Method for manufacturing electrodes, electrodes, dry coating compositions, batteries, and electronic circuits

Abstract

The present invention relates to a method for producing an electrode, comprising steps (A) of providing a conductive substrate, (B) of providing a coating composition comprising an electrode active material and optionally a binder, where the coating composition is a free-flowing powder, (C) of coating the conductive substrate provided in step (A) with the coating composition provided in step (B), and (D) of heating the coated conductive substrate obtained in step (C) and optionally compressing the coated conductive substrate (14), such as by calendaring. The present invention also relates to electrodes, dry coating compositions, batteries, and electric circuits.

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing electrodes, and more particularly to electrodes, dry coating compositions, batteries, and electrical circuits. [Background technology]

[0002] Ion batteries, such as lithium-ion batteries, typically have electrodes manufactured by dispersing powdered active material in a solvent. The dispersion is then doctor-bladed onto a current collector substrate, typically a metal foil, and subsequently dried to obtain the electrodes. Such coating processes require the use of solvents that are harmful to health and / or the environment.

[0003] Furthermore, as described, for example, by Kuang et al. in “Thick Electrode Batteries: Principles, Opportunities, and Challenges”, Adv. Energy Mater. 2019, 9, 1901457, applying a thicker layer of active material as a dispersion onto the current collector substrate to provide a thicker electrode and enable a higher battery energy surface density may result in several drawbacks due to the drying process caused by the evaporation of the solvent in the dispersion. In particular, if the drying process is too fast, it may lead to the formation of bubbles or poor contact between the current collector substrate and the active material. [Overview of the Initiative]

[0004] In light of the above, there is a need for a method of manufacturing electrodes that solves the aforementioned problems. [Means for solving the problem]

[0005] The present invention solves these problems with the method described in claim 1, the electrode described in claim 10, the coating composition described in claim 12, the battery described in claim 14, and the circuit described in claim 15.

[0006] The present invention relates to a method for manufacturing an electrode, comprising: (A) a step of preparing a conductive substrate; (B) a step of preparing a coating composition comprising an electrode active material and optionally a binder, wherein the coating composition is a free-flowing powder; (C) a step of coating the conductive substrate prepared in step (A) with the coating composition prepared in step (B); and (D) a step of heating the coated conductive substrate obtained in step (C) and optionally compressing the coated conductive substrate (14) by calendering or the like.

[0007] The above coating process utilizes a dry coating composition that is a freely flowing powder. Since it does not require a solvent to prepare a dispersion, it is more efficient in terms of cost and chemical consumption. In principle, these advantages can be realized for all types of conductive substrates. The conductive substrate can be a current collector substrate. The method of the present invention is particularly suitable for manufacturing three-dimensional (3D) electrodes. The freely flowing powder can easily penetrate cavities and pores, such as those in porous conductive substrates. Since the coating composition is a freely flowing powder, solvent evaporation is not required, simplifying the manufacturing process.

[0008] Along with the embodiments of the three-dimensional fiber network electrode, methods for manufacturing battery electrodes of various thicknesses are described here. However, the present invention is not limited to a three-dimensional fiber network and can also be realized using other conductive substrates.

[0009] According to the present invention, the conductive substrate prepared in step (A) is preferably a porous conductive substrate. By using a free-flowing powder as the coating composition, it can easily penetrate the pores of such a porous conductive substrate, making it possible to manufacture a thick electrode.

[0010] From the viewpoint of using a porous conductive substrate, the pores of the porous conductive substrate are preferably in the range of 0.1 to 2000 μm. The range of 0.1 to 2000 μm refers to the average pore diameter. More preferably, the range is 0.5 to 1000 μm, more preferably 1 to 1000 μm, even more preferably 2 to 500 μm, and even more preferably 5 to 500 μm. The average pore diameter can be determined by reproducing the 3D structure of the porous conductive substrate using a microcomputer tomograph and then evaluating the average pore diameter using the bubble point method. The bubble point method determines the maximum ball diameter that can fit into the cavity of the porous substrate, which is considered to be the pore diameter. More specifically, a point is placed at the center of the cavity of the conductive substrate, for example, between two fibers, and the radius of the bubble is increased from the point until it contacts each surface adjacent to the cavity of the conductive material, for example, both fibers. The diameter of the bubble corresponds to the pore diameter. If, under arbitrary given parameters, the bubble diameter contacts only one surface, for example, one fiber, then the center point is shifted in the direction of the surface that the bubble did not contact, for example, the fiber.

[0011] In particular, when the conductive substrate includes or consists of a three-dimensional network of metal fibers, the porosity of the conductive substrate is preferably in the range of 95% to 99.5% by volume, especially in the range of 96% to 99.4% by volume, and especially in the range of 97% to 99.0% by volume. Such high porosity allows for the addition of a large amount of electrode active material, reduces the proportion of inert components, and thereby improves battery performance per unit mass. Porosity can be determined by reproducing the fiber structure using a microcomputer tomograph and then evaluating the porosity using the bubble point method described herein.

[0012] The conductive substrate is not particularly limited. The present invention can be realized with many different conductive substrates. Certain advantages can be particularly realized when the conductive substrate has a three-dimensional structure, such as a woven fabric, nonwoven fabric, or network of fibers. The fibers should have sufficient conductivity, such as carbon nanotubes, graphite fibers, or metal fibers. The conductivity of the conductive substrate is 10 5It is preferable that the ratio be S / m or higher. There is no particular upper limit; it is determined by the material and structure of the conductive substrate.

