Method for the extraction of vinylidene fluoride polymers

Abstract

The present invention relates to a method for the extraction of vinylidene difluoride (VDF) polymers from a solid composition the method comprising: i) providing a solid composition comprising one or more VDF polymer ii) contacting said solid composition comprising one or more VDF polymer with a supercritical fluid in the presence of one or more polar aprotic co-solvent within a pressurized vessel at a pressure P above 120bar, preferably between 150 and 1500bar thereby dissolving said one or more VDF polymer and forming a VDF polymer solution and an extracted solid residue. The method can be used as a part of a recycling process for spent secondary batteries.

Description

DescriptionMethod for the extraction of vinylidene fluoride polymersTechnical Field

[0001] This application claims priority from the European Patent Application 24168854.8 filed on 2024-04-05, the whole content of this application being incorporated herein by reference for all purposes.

[0002] The invention pertains to a method for the extraction of vinylidene fluoride (VDF) based polymers from solid compositions comprising said polymers using a supercritical fluid in combination with a polar aprotic co-solvent at relatively high pressure. The method allows to selectively extract the VDF polymer(s) as a VDF polymer solution from a solid composition comprising the polymer and other materials, and to obtain the polymer(s) in pure form as a solid precipitate.

[0003] The method can be applied to any composition comprising VDF polymer, but it is particularly to extract VDF polymer(s) from solid compositions wherein the amount of VDF polymer(s) is relatively small with respect to the total amount of solid composition. The method can be used for example to extract and purify VDF polymers used in secondary batteries (e.g. in the electrodes or separators) directly from crushed discarded batteries or part of batteries. This represents a useful technology which allows to recover and reuse one of the components of spent secondary batteries in the context of a battery recycling process.

[0004] Notably the method of the invention is nondestructive for the solid composition so that the extracted solid residue resulting after the extraction of the VDF polymer(s) from the solid composition can be treated in subsequent steps in order to recover other valuable materials. For example in the case mentioned above wherein the solid composition comprising one or more VDF polymer is a crushed spent secondary battery the extracted solid residue may comprise valuable materials such as metals, metal oxides mixtures, metal salts, electrolytes, carbon derivatives and more which canbe recovered from said extracted residue and reused in the manufacturing of new batteries or in other applications.Background Art

[0005] The growing use of rechargeable batteries (also known as “secondary batteries”), typically Li-ion batteries, to power electric and electronic equipment including electric vehicles has increased the need to develop effective processes to recover the active materials from batteries which have reached the end of their life cycle in order to reuse them in new batteries or in other applications.

[0006] One critical aspect of battery recycling is that batteries consists of many different intermixed materials including metals (e.g. Al, Cu, Ni), metal oxides / salts (e.g. Li, Co, Ni, Mn, Fe), graphite, electrolytes and other materials. All these materials need to be separated from each other to allow recycling. Polymeric binders are typically used in secondary batteries to keep this material mixture together, in particular binders are used to bind the electrode active materials (i.e. the combination metals, metal oxides and salts which are the most valuable components in a battery) among them and to the current collectors.

[0007] In order to setup a process for recovering and reusing said battery constituent materials, the polymeric binder needs to be removed in order to “unbind” the other materials enabling easier recovery processes for the electrode active materials which are no longer bound by the polymeric binder and are therefore easier to separate..

[0008] Typically polymeric binders in secondary batteries include and often consist of polymers and copolymers of vinylidene fluoride (herein “VDF polymers”). Besides their use as binders, VDF polymers are also often used in secondary batteries in the coating of the battery separators.

[0009] In traditional processes for the recovery of electrode active materials from spent batteries, the polymeric binder is typically destroyed via thermal treatment (pyrolysis). Pyrolysis destroys the VDF polymers and also causes emission of harmful gases such as HF which require treatment, therefore battery recycling processes involving pyrolysis are not preferred.

[0010] VDF polymers are valuable polymers and there is an interest to recover them for reusing them, there is therefore a need for nondestructive methods which allow the extraction of VDF polymers from materials derived from spent secondary batteries.

