Electrochemical cell with gel-type electrolyte comprising a positive electrode prepared by dry method

Abstract

The present invention relates to a method for manufacturing positive electrodes by dry method comprising a gel polymer electrolyte for solid electrolyte battery, the electrochemical elements comprising them and their manufacture. Figure: None

Description

Title of the invention: Electrochemical element with gel-type electrolyte comprising a positive electrode prepared by dry method

[0001] The present invention relates to the technology of solid electrolyte lithium accumulators containing a gel polymer electrolyte and a solvent-free positive electrode.

[0002] An electrochemical element, also referred to as "element" in the following, comprises an electrochemical bundle consisting of an alternation of positive electrodes and negative electrodes framing a separator impregnated with electrolyte. Each positive electrode and each negative electrode consists of a metal current collector supporting on at least one of its faces at least one active material and generally a binder and an electronically conductive material.

[0003] Lithium-ion electrochemical elements are known from the state of the art. They are commonly used in many fields such as automobiles, telephony, electronic devices or aeronautics. Their operating principle is based on the reversible exchange of the lithium ion between a positive electrode (cathode), most often a lithium transition metal oxide or a lithium transition metal phosphate and a negative electrode (anode) for example made of graphite. The anode and the cathode are separated by a separator. The assembly formed by the anode, the cathode and the separator forms an electrochemical beam. This is impregnated with a liquid organic electrolyte often consisting of a mixture of alkyl carbonates in which a lithium salt is dissolved, for example lithium hexafluorophosphate LiPF6.

[0004] The liquid nature of the electrolyte leads to several drawbacks and we therefore seek to avoid the use of a liquid electrolyte.

[0005] All-solid-state technology refers to solid electrolyte cells, among which we distinguish between solid electrolytes proper (SSE) including inorganic solid electrolytes (ISE), solid polymer electrolytes (SPE) and composite polymer electrolytes (CPE), as well as quasi-solid electrolytes (QSSE) also called gel polymer electrolytes (GPE).

[0006] The gelled form has the particular advantage of preventing electrolyte leaks in the event of accidental opening of the cell container. It also prevents lithium dendrites that may form on the surface of the negative electrode from propagating towards the positive electrode and creating micro-short circuits that are detrimental to the life of the cell. Compared with a liquid electrolyte, a gelled electrolyte therefore improves the safety of the user of the cell.

[0007] The gelled electrolyte can typically be obtained by dissolving a polymer in an organic solvent. The gelled electrolyte is then brought into contact with the electrodes which gradually become impregnated with it.

[0008] FR3110776 describes a process for preparing a gelled electrolyte in Li-ion technology based on ionic liquid, poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) and solvent, it being understood that the gelled electrolyte is then deposited on the positive and / or negative active material, on either side of the separator. The polymer content relative to the mass of the liquid mixture is generally 10% by mass:

[0009] The application filed under number FR2207830 filed on July 29, 2022 describes a method for manufacturing in situ gelation of an electrochemical element by impregnation of an assembly consisting of a negative electrode, a separator and a positive electrode comprising an active material of the lithium manganese and iron phosphate (LMFP) type, with a liquid mixture comprising a solvent, a monomer, a lithium salt and a radical polymerization initiator, then crosslinking of the monomer. According to this method, after impregnation of the liquid in the electrode, the crosslinker is activated in order to cause the formation of a polymer network producing a gel capturing the liquid. The application describes more precisely a monomer content of 5% by mass relative to the quantity of liquid electrolyte.

[0010] However, each of these methods involves the prior preparation of the positive electrode by conventional solvent means, that is to say by depositing an ink in an organic solvent medium.

[0011] It has been observed that for electrolyte impregnation, when the monomer or polymer content is greater than or equal to 10% by mass, the viscosity of the liquid becomes too high and problems of impregnation of the electrode by the solution appear.

[0012] However, when the polymer content is this low, the safety aspect of the cells is not significantly increased compared to a non-gelled liquid electrolyte.

[0013] It is therefore desirable to increase the polymer content relative to that of liquid electrolyte in order to improve the safety of these cells. Other methods of manufacturing these electrodes are therefore to be considered to achieve polymer contents greater than 10% by mass relative to the liquid electrolyte content.

[0014] In addition, a very significant part of the manufacturing cost of an electrode is linked to its manufacturing process. Indeed, the solvent used to prepare the ink (based on active material, possible conductive fillers and binder) which will be coated on the strip to design the electrode, must be evaporated. This step therefore involves the use of energy-intensive ovens.

[0015] It is also desirable to limit the use of harmful solvents in an environmental approach.

[0016] Poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) has already been used in electrolytes: WO2019 / 053388 describes a solid polymer electrolyte comprising at least one lithium salt and poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)). However, the solid electrolyte layers are in the form of a solid polymer film, applied by co-lamination between the positive and negative electrode films.

[0017] An aim of the present invention is therefore to propose a new method of manufacturing a gel-type electrochemical element comprising positive electrodes prepared without solvent, making it possible to avoid the use of energy-intensive furnaces used for manufacturing the electrodes.

[0018] Another aim of the invention is also to propose a method for increasing the polymer content within the electrolyte, in particular greater than 10% by mass relative to the liquid electrolyte.

