Method and device for producing electrode of battery cell

Abstract

In a method for producing an electrode (40) of a battery cell, an electrode paste (22, 32) is first applied as a coating to a metal foil (20). The coating is subsequently dried in a dryer (42, 142), the drying conditions in the dryer (42, 142) being variable. According to the invention, in order to achieve in-line process monitoring, at least one property of the coating is measured in a non-contact manner by means of a measuring device (48a, 48b). The measuring device may include, for example, one or more spectrally confocal distance sensors or laser photo-thermal sensors. The temperature or another process condition in the dryer (42; 142) is varied if the value measured by the measuring device lies outside the nominal value range. Transitions between stages of the drying process can be detected by means of a measuring device in order to adjust the drying rate in a targeted manner, for example, depending on the progress of the drying process.

Description

BACKGROUND

[0001] 1. Field of the invention

[0002] The present invention relates to a method and a device for manufacturing electrodes for battery cells. The electrodes consist of a thin metal foil, to which electrode slurry is applied on both sides. The invention particularly relates to the drying of the applied electrode slurry.

[0003] 2. Description of the prior art

[0004] The electrodes of lithium-ion battery cells comprise a metal foil, which is coated on both sides with electrode slurry. In the case of a cathode, the metal foil consists of aluminum, and the electrode slurry consists of a mixture of chemically active substances (e.g. lithium metal oxides) with conductive carbon black, a binder and a solvent. In the case of an anode, the metal foil consists of copper, while the electrode slurry is usually a mixture of graphite, silicon, conductive carbon black, a binder and a solvent.

[0005] The thickness of the metal foil is usually in the range between 8 µm and 20 µm; the coating thickness on each side is approximately 70 µm to 1500 µm, with the thickness of anodes often being much greater than that of cathodes.

[0006] The initially still wet electrode slurry (English slot-die coating ) is applied to the metal foil by means of so-called slot die coating (English slurry ). Here, the metal foil, which is located on a drum, is guided past a slot die, which extends over the entire width of the metal foil. The electrode slurry, which has a solvent proportion of approximately between 40 and 60 percent by weight, is discharged under pressure from the slot die and pressed against the metal foil, to which it adheres on one side. The wet electrode slurry is then dried in a drying machine, so that the solvent is removed from the electrode slurry. In the drying process, the remaining constituents of the electrode slurry form a porous solid network.

[0007] After drying, the coating is compressed and cut by calendering.

[0008] The simplest concept is to coat and dry one side of the metal foil first and then to coat and dry the other side in the coating process. However, if a continuous process is to be realized, two drying machines with a length of up to 80 m are required. A more efficient, but technically more demanding method is to coat the metal foil on both sides and then to dry it. In this way, a continuous process can be realized with the aid of only one drying machine.

[0009] The drying of the electrode slurry is not only energy-intensive, but is also a critical process step for the later function of the battery cell. It has been shown that only by changing the process conditions during the drying process, the best results can be achieved. In the case of drying, for example, at a constant high heating rate, a certain negative influence on the adhesion of the electrode slurry on the metal foil can occur to such an extent that delamination occurs. This leads to a later reduction in the capacity of the battery cell or even to a malfunction.

[0010] Impingement jet dryers are usually used as dryers, which have several nozzles that direct hot air at the coating. The nozzles are arranged on opposite sides of the belt-shaped electrode along a conveying line; wherein the temperature and flow rate of the hot air emitted can be individually adjusted for each nozzle. In this way, the process conditions can be changed during drying. Alternatively, the belt-shaped electrode can be guided through several single dryers, in which different process conditions prevail.

[0011] At present, the process conditions in the different zones of the dryer (or in the several single dryers) are monitored during drying by means of sensors. The quantities measured include temperature, humidity and flow rate of the hot air directed at the respective coating.

[0012] In order to specify the process conditions in the zones or single dryers, a large number of experiments are carried out in advance, in which different time profiles of the process conditions are used. The finished electrodes are then investigated, on the basis of which the optimum process condition sequence is determined. In order to check the total thickness, either mechanical measuring devices such as scales or micrometer screws are used, or optical measuring devices based on the principle of laser triangulation or spectral confocal imaging are used. More detailed information about the porous network generated by drying can be determined by X-ray-based methods or by microscopic Raman spectroscopy (MRS), see, for example, the article by J. Kaiser et al., Process- and Product Development of Electrodes for Li-Ion Cells Process and Product Development of Electrodes for Li-Ion Cells (2014), Chemie Ingenieur Technik. 86. 10.1002 / cite.201300085. In these known methods, the adhesive / solvent system at the folded edge of the finished electrode is investigated.

