METHOD FOR PRODUCE A DRY FILM, ROLLING DEVICE, AS WELL AS DRY FILM AND SUBSTRATE COATED WITH THE DRY FILM

Description

[0001] The present invention relates to a method for producing a substrate, a rolling device, a dry film and a substrate coated with the dry film.

[0002] In the production of battery electrodes, 50 µm to 100 µm thick layers must be applied to metallic current collectors at high web speeds. This is typically achieved using wet-chemical roll-to-roll processes with suspensions of active materials in aqueous or organic solvents. This requires a high energy input for both dispersing the active material and drying the layer. There is a growing trend towards solvent-free, i.e., dry, coating processes. To achieve this, dry powder mixtures of active material, conductive additives, and suitable binders must be transformed into mechanically robust layers.

[0003] Prior art includes methods such as that disclosed in US 7352558 B2, in which the powder is compressed into a freestanding film. This typically involves a three-stage process: fibrillation of a dry powder mixture using an air jet mill, conveying of the powder into a calender gap where compression into the freestanding film takes place, and subsequent application of the freestanding film to a current collector. While this method allows for continuous processing, the multiple stages, and especially the intermediate step involving a difficult-to-handle freestanding film, are problematic.

[0004] In another method, disclosed in DE 10 2010 044 552 B4, the dry powder mixture is applied to the target substrate by electrostatic charging followed by heat treatment for mechanical stabilization using a thermoplastic binder. A disadvantage of this method is that the powder application requires subsequent compaction or mechanical stabilization by calendering, thus necessitating an additional process step. Furthermore, this method requires the use of planar target substrates.

[0005] From DE 10 2014 208 145 B3 a battery cell with a coated electrode and its manufacture is known.

[0006] US patent 2017 / 040591 A1 discloses a method for producing a dry film in which a dry powder mixture is processed into a dry film by a rolling device with a first roller and a second roller, wherein the first roller has a higher rotational speed than the second roller and the dry film is stored on the first roller.

[0007] US 2015 / 0224529 A1 concerns a device for the manufacture of a film product.

[0008] JP 2010 171366 A discloses a method for producing an electrode composition layer on a support, wherein an electrode composition layer containing an electrodeactive material and a binder is formed on the surface of a support by a drying process, and discloses a method for producing an electrode for an electrochemical element, wherein the support is peeled off the electrode composition layer.

[0009] US 2006 / 109608 A1 discloses a process for producing an energy storage product, comprising dry mixing of dry carbon particles and a dry binder, and dry fibrillating of the dry binder to create a structure within which the dry carbon particles are supported as a dry material.

[0010] The present invention therefore aims to propose a method that avoids the aforementioned disadvantages, enabling dry films to be applied to a substrate in an efficient and mechanically stable manner.

[0011] This problem is solved according to the invention by a method according to claim 1, a rolling device according to claim 9, a dry film according to claim 13, and an electrochemical element according to claim 15. Advantageous embodiments and further developments are described in the dependent claims and / or in the dependent aspects.

[0012] In a process for producing a dry film, a dry powder mixture is processed into the dry film by a rolling device comprising a first and a second roller. The first roller rotates at a higher peripheral speed than the second roller, and the resulting dry film is deposited on the first roller. The process produces a dry film with anisotropic fibrils. The process is characterized by maintaining a ratio of 10:5 to 10:1 between the peripheral speed of the first roller and the peripheral speed of the second roller.

[0013] The roller assembly, operating at different rotational speeds, achieves mechanical stabilization and film formation on the first roller, which rotates faster than the second. This prevents the formation of a free-standing film and allows for immediate further processing of the dry film carried or stored on the first roller.

[0014] Typically, after processing, the dry film is applied to a substrate by the two rollers, preferably laminated onto the substrate. Alternatively or additionally, this can be done during the dry film generation process. If the substrate has sufficient roughness, for example, if it is a metal wire mesh or carbon fiber mat, the dry film can also be pressed onto the substrate due to interlocking. However, it is also possible to detach the dry film from the first roller, for example, using a doctor blade. Due to the different roller speeds (which influence the spacing of the structures forming in the dry film) and the pressing force (which affects the height of these structures), the resulting dry film typically exhibits a fibril structure with a roughness Ra of 10 µm or less.

[0015] According to the invention, the ratio of the rotational speed of the first roller to the rotational speed of the second roller is maintained at a ratio of 10:5 to 10:1. Preferably, a ratio of 10:5 to 10:3, and particularly preferably 2:1, is maintained. This exerts a shear force on the powder in the gap between the two rollers, which causes fibril formation along the direction of travel. Thus, the dry film can be formed with a ribbed structure in which a periodicity of the structure is recognizable due to the different roller speeds.

