Method and process arrangement for manufacturing an electrode for a lithium-ion battery cell

Abstract

Method for manufacturing an electrode (K) for a lithium-ion battery cell, which is composed of a substrate film (1) with an active material layer (2) coated on one or both sides, wherein at least one process step is provided for adjusting the porosity of the active material layer (2) coated on the substrate film (1), in which a punch (33) with a plurality of microneedles (35) is moved with a setting force (FL) by a travel distance (Δz) in the direction of the active material layer (2), so that the microneedles (35) are driven into the active material layer (2), thereby increasing the porosity of the surface of the active material layer (2), characterized in that the setting force is a Lorentz force (FL), and that, for generating the Lorentz force (FL), the punch (33) is arranged as a current-carrying conductor in a magnetic field (B), whereby the Lorentz force (FL) acts on the punch (33).

Description

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

[0002] Such an electrode consists of a substrate film with an active material layer coated on one or both sides. The process for manufacturing such an electrode involves a coating process in which a coating paste (hereinafter referred to as slurry) is applied to the still uncoated substrate film. The active material applied to the substrate film has a high porosity. To increase the capacity in the battery cell, the substrate film coated with the active material is subjected to a calendering process. The active material is compressed by approximately 40% during the calendering process. The calendering process is similar to a rolling process in which the active material is compacted in a small deformation zone with a high calendering force.

[0003] The following problems occur during the calendering process: Due to the high calendering force, the porosity decreases more rapidly on the layer surface that comes into contact with the calendering rolls. For example, an anode consists of graphite as the active material. In addition, the binder SBR, the diluent CMS, and conductive carbon are used. After the calendering process, a density of 1.6 g / cm 3is sought in order to achieve a sufficiently high volumetric energy density of the cell. A high density also increases the conductivity of the cell. However, with increasing density, porosity decreases. The porosity decreases at the layer surface, which makes it more difficult for lithium ions to be incorporated into the graphite layers. This leads to lithium deposits (lithium plating) on the layer surface. This drastically shortens the lifespan of the cell. Since the active material deformation during calendering takes place in a very small area (depending on the diameter of the roller), there is a very high stress concentration in a small deformation area. This stress concentration leads to cracks in the active material. In summary, high compression during calendering is desirable; however, it is associated with problems such as lithium deposition.

[0004] Changes in pore structure for specific pore size ranges and deformation of the active material are crucial factors that negatively impact electrochemical performance. Surface roughness and open pores, which definitely influence the overall wetting behavior of the electrodes with electrolyte, vary with the compression ratio. Typically, calendering reduces porosity to as little as 30%; however, at such a target porosity, local porosity directly at the active material layer's surface can be reduced to as little as 20%. Therefore, the porosity is not evenly distributed across the layer thickness. Rather, the porosity at the layer's surface is significantly reduced compared to the porosity near the substrate film.

[0005] The excessive deformation of the active material particles at the film surface affects the electrochemical performance because the surface pores block lithium incorporation during electrochemical cycling, especially at high C rates.

[0006] At high compression rates (20% to 25%) during the calendering process, particularly large pores between 5 µm and 2 µm are closed by compression. The impact of these missing large pores and the overall lower total pore volume on the electrochemical behavior is one of the main causes of lithium plating, which drastically reduces cell lifetime. At high compression rates (i.e., reducing porosity to up to 20% to 25%), the large pores between 5 µm and 2 µm are closed, reducing the total pore volume of the electrode. The surface area blocked by the calendering-induced deformation of the active material particles creates a transport barrier for lithium ions, which drastically impacts the long-term performance of these electrodes at relatively high C rates.

[0007] Both the blockage of large pores and the significant reduction of total pore volume lead to a reduction in electrolyte / particle interfaces available for lithium ion transfer. Reducing the electrolyte-particle interfaces can cause electrolyte wetting problems, which also contributes to degraded electrochemical performance.