[0013] According to the present invention, the conductive substrate prepared in step (A) preferably contains a plurality of metal fibers forming a network of metal fibers. These metal fibers impart a porous structure to the conductive substrate, and its pores can be easily filled with a free-flowing powder coating composition in step (C). The metal fibers impart high conductivity and a high surface area on which electrode active material can be deposited. This makes it possible to keep the mass of the substrate low relative to the mass of the active material and to obtain a high energy density.

[0014] In the conductive substrate prepared in step (A), it is preferable that the metal fibers forming the network are in direct contact with each other. This improves the overall conductivity of the conductive substrate, even when the thickness of the conductive substrate is very large.

[0015] Preferably, the thickness of the conductive substrate prepared in step (A), especially when it has a three-dimensional structure, is 200 μm or more, more preferably 500 μm or more, even more preferably 550 μm or more, even more preferably 600 μm or more, even more preferably 750 μm or more, even more preferably 1000 μm or more, even more preferably 1500 μm or more, even more preferably 3000 μm or more, and most preferably 5000 μm or more. The thickness of the conductive substrate is not particularly limited when it has a three-dimensional structure. It may be 10 mm or less, preferably 8 mm or less, even more preferably 5 mm or less, and even more preferably 3 mm or less.

[0016] The contact points between the metal fibers constituting the conductive substrate prepared in step (A) can vary over a wide range. Preferably, the density of contact points is 1 mm -3 ~5000mm -3 This is within the range. More preferably, the density of contact points is 3 mm -3 ~2000mm -3 , more preferably 5 mm -3 ~500mm-3 within the above range. The density of contact points can also be regarded as the inter-fiber crosslink density, particularly when metal fibers are fixed to each other at the contact points, that is, when the metal fibers are directly fixed to each other and are in electrical contact with each other at the contact points. 1 mm -3 or more, particularly 5 mm -3 or higher contact point density achieves uniform potential distribution and avoids adverse effects such as high overvoltage or generation of local hot areas due to high resistance. On the other hand, 5000 mm -3 or less, particularly 2000 mm -3 or less, more particularly 500 mm -3 or lower contact point density is useful for imparting flexibility to the three-dimensional network of metal fibers forming the conductive substrate. As a result, even a considerably thick three-dimensional network, that is, even a network having a thickness of 200 µm or more, 500 µm or more, 550 µm or more, 600 µm or more, or 750 µm or more, can be deformed, for example rolled, without destroying the network.

[0017] It is also preferable that the volume fraction of metal fibers in the conductive substrate, particularly in a three-dimensional network of metal fibers, is 0.075 volume% or more, particularly 1.3 volume% or more, more particularly 2.0 volume% or more. In a network having a lower volume fraction, it may be difficult to uniformly distribute local potentials, which as a result may lead to the formation of hot spots and high overvoltage. Therefore, the battery life can be extended by the volume fraction of metal fibers in the three-dimensional network of metal fibers specified as above. The volume fraction of metal fibers in a three-dimensional network of metal fibers can be determined by reconstructing the fiber structure using microcomputer tomography and then evaluating the fraction using the bubble point method described herein.

[0018] The conductive substrate prepared in step (A), particularly when it comprises or consists of a three-dimensional network of metal fibers, has an electrical conductivity of 1×10 5 S / m or more, particularly 5×10 5 S / m or more, particularly 1×10 6not less than S / m, having a porosity in the range of 95 vol% to 99.5 vol%, particularly in the range of 96 vol% to 99.4 vol%, particularly in the range of 97 vol% to 99.0 vol%, and it is particularly preferable that the volume fraction of the metal fibers in the three-dimensional network of metal fibers is 0.075 vol% or more, particularly 1.3 vol% or more, particularly 2.0 vol% or more.

[0019] Preferably, in the conductive substrate, fibers, particularly metal fibers, are in direct electrical contact with each other so as to maximize conductivity. In this regard, it is particularly preferable that all metal fibers are sintered to other metal fibers without requiring an additional binder such as a polymer binder or solder, and it is most preferable that the metal fibers are directly sintered to other metal fibers. Therefore, it is further preferable that the metal fibers are fixed to each other without a polymer binder, because such polymer binders often have low conductivity and high-temperature performance.

[0020] It may be preferable that the metal fibers comprise at least one selected from copper, silver, gold, nickel, palladium, platinum, cobalt, iron, chromium, vanadium, titanium, aluminum, silicon, lithium, manganese, boron, combinations of the above, and alloys containing one or more of the above, for example, CuSn8, CuSi4, AlSi1, Ni, stainless steel, Cu, Al, or Vitrovac alloys. Vitrovac alloys are Fe-based and Co-based amorphous alloys. It may be particularly preferable that the metal fibers are made of copper, aluminum, or a stainless steel alloy. Different types of metal fibers can be combined with each other, as a result, the conductive substrate can comprise metal fibers made of, for example, copper, one or more stainless steel alloys, and / or aluminum. It is particularly preferable that the network is made of metal fibers composed of copper, aluminum, cobalt; or a stainless steel alloy containing copper, aluminum, silicon, and / or cobalt.

[0021] The fibers can be sintered together with each other, for example, as described in WO 2020 / 016240 A1.