[0016] The circumference of the first roller typically corresponds to the circumference of the second roller, resulting in a simple design with two rollers of the same diameter. However, it is also possible – for example, to achieve defined rotational speeds taking into account the respective circumferences – to design the first and second rollers with different diameters and thus different roller circumferences.

[0017] The rolling device can be designed as a calender rolling device. Further consolidation of the dry film can be achieved by using heated rollers. The formation of the fibril structure can be supported by heated rollers, or at least one heated roller. For a binder made of polytetrafluoroethylene (PTFE), the temperature of the at least one heated roller should be between 80 °C and 120 °C.

[0018] Preferably, the second roller, which rotates at a lower peripheral speed than the first roller, is provided with a modification, preferably a coating, on its surface that repels and / or reduces the adhesion of the forming dry film, thus facilitating its removal. The coating may comprise or consist of polytetrafluoroethylene (PTFE), silicone, and / or diamond-like carbon. Alternatively or additionally, the first roller may have a corresponding modification that promotes adhesion of the forming dry film. The modification may also be achieved by a roughened surface, for example, by giving the first roller a surface with a greater roughness than the second roller. In particular, the surface of the second roller may be polished smooth.

[0019] The dry film is typically applied to a substrate, preferably laminated, and moved at a speed corresponding to the rotational speed of the first roller for application or lamination. This enables a smooth transfer of the dry film from the first roller to the substrate due to the matched speeds.

[0020] Preferably, the substrate is moved over the first roller while the dry film is being formed on the substrate. This facilitates the direct formation of the dry film simultaneously with the movement of the substrate. If the substrate is guided directly over the first roller together with a primer film, an adhesion promoter layer can be applied to the substrate.

[0021] It may be provided that the dry film is formed by the first roller and the second roller with a line force of 100 N / cm to 10 kN / cm, preferably 400 N / cm, acting between the first roller and the second roller in the roller gap, in order to achieve sufficient mechanical consolidation of the dry film.

[0022] Typically, the substrate is made of or contains a metallic material in order to serve as an electrode for an energy storage unit.

[0023] Prior to laminating the dry film onto a surface intended for application or lamination, the substrate can be primed and / or bindered. This results in an improved bond. Preferably, a thermoplastic primer and / or binder is used for this purpose. Alternatively or additionally, a reactive primer or an adhesive can be used. The primer layer can comprise conductive carbon black and / or a thermoplastic component, preferably polyvinylpyrrolidone (PVP). The substrate is preferably made of expanded metal, a metal wire mesh, a nonwoven fabric, a substrate with a structured surface that allows for mechanical interlocking, or a metal foil, preferably a copper or aluminum foil.In a particularly preferred manner, a copper foil or an aluminum foil is used, onto which a carbon primer is applied.

[0024] The dry film is typically formed with a thickness of less than 500 µm, preferably less than 300 µm, and especially preferably less than 150 µm, in order to obtain a dry film that is as thin as possible and at the same time mechanically stable.

[0025] Typically, a dry powder mixture is used that contains polytetrafluoroethylene, a conductive additive (e.g., carbon nanotubes), porous carbon, a transition metal oxide, and / or sulfur. For a carbon / sulfur cathode, the dry powder mixture may contain porous carbon (e.g., porous carbon black or carbon nanotubes), sulfur, polytetrafluoroethylene, and optionally another conductive additive. For a lithium-ion electrode, in addition to polytetrafluoroethylene and an additional conductive additive, an active material can be used, preferably lithium iron phosphate (LFP), lithium manganese oxide (LMO), nickel manganese cobalt (NMC), nickel-rich lithium nickel manganese cobalt oxide (NMC 622 or NMC 811), lithium nickel cobalt aluminum oxide (NCA), lithium cobalt oxide (LCO), lithium manganese nickel oxide (LMNO) and / or lithium titanate (LTO).The described method is particularly advantageous for cathode materials in lithium-ion batteries, as these are rarely processed using aqueous solutions.

[0026] In a rolling device for carrying out the process, a dry powder mixture is fed from a powder conveyor into the roll gap between a first calender roll and a second calender roll. The first and second calender rolls are designed or driven such that the first calender roll has a higher peripheral speed than the second calender roll, and the first and second calender rolls rotate in opposite directions. The rolling device is suitable for forming a dry film with anisotropically structured fibrils. The rolling device is characterized in that it is configured to maintain a ratio of the peripheral speed of the first roll to the peripheral speed of the second roll of 10:5 to 10:1.