[0011] In some cases processes have been developed wherein the polymeric binders are extracted by dissolving them with large amounts of solvents and then recovered via evaporation of the solvents. A drawback of these methods is that a large amount of solvents is needed to extract VDF polymers. Also in many cases upon solvent evaporation the polymer does not precipitate from the solution but rather forms a gel from which the extraction of a pure polymer is difficult or impossible.

[0012] Supercritical fluids as supercritical CO2 have little or no capacity to dissolve VDF polymers when used alone. For example US2023257490 to Arkema describes a method in which VDF polymers are treated with supercritical CO2 in order to increase their purity. The document evidences how the VDF polymer are not dissolved at all when exposed to the supercritical fluid. According to the document, the supercritical fluid is able to strip some residual impurities from already purified VDF polymers.

[0013] WO2023187035 to Solvay Specialty Polymers Italy describes a process in which spent lithium batteries are treated with supercritical fluids in a pressurized reactor at a relatively mild pressure. In this case the supercritical fluid, upon release of pressure, causes the separation of the current collector from the electrode materials. Also in this document no effect of the supercritical fluid in dissolving the polymeric binders is reported.

[0014] The present invention aims at addressing the problems of the methods described above allowing the extraction of VDF polymer(s) from a solid composition comprising them, by using a supercritical fluid in combination with a polar aprotic co-solvent at a relatively high pressure.

[0015] Polar aprotic co-solvents when used alone are known as good solvents for VDF polymers, however they can only solubilize a relatively small amount of polymer and require to be heated to temperatures typically above 60- 70°C in order to be effective.

[0016] In the method of the present invention it was surprisingly found that it is possible to extract all or most of the VDF polymer(s) comprised in a solid composition by contacting said solid composition with a polar aprotic cosolvent in combination with a supercritical fluid (e.s. supercritical CO2) within a pressurized vessel at a relatively high pressure. Surprisingly the polar aprotic co-solvent and the supercritical fluid act synergistically extracting and dissolving the VDF polymer(s) resulting in a VDF polymer solution.

[0017] This VDF polymer(s) solution can be easily separated from the residual extracted material still within the pressurized vessel. Pure VDF polymer(s) can be precipitated from this VDF polymer solution in several ways. The preferred method according to the invention is to simply reduce the pressure in the vessel venting out the supercritical fluid (which can be recovered in a closed circuit). In this case the expansion of the gas induces a decrease in the temperature of the VDF polymer solution which causes the precipitation of the VDF polymer(s). Alternatively the VDF polymer solution can be evaporated or mixed with a non solvent for the polymer(s) to recover the VDF polymer as a solid.Summary of invention

[0018] The present invention relates to a method for the extraction of vinylidene difluoride (VDF) polymers from a solid composition the method comprising: i) providing a solid composition comprising one or more VDF polymer ii) contacting said solid composition comprising one or more VDF polymer with a supercritical fluid in the presence of one or more polar aprotic cosolvent within a pressurized vessel at a pressure P above 120bar, preferably between 150 and 1500bar thereby dissolving said one or more VDF polymer and forming a VDF polymer solution and an extracted solid residue.Detailed description of the Invention

[0019] As mentioned in the introduction, the method of the present invention is a method for the extraction of vinylidene difluoride polymers and copolymers from solid composition comprising such polymers.

[0020] In a first step of the method of the invention a solid composition comprising one or more VDF polymers is provided.

[0021] For “VDF polymer” it is intended a polymer or copolymer comprising a majority of recurring units derived from 1 ,1 , difluoro ethylene (vinylidene difluoride, or VDF). VDF based polymers suitable for use in the present invention comprise at least 50%, preferably at least 70%, more preferably at least 80%, most preferably at least 90% by moles of recurring units derived from VDF. All percentages are based on the total amount of recurring units in the polymer.

[0022] VDF polymers in the present invention may comprise 0.1 -10%, more preferably 0.2-7.5%, even more preferably 0.2-5% by moles, of recurring units which are derived from the polymerization of ethylenically unsaturated monomers including a polar group, preferably a carboxylic group. Examples of these non-fluorinated comonomers are notably hydrophilic (meth)acrylic monomers.

