluororesin.

[0024] The term "semi-crystalline" is intended herein to mean a polymer (A) that contains at least one crystallizable portion and at least one amorphous portion in the backbone and exhibits at least a first order reversible phase transition temperature, e.g. a melting point (solid-liquid transition).

[0025] The polymer (A) is preferably a semi-crystalline fluororesin having multiple endothermic peaks, the heat of fusion of the lower endothermic peak being at least 1 J / g.

[0026] Additionally, polymer (A) contains at least about 5 mole % VDF monomer.

[0027] Polymer (A) may optionally contain at least one other monomer. Suitable comonomers that may be included in polymer (A) include, but are not limited to, ethylene, propylene, isobutylene, fluorinated comonomers such as chlorotrifluoroethylene (CTFE), hexafluoropropylene (HFP), fluorodioxole, fluorodioxalane, perfluoroalkyl ethylene monomers (such as perfluorobutyl ethylene (PFBE), perfluorohexyl ethylene (PFHE) and perfluorooctyl ethylene (PFOE)), and perfluoroalkyl vinyl ether monomers (such as perfluoro(methyl vinyl ether) (PMVE), perfluoro(ethyl vinyl ether) (PEVE), and perfluoro(propyl vinyl ether) (PPVE)). Optional comonomers may be present in polymer (A) in an amount of about 0.001 mol% to about 10 mol%, about 0.01 mol% to about 7 mol%, or about 0.1 mol% to about 5 mol%.

[0028] According to another embodiment, polymer (A) consists essentially of repeat units derived from VDF and TFE.

[0029] Determination of the amount of VDF and TFE monomer repeat units in polymer (A) can be carried out by any suitable method, notably the NMR method.

[0030] The polymer (A) may further comprise other moieties such as defects, end groups, etc., which do not affect or impair its physicochemical properties.

[0031] The polymer (A) is obtained by polymerizing the VDF monomer, TFE and, optionally, further comonomers as defined above, in suspension in an organic medium or in aqueous emulsion, according to procedures known in the literature.

[0032] The procedure for preparing the polymer (A) comprises polymerization in an aqueous medium in the presence of the radical initiators VDF, TFE and, optionally, further comonomers as defined above, optionally in the presence of a chain transfer agent and a dispersing agent in the reaction vessel.

[0033] Generally, the process of the present invention is carried out at a temperature of at least 40°C, preferably at least 50°C, more preferably at least 60°C.

[0034] When the polymerization is carried out in suspension, the polymer (A) is typically provided in the form of a powder.

[0035] When the polymerization to obtain the polymer (A) is carried out in emulsion, the polymer (A) is typically provided in the form of an aqueous dispersion (D), which can be used as obtained directly by emulsion polymerization or after a concentration step. Preferably, the solids content of the polymer (A) in the dispersion (D) is in the range comprised between 10% and 50% by weight.

[0036] The polymer (A) obtained by emulsion polymerization can be isolated from the aqueous dispersion (D) by concentration and / or coagulation of the dispersion and can be obtained in powder form by subsequent drying.

[0037] The polymer (A) of the present invention is preferably obtained by emulsion polymerization in an aqueous polymerization medium, for example according to the procedures described in WO 2018 / 189090, in WO 2018 / 189091 and in WO 2018 / 189092.

[0038] In one embodiment of the present invention, the polymer (A) is advantageously obtained by emulsion polymerization, by a process carried out in the absence of any fluorosurfactant, for example according to the procedure described in WO 2019 / 076901.

[0039] As used herein, the terms "melting point", "melt temperature" and "melting temperature" are intended to define a melting endothermic peak as determined from differential scanning calorimetry (hereinafter also referred to as DSC). When a DSC curve shows multiple melting peaks (endothermic peaks), multiple melting temperatures (Tm) are determined.

[0040] The amount of polymer (A) that can be used in step B) of the method of the present invention is influenced by various factors. One such factor is the surface area and amount of the active material and the surface area and amount of the conductivity-imparting additive added to the electrode-forming composition. These factors are believed to be important because the binder particles provide a bridge between the conductive material particles and keep them in contact.