[0008] WO 2022 / 136810 A1 discloses a method for producing high-energy-density batteries. The method involves producing an electrode with a high charge per unit mass for a high-energy-density metal-ion battery. For this purpose, the electrolyte-impregnated solid electrode is prepared by mixing a salt, a solvent, a binder, and an active material to create a mechanically stable paste. US 2002 / 0182483 A1 discloses a method for producing a battery cell.A battery electrode plate is manufactured through an active material impregnation step of impregnating an entire porous core substrate shaped like a thin plate with an active material, a pressing step of performing pressing work on the core substrate to form a plurality of rail-shaped protrusions, a step of removing the active material to form core substrate exposed portions by applying ultrasonic vibrations to the rail-shaped protrusions, a flattening step of compressing the exposed portions of the core substrate to an identical level with the other portions, and a cutting step of cutting predetermined portions including the exposed portions of the core substrate.

[0009] The object of the invention is to provide a method and a process arrangement for producing an electrode for a lithium-ion battery cell, in which, compared to the prior art, a wetting process for wetting the active material with liquid electrolyte is accelerated and / or the performance of the battery cell is increased.

[0010] The object is solved by the features of claim 1 or 10. Preferred developments of the invention are disclosed in the subclaims.

[0011] The invention relates to a method for producing an electrode for a lithium-ion battery cell. The electrode consists of a substrate film with active material coated on one or both sides, said material having a predefined porosity. According to the characterizing part of claim 1, at least one process step is provided to adjust the porosity of the active material layer coated on the substrate film. In this process, a stamp having a plurality of microneedles is adjusted by a setting force in the direction of the active material layer by a certain travel distance. The microneedles of the stamp are thereby driven into the active material layer, increasing the porosity, particularly at the surface of the active material layer.

[0012] In the present concept, the electrode is compressed to the desired density by calendering. As already explained, calendering reduces porosity. However, this also leads to slower electrolyte wetting and thus to lower electrode conductivity. After calendering, the electrodes are cut to the desired shape using cutting operations. The electrodes are then stored in a storage tray and dried under vacuum at 120°C for approximately four hours. The purpose of this drying is to remove the moisture in the anode and the NMP residues in the cathode. During this four-hour drying process, the electrode is pierced according to the invention to create microporosity on the surface. This piercing process is initiated by the buildup of a Lorentz force. During the piercing process, an electrolyte can be injected into these newly formed micropores.This means that the electrode has micropores and the electrolyte is wetted during the drying process, which shortens the electrolyte soaking time during the formation process and also increases the conductivity of the lithium ions. Therefore, no additional process is required, as the piercing takes place during the drying process, during which the electrode is not subjected to any mechanical stress. The piercing process is achieved using the Lorentz force and can be regulated by controlling the electric current and the magnetic field.

[0013] The piercing process is as follows: First, the electrode sheets are stacked in a storage magazine. Between each two electrode sheets there is a stamping unit with a rubber stamping housing that is sealed on the outside and hollow on the inside. The stamping housing has micro-holes on its surface. Inside the stamping housing there is a stamping plate with needles on either side. These needles can protrude from the micro-holes in the stamping housing. The stamping plate is electrically conductive and has an electrical connection area that protrudes from the stamping housing and can be connected to a DC power supply. When de-energized, the needles of the stamping plate are out of contact with the electrode. Only when a Lorentz force is generated do the needles move up or down and protrude through micro-holes in the stamping housing.

[0014] In preparation for the inventive process step for increasing porosity, the electrodes are stacked in the storage magazine with stamping units in between. In addition, contact pressure is exerted on the electrode stack so that the stack is pressurized. The power connection of the stamping units is then connected to the DC power supply and supplied with a constant DC current. The complete assembly is placed in a vacuum drying cabinet. Electromagnets are mounted on the wall of the vacuum drying cabinet. Such electromagnets could also be mounted on the wall of the storage magazine. It is important that the polarity of the magnetic field can be switched at regular intervals. The vacuum drying process is now started. In parallel, direct current is applied to the power connections protruding from the stamping units. The direction of the electric current is therefore fixed.The direction of the magnetic field is perpendicular to the electric current. This means that the Lorentz force is directed either upwards or downwards, depending on the polarity of the magnetic field. The Lorentz force moves the stamping plates with the needles either upwards or downwards. The needles protrude from the stamping housing and pierce the electrode surface. The current is gradually increased. As a result, the penetration of the electrode coating gradually increases. This means that the microstructures formed in the coating do not develop all at once, but gradually. With each increase in the current, the depth of the microporosity increases.

