Double-slit die coater and method for coating an electrode active material paste using the same

Abstract

The dual-slit die coater according to the present disclosure is a dual-slit die coater including a lower slit and an upper slit, and is configured to extrude and coat an electrode active material slurry onto the surface of a continuously moving current collector through at least one of the lower slit or the upper slit. The dual-slit die coater includes a lower plate, an intermediate plate located on the lower plate, and an upper plate located on the intermediate plate. The lower slit is formed between the lower plate and the intermediate plate, and the upper slit is formed between the intermediate plate and the upper plate. Wherein the lower plate, the intermediate plate, and the upper plate have a lower die lip, an intermediate die lip, and an upper die lip, each of the lower die lip, the intermediate die lip, and the upper die lip forms a front end relative to the current collector, and the thickness of the lower die lip is greater than the thickness of the upper die lip and the thickness of the intermediate die lip.

Description

Technical Field

[0001] The present disclosure relates to a dual slit die coater capable of simultaneously forming a bilayer structure and a method of coating an electrode active material paste using the dual slit die coater. This application claims priority to Korean Patent Application No. 10-2020-0119919, filed in Korea on September 17, 2020, the disclosure of which is incorporated herein by reference. Background Art

[0002] With the continuous growth of technological development and the increasing demand for mobile devices, the demand for secondary batteries as an energy source has rapidly increased, and secondary batteries mainly include electrode assemblies, which are electrical energy generating elements. The electrode assembly includes a positive electrode, a separator, and a negative electrode stacked at least once, and the positive electrode and the negative electrode are manufactured by coating a positive electrode active material paste and a negative electrode active material paste on a current collector made of aluminum foil and a current collector made of copper foil, respectively, and drying them. Generally, secondary batteries include positive electrode active materials such as lithium cobalt oxide (LiCoO2) having a layered crystal structure, lithium manganese oxide (such as LiMnO2 having a layered crystal structure, LiMn2O4 having a spinel crystal structure), and lithium nickel oxide (LiNiO2). In addition, the negative electrode active material mainly includes carbon-based materials, and recently, with the growing demand for high-energy lithium secondary batteries, mixing with silicon-based materials and silicon oxide-based materials has been proposed, and the silicon-based materials and silicon oxide-based materials have an effective capacity at least 10 times higher than that of carbon-based materials. For the uniform charge / discharge characteristics of secondary batteries, it is necessary to uniformly coat the positive electrode active material paste and the negative electrode active material paste on the current collector.

[0003] To improve the performance of secondary batteries, attention has been directed to the development of an electrode structure having an active material layer with a bilayer structure on a current collector. To form an active material layer with a bilayer structure on a current collector, a dual slit die coater capable of simultaneously coating two types of electrode active material pastes can be used.

[0004] Figure 1 An example of a coating method using a dual slit die coater is shown, and Figure 2 is Figure 1 an enlarged view of part A in

[0005] Referring to Figure 1 and Figure 2 , a bilayer electrode active material layer can be simultaneously formed on the current collector 15 by conveying two types of electrode active material pastes 30, 40 from the dual slit die coater 20 while moving the current collector 15 by the rotation of the coating roller 10. The electrode active material pastes 30, 40 conveyed from the dual slit die coater 20 are coated on one surface of the current collector 15 to form an electrode active material layer.

[0006] The double-slit die coater 20 is constructed by assembling three plate members, namely, three die blocks 21, 22, and 23. Slits (two slits in total) are formed between adjacent die blocks to simultaneously convey two types of electrode active material slurries 30, 40 through outlets 24, 25 respectively communicating with the respective slits, so that the first electrode active material slurry 30 is first coated, and then the other second electrode active material slurry 40 is coated on the first electrode active material slurry 30 to simultaneously form a double-layer electrode active material layer. Reference numerals 26 and 27 denote manifolds in which the coating solution is received.

[0007] However, due to the use of the electrode active material slurries 30, 40 simultaneously conveyed from different outlets 24, 25, the process of using the double-slit die coater 20 has some problems in forming each electrode active material layer to a desired thickness.

[0008] The gap G from the outlets 24, 25 to the surface of the current collector 15 is the coating gap and is a very important variable that determines the coating quality of the electrode active material layer. Generally, the thickness of each electrode active material layer is affected by the amount of the electrode active material slurry conveyed through the outlets 24, 25, the type of the electrode active material slurry, and the coating gap. In addition, stable coating requires a uniform coating gap in the width direction (TD direction) of the current collector, and the coating gap deviation in the width direction seriously affects the coating width and the shape of the boundary of the uncoated area. The thickness of the electrode active material layer is a very small value of several tens of μm to several hundreds of μm, and even a change of several μm significantly affects the coating quality. Therefore, very strict management is required, and in order to stably achieve uniform coating in the width direction of the current collector, very strict control must be carried out to achieve uniform dimensional accuracy in the width direction. However, in order to increase the production volume, when the width of the double-slit die coater 20 increases with the increase in the width of the current collector, it becomes more difficult to perform uniform coating in the width direction, and thus more precise control of the coating gap is necessary.

[0009] In addition, due to the simultaneous conveyance of the electrode active material slurries 30, 40 from different outlets 24, 25, the coating process using the double-slit die coater 20 has problems such as leakage and edge rings. Among these problems, leakage refers to the instability caused by the loss of some coating solution upstream of the outer side of the die lip 21a, as Figure 2 shown. This belongs to the loss of the pre-metered coating solution, making it difficult to evaluate the final coating thickness. Due to leakage, the coating solution may stay for a long time and turn into a solid or there may be a coating thickness deviation in the width direction. In particular, when the coating gap G is reduced to several hundreds of μm and the coating solution is conveyed under high pressure to achieve thin film coating or reduce the coating thickness deviation in the width direction of the coating, the leakage may become more serious.

[0010] When drying the electrode active material slurries 30 and 40 after coating, the coating shape may change due to the surface tension of the liquid components of the slurries. Therefore, this factor should be considered during coating. For example, during drying, Marangoni flow occurs from the coating edge inward, and after drying, a thick edge pattern defect may appear, resulting in a thick edge. To prevent the thick edge pattern defect, the edge must be coated thinner. When the outlets 24 and 25 are closer to the current collector 15 at a smaller coating gap G, the edge can be coated thinner. However, when the coating gap G is small, leakage becomes more serious.

