Method for preparing an active electrode assembly
The method addresses solvent toxicity and process inefficiencies by co-laminating a homogeneous mixture with a microporous film to create a porous, adhesion-free active material for metal-ion batteries, enhancing energy density and safety.
Patent Information
- Authority / Receiving Office
- FR · FR
- Patent Type
- Applications
- Current Assignee / Owner
- COMMISSARIAT A LENERGIE ATOMIQUE ET AUX ENERGIES ALTERNATIVES
- Filing Date
- 2024-12-03
- Publication Date
- 2026-06-05
AI Technical Summary
Existing electrode fabrication processes for metal-ion batteries face issues such as the use of toxic solvents like N-methyl-2-pyrrolidone, high pressure leading to low porosity and energy density, high energy consumption, adhesion problems, and long drying times with aqueous coating, and poor porosity in aqueous extrusion.
A method involving co-lamination of a homogeneous mixture between a current collector and a microporous film, followed by drying, to create a porous active material without toxic solvents, ensuring high porosity and adhesion, using a process that includes extrusion with an aqueous solution and controlled rolling to achieve a thick, defect-free active material.
The method produces a highly porous, defect-free active material with improved adhesion, enabling higher energy density and efficient electrolyte impregnation, reducing solvent use and energy consumption, and enhancing battery safety by using a microporous film that prevents adhesion to rolling mill cylinders.
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Abstract
Description
Title of the invention: Method for preparing an active electrode assembly
[0001] The present invention relates to the field of electrode manufacturing for electrochemical elements. In particular, the invention concerns a method for preparing an active assembly comprising a porous active material for electrodes, notably used in metal-ion batteries. According to a second aspect, the invention relates to an active assembly obtained by said method.
[0002] Conventionally, electrode fabrication processes for metal-ion batteries by extrusion use an organic solvent to obtain a homogeneous mixture of the various components that is sufficiently fluid to facilitate their flow through the extruder. However, the most effective and widely used solvent in this context is N-methyl-2-pyrrolidone (or NMP), which is classified as a CMR substance (Carcinogenic, Mutagenic, Reprotoxic). Therefore, solvent extrusion preparation requires not only a solvent evaporation step but also specific treatment of the toxic solvent vapors.
[0003] An alternative to this type of extrusion is solvent-free extrusion. The homogeneous mixture is obtained by heating the polymers in the composition to their melting temperature to obtain a molten mixture and shaping it. However, this process has two major drawbacks for electrode preparation. First, during extrusion, the pressures inside the extruder and in the dies are high, resulting in compression of the extruded material. The recovered active layer thus exhibits low porosity, and therefore low ionic conductivity after impregnation with the electrolyte, compared to active layers obtained by an extrusion process with an organic solvent.On the other hand, since the viscosity of the mixture to be extruded must remain within pressure and torque ranges that can be supported by the equipment, a high molten polymer content relative to the electrochemically active material content is generally selected to allow the mixture to flow in the extruder, which negatively impacts the energy density.
[0004] In addition, since this solvent-free process requires bringing the polymers to their melting temperature, a large energy consumption is necessary.
[0005] Another known method for preparing active material for metal-ion battery electrodes involves aqueous coating using water-soluble polymers. However, the coating process requires a very fluid mixture of active components, which necessitates the use of a large quantity of water. The drying time of the water significantly increases the process cycle time and the risk of generating defects in the coated layer once dried.
[0006] According to yet another known but less frequently used possibility, it is possible to implement an aqueous extrusion process using water-soluble polymers. However, adhesion problems of the extruded mixture to the rolling mill rollers prevent obtaining a homogeneous and high-quality active layer.
[0007] One of the aims of the present invention is therefore to overcome at least one of the aforementioned drawbacks. To this end, and according to a first aspect, the invention proposes a method for preparing an active assembly for an electrode intended for electrochemical elements, in particular metal-ion batteries such as lithium-ion or sodium-ion batteries, the method comprising the following steps: a. supply of a homogeneous mixture comprising at least one electrochemically active material and an aqueous solution, the homogeneous mixture being intended, after drying, to form a porous active material for electrodes, b. shaping the homogeneous mixture by co-lamination between a current collector and a microporous film so as to form a three-layer assembly comprising the homogeneous mixture laminated in the form of a strip, one face of which is assembled to the current collector and the opposite face of which is assembled to the microporous film, the co-lamination partially reducing the amount of aqueous solution present in the homogeneous mixture, c. drying of the trilayer assembly so as to evaporate the residual aqueous solution and obtain an active assembly comprising a band of porous active material between the current collector and the microporous film.