[0015] When the magnetic polarity changes, the Lorentz force acts in the opposite direction, causing the electrode coating located on the other side of the punch unit to be pierced. The electrode coating is only pierced when an electric current flows through the punch plate located in the punch housing. The punch housing acts as a hold-down device, evenly pressing the electrode surface during the piercing process.

[0016] In addition to the piercing procedure, a small amount of electrolyte can be injected into the micropores. The piercing procedure combined with electrolyte injection can contribute to faster permeation. Piercing alone can also work independently of electrolyte injection.

[0017] The electric current can be increased gradually during the piercing process, so that microporosity on the electrode coating forms gradually rather than suddenly. In between, the current direction or the direction of the magnetic field is reversed. This causes the other side of the electrode to be punctured. This process continues during the vacuum drying process. Because the formation of microporosity occurs slowly and gradually, the active material is not damaged. After the vacuum drying process, the electrode is dried and also exhibits microporosity. These electrodes are then transferred to the cell assembly process, where they are stacked.

[0018] As mentioned above, a small amount of electrolyte can be injected into the pores during piercing. This not only makes the electrode microporous, but also wets it with electrolyte. The wetting process can be achieved in various ways. In a first embodiment, the storage tray can be filled with electrolyte solvents such as ethylene and propylene carbonate, which only boil at 200°C. Therefore, a vacuum drying temperature of 120°C would not affect the electrolyte. Once the microstructure has formed, the electrolyte is automatically injected into the pores.

[0019] In a second design variant, the hollow plunger housing is filled with an electrolyte solvent (cyclic carbonate such as EC, PC, or both). The electrolyte solvent has a higher viscosity because there is no linear carbonate present in the electrolyte solvent. Therefore, the electrolyte in the plunger housing does not drip from the micro-holes in the plunger housing. Once the needles are driven into the active material layer, the electrolyte is introduced into the resulting micropores.

[0020] In a third design variant, the storage magazine has a nozzle through which electrolyte is sprayed at regular intervals, thus wetting the electrode stack in the storage magazine. Here, the die housings have channels that serve as electrolyte reservoirs.

[0021] One aspect of the invention involves injecting electrolyte into micropores, allowing the electrolyte to penetrate deeply into the active electrode material. The piercing and electrolyte injection occur simultaneously during the drying process, eliminating the need for additional time for electrolyte soaking. Electrolyte injection after cell assembly remains unchanged. This electrolyte injection uses only solvent, not lithium salt.

[0022] During the vacuum drying process, the electrodes are pierced and wetted with electrolyte. A hollow stamp housing containing a stamp plate is placed between two adjacent electrodes of an electrode stack. The stamp housing is sealed on all sides and has micro-holes on both flat sides. The stamp plate has needles that may protrude from the stamp housing. The needles can be made of steel or ceramic. The stamp plate has a power connector that is connected to the busbar to generate direct current.

[0023] The magnetic field is applied perpendicular to the direction of current flow. Due to the magnetic field and the electric current, the magnetic plate is subject to the Lorentz force. The Lorentz force moves the stamping plate with the needles up or down (depending on the direction of the Lorentz force). As the Lorentz force builds up, micropores are created on the electrode surface. The electric current is gradually increased, so that the Lorentz force increases and microporosity gradually forms. The polarity of the magnetic field or the direction of the electric current is reversed, so that the Lorentz force acts in a different direction, resulting in the two electrodes being pierced. In this way, the change in the direction of the Lorentz force pierces both the upper and lower electrodes, causing microporosity to form on their surfaces.The electrode stack with the intermediate punch units is placed under mechanical prestress using a pressure element so that the Lorentz force counteracts the external force and only the electrode coating can penetrate. This prevents the electrode from simply being moved without piercing it. The punch housing can contain an electrolyte that is injected into the micropores during the piercing process. It is also possible to spray electrolyte into the magazine at regular intervals. Alternatively and / or additionally, the punch housing can also have external channels in which the electrolyte is stored. This electrolyte is injected into the pores via the needles.

[0024] After the drying process, the electrode undergoes a stacking process. The electrolyte already contains micropores, and these pores are wetted by the electrolyte. This wetted electrode helps reduce the electrolyte soaking time during the formation process.

