Method for manufacturing an electrode for a lithium-ion battery cell

Abstract

The invention relates to a method for producing an electrode for a lithium-ion battery cell, in which an active material layer (3) is coated onto a current collector foil (1), wherein a primer layer (5) is applied to the current collector foil (1) prior to coating the current collector foil (1) with the active material layer (3), which increases the adhesion of the active material layer (3) to the current collector foil (1). According to the invention, the application of the primer layer (5) comprises an incomplete combustion process in which soot particles (15) are deposited as a combustion product on the current collector foil (1), forming the primer layer (5).

Description

[0001] The invention relates to a method for manufacturing an electrode for a lithium-ion battery cell according to the preamble of claim 1.

[0002] A typical electrode for such a lithium-ion battery cell has a current collector foil coated with an active material layer. Before the current collector foil is coated with the active material layer, a primer layer is applied to the still metallic current collector foil. The primer layer increases the adhesion of the active material layer to the current collector foil. The primer layer usually consists of conductive carbon (for example, conductive carbon black) and a binder polymer, which enables adhesion to the current collector foil.

[0003] The following problems arise with the prior art method for applying the primer layer to the current collector foil: One initial problem concerns the fact that a primer layer is required but difficult and expensive to produce. The primer layer is a thin coating on the substrate, i.e., the electrode current collector foil. The primer layer's starting component consists of conductive carbon and a binder dispersed in a solvent. This primer layer component is applied to the current collector foil and then dried. In the next step of the process, an active material is applied to the primer layer of the current collector foil.

[0004] A second problem concerns the fact that the polymer used in the primer layer is an inactive material and does not contribute to conductivity. The polymers used are susceptible to solvent attack when not cured. This means that a primer layer without a cross-linked polymer would be useless. Thermoplastic resins or acrylic acid are used as polymers. For the anode, the resin should be hydrophobic so that it does not dissolve in water. For the cathode, it should be resistant to NMP. The polymer used lacks strength until it has cured. This means that conventional methods cannot immediately use the primer-coated electrode, as it must first cure. Most of the polymers used are less resistant to NMP. This limits the permissible polymers.

[0005] A third problem concerns the fact that the primer layer tends to be damaged upon contact with the rollers. The primer layer is a thin coating that can be damaged as soon as it comes into contact with rollers. From a manufacturing perspective, this means that it is not possible to coat the active material immediately after applying the primer layer. Instead, the current collector foil coated with the primer layer must first be stored and cured before it is coated with the active material.

[0006] Methods for producing an electrode for a lithium-ion battery cell are known from DE 10 2010 020 647 A1, DE 696 31 751 T2, US 2024 / 0 063 393 A1, and US 2022 / 0 344 672 A1. US 2017 / 0098817 A1 discloses a method for producing an electrode in which micrometer-sized anode or cathode material particles are mixed with aggregates of smaller conductive carbon black particles in two mixing steps using a liquid dispersion medium. One portion of the carbon black is mixed with the electrode particles to coat their surfaces with the smaller carbon black particles. Another portion of the carbon black is mixed with the originally coated electrode particles to form clusters of carbon black particles at the interfaces of the previously coated electrode particles.

[0007] The object of the invention is to provide a method for manufacturing an electrode for a lithium-ion battery cell, in which, compared to the prior art, a process-reliable application of the primer layer and / or an increased adhesion between the active material layer and the current collector foil is enabled.

[0008] The problem is solved by the features of claim 1. Preferred embodiments of the invention are disclosed in the dependent claims.

[0009] The invention relates to a method for manufacturing an electrode in which an active material layer is coated onto a current collector foil. Before coating the current collector foil with the active material layer, a primer layer is applied to the current collector foil, which increases the adhesion of the active material layer to the current collector foil. According to the characterizing part of claim 1, the application of the primer layer comprises an incomplete combustion process in which soot particles are deposited as a combustion product on the current collector foil, forming the primer layer. The deposition of the soot particles is advantageously carried out directly during the combustion process. In this case, the soot particles generated in the incomplete combustion process are directly, i.e.,during the combustion process, it is deposited on the current collector foil, forming the primer layer, through condensation of the combustion gas on the current collector foil.

[0010] The present invention comprises the innovations described below for producing a solvent-free, carbon-rich primer layer on aluminum and copper substrates for cathode and anode electrodes, respectively. There are various methods for producing a solvent-free primer layer, each of which is explained below.

