Multi-Slot Die Coater

Abstract

A multi-slot die coater has improved a deviation in a width direction of a slot gap caused by a structural feature including a thin die block. The multi-slot die coater including a lower slot and an upper slot includes a lower die block; an intermediate die block disposed on the lower die block to form the lower slot therebetween; and an upper die block disposed on the intermediate die block to form the upper slot therebetween, wherein the intermediate die block comprises a manifold that accommodates a coating solution, and is communicatively connected to the upper slot, and wherein a shortest distance from distances from any points on a bottom surface of the manifold to any points on the second surface is equal to or greater than 10 mm.

Claims

1. A multi-slot die coater comprising: a lower slot; an upper slot; a lower die block; an intermediate die block disposed on the lower die block to form the lower slot therebetween; and an upper die block disposed on the intermediate die block to form the upper slot therebetween, wherein the intermediate die block comprises a manifold that is a space provided from a first surface facing the upper die block toward a second surface that is an opposing surface of the first surface, the manifold accommodates a coating solution, and is communicatively connected to the upper slot, and wherein a shortest distance from a distances from any points on a bottom surface of the manifold to any points on the second surface is equal to or greater than 10 mm.

2. The multi-slot die coater of claim 1, wherein an angle formed by the lower slot and the upper slot is 20 degrees to 70 degrees.

3. The multi-slot die coater of claim 1, wherein the lower die block, the intermediate die block, and the upper die block respectively comprise a lower die lip, an intermediate die lip, and an upper die lip forming front end portions thereof, a lower discharge port communicatively connected to the lower slot is formed between the lower die lip and the intermediate die lip, and an upper discharge port communicatively connected to the upper slot is formed between the intermediate die lip and the upper die lip, the multi-slot die coater is configured to extrude and apply an electrode active material slurry on a surface of a continuously traveling substrate through at least one of the lower slot and the upper slot, and an angle formed by the first surface and the second surface is determined so that an angle formed by the lower discharge port and the upper discharge port is within a range in which an electrode active material slurry discharged from the upper discharge port and an electrode active material slurry discharged from the lower discharge port do not form a vortex immediately after being simultaneously discharged.

4. The multi-slot die coater of claim 1, wherein the shortest distance is determined so that a deviation of each slot gap obtained by measuring a slot gap of the lower slot and a slot gap of the upper slot for each position in a width direction of the multi-slot die coater is within 10 μm.

5. The multi-slot die coater of claim 1, wherein the multi-slot die coater is configured to extrude and apply an electrode active material slurry on a surface of a continuously traveling substrate through at least one of the lower slot and the upper slot, a direction in which the electrode active material slurry is discharged lies almost horizontally such that the multi-slot die coater is installed so that the first surface lies almost horizontally and an opposite side of the upper die block facing the first surface also lies almost horizontally, and surfaces of the lower die block, the intermediate die block, and the upper die block opposite to the direction in which the electrode active material slurry is discharged lie almost vertically.

6. The multi-slot die coater of claim 1, wherein the intermediate die block comprises a first intermediate die block and a second intermediate die block in face-to-face contact with each other up and down and slidably connected along a contact surface to be movable relative to each other, and wherein the second intermediate die block faces the upper die block, and the manifold is provided on the second intermediate die block.

7. The multi-slot die coater of claim 6, wherein the first intermediate die block is fixedly coupled to the lower die block, and the second intermediate die block is fixedly coupled to the upper die block.

8. The multi-slot die coater of claim 1, wherein the lower die block, the intermediate die block, and the upper die block respectively comprise a lower die lip, an intermediate die lip, and an upper die lip forming front end portions thereof, a lower discharge port communicatively connected to the lower slot is formed between the lower die lip and the intermediate die lip, an upper discharge port communicatively connected to the upper slot is formed between the intermediate die lip and the upper die lip, and a predetermined step is formed between the lower discharge port and the upper discharge port.

9. The multi-slot die coater of claim 1, further comprising: a first spacer interposed between the lower die block and the intermediate die block to adjust a width of the lower slot; and a second spacer interposed between the intermediate die block and the upper die block to adjust a width of the upper slot.

10. The multi-slot die coater of claim 1, wherein the lower die block comprises a first manifold that is a space provided from a surface facing the intermediate die block toward an opposite surface that is an opposing surface of the surface, accommodates a first coating solution, and is communicatively connected to the upper slot, and wherein the manifold included in the intermediate die block is a second manifold that accommodates a second coating solution.

11. The multi-slot die coater of claim 10, wherein a shortest distance among a distance from any points on a bottom surface of the first manifold to a surface of the lower die block is equal to or greater than 10 mm.

Description

DESCRIPTION OF DRAWINGS

[0034] The accompanying drawings illustrate a preferred embodiment of the present disclosure and together with the foregoing disclosure, serve to provide further understanding of the technical features of the present disclosure, and thus, the present disclosure is not construed as being limited to the drawings.

[0035] FIG. 1 is a schematic diagram showing an example of using a slot die coater according to the conventional art.

[0036] FIG. 2 is a cross-sectional view of a dual slot die coater according to the conventional art.

[0037] FIG. 3 is a schematic cross-sectional view of a multi-slot die coater according to an embodiment of the present disclosure.