[0013] Even if the optimum process conditions thus found are set and monitored during the drying operation by means of sensors, it still happens again and again that the electrodes are defective after drying. The economic damage caused by rejects is enormous, especially when these defects are only discovered in later battery cell tests. SUMMARY

[0014] It is an object of the present application to provide a method and a device for manufacturing electrodes for battery cells, by means of which such rejects due to defects are avoided or at least significantly reduced.

[0015] The solution according to the invention to achieve this object is a method for producing an electrode for a battery cell, wherein the method comprises the following steps:

[0016] a) applying an electrode slurry as a coating to a metal foil, thereby obtaining a coated metal foil;

[0017] b) drying the coating applied to the metal foil in a drying machine, wherein the drying conditions in the drying machine are variable;

[0018] c) non-contact measurement of at least one property of the coating by means of a measuring device for the purpose of inline process monitoring;

[0019] d) changing at least one of the process conditions in the drying machine if the value measured for the at least one property by means of the measuring device lies outside a nominal value range.

[0020] The invention is based on the insight that only by means of inline process monitoring, i.e. non-contact measurement of at least one property of the coating by means of a measuring device during the drying process, can defects leading to rejects be avoided. The reason for requiring non-contact measurement is that the coating is still deformable before the drying process is completed, so that in the case of contact unintended deformation cannot be ruled out.

[0021] Inline process monitoring is a method for real-time monitoring of a process or a device in progress. The concept of "inline" means that the monitoring is carried out without removal of a sample. By means of inline process monitoring deviations from the desired operating parameters can be quickly identified. This is a prerequisite for avoiding quality defects and the resulting rejects.

[0022] The at least one measured property of the coating can be, for example, the thickness of the coating. However, with the thickness alone it is difficult to correctly set the drying conditions. Therefore, it is preferred to determine the dryness of the coating from the at least one measured property, since this is a more important quantity for the optimum setting of the drying conditions.

[0023] At least one further property of the coating can be, in particular, the specular and / or diffuse reflectivity of the coating, the roughness of the surface of the coating, or the thermal conductivity of the coating. The roughness can be easily derived from the spatial dispersion of the measured thickness. Preferably, at least two of these properties of the coating are non-contact measured by means of the measuring device.

[0024] In general, the dryness of the coating during the drying process is not quantified, but given qualitatively. It has proven to be feasible to divide the drying process into the following five phases:

[0025] Phase (a): The coating forms a wet film;

[0026] Phase (b): The coating forms a completely filled capillary network;

[0027] Stage (c): The coating forms a partially filled capillary network;

[0028] Stage (d): Only isolated and mutually separated liquid residues are present in the coating; and

[0029] Stage (e): The coating forms a liquid-free film.

[0030] Of particular importance is the transition between stages (b) and (c) and between stages (c) and (d). Only in stage (c) is a slowing down of the drying required. To this end, it is necessary to deduce from the measured values at which point in time during the coating process the transition from stage (b) to stage (c) takes place. Once the transition between stages (c) and (d) has been identified, the drying can be accelerated again.

[0031] The slowing down or acceleration of the drying is usually accompanied by a reduction or increase in the temperature. In addition or as an alternative, the speed of the hot air directed at the coating can be reduced or increased.

[0032] In the case of a roughness of the surface or a quantity derived therefrom (for example the average roughness value Ra a The transition between stages (b) and (c) can be inferred with high reliability in the case of a roughness of the surface or a quantity derived therefrom (for example the average roughness value Ra

[0033] In the case of the at least one property being the reflectivity of the surface, this property can be deduced from the amplitude of the response of the coating to a thermal excitation.

[0034] In the case of the at least one property being the thermal conductivity of the coating, this property can be deduced from the phase shift of the response of the coating to a thermal excitation.

[0035] In particular, the transition between stages (b) and (c) can be inferred by determining the thermal conductivity of the coating. This transition can be identified in that a curve of the phase shift of the response of the coating to a thermal excitation or a quantity derived therefrom reaches a minimum value. The mass fractions of water, carbon and air in the coating change during the drying process and the thermal conductivities of these components differ significantly, so that an impregnation with water can be inferred.

[0036] Furthermore, in the case where the diffuse reflectivity of the surface first decreases and then increases until it reaches a final value, the transition between phase (c) and phase (d) can be inferred. Part of the reason for this noteworthy insight is that the surface of the non-dried coating is initially smooth and wet, which leads to a high specular reflectivity. When the capillary network starts to be emptied in phase (c), the specular reflectivity decreases because the diffuse reflection of the incident light increases at this point. Surprisingly, however, the diffuse reflectivity increases again as the emptying continues. Thus, with the aid of a sensor that measures the diffuse reflectivity (optionally together with the specular reflectivity), the transition between phase (c) and phase (d) can be identified.

