Method and device for producing electrodes for battery cells

Abstract

In a method for producing electrodes for battery cells, a metal foil (20) is guided past a first slot nozzle (12) while lying on a rotating roller (16). An electrode paste (22) exits the slot nozzle, whereby a front face (24) of the metal foil (20) is coated with the first electrode paste (22). The metal foil (20) is then guided past a second slot nozzle (30) without drying the first electrode paste (22), a second electrode paste (32) exiting the second slot nozzle, whereby a rear face (38) of the metal foil (20) is coated with the second electrode paste (32). In the process, the metal foil (20) is not supported against a stationary structure. According to the invention, the thickness d2) of the second electrode paste (32) on the rear face (38) of the metal foil (20) is regulated in a control loop which has a contactless thickness sensor (42, 44) as the measuring element.

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 using a slot-die coating system.

[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 typically has a thickness of between approximately 10 and 20 μm. The electrode paste is approximately 70 to 200 μm thick on each side and usually consists of a mixture of the chemically active substance, e.g., a lithium metal oxide, as well as conductive carbon black, a binder, and a solvent.

[0007] The anode consists of a metal foil made of copper and is approximately 8 to 15 μm thick. The electrode paste is usually a mixture of graphite, silicon, conductive carbon black, a binder, and a solvent. The coating thickness is on the order of 50 to 150 μm on each side.

[0008] The initially moist electrode paste is applied to the metal foil using a process known as slot-die coating. The metal foil is moved on a roller past a slot die that extends across the entire width of the metal foil. The electrode paste emerges from the slot die under pressure and is pressed against the metal foil, adhering to it on one side. The moist electrode paste is then dried in a dryer.

[0009] The homogeneity of the layer thickness is a crucial factor for the quality of the battery cells. Provided the slot die coating system generates a sufficiently high mass flow at a given metal foil conveying speed, the layer thickness of the electrode paste is equal to the distance between the slot die outlet and the metal foil.

[0010] Since both sides of the metal foil are to be coated, the front side of the metal foil is coated in a first coating pass and the back side of the metal foil is coated in a second coating pass.

[0011] If two slot-die coating systems and two dryers are available, the metal foil, coated and dried on one side, can be transferred directly to the second system after the first coating pass to apply and dry the electrode paste on the other side (Process A). This allows for a continuous process.

[0012] If only a slot die coating system is available, the electrode must be wound onto a spool in the first coating pass and unwound from this spool for the second coating pass (method B).

[0013] In both processes A and B, the electrodes have a very homogeneous layer thickness, since in both cases the distance between the slot nozzle and the metal foil guided on the roller is precisely defined and does not change during the process.

[0014] Although process A allows for approximately twice the throughput compared to process B, it also requires twice the equipment, namely two slot die coating systems and two dryers.

[0015] It was therefore proposed to coat the electrode on both sides in a single coating pass in a modified slot die coating system (Process C). In such a system, the front side is first coated with the electrode paste in the manner explained above. Directly afterwards, i.e. without drying, the metal foil coated on one side is passed over a second slot die which is arranged so that the back of the metal foil is coated with the electrode paste. Process C therefore only requires one drying process, which is carried out after the double-sided coating. This is a significant advantage because the dryers can be up to 80 m long and therefore require a lot of energy for heating. However, it has been shown that the thickness of the coating on the back of the metal foil varies, which cannot be tolerated for all quality requirements.

[0016] SUMMARY OF THE INVENTION

[0017] The object of the invention is to provide a method and a device with which both double-sided coated electrodes for battery cells can be efficiently produced in a single coating pass. Variations in coating thickness should be kept as small as possible.

[0018] A method for achieving this object comprises the following steps: a) a metal foil is guided past a first slotted nozzle, from which a first electrode paste emerges, lying on a rotating roller, whereby a front side of the metal foil is coated with the first electrode paste; b) the metal foil is guided past a second slotted nozzle, from which a second electrode paste emerges, whereby a back side of the metal foil is coated with the second electrode paste, without the metal foil being supported on a stationary structure.

[0019] According to the invention, the thickness of the second electrode paste on the back of the metal foil is controlled in a control circuit which has a non-contact thickness sensor as a measuring element.