[0022] According to the present invention, preferably, before being fixed to each other, the metal fibers of the conductive substrate prepared in step (A) exhibit an exothermic event when heated in DSC measurement, and said exothermic event releases energy in an amount of 0.1 kJ / g or more, more preferably 0.5 kJ / g or more, even more preferably 1.0 kJ / g or more, and most preferably 1.5 kJ / g or more. The absolute amount greatly depends on the metal or metal alloy used. The degree of the exothermic event can be determined by comparing DSC measurement values of the metal fibers before and after thermal equilibration. In other words, metal fibers exhibiting such an exothermic event are not in a thermodynamically equilibrium state at ambient temperature. During heating in DSC measurement, the metal fibers may transition from a metastable state to a more thermodynamically stable state, such as through crystallization, recrystallization, or other relaxation processes that reduce defects in the lattice of metal atoms. The exothermic event observed in metal fibers when heated, such as during DSC measurement, indicates that the metal fibers are not in a thermodynamically equilibrium state; for example, the metal fibers may be in an amorphous or nanocrystalline state containing defect energy and / or crystallization energy that is released during heating of the metal fibers as crystallization or recrystallization occurs. Such an event can be identified using methods such as DSC measurement. It has been found that the strength of a network of metal fibers exhibiting such an exothermic event is improved after the metal fibers are fixed to each other.

[0023] Optionally, the metal fibers of the conductive substrate have a non-circular cross-section, in particular a rectangular, square, partially circular, or elliptical cross-section having a major axis and a minor axis. Such a cross-section usually results in fibers that are not in a thermal equilibrium state, i.e., in a metastable state, which may be beneficial for some applications. The non-circular cross-section further increases the surface area per mass of the metal fibers, thereby also contributing to the performance of the electrode per mass.

[0024] In this regard, it is to be explicitly noted that the value of the minor axis must be smaller than the value of the major axis. In case the minor axis has a larger value, i.e., a longer length than the major axis, the definitions of "minor" and "major" shall simply be interchanged.

[0025] The ratio of the minor axis to the major axis is preferably in the range of 1 to 0.05, more preferably in the range of 0.7 to 0.1, and particularly preferably in the range of 0.5 to 0.1. As is generally known, the ratio of the lengths of the minor axis to the major axis of an ellipse becomes larger as the ellipse begins to resemble a circle, in which case the ratio will be 1. The smaller the value of the ratio, the flatter the ellipse. Therefore, the ratio of the minor axis to the major axis is particularly less than 1.

[0026] Alternatively, metal fibers can have a circular cross-section. In such a cross-section, the ratio of the "long" axis to the "short" axis will obviously be exactly 1. A circular cross-section has a more energetically favorable state compared to a cross-section with an aspect ratio of less than 1. Therefore, fibers with a circular cross-section are closer to an energetically equilibrium state than fibers with other cross-sectional shapes.

[0027] Preferably, the metal fibers of the conductive substrate are melted, particularly by vertical or horizontal melt spinning, to form a molten material of metal fibers 10 2 Kmin -1 This can be obtained by subjecting it to the above cooling rate. Such metal fibers produced by melt spinning may contain a spatially limited region of high-energy states (i.e., metastable states) due to the rapid cooling applied during the melt spinning process. Rapid cooling in this context refers to 10 2 K· min -1 Preferably 10 4 K· min -1 The above is more comfortable 10 5 K· min -1 This refers to the cooling rate described above. Therefore, the step (A) of preparing the conductive substrate preferably includes the production of metal fibers as described above and the formation of a network of metal fibers.

[0028] Furthermore, in the case of the conductive substrate prepared in process (A), the fibers obtained by melt spinning often have a rectangular or semi-elliptical cross-section, which is far from equilibrium and therefore preferable in certain application fields. Examples of melt spinning machines capable of producing such fibers are known, for example, from the unpublished international application PCT / EP2020 / 063026 and the published applications WO2016 / 020493A1 and WO2017 / 042155A1 (which are incorporated herein by reference).

[0029] In another example, at least some of the metal fibers among a group of metal fibers are amorphous, or at least some of the metal fibers among a group of metal fibers are nanocrystalline. Nanocrystalline metal fibers contain crystalline regions. When heated to a temperature of about 20–60% of the melting point of nanocrystalline metal fibers, these regions undergo recrystallization, resulting in an increase in the average size of the crystalline regions compared to the average size of the initial crystalline regions in the nanocrystalline metal fibers before heating. It is also possible to mix non-equilibrium fibers (e.g., nanocrystalline or amorphous fibers) with equilibrated fibers (e.g., annealed fibers).

[0030] The metal fibers that are preferably included in the conductive substrate prepared in step (A) may preferably have a length of 1.0 mm or more, and / or a width of 100 μm or less, and / or a thickness of 50 μm or less. With metal fibers having such dimensions, it is possible to manufacture a network in which the metal fibers are fixed to each other without having to heat the metal fibers to a temperature close to the melting point for more than 30 minutes. Conventional connection techniques, such as sintering, welding, or mixing techniques, require maintaining a temperature close to or slightly above the melting point of the metal for a relatively long period of time. This can cause the material of the metal fibers to melt or at least soften to some extent, and as a result, the metal fibers form a metal foil rather than a network, especially when relatively high pressure is applied during sintering. Since a network of metal fibers is not a metal foil, that is, the structure of the metal fibers used to manufacture the network of metal fibers is still recognizable in the network of metal fibers. Therefore, in a cross-sectional view of the network of metal fibers, there are voids between the metal fibers of the network, not parts of the metal fibers themselves.

[0031] Furthermore, the width of the metal fibers is preferably 80 μm or less, more preferably 70 μm or less, even more preferably 40 μm or less, and most preferably 10 μm or less. Furthermore, the thickness of the metal fibers is preferably 50 μm or less, more preferably 30 μm or less, even more preferably 10 μm or less, and most preferably 5 μm or less.