[0008] Thus, the aqueous preparation process of the invention avoids the use of potentially toxic organic solvents, thereby eliminating the need for vapor treatment steps. This process, which uses less (aqueous) solvent than the conventional process, also prevents defects related to the evaporation of the organic solvent, such as cracking, as well as poor adhesion to the current collector. It is therefore possible to shape the homogeneous mixture into a thick strip, resulting in a highly porous, high-grammage active material strip at the end of the process. In particular, this process makes it possible to achieve a defect-free active material thickness exceeding 500 micrometers, for example, up to 1 mm, and an electrode grammage of between 5 and 50 mg / cm², or even up to 100 mg / cm² if necessary.
[0009] The co-rolling step is carried out in a conventional rolling mill comprising two rolls, and the use of a microporous film provides an anti-adhesion effect of the homogeneous wet mixture on the rolls. Furthermore, the fact that this film is The microporous surface facilitates the removal and subsequent evaporation of the aqueous solution in steps b) and c) of the process. As will be seen later, evaporation is homogeneous across the surface and thickness of the homogeneous mixing strip.
[0010] This solution advantageously avoids covering the rolling mill cylinders with a protective film to prevent adhesion, or even using cylinders made of a non-stick material, which in practice is complicated because these solutions are not versatile.
[0011] Furthermore, the presence of residual aqueous solution in the homogeneous mixture after step b) of co-lamination subsequently allows for very good porosity in the active electrode material. This facilitates subsequent impregnation by the electrolyte. In addition, the presence of the residual aqueous solution allows the strip to be malleable; otherwise, the reduction in thickness occurs without a reduction in basis weight and only a reduction in the porosity obtained. Indeed, during co-lamination, the strip must be malleable to achieve the desired basis weight. Moreover, when the material is still wet, it is not truly porous. It must be dried to create porosity (the solvent releases its contents, leaving porosity).
[0012] During the co-lamination of the wet material according to step b), the porosity cannot be reduced since there is no porosity as such yet. The material is therefore spread on the current collector, reducing its thickness (reduction in basis weight). However, if the material were already porous (i.e., dry) at the time of co-lamination (compression of the material between the rollers), the porosity would be reduced without the material spreading on the current collector.
[0013] The porosity of the porous active material corresponds to the ratio of the volume occupied by the pores of the strip of porous active material to the total volume of the strip of porous active material.
[0014] According to one possibility, the co-rolling step b) is carried out by a rolling mill comprising two cylinders, one of which is covered by the microporous film and the other by the current collector, and between which the homogeneous mixture is introduced.
[0015] The gap between the two cylinders of the rolling mill is determined so as to shape the homogeneous mixture into a strip of a desired thickness, in particular between 50 and 500 micrometers
[0016] According to one provision, step b) of co-rolling includes a step of temperature-regulating the two cylinders of the rolling mill to a temperature Te higher than the ambient temperature, which contributes to the evaporation of some of the aqueous solution from the homogeneous mixture. This step is particularly useful if the material is not very malleable during co-rolling. It facilitates the flow of the material between the rollers and also begins to dry the material.
[0017] The temperature Te is lower than the temperature at which the properties of the microporous film degrade. The temperature Te is in particular between 40 and 95°C, especially between 50 and 80°C and for example between 55 and 70°C. According to one possibility, step b) includes laminating the homogeneous mixture onto the current collector prior to using the microporous film.
[0018] Step c) of drying the trilayer assembly is carried out under conditions that preserve the mechanical properties of the microporous film and the homogeneity of the porous active material of the electrode. In particular, step c) of drying the trilayer assembly is carried out at 60°C under vacuum. Step c) allows the residual aqueous solution from step b) to evaporate and reveals the full porosity of the porous active material.
[0019] According to one provision, the process includes before step c) a step b') of winding the three-layer assembly onto itself in the form of a coil.
[0020] This results in an active assembly in the form of a coil, which is easy to dry at least mostly, or even totally depending on its dimensions, and then to store and use.
[0021] Depending on the circumstances, a few ppm of water may remain in the trilayer assembly. However, since the electrodes are usually cut from the strip afterwards and individually dried before being assembled to create the electrochemical cell, this residual water is completely evaporated at this stage.
[0022] In this document, the term 'active assembly' means an assembly comprising a strip of porous active material for electrode, laminated between a current collector and a microporous film.
[0023] According to one provision, the process includes, before step a) of supplying the homogeneous mixture, carrying out a step i) of aqueous extrusion of constituents comprising at least, or being constituted by, the electrochemically active material, an electronic conductor, a water-soluble binder and an aqueous solution, so as to supply the homogeneous mixture at step a) at the extruder outlet.