[0019] For this purpose, the invention relates to an electrochemical element comprising: - A positive electrode comprising • at least one active ingredient; • a mechanical binder; • a solid gel-type electrolyte comprising • an ionically conductive gelling binder, • an electrolyte El comprising a lithium salt and a solvent,

[0020] such that said ionically conductive gelling binder is selected from polymers of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), poly(oxyethylene) (POE) and its derivatives, poly(trimethylene carbonate) (PTMC), polycaprolactone (PCL), polyethylene carbonate (PEC), polypropylene copolymer (PPC), polyacrylonitrile (PAN), acrylates, hydrogenated butadiene-acrylonitrile rubber (HNBR), and cellulose and its derivatives; • possibly a carbon electronic conductor; and • A porous separator whose pores are impregnated with an E2 gel-type electrolyte comprising a matrix which is a polymer obtained by crosslinking a monomer comprising at least two acrylate groups into which is incorporated a mixture comprising at least one solvent, at least one lithium salt and a radical polymerization initiator; • A negative electrode comprising an active material.

[0021] According to other advantageous aspects of the invention, said element comprises one or more of the following characteristics, taken in isolation or in all technically possible combinations: The electrochemical element

[0022] The term "electrochemical element" means an elementary electrochemical cell consisting of the positive electrode / separator / negative electrode assembly, making it possible to store the electrical energy supplied by a chemical reaction and to restore it in the form of current. The positive electrode

[0023] The term "positive electrode" designates the electrode where the electrons enter, and where the cations (Li+) arrive in discharge.

[0024] According to one embodiment, said electrode is prepared by a dry process. This term refers to a solvent-free process. It is therefore distinguished from electrodes typically consisting of an ink deposited on a current collector and the solvent of which is evaporated.

[0025] It is hereby specified that the term “solvent-free” refers to the solvent used for the process comprising the deposition of an ink, without prejudice to the solvent incorporated in the gelled electrolyte according to the invention.

[0026] According to the invention, the positive electrode is in gelled form.

[0027] The mechanical binder

[0028] The term “mechanical binder” refers to compounds capable of giving the electrode the cohesion of the different components and its mechanical strength on the current collector, and / or of giving a certain flexibility to the electrode for its implementation as an element.

[0029] Typically, said mechanical binder may be chosen from binders of the fibrillable fluoropolymer type such as polytetrafluoroethylene (PTFE) and its derivatives such as polyvinylidene tetrafluoroethylene fluoride (PVDF-TFE), in particular its copolymers such as chlorofluoroethylene, perfluoroalkoxy (PFA), polychlorotrifluoroethylene (PCTFE or PTFCE), fluorinated ethylene propylene (FEP), ethylene tetrafluoroethylene or poly(ethylene-co-tetrafluoroethylene) (ETFE), tetrafluoroethylene perfluoromethylvinylether (MFA).

[0030] According to one embodiment, said mechanical binder is chosen from polytetrafluoroethylene (PTFE) and polyvinylidene tetrafluoroethylene fluoride (PVDF-TFE) and their mixtures.

[0031] According to one embodiment, the mechanical binder represents from 0.1% to 10%, preferably from 0.5% to 5% by mass of the mass of the mixture constituting the positive electrode.

[0032] The electrolyte El

[0033] According to one embodiment, the solvent of the electrolyte E1 is chosen from saturated cyclic carbonates, unsaturated cyclic carbonates and linear carbonates, linear ethers, cyclic ethers and their derivatives such as trimethylolpropane triglycidyl ether (TTE), and their fluorinated derivatives.

[0034] Preferred saturated cyclic carbonates are propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), and a mixture thereof. An example of a fluorinated cyclic carbonate is monofluoroethylene carbonate (FEC). Vinylene carbonate can be used as an unsaturated cyclic carbonate. Preferred linear carbonates are dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), and a mixture thereof. Preferred linear ethers include dimethyl ether (DME), diethyl ether (DEE), and a mixture thereof. Preferred cyclic ethers include lactones, such as gamma-butyrolactone.

[0035] In a preferred embodiment, said at least one solvent is selected from the group consisting of cyclic carbonates, linear carbonates and a mixture thereof.

[0036] In a preferred embodiment, said at least one solvent comprises fluoroethylene carbonate (FEC).

[0037] In one embodiment, said at least one solvent is a mixture of a linear carbonate and fluoroethylene carbonate (FEC). The linear carbonate may be ethyl methyl carbonate (EMC).

[0038] According to one embodiment, the solvent comprises from 20 to 40% by volume of cyclic alkyl carbonate, optionally fluorinated such as monofluoroethylene carbonate (FEC), and from 60 to 80% by volume of linear alkyl carbonate.

[0039] According to one embodiment, the solvent comprises ethylene carbonate (EC) and ethyl methyl carbonate (EMC).

[0040] According to one embodiment, the electrolyte E1 comprises from 50 to 95% by weight of solvent, preferably between 60% and 85%, the percentage being relative to the weight of gelling polymer.

[0041] According to one embodiment, the electrode comprises from 15 to 35% by volume of EL electrolyte.

[0042] The ionically conductive gelling binder

[0043] The term "ionic conductive gelling binder" refers to agents capable of increasing the viscosity of the electrolyte El to give it the consistency of a gel, while allowing the conduction of lithium ions.