[0032] Regarding step (A), as described above, it is preferable to provide the conductive substrate on a support, especially when the conductive substrate has a porous structure. The support prevents the free-flowing powder from flowing out through the conductive substrate, and as a result, its pores or cavities can be filled with the coating composition. The support is not particularly limited. Preferably, the support is a non-adhesive support so that it can be easily removed after step (D) is performed. Furthermore, the support should be thermally stable under the conditions of step (D). Thermal stability in this context means that the support does not melt or decompose when step (D) is performed. That is, the support does not adhere to the conductive substrate and electrode active material after step (D) is performed. The support may be, for example, silicon-treated paper, a polymer sheet, a metal foil, or a nonwoven or woven sheet of fibers (e.g., natural fibers).

[0033] According to the present invention, in step (B) of preparing the coating composition, it is preferable to mix an electrode active material, optionally a polymer binder, and optionally a conductivity-enhancing additive (e.g., carbon black) to obtain a coating composition as a free-flowing powder. The coating composition may contain an electrode active material, a polymer binder, and a conductivity-enhancing additive (e.g., carbon black).

[0034] The coating composition prepared in step (B) is particularly preferably having a particle size smaller than the average pore diameter described above for the porous conductive substrate prepared in step (A). The particle size is preferably in the range of 80% or less, more preferably 50% or less, even more preferably 30% or less, and most preferably 20% or less, compared to the average pore diameter. The particle size of the free-flow coating composition can be determined by commonly applied methods such as laser diffraction or shifting.

[0035] The coating composition prepared in step (B) preferably contains a polymer binder as a binder. The polymer binder is not particularly limited and may be a fluorine-containing polymer, carboxymethylcellulose, styrene-butadiene rubber, or other rubber. The binder, especially the polymer binder, may preferably be present in an amount of 0 to 30% by weight of the coating composition, more preferably 0.5 to 20% by weight of the coating composition, even more preferably 1 to 15% by weight of the coating composition, and even more preferably 1.5 to 7% by weight of the coating composition. Such a fluorine-containing polymer binder has excellent electrochemical stability and can also provide good adhesion between the electrode active material and the conductive substrate. Suitable examples of fluorine-containing polymer binders include poly(vinylidene fluoride) (PVDF) and poly(tetrafluoroethylene) (PTFE), but other polymer binders can also be used. It is also possible to use PVDF derivatives such as PVDF-HFP (poly(vinylidene fluoride-co-hexafluoropropylene)) or PTFE derivatives. However, it is also possible to realize the present invention without using a binder, or by using a different binder. By avoiding the use of solvents, the risk of binder degradation is reduced, thereby making it possible to use many different types of polymers as binders. In this regard, the polymer binder is preferably thermoplastic.

[0036] Preferably, the glass transition temperature and / or melting point of the polymer binder, particularly the melting point, is greater than 30°C, more preferably greater than 50°C, and even more preferably greater than 70°C. When the glass transition temperature and / or melting point are above the above temperatures, the polymer binder is non-sticky at room temperature, thereby enabling the preparation of a free-flowing powder in step (B) without the use of an anti-caking agent. The glass transition temperature (Tg) and melting point (Tm) of the polymer binder can be measured using differential scanning calorimetry (DSC). During operation, secondary batteries, such as lithium-ion batteries, can become hot. Having the glass transition temperature and / or melting point within the above range prevents extensive softening and flowing of the binder during battery operation, thereby extending the life of the corresponding battery.

[0037] In step (B) of the method according to the present invention, it is preferable to prepare an electrode active material that can intercalate monovalent or polyvalent ions, particularly lithium ions.

[0038] Preferably, in step (B) of the method according to the present invention, the coating composition is prepared by mixing the components of the composition such that the electrode active material constitutes up to 100% by weight of the coating composition, more preferably up to 99% by weight of the coating composition, more preferably 70-99% by weight, even more preferably 80-98% by weight, and most preferably 80-95% by weight. By using a porous conductive substrate, particularly a network of metal fibers, the use of binders and conductivity-enhancing additives can be minimized, making it possible to use a very large amount of electrode active material. Therefore, when using a porous conductive substrate, the coating composition is prepared by mixing the components of the composition such that the electrode active material constitutes preferably 80-100% by weight of the coating composition, more preferably 90-100% by weight of the coating composition, even more preferably 95-100% by weight of the coating composition, even more preferably 98-100% by weight of the coating composition, even more preferably 99-100% by weight of the coating composition, and most preferably 100% by weight of the coating composition. A particularly preferred range is 95-98% by weight of the coating composition.

[0039] According to one preferred embodiment, the electrode active material is at least one selected from the group consisting of graphite, graphene, silicon, silicon carbide (SiC), and tin oxide (SnO), tin dioxide (SnO2), and lithium titanium oxide (LTO), or mixtures thereof. In another embodiment, the electrode material is a mixture of graphite and at least one selected from the group consisting of graphene, silicon, silicon carbide (SiC), and tin oxide (SnO), tin dioxide (SnO2), and lithium titanium oxide (LTO), particularly a mixture of graphite and silicon and / or silicon carbide. This is particularly preferred when electrodes manufactured by the method of the present invention are used as anodes in monovalent or polyvalent ion batteries, especially lithium-ion batteries.