[0020] The invention is based on the finding that the observed fluctuations in the coating thickness on the back of the metal foil are related to the fact that the metal foil cannot support itself with its front side on a roller or other stationary structure at the coating site because the first electrode paste applied to the front side has not yet dried. Due to this lack of support, the distance between the metal foil and the second slot die and, consequently, the layer thickness can be subject to temporal fluctuations. Such distance fluctuations can be triggered, for example, by pressure fluctuations when feeding the second electrode paste to the second slot die. Fluctuations in the tensile stress of the metal foil can also affect the distance between the metal foil and the slot die.It was also observed that tolerable thickness variations of the first electrode paste on the front side have such an adverse effect on the mass distribution of the electrode that intolerable variations in the distance between the metal foil and the second slot nozzle occur.

[0021] Since the pressure generated in slot-die coating systems to extrude the electrode pastes is built up by pumps, the resulting slight pressure fluctuations can only be avoided with great effort. Even small fluctuations in the tensile stress in the metal foil cannot be reliably eliminated, even though the movement of the metal foil past the stationary slot dies is intended to be uniform. Drying the electrode before it is fed to the second slot die in order to provide a roller or other support structure as with the first slot die is also not feasible, as this would require a second dryer and would lose one of the key advantages of Process C.

[0022] The invention therefore takes a different approach by controlling the thickness of the second electrode paste on the back of the metal foil in a control loop that uses a non-contact thickness sensor as the measuring element. If the control loop's actuator is already present, the control system essentially only requires the non-contact thickness sensor, which represents the measuring element in the control loop. With minimal additional hardware effort, fluctuations in coating thickness can be reduced to such an extent that even tight tolerances can be maintained.

[0023] The control loop's manipulated variable can, for example, be a mass flow exiting the second slot nozzle. This mass flow can be most easily adjusted using a pump that supplies the second electrode paste to the second slot nozzle, forming the control loop's actuator. Alternatively, additional actuators can be used. These actuators are located near the nozzle opening of the second slot nozzle and briefly change the pressure on the second electrode paste flowing paste by small amounts, or they can vary the mass flow of the electrode paste accordingly.

[0024] The tension (also known as strip tensile force) of the metal foil in the conveying direction in the area of ​​the second slot die can also be considered as a control variable for the control loop. The greater the tension of the metal foil there, the smaller the distance to the second slot die. A less taut metal foil, on the other hand, leads to an increase in the distance. This change in the distance between the second slot die and the metal foil also varies the coating thickness of the second coating. In this case, a first actuator can be used as the control element of the control loop, which exerts a tensile force on the electrode. Such an actuator can, for example, comprise a roller that acts on the front side in an area of ​​the metal foil that was not coated during step a). This roller can be moved in a direction that is not parallel to the metal foil with the help of control elements.

[0025] Another possible control variable is the position coordinate of the second slot nozzle in the direction perpendicular to the metal foil. If the second slot nozzle is arranged to move along a direction that is not parallel to the metal foil, the distance to the metal foil can be changed directly by moving the second slot nozzle. The control element of the control loop could then be, for example, a second actuator that moves the second slot nozzle.

[0026] The control loop can also be designed in such a way that several of the above-mentioned control variables are used simultaneously.

[0027] Because the electrode pastes are not transparent, it is difficult to measure the thickness of the second electrode paste on the back of the metal foil without contact using a single measurement. Therefore, the thickness sensor preferably comprises a first sensor unit that measures the thickness of the single-sided coated electrode and a second sensor unit that measures the thickness of the double-sided coated electrode. By calculating the difference, the thickness of the second electrode paste can be directly derived. If the thickness sensor comprises a third sensor unit that measures the thickness of the metal foil, the thickness of the first electrode paste can be calculated by calculating the difference.

[0028] For the purpose of thickness measurement, the first and second sensor units preferably each comprise two distance sensors, arranged in pairs on opposite sides of the electrode and measuring distances to the electrode. If the locations of the associated distance sensors are known, the thickness of the electrode can be directly deduced from the distance values. The distance sensors can be, for example, chromatic-confocal distance sensors. Interferometric distance sensors or distance sensors that emit X-rays, beta rays, ultrasound, or thermal waves are also possible.

[0029] Typically, the first and second electrode pastes in battery cells are identical. However, it is also possible to apply different electrode pastes to opposite sides of the metal foil. The thicknesses of the two coatings can also be different.

[0030] The object mentioned above is also achieved by a device for producing electrodes for battery cells, which device has a first slotted nozzle designed to apply a first electrode paste to the front side of a metal foil. A rotatably mounted roller is designed to guide the metal foil past the first slotted nozzle so that the metal foil rests on the roller during application of the first electrode paste. The device also has a second slotted nozzle designed to apply a second electrode paste to the back side of the metal foil. The electrode is not supported on the front side during coating of the back side of the metal foil. According to the invention, the device has a control device that has a contactless thickness sensor as a measuring element and is designed to control the thickness of the second electrode paste on the back side of the metal foil.