[0023] The hydrophilic (meth)acrylic monomer preferably complies to formula:wherein each of R1 , R2, R3, equal or different from each other, is independently a hydrogen atom or a C1-C3 hydrocarbon group, and ROH is a hydrogen or a C1-C5 hydrocarbon moiety comprising at least one hydroxyl group. Non limitative examples of hydrophilic (meth)acrylic monomers are notably acrylic acid, methacrylic acid, hydroxyethyl (meth)acrylate, hydroxypropyl(meth)acrylate; hydroxyethylhexyl (meth)acrylate, methyl(meth)acrylate; glycidyl(meth)acrylate.

[0024] The hydrophilic (meth)acrylic monomer is more preferably selected from: - hydroxyethylacrylate (HEA) of formula:- 2-hydroxypropyl acrylate (HPA) of either of formulae:- acrylic acid (AA) of formula:- and mixtures thereof.

[0025] Most preferably, the hydrophilic (meth)acrylic monomer is AA and / or HEA.

[0026] VDF polymers for use in the present invention may include other recurring units derived from other fluorinated comonomers such as vinylfluoride (VF1 ), chlorotrifluoroethylene (CTFE), hexafluoropropene (HFP), tetrafluoroethylene (TFE), trifluoroethylene (TrFE), (per)fluoroalkylvinylethers, perfluoroalkylethylenes (among these perfluorobutylethylene is preferred), (per)fluorodioxoles as described in U.S. Pat. No. 5,597,880, or mixtures thereof.

[0027] VDF polymers for use in the present invention may include other recurring units derived from non fluorinated comonomers such as ethylene, propylene, butylene, isopropylene, isobutylene and mixtures thereof.

[0028] A solid composition comprising one or more VDF polymers in the present invention is intended as any material which comprises one or more VDF polymers in a mixture with other materials. Such other materials can be of any nature, such as metals, metal oxides, metal salts, carbon derivatives, other polymers, liquid or solid electrolytes, organic materials and so on. The method of the invention is capable of extracting VDF polymers from a solid composition even if the composition only contains a small amount of such polymers, however the method of the invention also works well when the solid composition comprises a large amount of VDF polymers or consist essentially of VDF polymers. For example solid composition of the invention may contain 0.1 -99% by weight of VDF polymers.

[0029] The method of the invention is particularly useful when the solid composition comprising one or more VDF polymers is a material resulting from batteries (e.g. Li ion batteries) which have reached their end of life or a material which comes from industrial scraps deriving from the production of batteries such as e.g. production residues or off spec materials. As known to the skilled person, when a battery has reached its end of life and is, in other words, a “spent” battery (i.e. a battery which cannot be recharged anymore and / or cannot provide a sufficient amount of energy for a sufficient amount of time when recharged), it must be recycled so to recover the valuable materials contained therein. The same can be said for industrial scraps deriving from the production of batteries.

[0030] The most common processes to recover valuable materials from spent batteries include shredding the entire battery or some components of the battery in small pieces or in a powder, and then attempting to selectively dissolve or filter out the individual components using a combination of hydrometallurgical processes. However, as it can be read in the recent article “A critical review of current technologies for the liberation of electrode” (Science of the Total Environment 766 (2021 ) 142382 - Elsevier B.V.) there are still many competing methods, each one with its advantages and disadvantages, and the biggest challenge still resides in recovering materials which are sufficiently pure for being reused in new batteries, with a low environmental impact and a cost which is competitive with using newmaterials instead of recovered ones. As it can be appreciated from the cited article, as of today, a “standard” method for recycling materials from spent batteries has not yet been developed.

[0031] The method of the present invention is expected to provide a useful contribution in one of the critical parts of the recovery process i.e. the extraction of certain polymeric components from the spent batteries thus offering a contribution to reduce the environmental impact of the lifecycle of the batteries, in particular of lithium based secondary batteries.