[0040] According to another preferred embodiment, the electrode active material is at least one selected from the group consisting of lithium nickel manganese cobalt oxide (NMC), lithium nickel manganese oxide (NMO), lithium nickel cobalt aluminum oxide (NCA), lithium cobalt oxide (LiCoO2), and lithium iron phosphate (LFP). This is particularly preferred when the electrode manufactured by the method of the present invention is used as a cathode in a monovalent or polyvalent ion battery, especially a lithium-ion battery.

[0041] Regardless of whether the electrodes manufactured according to the present invention are intended to be used as the cathode or anode of a battery, as described above, the electrode active material preferably includes thermally pre-treated, i.e., calcined, graphite. It has been found that heat treatment of graphite makes it free-flowing, i.e., non-stick. For example, heat treatment can be performed by heating to 1000°C or higher in a protective gas atmosphere (such as argon) or in a vacuum. Although not bound by theory, it is assumed that the heat pre-treatment reduces the number of surface hydroxide groups and therefore reduces its affinity for water. This improves the free-flowing behavior of the graphite. If the manufactured electrode is intended for use as an anode, the graphite may constitute the majority or all of the electrode active material and may make up to 100% by weight of the coating composition. Alternatively, it may be used together with other suitable anode electrode active materials, such as those shown above. If the manufactured electrode is intended for use as a cathode, the graphite is preferably present in an amount of up to 20% by weight of the coating composition, more preferably up to 10% by weight of the coating composition, and even more preferably up to 5% by weight of the coating composition. While there are no particular restrictions on the amount of graphite used in the cathode electrode active material, it may be preferable to use at least 0.5% by weight, and more preferably at least 1.0% by weight, of the coating composition to obtain a certain improvement in the free-flow behavior of the coating composition.

[0042] Furthermore, in step (B) of the method according to the present invention, when preparing the coating composition, it is preferable that a conductivity-improving additive, such as carbon black, more preferably conductive carbon black, is included. Such carbon black can reduce the internal resistance of the final electrode. Such conductive carbon black can be obtained, for example, as Super-P carbon black. In the method of the present invention, the amount of such conductive carbon black is usually less than 10% by weight of the coating composition, more preferably less than 8% by weight of the coating composition, and even more preferably less than 6% by weight of the coating composition. In order to fully exhibit the effect of the conductivity-improving additive (e.g., carbon black), it is preferable to include it in an amount of at least 0.2% by weight of the coating composition, more preferably at least 0.5% by weight of the coating composition, and even more preferably at least 1.0% by weight of the coating composition.

[0043] In the method of the present invention, step (B) of mixing the components of the coating composition is not particularly limited and can be achieved by various mixing operations such as stirring, shaking, grinding (e.g., grinding in a mortar), milling (e.g., ball milling), etc. The mixing in step (B) is completed when each component is uniformly mixed. If the electrode active material contains or consists of graphite, it is preferable to mix the graphite and optionally other components by milling, especially ball milling, in order to activate the graphite. According to the present invention, in the step

[0044] In the method according to the present invention, the coating composition prepared in step (B) is dry, i.e., essentially free of water and other solvents. This allows the coating composition to be prepared as a free-flowing powder, which can be easily distributed onto a conductive substrate and enter structures provided thereon, such as pores or other cavities. Furthermore, evaporation of water or other solvents is not required.

[0045] Step (C) of the method according to the present invention preferably includes filling the pores or cavities of the conductive substrate prepared in step (A) with a free-flow coating material.

[0046] Filling such pores or cavities can be achieved by various methods. For example, a free-flowing powder can be dispersed on / inside a conductive substrate. Alternatively, a coating composition can be sprayed onto / inside a conductive substrate using a conventional sprayer or electrostatic sprayer.

[0047] To improve the filling of pores and cavities, it may be preferable in step (C) of the method according to the present invention that the conductive substrate is subjected to movement during or after coating with the coating composition prepared in step (B). Such movement may be shaking, vibration, rattling, or oscillation. Vibration can be achieved in step (C) by, for example, ultrasonic treatment.

[0048] In particular, when using a porous conductive substrate, it is preferable to fill the pores and / or cavities of the conductive substrate by coating in step (C). It is even more preferable to remove the supernatant coating composition, i.e., the coating composition that did not flow into the pores and / or cavities, before submitting the coated substrate to step (D). Removal can be achieved by skimming, for example, using a doctor blade. Skimming can also contribute to improved filling of pores and / or cavities. Therefore, the thickness of the coated conductive substrate will be essentially the same as that of an uncoated conductive substrate.

[0049] In the method of the present invention, it is preferable to compress the conductive substrate coated with the coating composition obtained after step (C) in step (D). Such compression can further shorten the process time. In addition to the economic advantages, compression also reduces the risk of binder spreading on the surface of the electrode active material and optional additives. Furthermore, compression allows for control of the residual porosity of the final electrode.

[0050] In step (D), it is particularly preferable to apply compression to a range of 10-70% of the thickness of the coated conductive substrate obtained after step (C). The degree of compression can be selected to adjust the desired residual porosity of the final electrode.