[0031] The considerations and advantageous embodiments mentioned for the method apply accordingly to the device.

[0032] In particular, a control variable of the control loop can be selected from the group consisting of: a mass flow emerging from the second slot nozzle, a tension of the metal foil in the conveying direction in the region of the second slot nozzle, and a position coordinate of the slot nozzle in a direction perpendicular to the metal foil.

[0033] Depending on the control variable used, an actuator of the control loop can be a pump configured to supply the second electrode paste to the second slotted nozzle. The pump can be used to change the flow rate. Alternatively or additionally, an actuator of the control loop can be a first actuator configured to exert a tensile force on the electrode. Furthermore, an actuator of the control loop can be a second actuator configured to move the second slotted nozzle in a direction perpendicular to the metal foil.

[0034] The thickness sensor may comprise a first sensor unit configured to measure the thickness of the single-sided coated electrode and a second sensor unit configured to measure the thickness of the double-sided coated electrode.

[0035] The first and the second sensor unit can each comprise two distance sensors which are arranged in pairs on opposite sides of the electrode and are configured to measure distances to the electrode.

[0036] The invention also relates to a measuring and control device for use in the device described above. The measuring and control device comprises, among other things, the thickness sensor and a controller, which can also be implemented as software and is adapted for use in the device.

[0037] BRIEF DESCRIPTION OF THE DRAWINGS

[0038] In the following, exemplary embodiments of the invention are explained in more detail with reference to the drawings. In these drawings:

[0039] Figure 1: a schematic and not to scale side view of a device according to the invention according to a first embodiment;

[0040] Figure 2: a top view of an electrode that is only partially covered by an electrode paste.

[0041] DESCRIPTION OF PREFERRED EMBODIMENTS

[0042] 1. First embodiment

[0043] Figure 1 shows a schematic side view of a first exemplary embodiment of an apparatus according to the invention, designated overall by 10, for producing electrodes for battery cells. The apparatus 10 is designed as a slot-die coating system and contains 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, which is unrolled from a storage roll (not shown), to the first slot die 12.

[0044] The first slotted 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 the description of the prior art and are also considered for the electrode paste 22. The same applies to the metal foil 20.

[0045] 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.

[0046] 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. Fixed does not mean that the thickness di must always be equal to this distance. Depending on the properties of the electrode paste 22, it can shrink or expand after application. In such cases, the thickness di of the electrode paste 22 differs from the said distance.

[0047] 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.

[0048] A pump 34, which supplies 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 cb of the electrode paste 32 on the back side 38 fluctuates significantly more than the thickness di of the first electrode paste 22.

[0049] These greater thickness fluctuations are due to the fact that the metal foil 20 cannot be supported on a roller or similar stationary structure during the coating of the back side 38, since the first electrode paste 22 on the front side 24 has not yet dried and therefore cannot be touched. The distance between the metal foil 20 and the nozzle opening 36 of the second support nozzle 30 is therefore not precisely defined during the coating of the back side 38, which leads to the undesirable thickness fluctuations.

[0050] In order to avoid such thickness fluctuations or at least to reduce them to a tolerable level, the device 10 has a measuring and control device 40 which, together with the control system explained above, forms a control loop.

[0051] For this purpose, the measuring and control device 40 comprises a thickness sensor with which the thickness of the second electrode paste 32 applied to the back 38 of the metal foil 20 can be measured without contact. In the illustrated embodiment, the thickness sensor comprises a first sensor unit 42 and a second sensor unit 44, each of which has a pair of opposing distance sensors 42a, 42b and 44a, 44b, respectively.

[0052] The distance between the distance sensors 42a, 42b is known with high accuracy, so that the thickness of the single-sided coated electrode passing between them can be derived from the distances measured by the distance sensors 42a, 42b. This thickness results from the distance between the distance sensors 42a, 42b minus the measured distances to the surface of the first electrode paste 22 or to the back 38 of the metal foil 20. In this way, the first sensor unit 42 can measure the thickness of the single-sided coated electrode quasi-continuously while the single-sided coated electrode is passed between the distance sensors 42a, 42b. Suitable distance sensors 42a, 42b include, for example, chromatic confocal distance sensors, which have very high measurement accuracy at sampling rates of approximately 35 kHz.