[0011] In a first embodiment, the current collector foil is passed over a yellow-glowing flame produced by the incomplete combustion of a carbon-containing material. Any combustible material can produce soot, including natural gas, liquefied petroleum gas, wood, naphthalene, oil, candle wax, gasoline, and diesel fuel. Such a yellowing of the flame results from the incomplete combustion of carbon and generates a large amount of soot. The soot-rich combustion gas produced during the incomplete combustion comes into contact with the current collector foil, condenses there, and forms a thin carbon layer (less than 1 micrometer) on the metallic current collector foil. In this way, a primer layer forms on the current collector foil, i.e., the copper and aluminum foil. This current collector foil is then coated with an active material, either by wet or dry coating.In wet coating, a slurry of active material, conductive carbon, and binder dispersed in solvent is applied to the primer layer using a slot nozzle. After coating, the coated current collector foil undergoes a drying process. The wet-coated current collector foil passes through the drying zone, which, according to the invention, is the same zone in which the primer layer was applied. This means that the heat generated by the yellow-colored, luminous flame is now used to dry the primer layer. Thus, according to the invention, the soot from the yellow-colored, luminous flame is used to produce the primer layer, and the resulting heat is used to dry the wet active material layer. Once the coating of one side of the current collector foil is complete, the same process is carried out for the other side of the current collector foil.

[0012] According to the invention, the primer layer is therefore not formed by wet coating, but by condensation of soot-rich combustion gases on the highly conductive metal surface of the current collector foil. Furthermore, the heat from the flame used in the combustion process is used to evaporate the solvent and to produce the dry active material coating. The primer layer produced by condensation of soot-rich vapors is solvent- and binder-free. It consists of pure carbon and provides good adhesion and conductivity between the active material layer and the current collector foil. It is important that a luminous (yellow) flame is used for this purpose. Unsaturated carbon compounds do not burn completely in this case and result in a flame with unburned or partially burned carbon particles.There are two possibilities for this: either long-chain hydrocarbons can be used as fuel, or access to oxygen can be restricted. Less oxygen means more incomplete combustion, less CO2 production, and therefore more soot formation. Such incomplete combustion generates less heat. Since the heat is used for a dual purpose—drying the active material layer—complete combustion can also occur after the primer layer has been formed, in addition to incomplete combustion. In this case, more heat is available for drying the wet active material layer.

[0013] To implement the combustion process described above, a primer coating station can have two combustion chambers connected in series in one production direction: In the first combustion chamber, a long-chain hydrocarbon (e.g., naphthalene) is burned in the presence of air and CO2. Incomplete combustion leads to increased soot formation. The soot-rich vapors condense on the metal surface of the substrate, forming the primer layer. The current collector foil with the primer layer is then passed through the second combustion chamber. Here, an aromatic compound (e.g., aliphatic hydrocarbons) is used as fuel. In the presence of air, this fuel is burned more or less completely, resulting in less soot formation. Complete combustion generates more heat, and the carbon is primarily converted to carbon dioxide.

[0014] The primer layer contains a water component that helps the carbon black particles adhere to the current collector foil. The primer-coated foil is then coated with the slurry using a slot die (against a support roller). It is important that the carbon black primer layer does not come into contact with a roller, thus preventing damage to the primer layer. Once the primer layer is formed, it is immediately wet-coated with the active material. The wet coating process is carried out in two layers. The first layer is rich in binders (SBR for the anode and PVDF for the cathode – both approximately 5%). The second layer has a binder content of approximately 2%. A higher binder concentration in contact with the primer layer results in stronger adhesion of the active material to the primer layer and of the primer layer to the current collector foil.

[0015] The wet active material layer is dried in the same combustion chamber where the primer layer was previously produced. The heat from the non-luminous flame (generated by complete combustion) creates a higher temperature at the beginning of the drying zone; here, most of the solvent evaporates from the active material layer. A yellow, luminous flame (generated by incomplete combustion), on the other hand, produces less heat, resulting in a lower temperature in the drying zone and gentler drying that prevents binder migration. It is important that the heat generated during the combustion process is used to dry the wet active material layer.