[0051] In step (D) of the method of the present invention, the temperature applied by heating is preferably set to be above the Tg of the binder, particularly the polymer binder. Particularly when compression is applied in step (D), the applied temperature is also preferably the Tm of the binder, particularly the polymer binder. Preferably, the applied temperature is between the Tg and Tm of the binder, particularly when compression is applied in step (D). It is preferably below the Tm of the binder in the range of 1°C to 50°C, more preferably 2°C to 30°C, and even more preferably 3°C to 20°C. For example, PVDF is a suitable polymer binder having a Tm of 177°C. Therefore, in the heating in step (D), it is preferable that heating is applied to provide a temperature in the range of 127°C to 176°C, more preferably 147°C to 175°C, and even more preferably 157°C to 174°C. In general, it is preferable to bring the temperature applied in step (D) close to the Tm of the binder. In this regard, temperatures of 50°C, 30°C, or 20°C below the binder's Tm are particularly preferred. A temperature close to the binder's Tm activates the binder, bonding the components of the coating composition together and also bonding with the conductive substrate. A temperature closer to the binder's Tm is more preferable because it allows for a shorter process time in step (D), thereby providing economic advantages. If the temperature applied in step (D) exceeds Tm, the binder melts. This can cause the binder to spread across the surface area of ​​the electrode active material and optionally included additives, such as carbon black. This can result in the electrode active material being covered by a binder film, potentially leading to insufficient bonding and reduced electrode performance. Therefore, the applied temperature is preferably at least 1°C, at least 2°C, or at least 3°C ​​lower than the binder's Tm. A specific difference between Tm and the applied temperature provides greater stability to the process and reduces the risk of accidental overheating, i.e., heating above Tm. Some materials have only a Tg and no Tm. For other materials, Tm is above the decomposition temperature (Tz). In any case, it is preferable to keep the temperature applied in process (D) below Tz.In this specification, Tz refers to the decomposition onset temperature determined by thermogravimetric analysis (TGA) under a protective gas atmosphere. Therefore, it is also preferable to set the applicable temperature between the binder's Tg and Tz, preferably at least 10°C above Tg, more preferably at least 20°C above Tg, and even more preferably at least 30°C above Tg but still below Tz.

[0052] Heating and compression can be performed sequentially. Alternatively, more preferably, heating and compression in step (D) of the method of the present invention are performed simultaneously, i.e., the coated conductive substrate is compressed at a high temperature. This can be achieved by hot pressing or by calendering the coated conductive substrate with a heated calender. The hot press or calender roll is heated to the applicable temperature. Heat is transferred by bringing the hot press or calender roll into contact with the coated substrate, either by direct contact or through a support such as those described above.

[0053] Step (D) of the method of the present invention can be carried out using various heating devices. Suitable heating devices include induction furnaces, infrared furnaces, high-temperature ceramic heating elements and / or zone furnaces, such as conveyor furnaces. The heating device may preferably be a continuous furnace or a batch furnace.

[0054] The heating in step (D) can also be carried out without compression, i.e., by heating the coated conductive substrate using the heating device described above. In such a case, temperatures above the binder's Tm can also be applied. In such a case, the applied temperature is preferably set to be 50°C, more preferably 30°C, and even more preferably 20°C below the binder's Tm, and preferably up to 20°C, more preferably up to 10°C, and even more preferably up to 5°C above the binder's Tm. Because there is no external pressure, the processing time is longer compared to when step (D) involves the application of such pressure. The uncompressed process allows for good control over the process, but the overall stability of the final electrode observed is higher when some compression is applied.

[0055] Furthermore, this application relates to an electrode comprising a conductive substrate that is dry-coated with a coating composition comprising an electrode active material and a polymer binder. Dry coating of the conductive substrate enables its manufacture without evaporation of the solvent. And, because it does not contain evaporation channels or bubbles, a higher volume density of the electrode active material is possible. Electrodes coated with dispersions have a structure determined by the evaporation of the solvent, such as evaporation channels and bubbles. Such structures are not present in electrodes obtained by the methods described herein that utilize dry coating.

[0056] Preferably, the electrode of the present invention can be obtained by the electrode manufacturing method described above and in the claims.

[0057] Furthermore, the present invention relates to a dry coating composition comprising a conductive substrate, an electrode active material, and a polymer binder, wherein the dry coating is a free-flowing powder. Since the coating composition is a dry coating composition, it is essentially free of water and solvents.

[0058] In contrast to other dry coating compositions, the amount of conductivity-enhancing additives, such as silver wire, carbon nanotubes, conductive carbon black, or conductive binders (in the case of cathodes, graphite may also be included for conductivity enhancement) can be kept to a minimum or even avoided entirely. In this regard, the content of the conductivity-enhancing additives is preferably 10% by weight or less, more preferably 8% by weight or less, and even more preferably 6% by weight or less, based on the total weight of the dry coating composition.

[0059] The dry coating composition of the present invention preferably does not contain conductive polymer binders, carbon nanotubes, and silver nanowires; that is, their total amount is preferably 0.1% by weight or less. Such conductive polymer binders are quite expensive and, as described above, are not necessary when the dry coating composition is used in combination with a porous conductive substrate.

[0060] Furthermore, the present invention relates to a battery including an electrode according to the present invention.

[0061] The details described above regarding conductive substrates and coating compositions in the context of the method of the present invention should be understood to apply in accordance with the electrodes and coating compositions of the present invention.