[0044] Said ionically conductive gelling binder may be chosen from poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and poly(oxyethylene) polymers. (POE) and its derivatives such as polyethylene glycol diacrylate (PEGDA), Poly(ethylene glycol) methyl ether acrylate (PEGMEA), trimethylolpropane triacrylate (TMPTA), Poly(trimethylene carbonate) (PTMC), polycaprolactone (PCL), polyethylene carbonate (PEC), polypropylene copolymer (PPC), polyacrylonitrile (PAN), acrylates, hydrogenated butadiene-acrylonitrile rubber (HNBR), and cellulose and its derivatives.

[0045] According to the invention, said gelling binder may be present in high concentrations within the electrolyte E1, in particular in mass concentration greater than 10%.

[0046] According to one embodiment, the mass ratio (ionic conductive gelling binder / electrolyte E1) is between 10 and 50%, preferably between 20 and 30%.

[0047] Lithium salts

[0048] The electrolyte El contains lithium salts.

[0049] According to one embodiment, said lithium salts of the electrolyte E1 and E2, identical or different, may be chosen from lithium perchlorate LiC104, lithium hexafluorophosphate LiPF6, lithium tetrafluoroborate LiBF4, lithium hexafluoroarsenate LiAsF6, lithium hexafluorantimonate LiSbF6, lithium trifluoromethanesulfonate LiCF3SO3, lithium bis(fluorosulfonyl)imide Li(FSO2)2N (LiFSI), lithium bis(trifluoromethanesulfonyl)imide LiN(CF3SO2)2 (LiTFSI), lithium tris(fluoromethanesulfonyl)methylide LiC(CF3SO2)3 (LiTFSM), lithium bis(pentafluoroethylsulfonyl)imide LiN(C2F(SO2)2 (LiBeTI), lithium-4,5-dicyano-2-(trifluoromethyl)imidazolide (LiTDI), lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LIDFOB), lithium tris(pentafluoroethyl)trifluorophosphate LiPF3(CF2CF3)3 (LiFAP), lithium difluorophosphate LiPO2F2, and mixtures thereof.

[0050] A first preferred mixture of lithium salts consists of LiPF6 and LiDFOB. A second preferred mixture of lithium salts consists of LiPF6, LiFSI and / or LiTFSI and LiDFOB. Preferably, this second mixture consists of LiPF6, LiFSI and LiDFOB.

[0051] According to one embodiment, the concentration of lithium salts in the electrolyte E1 is between 0.5 and 2 mol.L *.

[0052] Active ingredient

[0053] The active material of the positive electrode is not particularly limited. It can be selected from the following groups or mixtures thereof:

[0054] - nickel-manganese-cobalt (NMC) type compounds, in particular oxides lamellar with a high nickel content, that is to say typically those for which the molar ratio of nickel, reported to the total of the elements nickel, manganese and cobalt, is greater than or equal to 0.6, in particular greater than or equal to 0.8;

[0055] - nickel-cobalt-aluminum (NCA) type compounds

[0056] - lithium-iron-phosphate (LFP) type compounds;

[0057] - lithium vanadium fluorophosphate (LVPF) compounds, in particular those of LixVPO4F formula with 0.8 <x<l,2, ou l’un de ses dérivés de formule LixVi _ yMyPO4Fz où0,8<x<l,2 ; 0<y<0,5 ; 0,8<z<l,2 et M est choisi dans le groupe consistant en Ti, Al, Y, Cr, Cu, Mg, Mn, Fe, Co, Ni, et Zr ;

[0058] - lithium-iron-metal-phosphate (LFMP) type compounds, in particular of formula LixFei yMyPO4(LFMP) where M is selected from the group consisting of B, Mg, Al, Si, Ca, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Y, Zr, Nb and Mo; and 0.8 <x<l,2 ; 0<y<0,9 .

[0059] The conductive element

[0060] A conductive element may also be added for the preparation of the positive electrode. It may be selected from carbonaceous electronically conductive materials, such as graphite, activated carbon, carbon black, acetylene black, soot, graphene, carbon fibers, carbon nanotubes or a mixture thereof.

[0061] The mixture comprising at least one active material, the mechanical binder, the gel-type electrolyte comprising the ionically conductive gelling binder and the electrolyte E1 comprising a lithium salt and a solvent, and optionally the carbon electronic conductor constitutes the electrode formulation.

[0062] According to one embodiment, the electrodes may consist of a conductive support used as a current collector which is coated with the formulation according to the invention, after shaping.

[0063] Current collector means an element such as a pad, plate, sheet or other, made of conductive material, connected to the positive or negative electrode, and ensuring the conduction of the flow of electrons between the electrode and the terminals of the battery.

[0064] The current collector is preferably a two-dimensional conductive support such as a solid or perforated metal-based strip.

[0065] In the case of the positive electrode, the current collector is typically an aluminum foil, in particular covered with a carbon coating.

[0066] According to one embodiment, the positive electrode does not contain a radical polymerization initiator. The impregnated separator

[0067] The porous separator

[0068] The separator may be any type of ion-conducting porous separator. Particular mention may be made of separators used in Li-ion type cells. For example, the separator may consist of a layer of a material chosen from the group consisting of polypropylene (PP), polyethylene (PE), polytetrafluoroethylene (PTFE), polyacrylonitrile (PAN), polyester such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT), cellulose, polyimide, glass fibers. The separator may consist of several layers of the aforementioned materials, the layers being of the same or different nature.