[0016] During the drying of the active material layer, the evaporating solvent and carbon dioxide are extracted from the drying zone. The carbon dioxide is filtered and at least partially returned to the first combustion chamber to create incomplete combustion and thus more soot. The evaporated solvent (water for the anode and NMP for the cathode) is directed to the recycling chamber for reuse.

[0017] If the heat generated by the flame is insufficient to dry the wet active material layer, additional heat can be introduced into the drying zone.

[0018] In a second embodiment, a porous binder layer is applied to the current collector foil before the combustion process described above. The carbon black penetrates the pores of the binder layer, thereby making the binder layer conductive. The process for producing the carbon black is the same as described above.

[0019] In this option, the primer layer consists not only of carbon black particles, but of a combination of binder and carbon black particles. This results in better adhesion of the primer layer than in the first embodiment. In this case, a slot die with two openings or an active material layer consisting of two sublayers from the same slot die is not necessarily required.

[0020] The binder layer can consist of the same binder, such as PVDF for the cathode and SBR / CMC for the anode. It can also be made of adhesive resins (such as epoxy resins). In a preliminary process step, the binder is made porous by adding a low-boiling-point plasticizer. This plasticizer evaporates due to the heat of the flame. The pores created by the removal of the plasticizer are then filled by the carbon black particles. This makes the binder or adhesive resin electrically conductive. Plasticizers are hydrocarbon or water molecules that evaporate at temperatures around 100°C. The plasticizer can be removed in a first combustion chamber of the primer coating station; in a subsequent second combustion chamber, the carbon black particles are generated that penetrate the pores of the binder layer.

[0021] In this embodiment, a non-luminous flame (produced during complete combustion) is used in the first combustion chamber, generating more heat to remove the plasticizer. In the second combustion chamber, a luminous flame (produced during incomplete combustion) is used, producing more soot. This means that the arrangement of the combustion chambers is exactly the opposite of the first embodiment. Otherwise, the two embodiments are structurally identical.

[0022] In the second embodiment, the current collector foil can be provided with a binder or adhesive resin layer containing plasticizers. The porosity required in the second embodiment is achieved by evaporation of the plasticizer.

[0023] Alternatively, a binder layer containing carbon nanotubes can be used. This means that the coating is conductive from the outset. The carbon pigments from the carbon black contribute to the conductivity of the primer layer.

[0024] In a third embodiment, the current collector foil, i.e., a copper or aluminum foil, is activated by a cold plasma jet. This allows the coil surface to be cleaned of grease and dirt. Furthermore, the surface is activated in such a way that the carbon pigments from the soot adhere firmly to the metal surface. Here, we do not need a binding agent on the current collector foil to hold the soot particles, as is the case in the second embodiment.

[0025] In a fourth embodiment, the soot-rich gas produced during the combustion process is collected in a separate chamber and then redirected onto the current collector foil. This means that both sides of the current collector foil can be coated with carbon pigments simultaneously. Here, too, the coating must be applied to both sides using a simultaneous coating process; that is, both sides of the current collector foil are simultaneously provided with a primer layer and an active material layer.

[0026] In a fifth embodiment, the soot vapors produced during the combustion process are condensed. The soot particles are collected in the form of a paste (consisting of water and soot particles). The paste is then transferred to the current collector foil using a roller. This means that the flame does not directly contact the current collector foil during the combustion process. The carbon pigments are produced from soot vapors, but separately.

[0027] The invention offers the following advantages: The carbon-rich primer layer results in improved electrical conductivity of the electrode, enabling a battery cell with high power density. The primer layer reduces corrosion of the current collector foil. It is produced solvent-free, eliminating the need for solvent extraction and recycling. A very thin, uniform primer layer (approximately 1 µm) can be produced. The carbon coating is generated by a soot flame, allowing the soot particles to spread evenly across the current collector foil and, if necessary, penetrate the pores of a binder. The heat generated by both luminous and non-luminous flames is used to dry the coating and evaporate the solvent. The primer layer is applied immediately without contact with the roller.This means that the primer layer is not damaged. It also results in cost savings compared to conventional methods for producing a wet primer layer followed by drying.