[0024] According to one possibility, the electrochemically active material consists of a mixture of several electrochemically active electrode materials.
[0025] The composition of the homogeneous mixture is adapted to the electrochemically active material(s) considered, in order to perform extrusion. This concerns the choice of the polymer(s) constituting the water-soluble binder, the proportion of aqueous solution, etc.
[0026] In particular, the proportion of aqueous solution of the invention is determined to be just sufficient to obtain a homogeneous, pasty mixture, in other words, a "non-liquid mixture of several constituents," unlike an ink or a liquid suspension that is configured for use in a coating process and is therefore much more fluid. Indeed, the homogeneous mixture of the invention It does not flow back on itself like an ink used in a coating process. The homogeneous mixture of the invention is too pasty to be used in a coating process. This has the advantage of producing electrodes that are thicker than those produced by the coating process, in which the mixture spreads out, or thinner if too much of the mixture is deposited.
[0027] Advantageously, the extrusion step i) is carried out without the use of additional additives or solvents.
[0028] The extrusion of step (i) can be carried out at a temperature greater than or equal to the ambient temperature, more particularly at a temperature ranging from 20°C to 60°C, and for example between 20 and 30°C.
[0029] According to one provision, step i) of extrusion is carried out by an extruder comprising an outlet orifice suitable for supplying the homogeneous mixture in step a).
[0030] Generally, extrusion is carried out with an extruder that conveys the material to be extruded to the extruder outlet, in particular to an extruder outlet equipped with a die for initial shaping of the mixture, but which limits the pressure available in the process. Specifically, the extruder includes at least one mixing zone and at least one conveying zone. Preferably, the extruder used in step (i) is a co-rotating twin-screw extruder. The die can be a flat die or a rod die.
[0031] The constituents of the mixture can be introduced into the extruder individually or in the form of a premix.
[0032] According to one possibility, the outlet orifice is provided with a flat die configured to provide the homogeneous mixture in the form of a layer in step a), in particular with a thickness ranging from 0.1 mm to 5 mm, in particular from 0.5 mm to 2 mm.
[0033] According to one variant, the outlet orifice is provided with a wire die configured to supply the homogeneous mixture in the form of a bead in step a).
[0034] According to another variant, the rush die has an outlet to which is coupled a cutting system configured to form granules from the homogeneous mixing cord.
[0035] According to one possibility, the process includes an additional step ii) of extruding the homogeneous mixture granules in a second single-screw extruder followed by a complementary step iii) of shaping the homogeneous mixture in a flat die to provide the homogeneous mixture in the form of a layer in step a).
[0036] When step i) of extrusion and step b) of co-lamination are carried out continuously, the three-layer assembly is obtained in the form of a continuous strip.
[0037] When step i) of extrusion and step b) of co-lamination are carried out discontinuously the three-layer assembly comprising the porous active electrode material is in the form of a sheet.
[0038] According to one possibility, the microporous film comprises a porosity ratio of between 20 and 70%, in particular between 30 and 60%. This promotes the removal of part of the aqueous solution from step b) while leaving a residual quantity allowing the creation of sufficient porosity in the active material, for example between 20 and 40%, which makes it possible to obtain a sufficiently low tortuosity to allow the diffusion of the lithium ions present in the electrolyte after drying in step c).
[0039] According to one arrangement, the pores of the microporous film have an average diameter of between 0.020 micrometers and 0.070 micrometers, in particular between 0.025 and 0.050 micrometers.
[0040] It is envisaged that the microporous film will consist of a microporous membrane intended to separate the cathode and the anode of the cell in the batteries, the microporous membrane being permeable to the ions of the cell's electrolyte.
[0041] Said microporous membrane is also well known by those skilled in the art as a "separator" or "membrane separator".
[0042] According to one possibility, the microporous film comprises at least one layer of polypropylene and / or polyethylene.
[0043] According to one variant, the microporous film consists of a three-layer membrane of polypropylene and / or polyethylene.
[0044] According to one arrangement, the microporous film has a thickness between 10 and 120 micrometers and for example a thickness between 10 and 50 micrometers so as to obtain a compromise on the capacity for evaporation or elimination of the aqueous solution and on the mechanical resistance of the microporous film.
[0045] For example, the microporous film is a Celgard® C2325 separator, available from Celgard®, with a thickness between 15 and 40 micrometers. The porosity is between 25 and 50%, in particular 39%. The pore size is between 0.020 micrometers and 0.035 micrometers, for example 0.028 micrometers.