[0027] In addition to the dry powder mixture, a substrate, in particular a film or a net, can be guided through the roller gap.

[0028] For battery applications, for example, it may be necessary to perform so-called intermittent coating. This allows for coating in the form of strips perpendicular to the coating direction. The coating is suspended at regular and precisely timed intervals, resulting in uncoated strips. This precise timing, the accuracy of the resulting shapes, and the edge definition are limited. For instance, the process speed for intermittent dry film formation and coating onto a substrate can be reduced to half that of continuous coating (approximately < 30 m / min instead of approximately < 60 m / min).

[0029] The inventive method makes it possible to achieve such structuring by using an adhesion-promoting primer layer on the respective substrate surface. For this, it is necessary to first coat the substrate surface, onto which the dry film is to be formed, with the adhesion-promoting primer layer in the desired geometry. Since this is only a layer approximately 1 µm (dry) thick, it is simpler to structure the primer layer instead of the actual electrode layer (various printing or spraying methods). Any geometry (rectangular, but also round or other) is possible. The dry film layer would be formed in such a method (e.g., Fig.2The dry film is formed entirely on the faster-rotating first calender roll. However, it only forms in the areas where the substrate surface has a primer layer. The excess dry film (not transferred to the substrate surface) is removed from the first calender roll, reprocessed, and can be reused. This allows for the production of any geometry with good precision without reducing process speeds.

[0030] Two pairs of calender rolls with first and second calender rolls can also be arranged side by side in a mirror-symmetrical manner, such that a roll gap is formed between two first calender rolls, through which the dry film is applied to two sides of a substrate also guided through this roll gap, and the two first calender rolls have an opposite direction of rotation.

[0031] In a further embodiment, a second first calender roll rotating about an axis of rotation can be provided, onto which a dry film formed between the first and second calender rolls can be wound after exiting the roll gap. The two first calender rolls should have the same peripheral speed. In this embodiment, a substrate, preferably a film containing the dry film, can be guided through the roll gap between the first and second calender rolls and wound onto the second first calender roll.

[0032] According to the invention, a dry film is formed which has anisotropically formed fibrils. These fibrils preferably form anisotropically in the direction of travel of the first and second rollers due to shearing in the roller gap. The length of the fibrils is in the range between 0.1 µm and 1000 µm. Alternatively or additionally, the dry film with the fibrils can have a roughness Ra of less than 10 µm. The dry film is typically arranged on a substrate.

[0033] Preferably, an electrochemical storage device or an electrochemical converter has a dry film with the described properties or a substrate provided with the dry film with the described properties.

[0034] The dry film according to the invention is produced according to the described method; the described method is therefore designed for producing the dry film.

[0035] A non-flowable powder mixture can also be used for production. This non-flowability can be determined under the test conditions according to the German standard EN ISO 6186:1998 (August 1998 edition).

[0036] Exemplary embodiments of the invention are shown in the drawings and are described below with reference to the Figures 1 to 3 explained.

[0037] They show: Fig. 1 a schematic side view of a rolling device; Fig. 2 a Figure 1 corresponding view of a double rolling device and Fig. 3 Figure 1 corresponding view of the rolling device with substrate feed.

[0038] In Figure 1A schematic side view shows a rolling device in which a dry powder mixture stored in a powder conveyor 1 is conveyed from the powder conveyor 1 onto two chromium-plated calender rolls 2a and 2b of identical dimensions and is brought into a stable state by means of pressing and shearing forces. The first calender roll 2a is operated at a higher rotational speed than the second calender roll 2b, so that a dry film 3 remains on the first calender roll 2a after the combined pressing and shearing process.

[0039] In the illustrated embodiment, the dry powder used is premixed and comprises 90 wt.% Ketjenblack / sulfur (1:2 w / w), 3 wt.% polytetrafluoroethylene (PTFE), and 7 wt.% multiwalled carbon nanotubes (MWCNTs). A lithium-ion electrode typically uses 95 wt.% lithium manganese oxide, 3 wt.% of a conductive additive (in this case, multiwalled carbon nanotubes, MWCNTs), and 2 wt. Fibrillation of the dry powder mixture occurs in a calender gap located between the first roller 2a and the second roller 2b, thereby producing the continuous dry film 3.

