Patent Description:
Fluoropolymers are known in the art to be suitable as binders for the manufacture of electrodes and as separator coatings for the manufacture of composite separators for use in electrochemical devices such as secondary batteries.

Generally, techniques for manufacturing either positive or negative electrodes and also for coating separators involve the use of organic solvents for dissolving the fluoropolymer.

In the case of the manufacture of electrodes, the role of the organic solvent is typically to dissolve the fluoropolymer in order to bind the electro-active material particles to each other and to the metal collector upon evaporation of the organic solvent.

The fluoropolymer binder should properly bind the electro-active material particles to each other and to the metal collector so that these particles can chemically withstand large volume expansion and contraction during charging and discharging cycles.

In the manufacture of separators, a precursor solution is typically formulated as an ink or paste comprising a solid particulate material dispersed in a solution of a fluoropolymer binder in a suitable solvent.

The ink solution so obtained is usually disposed onto a surface of a non-coated inert support and the solvent is then removed from the solution layer to deposit a separator layer which adheres to the inert support. A solvent system is typically used to disperse the polymer binder, which generally comprises N-methyl pyrrolidone (NMP) or mixtures of N-methyl pyrrolidone and a diluting solvent such as acetone, propyl acetate, methyl ethyl ketone and ethyl acetate.

PVDF is the most widely used fluoropolymer in electrode binders. For instance, <CIT> discloses a polymeric electrolyte for use in lithium batteries comprising a vinylidene fluoride-hexafluoropropylene copolymer or a copolymer further comprising recurring units of at least one compound selected from the group consisting of acrylic acid and maleic acid monoalkylester.

Requirements to assure the good performance of an electrode are the good adhesion of the electrode towards the current collector and the good flexibility of the resulting electrode.

Similarly, a separator having a good adhesions of the coating onto the inert support and a good flexibility assures good performances.

This is of particular importance in applications, such as flexible batteries, wherein the electrochemical device components must be able to withstand bending while maintaining an excellent adhesion to guarantee cell operation.

For obtaining a good quality electrode deposited on the current collector a low viscosity electrode slurry is desired. This makes the manufacturing process easier.

It has been now surprisingly found that electrodes and separator with the desired properties described above are suitably provided by using a combination of at least two fluoropolymers having two distinctive characteristics.

In a first instance, the present invention pertains to a composition (C) comprising:.

In another aspect, the present invention provides a process for the preparation of the composition (C) as above detailed.

In a further aspect, the present invention provides a process for the preparation of an electrode comprising the steps of:.

In still another aspect, the present invention provides a method for the manufacture of a composite separator, notably suitable for use in an electrochemical device, said method comprising the following steps:.

The present invention further provides electrodes and separators for electrochemical devices comprising the composition (C) as defined above, and electrochemical devices comprising the same.

The Applicant has surprisingly found that electrochemical device components prepared by the use of the composition (C) as above detailed possess improved flexibility while maintaining an excellent adhesion to metal current collectors and to separators.

Furthermore, electrode slurry mixtures comprising the composition of the invention are characterized by a lower viscosity in comparison with slurries prepared by the use of compositions comprising exclusively a semi-crystalline fluoropolymer, which makes the coating of the current collectors easier.

Unless otherwise specified, in the context of the present invention the amount of a component in a composition is indicated as the ratio between the weight of the component and the total weight of the composition multiplied by <NUM> (also: "wt%").

As used herein, the term "semi-crystalline" means a fluoropolymer that has, besides the glass transition temperature Tg, at least one crystalline melting point on DSC analysis. For the purposes of the present invention a semi-crystalline fluoropolymer is hereby intended to denote a fluoropolymer having a heat of fusion of from <NUM> to <NUM> J/g, preferably of from <NUM> to <NUM> J/g, more preferably of from <NUM> to <NUM> J/g, as measured according to ASTM D3418-<NUM>.

As used herein, the terms "adheres" and "adhesion" indicate that two layers are permanently attached to each other via their surfaces of contact, e.g. classified as 5B to 3B in the cross-cut test according to ASTM D3359, test method B.

As used herein, the term "electrode" indicates a layer comprising a binder, generally formed of polymeric materials, and an electro-active compound.

For the purpose of the present invention, the term "electro-active compound" is intended to denote a compound which is able to incorporate or insert into its structure and substantially release therefrom alkaline or alkaline-earth metal ions during the charging phase and the discharging phase of an electrochemical device. The electro-active compound is preferably able to incorporate or insert and release lithium ions.

By the term "separator", it is hereby intended to denote a porous polymeric material which electrically and physically separates electrodes of opposite polarities in an electrochemical device and is permeable to ions flowing between them.