[0028] The primer coating process according to the invention can be integrated into the coating system. Here, the drying chamber is in operation not only during the solvent curing process but also during the primer layer coating. The waste heat from the luminous and non-luminous flames is utilized; thus, the additional heat required to evaporate the solvent from the active material layer is reduced. The primer coating is very carbon-rich, and therefore the electrical conductivity of the cell is increased more significantly than with a conventional primer coating, which consists largely of insulating binder or resin. Consequently, the proportion of conductive carbon black in the active material can be reduced. This increases the cell capacity and reduces the problem of calendered electrode rebound. The carbon black particles are practically free of metallic impurities.The fuel for a luminous flame can be selected to contain only low levels of metallic impurities. This reduces metallic impurities in the primer layer. A carbon dioxide-rich atmosphere in the drying zone reduces the risk of oxidation of the anode and cathode binders and active material. The current collector foil can be cleaned with cold plasma before coating with the primer layer. This results in better adhesion of the primer layer and less dirt and grease contamination on the current collector foil.

[0029] Exemplary embodiments of the invention are described below with reference to the accompanying figures. These show: Fig. Figures 1 to 7 are views illustrating the method according to the invention.

[0030] In the Fig. Figure 1 shows a completed electrode E for a lithium-ion battery cell. The electrode E has a current collector foil 1 coated on both sides with an active material layer 3. To increase the adhesion of the active material layer 3 to the current collector foil 1, the active material layer 3 is applied to the current collector foil 1 with an interposition of a primer layer 5. The current collector foil 1 extends laterally beyond the two active material layers 3 with an uncoated current collector tab 7. The coating of the current collector foil 1 with the active material layers 3 and the primer layers 5 is carried out in a process sequence, which is described below. Fig. 2 is described:

[0031] In the Fig. 2 shows a system diagram in which the current collector foil 1 is transported as a web material 9 via a conveyor section F1 to a deflection roller 11 and from there further in the opposite direction parallel to the first conveyor section F1 via a second conveyor section F2 in a conveying direction FR ( Fig. 1) is promoted. The contours of the current collector foil web material 9 are in the Fig. 1 indicated by dashed lines. In the first conveying section F1, the uncoated web material 9 passes through a primer coating station 13, where the primer layer 5 is applied to one of the two sides of the web material 9. In the area of ​​the downstream deflection roller 11, there is a wet coating station 16, where the active material layer 3 is applied to the primer layer 5 of the current collector foil 1. Subsequently, the coated web material 9 passes through a drying chamber 17, where the active material layer 3 is dried. In the further course of the process, the uncoated side of the web material 9 is coated with the primer layer 5 and the active material layer 3 in the same process sequence. The Fig. In addition to the system shown in 2, further processing stations are connected downstream, such as a calendering station and a cutting station, which, however, are not necessary for understanding the invention.

[0032] In the primer coating station 13, the primer layer 5 is applied in an incomplete combustion process, in which soot particles 15 are deposited directly onto the current collector foil 1 as a combustion product during the combustion process, forming the primer layer 5.

[0033] In the Fig. 2 The combustion process is carried out in two stages, with a first combustion stage taking place in a first combustion chamber 18 and a second combustion stage taking place in a downstream second combustion chamber 19. In the first combustion chamber 18, fuel 21 consisting of long-chain hydrocarbons, in particular naphthalene, is supplied. Air 23 and carbon dioxide 25 are also supplied. Based on these feedstocks, incomplete combustion is carried out in the first combustion chamber 18, in which the soot particles 15 dispersed in the combustion gas are conveyed directly towards the web material 9. The soot particles 15 are deposited on the web material 9 by condensation of the combustion gases. The introduction of carbon dioxide creates an oxygen-deficient atmosphere in the first combustion chamber 18 to ensure incomplete combustion.

[0034] In contrast, complete combustion takes place in the downstream combustion chamber 19, resulting in correspondingly reduced soot production and a correspondingly increased combustion temperature. This reduces the water content in the applied primer layer 5. The second combustion chamber 19 has an air supply 27 and a fuel supply 29, through which aromatic or aliphatic compounds are supplied as fuel. Combustion in the second combustion chamber 19 occurs with excess oxygen.

[0035] Immediately after the primer layer 5 is applied to the web material 9, the wet coating process is carried out, in which a coating paste 34 consisting of active material, conductive carbon, binder and solvent is applied to the primer layer 5 of the web material 9 via a coating nozzle 31. The deflection roller 11 acts as a counter-holder for the coating nozzle 31 and is therefore part of the wet coating station 16. In the Fig. 2 the wet coating station 16 is designed such that the active material layer 3 produced thereby has a two-layer structure, namely a binder-rich sublayer 33 which is coated in direct contact with the primer layer 5, and a binder-poor sublayer 35 which is applied to the binder-rich sublayer 33.

