Method and device for producing electrodes for battery cells

Abstract

The invention relates to a method for producing electrodes (40) for battery cells, in which method an electrode paste (22, 32) is first applied as a coating onto a metal foil (20). The coating is subsequently dried in a drier (42; 142), it being possible to change the drying conditions prevailing in the drier (42; 142). According to the invention, for the purpose of inline process monitoring, at least one property of the coating is measured contactlessly by a measuring device (48a, 48b). The measuring device may comprise, for example, one or more chromatic-confocal distance sensors or laser photometry sensors. If the values measured by the measuring device lie outside a target value range, the temperature or another process condition in the dryer (42; 142) is changed. The measuring device can be used to detect transitions between the phases of the drying process, so that, for example, the drying rate can be specifically adapted to the progress of the drying process.

Description

[0001] Method and device for producing electrodes for battery cells

[0002] BACKGROUND OF THE INVENTION

[0003] 1. Field of the invention

[0004] The invention relates to a method and a device for producing electrodes for battery cells. The electrodes consist of a thin metal foil to which an electrode paste is applied on both sides. The invention particularly relates to the drying of the applied electrode paste.

[0005] 2. Description of the state of the art

[0006] Electrodes for lithium-ion battery cells contain metal foils coated on both sides with an electrode paste. If the electrode is the cathode, the metal foil is made of aluminum, and the electrode paste is a mixture of the chemically active substance, e.g., a lithium metal oxide, as well as conductive carbon black, a binder, and a solvent. For the anode, the metal foil is made of copper, while the electrode paste is usually a mixture of graphite, silicon, conductive carbon black, a binder, and a solvent.

[0007] The metal foils typically have thicknesses ranging between 8 pm and 20 pm; the coatings are approximately 70 pm to 1,500 pm thick on each side, with the anode often being significantly thicker than the cathode.

[0008] The initially moist electrode paste (slurry) is applied to the metal foil using a slot-die coating process. The metal foil is guided on a roller past a slot die that extends across the entire width of the metal foil. The electrode paste, which has a solvent content of between approximately 40 and 60 percent by weight, emerges from the slot die under pressure and is pressed against the metal foil, to which it adheres on one side. The moist electrode paste is then dried in a dryer to remove the solvent from the electrode paste. The remaining components of the electrode paste form a porous, solid network upon drying.

[0009] After drying, the coatings are compacted and cut by calendering.

[0010] Conceptually, the simplest method is to first coat and dry one side of the metal foil, followed by the other side. However, if a continuous process is to be implemented, two dryers are required, which can be up to 80 meters long. More efficient, but technologically more demanding, are processes in which the metal foil is coated on both sides and then dried. In this way, a continuous process can be implemented with just one dryer.

[0011] Drying the electrode paste not only requires a lot of energy, but is also a critical process step for the subsequent function of the battery cells. It has been shown that optimal results can only be achieved if the process conditions are varied during the drying process. For example, if drying is carried out at consistently high heating rates, the adhesion of the electrode paste to the metal foil can be compromised to such an extent that delamination occurs. This leads to reduced capacity or even failure of the subsequent battery cells.

[0012] Impingement jet dryers are typically used as dryers. These dryers have multiple nozzles that direct hot air onto the coatings. The nozzles are arranged along the conveyor line on opposite sides of the strip-like electrode; the temperature and flow rate of the escaping hot air can be adjusted separately for each nozzle. This allows the process conditions to be varied during drying. Alternatively, the strip-like electrode can be passed through several individual dryers, each with different process conditions.

[0013] The process conditions in the various zones of the dryer (or in several individual dryers) are currently monitored during drying using sensors. The measured variables include temperature, humidity, and the flow rate of the hot air directed at the respective coating. To determine the process conditions in the zones or individual dryers, numerous tests are conducted in advance with different temporal sequences of process conditions. The finished electrode is then examined to determine the optimal sequence of process conditions. To check the total thickness, either mechanical measuring devices such as dial indicators or micrometer screws or optical measuring devices based on the principle of laser triangulation or chromatic confocal imaging are used.More precise information on the porous network created by drying can be obtained using X-ray-based methods or micro-Raman spectroscopy (MRS), as described, for example, in a paper by J. Kaiser et al., "Process and Product Development of Electrodes for Li-Ion Cells," (2014), Chemie Ingenieur Technik. 86. 10.1002 / cite.201300085. In these known methods, the binder / solvent system is examined at a fracture edge of the finished electrode.