By the term "electrochemical device", it is hereby intended to denote an electrochemical cell comprising a positive electrode, a negative electrode and a liquid electrolyte, wherein a monolayer or multilayer separator is adhered to at least one surface of one of said electrodes.

Non-limitative examples of suitable electrochemical devices include, notably, secondary batteries, especially, alkaline or an alkaline-earth secondary batteries such as lithium ion batteries, and capacitors, especially lithium ion-based capacitors and electric double layer capacitors ("supercapacitors").

For the purpose of the present invention, by "secondary battery" it is intended to denote a rechargeable battery. Non-limitative examples of secondary batteries include, notably, alkaline or alkaline-earth secondary batteries.

By the term "recurring unit derived from vinylidene difluoride" (also generally indicated as vinylidene fluoride <NUM>,<NUM>-difluoroethylene, VDF), is intended to denote a recurring unit of formula (I):.

The hydrophilic (meth)acrylic monomer (MA) preferably complies with formula (II) here below:
<CHM>
wherein each of R<NUM>, R<NUM> and R<NUM>, equal to or different from each other, is independently a hydrogen atom or a C<NUM>-C<NUM> hydrocarbon group.

Non-limitative examples of hydrophilic (meth)acrylic monomers (MA) include, notably:.

Still more preferably, the hydrophilic (meth)acrylic monomer (MA) is acrylic acid (AA).

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

In the rest of the text, the expression "fluorinated monomers" is understood, for the purposes of the present invention, both in the plural and the singular, that is to say that they denote both one or more than one fluorinated monomers as defined above.

Should the fluorinated monomer (FM) comprise at least one hydrogen atom, it is designated as hydrogen-containing fluorinated monomer.

Should the fluorinated monomer (FM) be free of hydrogen atoms, it is designated as per(halo)fluorinated monomer.

The fluorinated monomer (FM) may further comprise one or more other halogen atoms (Cl, Br, I).

Non-limiting examples of suitable fluorinated monomers (FM) include, notably, the followings:.

The fluorinated monomer (FM) is preferably hexafluoropropylene (HFP).

The inventors have found that best results are obtained when the polymer (F1) is a linear semi-crystalline co-polymer.

The term "linear" is intended to denote a co-polymer made of substantially linear sequences of recurring units from (VDF) monomer and (MA) monomer; polymer (F1) is thus distinguishable from grafted and/or comb-like polymers.

Determination of mole percentage of recurring units derived from at least one functional hydrogenated monomer in the polymer (F1) can be performed by any suitable method. Mention can be notably made of acid-base titration methods or NMR methods.

The polymer (F1) more preferably comprises recurring units derived from:.

The polymer (F1) is typically obtainable by emulsion polymerization or suspension polymerization of at least one vinylidene difluoride monomer and at least one hydrophilic (meth)acrylic monomer.

The polymer (F2) is typically obtainable by emulsion polymerization or suspension polymerization of at least one vinylidene difluoride monomer and at least one fluorinated monomer (FM).

In the case polymer (F2) contains recurring units derived from at least one hydrophilic (meth)acrylic monomer (MA), said recurring units (MA) are preferably comprised in an amount at least <NUM> % by moles, more preferably at least <NUM>% by moles, even more preferably at least <NUM>%, and at most <NUM>% by moles, more preferably at most <NUM>% by moles, even more preferably at most <NUM>% by moles with respect to the total moles of recurring units of polymer (F2).

The inventors have found that a substantially random distribution of monomer (MA) within the polyvinylidene fluoride backbone of polymer (F1) advantageously maximizes the effects of the monomer (MA) on both adhesiveness and/or hydrophilic behaviour of the resulting copolymer, even at low levels of monomer (MA) in the composition, without impairing the other outstanding properties of the vinylidene fluoride polymers, e.g. thermal stability and mechanical properties.

At least another fluorinated monomer (FM2) different from (FM) and of VDF may be included in polymers (F1) and (F2).

Such monomer (FM2) can include at least one conventionally used monomer copolymerizable with vinylidene fluoride, such as, but not limited to, vinyl fluoride, trifluoroethylene, trifluorochloroethylene (CTFE), tetrafluoroethylene (TFE), hexafluoropropylene (HFP), and fluoroalkyl vinyl ether and their mixtures.

The amount of monomer (FM2) in polymer (F1) and in polymer (F2) is preferably below <NUM> mol%, more preferably below <NUM> mol% or below <NUM> mol% over the total number of moles of recurring units in polymer (F1) or polymer (F2), respectively.