[0037] It turns out that the roughness and the reflectivity of the surface have only a small correlation with the layer thickness. In other words, in most cases, it is not possible to reliably identify whether a transition between phase (b) and (c) or between phase (c) and (d) has occurred from a change in the layer thickness.

[0038] The threshold value for the roughness, above which the transition between phase (b) and phase (c) can be identified, and the final value for the diffuse reflectivity, at which the transition between phase (c) and phase (d) can be identified, are preferably determined experimentally by preliminary tests. The same applies analogously to the thermal conductivity.

[0039] For optimal in-line process monitoring, the measuring device should measure the at least one property of the coating layer quasi-continuously during the drying process. However, it is also possible to carry out the measurements intermittently at preset time intervals.

[0040] Preferably, the measuring device is a measuring element of a control loop, which adjusts the process conditions in the drying machine in accordance with the measured at least one property. The actuating element of the control loop can be, for example, a heating device, which can change the temperature in the drying machine or in a drying machine zone, and / or a fan, which can change the speed of the hot air in the drying machine.

[0041] However, adjustment is not mandatory, because, for example, measurements can also be carried out at greater intervals, which trigger different preset control operations, which cannot be called adjustment in the strict sense.

[0042] The application also relates to a device for producing an electrode for a battery cell. The device comprises a coating device which is configured to apply an electrode slurry as a coating on a metal foil, thereby obtaining a coated metal foil. The device also comprises a drying machine which either has several individual drying machine units or has drying machine regions which are arranged in a common housing. In the drying machine units, or in the drying machine regions, different process conditions can be produced. According to the application, the device has a measuring device which is configured to contactlessly measure at least one property of the coating during drying. A control or regulating device is configured to change at least one of the process conditions in the drying machine in the event of a value measured by the measuring device for the at least one property lying outside a nominal value range, for in-line process monitoring.

[0043] The measuring device preferably has several distance sensors, which can in particular be spectral confocal distance sensors or laser pyroelectric sensors, which are configured to determine the phase shift and the amplitude from the response to a thermal excitation.

[0044] The application also relates to the use of spectral confocal distance sensors, or to the use thereof for measuring the thickness, the roughness and the reflectivity of a coating in the method according to the application. It has turned out that all three quantities can be measured simultaneously with the aid of such sensors. Since the measuring beam is perpendicular to the coating, the total reflectivity, which consists of the specular reflectivity and the diffuse reflectivity, can be inferred from the median of the intensity. If two such distance sensors are arranged on opposite sides of the electrode, the total thickness of the electrode can be measured by differencing. Since the thickness of the metal foil is known or can easily be measured in advance, the total thickness of the two coatings can be inferred therefrom. The measuring accuracy of such sensors is sufficient to enable a contactless measurement of the roughness.

[0045] The application also relates to the use of laser pyroelectric sensors in the method described above. BRIEF DESCRIPTION OF DRAWINGS

[0046] Embodiments of the application are described in detail below with reference to the drawings. In which:

[0047] Figure 1 : schematic side view, not to scale, of a device according to the application according to a first embodiment;

[0048] Figure 2 : longitudinal section of an impinging jet dryer, in which several sensors detect the dryness of the coating according to the application;

[0049] Figures 3a to 3d : diagram for five drying stages;

[0050] Figure 4 : diagram of the roughness R and the total thickness of the coating as a function of time during drying;

[0051] Figure 5 Graph of the intensity I measured by the spectroscopic confocal distance sensor and the total thickness of the coating as a function of time during drying;

[0052] Figure 6 Schematic top view of an electrode and a stationary measuring unit with several distance sensors;

[0053] Figure 7 is a schematic top view similar to Figure 6 but shows a measuring unit arranged in a longitudinally movable manner;

[0054] Figure 8 is a schematic top view similar to Figure 6 but shows two stationary measuring units;

[0055] Figure 9 shows a drying machine according to a further embodiment with several mutually independent drying machine units

[0056] Figure 10 Graph of the phase shift of the thermal excitation response as a function of time during drying;

[0057] Figure 11 Graph of the amplitude of the thermal excitation response as a function of time during drying;

[0058] Figure 12 Graph of the phase shift of the thermal excitation response as a function of time during several drying operations. DETAILED DESCRIPTION

[0059] 1. First embodiment

[0060] Figure 1 schematic side view of a first embodiment of a component of a device for manufacturing electrodes for battery cells, designated as a whole by 10. The device 10 has a slot die coating apparatus 11 comprising a first slot die nozzle 12, the nozzle opening 14 of which is arranged at a precisely specified distance from the surface of a drum 16. The drum 16 is rotated during coating in the direction of rotation indicated by arrow 18, and a metal foil 20 of thickness d F unwound from a storage reel, not shown, is fed to the first slot die nozzle 12.