[0041] For the purposes of the present invention, the term "electrode active material" is intended to mean a compound capable of incorporating or intercalating into its structure and subsequently releasing therefrom alkali or alkaline earth metal ions during the charging and discharging stages of an electrochemical cell. The electrode active material is preferably capable of incorporating or intercalating and releasing lithium ions.

[0042] The nature of the electrode active material in the electrode-forming composition (C) varies depending on whether the composition is used to manufacture a negative electrode (anode) or a positive electrode (cathode).

[0043] When forming a positive electrode for a lithium ion secondary battery, the electrode active material (AM) can include a complex metal chalcogenide of the formula LiMQ2 (wherein M is at least one metal selected from transition metals such as Co, Ni, Fe, Mn, Cr, and V, and Q is a chalcogen such as O or S). Among these, it is preferable to use a lithium-based complex metal oxide of the formula LiMO2 (wherein M is the same as defined above). Preferred examples of these include LiCoO2, LiNiO2, LiNi x Co 1-xO2 (0 < x < 1) and spinel-structured LiMn2O4 can be mentioned.

[0044] As an alternative form, when forming a positive electrode for a lithium-ion secondary battery, further, the electrode active material (AM) has the formula M1M2(JO4) f E 1-f (wherein M1 is lithium and can be partially substituted by another alkali metal corresponding to less than 20% of the M1 metal, M2 is a transition metal of +2 oxidation level selected from Fe, Mn, Ni or mixtures thereof, and can be partially substituted by one or more additional metals of +1 to +5 oxidation levels corresponding to less than 35% of the M2 metal including 0, JO4 is any oxyanion, J is any of P, S, V, Si, Nb, Mo or combinations thereof, E is a fluoride, hydroxide or chloride anion, and f is usually the molar fraction of the JO4 oxyanion included in the range of 0.75 to 1) and may include a lithiated or partially lithiated transition metal oxyanion-based electroactive material.

[0045] M1M2(JO4) as defined above f E 1-f The electroactive material preferably is phosphate-based and may have an ordered or modified olivine structure.

[0046] More preferably, the electrode active material (AM) when forming the positive electrode has the formula Li 3-x M’ y M’’ 2-y (JO4)3 (where 0 ≤ x ≤ 3, 0 ≤ y ≤ 2, M’ and M’’ are the same or different metals, at least one of which is a transition metal, JO4 is preferably PO4 which may be partially substituted by another oxyanion, and J is any of S, V, Si, Nb, Mo or combinations thereof). Even more preferably, the electrode active material has the formula Li(Fe x Mn 1-xLiFePO4 is a phosphate-based electroactive material (i.e., lithium iron phosphate of formula LiFePO4), where 0≦x≦1, and x is preferably 1.

[0047] When forming a negative electrode for a lithium ion secondary battery, the electrode active material (AM) may preferably include one or more carbon-based materials and / or one or more silicon-based materials.

[0048] In some embodiments, the carbon-based material may be selected from graphite, such as natural or artificial graphite, graphene, or carbon black.

[0049] These materials may be used alone or as a mixture of two or more thereof.

[0050] The carbon-based material is preferably graphite.

[0051] The silicon-based compound may be one or more selected from the group consisting of chlorosilanes, alkoxysilanes, aminosilanes, fluoroalkylsilanes, silicon, silicon chloride, silicon carbide, and silicon oxide.

[0052] More specifically, the silicon-based compound may be silicon oxide or silicon carbide.

[0053] When present in the electrode active material, the silicon-based compound is included in an amount ranging from 1 to 60% by weight, preferably from 5 to 30% by weight, based on the total weight of the electroactive compound.

[0054] One or more optional conductivity-imparting additives may be added to improve the electrical conductivity of the resulting electrodes made from the compositions of the present invention. Conductive agents for batteries are known in the art.

[0055] Examples of these may include carbon-based materials such as carbon black, graphite fine powder, carbon nanotubes, graphene, or fibers, or metal fine powders or fibers such as nickel or aluminum. The optional conductive agent is preferably carbon black. Carbon black is available, for example, under the trademarks Super P® or Ketjenblack®.