[0036] A key aspect of the invention is that the two combustion chambers 18, 19 are in thermal connection with the drying chamber 17; in the Fig. In this respect, the two combustion chambers 18 and 19, as well as the drying chamber 17, are even components of a common process chamber. In this way, waste heat from the combustion chambers 17 and 19 is used for the drying process. The drying chamber 17 also has an extraction device 37, through which exhaust gas is extracted from the drying chamber and recycled. The exhaust gas consists essentially of vaporized solvents and carbon dioxide. The carbon dioxide is filtered from the exhaust gas in a filter system 39 and returned to the carbon dioxide supply 25 of the first combustion chamber 17 via a return line 41.

[0037] In the plant diagram of the Fig. 2 is the inlet area of ​​the drying chamber 17, which is thermally connected to, in particular, the second combustion chamber 19. The inlet area of ​​the drying chamber 17 is therefore subjected to a high amount of heat energy, so that a large portion of the solvent / carbon dioxide evaporates within the drying chamber inlet area. In contrast, the outlet area of ​​the drying chamber 17 is thermally connected to, in particular, the first combustion chamber 19. This is subjected to a reduced amount of heat compared to the inlet area of ​​the drying chamber, thus preventing damage to the dried primer layer 5.

[0038] The one in Fig. The drying chamber 17 shown also has heating units 43, for example heating nozzles, which may be used to increase the heat input into the drying chamber 17.

[0039] By in the Fig. 1 or Fig. In the embodiment shown in Figure 2, the primer layer 5 is made from the carbon black particles 15, but without a binder polymer. To increase the adhesion of the active material layer 3 to the current collector foil 1, the embodiment of the Fig. 3 Each of the primer layers 5 has a polymer-based binder material 45 with pores 47 in which soot particles 15 are embedded. The in the Fig. The 3 primer layers shown are based on the information in the Fig. The process sequence indicated in 4 to 6 can be produced as follows: Accordingly, the web material 9 is first provided with a binder material layer 49 containing embedded plasticizer particles 51 ( Fig. 4) In a subsequent combustion process, a substantially complete combustion is first carried out in a first combustion chamber 18 at a process temperature at which the plasticizer particles 51 evaporate from the binder material layer 49. This forms the remaining free pores 47, as shown in the Fig. 5 is indicated. In the further course of the process, incomplete combustion occurs, in which soot particles 15, as a combustion product, are deposited in the still free pores 47 of the binder material layer 49 ( Fig. 6), namely forming the primer layer 5 ( Fig. 7). The same process sequence is also carried out for the opposite side of the web material 9 (not shown).

[0040] This is followed by a wet coating process and a drying process, which are essentially identical to those in the previous example.

[0041] In the Fig. Figure 8 shows a further embodiment for applying a primer layer 5 to the web material 9. Accordingly, the combustion gas produced in a combustion process is condensed into a carbon black paste 53, which consists of water and carbon black particles 15. The carbon black paste 53 is applied to the web material 9 in an application process, forming the primer layer 5. This is done in the Fig. 8 via application rollers 55, which are slightly immersed in the soot paste 53, and apply the soot paste 53 to the web material 9 on the diametrically opposite side. Reference symbol list 1 Current collector foil 3 Active material layer 5 Primer layer 7. Discharge plume 9 Rail goods 11 Pulley 13 Primer coating station 15 soot particles 16 wet coating stations 17 Drying chamber 18, 19 combustion chambers 21 Fuel supply 23 Air supply 25 CO2 supply 27 Air supply 29 Fuel supply 31 Coating nozzle 33 binder-rich sublayer 34 Coating paste or slurry 35 binder-free sublayer 37 Extraction unit 39 Filter system 41 Return line 43 Heating unit 45 Binder material 47 pores 49 Binder material layer 51 plasticizer particles 53 Soot paste 55 Application roller E electrode F1, F2 conveyor sections FR Direction of Conveyance