[0061] The first slot die nozzle 12 is connected by a pump, not shown, with a likewise not shown storage container for a first electrode slurry 22, which is wet and has a granular consistency. Possible compositions of the electrode slurry, which have already been described above in connection with the prior art, can also be used for the electrode slurry 22. The same applies to the metal foil 20.

[0062] A pump pumps the first electrode paste 22 from a storage container and feeds it to the nozzle opening 14 at a uniform pressure. The nozzle opening 14 is slit-shaped and extends over the entire width of the metal foil 20 perpendicular to the paper plane.

[0063] 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 drum 16 and the mass flow of the first electrode paste 22 are correctly matched to each other, a layer consisting of the first electrode paste 22 having a uniform thickness dl is produced on the front side 24 of the metal foil 20. The thickness dl is specified by the distance between the nozzle opening 14 and the metal foil 20, the positions of which are precisely defined by the stationary drum 16. However, the specification does not mean that the thickness dl no longer varies. Depending on the properties of the electrode paste 22, it can shrink or swell after the application.

[0064] A second slit-shaped nozzle 30 is arranged in the conveying direction after the first slit-shaped nozzle 12, which coats the back side of the metal foil 20 with a second electrode paste 32. A pump feeding the second electrode paste 32 to the nozzle opening 36 of the second slit-shaped nozzle 30 feeds the second electrode paste from a not shown storage container to the nozzle opening 36 of the second slit-shaped nozzle 30. The mass flow of the second electrode paste 32 and the conveying speed of the metal foil 20 are also matched to each other here, so that the back side 38 of the metal foil 20 is uniformly coated with the second electrode paste 32. However, the thickness d2 of the electrode paste 32 on the back side 38 fluctuates much more than the thickness dl of the first electrode paste 22, since the metal foil 20 cannot be supported on a drum or a similar stationary structure during the coating of the back side 38.

[0065] The metal foil, which has now been coated on both sides and is referred to in the following as electrode 40, is fed to a dryer 42 by means of known and thus not shown conveying means in order to carry out the next process step. The electrode paste 22, 32 is soft before drying, so that a drive wheel or the like should be avoided from contacting the electrode paste. In the case, for example, of uncoated, longitudinally extending strips on the metal foil 20, a drive wheel or skid can be clamped there.

[0066] The dryer 42 comprises a housing 44, in which controllable heating devices 46, a thermometer 47 and two distance sensors 48a, 48b are provided, which are based on the principle of spectral confocal measurement and have a very high measurement accuracy at a sampling rate of approximately 35 kHz. Details about such sensors are described, for example, in DE 10 2006 017 400 Al. The total thickness d G of the coating is measured quasi-continuously during the drying of the electrode 40 by the distance sensors 48a, 48b. The distance between the two distance sensors 48a, 48b and the thickness d Fis known that by subtracting the measured distance value from the distance and the thickness d of the metal foil 20 F the total thickness d of the two coatings is obtained G = d1 + d2. To improve the measurement accuracy, a calibration can be carried out beforehand using a thickness standard. The components in the housing 44 are connected to a regulator 50, which in the embodiment shown is arranged outside the housing 44. The sensitive parts of the distance sensors 48a, 48b can also be arranged outside the housing 44, so as to avoid being subjected to the high temperatures in the drying machine 42. In the case of distance sensors based on the principle of spectral confocal measurement, for example, only one measurement head containing the objective lens (possibly water-cooled) can be located in the drying machine 42, while the spectrometer containing the electronic evaluation device is arranged outside the housing 44 and is connected to the measurement head by means of an optical fiber.

[0067] In the embodiment shown, the drying speed of the coating mainly depends on the temperature T in the drying machine 42, which is measured by the thermometer 47. On the basis of the total thickness of the coating measured by the distance sensors 48a, 48b, it is possible to make an approximate inference about the dryness of the coating. If the coating dries too quickly or too slowly, this has an adverse effect on the properties of the electrode 40.

[0068] The regulator 50 therefore regulates the temperature T in the drying machine 42 in such a way that the thickness of the coating lies within the nominal value range. The distance sensors 48a, 48b are the measuring elements, and the heating device 46 is the actuating element of the regulating loop. The regulation reduces the probability that the coating will not adhere correctly to the metal foil 20 because it dries too slowly or too quickly, which in the long term leads to delamination and thus to functional failure.