[0056] When present, the conductive agent is different from the carbon-based material described above.

[0057] The amount of the optional conductive agent is preferably 0 to 30% by weight of the total solids in the electrode-forming composition. In particular, for the cathode-forming composition, the optional conductive agent is typically 0 to 10% by weight, more preferably 0 to 5% by weight, of the total solids in the composition.

[0058] For anode-forming compositions that do not include a silicon based electroactive compound, the optional conductive agent is typically present in an amount of from 0% to 5% by weight, more preferably from 0% to 2% by weight, of the total amount of solids in the composition, while for anode-forming compositions that include a silicon based electroactive compound, it has been found beneficial to incorporate a larger amount of the optional conductive agent, typically from 0.5 to 30% by weight of the total amount of solids in the composition.

[0059] In step B), mixing the electrode active material (AM), the polymer (A) as defined above, and optionally at least one conductive agent is carried out by dry mixing these components without adding any solvent, liquid, processing aid, etc. to the particle mixture. Dry mixing can be carried out, for example, in a mill, mixer or blender (such as a V-blender equipped with a high-strength stirring bar) until a uniform dry mixture is formed. The resulting material can be calendered multiple times to produce a conductive film of desired thickness and density. Those skilled in the art will recognize after reading this specification that the mixing time may vary based on batch size, material, particle size, density, and other properties and still remain within the scope of the present invention.

[0060] In step C) of the process of the present invention, the powdery dry mixture obtained in step B) is subjected to a mechanical compression step to obtain a self-supporting dry film.

[0061] The compaction of the dry mixture obtained in step B) can be carried out as a mechanical compaction, for example by means of a roller compactor or a tablet press, but it may also be carried out as a rolling, build-up or by any other technique suitable for this purpose.

[0062] The mechanical compression step may be associated with a thermal consolidation step. The combination of pressure and heat treatment allows for thermal consolidation at lower temperatures than if it were carried out alone.

[0063] In one embodiment, the mechanical pressing step is carried out by pressing, preferably by pressing the dry mixture obtained in step B) between two metal foils.

[0064] The compression step may be carried out at a temperature below the maximum melting temperature of polymer (A).

[0065] The compression step is conveniently carried out at a temperature below 200°C, preferably below 180°C.

[0066] In step D), the dry film obtained in step C) is applied onto a conductive substrate to form an electrode.

[0067] The sheet of substrate material may comprise a metal foil, in particular an aluminium foil.

[0068] Due to the improved adhesion of the polymer (A), the dry film obtained in step C) can be applied onto a conductive substrate without the need for any primer or adhesive layer.

[0069] The electrode (E) of the present invention is particularly suitable for use in electrochemical devices, in particular in secondary batteries.

[0070] In one aspect, the present invention provides an electrochemical device that is a secondary battery, the secondary battery comprising: -Positive and negative electrodes Including, Here, at least one of the positive electrode and the negative electrode is an electrode (E) according to the present invention.

[0071] Preferably, the electrochemical device comprises: -Positive and negative electrodes A secondary battery comprising: Here, the positive electrode is the electrode (E) according to the invention.

[0072] The secondary battery of the present invention is preferably an alkaline secondary battery or an alkaline earth secondary battery.

[0073] The secondary battery of the present invention is more preferably a lithium ion secondary battery.

[0074] Electrochemical devices according to the present invention can be prepared by standard methods known to those skilled in the art.

[0075] To the extent that the disclosures of any patents, patent applications, and publications incorporated herein by reference conflict with the statements in this application to the extent that any term may be unclear, the statements herein shall control.

[0076] The present invention will now be described with reference to the following examples, the purposes of which are merely illustrative and are not intended to limit the scope of the invention. EXAMPLES

[0077] raw materials PTFE: PTFE homopolymer powder having a specific gravity of 2160, measured according to ASTM D792, and a rheometric pressure of 9.50 MPa, measured according to ASTM D4895; Lithium iron phosphate, LFP, available as Life Power, manufactured by Johnson Matthey; Carbon black available as SC65 available from Imerys SA; Galden HT80 available from Solvay Materials.