[0014] Even if the optimal process conditions found in this way are set and monitored by sensors during the drying process, it still happens again and again that the electrode exhibits defects after drying. Especially if these defects are only discovered during subsequent testing of the battery cells, the economic damage caused by the rejection is significant.

[0015] SUMMARY OF THE INVENTION

[0016] The object of the invention is to provide a method and a device for producing electrodes for battery cells, with which such defect-related rejects can be avoided or at least noticeably reduced.

[0017] This object is achieved by a method for producing electrodes for battery cells, wherein the method comprises the following steps: a) an electrode paste is applied to a metal foil as a coating, whereby a coated metal foil is obtained; b) the coating applied to the metal foil is dried in a dryer, wherein the drying conditions prevailing in the dryer can be changed; c) for the purpose of inline process monitoring, at least one property of the coating is measured non-contact by a measuring device; d) if values ​​measured by the measuring device for the at least one property lie outside a target value range, at least one of the process conditions in the dryer is changed.

[0018] The invention is based on the realization that rejects due to defects can only be avoided if at least one property of the coating is measured non-contact by a measuring device during inline process monitoring, i.e., during the drying process. The requirement for non-contact measurement arises from the fact that the coating is still deformable until drying is complete, so undesirable deformation cannot be ruled out in the event of contact.

[0019] Inline process monitoring is a method of real-time monitoring of an ongoing process or system. The term "inline" means that monitoring takes place without taking samples. Inline process monitoring allows deviations from the desired operating parameters to be quickly detected. This is a prerequisite for preventing quality defects and resulting rejects from occurring in the first place.

[0020] The at least one measured property of the coating could be, for example, its thickness. However, the thickness alone makes it difficult to correctly adjust the drying conditions. Therefore, the degree of drying of the coating is preferably determined from the at least one measured property, as this represents the more significant parameter for optimally adjusting the drying conditions.

[0021] The at least one further property of the coating can be, in particular, a specular and / or scattering reflectivity of the coating, a roughness of the surface of the coating, or a thermal conductivity of the coating. The roughness can be easily derived from the spatial dispersion of the measured thicknesses. Preferably, at least two of these properties of the coating are measured non-contact by a measuring device. The degree of drying of the coating during the drying process is generally not specified quantitatively, but qualitatively. A division of the drying process into the following five phases has proven to be effective:

[0022] Phase (a): the coating forms a moist film;

[0023] Phase (b): the coating forms a completely filled capillary network;

[0024] Phase (c): the coating forms a partially filled capillary network;

[0025] Phase (d): only individual and separate liquid residues remain in the coating; and

[0026] Phase (e): the coating forms a liquid-free film.

[0027] The transitions between phases (b) and (c), as well as between phases (c) and (d), are particularly critical. Drying should only be slowed down in phase (c). To do this, it is necessary to determine from the measured values ​​when the transition between phases (b) and (c) occurs during the coating process. Once the transition between phases (c) and (d) has been detected, drying can be accelerated again.

[0028] Slowing down or accelerating drying is usually accompanied by a decrease or increase in temperature. Additionally or alternatively, the speed of the hot air directed onto the coating can be decreased or increased.

[0029] The transition between phase (b) and phase (c) can be determined with high reliability if the roughness of the surface or a value derived from it (e.g. the time derivative of the mean roughness value R a) exceeds a threshold value. The decreasing filling of the capillary network is noticeable in that the surface becomes significantly rougher. In addition, the specular reflectivity decreases during this transition and can be easily measured using a suitable measuring instrument. The roughness can be described by one or more of the usual values ​​used to quantify roughness, e.g. the mean roughness Ra, the root mean square roughness Rq or the average roughness depth Rz. If at least one of the properties is the reflectivity of the surface, this can be derived from the amplitude of a response to a thermal excitation of the coating.

[0030] If the at least one property is the thermal conductivity of the coating, this can be derived from a phase shift of a response to a thermal excitation of the coating.

[0031] The thermal conductivity of the coating can be determined, in particular, to infer the transition between phase (b) and phase (c). This transition is recognizable when the phase shift of a response to a thermal excitation of the coating or a parameter derived from it reaches a minimum value. Since the mass fractions of water, carbon, and air in the coating change during the drying process and these components have significantly different thermal conductivities, conclusions can be drawn about the water impregnation.

[0032] The transition between phase (c) and phase (d) can also be inferred when the scattering reflectivity of the surface increases after a prior decrease until a final value is reached. This remarkable finding can be partially explained by the fact that the surface of the undried coating is initially smooth and moist, which leads to high specular reflectivity. As the capillary network begins to empty in phase (c), the specular reflectivity decreases, as there is now increased scattering reflection of the incident light. Surprisingly, however, the scattering reflectivity then increases again as emptying continues. With sensors that measure the scattering reflectivity (possibly together with the specular reflectivity), the aforementioned transition between phases (c) and (d) can thus be detected.