[0069] After drying, the electrode 40, which can have a length of up to 2000 m and a width of 1 m, is compressed by calendering in a known manner and is further processed by longitudinal cutting.

[0070] 2. Second embodiment

[0071] In order to give the electrode paste 22, 32 of the finished electrode 40 the optimum structure and to adhere it perfectly to the metal foil 20, it is advantageous to slow down the drying temporarily. To this end, the drying rate in the drying machine 42 must be reduced by a suitable amount at the correct point in time. If the drying is always carried out at a low drying rate, the drying time is prolonged, which is disadvantageous in terms of the economics of the process.

[0072] Figure 2An impinging jet dryer 142 is schematically shown, by means of which the drying rate can be flexibly varied during drying. The impinging jet dryer 142 comprises a housing 44 through which the electrode 40 is guided. On each side of the electrode 40, a number of hot air nozzles 52 are provided from which hot air 54 is discharged, which is directed at the passing strip electrode 40. In the shown embodiment, 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. By correspondingly varying the temperature T and / or the speed of the discharged hot air 54, different drying rates can be achieved in different regions of the impinging jet dryer 142.

[0073] Figures 3a to 3e Different drying rates or stages during drying are illustrated by way of example for an anode electrode paste. In these figures, the large white particles 56 represent graphite particles, the small circles 58 represent SBR particles (SBR stands for styrene butadiene rubber), the thin lines 60 represent carboxymethyl cellulose (CMC), and the remaining particles 62 represent carbon black particles. Styrene-Butadiene Rubber

[0074] A solution which has proven to be feasible and is widely used in the professional field is to divide the drying process into the following five stages:

[0075] Figure 3a Stage (a): The coating forms a wet film;

[0076] Figure 3b Stage (b): The coating forms a completely filled capillary network;

[0077] Figure 3c Stage (c): The coating forms a partially filled capillary network;

[0078] Figure 3d Stage (d): Only isolated and mutually separated liquid residues are present in the coating; and

[0079] Figure 3e Stage (e): The coating forms a film without liquid.

[0080] During stage (c), the drying should be slowed down. For this purpose, the transitions between stages (b) and (c) and between stages (c) and (d) need to be reliably identified. According to the total thickness d G measured by the distance sensors 48a, 48b, the time points of these transitions cannot be reliably determined. As the degree of drying increases, the total thickness d G continually decreases. However, at which total thickness d G the transitions between the stages occur depends, primarily and unpredictably, on the thicknesses d1 and d2 of the newly applied electrode paste 22, 32. ​

[0081] Thus, in addition to the distance values (or the total thicknesses d G derived therefrom) measured by the distance sensors 48a, 48b, additional information is provided to the regulator 50, which is used to determine the dryness and to identify the transitions between the phases (b) and (c) and between (c) and (d).

[0082] For identifying the transition between the phases (b) and (c), the roughness of the facing surface is determined from the measured distance values. It is exploited here that, in the case of stationary distance sensors 48a, 48b, the electrode 40 passes by the distance sensors 48a, 48b continuously at a constant speed. In this way, a large number of adjacent measurement points are obtained, from which a measured variable for the roughness can be derived. By comparing Figure 3b and Figure 3c it can be seen that the roughness of the surface increases abruptly at the transition between the phases (b) and (c).

[0083] Figure 4 This is shown in connection with a diagram, in which the total thickness d G (dotted line) and a measured variable describing the roughness, for example the mean roughness value R a (solid line), are shown schematically over time t during drying. The mentioned continuous decrease of the thickness d G can be seen. Although the thickness decrease is not constant, the change in slope is not a reliable indicator of the transition between the phases (b) and (c) for the reasons mentioned above.

[0084] The increase of the mean roughness value R a as shown in Figure 3b and Figure 3c can be determined well quantitatively from the diagram of Figure 4 . For example, the point in time t bc at which the rate of increase of the mean roughness value R a , i.e. the first time derivative, reaches a maximum (see vertical dotted line) can be specified as the transition between the phases (b) and (c). From the point in time t bc onwards, the heating rate can be reduced, for example by reducing the temperature and / or the speed of the discharged hot air.

[0085] During the phase (c) and in the subsequent phases (d) and (e), the roughness increases only slightly. From Figure 3c and Figure 3d it can be seen that the transition between the phases (c) and (d) cannot be reliably identified from the roughness.