[0069] Electrolyte E2

[0070] The separator is impregnated with gelled electrolyte E2 which is obtained by crosslinking a monomer.

[0071] The term "crosslinking" refers to the formation of a polymer having a three-dimensional structure.

[0072] The monomer

[0073] Said monomer may advantageously be of acrylate type. Typically, it comprises at least two acrylate groups.

[0074] The term "acrylate groups" includes in the following methacrylate groups. The monomer typically comprises two, three or four acrylate groups. In a preferred embodiment, crosslinking occurs only in the presence of the monomer comprising at least two acrylate groups. The crosslinked polymer comprises at least one acrylate group. According to one embodiment, the reaction mixture does not comprise a comonomer. Examples of monomers comprising two acrylate groups are poly(ethylene glycol) diacrylate (PEGDA), poly(ethylene glycol) dimethacrylate (PEGDMA), poly(propylene glycol) diacrylate (PPGDA) and poly(propylene glycol) dimethacrylate (PPGDMA). Examples include di(propylene glycol) diacrylate, 1H,1H,6H,6H-perfluoro-1,6-hexyl diacrylate, 1H,1H,5H,5H-perfluoropentane-1,5-diyl diacrylate, and 2,2,3,3-tetrafluoro-1,4-butyl diacrylate.Poly(ethylene glycol) diacrylate (PEGDA) may have a number-average molecular weight ranging from 5000 to 10000 g / mol or from 6000 to 8000 g / mol. It may also have a molecular weight between 500 and 1000 g / mol, preferably between 600 and 800 g / mol. It has been observed that low molecular weight monomers promote the impregnation of the electrode pores by the gelled electrolyte.

[0075] Examples of monomers having three acrylate groups are trimethylolpropane triacrylate (TMPTA), pentaerythritol triacrylate, trimethylolpropane ethoxylate triacrylate, and trimethylolpropane propoxylate triacrylate (TPPTA).

[0076] An example of a monomer having four acrylate groups is pentaerythritol tetraacrylate (PETEA).

[0077] Among the monomers cited above, a preferred monomer of the electrolyte E2 is trimethylolpropane triacrylate (TMPTA).

[0078] The initiator

[0079] The initiator designates a radical polymerization initiator of thermal or photochemical type.

[0080] Said initiator may be a compound comprising one or more azo groups. Preferred initiators are those which are activated by a temperature ranging from 50 to 100°C or from 60 to 90°C or from 70 to 80°C. Moderate temperatures ranging from 55 to 75°C are preferred. A thermal initiator for radical polymerization comprising an azo group is razobisisobutyronitrile (AIBN).

[0081] Lithium salts

[0082] Electrolyte E2 contains lithium salts.

[0083] According to one embodiment, said lithium salts of the electrolyte E2 may be identical to or different from those of the electrolyte E1 as defined above.

[0084] They may be chosen from lithium perchlorate LiC104, lithium hexafluorophosphate LiPF6, lithium tetrafluoroborate LiBF4, lithium hexafluoroarsenate LiAsF6, lithium hexafluorantimonate LiSbF6, lithium trifluoromethanesulfonate LiCF3SO3, lithium bis(fluorosulfonyl)imide Li(FSO2)2N (LiFSI), lithium bis(trifluoromethanesulfonyl)imide LiN(CF3SO2)2 (LiTFSI), lithium tris(fluoromethanesulfonyl)methylide LiC(CF3SO2)3 (LiTFSM), lithium bis(pentafluoroethylsulfonyl)imide LiN(CF3SO2)2 (LiBeTI), lithium-4,5-dicyano-2-(trifluoromethyl)imidazolide (LiTDI), lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LIDFOB), lithium tris(pentafluoroethyl)trifluorophosphate LiPF3(CF2CF3)3 (LiFAP), lithium difluorophosphate LiPO2F2, and mixtures thereof.

[0085] A first preferred mixture of lithium salts consists of LiPF6 and LiDFOB. A second preferred mixture of lithium salts consists of LiPF6, LiFSI and / or LiTFSI and LiDFOB. Preferably, this second mixture consists of LiPF6, LiFSI and LiDFOB.

[0086] The solvent

[0087] According to one embodiment, said solvent of the electrolyte E2 is a cyclic carbonate or a linear carbonate or a mixture thereof, in particular fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC) and a mixture thereof.

[0088] In a preferred embodiment, said at least one solvent comprises fluoroethylene carbonate (FEC). In one embodiment, said at least one solvent is a mixture of a linear carbonate and fluoroethylene carbonate (FEC). The linear carbonate may be ethyl methyl carbonate (EMC).

[0089] According to one embodiment, the solvent comprises from 20 to 40% by volume of cyclic alkyl carbonate, optionally fluorinated such as monofluoroethylene carbonate (FEC), and from 60 to 80% by volume of linear alkyl carbonate.

[0090] According to one embodiment, the solvent comprises ethylene carbonate (EC) and ethyl methyl carbonate (EMC).

[0091] In one embodiment, said at least one solvent is a mixture of fluoroethylene carbonate (FEC) and ethyl methyl carbonate (EMC) and the salt mixture consists of LiPF6 and LiDFOB.