[0046] According to one possibility, the water-soluble binder of the homogeneous mixture consists of one or more water-soluble polymers present in a proportion between 0.5% by weight and a value strictly less than 10% by weight relative to the total weight of the constituents entering into the composition of the porous active material of the electrode in the dry state.
[0047] According to one provision, the proportion of aqueous solution in the homogeneous mixture of step a) is between 3% and 20% by weight relative to the total weight of the homogeneous mixture dry extract, and for example between 5% and 15% by weight.
[0048] This proportion of aqueous solution is determined to be sufficient to solubilize the water-soluble polymers in the mixture; an incorrect distribution of these polymers could lead to adhesion and performance problems. electrochemicals. This proportion also reduces the viscosity of the homogeneous mixture, which remains pasty and is suitable for proper flow in the extruder. Furthermore, the evaporation of water in step c) generates high porosity in the active material of the electrode, which is essential for the use of liquid electrolyte in Li-ion batteries. This proportion, much lower than that used in an aqueous coating process, advantageously limits the evaporation time during drying step c).
[0049] According to one possibility, the homogeneous mixture is obtained in the presence of at least 70% by weight, in particular at least 80% by weight, more particularly at least 90% by weight, for example at least 94% by weight of electrochemically active material, relative to its total weight.
[0050] According to a particular embodiment of the invention, the homogeneous mixture is intended for a lithium-ion battery (or LiNixMnyCozO2 with x+y+z = 1) and comprises or is made up of an electrochemically active NMC (Nickel Manganese Cobalt, of formula LiNixMnyCozO2) material, for example at 96% by weight, in particular Li(Nio.6Mn0.2Coo.2)O2 (NMC622® from Targray® - product code SNMC 03006), a carbon black electronic conductor, such as SUPER C65, in particular at 2% by weight, and a water-soluble binder comprising or being made up of carboxymethyl cellulose CMC, for example at 2% by weight, in particular CMC Ashland® CRT 10000PA, and an aqueous latex solution, such as an SBR latex solution (acronym (Anglo-Saxon Styrene Butadiene Rubber). In this particular example, the proportion of aqueous solution is 11% by weight relative to the total weight of the homogeneous dry extract mixture.
[0051] In the formula Li(Nio.6Mn0.2Coo.2)O2 (NMC622® from the company Targray®) the electrochemically active material of NMC comprises at least 60% Nickel, 20% Mn and 20% Co. In other words x = 0.6, y = 0.2 and z = 0.2 in the formula LiNixMnyCozO2.
[0052] According to one provision, the process for preparing the active assembly includes, after step c), a step of depositing d) a second film. This can be useful in cases where the microporous film has been damaged, for example, perforated during step b) of co-lamination, and there is a risk of short circuit. The second film is advantageously a cell separator.
[0053] Alternatively, the process includes, before step d), carrying out a step e) of removing the microporous film. The second film is then deposited directly onto the porous active material. Thus, the process of the invention makes it possible to retain the separator (microporous film) from the co-lamination step b) until the cell assembly. If If this one is damaged during the process, it is replaced by another new separator (the second film).
[0054] According to a second aspect, the invention provides an active assembly, in particular obtained by the process as previously described. Said active assembly comprises a strip of porous active material for an electrode between a current collector and a microporous film. It has a thickness of between 50 micrometers and 500 micrometers, preferably between 200 and 500 micrometers. These values make it possible to obtain electrodes with a high basis weight.
[0055] According to one possibility, the active assembly comprises a surface mass of porous active material of between 5 and 50 mg / cm2.
[0056] It is understood in this document that the expression "surface mass of the porous active material" corresponds to the 'weight of the electrode'.
[0057] According to other features, the preparation process of the invention comprises one or more of the following optional features considered alone or in combination:
[0058] - The aqueous solution consists of demineralized water and / or distilled water.
[0059] - By homogeneous mixture is meant a mixture of homogeneous composition. In other words, by homogeneous mixture we mean a mixture whose composition is identical in every elementary volume of the mixture.
[0060] - The temperature Te of the two rolling mill cylinders at co-rolling step b) is 25°C.
[0061] - The porosity of the porous active material is between 20 and 40% and in particular between 25 and 35%.
[0062] - The electronic conductor is chosen from carbon fibers, black carbon, carbon nanotubes and their mixtures.
[0063] - The current collector is selected from a metal strip or a grid metallic. For example, copper, aluminum, nickel, carbon felt, or stainless steel can be used as a current collector for a positive electrode; and copper or steel, processed into a cut sheet, foam metal, or rolled sheet plate, for example, can be used as a current collector for a negative electrode.