In a preferred embodiment of the invention, (F1) is a copolymer of VDF-MA in which the content of hydrophilic (meth)acrylic monomer of formula (I) is comprised in an amount of between <NUM> to <NUM> mole% with respect to the total moles of recurring units of polymer (F1).

More preferably, the hydrophilic (meth)acrylic monomer (MA) is a hydrophilic (meth)acrylic monomer of formula (II), still more preferably it is acrylic acid (AA), and (F1) is a VDF-AA copolymer.

In a preferred embodiment of the invention, (F2) is a copolymer of VDF with a fluorinated monomer, wherein.

the fluorinated monomer (FM) is hexafluoropropylene (HFP) and (F2) is a VDF-HFP copolymer.

In composition (C) polymer (F1) preferably forms at least the <NUM>% by weight over the total weight of composition (C), and polymer (F2) forms at most <NUM>% by weight over the total weight of composition (C).

Composition (C) is typically provided in the form of powder.

In another aspect, the present invention provides a process for the preparation of the composition (C) as above detailed, said process comprising the step of mixing the at least one polymer (F1) with the at least one polymer (F2).

In a preferred embodiment of the present invention, polymer (F2) has an intrinsic viscosity measured in dimethylformamide at <NUM> which is lower than that of polymer (F1), preferably lower than <NUM> dl/g, more preferably lower than <NUM> dl/g.

The metal substrate is generally a foil, mesh or net made of a metal such as copper, aluminum, iron, stainless steel, nickel, titanium or silver.

Generally, techniques for manufacturing an electrode involve the use of solvents, e.g. organic solvents, such as N-methyl-<NUM>-pyrrolidone (NMP), for dissolving VDF polymer binders and homogenizing them with a powdery electrode material and all other suitable components to produce a paste to be applied to a metal collector (e.g. an aluminium collector). Non-limiting examples of electrodes and methods for their manufacturing are described in <CIT>, and references cited therein.

Under step ii. of the process of the invention, suitable non-aqueous solvents include, notably, the followings: N-methyl-<NUM>-pyrrolidone (NMP), N,N-dimethylformamide, N,N-dimethylacetamide, dimethylsulfoxide, hexamethylphosphamide, dioxane, tetrahydrofuran, tetramethylurea, triethyl phosphate, trimethyl phosphate and mixtures thereof.

Under step iii. of the process of the invention, the electrode slurry mixture may further comprise:.

Non-limitative examples of suitable additives include, notably, electroconductivity-imparting additives and/or thickeners.

In step iv. of the process of the invention, the electrode slurry mixture provided by step iii. is applied onto at least one surface of a metal substrate by techniques commonly known in the art such as by casting, brush, roller, ink jet, squeegee, foam applicator, curtain coating, vacuum coating, spraying.

In still another aspect, the present invention provides a method for the manufacture of a composite separator notably suitable for use in an electrochemical device, said method comprising the following steps:.

In step III. of the method of the invention, the composition (C) is typically applied onto at least one surface of the porous substrate by a technique selected from casting, spray coating, roll coating, doctor blading, slot die coating, gravure coating, inkjet printing, spin coating and screen printing, brush, squeegee, foam applicator, curtain coating, vacuum coating.

Non-limitative examples of suitable porous substrate include, notably, porous membranes made from inorganic, organic and naturally occurring materials, and in particular made from nonwoven fibers (cotton, polyamides, polyesters, glass), from polymers (polyethylene, polypropylene, poly(tetrafluoroethylene), poly(vinyl chloride), and from certain fibrous naturally occurring substances (e.g. asbestos).

Advantageous results have been obtained when the porous support was a polyolefin porous support, e.g. a polyethylene or a polypropylene porous support.

of the process of the invention, the coating composition layer is dried.

Drying temperature is typically comprised between <NUM> and <NUM>, preferably between <NUM> and <NUM>.

The invention will be now described with reference to the following examples, whose purpose is merely illustrative and not intended to limit the scope of the invention.

Polymer (F1): VDF-AA (<NUM>% by moles) polymer having an intrinsic viscosity of <NUM> dl/g in DMF at <NUM> (hereinafter Polymer (F1)-A).

Polymer (F1): VDF-AA (<NUM>% by moles) polymer having an intrinsic viscosity of <NUM> dl/g in DMF at <NUM> (hereinafter Polymer (F1)-B).

Polymer (F2): VDF-HFP (<NUM>% by moles) polymer having an intrinsic viscosity of <NUM> dl/g in DMF at <NUM>.

Determination of intrinsic viscosity of the polymer (in DMF at <NUM>) Intrinsic viscosity [η] was determined using the following equation on the basis of the dropping time, at <NUM>, of a solution obtained by dissolving polymer (F1) or polymer (F2) in dimethylformamide at a concentration of about <NUM>/dl, in an Ubbelhode viscosimeter: <MAT>.