[0092] Said separator impregnated with the gel electrolyte E2 can be prepared in particular by applying and / or adapting the methods described in the application filed under number FR2207830. Illustratively, appropriate quantities of the monomer, said at least one solvent, said at least one lithium salt and the thermal initiator are mixed to produce a liquid mixture. Generally, the mass percentage of the monomer ranges from 1 to 20% or from 5 to 10% or from 2 to 7% for a total mass of said at least one solvent, said at least one lithium salt and the radical polymerization thermal initiator of 100%. A low percentage of monomer makes it possible to obtain a lower viscosity of the liquid mixture, which improves the impregnation of the electrodes. The total amount of the thermal initiator(s) for radical polymerization may represent from 0.1 to 3% by mass, ideally from 1 to 1.5% by mass relative to the mass of monomer.The separator may be immersed in the liquid mixture at room temperature. A rest period of generally 12 to 24 hours is preferably observed to improve the penetration of the liquid mixture into the pores of the separator. The temperature may then be increased to a value sufficient to activate the initiator and cause crosslinking of the monomer. The temperature is generally set in the range of 50 to 100°C, preferably 60 to 70°C. It may be set in the range of 60°C to 70°C when the radical initiator is azobisisobutyronitrile (AIBN). The heat is maintained for 2-24 hours, generally 5 to 15 hours depending on the quantities of monomer and radical initiator used.

[0093] The degree of progress of crosslinking can be estimated by measuring the percentage of residual monomers in the reaction medium at different times during the reaction. Residual acrylate monomers are characterized by the presence of a C=C double bond in the vinyl group. This double bond can be detected by Fourier transform infrared spectroscopy (FT-IR). This technique can be used by those skilled in the art to measure the amount of residual monomers. The use of the FT-IR technique makes it possible to check whether the reaction mixture still contains compounds having a C=C double bond. Those skilled in the art are able to know when to stop applying heat. The crosslinking conditions are chosen to minimize the amount of residual monomers. The presence of a small amount of residual monomers can have a detrimental effect on the operation of the element. The negative electrode

[0094] The term "negative electrode" designates when the accumulator is discharging, the electrode functioning as an anode and when the accumulator is charging, the electrode functioning as a cathode, the anode being defined as the electrode where an electrochemical oxidation reaction takes place (emission of electrons), while the cathode is the seat of the reduction. The term negative electrode also designates the electrode from which the electrons leave, and from which the cations (Li+) are released in discharge.

[0095] The negative electrode typically comprises at least one active material, optionally a binder, optionally one or more co-binders, and optionally gel electrolyte of type E2 and / or EL.

[0096] The active material(s) of the negative electrode may be chosen from the group consisting of lithium, a lithium-based alloy, graphite, silicon, mixtures of graphite and silicon, and titanium oxides. In particular, the active material of the anode is lithium or a lithium-based alloy. Process

[0097] According to another object, the present invention also relates to a method for preparing an element according to the invention.

[0098] The method can be carried out according to two alternatives detailed below.

[0099] According to a first alternative, said method comprises: - Preparation of the positive electrode, including: • The fibrillation of a mixture of the active material, the mechanical binder, the ionically conductive gelling binder, and possibly the electronically conductive material, in powder form; • Shaping the fibrillated mixture into a film and depositing said film on a positive electrode current collector; - The assembly of said positive electrode thus formed with said separator impregnated with the gel electrolyte E2, and said negative electrode; - The impregnation of said positive electrode by said electrolyte El in liquid form comprising said lithium salt and said solvent; and - The gelling of said positive electrode by thermal activation.

[0100] According to this first alternative, the element is prepared by gelling the positive electrode in situ, i.e. pre-assembled within an element.

[0101] According to this alternative, the active material, the mechanical binder, the ionically conductive gelling binder, and optionally the conductive material ingredients of the positive electrode formulation are each in powder form and mixed.

[0102] The mixing can in particular be carried out with a planetary mixer.

[0103] At the end of the fibrillation (see below), the mixture obtained is typically in the form of granules or agglomerated powder.

[0104] After shaping the positive electrode, it is assembled with the other constituents of the element (separator and negative electrode).

[0105] The impregnation of the positive electrode is then carried out using electrolyte El in liquid form. This can be carried out by the usual impregnation methods conventionally used for Li-ion type electrochemical systems. This impregnation can for example be carried out by filling, for example by dipping or immersing said positive electrode.

[0106] A schematic representation of this first alternative is illustrated by [Fig.l].

[0107] As shown in [Fig.l], the process can be broken down into 3 steps.

[0108] In a first step A, the mixture in powder form is introduced into a extruder 3, a planetary mixer 4 or an internal mixer 5 in which it is fibrillated. Typically, the fibrillated product obtained may be in the form of flakes or granules (1) or agglomerated powder (2).

[0109] In a second step B, the mixture thus fibrillated is introduced into a calender 7 for shaping. [Fig.l] here only represents the calendering of agglomerated powder, but the shaping can also be done on the fibrillated mixture in the form of flakes or granules, indifferently. In the case of flakes or granules, it may be appropriate to deagglomerate the mixture to facilitate the manufacture of the film.

[0110] At the end of this shaping, a film 8 is obtained which is deposited on a current collector.

[0111] In a third step, the film 8 comprising pores 13 is assembled with the separator impregnated with gelled electrolyte E2 11 and a negative electrode 12, thus constituting a precursor of positive electrode 10. The pores 13 are then impregnated with liquid electrolyte E1, then gelled, so as to form the positive electrode 10'.