[0011] There is a window margin between the leakage area and the side ring area. The wider the window margin, the higher the productivity. Since the above-mentioned coating gap G significantly affects the size and shape of the coating beads formed between the current collector 15 and the die lip 21a during coating, conventional slit coating processes prevent leakage by repeatedly adjusting initial conditions such as the coating gap, the properties of the coating solution, and the flow volume and rate of the coating solution. However, it is difficult to set the initial conditions and it takes a long time to find the optimal processing conditions. Accordingly, a wider window margin makes it easier to control the coating gap or set the initial conditions. SUMMARY OF THE INVENTION

[0012] TECHNICAL PROBLEM

[0013] The present disclosure is designed to solve the above problems. Therefore, the present disclosure aims to provide a dual-slit die coater having a high productivity by reducing the occurrence of leakage and widening the window margin, and a method of coating an electrode active material slurry using the dual-slit die coater.

[0014] However, the problems to be solved by the present disclosure are not limited to the above problems, and those skilled in the art will clearly understand other problems from the following detailed description.

[0015] TECHNICAL SOLUTION

[0016] To solve the above problems, a dual-slit die coater according to the present disclosure is a dual-slit die coater including a lower slit and an upper slit, for extrusion coating an electrode active material slurry on the surface of a continuously moving current collector through at least one of the lower slit or the upper slit. The dual-slit die coater includes a lower plate, an intermediate plate located on the lower plate, and an upper plate located on the intermediate plate. The lower slit is formed between the lower plate and the intermediate plate, and the upper slit is formed between the intermediate plate and the upper plate. Wherein the lower plate, the intermediate plate, and the upper plate have a lower die lip, an intermediate die lip, and an upper die lip, and each of the lower die lip, the intermediate die lip, and the upper die lip forms a front end with respect to the current collector, and the thickness of the lower die lip is greater than the thickness of the upper die lip and the thickness of the intermediate die lip.

[0017] In the present disclosure, the thickness of the lower die lip: the thickness of the upper die lip may be 1.2:1 or greater.

[0018] In the present disclosure, the thickness of the lower die lip: the thickness of the middle die lip may be 1.2:1 or greater.

[0019] In the present disclosure, a lower outlet communicating with the lower slit may be formed between the lower die lip and the middle die lip, and an upper outlet communicating with the upper slit may be formed between the middle die lip and the upper die lip. The electrode active material paste forming the lower paste layer may be transported onto the current collector through the lower outlet, and the electrode active material paste forming the upper paste layer may be transported onto the lower paste layer on the current collector through the upper outlet. The upper outlet is spaced apart from the lower outlet downstream in the coating direction.

[0020] In the present disclosure, the thickness of the thinner die lip between the upper die lip and the middle die lip is preferably 80 μm or greater.

[0021] In the present disclosure, the distance between the current collector and the upper die lip may be greater than the distance between the current collector and the lower die lip and the distance between the current collector and the middle die lip.

[0022] In the present disclosure, preferably, the current collector is moved by the rotation of a circular coating roll having a diameter of 350 to 400 mm, and the current collector has a curvature according to the coating roll, and the thicknesses of the lower die lip, the middle die lip, and the upper die lip are such that the average increase in curvature at positions corresponding to the top of the lower plate, the bottom of the middle plate, the top of the middle plate, and the bottom of the upper plate does not exceed 50 μm. The top of the lower plate is the region of the lower die lip located on the most downstream side along the moving direction of the current collector, the bottom of the middle plate is the region of the middle die lip located on the most upstream side along the moving direction of the current collector, the top of the middle plate is the region of the middle die lip located on the most downstream side along the moving direction of the current collector, and the bottom of the upper plate is the region of the upper die lip located on the most upstream side along the moving direction of the current collector.

[0023] In the present disclosure, the thickness range of the upper die lip may be 0.08 to 4.5 mm, the thickness range of the middle die lip may be 0.08 to 8.8 mm, and the thickness range of the lower die lip may be 1 to 4.5 mm.

[0024] A method for coating an electrode active material paste according to the present disclosure includes forming an electrode active material paste on a current collector by using a dual slit die coater according to the present disclosure while supplying the electrode active material paste while moving the current collector from the lower die lip to the upper die lip.

[0025] In the method for coating an electrode active material slurry according to the present disclosure, the electrode active material slurry layer can be formed to have a thickness of 60 μm or greater.

[0026] In the method for coating an electrode active material slurry according to the present disclosure, the electrode active material slurry can have a viscosity of 1000 cps or greater.

[0027] Another method for coating an electrode active material slurry according to the present disclosure is described below. The method for coating an electrode active material slurry uses a dual-slit die coater including a lower slit and an upper slit for simultaneously extrusion-coating two types of electrode active material slurries on the surface of a continuously moving current collector through the lower slit and the upper slit. The dual-slit die coater includes a lower plate, an intermediate plate located on the lower plate, and an upper plate located on the intermediate plate. The lower slit is formed between the lower plate and the intermediate plate, and the upper slit is formed between the intermediate plate and the upper plate. Wherein the lower plate, the intermediate plate, and the upper plate have a lower die lip, an intermediate die lip, and an upper die lip, and each of the lower die lip, the intermediate die lip, and the upper die lip forms a front end relative to the current collector, and the thickness of the lower die lip is greater than the thickness of the upper die lip and the thickness of the intermediate die lip. The method includes simultaneously conveying two types of electrode active material slurries on a current collector moving in a direction from the lower die lip to the upper die lip through a lower outlet and an upper outlet to form a double-layer structure including a lower slurry layer and an upper slurry layer coated on the lower slurry layer, wherein the lower outlet communicating with the lower slit is formed between the lower die lip and the intermediate die lip, the upper outlet communicating with the upper slit is formed between the intermediate die lip and the upper die lip, and the upper outlet is spaced apart from the lower outlet downstream in the coating direction.

[0028] Here, the flow rate ratio of the electrode active material slurry forming the lower slurry layer to the electrode active material slurry forming the upper slurry layer can be 1:1.

[0029] Advantageous Effects

[0030] According to the present disclosure, among the lower die lip, the intermediate die lip, and the upper die lip of the dual-slit die coater, the lower die lip has the largest thickness. As the thickness of the lower die lip increases, the window margin becomes wider. Accordingly, according to the present disclosure, the productivity increases, and the dynamic contact line can be used for a wider range of coating applications according to the desired coating product and quality.