[0038] FIG. 4 is a schematic exploded perspective view of a multi-slot die coater according to an embodiment of the present disclosure.

[0039] FIG. 5 is a schematic cross-sectional view of a multi-slot die coater according to another embodiment of the present disclosure.

[0040] FIG. 6 is a diagram illustrating a case in which a positional difference between an upper discharge port and a lower discharge port occurs due to a relative movement between a lower die block and an upper die block in the multi-slot die coater of FIG. 5.

[0041] FIG. 7 shows a coating aspect according to a change in an angle of a die block in a comparative example.

[0042] FIG. 8 is a cross-sectional view of side and center parts in a width direction in the comparative example.

[0043] FIG. 9 is a graph of a slot gap in a TD according to a fastening strength in the comparative example.

BEST MODE

[0044] Hereinafter, preferred embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. Prior to the description, it should be understood that the terms used in the specification and the appended claims should not be construed as limited to general and dictionary meanings, but interpreted based on the meanings and concepts corresponding to technical aspects of the present disclosure on the basis of the principle that the inventor is allowed to define terms appropriately for the best explanation. Therefore, the description proposed herein is just a preferable example for the purpose of illustrations only, not intended to limit the scope of the disclosure, so it should be understood that other equivalents and modifications could be made thereto without departing from the scope of the disclosure.

[0045] A multi-slot die coater according to an embodiment of the present disclosure may include two or more slots. Basically, the multi-slot die coater is an apparatus including a lower slot and an upper slot and coating a coating solution in a double layer on a substrate. The ‘substrate’ described below is a current collector and the coating solution is an ‘electrode active material slurry’. Both a first coating solution and a second coating solution are electrode active material slurries, and may mean electrode active material slurries that have the same or different composition (types of an active material, a conductive material, and a binder), content (an amount of each of the active material, the conductive material, and the binder), or physical properties. The multi-slot die coater according to an embodiment of the present disclosure is optimized for electrode manufacturing in which coating is performed by simultaneously or pattern- coating is performed by alternately applying two or more types of electrode active material slurries. However, the scope of the present disclosure is not necessarily limited thereto. For example, the substrate may be a porous scaffold constituting a separation membrane, and the first coating solution and the second coating solution may be organic matters having different compositions or physical properties. That is, when thin film coating is required, any substrate, any first coating liquid, and any second coating liquid may be good.

[0046] The multi-slot die coater may usually be made of a SUS material. In general, since liquid leakage easily occurs on a coupling surface of an SUS assembly, a rubber ring or other soft material is placed between components to seal the coupling surface and suppress the leakage. However, such a sealing method is not suitable for controlling a uniform assembly form (e.g., an assembly deviation less than 10 μm), and thus it is difficult to apply the sealing method to the multi-slot die coater.

[0047] For this reason, in the multi-slot die coater, a die block processed with a very high precision (straightness and flatness ±5 μm) is bolt-fastened and assembled. Bolt-fastening is performed at a high pressure of about 200 to 350 N since the liquid leakage should be prevented. However, such high pressure bolt-fastening causes a stress imbalance to minutely occur, and accordingly, deformation of the block die may be induced, and deformation or torsion of the die block is also caused by the pressure of a coating solution supplied during coating. The inventors of the present disclosure have led to the present disclosure by discovering that when the die block is manufactured so that a distance between a manifold provided in the die block and a surface of the die block is a distance that causes deformation beyond the acceptable level, an amount of deformation increases and a non-uniform coating amount is coated, and conducting research to determine an optimal distance.

[0048] The present disclosure proposes the distance (the shortest distance) between the manifold provided in the die block and the surface of the die block, which causes only the acceptable level of deformation.

[0049] FIG. 3 is a schematic cross-sectional view of a multi-slot die coater according to an embodiment of the present disclosure. FIG. 4 is a schematic exploded perspective view of a multi-slot die coater according to an embodiment of the present disclosure.

[0050] A multi-slot die coater 100 according to an embodiment of the present disclosure is a dual slot die coater including a lower slot 101 and an upper slot 102 and is an apparatus capable of simultaneously or alternately coating two types of same or different coating solutions on a substrate 300 through the lower slot 101 and the upper slot 102. Referring to FIGS. 3 and 4, the multi-slot die coater 100 includes a lower die block 110, an intermediate die block 120 disposed on an upper portion of the lower die block 110, and an upper die block 130 disposed on an upper portion of the intermediate die block 120.

[0051] In FIG. 3, the multi-slot die coater 100 is installed in a substantially horizontal direction (X direction) in which an electrode active material slurry which is a coating solution is discharged (approximately: ±5 degrees).

[0052] The intermediate die block 120 is a block located in the middle of blocks constituting the multi-slot die coater 100, and is a block disposed between the lower die block 110 and the upper die block 130 to form a dual slot. A cross section of the intermediate die block 120 of the present embodiment is a right triangle, but is not necessarily limited to such as shape. For example, the cross section may be provided as an isosceles triangle.