[0112] It is understood that the electrodes also include current collectors, not shown here.

[0113] According to a second alternative, said method comprises: - The preparation of a positive electrode composition comprising the mixture of said active material, mechanical binder, ionic conductive gelling binder, and optionally electronic conductive material with said electrolyte El in liquid form comprising said lithium salt and said solvent; - The fibrillation and gelation of said mixture; - Shaping the gelled fibrillated mixture into a film and depositing said film on a current collector; - The assembly of said collector thus coated with said separator impregnated with the gel electrolyte E2, and said negative electrode.

[0114] According to this second alternative, the element is prepared by assembling its previously prepared constituents. More particularly, according to this alternative, the positive electrode is previously gelled and shaped.

[0115] The method according to the invention according to this second alternative is illustrated in [Fig.2].

[0116] As shown in [Fig.2], the process can be broken down into 3 steps.

[0117] In a first step A, the mixture consisting of the active material, binder mechanical, ionic conductive gelling binder, and optionally electronically conductive material with said electrolyte El is introduced into an extruder 3, a planetary mixer 4 or an internal mixer 5 and heated in which it is fibrillated and gelled.

[0118] Typically, the fibrillated and gelled product obtained can be in the form of gelled powder or gelled flakes, or gelled granules (6').

[0119] In a second step B, the mixture 6' thus fibrillated and gelled is introduced into a calender 7 for shaping.

[0120] At the end of this shaping, a gelled film 8' is obtained and deposited on a current collector.

[0121] In a third step, the gelled film 8' is assembled with the separator impregnated with gelled electrolyte E2 11 and a negative electrode 12, thus constituting a positive electrode 10'.

[0122] It is understood that the electrodes also include current collectors, not shown here.

[0123] Fibrillation

[0124] According to either alternative, fibrillation refers to mixing under mechanical stress, aimed at fibrillating the fluoropolymer-type mechanical binder.

[0125] Fibrillation may typically be achieved by extrusion and / or by mixing within a mixer, such as an internal mixer or a planetary mixer.

[0126] “Extrusion” means a thermomechanical process in which the formulation is forced to pass through a barrel, under the action of pressure and / or rotation of the screws and heat.

[0127] The extrusion step can be adapted according to several parameters, such as the mixing temperature, the type of extruder screw profile, the type of extruder die, the rotation speed and / or the length of the screws.

[0128] According to one embodiment, the fibrillation can be carried out with a single- or twin-screw type extruder, preferably a co-rotating twin-screw.

[0129] According to one embodiment, the screw profile used in the extruder is of the shearing type in order to fibrillate the fluoropolymer in the extruder. The screw profile may contain one or more mixing zones. The number of mixing zones typically depends on the number of introduction zones. The position of the mixing zones in the extruder generally depends on the number of material introduction zones. After each material introduction zone, a mixing zone may be added.

[0130] Typically, the type of screw element for shearing the material can be adapted to the type of active material contained in the premix. If the active material is sensitive to shear, it is preferable to favor low or medium shear elements. If the active material is not very sensitive to shear, it is possible to use low, medium or high shear elements.

[0131] The rotation speed of the screw is generally the same throughout the screw. It is generally recommended to rotate it between 100 rpm and 1000 rpm, in particular between 100 and 750 rpm. The rotation speed of the screw is generally adapted according to the desired material flow rate at the extruder outlet. The lower the rotation speed of the screw, the lower the output flow rates will be. Note that low rotation speeds result in longer residence times in the extruder. In such a case, if the material input flow rate is high, there may be a risk of clogging the extruder. In the case of a high screw rotation speed, the output flow rates may fluctuate if the incoming material flow rates are too low.

[0132] The fibrillation step can typically be carried out at a temperature between 40°C and the degradation temperature of the fluoropolymer.

[0133] By way of illustration, for PTFE, the degradation temperature is approximately 350°C (under shear) and the melting temperature is approximately 330°C (this value may vary depending on the grade of PTFE), it being understood that due to the stresses exerted, the extrusion temperature is lower, preferably lower than or equal to 260°C.

[0134] When fibrillation and gelation are carried out simultaneously, the temperature of the fibrillation step can advantageously be adjusted to avoid degradation of one or more ingredients of the formulation. Thus, the appropriate fibrillation temperature can be in particular less than 200°C, or even less than 100°C.

[0135] In the case where the fibrillation is carried out by an internal mixer, the same conditions as for the extruder can be implemented.

[0136] Gelation

[0137] This step of gelling the electrolyte El can be initiated by thermal activation by increasing the temperature and / or by photochemical activation, for example by UV irradiation, in particular by irradiation using a UV lamp. with wavelength between 200 and 500 nm, preferably between 300 and 400 nm.

[0138] The thermal activation can be carried out at a temperature between 40 and 200°C, preferably between 40 and 130°C, and in particular between 40 and 100°C.

[0139] According to an embodiment of the second alternative, the fibrillation and the gelation can be carried out in the same equipment, at the gelation temperature discussed above. They can be carried out simultaneously, or successively.

[0140] The formatting

[0141] According to either of the alternatives, the shaping of the formulation aims to put the formulation in the form of a film, and / or to deposit the formulation and / or the film thus obtained on a current collector.