[0031] According to the present disclosure, the leakage limit can be increased. Additionally, the side ring region can be reduced. When the coating gap is decreased, the dynamic contact line moves in a direction opposite to the coating direction, and when it exceeds a predetermined level, leakage occurs. However, the present disclosure achieves a wider window margin by increasing the thickness of the lower die lip, thereby preventing leakage even when the coating gap is decreased. This is because the electrode active material slurry does not leak and a relatively large amount of the electrode active material slurry remains in the lower die lip. According to the present disclosure, leakage can be prevented when the coating gap is small or when the slurry is supplied in a relatively large amount relative to the moving speed of the current collector, thereby forming an electrode active material slurry layer with good coating quality.

[0032] According to the present disclosure, leakage can be prevented without repeatedly adjusting initial conditions (such as the coating gap, the properties of the coating solution, and the volume and speed of the coating solution flow) to prevent leakage. In order to prevent a thick edge caused by Marangoni flow from the edge during drying, the edge must be coated thinner during the coating step. However, when the distance between the coating roll and the die lip or the coating gap is small, leakage becomes more severe. According to the present disclosure, since leakage is prevented even when the coating gap is decreased, pattern defects such as thick edges can be prevented. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The drawings illustrate preferred embodiments of the present disclosure and are used in conjunction with the detailed description of the present disclosure to provide a further understanding of the technical features of the present disclosure. Therefore, the present disclosure is not to be construed as limited to the drawings.

[0034] Figure 1 is a schematic cross-sectional view of a conventional double-slit die coater.

[0035] Figure 2 is Figure 1 an enlarged view of part A in

[0036] Figure 3 is a schematic cross-sectional view of a double-slit die coater according to an embodiment of the present disclosure.

[0037] Figure 4 is a schematic exploded perspective view of a double-slit die coater according to an embodiment of the present disclosure.

[0038] Figure 5 is Figure 3 an enlarged view of part B of

[0039] Figure 6 is a diagram showing the thickness range of each die lip in a double-slit die coater according to an embodiment of the present disclosure.

[0040] Figure 7 (a), (b), and (c) are simulation diagrams of multiple experimental examples. Detailed implementation mode

[0041] Hereinafter, preferred implementation modes of the present disclosure will be described in detail with reference to the accompanying drawings. Before the description, it should be understood that the terms or phrases used in the specification and the appended claims should not be construed as being limited to the general and dictionary meanings, but should be interpreted based on the meanings and concepts corresponding to the technical aspects of the present disclosure on the basis of the principle that allows the inventor to appropriately define the terms for the best interpretation. Therefore, the implementation modes described herein and the illustrations in the drawings are only some preferred implementation modes of the present disclosure, and do not fully describe the technical features of the present disclosure. Therefore, it should be understood that various other equivalents and modifications can also be made to the present disclosure when submitting this application.

[0042] The double-slit die coater of the present disclosure is a device including a lower slit and an upper slit to coat a double-layer coating solution on a substrate. The'substrate' described below is a current collector, and the coating solution is an 'electrode active material slurry'. The slurries transported through the lower slit and the slurries transported through the upper slit can be electrode active material slurries having the same or different compositions (types of active materials, conductive materials, and binders), amounts (amounts of active materials, conductive materials, and binders), or properties. The double-slit die coater of the present disclosure is optimized for electrodes manufactured by simultaneously coating two types of electrode active material slurries, or pattern coating by alternately coating two types of electrode active material slurries, or intermittent coating by alternately supplying and stopping two types of electrode active material slurries. However, the scope of the present disclosure is not necessarily limited thereto.

[0043] Figure 3 is a schematic cross-sectional view of a double-slit die coater according to an implementation mode of the present disclosure. Figure 4 is a schematic exploded perspective view of a double-slit die coater according to an implementation mode of the present disclosure. Figure 5 is Figure 3 an enlarged view of part B of

[0044] Referring to Figures 3 to 5 , the double-slit die coater 100 according to the present disclosure includes a lower slit 101 and an upper slit 102, and is a device capable of simultaneously, alternately, or intermittently coating the same type of electrode active material slurry or two different types of electrode active material slurries on the current collector 300 through the lower slit 101 and the upper slit 102.

[0045] The double slit die coater 100 includes a lower plate 110, an intermediate plate 120 located on the lower plate 110, and an upper plate 130 located on the intermediate plate 120. The lower plate 110, the intermediate plate 120, and the upper plate 130 are assembled together by fasteners such as bolts. The lower plate 110 is the lowermost block in the blocks of the double slit die coater 100, and the surface facing the intermediate plate 120 is inclined at an angle of about 30° to 60° toward the bottom surface (X-Z plane).

[0046] The lower slit 101 may be formed at a position where the lower plate 110 and the intermediate plate 120 face each other. For example, a first spacer 113 is inserted between the lower plate 110 and the intermediate plate 120 to form a gap therebetween, and the lower slit 101 corresponding to the channel for the flow of the first electrode active material paste 150 may be formed. In this case, the thickness of the first spacer 113 determines the vertical width (Y-axis direction, slit gap) of the lower slit 101.

[0047] As Figure 4 shown, the first spacer 113 has a first opening portion 113a that is cut in a region, and may be inserted into the remaining portions of the facing surfaces of the lower plate 110 and the intermediate plate 120 except for one side in the edge region. Accordingly, the lower outlet 101a (through which the first electrode active material paste 150 appears) is formed only between the front end of the lower plate 110 and the front end of the intermediate plate 120. The front end of the lower plate 110 and the front end of the intermediate plate 120 are respectively defined as the lower die lip 111 and the intermediate die lip 121. In other words, the lower outlet 101a is formed by the interval between the lower die lip 111 and the intermediate die lip 121.

[0048] For reference, the first spacer 113 acts as a gasket to prevent the first electrode active material paste 150 from leaking through the gap between the lower plate 110 and the intermediate plate 120 except for the region where the lower outlet 101a is formed. Therefore, the first spacer 113 is preferably made of a material having a sealing ability.