[0053] A first surface 120a of the intermediate die block 120 facing the upper die block 130 lies almost horizontally, and a surface 130d (that is, a surface forming an upper surface of an outer circumferential surface of the multi-slot die coater 100) opposite to a surface 130b of the upper die block 130 facing the first surface 120a also lies almost horizontally. In this way, the first surface 120a and the opposite surface 130d are almost parallel to each other. And a surface 110d (that is, a surface forming a lower surface of the outer circumferential surface of the multi-slot die coater 100) opposite to a surface 110b of the lower die block 110 facing the intermediate die block 120 also lies almost horizontally, and this surface is a bottom surface 110d (X-Z plane). Surfaces of the lower die block 110, the intermediate die block 120, and the upper die block 130 which are opposite to a direction in which the electrode active material slurry is discharged, that is, rear surfaces 110c, 120c, and 130c, lie almost vertically (Y direction).

[0054] In a surface forming the outer circumferential surface of the multi-slot die coater 100 in the lower die block 110 and the upper die block 130 which are the outermost die blocks, the bottom surface 110d of the lower die block 110 and the upper surface 130d of the upper die block 130 manufactured to be almost vertical to the rear surfaces 110c and 130c may be used. And, the first surface 120a of the intermediate die block 120 manufactured to be almost vertical to the rear surface 120c may be used. In such die blocks 110, 120, and 130, since corners formed by surfaces are formed at right angles, there is a right angle portion in the cross section, and since a vertical or horizontal surface may be used as a reference surface, manufacturing or handling of the die blocks 110, 120, and 130 are easy and the precision thereof is guaranteed. In addition, a state in which the lower die block 110, the intermediate die block 120, and the upper die block 130 are combined has an approximately rectangular parallelepiped shape as a whole, and only a front portion from which the coating solution is discharged is inclined toward the substrate 300. This is advantageous in that a shape after assembly is approximately similar to that of a slot die coater including a single slot (e.g., 30 in FIG. 1) so that a slot die coater stand, etc. may be shared.

[0055] The multi-slot die coater 100 may further include two or more fixing units 140 provided on the rear surfaces 110c, 120c, and 130c thereof. The fixing units 140 are provided for fastening between the lower die block 110 and the intermediate die block 120 and for fastening between the intermediate die block 120 and the upper die block 130. A plurality of fixing units 140 may be provided in the width direction of the multi-slot die coater 100. Bolts are fastened to the fixing units 140, through which the lower die block 110, the intermediate die block 120, and the upper die block 130 are assembled with each other.

[0056] The lower die block 110, the intermediate die block 120, and the upper die block 130 are not necessarily limited to shapes of the above examples, and may be configured, for example, as vertical dies with the direction in which the electrode active material slurry is discharged as an upper direction and the rear surfaces 110c, 120c, and 130c as bottom surfaces.

[0057] The die blocks 110, 120, and 130 are made of, for example, a SUS material. Materials that are easy to process, such as SUS420J2, SUS630, SUS440C, SUS304, and SUS316L, may be used. SUS has advantages in that it is easy to process, inexpensive, has high corrosion resistance, and may be manufactured in a desired shape at low cost.

[0058] The lower die block 110 is a lowermost block among the blocks constituting the multi-slot die coater 100, and the surface 110b facing the intermediate die block 120 has an inclined shape to form an angle of approximately 20 degrees to 60 degrees with respect to the bottom surface 110d.

[0059] The lower slot 101 may be formed where the lower die block 110 and the intermediate die block 120 face each other. For example, a first spacer 113 is interposed between the lower die block 110 and the intermediate die block 120 to provide a gap therebetween, so that the lower slot 101 corresponding to a passage through which the first coating solution 50 may flow may be formed. In this case, the thickness of the first spacer 113 determines the vertical width (Y-axis direction and a slot gap) of the lower slot 101. However, conventional die blocks were vulnerable to deformation and torsion, making it difficult to maintain the slot gap.

[0060] As shown in FIG. 4, the first spacer 113 includes a first opening portion 113a in which one area is cut and may be interposed in the remaining portion except for one side in a border area of an opposite surface of each of the lower die block 110 and the intermediate die block 120. Accordingly, a lower discharge port 101a through which the first coating solution 50 may be discharged to the outside is formed only between a front end portion of the lower die block 110 and a front end portion of the intermediate die block 120. The front end portion of the lower die block 110 and the front end portion of the intermediate die block 120 are defined as a lower die lip 111 and an intermediate die lip 121, respectively. In other words, the lower discharge port 101a may be formed by making the lower die lip 111 and the intermediate die lip 121 spaced apart from each other.

[0061] For reference, the first spacer 113 functions as a gasket to prevent the first coating solution 50 from leaking into a gap between the lower die block 110 and the intermediate die block 120 except for the area where the lower discharge port 101a is formed, and thus the first spacer 113 is preferably made of a material having sealing properties.

[0062] The lower die block 110 includes a first manifold 112 having a predetermined depth on the surface 110b facing the intermediate die block 120 and communicatively connected to the lower slot 101. The first manifold 112 is a space provided from the surface 110b of the lower die block 110 facing the intermediate die block 120 toward the opposite surface 110d that is opposite to the surface 110b. The first manifold 112 is connected to a first coating solution supply chamber (not shown) installed outside through a supply pipe to receive the first coating solution 50. When the first coating solution 50 is filled in the first manifold 112, the flow of the first coating solution 50 is induced along the lower slot 101 and discharged to the outside through the lower discharge port 101a.

[0063] The upper die block 130 is disposed to face the first surface 120a which is an upper surface of the intermediate die block 120 that is horizontal with respect to a bottom surface. The upper slot 102 is thus formed where the intermediate die block 120 and the upper die block 130 face to each other.