[0062] Herein, the present invention will be described in more detail by various examples of the networks and methods of the present invention, with reference to the accompanying drawings and figures, although these are merely illustrative. The drawings show the following: [Brief explanation of the drawing]

[0063] [Figure 1] This is an exemplary schematic diagram of the process according to the present invention. [Figure 2] This graph shows the capacitance versus voltage of the electrodes in Example 1 and Comparative Example 1. [Figure 3] This graph shows the cycle behavior of the electrodes in Example 1 and Comparative Example 1. [Figure 4]This is a scanning electrode microscope image of the electrode according to Example 1. [Figure 5] This is a scanning electrode microscope image of the electrode according to Example 5. [Modes for carrying out the invention]

[0064] Figure 1 schematically shows the preparation of electrode 1 by the method of the present invention. First, in step (A), a conductive substrate 10 having a three-dimensional structure is prepared. Figure 1 shows a cross-sectional view of the conductive substrate, where the black lines indicate a network of fibers. These fibers may be metal fibers, carbon fibers, or carbon nanotubes that impart high conductivity. The coating composition 12 is prepared in step (B) and includes an electrode active material and a binder, as well as optional additives, such as graphite and carbon black if not an electrode active material. In step (C), the conductive substrate 10 is coated with the coating composition to obtain a coated conductive substrate 14, on which a portion of the coating composition exists as supernatant 16. To improve the pore-filling properties of the conductive substrate 10, it may be subjected to vibration or other movements when filling with the coating composition 12. The supernatant 16 is removed using a doctor blade 18. Subsequently, the coated conductive substrate 14 is subjected to hot calendering between two hot calender rolls 20, thereby heating and compressing the conductive substrate in a single step to obtain the manufactured electrode. This corresponds to step (D) of the present invention.

[0065] Figure 2 shows the voltage-capacitance ratio of the two electrodes. As will be further described below in Example 1, one electrode was manufactured using the dry coating method according to the present invention. The data corresponding to the electrode according to the present invention is shown by the black line. In addition, another electrode (Comparative Example 1) was manufactured using the same conductive substrate and a coating composition that differed only in that it was applied in slurry form, by coating it using a conventional method with dispersion technology and then evaporating it. This electrode corresponds to Comparative Example 1. The data corresponding to the electrode according to Comparative Example 1 is shown by the gray line. Figure 3 shows the cycle behavior of the electrode in Figure 2. As can be easily seen, the performance behavior is similar.

[0066] Figure 4 shows a scanning electrode microscope image of the electrode of Example 1. Its conductive substrate consists of copper fibers, particularly CuSi3 fibers containing 3 wt% silicon, which sinter together to form a conductive network. In the microscope image, the metal fibers 100 can be identified as the central gray region. The graphite particles 102 appear as a dark plate, but the binder 104, which contains conductive carbon black, can be identified as white flakes. In Figure 4, reference numerals are shown only for some of the graphite particles 102 and binder particles 104.

[0067] Figure 5 shows a scanning electrode microscope examination of the electrode of Example 5. Its conductive substrate consists of aluminum fibers that sinter together to form a conductive network. In the microscope image, the metal fibers 100 can be identified as a smooth gray area in the left portion of the microscope image. The binder 104 can be identified as white flakes because it contains conductive carbon black. The electrode active material NCA 106 can be identified as light gray spherical particles. The graphite particles 102 can also be identified as a dark plate. In Figure 5, reference numerals are shown only for the graphite particles 102, the binder particles 104, and some of the NCA particles 106.

[0068] The present invention will be further described below with reference to examples.

[0069] In Examples 1 and 2, a network of copper fibers was used as the conductive substrate. In Examples 3 to 5, a network of aluminum fibers was used as the conductive substrate. The metal fiber network can be obtained as described in International Publication No. 2020 / 016240A1.

[0070] Coating compositions for preparing electrodes from these metal fiber networks were obtained by preparing the components shown in Table 1 in powder form and thoroughly mixing them in a mortar. Additive 1 is included in the binder. The coating compositions were dry, i.e., essentially free of water and other solvents, and were in the form of a free-flowing powder. The compositions shown in Table 1 are suitable for coating 2D electrodes, i.e., metal foil, and 3D electrodes, i.e., porous conductive substrates, such as networks of metal fibers. A better comparison can be made by using the same coating composition for both types of conductive substrates. Nevertheless, in the case of 3D electrodes, it is possible to use much smaller amounts of binder and additives, or even none at all.

[0071] [Table 1]

[0072] In Table 1, the values ​​in parentheses represent weight percentages based on 100% by weight of the entire coating composition. Graphite is synthetic graphite powder less than 20 μm (SigmaAldrich (Art.Nr.282863)) and is calcined graphite. PVDF is poly(vinylidene fluoride) (AlfaAesar (Art.Nr.44080)). Super-P is conductive carbon black (Alfa Aesar (Art.Nr.H30253)). LTO is lithium titanate (Targray (Art.Nr.SLTO102)), NMC622 is lithium nickel manganese cobalt oxide (Targray (Art.Nr.SNMC03006)), and NCA is lithium nickel cobalt aluminum oxide (MTI Corp. (Art.Nr.Lib-LNCA810)).

[0073] A conductive substrate was placed on a silicon-treated paper as a support to prevent the coating composition, in the form of a free-flowing powder, from flowing through the conductive substrate. For each example, the corresponding coating composition shown in Table 1 was scattered on the corresponding conductive substrate. The supernatant coating composition was then removed with a doctor blade. Next, the coated conductive substrate samples were subjected to two different types of heat treatment as described below.

[0074] Type 1 heat treatment: Each sample from Examples 1-5 was heated in a furnace to 170°C for 60 minutes. After cooling to room temperature, the samples were compressed by calendering, reducing their thickness by approximately 50%. The thickness of the samples after calendering ranged from 150 μm to 200 μm.

[0075] Second type of heat treatment: Each of the samples from Examples 1 to 5 was subjected to hot calendering using a calender roll at a temperature of 160°C and a speed of 5 m / min. During calendering, a 50% compression of the sample was achieved, resulting in a sample thickness of 150 μm to 200 μm after calendering.