[0086] In order to be able to reliably identify this transition, the distance sensors 48a, 48b also detect the intensity of the measuring light, which is incident perpendicularly to the surface of the coating and is partially reflected by the surface. Due to the perpendicular incidence, the distance sensors 48a, 48b measure both the specularly reflected portion and the diffusely reflected portion of the reflected light.

[0087] Figure 5 The development of the intensity I (or more precisely the median value thereof) measured by the distance sensors 48a, 48b during drying is shown in a diagram similar to Figure 4 During phases (a) and (b), the intensity is high and only falls slightly, since the liquid components in the electrode paste 22, 32 lead to a smooth surface, which specularly reflects relatively much of the measuring light. In phase (c), the surface is rough, so that it specularly reflects significantly less of the measuring light, which manifests itself in a significant drop in the measured intensity I. Surprisingly, however, towards the end of phase (c), the reflection on the surface rises again, causing the measured intensity I to rise again. At the transition between phases (c) and (d), this rise ends. That is, if the intensity I first falls and then rises until it reaches an end value, the point in time at which this end value (or the end of the rise) is reached can be taken as the point in time t cd at which the transition between phases (c) and (d) occurs. cd From this point in time t cd , the drying can be accelerated again, for example to the original values during phases (a) and (b).

[0088] Thus, in the embodiment shown, the total thickness d G , the roughness of the surface and its reflectivity can be determined by means of the distance sensors 48a, 48b alone, and on the basis thereof the degree of drying can be determined. In particular, it can be judged when the transitions between phases (b) and (c) and between phases (c) and (d) occur, which are important for process control.

[0089] In Figure 2 , pairs of distance sensors 48a, 48b are arranged in succession in the conveying direction in the impinging jet dryer 142. The number of distance sensors 48a, 48b required and their ideal arrangement along the conveying path depend on a large number of parameters of the individual case. In general, it is sufficient to provide only one pair of distance sensors 48a, 48b in the lateral direction of the electrode (i.e. perpendicular to the plane of the paper of Figure 2 . Depending on the number and arrangement of the hot air nozzles 52, however, the degree of drying can vary in the lateral direction. In these cases, it is preferable to arrange pairs of distance sensors 48a, 48b at different positions in the lateral direction, as is shown in Figure 6 . It can be seen therefrom that the coating facing upwards towards the observer does not completely cover the metal foil 20, but leaves uncoated longitudinal strips 72 and lateral strips 74.

[0090] 76 represents a measuring station, which has a total of 6 distance sensors 48a, which are distributed in the transverse direction in the range of the coated section of the electrode 40.

[0091] In order to monitor the drying at a specific point on the electrode 40, the measuring station 76 can be caused to travel in synchronism with the electrode 40, as is shown in Figure 7 The common direction of transport is indicated by the arrow 78.

[0092] Alternatively, a number of measuring stations 76a, 76b can be provided, the measurement results of which are correlated in such a way that they correspond to a specific point on the electrode, cf. Figure 8 At a later position of the electrode 40, indicated by a dashed line, the point measured by the first measuring station 76a is directly below the second measuring station 76b.

[0093] In the case of a double-sided coated metal foil 20, in order to measure the total thickness d G of the coating, two distance sensors 48a, 48b must be provided for each measuring point. It has turned out, however, that the transitions between the phases (b) and (c) and (c) and (d) can be recognized on the basis of the roughness or the measured intensity alone; knowledge of the total thickness d G improves the reliability of the recognition, but is not necessary. If the total thickness d G is not required, the measurement can be carried out by means of only one distance sensor 48a or 48b. In this case, it measures the distance value only in order to determine the roughness average R a and the intensity I.

[0094] In the case of an apparatus in which the second coating is applied only after the first coating has been dried, two dryers are required, which dry only one coating each. In these cases, too, the measurement by means of only one distance sensor 48a or 48b is sufficient.

[0095] 3. Third embodiment

[0096] The statements made in connection with the first and second embodiments apply equally to the third embodiment. Figure 2 and Figures 3a to 3e The statements made in connection with the first and second embodiments apply equally to the third embodiment.

[0097] In this embodiment, laser pyroelectric sensors are used instead of the spectroscopic confocal distance sensors to provide the regulator 50 with information which is used to determine the degree of drying and to recognize the transitions between the phases (b) and (c) and (c) and (d).

[0098] EP 3 017 274 A1, for example, reveals the working principle of this type of laser-thermal sensor. In this sensor, a laser is used to thermally excite the surface of a component, and the temperature response of the component is recorded by a near-infrared photodiode. The excitation is modulated. Therefore, the temperature response is also modulated. Two important characteristic parameters of the temperature response can be evaluated: the amplitude and phase shift of the modulation, which are related to the excitation.