[0033] It has been shown that surface roughness and reflectivity depend only slightly on layer thickness. In other words, in most cases, the change in layer thickness does not reliably determine whether transitions between phases (b) and (c) or between phases (c) and (d) have occurred. The roughness threshold value, above which a transition between phase (b) and phase (c) can be detected, and the final value at which the scattering reflectivity can be reached, indicating the transition between phase (c) and phase (d), are preferably determined experimentally through preliminary tests. The same applies to thermal conductivity.

[0034] For optimal inline process monitoring, the measuring system should measure at least one property of the coating quasi-continuously during the drying process. However, intermittent measurements at specified intervals are also possible.

[0035] It is preferred if the measuring device is a measuring element of a control loop that regulates the process conditions in the dryer depending on the measured at least one property. The actuator of the control loop can be, for example, a heating device with which the temperature in a dryer or a drying zone can be changed, and / or a fan with which the speed of the hot air in the dryer can be changed.

[0036] However, control is not mandatory because, for example, measurements can be carried out at longer intervals, which trigger different predefined control processes, which strictly speaking is not called control.

[0037] The invention further relates to a device for producing electrodes for battery cells. The device comprises a coating device configured to apply an electrode paste as a coating to a metal foil, thereby obtaining a coated metal foil. The device further comprises a dryer having either several separate dryer units or dryer zones arranged in a common housing. Different process conditions can be generated in the dryer units or the dryer zones. According to the invention, the device comprises a measuring device configured to measure at least one property of the coating in a non-contact manner during drying.A control or regulating device is configured to change at least one of the process conditions in the dryer for the purpose of inline process monitoring if values ​​measured by the measuring device for at least one property lie outside a target value range. The measuring device preferably has several distance sensors, which can in particular be chromatic confocal distance sensors or laser photothermal sensors, which are configured to determine a phase shift and an amplitude from the response to a thermal excitation.

[0038] The invention further relates to the use of a chromatic-confocal distance sensor or one for measuring the thickness, roughness, and reflectivity of the coating in a method according to the invention. It has been shown that all three variables can be measured simultaneously with such sensors. Since the measuring light beam is directed perpendicularly onto the coating, the total reflectivity, which is composed of the specular and scattered reflectivity, can be deduced from the median intensity. If two such distance sensors are arranged on opposite sides of the electrode, the total thickness of the electrode can be measured by taking the difference. Since the thickness of the metal foil is known or can be easily measured in advance, the total thickness of the two coatings can be derived from it. The measurement accuracy of such sensors is so high that they can even be used to measure roughness without contact.

[0039] The invention also relates to the use of a laser photometry sensor in the aforementioned method.

[0040] BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In the following, exemplary embodiments of the invention are explained in more detail with reference to the drawings. In these drawings:

[0042] Figure 1: a schematic and not to scale side view of a device according to the invention according to a first embodiment;

[0043] Figure 2: a longitudinal section through an impact jet dryer in which, according to the invention, several sensors detect the degree of drying of the coating;

[0044] Figures 3a to 3d are schematic representations of the five drying phases;

[0045] Figure 4 shows a diagram in which the roughness R and the total thickness of the coating are plotted against time during drying; Figure 5 shows a diagram in which the intensity I measured by a chromatic confocal distance sensor and the total thickness of the coating are plotted against time during drying;

[0046] Figure 6 is a schematic plan view of an electrode and a fixed measuring unit with several distance sensors;

[0047] Figure 7 is a schematic plan view similar to Figure 6, but with a longitudinally movable measuring unit;

[0048] Figure 8 is a schematic plan view similar to Figure 6, but with two fixed measuring units;

[0049] Figure 9 shows a dryer according to another embodiment with several separate dryer units

[0050] Figure 10 is a diagram showing the phase shift of the thermal excitation response versus time during drying;

[0051] Figure 11 is a diagram showing the amplitude of the thermal excitation response versus time during drying;

[0052] Figure 12 is a diagram showing the phase shift of the thermal excitation response over time during several drying processes.