[0064] Like the lower slot 101 described above, a second spacer 133 may be interposed between the intermediate die block 120 and the upper die block 130 to provide a gap therebetween. Accordingly, the upper slot 102 corresponding to a passage through which a second coating solution 60 may flow is formed. In this case, a vertical width (Y-axis direction and a slot gap) of the upper slot 102 is determined by the second spacer 133. However, conventional die blocks were vulnerable to deformation and torsion, making it difficult to maintain the slot gap.

[0065] In addition, the second spacer 133, which also has a structure similar to that of the above-described first spacer 113, includes a second opening portion 133a in which one area is cut, and may be interposed in the remaining portion except for one side in a border area of an opposite surface of each of the intermediate die block 120 and the upper die block 130. Similarly, a circumferential direction of the second spacer 133 except for the front of the upper slot 102 is blocked, and the upper discharge port 102a is formed only between the front end portion of the intermediate die block 120 and a front end portion of the upper die block 130. The front end portion of the upper die block 130 is defined as an upper die lip 131. In other words, the upper discharge port 102a may be formed by making the intermediate die lip 121 and the upper die lip 131 spaced apart from each other.

[0066] In addition, the intermediate die block 120 includes a second manifold 132 having a predetermined depth on the surface 120a facing the upper die block 130 and communicatively connected to the upper slot 102. The intermediate die block 120 has a second surface 120b that is an opposite surface of the first surface 120a. The second surface 120b is also a surface of the intermediate die block 120 facing the lower die block 110. The second manifold 132 is a space provided from the first surface 120a toward the second surface 120b. Although not shown in the drawings, the second manifold 132 is connected to a supply chamber of the second coating solution 60 installed outside through a supply pipe to receive the second coating solution 60. When the second coating solution 60 is supplied from the outside along the supply pipe in the shape of a pipe and filled in the second manifold 132, the flow of the second coating solution 60 is induced along the upper slot 102 communicatively connected to the second manifold 132 and discharged to the outside through the upper discharge port 102a.

[0067] A shape of the second manifold 132 may vary. The cross-sectional area and the length in the width direction may also vary. The bottom surface of the second manifold 132 may or may not be parallel to the second surface 120b, and may or may not include an inclined portion.

[0068] The upper slot 102 and the lower slot 101 form a certain angle, and the angle may be approximately 20 degrees to 70 degrees. The upper slot 102 and the lower slot 101 may intersect each other at one point, and the upper discharge port 102a and the lower discharge port 101a may be provided near the intersection point. Accordingly, discharge points of the first coating solution 50 and the second coating solution 60 may be concentrated approximately at one point.

[0069] According to the multi-slot die coater 100 having such a configuration, a rotatably provided coating roll 200 is disposed in front of the multi-slot die coater 100, the substrate 300 to be coated by rotating the coating roll 200 is driven, the first coating solution 50 and the second coating solution 60 are continuously contacted with the surface of the substrate 300 so that the substrate 300 may be coated in a double layer. Alternatively, supply and interruption of the first coating solution 50 and supply and interruption of the second coating solution 60 are alternately performed so that pattern-coating may be intermittently performed on the substrate 300.

[0070] Here, distances from any points on the bottom surface of the second manifold 132 to any points on the second surface 120b is important, and among the distances, a shortest distance P should be equal to or greater than 10 mm. Here, the shortest distance P is determined so that a deviation of each slot gap obtained by measuring a slot gap of the lower slot 101 and a slot gap of the upper slot 102 for each position in the width direction of the multi-slot die coater 100 is within 10 μm. The deviation of the slot gap may indicate a difference between a largest slot gap and a smallest slot gap. Usually, the slot gap is the largest at the center in the width direction, and the slot gap is smaller toward the side in the width direction. When the deviation of each slot gap is greater than 10 μm, it is determined that loading in the width direction is non-uniform. Since the non-uniform loading in the width direction leads to a non-uniform capacity when an electrode is manufactured as a secondary battery in the future, it is not preferable in terms of the characteristics of the secondary battery. Accordingly, in an embodiment of the present disclosure, the acceptable level is managed so that the deviation of the slot gap is within 10 μm on the basis that the deviation of the slot gap is 10 μm. 10 mm that is the minimum value of the shortest distance P may be determined in consideration of the deviation of the slot gap.

[0071] Meanwhile, an angle θ between the first surface 120a of the intermediate die block 120 facing the upper die block 130 and the second surface 120b of the intermediate die block 120 facing the lower die block 110, that is, an angle of the intermediate die block 120, is preferably within a range in which an electrode active material slurry discharged from the upper discharge port 102a and an electrode active material slurry discharged from the lower discharge port 101a do not form a vortex immediately after being simultaneously discharged.

[0072] When the angle θ is too small, the intermediate die block 120 is so thin as to be vulnerable to deformation and torsion. And it is difficult to set the shortest distance P to be equal to or greater than 10 mm. When the depth of the second manifold 132 is fixed, since the intermediate die block 120 is thicker as the angle θ increases, the shortest distance P increase, which will be advantageous in terms of deformation and torsion.