[0040] The rotational speeds of the first roller 2a and the second roller 2b are in a range between 10:9 and 10:4, in the illustrated embodiment at 2:1, namely either 10 mm / s:5 mm / s or 20 mm / s:10 mm / s. In further embodiments, depending on the parameter range and powder state, rotational speeds of 80 mm / s:40 mm / s can also be used. Higher rotational speeds result in thinner dry films with a less pronounced ribbing structure or less pronounced fibrils, i.e., a lower surface roughness Ra. In the illustrated embodiment, the fibrils have an average length of 10 µm and are anisotropic in the direction of rotation of rollers 2a and 2b.The rotational speeds exert a shear force on the powder in the roller gap, which has a width of 50 µm in the illustrated embodiment but can also be between 10 µm and 300 µm. This shear force causes fibril formation along the direction of travel. This results in mechanical stabilization and film formation on the first roller 2a, which rotates at a higher speed, and prevents the formation of a free-standing film (although this can be achieved if necessary by mechanical removal, for example, using a doctor blade). Instead, a dry film 3 supported on the faster roller 2a is obtained, which is particularly advantageous for dry films with a thickness of less than 200 µm due to their limited mechanical stability.

[0041] In the illustrated embodiment, the first roller 2a and the second roller 2b can each be heated to a temperature of 100 °C. Furthermore, the first roller 2a can be provided with an adhesion-promoting surface to which the dry film 3 adheres, while the second roller 2b has a surface that reduces the adhesion of the dry film 3. The line force acting between the first roller 2a and the second roller 2b is 400 N in the illustrated embodiment.

[0042] By subsequent lamination onto a current collector provided with thermoplastic primer or binder, the dry film 3 can be removed from the first roller 2a and thus, for example, a solvent-free manufactured electrode can be generated.

[0043] In Figure 2 is in a Figure 1 The corresponding view shows an exemplary embodiment in which a symmetrical structure consisting of two of the in Figure 1The rolling apparatus shown is present. Recurring features in this figure, as well as in the following figure, are identified by identical reference numerals.

[0044] In the illustrated embodiment, the substrate 4 is guided between two roller devices arranged in a mirror-symmetrical manner. The two first rollers 2a, each carrying one of the dry films 3, face each other, so that the substrate 4 can be coated with the dry film 3 on both sides, since both surfaces face one of the rollers 2a. For this purpose, the substrate 4 is moved at a speed that corresponds exactly to the peripheral speed of the two first rollers 2a. In the illustrated embodiment, the two roller devices are identical in construction except for their mirror-symmetrical arrangement; in particular, they have the same dimensions and are operated at the same rotational speeds or peripheral speeds. In further embodiments, dry films 3 with different compositions can also be applied to the substrate 4 by... Figure 2 In the illustrated embodiment, however, the dry films 3 are identical.

[0045] The described method also allows the production of an electrode with alternative current collectors as substrate 4, e.g., perforated substrates with low basis weight such as perforated metal foils or conductive fabrics. In the Figure 2 In the illustrated embodiment, the substrate 4 is an aluminum foil with a carbon primer as a coating on both sides.

[0046] This enables continuous film production for battery electrodes for primary and secondary batteries, e.g. lithium-ion batteries, lithium-sulfur batteries, sodium-sulfur batteries, solid-state batteries, supercapacitor electrodes, electrodes for fuel cells, electrodes for electrolysis cells, electrodes for other electrochemical elements, but also filter membranes or adsorptive coatings through the use of porous particles, decorative layers, optical layers for absorption and / or layers made of moisture-sensitive or solvent-sensitive materials.

[0047] Figure 3 shows in a Figure 1A further embodiment of the invention is shown in the corresponding schematic side view, in which the substrate 4 is wound onto a substrate roll 5 in the form of a film and is introduced into the roller gap in film form, so that the forming dry film 3 is laminated directly onto the substrate 4 in the roller gap. In this embodiment, the dry film 3 is no longer stored directly, i.e., in direct contact, on the first roller 2a, but is only guided indirectly on the first roller 2a and wound onto a further roller 2a.

[0048] The described process therefore allows electrode production directly from a premixed dry film powder without additional fibrillation steps, thus eliminating the need to form a free-standing film. The process can be used for pre-fibrillation, which increases the mechanical stability of the dry film. Furthermore, the free-standing film can be created by detaching it from the carrier roll. The loading and density can be adjusted by the rotational speed of the first roll 2a and the second roll 2b, and by the pressing force acting in the direction of the calender gap or roll gap. Dry film formation is self-metering, and the resulting layer thickness is determined by the pressing force applied to the two rolls 2a and 2b.Pre-dosing is achieved by continuously feeding a specific quantity of powder (adapted to process parameters), for example via powder conveyor 1 or a conveying substrate. This also allows for influencing the layer thickness.

[0049] The mechanical stability of the dry film 3 is determined by the pressing forces and rotational speeds (shear rates) used. Compared to freestanding films that were merely pressed in the roller gap at the same rotational speeds of rollers 2a and 2b, the dry films 3 produced by the proposed method exhibit significantly increased mechanical stability.