[0032] VDF Polymers are used in batteries typically as binders for the electrode materials. VDF polymers are a common choice due to their stability, mechanical properties and chemical resistance. A typical composition for an electrode material includes 80% to 99% by weight of electro active materials, and 1 to 20%wt of binder, based on the total weight of electrode material. Sometimes VDF polymers are used in secondary batteries also as structural components within the battery or as coating for the separator.

[0033] As mentioned above in some cases batteries or some of their constituent parts can be shredded or pulverized generating the so-called “black mass”. In other cases batteries can be disassembled and their most valuable components, typically the electrode active materials (i.e. the mixture of metal oxides and binders which is distributed on the current collector) is scraped from the current collector.

[0034] In all cases these materials recovered from spent batteries can be used as starting solid composition if they comprise one or more VDF polymer.

[0035] As mentioned above a solid composition for use in the present invention contains may in general contain 0.1 -99% by weight of VDF polymers. However the method of the invention is particularly suitable for the extraction of VDF polymers from compositions which comprise a small amount of polymer, therefore, preferably a solid composition for use in the present invention comprises from 0.1 to 20%, more preferably from 0.2 to 10%, even more preferably from 0.3 to 5% by weight of VDF polymers. Also preferably a solid composition for use in the present invention comprises from 1 to 99%, preferably from 10 to 90% by weight of metals and metal oxides.

[0036] In order to facilitate diffusion of the supercritical fluid and of the co-solvent within the particles of the solid composition and obtain a faster extraction it is preferred that the solid composition of the invention is provided in finely divided form, such as pulverized form. However the method of the invention can also be applied using the solid composition in any other form such as pellets, granules, layers, films and more. In general, the method will require more time if the solid material is provided in a form which has a higher surface to volume ratio.

[0037] In a second step of the process of the present invention the solid composition comprising one or more VDF polymer is contacted with one or more supercritical fluid in the presence of one or more polar aprotic cosolvent.

[0038] For “supercritical fluid” it is intended a fluid in supercritical conditions, i.e. at a pressure (P) and temperature (T) above its critical point defined by a critical temperature (Tc) and pressure (Pc). The supercritical fluid can be preferably selected so to have a certain affinity for the binder, in terms of polarity, and to be non-reactive with any of the components of the electrode. Preferred supercritical fluid in the method of the present invention have a critical density in the range 0.470-0.220 g / ml. Examples of more preferred supercritical fluids to be used herein are CO2, H2O, CH3OH, CH3CH2OH, (CH3)2CO, N2O. The use of supercritical NH3 and in general of chemical compounds having basic properties although per se effective, is not advisable it can negatively interact with VDF based polymers. Most preferably the supercritical fluid is supercritical CO2.

[0039] Suitable polar aprotic co-solvents are for example N-Methyl-pyrrolidone, N- butyl-pyrrolidone, dimethylsulfoxide, y-valerolactone, y-butyrolactone, dimethylacetamide, dimethylformamide, diesters such as Rhodiasolv® RPDE, triethyl phosphate. Preferred co-solvents do not comprise nitrogen atoms. More preferred co-solvents are selected from dimethylsulfoxide y- valerolactone, y-butyrolactone and triethyl phosphate, most preferred co-solvent is triethyl phosphate. An additional advantage of the preferred cosolvents is that they are environmentally friendly.

[0040] The sequence of steps by which the solid compositions comprising one or more VDF polymer is put in contact with the supercritical fluid and the cosolvent is not critical. Naturally, since the conditions for a supercritical fluid require high pressure, the method will be performed within a pressurized vessel, for example a supercritical fluid extractor or an autoclave. The solid composition may be placed in a suitable container within the pressurized vessel or the pressurized vessel itself may act as a container. In a preferred embodiment the solid composition is separated from the container or the vessel surface by filtering means with have a sufficiently tight mesh to prevent the solid composition form passing through, but which are permeable to fluids.

[0041] Once the solid composition is sealed within the vessel, said vessel can be heated and pressurized with the desired gas up to a T and P above the critical point of said gas so that the gas transitions to its supercritical phase.