[0078] Polymer (A-1) In a 2.2 liter AISI 316 steel vertical autoclave equipped with baffles and agitator operating at 600 rpm, 1.25 liters of demineralized water were introduced. The temperature was then brought to the reaction temperature of 80° C.; once this was reached, 50 g of a solution of c-C6O4 ammonium salt dissolved in demineralized water at a concentration of 35% VDF was introduced, which raised the pressure to 19.5 bar. A pure gas mixture of 20% VDF and 80% TFE in molar amounts was then added via a compressor until a pressure of 20 absolute bar was reached. 20 ml of a solution of ammonium persulfate (APS) in demineralized water at a concentration of 3% by weight were then fed. The polymerization pressure was kept constant by feeding the above mixture; when 150 g of gas had been fed, the reactor was cooled at room temperature, then stripped and the polymer was discharged in latex form.

[0079] Polymer (A-2) The same procedure was followed as for Example Polymer (A-1), with the following exceptions: - Agitator running at 650 Rpm, - the amount of TFE that produces a pressure change of 3.5 bar, Pressurization of the reactor with pure VDF to reach a polymerization pressure of -20 bar. - 20 g of a solution of c-C6O4 ammonium salt dissolved in demineralized water at a concentration of 35%, - A solution of ammonium persulfate (APS) in demineralized water at a concentration of 3% by weight in 30 ml. -Once the reaction has started, the pressure is reduced to 5 bar absolute. Once this pressure was reached, the reactor was repressurized with a mixture of 80% TFE and 20% VDF in molar amounts, then the above mixture was fed in an amount of 50 g.

[0080] Polymer (A-3) The same procedure was followed as for Example Polymer (A-1), with the following exceptions: - Agitator at 650 rpm, -Introduction of VDF, which produces a pressure change of 7.8 bar; - 20 g of a solution of c-C6O4 ammonium salt dissolved in demineralized water at a concentration of 35%, - A solution of ammonium persulfate (APS) in demineralized water at a concentration of 3% by weight in 30 ml. Then a gaseous mixture of VDF / TFE in a molar ratio of 75% / 25% was added via a compressor until a pressure of 20 bar absolute was reached. Once the polymerization started, 250 g of the above mixture was fed into the reactor, then the gas mixture was switched to pure TFE and 250 g of pure TFE was fed into the reactor.

[0081] Polymer (A-4) The same procedure was followed as for the example polymer (A-3), except that the solution of c-C6O4 ammonium salt was not fed into the reactor.

[0082] Polymer (C-1) - Comparative Example The same procedure was followed as for the polymer (A-1) of the example, with the following exceptions: introduction of 1.4 liters of demineralized water, -650rpm stirring, Introduction of VDF, which produces a pressure change of -8 bar; - 30 g of a solution of c-C6O4 ammonium salt dissolved in demineralized water at a concentration of 35%, pressurized to a working pressure of 20 bar absolute by introducing a gas mixture of 80% TFE and 20% VDF in molar quantities, -500 g of the above gas mixture.

[0083] Polymer (C-2) - Comparative Example The same procedure was followed as for comparative polymer (C-1), with the following exceptions: - Use of a 5 liter reactor, - introduction of 3.5 liters of demineralized water, -650rpm stirring, -65℃ working temperature, -Introduction of VDF, which produces a pressure change of 10 bar; - 50 g of a solution of c-C6O4 ammonium salt dissolved in demineralized water at a concentration of 35%, - 35 ml solution of ammonium persulfate (APS) in demineralized water at a concentration of 3% by weight. pressurized to a working pressure of 20 bar absolute by introducing a gas mixture of 80% TFE and 20% VDF in molar quantities, -500 g of the above gas mixture.