[0025] The needles can be made of metal or ceramic. The advantage of ceramic is that no metallic contamination occurs during the hole-piercing process. The disadvantage is that the ceramic needles must be thicker, approximately 10 to 15 µm, otherwise the pores will not gradually expand with increasing Lorentz force. The length of the needles determines the depth of the micropores.

[0026] The invention achieves the following advantages: Lithium plating, which closes the micropores of the active material layer, can be prevented. This results in a high electrode density and, at the same time, high porosity. The pore volume is increased after calendering by piercing during the drying process. Parallel to the formation of micropores, these pores are also wetted by the electrolyte. This drastically shortens the electrolyte impregnation time during the formation process. This also shortens the overall process time for battery cell production. A high volumetric energy density of the electrode is achieved without compromising the pore volume.The electrical conductivity of the active material increases due to the high density achieved after calendering and the low tortuosity due to the micropores, which provide a good path for the movement of lithium through the electrolyte. The piercing and wetting process can be easily integrated into the existing drying process. No additional time is required for the piercing and wetting process, as it occurs in parallel with the vacuum drying process. The formation time is shortened by one to two days because the electrolyte requires less time to soak. The depth of the micropores gradually increases with increasing Lorentz force. Therefore, this method is more suitable than machining micropores by texture-forming calender rolls. The micropores are created throughout the drying process under the influence of the Lorentz force.The Lorentz force can be precisely controlled by controlling the magnetic field and / or the direct electric current. Both the upper and lower electrodes are pierced by changing the direction of the Lorentz force. If no current flows, no Lorentz force is generated. In this case, the needles are located inside the plunger housing. The plunger housing is made of an elastically compliant elastomer material and therefore acts like a spring. A good relaxation time can be maintained between punctures to allow the electrolyte time to penetrate the pores.

[0027] An embodiment of the invention is described below with reference to the attached figures.

[0028] They show: Fig. 1 shows a perspective view of a finished electrode for a lithium-ion battery cell; Fig. 2 to 10 views illustrating a process sequence for manufacturing the electrode.

[0029] In the Fig. 1 shows a finished cathode K for a lithium-ion battery cell. The cathode K has a central substrate foil 1 made of, for example, aluminum, which is coated on both sides with an active material layer 2. Fig. 1, the carrier foil 1 is formed with a conductor tab 3 that laterally projects beyond the two active material layers 2. The main component of the active material layer 2 is a lithium metal oxide. The active material layer also contains PVDF as a binder.

[0030] Below is a manufacturing process for the production of the Fig. 1 shown cathode K using the Fig. 2 to 8. Accordingly, in the Fig. 2 shows a coating station 5 and a downstream drying station 7. At the entrance of the coating station 5 there is a winding 9 (hereinafter coil) made of an uncoated substrate film 1. This is guided in a continuous process through the coating station 5 and through the drying station 7. The coating station 5 has an application tool 11 with which a coating paste, i.e. a viscous starting component of the active material (hereinafter slurry), is applied to the substrate film 1, forming an electrode track 13. In the Fig. 2, in the coating station 5, only the upper side of the substrate film 1 is coated with the slurry and then dried in the drying station 7. The underside of the substrate film 1 can (although not shown) be coated and dried in the same way.

[0031] In the drying station 7, the electrode web 13 is dried using hot air nozzles 15, thereby removing solvent, particularly NMP, from the active material layer 2. At the exit of the drying station 7, the coated and dried electrode web 13 is rewound into a coil 17.

[0032] The coil 17 with the dried and coated substrate film 1 is conveyed to a calendering station 19 ( Fig. 3). In the calendering station 19, the coated substrate film 1 is guided through the nip of a pair of rollers 21, so that the porosity of the active material layer 2 is reduced to a predefined value. After the calendering process, the thus treated electrode web 13 is rewound onto a coil 23, which is subjected to a cutting process ( Fig. 4). According to the Fig. 4, the electrode web 13 unwound from the coil 23 is separated and cut to size into cathodes K by a longitudinal cut (slitting) as well as by cross cuts and by lateral cuts (notching). The separated cathodes K are then transported to a deposition station 25 ( Fig. 5), in which the cathodes K are stacked in a storage magazine 26. The storage station 25 has a vacuum dryer (not shown) by means of which the electrode stack is vacuum-dried. Furthermore, the storage station 25 has a suction device (not shown) by means of which evaporated residual solvent and residual moisture can be extracted and recycled.