[0073] The upper limit of the angle θ may be determined as follows. The sum of all of an angle between the surface 130b of the upper die block 130 facing the first surface 120a and the other surface 130a adjacent to the surface 130b, that is, the angle of the upper die block 130, the angle of the lower die block 110 that is an angle between the surface 110b of the lower die block 110 facing the second surface 120b and the other surface 110a adjacent to the surface 110b, and the angle of the intermediate die block 120 may be set a maximum of 180 degrees. Then, the upper limit of the angle of the intermediate die block 120 is a value obtained by subtracting the angle of the upper die block 130 and the angle of the lower die block 110 from 180 degrees. The angle of the upper die block 130 and the angle of the lower die block 110 may be determined in various ways.

[0074] However, since the angle θ also affects an angle between the lower discharge port 101a and the upper discharge port 102a, the angle θ may not be large. Therefore, in an embodiment of the present disclosure, the angle between the lower discharge port 101a and the upper discharge port 102a may be determined within a range in which an electrode active material slurry discharged from the upper discharge port 102a and an electrode active material slurry discharged from the lower discharge port 101a do not form a vortex immediately after being simultaneously discharged, and accordingly, the angle θ formed between the first surface 120a and the second surface 120b may be determined.

[0075] As such, when the depth of the second manifold 132 is fixed, the shortest distance P may be controlled by adjusting the angle θ. When the angle θ needs to be fixed, the shortest distance P may be controlled by adjusting the depth of the second manifold 132. The shape of the intermediate die block 120, the thickness of the intermediate die block 120 (the angle θ of the intermediate die block 120), and the shape of the second manifold 132, particularly, the depth, are factors that may be changed at any time. Therefore, there is an advantage that various equipment designs are possible by freely changing these factors while keeping in mind that the shortest distance P is equal to or greater than 10 mm.

[0076] The shortest distance P, which is a distance from the bottom surface of the second manifold 132 in the intermediate die block 120 to the surface of the intermediate die block 120, is proposed to be managed as the shortest distance P in the intermediate die block 120 that may be structurally the thinnest. However, a distance between the bottom surface of the first manifold 112 provided in the lower die block 110 and the surface of the lower die block 110 may also be equal to or greater than 10 mm. Similarly, whether the manifold is provided on any die block such as the upper die block, the intermediate die block, or the lower die block, the length of the nearest distance among distances from any points on a surface forming the bottom surface of the manifold to any points on a neighboring surface may be equal to or greater than 10 mm. Here, the neighboring surface is an outer surface of the die block on which the manifold is provided.

[0077] As described above, according to the present disclosure, the shortest distance P among the distances between the bottom surface of the second manifold 132 formed in the die block, in particular the intermediate die block 120, and the surface 120b of the intermediate die block 120 is proposed, so as to improve the deformation or torsion of the die block which is structurally vulnerable due to a thin thickness. The present disclosure proposes that the shortest distance P is equal to or greater than 10 mm to cause only the acceptable level of deformation. According to the present disclosure, even considering that the die block is deformed by the force of the fastening bolt and the pressure of the discharged electrode active material slurry, there is an effect of uniformly controlling the coating amount by maintaining a uniform (±2%) slot gap.

[0078] When a bolt fastening pressure is lowered by reducing the bolt force to minimize the deformation of the die block, the electrode active material slurry may leak from a coupling surface between die blocks. Since the die block of the present disclosure manages the control factor that is the shortest distance that minimizes deformation, the die block may be used without leakage of the electrode active material slurry by sufficiently applying the bolt force.

[0079] Conventionally, a slot gap was changed even by the pressure of the electrode active material slurry, and in particular, there was a big difference in the slot gap between the side and the center in the TD. Since the die block of the present disclosure has the shortest distance P that minimizes deformation, even if deformation occurs due to the pressure of the supplied electrode active material slurry, the deformation occurs within the acceptable level, and thus it is possible to obtain a coating product of uniform quality, in particular, an electrode for a secondary battery by using the multi-slot die coater with the uniform slot gap.

[0080] When the shortest distance P is equal to or greater than 10 mm, the effect of maintaining the slot gap in the width direction uniformly is excellent even though the cross-sectional area of the second manifold 132 is equal to or greater than 300 mm.sup.2, the length in the width direction length is equal to or greater than 250 mm, and the pressure of the second coating solution 60 inside the second manifold 132 is tens to hundreds of kPa. In the past, there was a problem that the slot gap is easily changed even by a slurry discharge pressure. According to the present disclosure, it is possible to suppress the change in the slot gap due to the slurry discharge pressure by merely controlling the shortest distance P to be equal to or greater than a certain level. Conventionally, deformation of the die block of the multi-slot die coater is affected by the discharge pressure of a slurry. Since the amount of deformation is proportional to the pressure but also proportional to the area to which the pressure is applied, when a manifold having a large cross-sectional area is used, there was a problem in that the amount of deformation increases. The present disclosure is innovative because the present disclosure proposes that it is possible to suppress the change in the slot gap only as long as the shortest distance P is equal to or greater than 10 mm, regardless of the cross-sectional area of the manifold or the slurry discharge pressure.

[0081] As described above, according to the present disclosure, it is possible to limit the minimum acceptable range of deformation and torsion by controlling the distance from the bottom surface of the manifold to the surface of the die block. Even if the discharge pressure of the electrode active material slurry increases, and even if a larger and wider manifold is used, the effect of maintaining the once adjusted slot gap is excellent. This has the effect of securing coating process ability and securing reproducibility.