[0042] The vessel needs to be pressurized to a pressure P of at least 120bar, preferably of 150-1500 bar, more preferably of 300-1000 bar, most preferably of 310-900 bar.

[0043] The temperature at which the treatment is performed is not critical. In general it is preferred to operate at a temperature in °C between Tc (i.e. the critical temperature of the selected supercritical fluid expressed) and Tc+400°C. E.g. for CO2 suitable temperatures are 30 to 430°C; for H2O 100 to 500°C, for CH3OH from 40 to 440°C and for CH3CH2OH from 50 to 450°C. Preferably the T of the treatment is comprised from Tc and Tc+150°C, more preferably between Tc+10°C and Tc+70°C.

[0044] The preferred supercritical fluid for use in the present invention is CO2. In this case the preferred temperature T is from 30°C to 430°C, preferably from 40°C to 180°C, more preferably from 40°C and 100°C, even more preferably between 40 and 80°C.

[0045] When CO2 is used it may be (at least in part) introduced in the pressurized vessel in solid form (dry ice) before sealing for easier handling, as it can turnto liquid and then gas / supercritical fluid state during the performance of the process at the conditions selected.

[0046] The co-solvent can be added in the vessel or container wherein the solid composition is placed before sealing the reactor or, preferably, after the reactor has reached the supercritical P and T. More preferably the cosolvent is added to the container stepwise

[0047] After the pressurized vessel is sealed the temperature is brought to the desired level and the pressure is increased typically via an inlet using the supercritical fluid chosen. The co-solvent, if not already present in the vessel may be introduced through the same or a different inlet.

[0048] Once the solid composition has been contacted with both the supercritical fluid and the co-solvent, an extraction time is required to allow the extraction of the VDF polymers. Optional agitation means may be provided during this time, but they are not essential.

[0049] Once the extraction time is passed a VDF polymer solution is formed within the container or vessel with the solid composition, such solution will comprise co-solvent, supercritical fluid and VDF polymers. The residual solid composition, after the VDF polymer extraction, is called herein “extracted solid residue”.

[0050] A preferred method for precipitating the VDF polymer from the VDF polymer solution is to release the pressure of the pressurized vessel and cooling said VDF polymer solution to a temperature below 35°C, preferably between -50°C and 35°C, thereby directly causing the precipitation of said VDF polymer from said VDF polymer solution. Pressure release can be conducted by opening an outlet. The supercritical fluid will exit the outlet in gas or liquid form and can be collected and reused. At the same time the supercritical fluid will leave the VDF polymer solution, and the expansion of the gas will typically cause a cooling of the entire system (if needed cooling means may be used to further cool the system, but in general it will not be necessary). We have surprisingly found that upon releasing the supercritical fluid and at the same time cooling the system down to a T below 35°C, preferably between -50°C and 35°C, more preferably between 0 and 30°Cthe VDF polymer precipitates as a pure white solid which can be filtered out washed e.g. with water and dried to provide a solid VDF polymer ready for being reused.

[0051] Optionally the co-solvent can be recovered for further use via evaporation.

[0052] Alternatively the VDF polymer can be precipitated from the VDF polymer solution with other typical procedures such as complete evaporation of the solvents (supercritical fluid and co-solvent) or by mixing the VDF polymer solution (before or after release of the supercritical fluid) with a non solvent for the polymer (i.e. a liquid in which said VDF polymer is not soluble). Preferably said non-solvent is miscible with said co-solvent and is selected from water, alcohols or mixtures thereof.

[0053] The process is environmentally friendly as the supercritical fluids and cosolvent used can be entirely recovered and reused.