Four compositions comprising different amounts of polymer (F1)-A or polymer (F1)-B and polymer (F2) were prepared in a Henschel mixer, model FML <NUM> with a command of MINICON S210, for <NUM> at 1500rpm. The powders were all charged at room temperature.

Composition (a) comprises <NUM>% by weight of polymer (F1)-A and <NUM>% by weigh of polymer (F2) with respect to the total weight of the composition. Composition (b) comprises <NUM>% by weight of polymer (F1)-A and <NUM>% by weigh of polymer (F2) with respect to the total weight of the composition. Composition (c) comprises <NUM>% by weight of polymer (F1)-A) and <NUM>% by weigh of polymer (F2) with respect to the total weight of the composition. Composition (d) comprises <NUM>% by weight of polymer (F1)-B and <NUM>% by weigh of polymer (F2) with respect to the total weight of the composition.

Electrode slurry mixtures were prepared by dissolving, respectively, <NUM> gram of powder of composition (a), of composition (b), of composition (c), of composition (d), of polymer (F1)-A, of polymer (F1)-B and of polymer (F2) in <NUM> grams of NMP under stirring at room temperature, obtaining clear solutions. Under moderate stirring, using a Dispermat mixing device, <NUM> gram of carbon black (C-NERGY Super C65 from Imerys) and <NUM> grams of LiCoO<NUM> (Cellcore® LCO D10 from Umicore) were added and the slurries were thoroughly mixed to ensure a good homogeneity.

The percentage of solid in the electrode slurries was of <NUM>% by weight, the composition (C) representing the <NUM>% by weight of the total solid components, carbon black being the <NUM>% by weight and LiCoO<NUM> being the <NUM>% by weight.

This was measured using a rotational rheometer Model Rheolab® QC from Anton Paar. The measurements were performed at <NUM>. The viscosity values are reported at a shear rate of <NUM>,<NUM>-<NUM>.

The cathode was prepared by casting on an AI metal foil the slurry previously prepared. The coating was finally dried in vacuum oven at <NUM>° C for enough time to ensure solvent removal. The coating thickness was set in order to obtain a final electrode thickness around <NUM>.

Peeling tests were performed by following the standard ASTM D903 to evaluate the adhesion of the electrode mixture coating on the metal foil.

Electrode flexibility was evaluated through a Mandrel Bend Test Method, derived from ASTM D <NUM>-<NUM>. Test strips of the electrode properly sized and conditioned, were bent <NUM>° over a <NUM>-mm diameter mandrel (rod) several times until cracks become visible in the electrode. The higher the number of bending times, the higher is the electrode flexibility.

The results show that the compositions according to the present invention (compositions (a) to (d)) have good values of adhesion to the electrode, comparable to that of polymer (F1) alone.

Claim 1:
A composition (C) comprising:
- at least one semi-crystalline fluoropolymer [polymer (F1)] comprising recurring units derived from vinylidene fluoride (VDF) in an amount of at least <NUM> % by mole with respect to the total moles of recurring units of polymer (F1), and recurring units derived from at least one functional hydrogenated monomer comprising at least one hydrophilic (meth)acrylic monomer (MA) of formula (I):
<CHM>
wherein:
- R<NUM>, R<NUM> and R<NUM>, equal to or different from each other, are independently selected from a hydrogen atom and a C<NUM>-C<NUM> hydrocarbon group, and
- ROH is a hydrogen atom or a C<NUM>-C<NUM> hydrocarbon moiety comprising at least one hydroxyl group,
in an amount of at least <NUM> % by mole, preferably at least <NUM> % by mole, even more preferably at least <NUM> % by mole, and not more than <NUM> % by mole with respect to the total moles of recurring units of polymer (F1),
said polymer (F1) having an intrinsic viscosity measured in dimethylformamide at <NUM> higher than <NUM> dl/g, preferably higher than <NUM> dl/g, even more preferably higher than <NUM> dl/g and lower than <NUM> dl/g; and
- at least one fluoropolymer [polymer (F2)], different from (F1), comprising recurring units derived from vinylidene fluoride (VDF) in an amount of at least <NUM> % by mole with respect to the total moles of recurring units of polymer (F2), and recurring units derived from at least one fluorinated monomer (FM) different from vinylidene fluoride in an amount of between <NUM> % by mole and <NUM>% by mole % with respect to the total moles of recurring units of polymer (F2),
wherein polymer (F1) forms at least the <NUM>% by weight over the total weight of the composition (C) and polymer (F2) forms at most <NUM>% by weight over the total weight of the composition (C).