Claims

[1] Method for producing an electrode for a lithium-ion battery cell in which an active material layer (3) is coated on a current collector foil (1), wherein a primer layer (5) is applied to the current collector foil (1) prior to coating the current collector foil (1) with the active material layer (3), which increases the adhesion of the active material layer (3) to the current collector foil (1), characterized by , that the application of the primer layer (5) involves an incomplete combustion process in which soot particles (15) are deposited as a combustion product on the current collector foil (1) to form the primer layer (5). [2] Method according to claim 1, characterized by, that the soot particles (15) produced in the incomplete combustion process are deposited directly as a combustion product, i.e. during the combustion process, on the current collector foil (1) forming the primer layer (5), by condensing the combustion gas on the current collector foil (1). [3] Method according to claim 1 or 2, characterized by , that immediately after the application of the primer layer (5) the active material layer (3) is applied to the primer layer (5) in wet or dry coating, and that in particular a process sequence is carried out in which - in wet coating, a coating paste (34) consisting of active material, conductive carbon, binder and solvent is applied to the primer layer (5) of the current collector foil (1) via at least one coating nozzle (31), and - in a drying process the current collector foil (1) coated with the active material is dried by evaporating the solvent, and in particular the drying process is carried out at least partially using the waste heat from the combustion process, and / or in particular the active material layer (3) produced in the wet coating has a two-layer structure, namely a binder-rich sublayer (33) in direct contact with the primer layer (5) and a binder-poor sublayer (35) which is applied on the binder-rich sublayer (33). [4] Method according to any of the preceding claims, characterized by that the combustion process takes place in two stages, namely with - a first combustion stage to produce the soot particles (15) as a combustion product, which is deposited on the current collector foil (1) to form the primer shaft (5), and with - a second combustion stage downstream in the process, in which combustion is essentially complete compared to the first combustion stage, with correspondingly reduced soot production and increased combustion temperature. [5] Method according to any one of the preceding claims, characterized by , that in incomplete combustion, long-chain hydrocarbons, especially naphthalene, are used as fuel, and / or combustion is carried out under oxygen deficiency, and / or that in complete combustion, aromatic or aliphatic hydrocarbons, especially naphthalene, are used as fuel, and / or combustion is carried out under oxygen excess. [6] Method according to any one of claims 3 to 5, characterized by, that the application of the primer layer (5), the active material coating and the drying process are carried out in a continuous process sequence, in which the current collector foil (1) is conveyed as web material (9) via a first conveying section (F1) to a deflecting roller (11) and in the opposite direction in a second conveying section (F2), and that in particular - the first conveying section (F1) leads through at least one combustion chamber (18, 19) in which soot particles (15) are applied to the current collector foil (1) by incomplete combustion, forming the primer layer (5), - the deflection roller (11) forms a counter-holder for a coating nozzle (31), by means of which the coating nozzle (31) is applied to the primer layer (5) of the current collector foil (1) in the wet coating process, - the second conveying section (F2) leads through a drying chamber (17) in which the drying process takes place, and in particular the combustion chamber (18, 19) and the drying chamber (17) are thermally connected so that the waste heat from the combustion process can be used in the drying process. [7] Method according to any one of the preceding claims, characterized by , that to increase the adhesion between the active material layer (3) and the current collector foil (1), the primer layer (5) has a binder material (45), and that the binder material (45) is formed with pores (47) in which carbon black particles (15) are embedded, and that in particular a process sequence is carried out to produce the primer layer (5) in which - a binder material layer (49) with embedded plasticizers (51) is applied to the current collector foil (1), - in the subsequent combustion process, the plasticizers (51) evaporate from the binder material layer (49), forming the pores (47), and - soot particles (15) produced during the further course of the combustion process are deposited in the pores (47) of the binder material layer (49). [8] Method according to any one of claims 1 to 6, characterized by , that to increase the adhesion between the active material layer (3) and the current collector foil (1), a cleaning process, in particular a cold plasma jet treatment, is carried out before the combustion process to remove impurities, such as grease, from the surface of the current collector foil (1). [9] Method according to claim 1, characterized by, that the combustion gas produced in the combustion process is collected in a separate reservoir and distributed from there onto the current collector foil (1) to form the primer layer (5), so that in particular the primer layer (5) can be applied simultaneously to both sides of the current collector foil (1). [10] Method according to claim 1, characterized by , that the combustion gas produced in the combustion process is condensed into a soot paste (53), and that the soot paste (53) is applied to the substrate film (1) in an application process forming the primer layer (5).

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