[0053] For very wide metal foils 20, the distance sensors 42a, 42b can also be designed as line sensors, or several point-measuring distance sensors can be arranged side by side along a direction perpendicular to the paper direction. However, larger thickness fluctuations often occur only in the longitudinal and not the transverse direction of the electrode, so that a single pair of point-measuring distance sensors 42a, 42b may be sufficient.

[0054] The second sensor unit 44 with the distance sensors 44a, 44b is constructed in the same way and thus provides the thickness dE of the electrode coated on both sides at this point.

[0055] The thickness dz of the second electrode paste 32 on the back side 38 of the metal foil 20 can be determined by subtracting the thickness of the single-sided coated electrode measured by the first sensor unit 42 from the thickness dE of the double-sided coated electrode measured by the second sensor unit 44.

[0056] If the thickness of the first electrode paste 22 is sufficiently constant, the measurement data generated by the sensor units 42, 44 do not need to be temporally correlated. If this assumption is not met, the thicknesses measured by the sensor units 42, 44 must be synchronized accordingly via the conveying speed of the metal foil 20 ("same spot measurement").

[0057] The calculation of the thickness dz of the second electrode paste 32, as explained above, is performed in a controller 50, to which the measurement data from the distance sensors 42a, 42b and 44a, 44b are fed. If the measured actual value of the thickness dz deviates noticeably from a predetermined target value, the controller 50 calculates a control variable for an actuator that ensures a reduction in the thickness deviation.

[0058] In the illustrated embodiment, the actuator is the pump 34, which feeds the second electrode paste 32 to the second slotted nozzle 30. If the measured actual value of the thickness of the second electrode paste 32 is, for example, below the target value, the pump 34 is controlled such that it delivers more mass, whereby the mass of second electrode paste 32 emerging from the slotted nozzle 30 per unit of time, and thus the thickness, increases. If, however, the thickness sensor detects that the target thickness has been exceeded, the pumping power of the pump 34 is reduced accordingly by the controller 50. The control ensures that the second electrode paste 32 is applied with a uniform thickness dz despite the lack of support on the front side of the metal foil 20. In the illustrated embodiment, the device 10 also contains a third sensor unit 52, which also comprises a pair of opposing distance sensors 52a, 52b.The third sensor unit 52 measures the thickness dp of the metal foil 20 before the front side 24 is coated by the first slot nozzle 12. This makes it possible to measure the thickness di of the layer of first electrode paste 22 applied to the front side 24 by forming the difference. Although regulation of the thickness di is generally not necessary, tolerable fluctuations in the thickness di of the first electrode paste 22 can, however, affect the coating with the second electrode paste 32 due to the associated mass shifts. The controller 50 can incorporate fluctuations in the thickness di when determining the manipulated variable for the pump 34.

[0059] 2. Further examples

[0060] In Figure 1, 60 and 62 indicate actuators with which the second slotted nozzle 30 can be moved at least approximately perpendicular to the metal foil 20 by small distances. In this way, the distance between the metal foil and the nozzle opening 36 of the second slotted nozzle 30, and thus the layer thickness of the second electrode paste 32, can be changed. The actuators 60 and 62 represent control loop elements with which the controller 50 can influence the thickness dz. These control elements can be provided in addition to or as an alternative to the pump 34.

[0061] Another possible actuator is indicated in Figure 1 by a roller 70 shown in dashed lines, which can be moved by adjusting elements (not shown), as indicated by a double arrow. Such a roller 70 may only touch the electrode in areas where no electrode paste covers the metal foil 20. Such uncovered and usually strip-shaped areas are shown in a plan view in Figure 2 and designated 72 (longitudinal strip) and 74 (transverse strip). The roller 70 can act on the longitudinal strip 72 extending in the longitudinal direction of the electrode, thereby generating tensile stress in the metal foil 20. To change the tensile stress, the adjusting elements move the roller 70, causing it to move slightly toward or away from the metal foil 20. The roller 70 therefore acts as an actuator, exerting a tensile force on the metal foil 20.The tensile force changes the tensile stress in the metal foil and thus the distance between the metal foil 20 and the nozzle opening 36 of the second slot nozzle 30. This change in distance has a direct effect on the layer thickness of the second electrode paste 32.

[0062] In general, it is sufficient for the controller 50 to act on only one of the aforementioned actuators. In principle, however, it is also possible to provide several different actuators.