[0076] After heat treatment, 14mm diameter discs were punched out from the sample. Both types of heat treatment were successful in preparing electrodes, but the second type of heat treatment (hot calendering) resulted in better adhesion between the electrode active material and the conductive substrate. Only a small amount of electrode active material was lost when punching out the disc. In contrast, when the first type of heat treatment (heating without compression followed by compression at room temperature) was applied, the loss of electrode active material during punching out the disc was observed at the edges of the punched disc, but no active material was lost at the center of the disc, meaning that an acceptable electrode was obtained there as well. The 14mm discs were tested with metallic lithium in a coin cell.

[0077] Furthermore, as Comparative Example 1, a network of metal fibers used in Example 1 as a conductive substrate was coated using a general wet coating process. In the wet coating process, the coating compositions used in Examples 1 to 5 were dispersed in acetone. Electrodes were prepared as described in International Publication No. 2020 / 016240A1.

[0078] The electrodes of Example 1 and Comparative Example 1 exhibited equivalent performance in terms of capacity and cycle behavior, as shown by the measurements in Figures 2 and 3. Furthermore, in terms of mechanical stability, the performance of these electrodes was equivalent, especially when the second type of heat treatment (hot calendering) was applied. Nevertheless, the electrode of Example 1 can be prepared using solvent-free dry coating; that is, no solvents harmful to health and / or the environment are required to prepare the electrode of Example 1. Moreover, since the electrode of the present invention is prepared using dry coating, solvent evaporation is unnecessary, and as a result, a higher volume packing density of the electrode can be obtained because there are no channels and bubbles caused by solvent evaporation.

[0079] The electrodes of Examples 1-5 also showed improved stability compared to the two-dimensional electrodes, as observed for the three-dimensional electrodes disclosed in International Publication No. 2020 / 016240A1. Therefore, when the electrodes of Examples 1-5 were bent (90° to one side, 180° to the other side, and then returned to their original position), only 1-3% by weight of the electrode active material was lost. The loss observed in Example 1 did not increase even when the amount of binder was reduced, as in Example 2. Furthermore, the electrodes of Examples 3-5 showed similarly low active material loss when subjected to bending. In contrast, when two-dimensional electrodes (copper or aluminum foil as the conductive substrate) were subjected to the same bending, the electrode active material peeled off from the conductive substrate, resulting in a loss of electrode functionality. [Explanation of Symbols]

[0080] 1 electrode 10 Conductive base material 12 Coating composition 14 Coated conductive substrate 16. Supernatant 18 Doctor Blade 20 Calendar Roll 100 Metal Fibers 102 Graphite particles 104 Binder 106 NMC particles

Claims

1. A method for manufacturing an electrode (1), Step (A) involves preparing a conductive substrate (10) which is a porous conductive substrate, Step (B) is a step of preparing a coating composition (12) comprising an electrode active material and optionally a binder (104), wherein the coating composition (12) is a free-flowing powder. Step (C) involves coating the conductive substrate (10) prepared in step (A) with the coating composition (12) prepared in step (B), The process includes heating the coated conductive substrate (14) obtained in step (C), and optionally compressing the coated conductive substrate (14) in step (D). The heating in step (D) above is set such that (i) the temperature of the binder (104) is above Tg but below Tm, If the Tm of the binder (104) exceeds the Tz of the binder (104), then (ii) the heating is set so that the Tg of the binder (104) exceeds the Tz of the binder (104), A method for manufacturing an electrode (1), comprising the step of removing the supernatant coating composition, i.e., the coating composition that did not flow into the pores and / or cavities of the porous conductive substrate, before subjecting the coated conductive substrate (14) to step (D), such that the thickness of the coated conductive substrate is essentially the same as that of an uncoated conductive substrate.

2. The method according to claim 1, wherein the conductive substrate (10) includes a plurality of metal fibers (100) that form a network of metal fibers (100).

3. The method according to claim 2, wherein the metal fibers (100) are in direct contact with each other.

4. The method according to claim 1 or 2, wherein the thickness of the conductive substrate (10) is 200 μm or more.

5. The method according to claim 1 or 2, wherein step (B) of preparing the coating composition (12) includes mixing the electrode active material, polymer binder (104), and optionally carbon black to obtain the coating composition as a free-flowing powder.

6. The method according to claim 1 or 2, wherein the electrode active material includes calcined graphite (102).

7. The method according to claim 1 or 2, wherein step (D) further comprises compressing the coated conductive substrate (14).

8. In step (D), the coated conductive substrate (14) is subjected to hot compression. The above heating is (i) set to a temperature above the binder's Tg but below the binder's Tm by 50°C, and / or The method according to claim 7, wherein the heating is (ii) set to be 10°C above the Tg of the binder but below the Tz of the binder.

9. An electrode (1) that can be obtained by the method of claim 1 or 2, An electrode (1) comprising a coated conductive substrate (14), wherein the coated conductive substrate (14) is dry-coated with a coating composition (12) comprising an electrode active material and a polymer binder.

10. A dry coating composition (12) for preparing the electrode (1) described in claim 9, and / or for use in the method described in claim 1 or 2, comprising an electrode active material and a polymer binder (104), and being a free-flowing powder.

11. The dry coating composition (12) according to claim 10, wherein the content of the conductivity-improving additive is 10% by weight or less based on the total weight of the dry coating composition.

12. A battery comprising the electrode (1) according to claim 9.

13. An electrical circuit including a battery as described in claim 12.

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