[0053] DESCRIPTION OF PREFERRED EMBODIMENTS

[0054] 1. First embodiment

[0055] Figure 1 shows a schematic side view of a first exemplary embodiment of parts of an apparatus according to the invention, designated overall by 10, for producing electrodes for battery cells. The apparatus 10 comprises a slot-die coating system 11 with a first slot die 12, the nozzle opening 14 of which is arranged at a precisely defined distance from the surface of a roller 16. During coating, the roller 16 rotates in a direction of rotation indicated by an arrow 18 and feeds a metal foil 20 of thickness dp to the first slot die 12, which foil is unrolled from a storage roll (not shown).

[0056] The first slot nozzle 12 is connected via a pump (not shown) to a reservoir (also not shown) for a first electrode paste 22, which is moist and has a granular consistency. Possible compositions for electrode pastes are described above in connection with the description of the prior art and are also considered for the electrode paste 22. The same applies to the metal foil 20.

[0057] The pump pumps the first electrode paste 22 from the reservoir and supplies it with uniform pressure to the nozzle opening 14. The nozzle opening 14 is slit-shaped and extends perpendicular to the paper plane across the entire width of the metal foil 20.

[0058] The pump presses the first electrode paste 22 out of the nozzle opening 14, whereby the first electrode paste 22 is applied to the metal foil 20 and adheres there. If the rotational speed of the roller 16 and the mass flow of the first electrode paste 22 are correctly coordinated, a layer of the first electrode paste 22 with a uniform thickness di is formed on the front side 24 of the metal foil 20. The thickness di is determined by the distance between the nozzle opening 14 and the metal foil 20, the position of which is precisely defined by the stationary roller 16. However, "fixed" does not mean that the thickness di will no longer change. Depending on the properties of the electrode paste 22, it can shrink or expand after application.

[0059] In the conveying direction behind the first slot nozzle 12, a second slot nozzle 30 is arranged, which coats the back of the metal foil 20 with a second electrode paste 32.

[0060] A pump, which feeds the second electrode paste 32 to the second slot nozzle 30, conveys the second electrode paste from a reservoir (not shown) to a nozzle opening 36 of the second slot nozzle 30. The mass flow of the second electrode paste 32 and the conveying speed of the metal foil 20 are also coordinated here such that the back side 38 of the metal foil 20 is evenly coated with the second electrode paste 32. However, the thickness dz of the electrode paste 32 on the back side 38 fluctuates significantly more than the thickness di of the first electrode paste 22, since the metal foil 20 cannot support itself on a roller or a similar stationary structure when the back side 38 is coated. The metal foil, now coated on both sides and referred to below as electrode 40, is fed to a dryer 42 for the next process step with the aid of conveying means that are known per se and are therefore not shown.Since the electrode pastes 22, 32 are soft before drying, contact with the electrode paste by drive wheels or similar objects should be avoided. For example, if longitudinal stripes on the metal foil 20 remain uncoated, they may be damaged by drive wheels or skids.

[0061] The dryer 42 contains a housing 44 in which a controllable heating device 46, a thermometer 47, and two distance sensors 48a, 48b are arranged. These sensors are based on the chromatic confocal measuring principle and have very high measurement accuracy at sampling rates of approximately 35 kHz. Details on such sensors can be found, for example, in DE 10 2006 017 400 A1. The distance sensors 48a, 48b measure the total thickness dc of the coatings quasi-continuously during the drying of the electrode 40. Since the distance between the two distance sensors 48a, 48b and the thickness dp of the metal foil 20 are known, the total thickness dc = di + dz of the two coatings results from the said distance minus the measured distance values ​​and the thickness dp of the metal foil 20. To increase the measurement accuracy, a calibration with a thickness standard can be carried out in advance.The components in the housing 44 are connected to a controller 50, which in the illustrated embodiment is arranged outside the housing 44. Sensitive parts of the distance sensors 48a, 48b can also be arranged outside the housing 44 in order to avoid exposing them to the high temperatures in the dryer 42. In the case of distance sensors based on the chromatic-confocal measuring principle, for example, only one (possibly water-cooled) measuring head with the lens can be located in the dryer 42, while the spectrometer with the evaluation electronics is arranged outside the housing 44 and connected to the measuring head via an optical fiber.

[0062] In the illustrated embodiment, how quickly the coatings dry depends primarily on the temperature T in the dryer 42, which is measured by the thermometer 47. The total thickness of the coatings measured by the distance sensors 48a, 48b allows approximate conclusions to be drawn about the degree of drying of the coatings. If the coatings dry too quickly or too slowly, this can have a detrimental effect on the properties of the electrode 40. The controller 50 therefore regulates the temperature T in the dryer 42 so that the thickness of the coatings lies within a target value range. The distance sensors 48a, 48b represent the measuring elements and the heating device 46 represents the actuator of the control loop. The control reduces the probability that the coatings will not adhere correctly to the metal foil 20 due to drying too slowly or too quickly, which can later lead to delamination and thus to functional failures.