[0049] The lower plate 110 includes a first manifold 112 having a predetermined depth on the surface facing the intermediate plate 120, and the first manifold 112 communicates with the lower slit 101. Although not shown in the figure, the first manifold 112 is connected to a first electrode active material paste supply chamber (not shown) installed outside via a supply pipe and is supplied with the first electrode active material paste 150. When the first manifold 112 is filled with the first electrode active material paste 150, the flow of the first electrode active material paste 150 is guided along the lower slit 101 and the first electrode active material paste 150 flows out from the lower outlet 101a.

[0050] The intermediate plate 120 is a block disposed in the middle of the die blocks of the double slit die coater 100, and is inserted between the lower plate 110 and the upper plate 130 to form a double slit. The cross-section of the intermediate plate 120 in this embodiment is a right triangle, but it is not necessarily limited thereto. For example, the cross-section of the intermediate plate 120 can be an isosceles triangle, for example.

[0051] The upper plate 130 is positioned facing the upper surface of the intermediate plate 120 parallel to the bottom surface. As described above, the upper slit 102 is formed at the position where the intermediate plate 120 and the upper plate 130 face each other.

[0052] In the same manner as the lower slit 101 described above, the second spacer 133 can be inserted between the intermediate plate 120 and the upper plate 130 to form a gap therebetween. Accordingly, the upper slit 102 corresponding to the channel for the flow of the second electrode active material slurry 160 is formed. In this case, the vertical width (Y-axis direction, slit gap) of the upper slit 102 is determined by the second spacer 133.

[0053] In addition, the second spacer 133 having a structure similar to that of the first spacer 113 has a second opening portion 133a cut in a region, and is inserted into the remaining portions of the facing surfaces of the intermediate plate 120 and the upper plate 130 except for one side in the edge region. Similarly, the circumferential direction except for the front side of the upper slit 102 is blocked, and the upper outlet 102a is formed only between the front end of the intermediate plate 120 and the front end of the upper plate 130. The front end of the upper plate 130 is defined as the upper die lip 131. In other words, the upper outlet 102a is formed by the interval between the intermediate die lip 121 and the upper die lip 131.

[0054] In addition, the upper plate 130 includes a second manifold 132 having a predetermined depth on the surface facing the intermediate plate 120, and the second manifold 132 communicates with the upper slit 102. Although not shown in the figure, the second manifold 132 is connected to a supply chamber for the second electrode active material slurry 160 installed outside via a supply pipe and is supplied with the second electrode active material slurry 160. When the second electrode active material slurry 160 is supplied from an external source along the supply pipe and the second manifold 132 is filled with the second electrode active material slurry 160, the flow of the second electrode active material slurry 160 is guided along the upper slit 102 communicating with the second manifold 132 and the second electrode active material slurry 160 flows out from the upper outlet 102a.

[0055] The upper slit 102 and the lower slit 101 form a predetermined angle, and the angle can be about 30° to 60°. The upper slit 102 and the lower slit 101 can intersect at a point, and the upper outlet 102a and the lower outlet 101a can be provided near the intersection point. Accordingly, the positions where the first electrode active material slurry 150 and the second electrode active material slurry 160 appear can converge to approximately one point.

[0056] The first manifold 112 and the second manifold 132 are respectively formed in the lower plate 110 and the upper plate 130. In this case, the structurally weakest intermediate plate 120 can be less likely to deform.

[0057] Meanwhile, the dual-slit die coater 100 can further include a first valve for opening / closing the conveyance through the lower outlet 101a, a second valve for opening / closing the conveyance through the upper outlet 102a, and a valve control unit for controlling the opening / closing of the first and second valves.

[0058] According to the dual-slit die coater 100 having the above-described configuration, the rotatable coating roll 200 is positioned on the front side of the dual-slit die coater 100, and the coating roll 200 can rotate to move the current collector 300 to be coated while continuously contacting the first electrode active material slurry 150 and the second electrode active material slurry 160 with the surface of the current collector 300, whereby the current collector 300 can be coated with a double-layer structure simultaneously. Alternatively, the supply and stop of the first electrode active material slurry 150 and the supply and stop of the second electrode active material slurry 160 can be alternately performed by the on / off control of the first valve and the second valve by the valve control unit, and pattern coating can be intermittently formed on the current collector 300.

[0059] The first electrode active material slurry 150 is coated on the current collector 300 to form a lower slurry layer, and almost simultaneously, the second electrode active material slurry 160 is coated on the lower slurry layer to form an upper slurry layer.

[0060] Further referring to Figure 5 , the structure of the die lip of the dual-slit die coater according to an embodiment of the present disclosure and a method of coating an electrode active material slurry using the dual-slit die coater will be described in detail. The dual-slit die coater 100 according to the present disclosure has different lip thicknesses between the upper plate / intermediate plate / lower plate.

[0061] The lower die lip thickness D1 is greater than the upper die lip thickness D3, and the lower die lip thickness D1 is greater than the intermediate die lip thickness D2. Accordingly, the lower die lip thickness D1 is greater than the average thickness of the upper die lip thickness D3 and the intermediate die lip thickness D2. As described above, the lower die lip thickness D1 is the largest. D1 > D2, D1 > D3, D1 > (D2 + D3) / 2. The upper die lip thickness D3 can be equal to the intermediate die lip thickness D2.

[0062] The thickness D1 of the lower die lip: the thickness D3 of the upper die lip can be 1.2:1 or greater. That is to say, the thickness D1 of the lower die lip can be at least 1.2 times the thickness D3 of the upper die lip. When the thickness D1 of the lower die lip is greater than the thickness D3 of the upper die lip, the window margin becomes wider as expected, but when the thickness D1 of the lower die lip is at least 1.2 times the thickness D3 of the upper die lip, leakage can be prevented. When the thickness D1 of the lower die lip is equal to the thickness D3 of the upper die lip or the thickness D3 of the upper die lip is greater than the thickness D1 of the lower die lip, leakage occurs.

[0063] The thickness D1 of the lower die lip: the thickness D2 of the middle die lip can be 1.2:1 or greater. That is to say, the thickness D1 of the lower die lip is at least 1.2 times the thickness D2 of the middle die lip. When the thickness D1 of the lower die lip is greater than the thickness D2 of the middle die lip, the window margin becomes wider as expected, but when the thickness D1 of the lower die lip is at least 1.2 times the thickness D2 of the middle die lip, leakage can be prevented. When the thickness D1 of the lower die lip is equal to the thickness D2 of the middle die lip, leakage occurs. When the thickness D2 of the middle die lip is greater than the thickness D1 of the lower die lip, no leakage occurs, but pattern defects occur.