[0064] - The metal grid includes gaps of larger dimensions than those of the pores of the microporous film. Therefore, the grid cannot be considered equivalent to the microporous film / separator, which is also composed of plastic.
[0065] - The electrochemical element in which the active assembly is implemented according to The invention is an electrode for a rechargeable electrochemical accumulator, in particular a lithium accumulator or battery.
[0066] - The water-soluble polymer comprises carboxymethyl cellulose and / or acid polyacrylic.
[0067] - The homogeneous mixture comprises Hard Carbon (well known to man to the art under the Anglo-Saxon name "hard carbon", such as Carbotron®P (available from Kureha Battery Materials Japan Co.) 97% by weight, as an electrochemically active material, and sodium carboxymethyl cellulose as a water-soluble polymer, including Aqualon™ sodium carboxymethylcellulose available from supplier Ashland CRT 10000PA or blanose™ sodium carboxymethylcellulose CMC, for example 3% by weight, and 65% dry extract.
[0068] - SBR (English acronym for Styrene Butadiene Rubber) added to the mixture The above helps to improve its membership.
[0069] Other aspects, objects and advantages of the present invention will become more apparent upon reading the following description of an embodiment thereof, given by way of non-limiting example and with reference to the following figures in which:
[0070] [Fig-1] represents a diagram illustrating the process of preparing an active assembly according to an embodiment of the invention.
[0071] [Fig.2] represents a homogeneous mixture layer supplied at the outlet of a flat die according to an embodiment of the invention.
[0072] [Fig.3] represents a homogeneous mixing cord supplied at the outlet of a reed die according to another embodiment of the invention.
[0073] [Fig.4] illustrates the different effects during co-lamination with a microporous film according to the invention and with a non-porous film.
[0074] [Fig.5] represents a step of removing the microporous film according to an embodiment of the invention.
[0075] [Fig.6] represents an electrode obtained in the presence of a non-porous film during co-lamination according to an example not forming part of the invention.
[0076] [Fig.7] represents an electrode obtained using a microporous film according to an embodiment of the invention.
[0077] [Fig.8] represents a first charge / discharge cycle of a Li-ion battery made from a positive electrode according to the invention.
[0078] [Fig.9] represents a first charge / discharge cycle of a Li-ion battery using a new separator in place of the microporous separator.
[0079] [Fig. 10] represents a first charge / discharge cycle of a Li-ion battery made from a coated positive electrode and a new separator.
[0080] Figure 1 illustrates the process for preparing an active electrode assembly 100 according to the invention, which comprises an extrusion (i) to provide a homogeneous paste-like mixture 1 (step a), a step (b) of co-laminating the mixture between a current collector and a microporous film, followed by a step (c) of drying. Firstly,In order to provide a homogeneous mixture 1 in step a), the process includes a step i) of aqueous extrusion of a paste-like electrode mixture comprising at least one electrochemically active material. The extruder 2 considered in the invention is a co-rotating twin-screw extruder and ensures the obtaining of a homogeneous mixture 1 of the constituents, despite a small amount of aqueous solution in the mixture, which is therefore paste-like. Shown in [Fig. 1], the outlet of the extruder 2 is equipped with an optional flat die 3 allowing the homogeneous mixture 1 to be initially shaped into a layer ([Fig. 2]) before being supplied in step a) of the process. The layer is then laminated in step b) between a current collector 4 and a microporous film 5 so as to obtain a three-layer assembly 6 in which the mixture 1 is strongly adhered to the collector 4.This three-layer assembly 6 is wound upon itself before the drying of the mixture 1 in step c) so as to obtain a strip 9 of porous active material 7 in the active assembly 100 for the electrode. Advantageously, the microporous film 5 is a separator 8 for the electrode (microporous membrane), which facilitates cell fabrication.
[0081] This configuration offers an additional advantage as it allows for better cohesion of the complete system (negative electrode / separator 8 / positive electrode), thus improving safety during battery operation. If removal of the separator 8 were nevertheless necessary (for example, to further densify the electrode or in case of perforation of the separator 8 during the process), its microporous nature limits the adhesion of material to the separator 8 during removal, the electrode inhomogeneity defects as explained below, and consequently, problems related to poor adhesion to the current collector 4.