[0054] Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.EXAMPLES

[0055] Material Used : Black Mass obtained from and containing about 1 % w of PVDF:

[0056] Example 115g of pulverized black mass are inserted in the sample holder of extractor Series SFT- 120 by Supercritical Fluids Technologies. The extractor is pressurized with CO2 to P: 400bar and heated to T: 60°C.TriEthylPhosphate (TEP) as co-solvent is dosed with an Agilent pump stepwise to a total amount of 7mL. After 45min of extraction time the pressure is released. The sample holder is cooled down to 15°C. Once the pressure is fully released 0.15g of a white solid were filtered out of the residual co-solvent, washed with water and dried. The white solid was tested with FT IR, TGA and DSC and confirmed to be PVDF with a purity >98%.

[0057] Example 2

[0058] The same process of example 1 was used but without the addition of cosolvent. No white solid precipitation is observed. PVDF not extracted from black mass.

Claims

What is claimed is:Claim 1A method for the extraction of vinylidene difluoride (VDF) polymers from a solid composition the method comprising: i) providing a solid composition comprising one or more VDF polymer ii) contacting said solid composition comprising one or more VDF polymer with a supercritical fluid in the presence of one or more polar aprotic co-solvent within a pressurized vessel at a pressure P above 120bar, preferably between 150 and 1500bar thereby dissolving said one or more VDF polymer and forming a VDF polymer solution and an extracted solid residue.Claim 2A method according to claim 1 further comprising the steps of releasing the pressure of said pressurized vessel and cooling said VDF polymer solution to a temperature below 35°C, preferably between -50°C and 35°C, thereby directly causing the precipitation of said VDF polymer from said VDF polymer solution.Claim 3A method according to claim 1 wherein said VDF polymer solution is mixed with a non solvent for the polymer thereby directly causing the precipitation of said VDF polymer from said VDF polymer solution.Claim 4A method according to any preceding claim wherein said co-solvent is free from nitrogen atoms.Claim 5A method according to any preceding claim wherein said co-solvent is selected from dimethylsulfoxide y-valerolactone, y-butyrolactone and triethyl phosphate and is preferably triethyl phosphate.Claim 6A method according to any preceding claim wherein said solid composition comprising one or more VDF polymer also comprises one or more of metal oxides and / or metal salts.Claim 7A method according to any preceding claim wherein said solid composition comprising one or more VDF polymer is a material deriving from spent secondary batteries, preferably spent lithium ion batteries and is more preferably black mass.Claim 8A method according to any preceding claim wherein said solid composition comprising one or more VDF polymer comprises 0.1-99%, preferably 0.1-20%, more preferably 0.2-10%, even more preferably 0.3-5% by weight of VDF polymers.Claim 9A method according to any preceding claim wherein said one or more VDF polymer comprises at least 50%, preferably at least 70%, more preferably at least 80%, most preferably at least 90% by moles of recurring units derived from VDF and 0.1-10%, more preferably 0.2-7.5%, even more preferably 0.2-5% by moles, of recurring units derived from (meth)acrylic monomers of formula:wherein each of R1 , R2, R3, equal or different from each other, is independently a hydrogen atom or a C1-C3 hydrocarbon group, and ROH is a hydrogen or a C1-C5 hydrocarbon moiety comprising at least one hydroxyl group.Claim 10A method according to claim 9 wherein said (meth)acrylic monomer is selected from acrylic acid, methacrylic acid, hydroxyethyl (meth)acrylate, hydroxypropyl(meth)acrylate; hydroxyethylhexyl (meth)acrylates, methyl(meth)acrylate; glycidyl(meth)acrylate.Claim 11A method according to any preceding claim wherein said supercritical fluid is selected from CO2, H2O, CH3OH, CH3CH2OH, (CH3)2CO, and is preferably supercritical CO2.Claim 12A method according to any preceding claim wherein said solid composition comprising one or more VDF polymer is contacted with a supercritical fluid in the presence of one or more polar aprotic co-solvent at a temperature T, wherein said temperature T expressed in °C is comprised between Tc and Tc+400°C, wherein Tc is the critical temperature of said supercritical fluid.Claim 13A method according to claim 11 wherein said supercritical fluid is supercritical CO2 and said temperature T is from 30°C to 430°C.Claim 14The use of a method according to claims 1-13 to reduce the environmental impact of the recycling process of spent batteries.

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