[0142] Shaping may typically be accomplished by calendering using a heated roller calender, typically having a different or similar roller speed, or an external roller mixer.

[0143] The film thus obtained can then be deposited on a current collector to form an electrode.

[0144] According to another object, the present invention also relates to an electrochemical module comprising the stack of at least two elements according to the invention, each element being electrically connected with one or more other element(s).

[0145] The term “module” therefore designates here the assembly of several electrochemical elements, said assemblies being able to be in series and / or parallel.

[0146] Another object of the invention is also a battery comprising one or more modules according to the invention.

[0147] The term “battery” or accumulator means the assembly of several modules according to the invention.

[0148] The invention will appear more clearly on reading the description which follows, given solely by way of non-limiting example, and made with reference to the drawings. Figures

[0149] [Fig.l] [Fig.l] illustrates the method according to the invention according to the first alternative.

[0150] [Fig.2] [Fig.2] illustrates the method according to the invention according to the second alternative.

[0151] [Fig.3] [Fig.3] schematically represents the structure of a positive electrode prepared using the dry process before introduction of the liquid electrolyte part.

[0152] [Fig.4] [Fig.4] illustrates the structure of a positive electrode prepared by the dry process after introduction of the liquid electrolyte part and gelation of EL

[0153] [Fig.5] [Fig.5] illustrates the study of the gelation range of a PVDF-HFP binder with a liquid electrolyte.

[0154] [Fig.6] [Fig.6] (A and B) illustrates the cycling results at 25°C for a battery button prepared in the examples. Examples Example 1: Preparation of the positive electrode

[0155] The formulation used for the production of positive electrodes in the dry process for an element based on polymer electrolyte comprises: active material of type NMC622 or 811, an electronic conductor (carbon black), a mechanical binder (PTFE) and PVDF-HFP as an ionic conductive gelling binder.

[0156] In a first step, the NMC, the CB and the two binders are mixed in order to be distributed homogeneously.

[0157] This step is carried out in a planetary mixer.

[0158] Then, the mechanical binder is fibrillated. This step can be carried out either in a planetary mixer or in a twin-screw extruder or an internal mixer.

[0159] During this step, the mechanical binder is transformed, it goes from spherical particle to fiber. This allows the electrode to be given its mechanical strength.

[0160] The second step consists of shaping the electrode and depositing it on a current collector. A calender is then used.

[0161] [Fig.3] represents the structure after shaping, comprising the active material 1 and the conductor 3, and the gelling binder 4 distributed in the network of fibers consisting of the fibrillated mechanical binder 2. This formulation is deposited on a current collector 5.

[0162] Once the electrode is made ([Fig.3]), it is assembled then impregnated with a liquid electrolyte.

[0163] In contact with the gelling ionic conductive binder, brought to 60°C, the liquid and the latter will form a gel represented in [Fig.4]: a matrix 6 consisting of the gelling binder, the lithium salts and the solvent incorporates the network of fibrils of the mechanical binder 3 and within which the active material 1 and the conductor 3 are distributed. The gel thus formed is deposited on a current collector 5.

[0164] The gelling binder content and the porosity proportion of the electrode (therefore of liquid electrolyte which will be introduced) being controlled, it is therefore possible to modulate the gelling polymer / liquid electrolyte ratio and thus define the desired polymer content.

[0165] As shown in [Fig.4], after shaping the electrode, it has a good mechanical structure due to the fibrous PTFE binder which allows the different particles to be held together.

[0166] The porosity content of the electrode is fixed during the implementation of the electrode, this depending on the ratio of gelling polymer electrolyte / desired liquid electrolyte. Furthermore, since PTFE has no particular affinity with the electrolyte, it can therefore correctly play its role as a mechanical binder.

[0167] Once the liquid electrolyte is introduced, the system is heated to 60°C in order to gel the ionic conductive binder (PVDF-HFP). A gel polymer (GPE, Gel Polymer Electrolyte) is then formed within the positive electrode. Example 2: Properties

[0168] In order to ensure the good gelation of the PVDF-HFP polymer, different ex-situ tests were first carried out: several types of PVDF-HFP were chosen as well as different types of liquid electrolytes. For each case, different ratios of PVDF-HFP / liquid electrolyte were tested in order to determine the range in which the polymer is capable of gelling. In addition, the impact of temperature on the gelling capacity of these systems was studied.

[0169] For a given liquid electrolyte, whatever the type of PVDF-HFP, it has been shown that gelation occurs advantageously from a PVDF-HFP content of the order of 10% by mass and up to 30% by mass.

[0170] Indeed, below 10%, the mixture may be liquid whereas above 30%, the quantity of liquid electrolyte may not be sufficient to soak all of the polymer.

[0171] The gel structure also evolves with the PVDF-HFP content. [Fig.5] illustrates for a given PVDF-HFP how the gel structure evolves as a function of the polymer / liquid electrolyte ratio. Example 3: Electrochemical characterizations

[0172] In order to electrochemically test the prepared positive electrodes, these electrodes were gelled (introduction of liquid electrolyte at 25°C for 2 hours then the electrode is heated to 60°C to activate gelling) then assembled with a separator soaked in E2 electrolyte and a counter-electrode made of Li metal.