[0082] Using such a multi-slot die coater, it is possible to uniformly form a coating layer, in particular, an electrode active material layer, to a desired thickness, and preferably, simultaneous coating of two or more types of electrode active material slurries is possible, and thus there are effects that both performance and productivity are excellent.

[0083] Meanwhile, in the present embodiment, the case of applying the coating solution in two layers or the case of performing pattern-coating by alternately supplying the coating solution has been described as an example, but it will be understood without separate explanation that it is also applied to the case where three or more layers are simultaneously applied by providing three or more slots. It will be understood without detailed explanation that four or more die blocks are required to provide three or more slots.

[0084] Next, another embodiment of the present disclosure will be described with reference to FIGS. 5 and 6. The same reference numerals as in the above-described embodiment denote the same members, and redundant descriptions of the same members will be omitted, and differences from the above-described embodiment will be mainly described.

[0085] In the above-described embodiment, the intermediate die block 120 includes one block, so that relative positions of the upper discharge port 102a and the lower discharge port 101a may not be variably adjusted, but according to another embodiment of the present disclosure, the relative positions of the upper discharge port 102a and the lower discharge port 101a may be easily adjusted.

[0086] To this end, in a multi-slot die coater 100′ according to another embodiment of the present, the intermediate die block 120 includes a first intermediate die block 122 and a second intermediate die block 124, and the first intermediate die block 122 and the second intermediate die block 124 face-to-face contact with each other up and down, but slide along a contact surface to be movable relative to each other. And, the first intermediate die block 122 is fixedly coupled to the lower die block 110 by bolt coupling, etc., and the second intermediate die block 124 is fixedly coupled to the upper die block 130 by bolt coupling, etc. Accordingly, the first intermediate die block 122 and the lower die block 110 may move integrally, and the second intermediate die block 124 and the upper die block 130 may move integrally.

[0087] That is, like the multi-slot die coater 100, the multi-slot die coater 100′ further includes two or more first fixing units 140′ provided on the rear surfaces 110c, 120c, and 130c thereof. The first fixing units 140′ are provided for fastening the lower die block 110 and the first intermediate die block 122 and for fastening the second intermediate die block 124 and the upper die block 130. A second fixing unit 140″ fastens the first intermediate die block 122 and the second intermediate die block 124, and serves to fasten the lower die block 110 and the upper die block 130. The second fixing unit 140″ is installed at a certain level of assembly tolerance (a range of about 300 μm to about 500 μm) in consideration of the fact that the first intermediate die block 122 and the second intermediate die block 124 must be relatively movable. That is, the second fixing unit 140″ fixes the first intermediate die block 122 and the second intermediate die block 124, so as to prevent a movement above a certain level therebetween by allowing the first intermediate die block 122 and the second intermediate die block 124 to move forward or backward to be slidable and fixed, and allows a fine movement due to the assembly tolerance.

[0088] In the multi-slot die coater 100′, if necessary, the two discharge ports 101a and 102a may be spaced apart from each other in a horizontal direction to be arranged back and forth.

[0089] That is, as shown in FIGS. 5 and 6, a separate apparatus for adjusting the shape of the multi-slot die coater 100′ is used, or an operator may make the relative movement of the lower die block 110 and the upper die block 130 by hand (the second fixing unit 140″ is omitted in FIG. 6 for convenience of illustration).

[0090] For example, in a state where the lower die block 110 does not move and is left as it is, a step between the lower discharge port 101a and the upper discharge port 102a may be formed by moving the upper die block 130 along a sliding surface by a certain distance D backward or forward opposite to a discharge direction of the coating solutions 50 and 60. Here, the sliding surface means an opposite surface of the first intermediate die block 122 and the second intermediate die block 124.

[0091] The width D of the step formed as described above may be determined within the range of approximately several hundred micrometers to several millimeters, which may be determined according to the physical properties and viscosity of the first coating solution 50 and the second coating solution 60 formed on the substrate 300, or a desired thickness for each layer on the substrate 300. For example, as the thickness of a coating layer to be formed on the substrate 300 increases, a numerical value of the width D of the step may increase.

[0092] In addition, as the lower discharge port 101a and the upper discharge port 102a are arranged at positions spaced apart from each other in the horizontal direction, there is no concern that the second coating solution 60 discharged from the upper discharge port 102a flows into the lower discharge port 101a or the first coating solution 50 discharged from the lower discharge port 101a flows into the upper discharge port 102a.

[0093] That is, there is no concern that the coating solution discharged through the lower discharge port 101a or the upper discharge port 102a is blocked by a surface forming the step formed between the lower discharge port 101a and the upper discharge port 102a and flows into the other discharge port, whereby a more smooth multi-layer active material coating process may proceed.

[0094] The multi-slot die coater 100′ according to another embodiment of the present disclosure as described above may be adjusted simply by the sliding movement of the lower die block 110 and/or the upper die block 130, in a case where the relative position between the lower discharge port 101a and the upper discharge port 102a needs to be changed, and there is no need to disassemble and reassemble each of the die blocks 110, 120, and 130, and thus process ability may be greatly improved.