[0084] Polymer Post-Treatment The polymers (A-1), (A-2), (A-3), (A-4), (C-1) and (C-2) in powder form are - Aluminum sulfate (Al2(SO4)3) with a concentration of 2 g / l calculated based on the amount of water in the coagulation vessel, - Ammonium carbonate ((NH4)2CO3) at a concentration of 8 g / l calculated based on the amount of water in the coagulation vessel, or -Nitric acid (HNO3), 25 ml of 65% solution calculated based on the amount of water in the coagulation vessel The latexes were obtained by cryogenic or electrolytic coagulation using

[0085] After coagulation had taken place, each polymer was washed with demineralized water at room temperature. After coagulation and washing, the resulting powder was then dried in a ventilated oven at a temperature of at least 80° C. and less than 120° C. for 24 hours.

[0086] The melting temperatures and fusion enthalpies (ΔH) of polymers (A-1) to (A-4), (C-1) and (C-2), characterized by DSC according to the ASTM D3418 standard, are reported in Table 1.

[0087] [Table 1]

[0088] Metal substrate lamination Films containing any of the polymers (A-1) to (A-4), (C-1) and (C-2) were obtained by compressing the powder composition between two aluminum foils. The lamination process was carried out at a temperature below 180° C. and a pressure of 160 bar. The material was left in the heating device for 6 minutes and then pressed at the operating pressure with 6 degassing steps (by manually releasing the pressure) for a total of 180 seconds. The sample was then cooled at room temperature by a cold press. The sample was left in the cold press at the same pressure as in the first step for a period comprised between 4 and 6 minutes.

[0089] Adhesion evaluation and measurement Adhesion evaluation and measurements were carried out between the film obtained as above and the aluminum foil in the three-layer structure (metal foil / film / metal foil) obtained after lamination according to ASTM D 1876. The adhesion values ​​are reported in Table 2.

[0090] [Table 2]

[0091] No adhesion was observed with PTFE when used under the same conditions.

[0092] Processability The processability of the materials was examined using a Capillary Rheometer (Goettefert Rheograph 2003) at shear rates of 10 s-1 and 100 s-1 and a temperature of 180 °C using a die (L / D 10, diameter 1 mm, operating at an angle of 20°) to obtain extrudates of the blends.

[0093] The applicant has surprisingly discovered that polymer (A) may be suitably processed at temperatures well below its higher melting point, thus allowing binder extrudates to be obtained by a convenient process.

[0094] Example 1: Preparation of an electrode using polymer (A-4) A dry mixture of 1.8 g of LFP and 0.1 g of SC65 was prepared by grinding the two powders in a ceramic mortar with a pestle for 2 minutes.

[0095] 1 ml of Galden was added to the powder mixture and the complex was mixed in a Vortex mixer at 20000 rpm for 0.5 minutes. A homogenous paste was obtained.

[0096] 0.1 g of polymer (A-4) was added to the homogenous paste and the mixture was mixed in a Vortex mixer at 16000 rpm for 0.5 min. A homogenous composite was obtained.

[0097] The composite was then mixed in a ceramic mortar with a pestle for an additional 5 minutes to fibrillate the polymer.

[0098] 0.6 g of the complex was finally pressed in a flat press at 24 tonnes for 5 minutes to obtain tablets of 20 mm diameter and 0.9 mm thickness.

[0099] This procedure was repeated to prepare three tablets.

[0100] The tablets were dried in a vacuum oven (Buchi) at 90° C. under reduced pressure for 2 hours to obtain the electrode composition.

[0101] The resulting positive electrode had the following composition by weight: 90% LFP, 5% polymer (A-4) and 5% carbon black. Thus, the electrode EC1 was obtained.

[0102] Example 2 - PTFE A dry mixture of 1.8 g of LFP and 0.1 g of SC65 was prepared by grinding the two powders in a ceramic mortar with a pestle for 2 minutes.

[0103] 1 ml of Galden was added to the powder mixture and the complex was mixed in a Vortex mixer at 20000 rpm for 0.5 minutes. A homogenous paste was obtained.

[0104] 0.1 g of PTFE was added to the homogenous paste and the mixture was mixed in a Vortex mixer at 16000 rpm for 0.5 minutes. A homogenous composite was obtained.

[0105] The composite was then mixed in a ceramic mortar with a pestle for an additional 5 minutes to fibrillate the polymer.