[0099] Figure 10 A graph is shown, schematically illustrating a typical curve of phase shift P during the drying process. During stages a and b, the water content in the coating decreases, thus increasing the mass percentage of graphite. Graphite has better thermal conductivity than water, therefore the thermal conductivity of the coating increases, which leads to a decrease in phase shift.

[0100] In stage c, the capillary network begins to empty, so the water content is gradually replaced by air. Air has much lower thermal conductivity than water, therefore the overall thermal conductivity of the coating decreases again, and the phase shift P increases accordingly. At time point t... bc The phase shift reaches its minimum at the transition between stages (b) and (c), where the amount of water completely filling the capillary network is minimal, and air is not yet present. Therefore, this transition can be precisely identified by determining the minimum value of the phase shift P curve. Near the end of stage (d), where only very scattered and separated liquid residues remain, and in the completely dry stage (e), thermal conductivity stabilizes, and the phase shift P thus stabilizes at a constant value P. f .

[0101] Therefore, phase shift is roughly a measure of the coating's thermal conductivity. Thermal conductivity depends to a large extent on the mass percentage of water within the coating.

[0102] The facts show that this value P f It is largely independent of factors that are sensitively responded to by other sensors, such as the coating dilution at the start of the process and the total thickness.

[0103] Figure 11 This is a graph illustrating the phase shift P for several drying processes, where dilution, thickness, and drying process vary. Although these curves differ over time, they all terminate at the same plateau. Therefore, the value P... f It can be used as a reliable indicator of the end of the drying process.

[0104] Figure 12 A graph is shown, schematically illustrating a typical curve of the amplitude A of the temperature response during the drying process. Here, similar to the intensity measured using a spectral confocal sensor in Example 2, the amplitude A depends primarily on reflectivity. Therefore, this curve is similar to... Figure 4The curve is similar, where the thermal response also has a component originating from the reflection from below the surface, and is thus more sensitive to the drying process at greater depths in the coating.

[0105] During phases (a) and (b), the drying / vaporization mainly leads to a reduction of the layer thickness, where the surface and the gap remain wet. Therefore, the reflectivity only decreases very slowly, which leads to a slow increase of the amplitude A. In the laser pyrometric method, the high reflectivity of the surface means a poor absorption and, in turn, a small temperature response (low amplitude). As soon as the thickness stabilizes in phase (c), the vaporization starts to cause drying, especially near the surface, where the reflectivity of the surface rapidly decreases. Therefore, a sharp rise can be observed from the time point t bc .

[0106] In phase (d), the surface is already substantially dry, and the vaporization mainly takes place from the inside. That is, the reflectivity of the surface stabilizes, and a stabilization of the amplitude A of the temperature response results. From this stabilization, the transition between phases (c) and (d) can be identified.

[0107] Therefore, the amplitude is generally a measure of the reflectivity of the coating.

[0108] It turns out that, depending on the frequency of the modulation of the excitation, the effects in different depths of the coating play a decisive role for the temperature response. When measuring curves as shown in Figures 10 to 12 , an excitation frequency of 2000 - 4000 Hz is used, so that mainly information about the drying state of the surface of the coating is obtained. But if the excitation is performed with a frequency of 20 - 400 Hz, information about the inside of the material is obtained. In this way, the drying state both on the surface of the coating (laser modulation between 2000 - 4000 Hz) and in the inside of the coating (laser excitation between 20 - 400 Hz) can be determined unambiguously.

[0109] The laser pyrometric sensor can be equipped for the impinging jet dryer 142 as shown in Figure 2 , similar to the second embodiment described above, instead of a distance sensor, in order to carry out the method described here. In this case, the regulation is carried out in accordance with the values of the phase shift P and the amplitude A.

[0110] 4. Embodiment

[0111] Figure 9 A fourth embodiment is shown, in which elements of the first and second embodiments or the first and third embodiments described above are combined. In this embodiment, the dryer 42 has three dryer units 42a, 42b, 42c, which are each structured as shown in Figure 2The shown dryer is identical. The regulator 50 performs a common regulation of all three dryer units 42a, 42b, 42c. Different drying conditions can be set in the dryer units 42a, 42b, 42c. Thereby, the drying can be slowed down temporarily, as in the second embodiment, and in particular in phase (c).

[0112] As Figure 9 The shown dryer with several dryer units 42a, 42b, 42c is particularly suitable for cases where it is desired to avoid that the coating is subjected to strong air movements, as is the case with the impingement jet dryer 142. The hot air can stay in the dryer units 42a, 42b, 42c, so that the drying rate is set only by the temperature T in the respective unit.