[0063] After drying, the electrode 40, which can be up to 2,000 m long and 1 m wide, is compacted in a conventional manner by calendering and further processed by longitudinal cutting.

[0064] 2. Second embodiment

[0065] To ensure that the electrode pastes 22, 32 of the finished electrode 40 have an optimal structure and adhere perfectly to the metal foil 20, it is beneficial to temporarily slow down the drying process. To achieve this, the drying rate in the dryer 42 must be reduced by the correct amount at the right time. Consistently drying at a low drying rate will lengthen the drying time, which is detrimental to the economic efficiency of the process.

[0066] Figure 2 schematically shows an impingement jet dryer 142 with which the drying rate can be flexibly varied during drying. The impingement jet dryer 142 comprises a housing 44 through which the electrode 40 is passed. On each side of the electrode 40, several hot air nozzles 52 are arranged, from which hot air 54 emerges, which is directed onto the band-shaped electrode 40 moving past. In the illustrated embodiment, both the temperature T and the speed of the hot air 54 can be individually adjusted for each hot air nozzle 52 in a manner not shown in detail. Different drying rates can be achieved in different zones of the impingement jet dryer 142 by changing the temperature T and / or the speed of the escaping hot air 54 accordingly.

[0067] Figures 3a to 3e illustrate different degrees of drying or phases during the drying process using an anode electrode paste as an example. In these figures, the large white grains 56 indicate graphite particles, small circles 58 indicate SBR particles (SBR stands for styrene-butadiene rubber), thin threads 60 indicate carboxymethylcellulose (CMC), and the remaining particles 62 indicate carbon black particles.

[0068] A division of the drying process into the following five phases has proven to be successful and has become accepted in the professional world:

[0069] Figure 3a - Phase (a): the coating forms a moist film;

[0070] Figure 3b - Phase (b): the coating forms a completely filled capillary network;

[0071] Figure 3c - Phase (c): the coating forms a partially filled capillary network;

[0072] Figure 3d - Phase (d): only individual and separated liquid residues are present in the coating; and

[0073] Figure 3e - Phase (e): the coating forms a liquid-free film.

[0074] During phase (c), drying should be slowed down. This requires reliable detection of the transition between phases (b) and (c), as well as between phases (c) and (d). The time of these transitions cannot be reliably determined from the total thickness dc measured by the distance sensors 48a, 48b. While the total thickness dc decreases continuously with increasing degree of drying, the total thickness dc at which the aforementioned transitions between the phases occur depends, among other things, in a difficult-to-predict manner on the thicknesses di and dz of the freshly applied electrode pastes 22, 32.

[0075] Therefore, in addition to the distance values ​​measured by the distance sensors 48a, 48b (or the total thickness dc already derived therefrom), additional information is supplied to the controller 50 with which the degree of drying can be determined and the aforementioned transitions between phases (b) and (c) as well as (c) and (d) can be detected.

[0076] To detect the transition between phases (b) and (c), the roughness of the respective facing surface is determined from the measured distance values. This takes advantage of the fact that, with stationary distance sensors 48a, 48b, the electrode 40 is continuously moved past the distance sensors 48a, 48b at a constant speed. This results in a large number of adjacent measurement points from which a roughness measurement value can be derived. As can be seen by comparing Figures 3b and 3c, the roughness of the surface increases abruptly during the transition between phases (b) and (c).

[0077] Figure 4 illustrates this using a diagram in which the total thickness dc (dashed line) and a roughness-describing parameter such as the mean roughness value R a (solid line) is shown schematically as a function of time t during drying. The aforementioned continuous decrease in thickness dc can be seen. Although the thickness decrease is not constant, the change in gradient is not a reliable indicator of the transition between phases (b) and (c) for the reasons mentioned above.

[0078] The increase in the mean roughness value R seen in Figures 3b and 3c a However, it can be quantitatively determined using the diagram in Figure 4. The moment of transition between phases (b) and (c) can be defined as the time tbc, at which the increase in the mean roughness value R a(ie, the first derivative over time) is maximum, see the vertical dashed line. From time tbc onward, the heating rate can be reduced, for example, by reducing the temperature and / or the velocity of the incident hot air.

[0079] During phase (c) and the subsequent phases (d) and (e), the roughness increases only slightly. As can be seen from Figures 3c and 3d, the transition between phases (c) and (d) cannot be reliably identified based on the roughness.