Claims

PATENT CLAIMS 1. A method for producing electrodes for battery cells, the method comprising the following steps: a) a metal foil (20), resting on a rotating roller (16), is guided past a first slotted nozzle (12) from which a first electrode paste (22) emerges, whereby a front side (24) of the metal foil (20) is coated with the first electrode paste (22); b) the metal foil (20), without drying the first electrode paste (22), is guided past a second slotted nozzle (30), from which a second electrode paste (32) emerges, whereby a rear side (38) of the metal foil (20) is coated with the second electrode paste (32) without the metal foil (20) being supported on a stationary structure; characterized in that the thickness (dz) of the second electrode paste (32) on the back (38) of the metal foil (20) is controlled in a control circuit which has a contactless thickness sensor (42, 44) as a measuring element.

2. Method according to claim 1, characterized in that a control variable of the control loop is selected from the group consisting of: a mass flow emerging from the second slot nozzle (30), a tension of the metal foil (20) in the conveying direction in the region of the second slot nozzle (30), and a location coordinate of the second slot nozzle (30) in a direction perpendicular to the metal foil (20).

3. Method according to claim 2, characterized in that the manipulated variable is the mass flow emerging from the second slot nozzle (30), and that an actuator of the control circuit is a pump (34) which supplies the second electrode paste (32) to the second slot nozzle (30).

4. Method according to claim 2, characterized in that the manipulated variable is the tension of the metal foil (20) in the conveying direction in the region of the second slot nozzle (30), and that an actuator of the control circuit is a first actuator (70) which exerts a tensile force on the electrode.

5. The method according to claim 4, characterized in that the first actuator comprises a roller (70) which engages the front side (24) in a region (72) of the metal foil (20) which was not coated during step a).

6. Method according to one of the preceding claims, characterized in that the thickness sensor comprises a first sensor unit (42) which measures the thickness of the electrode coated on one side, and a second sensor unit (44) which measures the thickness (ds) of the electrode coated on both sides.

7. The method according to claim 6, characterized in that the first and the second sensor unit (42, 44) each comprise two distance sensors (42a, 42b; 44a, 44b) which are arranged in pairs on opposite sides of the electrode and measure distances to the electrode.

8. The method according to claim 7, wherein the distance sensors are chromatic-confocal measuring distance sensors (42a, 42b; 44a, 44b).

9. The method according to claim 7, wherein the distance sensors are interferometrically measuring distance sensors.

10. Device (10) for producing electrodes for battery cells, comprising a first slotted nozzle (12) configured to apply a first electrode paste (22) to a front side (24) of a metal foil (20), a rotatably mounted roller (16) configured to guide the metal foil (20) past the first slotted nozzle (12) so that the metal foil (20) rests on the roller (16) during the application of the first electrode paste (22), a second slotted nozzle (30) configured to apply a second electrode paste (32) to a back side (38) of the metal foil (20), wherein the electrode is not supported on the front side (24) during the coating of the back side (38) of the metal foil (20), characterized by a control device (40) which has a contactless thickness sensor (42, 44) as a measuring element and is designed to control the thickness of the second electrode paste (32) on the back (38) of the metal foil (20).

11. Device according to claim 10, characterized in that a manipulated variable of the control device (40) is selected from the group consisting of: a mass flow emerging from the second slot nozzle (30), a tension of the metal foil (20) in the conveying direction in the region of the second slot nozzle (30), and a location coordinate of the second slot nozzle (30) in a direction perpendicular to the metal foil (20).

12. Device according to claim 10 or 11, characterized in that the thickness sensor comprises a first sensor unit (42) which is designed to measure the thickness of the electrode coated on one side, and a second sensor unit (44) which is designed to measure the thickness (ds) of the electrode coated on both sides.

13. Device according to claim 12, characterized in that the first and the second sensor unit (42, 44) each comprise two distance sensors (42a, 42b; 44a, 44b) which are arranged in pairs on opposite sides of the electrode and are adapted to measure distances to the electrode.

14. Device according to claim 13, characterized in that the distance sensors are chromatic-confocal measuring distance sensors (42a, 42b; 44a, 44b).

15. Device according to claim 13, characterized in that the distance sensors are interferometrically measuring distance sensors.

16. Measuring and control device (40) for use in the device according to one of claims 10 to 15.

17. Use of a non-contact thickness sensor (42, 44) for measuring the thickness of the second electrode paste (32) on the back of the metal foil (20) in a method according to one of claims 1 to 9.

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