[0064] According to the present disclosure, the thickness of the lower die lip 111 is the largest. The inventors have found that when the lower die lip 111 is thicker, the window margin is wider. Accordingly, the coating gap can be more easily controlled or the initial conditions can be set. Thus, according to the present disclosure, the productivity is increased, and dynamic contact line coating can be used in a wide range of coating applications according to the desired coating product and quality.

[0065] According to the present disclosure, as the window margin becomes wider, the leakage limit can be increased. In addition, the side ring area can be reduced. When the coating gap is reduced, the dynamic contact line moves in a direction opposite to the coating direction, and when it exceeds a predetermined level, leakage occurs, but the present disclosure can reduce leakage by increasing the thickness of the lower die lip 111. This is because the electrode active material slurry does not leak and a large amount of the electrode active material slurry remains in the lower die lip 111. According to the present disclosure, even when the coating gap is small or the slurry is supplied in a large amount relative to the moving speed of the current collector 300, leakage can be reduced.

[0066] According to the dual-slit die coater and the method of coating an electrode active material slurry using the dual-slit die coater according to the present disclosure, forming a double-layer structure of the active material layer on the current collector can increase the process efficiency and reduce the defect rate.

[0067] Preferably, the thickness conditions of the lower die lip 111, the upper die lip 131, and the intermediate die lip 121 are satisfied, and the thickness of the thinnest die lip among the upper die lip 131 and the intermediate die lip 121 is equal to or greater than 80 μm (0.08 mm). When the thickness of the thinnest die lip is less than 80 μm, the die plate is too thin and the slit gap changes, making it difficult to achieve uniform coating. Therefore, the thickness is at least 80 μm. When the thickness of the thinnest die lip is 80 μm, the thickness of the lower die lip 111 can be equal to or greater than 96 μm, which is 1.2 times the thickness of the thinnest die lip of 96 μm.

[0068] Preferably, the thickness of the thinnest die lip is equal to or greater than 800 μm. This value can be considered in view of the pressure of the flowing out electrode active material slurry. More preferably, the thickness of the thinnest die lip is equal to or greater than 1 mm. This can be a value set for precision machining. When the thickness of the thinnest die lip is 800 μm, the thickness of the lower die lip 111 can be equal to or greater than 0.96 mm, which is 1.2 times the thickness of the thinnest die lip. More preferably, the thickness of the thinnest die lip is equal to or greater than 1 mm. This can also be a value set for precision machining. The distance H3 between the current collector 300 and the upper die lip 131 can be greater than the distance H2 between the current collector 300 and the intermediate die lip 121 and the distance H1 between the current collector 300 and the lower die lip 111. This distance difference can be formed by moving the upper die lip 131 (which is the lip of the upper plate 130) backward away from the current collector 300 in a direction opposite to the conveying direction of the lower die lip 111 (which is the lip of the lower plate 110) and the intermediate die lip 121 (which is the lip of the intermediate plate 120) to form a lip step. The loading out area can be reduced by the step between the lips. The distance H1 between the current collector 300 and the lower die lip 111 can be equal to the distance H2 between the current collector 300 and the intermediate die lip 121.

[0069] To form the lip step, the dual-slit die coater 100 can further include a control unit for separately moving the upper die lip 131 backward after linearly aligning and placing the lower die lip 111, the intermediate die lip 121, and the upper die lip 131 relative to the current collector 300. In this configuration, the upper die lip 131 is disposed at a more backward position from the current collector 300 compared to the lower die lip 111 and the intermediate die lip 121.

[0070] Accordingly, the predetermined step is formed between the lower outlet 101a and the upper outlet 102a. The step is the result of subtracting the distance H1 between the current collector 300 and the lower die lip 111 from the distance H3 between the current collector 300 and the upper die lip 131. The lower outlet 101a and the upper outlet 102a are spaced apart from each other in the horizontal direction by this step, thereby preventing the second electrode active material slurry 160 flowing out from the upper outlet 102a from entering the lower outlet 101a, or the first electrode active material slurry 150 flowing out from the lower outlet 101a from entering the upper outlet 102a. In addition, the intermediate die lip 121 presses the first electrode active material slurry 150 downward, but the upper die lip 131 does not press the second electrode active material slurry 160 downward, so the width of the lower slurry layer coated with the first electrode active material slurry 150 is equal to the width of the upper slurry layer coated with the second electrode active material slurry 160.

[0071] As shown in the figure, when setting the lip position, the upper outlet 102a is spaced apart from the lower outlet 101a downstream in the coating direction. By simultaneously transporting the first electrode active material slurry 150 and the second electrode active material slurry 160 via the lower outlet 101a and the upper outlet 102a while moving the current collector 300 from the lower die lip 111 to the upper die lip 131, an active material layer with a double-layer structure can be formed on the current collector 300.

[0072] The first electrode active material slurry 150 gushing out from the lower outlet 101a is coated on the current collector 300 to form a lower slurry layer. At the same time, the second electrode active material slurry 160 gushing out from the upper outlet 102a is coated on the lower slurry layer to form an upper slurry layer. The double-slit die coater 100 of the present disclosure can form a double-layer structure including an upper slurry layer located on the lower slurry layer.

[0073] The average thickness of the lower slurry layer formed by the first electrode active material slurry 150 emerging through the lower outlet 101a and the average thickness of the upper slurry layer formed by the second electrode active material slurry 160 emerging through the upper outlet 102a can each be 60 μm or greater. Each average thickness can be 200 μm or less. Generally, the average particle size of the secondary battery active material is about 10 μm, but this particle size follows a common normal distribution, so usually d(90) or d(max) is greater than 10 μm. (Here, the average particle size is d(50), and can be defined as the particle size corresponding to 50% cumulative mass percentage in the particle size distribution curve measured by laser diffraction. It will be understood that d(90) means that 90% of the total particles have a particle size equal to or less than the above value). Since the slurry layer includes the active material, it is difficult to form the slurry layer to a thickness less than 40 μm. Additionally, when the thickness of the slurry layer is equal to or greater than 60 μm, coating can be smoothly performed without usually having the active material clogged at the coating gap. Further, when the thickness of the slurry layer is equal to or greater than 200 μm, it can be advantageous, but it is difficult to actually achieve a coating amount greater than 200 μm for practical use in a secondary battery.