[0082] The homogeneous mixture 1 used for the invention is formed from at least one electrochemically active material of LiNixMnyCozO2 with x+y+z = 1 or NMC, an aqueous solution, an electronic conductor such as carbon black, and a water-soluble binder, such as latex in solution in the aqueous solution. The solid constituents of the mixture are added in the form of premixed powders, and the latex solution is diluted in a predetermined amount of aqueous solution so as to obtain the desired total proportion of aqueous solution in the mixture 1.
[0083] According to another option, the constituents are added independently of each other.
[0084] In the example shown, a flat die 3 is combined with the orifice of the extruder 2. The outlet of the extruder 2 could be without a die, particularly when the pressure is very high, and a rod die could be used instead. In this case, the homogeneous mixture 1 is extruded in the form of a rod or "cord" ([Fig. 3]).
[0085] In the illustrated example, the flat die 3 exerts pressure on the mixture 1 so as to flatten it into a layer approximately 500 µm thick. Then, step b) allows the homogeneous mixture 1 to be coated onto a current collector 4 to obtain a strip of homogeneous mixture 1 of reduced thickness. The thickness of the strip can be fine-tuned by adjusting the gap between the two cylinders of the rolling mill L to obtain the desired electrode weight. To prevent the homogeneous paste mixture 1 from adhering to the second cylinder, a microporous film 5 is simultaneously introduced between the mixture 1 and an upper cylinder of the rolling mill L so as to form the three-layer assembly 6. The presence of the microporous film 5 also allows the homogeneous removal of part of the aqueous solution at this stage, unlike lamination processes with a non-porous film 5' which generates a demixing in the strip.
[0086] Indeed, as illustrated in [Fig. 4]-B, in the presence of a standard non-porous anti-adhesive film, a demixing phenomenon occurs and the constituents regroup below the surface, thus generating whitish traces 11 that can be seen with the naked eye (refer also to [Fig. 6] illustrating the electrode obtained). Water does not evacuate as well from the homogeneous mixture, which can generate inhomogeneities in the thickness, length, and width of the band 9 of active material 7. As illustrated in [Fig. 4]-A, in the presence of the microporous film 5, the constituents remain homogeneously mixed while part of the aqueous solution escapes through the micropores.
[0087] The microporous film 5 comprises a porosity of approximately 40% and an average pore size of approximately 0.04 µm. The porosity can vary between approximately 20 and 70% and the pore size can vary between 0.020 micrometers and 0.070 micrometers, so as to allow optimal and homogeneous removal of the aqueous solution.
[0088] Since the homogeneous mixture 1 is free of organic solvent, it is not necessary to recover it by evaporation for example, nor to treat its vapors.
[0089] The drying of the resulting three-layer assembly 6 is carried out under vacuum, in particular at a temperature of approximately 60°C. The choice of temperature is a compromise between the evaporation rate of the residual aqueous solution, allowing the porous active material 7 for the electrode to be obtained, and the temperature at which the mechanical properties of the microporous separator 8 degrade.
[0090] The water-soluble binder used can be an aqueous latex solution; its proportion in the homogeneous mixture 1 is determined by the fluidity, the electrochemically active material used, the electrode thickness, and the desired weight. It can be included in the composition up to a value of less than 10% by weight of the dry extract. The aqueous solution has a variable proportion between 3 and 20% by weight of the dry extract, here approximately 15%.
[0091] After step b) of co-lamination, the three-layer assembly 6 is wound with the separator 8 ([Fig. 2]). This produces a "coil". This coil is left to dry to evaporate any residual water. Once dry, the coil is unwound to obtain the electrode. The separator 8 can then be kept for use in the assembly of the Li-ion battery, or it can be removed manually by simply pulling it off. The separator 8, made of a microporous material specifically designed to prevent adhesion to the cylinders, prevents any active material from adhering to it. Next, with or without the separator 8, the electrode is cut to the desired dimensions following a precise protocol commonly used in the laboratory. The battery is then assembled following the same protocol established in the laboratory, and a new separator 8' can be added at this point.
[0092] The operating results are now described with a particular example of an NMC electrode from a homogeneous mixture 1 made according to the invention as follows:
[0093] In the 2-screw co-rotating extruder, the following constituents are introduced, in % mass: 94% of NMC nominal composition Li(Nio.6Mno.2Coo.2)O2 (available under the name HX12TH® from UMICORE®), 3% of SUPER C65 (TIMCAL®), 1% of CMC (CTR 10000PA, DOW®), and 2% of LATEX (styrene-butadiene copolymer SBR (styrene-butadiene rubber), TRD105A, JSR, ZEON®)
[0094] The CMC polymer, solubilized in water within the extruder 2, gives the mixture rheological properties suitable for the extrusion technique. This polymer is also intended to ensure the cohesion of the porous active material 7 for the electrode.