[0113] 5. Embodiment

[0114] It is likewise preferred to combine the distance sensor as described in the second embodiment with the laser light-heat sensor as described in the third embodiment in one device. In this case, the phase transition and the regulation can be determined from any combination of the quantities thickness, reflectivity, roughness, phase shift of the temperature response, amplitude of the temperature response.

Claims

1. A method of manufacturing an electrode (40) of a battery cell, wherein the method comprises the following steps: a) applying an electrode slurry (22, 32) as a coating to a metal foil (20), thereby obtaining a coated metal foil; b) drying the coating applied to the metal foil (20) in a drying machine (42; 142), wherein the drying conditions in the drying machine (42; 142) are changeable; characterized in that, for the purpose of inline process monitoring, at least one property of the coating is non-contact measured by a measuring device (48a, 48b), and, in the event that the value measured by the measuring device for the at least one property lies outside a nominal value range, at least one of the process conditions in the drying machine (42; 142) is changed. The dryness of the coating is determined from the at least one measured property. The dryness of the coating during the drying process is describable by the following five stages which are passed in succession during the drying process, namely stage (a), in which the coating forms a wet film, stage (b), in which the coating forms a completely filled capillary network, stage (c), in which the coating forms a partially filled capillary network, stage (d), in which only isolated and mutually separated liquid residues are present in the coating, and stage (e), in which the coating forms a film which is free of liquid, and, from the measured value, the time at which the transition between the stages (b) and (c) and / or the transition between the stages (c) and (d) occurs during the coating process is derived. The roughness of the surface of the coating is derived from the spatial dispersion of the measured thickness. In the event that the roughness of the surface or a quantity derived therefrom exceeds a threshold value, the transition between the stages (b) and (c) is inferred.

2. The method of claim 1, wherein, The reflectivity of the surface is derived from the amplitude of the response of the coating to a thermal excitation, and / or the thermal conductivity of the surface is derived from the phase shift of the response of the coating to a thermal excitation.

3. The method of claim 2, wherein, In the event that a curve of the phase shift of the response of the coating to a thermal excitation or a quantity derived therefrom reaches a minimum value, the transition between the stages (b) and (c) is inferred.

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

5. The method according to any of the preceding claims, characterized in that, at least two properties of the coating are non-contact measured by a measuring device (48a, 48b), and the two properties are selected from the group consisting of: a thickness (d G ) of the coating, a reflectivity of the coating, a roughness of a surface of the coating, and a thermal conductivity of the coating.

6. The method according to claim 4 or 5, characterized in that, In the event that the transition between the stages (b) and (c) is inferred, the drying is slowed down.

7. The method according to any one of claims 4 to 6 back referencing claim 3, characterized in that, In the event that the diffuse reflectivity of the surface initially falls and then continuously rises until a final value is reached, the transition between the stages (c) and (d) is inferred.

8. The method according to claim 4 or 5, characterized in that, 12. The method according to claim 11, in the event that the transition between the stages (c) and (d) is inferred, the drying is accelerated.

9. The method according to claim 4, 5 or 8 and back-cited claims 3, characterized in that, The measuring device (48a, 48b) is a measuring element of a control loop which adjusts the process conditions in the drying machine (42; 142) as a function of the measured at least one property.

10. The method according to claim 6 or 9, characterized in that, 14. A device (10) for manufacturing an electrode (40) of a battery cell, having 11. The method according to any one of claims 4 to 10 back referencing claim 3, characterized in that, ​ ​ 13. The method according to any of the preceding claims, characterized in that, ​ ​ A coating device (11) is configured to apply an electrode paste (22, 32) as a coating on a metal foil (20) so as to obtain a coated metal foil, A drying machine (42; 142) either has several independent drying machine units (42a, 42b, 42c) or has drying machine zones arranged in a common housing (44), in which different process conditions can be produced in the drying machine units or in the drying machine zones, characterized in that A measuring device (48a, 48b) is configured to measure at least one property of the coating non-contact during the drying, and A control or regulating device (50) is configured to change at least one of the process conditions in the drying machine in order to achieve an inline process monitoring if the value measured by the measuring device for the at least one property lies outside a nominal value range.

15. The apparatus of claim 14, wherein, The measuring device has several optical spectrum confocal distance sensors (48a, 48b) which are configured to determine the roughness of the surface of the coating from the measured distance.

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

17. The apparatus of claim 14, wherein, The measuring device has several laser pyroelectric sensors which are configured to determine the phase shift and the amplitude from the response to a thermal excitation.

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

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