[0074] For example, the method of coating an electrode active material slurry of the present disclosure can be applied to the manufacture of a positive electrode of a secondary battery. The positive electrode includes a current collector and a positive electrode active material layer formed on the surface of the current collector. The current collector can include any material exhibiting conductivity, such as Al, Cu, and an appropriate material can be used according to the polarity of the current collector of the electrode well-known in the field of secondary batteries. The positive electrode active material layer can further include at least one of positive electrode active material particles, a conductive material, or a binder. Additionally, the positive electrode can further include various types of additives to enhance or improve electrical and chemical properties.

[0075] The active material is not limited to a specific type and can include any material for the positive electrode active material of a lithium-ion secondary battery. Non-limiting examples thereof can include at least one of a layered compound or a compound having one or more transition metal substitutions, such as lithium manganese composite oxides (LiMn2O4, LiMnO2), lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2); lithium manganese oxides having the chemical formula Li 1+x Mn 2-x O4 (x is 0 to 0.33), LiMnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides, such as LiV3O8, LiV3O4, V2O5, Cu2V2O7; represented by the chemical formula LiNi 1-x M xNi-site lithium nickel oxide represented by O2 (M = Co, Mn, Al, Cu, Fe, Mg, B or Ga, x = 0.01 to 0.3); lithium manganese composite oxide represented by the chemical formula LiMn 2-x M x O2 (M = Co, Ni, Fe, Cr, Zn or Ta, x = 0.01 to 0.1) or Li2Mn3MO8 (M = Fe, Co, Ni, Cu or Zn); LiMn2O4 in which an alkaline earth metal ion partially replaces Li in the chemical formula; a disulfide compound; or Fe2(MoO4)3. In the present disclosure, the positive electrode may include a solid electrolyte material, such as at least one of a polymer-based solid electrolyte, an oxide-based solid electrolyte, or a sulfide-based solid electrolyte.

[0076] Generally, 1 wt% to 20 wt% of a conductive material may be added based on the total weight of the mixture including the electrode active material. The conductive material is not limited to a specific type and may include any material having conductive properties without causing any chemical changes to the corresponding battery. For example, the material is at least selected from: graphite, such as natural graphite or artificial graphite; carbon black, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking carbon black; conductive fibers, such as carbon fibers or metal fibers; metal powders, such as carbon fluoride, aluminum and nickel powders; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; and conductive materials, such as polyphenylene derivatives.

[0077] The binder is not limited to a specific type and may include any material that helps to bond the active material and the conductive material and bond to the current collector. For example, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, and various copolymers of the foregoing substances. Generally, 1 wt% to 30 wt% or 1 wt% to 10 wt% of the binder may be included based on 100 wt% of the electrode layer.

[0078] The electrode may be a negative electrode. The negative electrode includes a current collector and a negative electrode active material layer formed on the surface of the current collector. The negative electrode active material layer may further include at least one of negative electrode active material particles, a conductive material, or a binder. Additionally, the negative electrode may further include various additives to enhance or improve electrical and chemical properties.

[0079] The negative electrode active material may include: carbon materials, such as graphite, amorphous carbon, diamond-phase carbon, fullerenes, carbon nanotubes, and carbon nanohorns; lithium metal materials; alloy-based materials, such as silicon- or tin-based alloy materials; oxide-based materials, such as Nb2O5, Li5Ti4O12 , TiO2, or a composite of the above materials. For the conductive material, binder, and current collector of the negative electrode, reference can be made to the positive electrode.

[0080] In particular, the electrode manufactured according to the present disclosure is preferably a positive electrode. Preferably, the positive electrode has a structure in which a lower active material layer and an upper active material layer are sequentially formed on the current collector. The lower active material layer contains a relatively large amount of conductive material, while the upper active material layer contains a relatively small amount of conductive material. In this case, the amount of the conductive material in the lower active material layer can be adjusted in the range between 0.5 wt% and 5 wt%. By reducing the amount of the conductive material in the upper active material layer, the amount of the active material on the electrode surface can be increased and the conductivity can be reduced to a predetermined level. In particular, when the amount of the conductive material in the upper active material layer is controlled to a very low level of 0.02 wt%, the heat generation reaction can be reduced in the case of an internal short circuit of the battery.

[0081] In another example, the average particle size P1 of the active material forming the lower active material layer is in the range between 50 and 95% of the average particle size P2 of the active material forming the upper active material layer. In this case, applying the active material with a small particle size to the lower active material layer and the active material with a large particle size to the upper active material layer makes it easy for the electrolyte solution to wet and induces smooth movement of ions or holes.

[0082] Here, the flow rate ratio of the first electrode active material slurry 150 to the second electrode active material slurry 160 may be 1:1. The first electrode active material slurry 150 and the second electrode active material slurry 160 may include graphite, a conductive material, CMC, and a binder. The viscosities of the first electrode active material slurry 150 and the second electrode active material slurry 160 may be equal to or higher than 1000 cps. The viscosity of the slurry for forming a secondary battery electrode may be 2000 cps to 30000 cps. For example, the negative electrode active material slurry may have 2000 cps to 4000 cps. The positive electrode active material slurry may have a viscosity of 8000 cps to 30000 cps. Since the dual slit die coater 100 of the present disclosure is configured to coat a coating solution having a viscosity of 1000 cps or higher, the structure of the dual slit die coater 100 of the present disclosure is different from that of a device for coating a coating solution having a lower viscosity, and cannot be obtained by modifying such a device. Examples of the coating solution having a lower viscosity include commonly used resin solutions, such as photosensitive latex, magnetic fluid, anti-reflection solution, anti-glare solution, a solution for increasing the viewing angle, and a dye solution for a color filter. Since the dual slit die coater of the present disclosure is configured to coat a slurry including a secondary battery active material having an average particle size of about 10 μm, the structure of the dual slit die coater 100 of the present disclosure is different from that of a device for coating a coating solution of particles not having the above particle size, and cannot be obtained by modifying such a device. The dual slit die coater 100 of the present disclosure is optimal for a coater for electrodes.