[0106] 0.6 g of the complex was finally pressed in a flat press at 24 tonnes for 5 minutes to obtain tablets of 20 mm diameter and 0.95 mm thickness.

[0107] This procedure was repeated to prepare three tablets.

[0108] The tablets were dried in a vacuum oven (Buchi) at 90° C. under reduced pressure for 2 hours to obtain the electrode composition.

[0109] The positive electrode obtained had the following composition by weight: 90% LFP, 5% PTFE and 5% carbon black. Electrode CE1 was thus obtained.

[0110] Electrical Resistance Measurement Electrical resistance measurements of EC1 and CE1 were carried out in ElCel between two stainless steel plates (18 mm diameter) by applying a perturbation of 10 mV rms at frequencies between 1 Hz and 200 kHz recording 10 points per decade. The electrical resistance results were taken from the intercept of the complex impedance spectrum with the x-axis. The average resistance of the two samples, calculated as the average of the measurements repeated in triplicate, is reported in Table 3. The values ​​are similar for all compositions.

[0111] [Table 3]

[0112] The data show that the binders of the present invention have improved adhesion to current collectors while retaining good electrical properties, and therefore may be suitably used as binders for electrodes.

Claims

1. A binder composition [binder (B)] for use in preparing an electrode for an electrochemical device, comprising: - repeat units derived from vinylidene fluoride (VDF), - a repeating unit derived from tetrafluoroethylene (TFE) A VDF-based copolymer [polymer (A)] containing A binder composition [binder (B)] characterized in that the polymer (A) has at least two melting points, one of which is lower than 220°C and the other of which is higher than 250°C.

2. 2. The binder (B) of claim 1, wherein the polymer (A) has one melting point below 180°C and one melting point above 300°C.

3. 2. The binder (B) of claim 1, wherein the polymer (A) further contains at least one other monomer selected from the group consisting of ethylene, propylene, isobutylene, fluorinated comonomers such as chlorotrifluoroethylene (CTFE), hexafluoropropylene (HFP), fluorodioxole, fluorodioxalane, perfluoroalkyl ethylene monomers (e.g., perfluorobutylethylene (PFBE), perfluorohexylethylene (PFHE), and perfluorooctylethylene (PFOE)), and perfluoroalkyl vinyl ether monomers (e.g., perfluoro(methyl vinyl ether) (PMVE), perfluoro(ethyl vinyl ether) (PEVE), and perfluoro(propyl vinyl ether) (PPVE)).

4. An electrode-forming composition [composition (C)] for use in producing an electrode for an electrochemical device, comprising: a) at least one electrode active material (AM); b) a binder (B) according to claim 1; c) optionally at least one conductive agent; An electrode-forming composition [composition (C)] comprising:

5. A method for producing an electrode [electrode (E)] for an electrochemical cell, comprising the steps of: A) providing a VDF-based copolymer [polymer (A)] according to claim 1; -B) dry-mixing, in the absence of a solvent, at least one electrode active material (AM), said polymer (A) as defined above, and, optionally, at least one conductive agent to provide a dry electrode-forming composition [composition (C)]; -C) feeding the composition (C) obtained in step B) into a press to form a self-supporting dry film; and -D) applying said dry film to a conductive substrate to form said electrode. A method comprising:

6. 6. The method according to claim 5, wherein in step B) the dry mixing is carried out, for example in a mill, mixer or blender, until a homogeneous dry mixture is formed.

7. 6. The process according to claim 5, wherein step C) is carried out at a temperature of not more than 200°C, preferably at a temperature below 180°C.

8. An electrode (E) for a secondary battery obtainable by the method according to claim 5.

9. Electrochemical device, such as a secondary battery or a capacitor, comprising at least one electrode (E) according to claim 8.

10. The electrochemical device is - Positive and negative electrodes A secondary battery comprising:

10. The electrochemical device according to claim 9, wherein at least one of the positive electrode and the negative electrode is the electrode (E) according to claim 8.

11. The electrochemical device is - Positive and negative electrodes A secondary battery comprising:

10. The electrochemical device according to claim 9, wherein the positive electrode is the electrode (E) according to claim 8.