[0050] Only features of the various embodiments disclosed in the exemplary embodiments can be combined and claimed individually.

Claims

1. A method for producing a dry film (3), in which a dry powder mixture is processed into the dry film (3) by a rolling device having a first roll (2a) and a second roll (2b), wherein the first roll (2a) has a higher rotational peripheral speed than the second roll (2b) and the dry film (3) is supported on the first roll (2a), wherein a dry film (3) is formed which has anisotropically formed fibrils, characterised in that a ratio of the rotational peripheral speed of the first roll (2a) to the rotational peripheral speed of the second roll (2b) of 10:5 to 10:1 is maintained.

2. A method according to claim 1, characterised in that a ratio of the rotational peripheral speed of the first roll (2a) to the rotational peripheral speed of the second roll (2b) of 10:5 to 10:3, particularly preferably 2:1, is maintained.

3. A method according to any one of the preceding claims, characterised in that a non-flowable powder mixture is used for producing a dry film.

4. A method according to any one of the preceding claims, characterised in that the first roll (2a) is provided with an adhesion-enhancing modification, and / or the second roll (2b) is provided with an adhesion-reducing modification.

5. A method according to any one of the preceding claims, characterised in that the dry film (3) is applied to a substrate (4), wherein, preferaby, the substrate (4) i) is moved at a speed corresponding to the rotational peripheral speed of the first roll (2a) for the application; and / or ii) is moved over the first roll (2a) while the dry film (3) is formed on the substrate (4); and / or iii) consisting of a metallic material is used; and / or iv) prior to the lamination of the dry film (3) on a respective surface, is provided with a, preferably thermoplastic, primer, and / or a, preferably thermoplastic, binder; and / or v) consisting of an expanded metal, a metal wire mesh, a nonwoven fabric, a copper foil or an aluminium foil with an applied carbon primer is used.

6. A method according to any one of the preceding claims, characterised in that the dry film (3) i) is formed by the first roll (2a) and the second roll (2b) with a linear force, acting in the nip between the rolls, of 100 N / cm to 10 kN / cm, preferably 400 N / cm; and / or ii) is formed with a thickness of less than 500 µm, preferably less than 300 µm, particularly preferably less than 150 µm.

7. A method according to any one of the preceding claims, characterised in that a dry powder mixture is used which comprises polytetrafluoroethylene, a conductive additive, porous carbon, a transition metal oxide and / or sulphur.

8. A method according to any one of the preceding claims, characterised in that, with an adhesion-enhancing primer layer on respective substrate surface regions for the formation ofa structuring of the dry film (3), preferably in the form of strips or in the shape of rectangle, and particularly preferably in the direction of coating, is achieved.

9. A rolling device for implementing the method according to any one of the preceding claims, wherein a dry powder mixture is introduced into the nip between a first calender roll (2a) and a second calender roll (2b), and the first calender roll and second calender roll (2a and 2b) are configured or driven such that the first calender roll (2a) has a higher rotational peripheral speed than the second calender roll (2b), and the first calender roll and second calender roll (2a and 2b) respectively have an opposite direction of rotation, wherein the rolling device is suitable for forming a dry film (3) having anisotropically formed fibrils, characterised in that the rolling device is configured to maintain a ratio of the rotational peripheral speed of the first roll (2a) to the rotational peripheral speed of the second roll (2b) of 10:5 to 10:1.

10. A rolling device according to the preceding claim, characterised in that, in addition to the dry powder mixture, a substrate (4) is passed through the nip.

11. A rolling device according to claim 9, characterised in that two calender roll pairs comprising first and second calender rolls (2a and 2b) are arranged side by side in a mirror symmetry such that a nip is formed between two first calender rolls (2a), through which a dry film (3) is provided to two sides of a substrate (4) which is likewise fed through this nip, and the two first calender rolls (2a) have an opposite direction of rotation.

12. A rolling device according to claim 9, characterised in that a further first calender roll (2a), which rotates about a rotational axis, is present, onto which a dry film formed between the first calender roll (2a) and the second calender roll (2b) can be wound following exit from the nip, and the two first calender rollers (2a) have the same rotational peripheral speed.

13. A dry film produced by method according to any of claims 1 to 8, characterised in that the fibrils have a length of 0.1 µm to 1000 µm.

14. A dry film according to claim 13, characterised in that the dry film (3) is applied to a substrate (4).

15. An electrochemical storage device or electrochemical converter, comprising a dry film according to any one of claims 13 or 14.