[0083] Although Figure 5 the illustrated view does not consider the curvature of the coating roller 200, in reality, the current collector 300 placed on the coating roller 200 also has a curvature due to the curvature of the coating roller 200, and thus the coating gap between the die lips 111, 121, 131 and the current collector 300 varies according to the position. When the lower die lip 111 is thick, the window margin is wide, but the deviation of the coating gap increases. The same applies to the other die lips 121, 131. Accordingly, the inventors propose a preferred thickness range for each of the die lips 111, 121, 131, including the thickest lower die lip 111. That is, since the coating roller 200 is circular, when the die lips 111, 121, 131 are too thick, the coating gap at each position changes. In order to achieve coating uniformity while preventing a thick edge by making the lower die lip 111 the thickest, the present disclosure proposes the maximum value of the thickness of each of the die lips 111, 121, 131.

[0084] Figure 6 is a view showing the thickness range of each die lip in the dual slit die coater according to an embodiment of the present disclosure.

[0085] The die lips 111, 121, and 131 are linearly aligned. The region of the lower die lip 111 that is located on the most upstream side along the moving direction of the current collector is defined as the lower plate bottom 111a, and the region of the lower die lip 111 that is located on the most downstream side is defined as the lower plate top 111b. The region of the intermediate die lip 121 that is located on the most upstream side along the moving direction of the current collector is defined as the intermediate plate bottom 121a, and the region of the intermediate die lip 121 that is located on the most downstream side is defined as the intermediate plate top 121b. The region of the upper die lip 131 that is located on the most upstream side along the moving direction of the current collector is defined as the upper plate bottom 131a, and the region of the upper die lip 131 that is located on the most downstream side is defined as the upper plate top 131b.

[0086] The intermediate plate bottom coating gap G2a is the coating gap at the intermediate plate bottom 121a, which is the target coating gap PV + the curvature increase at the position corresponding to the intermediate plate bottom 121a. Similarly, the lower plate top coating gap G1b is the coating gap at the lower plate top 111b, which is the target coating gap PV + the curvature increase at the position corresponding to the lower plate top 111b. The upper plate bottom coating gap G3a is the coating gap at the upper plate bottom 131a, which is the target coating gap PV + the curvature increase at the position corresponding to the upper plate bottom 131a. Similarly, the intermediate plate top coating gap G2b is the coating gap at the intermediate plate top 121b, which is the target coating gap PV + the curvature increase at the position corresponding to the intermediate plate top 121b.

[0087] When the average value of the curvature increase at the position corresponding to the intermediate plate bottom 121a and the curvature increase at the position corresponding to the lower plate top 111b is AV', and the average value of the curvature increase at the position corresponding to the upper plate bottom 131a and the curvature increase at the position corresponding to the intermediate plate top 121b is AV, AV' and AV change depending on the diameter of the coating roll 200, the thickness of each die lip 111, 121, 131, the thickness of the spacers 113, 133 used, and the part of the dual-slit die coater 100 that is aligned with the center of the coating roll 200.

[0088] In the case where the thickness of the spacers 113, 133 is 0.2 to 3 mm, the diameter of the coating roll 200 is 350 to 400 mm, and the center of the coating roll 200 is aligned with the upper plate bottom 131a or the lower plate top 111b, when AV' and AV are equal to or less than 50 μm, the coating quality is not affected. When AV' and AV do not exceed 50 μm, the maximum value of the thickness D3 of the upper die lip 131 is 4.5 mm. The maximum value of the thickness D2 of the intermediate die lip 121 is 8.8 mm. The maximum value of the thickness D1 of the lower die lip 111 is 4.5 mm.

[0089] Accordingly, when considering the minimum values of each die lip described above together, the thickness D3 of the upper die lip 131 can be in the range of 0.08 to 4.5 mm, the thickness D2 of the middle die lip 121 can be in the range of 0.08 to 8.8, and the thickness D1 of the lower die lip 111 can be in the range of 1 to 4.5 mm.

[0090] Hereinafter, the present disclosure will be described in more detail through experimental examples.

[0091] (Experimental Example)

[0092] For the evaluation of electrode quality, the varying thicknesses of the following lower die lip 111, middle die lip 121, and upper die lip 131 form an electrode active material slurry layer with a double-layer structure. The electrode active material slurries 150, 160 are a mixture of graphite: conductive material: CMC: binder in a ratio of 95:1:1:3 in water. The coating gap is 80 μm, and the flow rate ratio of the upper layer slurry and the lower layer slurry is 1:1. The coating rate is 40 m / minute.

[0093] Table 1 summarizes the die lip thickness and thickness ratio, as well as the electrode quality results.

[0094] [Table 1]

[0095]

[0096] In Experimental Example 1, contrary to the present disclosure, the thicknesses of the upper die lip 131, middle die lip 121, and lower die lip 111 are all equal to 1 mm. The results of simulation and actual testing show leakage. The Carreau - Yasuda viscosity model was used in the simulation.

[0097] In Experimental Example 2, the thickness of each of the upper die lip 131, middle die lip 121, and lower die lip 111 is 2 mm. The thickness of each die lip is twice that of Experimental Example 1, but their ratios are all equal to 1:1:1. In the same manner as Experimental Example 1, the results of simulation and actual testing show leakage. As the simulation result of Experimental Example 2, Figure 7 (c) shows the leakage of the first electrode active material slurry 150 in the lower die lip 111.

[0098] In Experimental Examples 3 to 5, the lower die lip 111 is the thickest. That is, the die lip thickness requirements of the present disclosure are met. The results of simulation and actual testing in Experimental Examples 3 to 5 show that there is no leakage or edge ring, and the coating quality is good. Figure 7 (a) shows Experimental Example 3, and Figure 7(b) shows Experimental Example 4. The arrow indicates the dynamic contact line. The lower die lip 111 of Experimental Example 4 is thicker than that of Experimental Example 3. In Experimental Example 4, it can be seen that the dynamic contact line is set at the inner position of the lower die lip 111. That is to say, the window margin is wider. Thus, according to the present disclosure, as shown in Experimental Examples 3 to 5, by using the increased thickness of the lower die lip 111, leakage can be prevented and a wider window margin can be achieved. The thickness of the lower die lip 111 is not increased without limit, but is adjusted according to the desired coating product and quality during coating so that the dynamic contact line is placed at an appropriate position.

[0099] In Experimental Example 6, contrary to the present disclosure, the upper die lip 131 is the thickest. The simulation results show that leakage occurs.