[0095] Even the multi-slot die coater 100′ including four die blocks has the shortest distance among the distances between the bottom surface of the manifold and the surface of the die block as a control factor.

[0096] In particular, since the second manifold 132 is convexly formed from the surface 124a where the second intermediate die block 124 faces the first intermediate die block 122 toward an opposite surface of the surface 124a which is the second surface 124b where the second intermediate die block 124 faces the first intermediate die block 122, the shortest distance P among the distances from any points on the bottom surface of the second manifold 132 to any points on the second surface 124b is also set to be equal to or greater than 10 mm.

[0097] In the multi-slot die coater 100′ of the present embodiment, since the number of die blocks increases by one compared to the multi-slot die coater 100 of the above-described embodiment, the thickness of the die block may be thinner when the overall volume is maintained. Even so, the shortest distance P is to be equal to or greater than 10 mm.

Consideration of Upper Slot Angle and Lower Slot Angle

[0098] Hereinafter, the dual slot die coater according to the conventional art is described as a comparative example, and the effect of the dual slot die coater according to an embodiment of the present disclosure is described.

[0099] As mentioned above, the original dual slot die coater is structurally vulnerable to deformation and torsion due to a thin thickness of each die block. When the size of the die block is vaguely increased (change in the angle), the discharge direction is changed, which causes a problem in deterioration of coating process ability.

[0100] In the conventional dual slot die coater, in order to confirm the influence of a slot inclination due to the change in the angle of the die block on the electrode active material slurry coating stability, the upper slot and the lower slot were analyzed by inclining each of the upper slot and the lower slot by 30 degrees in a direction away from the intermediate die block.

[0101] FIG. 7 shows a coating aspect according to a change in an angle of a die block in a comparative example.

[0102] Case 1 is that the upper slot has an angle of 120 degrees with respect to a surface of a substrate and the lower slot has an angle of 90 degrees with respect to the surface of the substrate by inclining the upper slot by 30 degrees in a direction away from the intermediate die block, Case 2 is that all slots are upright with respect to the surface of the substrate so that each of the upper slot and the lower slot has an angle of 90 degrees with respect to the surface of the substrate, and Case 3 is that the upper slot has an angle of 90 degrees with respect to the surface of the substrate and the lower slot has an angle of 60 degrees with respect to the surface of the substrate by inclining the lower slot by 30 degrees in a direction away from the intermediate die block. A state of each of electrode active material slurry coating layers is as shown in FIG. 7. Positions of a coating bead and a separation point are shown in FIG. 7. FIG. 7 shows the coating aspect according to the change in the angle of the die block in a comparative example. In FIG. 7, (a), (b), and (c) represent case 1, case 2, and case 3, respectively.

[0103] The positions of the coating bead and the separation point for each case are summarized in Table 1.

TABLE-US-00001 TABLE 1 Position of Position of Angle of Angle of coating bead separation point lower slot upper slot (μm) (μm) Case 1 90 degrees 120 degrees  165 102 Case 2 90 degrees 90 degrees 115 62 Case 3 60 degrees 90 degrees 49 61

[0104] Coating stability may be determined by the positions of the coating bead and the separation point.

[0105] As a result of changing the angle of the upper slot and the angle of the lower slot as described above, as to the coating stability, case 2 was the same as or better than case 3 and case 1 was the worst (case 2≥case 3>case 1). Compared to case 2 in which all slots are upright, case 1 and case 2 relate to a V-shaped solution supply method by inkling either slot. Such a solution supply method has a problem in that it is difficult to form a double layer because intermixing occurs due to vortex formation in a region where upper and lower slurries meet. When the size of the die block is increased and the angle is changed as described above, the discharge direction is changed, which causes deterioration of coating process ability.

[0106] As to deformation vulnerability, case 1 and case 3 were similar, and were very vulnerable compared to case 2 (case 1=case 3>>case 2). That is, when the angle is changed as in case 1 and case 3, the intermediate die block is so thin as to be vulnerable. That is, even if deformation and torsion are improved by increasing thicknesses of the upper/lower die blocks located outside among the three die blocks, it is still difficult to compensate for the deformation of the structurally most vulnerable intermediate die block. Therefore, it may be seen that it is difficult to block deformation and torsion through a simple change in the angle. In the present disclosure, a minimum acceptable range of deformation and distortion is limited by controlling the distance between the bottom surface of the manifold and the surface of the die block, and thus the present disclosure is superior to the conventional art.

[0107] Confirmation of problem of deviation of slot gap of conventional dual slot die coater

[0108] FIG. 8 is a cross-sectional view of side and center parts in a width direction in case 3 among the comparative examples.

[0109] FIGS. 8(a) and 8(b) show cross-sections of a dual slot die coater of the side and the center parts, respectively. In the drawings, a degree of deformation is indicated for each position in the die block. The degree of deformation is {circle around (1)}<{circle around (2)}<{circle around (3)}.