[0080] To reliably detect this transition, the distance sensors 48a, 48b also measure the intensity of the measuring light that strikes the coating surfaces perpendicularly and is partially reflected by them. Due to the perpendicular incidence of the light, the distance sensors 48a, 48b measure both the specular and scattered portions of the reflected light.

[0081] Figure 5 shows, in a diagram similar to Figure 4, how the intensity I measured by a distance sensor 48a, 48b (or more precisely its median) changes during drying. During phases (a) and (b), the intensity is high and decreases only slightly, since the liquid components in the electrode pastes 22, 32 result in a smooth surface that specularly reflects a comparatively large amount of measurement light. In phase (c), the surface is rough and therefore specularly reflects significantly less measurement light, which is noticeable in a significant decrease in the measured intensity I. Surprisingly, however, towards the end of phase (c), there is an increase in reflection at the surface, causing the measured intensity I to rise again. This increase ends at the transition between phases (c) and (d).Thus, if the intensity I increases again after a decrease until a final value is reached, the time at which this final value is reached (or the increase ends) can be considered time tcd, at which the transition between phases (c) and (d) takes place. From this time tcd, drying can be accelerated again, e.g., to the original value during phases (a) and (b).

[0082] In the illustrated embodiment, it is thus possible to determine the total thickness dc, the roughness of the surfaces, and their reflectivity using only the distance sensors 48a, 48b, and from this to determine the degree of drying. In particular, it is possible to determine when the transitions between phases (b) and (c) and phases (c) and (d) occur, which are important for process control.

[0083] In Figure 2, several pairs of distance sensors 48a, 48b are arranged one behind the other along the conveying direction in the impact jet dryer 142. How many distance sensors 48a, 48b are required and where they should ideally be arranged along the conveying path depends on numerous parameters of the individual case. It is often sufficient if only one pair of distance sensors 48a, 48b is arranged in the transverse direction of the electrode (i.e. perpendicular to the paper plane in Figure 2), e.g. in the middle of the electrode 40. Depending on the number and arrangement of the hot air nozzles 52, however, the degree of drying in the transverse direction can vary. In these cases, it can be advantageous if pairs of distance sensors 48a, 48b are arranged at different positions in the transverse direction, as illustrated in Figure 6. There it can be seen that the coating facing upwards towards the viewer does not completely cover the metal foil 20, but that longitudinal and transverse stripes 72 and 74, respectively, are formed.74 remain, which are uncoated. Reference numeral 76 indicates a measuring station comprising a total of six distance sensors 48a distributed transversely across the coated sections of the electrode 40.

[0084] To monitor the drying at specific points on the electrode 40, the measuring station 76 can be moved synchronously with the electrode 40, as illustrated in Figure 7. The common conveying direction is indicated by arrows 78.

[0085] Alternatively, several measuring stations 76a, 76b can be provided, the measurement results of which are correlated in such a way that they can be assigned to specific points on the electrode, see Figure 8. At the later position of the electrode 40 indicated by dashed lines, the points measured by the first measuring station 76a are located exactly below the second measuring station 76b.

[0086] To measure the total thickness dc of the coating, two distance sensors 48a, 48b must be provided per measuring point for metal foils 20 coated on both sides. However, it has been shown that the transition between phases (b) and (c) and between phases (c) and (d) can be detected solely based on the roughness or the measured intensity; knowledge of the total thickness dc improves the reliability of detection, but is not absolutely necessary. If the total thickness dc is not required, the measurement can be performed with only one distance sensor 48a or 48b. This then measures the distance values ​​solely to determine the roughness average value R a and intensity I.

[0087] In systems where the second coating is applied after the first coating has dried, two dryers are required, each drying only one coating. Even in these cases, it is sufficient to perform the measurement with only one distance sensor 48a or 48b.

[0088] 3. Third embodiment

[0089] The explanations for Figures 2 and 3a to e also apply to the third embodiment.

[0090] In this embodiment, laser photothermal sensors are used instead of chromatic confocal distance sensors to supply information to the controller 50 with which the degree of drying can be determined and the aforementioned transitions between phases (b) and (c) as well as (c) and (d) can be detected.

[0091] The functionality of such laser photothermal sensors is known, for example, from EP 3 017 274 A1. A component surface is thermally excited with a laser, and the component's temperature response is recorded via a near-infrared photodiode. The excitation is modulated. The temperature response is therefore also modulated. Two important parameters of the temperature response can be evaluated: the amplitude and the phase shift of the modulation relative to the excitation.