[0100] In Experimental Example 7, contrary to the present disclosure, the thickness of the middle die lip 121 is the largest. The simulation results show that no leakage occurs, but in actual tests, a thick edge pattern defect appears, resulting in a thick electrode edge.

[0101] Although the present disclosure has been described with respect to a limited number of embodiments and drawings, the present disclosure is not limited thereto, and it will be apparent to those skilled in the art that various changes and modifications can be made to the present disclosure within the technical scope of the present disclosure and the scope of the appended claims and their equivalents.

[0102] Although terms indicating directions, such as up, down, left, and right, are used herein, these terms are only used for convenience of explanation, and it will be apparent to those skilled in the art that these terms can be changed according to the position of the target object or the observer.

Claims

1. A dual-slit die coater including a lower slit and an upper slit for extruding and coating an electrode active material slurry onto the surface of a continuously moving current collector through at least one of the lower slit or the upper slit, the dual-slit die coater comprising: a lower plate, an intermediate plate located on the lower plate, and an upper plate located on the intermediate plate, the lower slit being formed between the lower plate and the intermediate plate, and the upper slit being formed between the intermediate plate and the upper plate, wherein the lower plate, the intermediate plate, and the upper plate have a lower die lip, an intermediate die lip, and an upper die lip, and each of the lower die lip, the intermediate die lip, and the upper die lip forms a front end relative to the current collector, and the thickness of the lower die lip is greater than the thickness of the upper die lip and the thickness of the intermediate die lip, wherein the ratio of the thickness of the lower die lip to the thickness of the upper die lip is 1.2:1 or greater, or wherein the ratio of the thickness of the lower die lip to the thickness of the intermediate die lip is 1.2:1 or greater, and wherein the thickness of the thinner die lip between the upper die lip and the intermediate die lip is 800 μm or greater.

2. The dual-slit die coater according to claim 1, wherein a lower outlet communicating with the lower slit is formed between the lower die lip and the intermediate die lip, an upper outlet communicating with the upper slit is formed between the intermediate die lip and the upper die lip, the electrode active material slurry forming a lower slurry layer is transported to the current collector through the lower outlet, and the electrode active material slurry forming an upper slurry layer is transported to the lower slurry layer on the current collector through the upper outlet, and the upper outlet is spaced apart from the lower outlet downstream in the coating direction.

3. The dual-slit die coater according to claim 1, wherein the distance between the current collector and the upper die lip is greater than the distance between the current collector and the lower die lip and the distance between the current collector and the intermediate die lip.

4. The dual-slit die coater according to claim 1, wherein the current collector moves by the rotation of a circular coating roll having a diameter of 350 to 400 mm, and the current collector has a curvature according to the curvature of the coating roll, and the thicknesses of the lower die lip, the intermediate die lip, and the upper die lip are such that the average increase in curvature at positions corresponding to the top of the lower plate, the bottom of the intermediate plate, the top of the intermediate plate, and the bottom of the upper plate does not exceed 50 μm, the top of the lower plate being the region of the lower die lip located on the most downstream side along the moving direction of the current collector, the bottom of the intermediate plate being the region of the intermediate die lip located on the most upstream side along the moving direction of the current collector, the top of the intermediate plate being the region of the intermediate die lip located on the most downstream side along the moving direction of the current collector, and the bottom of the upper plate being the region of the upper die lip located on the most upstream side along the moving direction of the current collector.

5. The dual-slit die coater according to claim 1, wherein the thickness of the upper die lip ranges from 0.8 to 4.5 mm, the thickness of the middle die lip ranges from 0.8 to 8.8 mm, and the thickness of the lower die lip ranges from 1 to 4.5 mm.

6. A method for coating an electrode active material slurry, comprising: forming an electrode active material slurry on the current collector by using the dual-slit die coater according to any one of claims 1 to 5 while supplying the electrode active material slurry while moving the current collector from the lower die lip to the upper die lip.

7. The method for coating an electrode active material slurry according to claim 6, wherein the electrode active material slurry layer is formed to have a thickness of 60 μm or greater.

8. The method for coating an electrode active material slurry according to claim 6, wherein the electrode active material slurry has a viscosity of 1000 cps or greater.

9. A method for coating an electrode active material slurry, the method using a dual-slit die coater including a lower slit and an upper slit for simultaneously extruding and coating two types of electrode active material slurries on the surface of a continuously moving current collector through the lower slit and the upper slit, the dual-slit die coater including a lower plate, an intermediate plate located on the lower plate, and an upper plate located on the intermediate plate, the lower slit being formed between the lower plate and the intermediate plate, and the upper slit being formed between the intermediate plate and the upper plate, wherein the lower plate, the intermediate plate, and the upper plate have a lower die lip, an intermediate die lip, and an upper die lip, and each of the lower die lip, the intermediate die lip, and the upper die lip forms a front end with respect to the current collector, and the thickness of the lower die lip is greater than the thickness of the upper die lip and the thickness of the intermediate die lip, wherein the ratio of the thickness of the lower die lip: the thickness of the upper die lip is 1.2:1 or greater, or wherein the ratio of the thickness of the lower die lip: the thickness of the intermediate die lip is 1.2:1 or greater, and wherein the thickness of the thinner die lip between the upper die lip and the intermediate die lip is 800 μm or greater, the method comprising: simultaneously conveying the two types of electrode active material slurries onto the current collector moving in the direction from the lower die lip to the upper die lip through a lower outlet and an upper outlet to form a double-layer structure including a lower slurry layer and an upper slurry layer coated on the lower slurry layer, wherein the lower outlet communicating with the lower slit is formed between the lower die lip and the intermediate die lip, the upper outlet communicating with the upper slit is formed between the intermediate die lip and the upper die lip, and the upper outlet is spaced apart from the lower outlet downstream in the coating direction.

10. The method for coating an electrode active material slurry according to claim 9, wherein each of the lower slurry layer and the upper slurry layer is formed to have a thickness of 60 μm or greater.

11. The method for coating an electrode active material slurry according to claim 9, wherein the electrode active material slurry has a viscosity of 1000 cps or greater.

12. The method for coating an electrode active material slurry according to claim 9, wherein the flow rate ratio of the electrode active material slurry forming the lower slurry layer to the electrode active material slurry forming the upper slurry layer is 1:1.

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