[0173] The SEL that was prepared comprises a standard celgard separator soaked with a solution containing liquid electrolyte (about 90%) and a monomer, TMPTA (about 10%).

[0174] The latter was polymerized and crosslinked using thermal activation (12h at 70°C).

[0175] Considering the weights used (21-24mg / cm2) and the Li metal counter electrode, the regimes for formation (C / 20 and D / 20) and for cycling (C / 10 and D / 10) were deliberately chosen to be slow. The theoretical capacity of the system is 7.5mAh and the actual capacity recovered during formation carried out at 25°C is 7.4mAh.

[0176] The cycling results at 25°C are described in [Fig.6].

Claims

Claims

1. Electrochemical element comprising: - A positive electrode comprising • at least one active material; • a mechanical binder; • a solid gel-type electrolyte comprising • an ionically conductive gelling binder, • an electrolyte E1 comprising a lithium salt and a solvent, such that said ionically conductive gelling binder is chosen from polymers of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), poly(oxyethylene) (POE) and its derivatives, poly(trimethylene carbonate) (PTMC), polycaprolactone (PCL), polyethylene carbonate (PEC), polypropylene copolymer (PPC), polyacrylonitrile (PAN), acrylates, hydrogenated butadiene-acrylonitrile rubber (HNBR), and cellulose and its derivatives; • optionally a carbon electronic conductor;and - A porous separator whose pores are impregnated with an E2 gel-type electrolyte comprising a matrix which is a polymer obtained by crosslinking a monomer comprising at least two acrylate groups into which is incorporated a mixture comprising at least one solvent, at least one lithium salt and a radical polymerization initiator; - A negative electrode comprising an active material.;

2. Element according to claim 1 such that the mass ratio (ionic conductive gelling binder / electrolyte E1) is between 10 and 50%, preferably between 20 and 30%.

3. Element according to claim 1 or 2 such that said mechanical binder is chosen from polytetrafluoroethylene (PTFE) and polyvinylidene tetrafluoroethylene fluoride (PVDF-TFE) and mixtures thereof.

4. Element according to any one of the preceding claims such that said lithium salts of the electrolytes E1 and E2, identical or different, are chosen from lithium perchlorate LiC104, lithium hexafluorophosphate LiPF6, lithium tetrafluoroborate LiBF4, lithium hexafluoroarsenate LiAsF6, lithium hexafluorantimonate LiSbF6, lithium trifluoromethanesulfonate LiCF3SO3, lithium bis(fluorosulfonyl)imide Li(FSO2)2N (LiFSI), lithium bis(trifluoromethanesulfonyl)imide LiN(CF3SO2)2 (LiTFSI), lithium tris(fluoromethanesulfonyl)methylide LiC(CF3SO2)3 (LiTFSM), lithium bis(pentafluoroethylsulfonyl)imide LiN(C2F(SO2)2 (LiBeTI), lithium-4,5-dicyano-2-(trifluoromethyl)imidazolide (LiTDI), lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LIDFOB), lithium tris(pentafluoroethyl)trifluorophosphate LiPF3(CF2CF3)3 (LiFAP), lithium difluorophosphate LiPO2F2,and their mixtures.,

5. An element according to any preceding claim such that the positive electrode does not contain a radical polymerization initiator.

6. Element according to any one of the preceding claims, such that the solvent of the electrolyte E1 is chosen from saturated cyclic carbonates, unsaturated cyclic carbonates and linear carbonates, ethers and their derivatives.

7. An electrochemical element according to any preceding claim, wherein said solvent of electrolyte E2 is a cyclic carbonate or a linear carbonate or a mixture thereof, including fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC) and a mixture thereof.

8. An electrochemical element according to any preceding claim, wherein the radical polymerization initiator present in the separator is azobisisobutyronitrile (AIBN).

9. An electrochemical element according to any preceding claim, wherein said monomer of electrolyte E2 is trimethylolpropane triacrylate (TMPTA).

10. A method of preparing an element according to any one of the preceding claims comprising: - The preparation of the positive electrode, comprising: • The fibrillation of a mixture of the active material, the mechanical binder, the ionically conductive gelling binder, and optionally the electronically conductive material in powder form; • The shaping of the fibrillated mixture in the form of a film and the deposition of said film on a positive electrode current collector; - The assembly of said positive electrode thus formed with said separator impregnated with the gel electrolyte E2, and said negative electrode; - The impregnation of said positive electrode with said electrolyte El in liquid form comprising said lithium salt and said solvent; and - The gelling of said positive electrode by thermal activation.

11. A method of preparing an element according to any one of claims 1 to 9 comprising: - The preparation of a positive electrode composition comprising the mixture of said active material, mechanical binder, ionic conductive gelling binder, and optionally electronic conductive material with said electrolyte E1 in liquid form comprising said lithium salt and said solvent; - The fibrillation and gelling of said mixture; - The shaping of the gelled fibrillated mixture in the form of a film and the deposition of said film on a current collector; - The assembly of said collector thus coated with said separator impregnated with the gel electrolyte E2, and said negative electrode.

12. A method according to claim 10 or 11 such that the fibrillation is carried out by extrusion and / or by mixing within an internal mixer or a planetary mixer.

13. A method according to claim 10, 11 or 12 such that the gelation comprises thermal activation by increasing the temperature between 40 and 200°C, preferably between 40 and 130°C.

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