[0033] A core of the invention is that during the vacuum drying process an additional process step takes place, with which the porosity on the surface of the active material layers 2 of the cathodes K is increased. To carry out this process step, the electrode stack stacked in the storage magazine 26 is located in a magnetic field B, which is generated by means of a permanent magnet M ( Fig. 6a), transverse to the stacking direction of the electrode stack. In addition, the electrodes K are stacked one above the other in the electrode stack with the interposition of stamping units 29, of which one stamping unit 29 is in the Fig. 7 or Fig. 8 is indicated enlarged. Accordingly, the stamp unit 29 has a flat stamp housing 31 made of a flexible elastomer material. In the interior of this housing, a stamp plate 33 is adjustable in the stacking direction over a travel distance Δz. The stamp plate 33 has a plurality of microneedles 35 on each of its two flat sides. In addition, all stamp units 33 in the electrode stack are connected to an electrical circuit SK ( Fig. 5), in which the stamping plates 33 are connected via power connections to lateral busbars 34 and can be supplied with direct current. When the stamping plate 33 is energized, a Lorentz force F builds up in the magnetic field B of the storage magazine 26. L which acts on the stamp plate 33. As can be seen from the Fig. 5, Fig. 6a and Fig. As can be seen in Figure 6b, the magnetic field B is oriented transversely to the current flow I through the stamping plate 33, whereby the Lorentz force F Lperpendicular to the magnetic field B and perpendicular to the current flow I. With the help of the Fig. The direction of the Lorentz force can be identified using the three-finger rule of the left hand indicated in Figure 6b. Accordingly, the thumb points in the direction of current flow, while the index finger points in the direction of the magnetic field B, and the middle finger indicates the direction of the Lorentz force. As shown in the Fig. As indicated in Figure 6a, the magnetic field B runs from the north pole N to the south pole S of the permanent magnet M.

[0034] If there is no current supply to the respective stamp plate 33, it is located in the Fig. 7. In the neutral position, the microneedles 35 of the stamp plate 33 are arranged inside the housing. The stamp housing 31 has a plurality of microholes 37 on each of its two flat sides. When current is applied to the stamp plate 33, the Lorentz force F K, whereby the stamp plate 33 is adjusted by the travel distance Δz in the stacking direction S. The microneedles 35 of the stamp plate 33 are therefore guided through the microholes 37 to the outside of the housing and driven into the active material layer 2 of the adjacent cathode K. When the direction of current flowing through the stamp plate 33 is reversed, the Lorentz force F L in the opposite direction. Accordingly, the stamp plate 33 is displaced by the travel distance Δz in the opposite direction, whereby the microneedles 35 are driven into the active material layer 2 of the cathode K arranged on the other side.

[0035] In order to ensure a reliable process for increasing the porosity, the storage magazine 26 has a pressure element 39 ( Fig. 5), which applies a predefined mechanical prestress to the electrode stack. Each of the stamp housings 31 therefore presses with its two flat sides against the active material layers 2 of the adjacent cathodes K in the manner of a hold-down device.

[0036] A further core of the invention is that during the vacuum drying process, an electrolyte wetting process takes place, in which a liquid electrolyte E penetrates into the pores of the active material layer 2. To carry out the electrolyte wetting process, the housing interior of the stamp housing 31 can be filled with liquid electrolyte or electrolyte solvent, as described in the Fig. 9. This allows the electrolyte E to penetrate directly into the pores of the active material layer 2 via the microneedles 35 of the stamp plate 33 during the piercing process.