[0095] The electrochemically active NMC material implemented in this example consists of particles having a diameter less than or equal to 21 pm. Its density is greater than or equal to 4.7 g / cm3 and the specific surface area is 0.4 m2 / g, and as indicated by the formula Li(NiO.6MNiO.2CoO.2)O2, the nickel content is 60%, the manganese content is 20% and the cobalt content is 20%.
[0096] The powders of the different components are premixed before being introduced into the extruder 2, and the aqueous solution is introduced downstream by means of a peristaltic pump. Alternatively, the powders can also be fed independently into the extruder 2 using gravimetric feeders.
[0097] The SBR latex solution is introduced simultaneously with the aqueous solution. The solution is pre-diluted so that the final dry extract of the mixture is as expected, here 89%. The proportion of total aqueous solution is 11% of the dry extract. The latex solution provides adhesion to the active assembly 100. According to In an alternative implementation, the latex solution could also be introduced downstream of extruder 2.
[0098] The constituents are mixed in the extruder 2 using mixing elements mounted on the two co-rotating screws according to step i) of the process. The homogeneous mixture 1 thus obtained is pushed towards the outlet of the extruder 2 by means of conveying and transporting elements mounted on the two co-rotating screws according to step a).
[0099] The outlet of the twin-screw extruder 2 is equipped with a flat die 3 which allows a first shaping of the mixture 1 into a layer with a width of 3 cm and a thickness of 500 µm in the present case.
[0100] The homogeneous mixing layer 1 is introduced directly into the rolling mill L, between the current collector 4 and the separator 8 (microporous film 5) according to step b) of the process, so as to obtain the three-layer assembly 6. The separator 8 used here is the Celgard® C2325, 25 µm thick, with a porosity of 39% and an average pore size of 0.028 µm. The rollers of the rolling mill L are temperature-controlled, which allows some of the aqueous solution to evaporate during this co-rolling step.
[0101] The rollers are brought together until the thickness of the trilayer assembly 6 is reached, and thus the desired final electrode weight is achieved. In the example described, the thickness of the trilayer assembly 6 is 24.45 mg / cm² with a rolling gap of 120 µm and an electrode thickness of 111 micrometers (including the separator thickness and 20 micrometers of current collector).
[0102] The three-layer assembly 6 is rolled up on itself and then dried in a vacuum oven at 60°C (step c).
[0103] After drying and therefore after removal of the residual aqueous solution, a band 9 of porous active material 7 is obtained between the current collector 4 and the separator 8 which forms the active assembly 100. The separator 8 can be retained for the assembly of the cell.
[0104] Due to concerns about potential short circuits caused by possible perforations of the separator 8 during lamination, a new separator 8' can be added after removing the one supplied during the co-lamination in step b). As illustrated in [Fig. 5], the initial separator 8 can also be removed to use the electrode as a conventional electrode (made by coating), calendered, and cut if necessary. Another possibility is to remove the initial separator 8 if it has been damaged and replace it with a new separator 8'. Indeed, the microporous film or separator 8, made of a specific material that prevents adhesion during co-lamination (for example, plastic), allows, depending on the composition of the active material 7 and its surface condition, the active material 7 to prevent adhesion to the film 8.
[0105] The use of a non-porous film was tested in the same process to identify the differences obtained with a microporous film during the co-lamination and drying steps b) and c). Eight different non-porous films were tested. Photographs of the assemblies obtained after drying are shown in [Fig. 6].
[0106] The surfaces of the assemblies show whitish spots characteristic of inhomogeneities in the band 9 of porous active material 7 for electrode: the aqueous solution is trapped between the current collector 4 and the non-porous film, which causes a migration of the binder (latex) in the thickness of the band 9 and the formation of inhomogeneities as illustrated in [Fig.4] - B. The water does not evacuate as well from the homogeneous mixture which can generate inhomogeneities in the thickness, length and width of the active material which can generate adhesion defects on the collector 4.
[0107] Conversely, when a microporous film 5 or separator 8 is used during the process, these surface inhomogeneities are not formed. The photograph in [Fig. 7] illustrates the active assembly 100 obtained with the use of a microporous separator 8. No whitish trace is visible; the surface exhibits a homogeneous coloration.
[0108] In order to evaluate the behavior of the electrode formed from the active assembly 100 according to the invention, training cycles (1st charge / discharge cycle) were carried out with accumulators obtained by different techniques and the results are shown in figures 8 to 10.