[0092] Figure 10 shows a diagram schematically illustrating a typical phase shift P during the drying process. During phases a and b, the water content in the coating decreases, so the mass fraction of graphite increases in percentage. Since graphite has better thermal conductivity than water, the thermal conductivity of the coating increases, leading to a decrease in the phase shift.

[0093] In phase c, the capillary network begins to empty, so that the water content is gradually replaced by air. Air, in turn, has a significantly poorer thermal conductivity than water, so that the overall thermal conductivity of the coating decreases again and the phase shift P increases. A minimum of the phase shift during the transition between phases (b) and (c) at time tbc is reached when the minimum amount of water is present that completely fills the capillary network and no air is yet present. This transition can therefore be easily identified by determining the minimum of the value curve of the phase shift P. Towards the end of phase (d), in which only very isolated and separate liquid residues are present, and in the completely dry phase (e), the thermal conductivity and thus the phase shift P stabilizes at a constant value Pf.

[0094] The phase shift is therefore essentially a measure of the thermal conductivity of the coating. Thermal conductivity depends significantly on the mass fraction of water within the coating. It has been shown that this value Pf is largely independent of factors such as the dilution of the coating at the beginning of the process and the total thickness, to which other sensors are sensitive.

[0095] Figure 11 shows a diagram of schematic phase shift P curves for drying processes in which the thinning, thickness, and drying process are different. While the curves differ over time, they all end at the same final plateau. The value Pf can therefore be used as a reliable indicator of the end of the drying process.

[0096] Figure 12 shows a diagram schematically depicting a typical curve of the amplitude A of the temperature response during the drying process. Similar to the intensity measured using a chromatic confocal sensor in Example 2, the amplitude A is primarily dependent on reflectivity. The curve is therefore similar to that shown in Figure 4, although the thermal response also includes components resulting from reflections below the surface and therefore reacts more sensitively to the drying process, even deeper within the coating.

[0097] During phases (a) and (b), drying / evaporation primarily causes a decrease in layer thickness, while the surface and the interstices remain wetted. Reflectivity therefore decreases only very slowly, resulting in a slow increase in amplitude A. High surface reflectivity in the laser photothermal process means poorer absorption and thus a smaller temperature response (lower amplitude). As soon as the thickness stabilizes in phase (c), evaporation begins to cause drying, primarily near the surface, whose reflectivity decreases rapidly during this phase. Therefore, a sharp increase is observed from time tbc onwards.

[0098] In phase (d), however, the surface is already largely dry, and evaporation occurs primarily from the interior. The surface reflectivity thus stabilizes, leading to a stabilization of the amplitude A of the temperature response. The transition between phases (c) and (d) can be recognized by this stabilization.

[0099] The amplitude is therefore essentially a measure of the reflectivity of the coating. It has been found that, depending on the frequency of the excitation modulation, effects at different depths of the coating contribute significantly to the temperature response. When measuring the curves shown in Figures 10 to 12, an excitation frequency of 2000–4000 Hz was used, which primarily provides information on the drying state of the coating surface. However, excitation at 20–400 Hz provides information from the interior of the material. This allows the drying state to be clearly determined both on the coating surface (laser modulation between 2000–4000 Hz) and inside the coating (laser excitation between 20–400 Hz).

[0100] The impingement jet dryer 142 shown in Figure 2 can be equipped with laser photothermal sensors instead of distance sensors, analogous to the second embodiment described above, to implement the method described here. Control is then carried out based on the values ​​of the phase shift P and the amplitude A.

[0101] 4. Example

[0102] Figure 9 shows a fourth embodiment in which elements of the first and second or the first and third previously described embodiments are combined. In this embodiment, the dryer 42 has three dryer units 42a, 42b, 42c, each constructed like the dryer shown in Figure 2. The controller 50 controls all three dryer units 42a, 42b, 42c jointly. Different drying conditions can be set in the dryer units 42a, 42b, 42c. As in the second embodiment, the drying process can be temporarily slowed down, particularly in phase (c).

[0103] The dryer shown in Figure 9 with multiple dryer units 42a, 42b, 42c is particularly advantageous when the coating is not to be exposed to strong air movements, as is the case with the impingement jet dryer 142. The hot air can remain at rest in the dryer units 42a, 42b, 42c, so that the drying rate is adjusted only by the temperature T in the respective unit. 5th Embodiment

[0104] It is also preferable to combine distance sensors as described in the second embodiment and laser photometry sensors as described in the third embodiment in one system. The determination of the phase transitions and the control can then be carried out based on the parameters thickness, reflectivity, roughness, phase shift of the temperature response, and amplitude of the temperature response in any combination.