[0037] In the Fig.10 shows a stamp unit 29 according to a second embodiment. Accordingly, the stamp housing 31 has receiving grooves 39 on each of its two flat sides, into the groove bottoms of which the microholes 37 open. The receiving grooves 39 are filled with electrolyte E during the vacuum drying process. When the microneedles 35 are driven into the active material layer 2 of the adjacent cathode K, the electrolyte E is simultaneously injected from the receiving grooves 39 into the pores of the adjacent active material layer 2. List of reference symbols 1 substrate film 2 active material layer 3 arrester lug 5 coating stations 7 Drying station 9 Coil 11 Application tool 13 Electrode track 15 Hot air nozzle 17 Coil 19 Calendering station 21 pairs of rollers 23 coils 24 cutting stations 25 storage stations 26 storage magazine 29 Stamp unit 31 stamp housing 33 Stamp plate 34 Busbar 35 microneedles 37 microhole 39 mounting groove K cathode Δz travel F L Lorentz force S Stacking direction E Electrolyte B Magnetic field I Current flow SK circuit QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] WO 2022 / 136810 A1

[0008] US 2002 / 0182483 A1

[0008]

Claims

[1] Method for producing an electrode (K) for a lithium-ion battery cell, which is constructed from a substrate film (1) with an active material layer (2) coated on one or both sides, characterized by that for adjusting the porosity of the active material layer (2) coated on the substrate film (1) at least one process step is provided, in which a stamp (33) with a plurality of microneedles (35) is pressed with a setting force (F L ) is adjusted by a travel distance (Δz) in the direction of the active material layer (2), so that the microneedles (35) are driven into the active material layer (2), specifically increasing the porosity, in particular of the surface of the active material layer (2). [2] Method according to claim 1, characterized by that the setting force is a Lorentz force (F L ) and that the Lorentz force (F L) the stamp (33) is arranged as a current-carrying conductor in a magnetic field (B), whereby the Lorentz force (F L ) works. [3] Method according to claim 1 or 2, characterized by that the stamp (33) is a component of a stamp unit (29) with a stamp housing (31) made of, in particular, elastically flexible elastomer material, in the interior of which the stamp (33) can be adjusted over the adjustment path (Δz), and that in particular the stamp housing (31) has micro-holes (37) through which the micro-needles (35) of the stamp (33) can be guided to the outside of the housing, provided that the Lorentz force (F L ) acts on the stamp (33). [4] Method according to claim 3, characterized bythat to carry out the process step, at least one electrode (K) and one stamp unit (29) are stacked one above the other in a storage magazine (26), and that by applying the magnetic field (B) and by energizing the stamp (33), the Lorentz force (F L ) acts on the stamp (33), whereby its microneedles (35) are driven in the stacking direction (S) through the microholes (37) of the stamp housing (31) to the outside of the housing into the active material layer (2) of the electrode (K). [5] Method according to claim 4, characterized by that a mechanical prestress is applied to the stack located in the storage magazine (26) by means of a pressing element (39), whereby the stamp housing (31) is pressed against the active material layer (2) of the adjacent electrode (K) in the manner of a hold-down device. [6] Method according to one of claims 4 or 5, characterized by that the procedure comprises the following process steps: - coating process in which a liquid starting component of the active material, i.e. slurry, is coated onto the substrate film (1), specifically to form an electrode track (13); - drying process in which the electrode track (13) is dried, in particular the moisture and / or the solvent is removed from the active material layer (2); - calendering process in which the porosity of the active material layer (2) is reduced to a predefined value; - Cutting process in which electrodes (K) are separated and cut from the electrode web (13) by means of cutting operations; - depositing process in which the electrodes (K) are stacked in the deposit magazine (26); and - Vacuum drying process in which residual moisture and / or residual solvent are removed from the electrodes (K) stacked in the storage magazine (26). [7] Method according to claim 6, characterized bythat in the deposition process the electrodes (K) are each stacked with the interposition of at least one stamp unit (29). [8] Method according to claim 6 or 7, characterized by that the vacuum drying process takes place simultaneously with both the process step for increasing the porosity in the active material layer (2) and with an electrolyte wetting process in which the electrolyte (E) is injected into the pores of the active material layer (2). [9] Method according to claim 8, characterized by that in order to carry out the wetting process, the storage magazine (26) is filled with electrolyte (E), and / or only the housing interior of the stamp housing (31) is filled with electrolyte (E), whereby the electrolyte (E) reaches the pores of the active material layer (2) directly via the microneedles (35) of the stamp (33). [10] Process arrangement for carrying out a method for manufacturing an electrode (K) according to one of the preceding claims.

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