[0109] The positive electrodes in Figures 8 and 9 were manufactured by extrusion, while the positive electrodes in [Fig. 10] were made by coating. The separator 8 in [Fig. 8] was used as a microporous film 5 for lamination, while the separator in Figures 9 and 10 was replaced by a new separator 8'.
[0110] The formation cycle of all these cells is carried out at 60°C, with a CC step at C / 10 followed by a CV step at 4.2V. As shown in Figures 8 to 10, the results illustrate that it is possible to produce a Li-ion battery with a positive electrode extruded in aqueous solution and a separator 8 used as a microporous film 5 during the lamination step.
[0111] Thus, the invention provides a decisive improvement to the prior art, by providing a method for preparing an active assembly 100 for electrode, comprising a band 9 of homogeneous porous active material 7 adhered to a current collector 4 and to a microporous film 5, such as a cell separator 8, by a simplified implementation, not requiring the use of organic solvent or even the change of the separator 8 during assembly in the cell.
[0112] Electrodes exhibiting good performance after impregnation with an electrolyte are thus obtained. Furthermore, obtaining a separator 8 laminated onto the porous active material 7 improves battery safety without degrading its performance. Indeed, the bonding of the separator 8 to the porous active material 7 of the electrode reduces the possible movements of the latter, particularly in the event of battery swelling, which therefore limits the risk of short circuit.
Claims
Demands
1. A method for preparing an active assembly (100) for an electrode intended for electrochemical elements, in particular metal-ion batteries, the method comprising the following steps: a. supplying a homogeneous mixture (1) comprising at least one electrochemically active material and an aqueous solution, the homogeneous mixture (1) being intended, after drying, to form a porous active material (7) for an electrode, b. shaping the homogeneous mixture (1) by co-lamination between a current collector (4) and a microporous film (5) so as to form a three-layer assembly (6) comprising the homogeneous mixture (1) laminated in the form of a strip, one face of which is assembled to the current collector (4) and the opposite face of which is assembled to the microporous film (5), the co-lamination partially reducing the amount of aqueous solution present in the homogeneous mixture (1), c.drying of the trilayer assembly (6) so as to evaporate the residual aqueous solution and obtain an active assembly (100) comprising a band (9) of porous active material (7) between the current collector (4) and the microporous film (5).
2. A method according to claim 1, wherein prior to step a) of supplying the homogeneous mixture (1), carrying out a step i) of aqueous extrusion of constituents comprising at least the electrochemically active material, an electronic conductor, a water-soluble binder and an aqueous solution, so as to supply the homogeneous mixture (1) in step a).
3. A method according to any one of the preceding claims wherein the microporous film (5) comprises a porosity ratio of between 20 and 70%, in particular between 30 and 60%.
4. A method according to any one of the preceding claims wherein the pores of the microporous film (5) have an average diameter of between 0.020 micrometers and 0.070 micrometers, in particular between 0.025 and 0.050 micrometers.
5. A method according to any one of the preceding claims, wherein the microporous film (5) is constituted by a microporous membrane (8) intended to separate the cell cathode and anode in batteries, the microporous membrane being permeable to ions of the cell electrolyte.
6. A method according to any one of claims 2 to 5, wherein the water-soluble binder of the homogeneous mixture (1) consists of one or more water-soluble polymers present in a proportion of between 0.5% by weight and a value strictly less than 10% by weight relative to the total weight of the constituents forming part of the porous active material (7) of the electrode in the state
7. □CC. A process according to any one of the preceding claims, wherein the proportion of aqueous solution in the homogeneous mixture (1) of step a) is between 3% and 20% by weight relative to the total weight of the homogeneous mixture (1) dry extract, and for example between 5% and 15% by weight.
8. A method according to any one of the preceding claims, wherein the homogeneous mixture (1) is intended for a lithium ion battery and comprises an electrochemically active NMC material, an electronic carbon black conductor, for example SUPER C65, and a water-soluble binder comprising carboxymethyl cellulose (CMC) and an aqueous latex solution.
9. Active assembly (100), in particular obtained by the process according to any one of claims 1 to 8, which comprises a strip (9) of porous active material (7) for electrode between a current collector (4) and a microporous film (5), and has a thickness of between 50 micrometers and 500 micrometers, preferably a thickness of between 200 and 500 micrometers.
10. Active assembly (100) according to claim 9, which comprises a surface mass of porous active material (7) of between 5 and 50 mg / cm2.
11. Active assembly (100) according to claim 9 or 10, wherein the porosity of the porous active material (7) is between 20 and 40%, and in particular between 25 and 35%.