Claims

PATENT CLAIMS 1. A method for producing electrodes (40) for battery cells, the method comprising the following steps: a) an electrode paste (22, 32) is applied to a metal foil (20) as a coating, whereby a coated metal foil is obtained; b) the coating applied to the metal foil (20) is dried in a dryer (42; 142), wherein drying conditions prevailing in the dryer (42; 142) are variable; characterized in that, for the purpose of inline process monitoring, at least one property of the coating is measured non-contact by a measuring device (48a, 48b), and that if values ​​measured by the measuring device for the at least one property lie outside a target value range, at least one of the process conditions in the dryer (42; 142) is varied.

2. Method according to claim 1, characterized in that a degree of drying of the coating is determined from the at least one measured property.

3. Method according to claim 2, characterized in that the degree of drying of the coating during the drying process can be described by the following five phases, which are passed through successively during the drying process, namely phase (a), in which the coating forms a moist film, phase (b), in which the coating forms a completely filled capillary network, phase (c), in which the coating forms a partially filled capillary network, phase (d), in which only individual and separate liquid residues are present in the coating, and phase (e), in which the coating forms a liquid-free film, and that the measured values ​​are used to derive when the transition between phases (b) and (c) and / or when the transition between phases (c) and (d) takes place in the coating.

4. Method according to one of the preceding claims, characterized in that the at least one property is selected from the group consisting of: a thickness (dc) of the coating, a reflectivity of the coating, a roughness of the surface of the coating, and a thermal conductivity of the coating.

5. Method according to one of the preceding claims, characterized in that at least two properties of the coating are measured contactlessly by a measuring device (48a, 48b), and the two properties are selected from the group consisting of: a thickness (dc) of the coating, a reflectivity of the coating, a roughness of the surface of the coating, and a thermal conductivity of the coating.

6. Method according to claim 4 or 5, characterized in that the roughness of the surface of the coating is derived from a spatial scattering of the measured thicknesses.

7. Method according to one of claims 4 to 6 with reference to claim 3, characterized in that the transition between phase (b) and phase (c) is inferred when the roughness of the surface or a value derived therefrom exceeds a threshold value.

8. Method according to claim 4 or 5, characterized in that the reflectivity of the surface is derived from the amplitude of a response to a thermal excitation of the coating, and / or that the thermal conductivity of the coating is derived from a phase shift of a response to a thermal excitation of the coating.

9. Method according to claim 4, 5 or 8 with reference back to claim 3, characterized in that the transition between phase (b) and phase (c) is inferred when the course of the phase shift of a response to a thermal excitation of the coating or a variable derived therefrom reaches a minimum value.

10. The method according to claim 6 or 9, characterized in that the drying is slowed down when the transition between phase (b) and phase (c) has been concluded.

11. Method according to one of claims 4 to 10 with reference back to claim 3, characterized in that the transition between phase (c) and phase (d) is inferred when the scattering reflectivity of the surface, after a preceding decrease, increases until a final value is reached.

12. The method according to claim 11, characterized in that the drying is accelerated when the transition between phase (c) and phase (d) has been concluded.

13. Method according to one of the preceding claims, characterized in that the measuring device (48a, 48b) is a measuring element of a control circuit which regulates the process conditions in the dryer (42; 142) as a function of the measured at least one property.

14. A device (10) for producing electrodes (40) for battery cells, comprising a coating device (11) configured to apply an electrode paste (22, 32) to a metal foil (20) as a coating, thereby obtaining a coated metal foil, a dryer (42; 142) having either a plurality of separate dryer units (42a, 42b, 42c) or dryer zones arranged in a common housing (44), wherein different process conditions can be generated in the dryer units or the dryer zones, characterized by a measuring device (48a, 48b) configured to measure at least one property of the coating during drying in a contactless manner, and by a control or regulating device (50) configured to change at least one of the process conditions in the dryer for the purpose of inline process monitoring, if values ​​measured by the measuring device for at least one property lie outside a target value range.

15. Device according to claim 14, characterized in that the measuring device has a plurality of chromatic-confocal distance sensors (48a, 48b) which are designed to determine a roughness of the surface of the coating from measured distances.

16. Use of a chromatic confocal distance sensor (48a, 48b) in a method according to one of claims 1 to 13.

17. Device according to claim 14, characterized in that the measuring device has a plurality of laser photometry sensors which are designed to determine a phase shift and an amplitude from the response to a thermal excitation.

18. Use of a laser photometric sensor in a method according to one of claims 1 to 13.

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