[0110] Referring to FIG. 8(a), it may be seen that the degree of deformation of a portion corresponding to the second intermediate die block in the side part is slightly deformed to {circle around (2)}. Referring to FIG. 8(b), it may be seen that the degree of deformation reaches {circle around (3)} near a lip of the second intermediate die block as the deformation further deepens in the center part. Also, it may be seen in the center part that deformation of the degree of deformation {circle around (2)} occurred in a portion corresponding to near a lip of the first intermediate die block. As such, since the deformation of the die block is more severe in the center part, it may be confirmed that a slot gap of the upper slot in the center part is significantly different from a slot gap of the upper slot in the side part. In addition, it may be seen that since the deformation of the first intermediate die block is small in the side part, a change in the slot gap of the lower slot is small compared to a change in the slot gap of the upper slot, whereas since the first intermediate die block is also deformed in the center part, the change in the slot gap of the lower slot also reaches a level not to be ignored.

[0111] As described above, in the conventional dual slot die coater, a difference in the slot gap occurs in the side part and the center part in the width direction, which causes a deviation of the slot gap, and it was difficult to reduce the deviation of the slot gap. However, in the present disclosure, the deviation of the slot gap may be managed within an acceptable level by limiting the shortest distance P in the second intermediate die block that may be the thinnest among the die blocks.

[0112] In the conventional dual slot die coater, slot gaps (the size of a discharge port between the upper die block and the intermediate die block, that is, a gap of the upper slot) according to respective bolt fastening strengths when assembling the upper die block and the intermediate die block are measured in the TD which is the width direction of the die block perpendicular to a traveling direction and are summarized in FIG. 9 and Table 2. FIG. 9 is a graph of a change in the slot gap in the TD according to the fastening strength in the comparative example. In the graph, the X-axis represents a TD displacement measured from one end of the die block, and the Y-axis represents the size of the slot gap.

TABLE-US-00002 TABLE 2 Slot gap Bolt Maximum − Torque Maximum Minimum Minimum (N) (μm) (μm) (μm) Sample 1 350 773 734 39 Sample 2 150 777 760 13 Sample 3 200 778 761 12

[0113] Upon assembling the upper die block and the intermediate die block as in sample 1, when the bolt torque of 350 N is used, a maximum-minimum difference of the slot gap is 39 μm. When the fastening strength is changed to 150 N and 200 N in the order of samples 2 and 3, the maximum-minimum difference is reduced to 13 μm and 12 μm. As described above, in the related art, the change in the slot gap is easy according to the fastening strength. Conventionally, even if the fastening strength was set at 200 N and tightly managed to assemble the upper die block and the intermediate die block, the maximum-minimum difference of 12 μm had to be endured. However, in an embodiment of the present disclosure, the shortest distance P is determined so that the deviation of the slot gap of the multi-slot die coater is within 10 μm. When the deviation of each slot gap is greater than 10 μm, it is determined that loading in the width direction is non-uniform. Since the non-uniform loading in the width direction leads to a non-uniform capacity when an electrode is manufactured as a secondary battery in the future, it is not preferable due to the characteristics of the secondary battery. Accordingly, in the embodiment of the present disclosure, an acceptable level is managed so that the deviation of the slot gap is within 10 μm on the basis that the deviation of the slot gap is 10 μm. Whether there are three or four die blocks, it is determined that the distance between the bottom surface of the manifold and the surface of the die block on which the manifold is provided is equal to or greater than 10 mm in consideration of the deviation of the slot gap.

Confirmation of Effect of Lower Limit of Shortest Distance in Second Intermediate Die Block in Multi-Slot Die Coater Including 4 Die Blocks

[0114] In the same structure as the multi-slot die coater 100′ described with reference to FIG. 5, with respect to cases where the shortest distance in the second intermediate die block is 9 mm (Comparative Example) and 10.5 mm (Embodiment), the deviation of the slot gap of the upper slot and the deviation of the slot gap of the lower slot were measured and shown in Table 3 below. Here, a target value of the slot gap was set to 1 mm, and degrees to which the slot gap increased in the center compared to the side in the width direction according to various discharge pressures of a slurry were measured and summarized.

TABLE-US-00003 TABLE 3 Deviation of slot gap Deviation of slot gap of upper slot (μm) of lower slot (μm) Distance Pressure (kPa) 9 mm 10.5 mm 9 mm 10.5 mm 34 5.2 3.4 4.7 3.1 54 8.1 5.3 7.2 4.8 74 10.7 7.3 9.4 6.6 94 13.3 9.7 11.6 8.8

[0115] In an embodiment of the present disclosure, the acceptable level is on the basis that the deviation of the slot gap of 10 μm. In Table 3, until the pressure is 34 or 54 kPa, even if the shortest distance in the second intermediate die block is 9 mm, there is no problem because the deviation is within 10 μm. The discharge pressure of the slurry may vary depending on the coating conditions (a loading amount and a coating speed), and in general, a pressure equal to or less than 100 kPa is used in a secondary battery. From 74 kPa which is close to 100 kPa, when the shortest distance in the second intermediate die block is 9 mm, the deviation of the slot gap of the upper slot exceeds the acceptable level. In particular, at 94 kPa which is very close to 100 kPa, when the shortest distance in the second intermediate die block is 9 mm, both the deviation of the slot gap of the upper slot and the deviation of the slot gap of the lower slot greatly are beyond 10 μm. In contrast, when the shortest distance in the second intermediate die block is 10.5 mm, both the deviation of the slot gap of the upper slot and the deviation of the slot gap of the lower slot are within 10 μm. Therefore, the shortest distance in the second intermediate die block should be greater than 9 mm and closer to 10.5 mm so that the slot gap may be managed within an acceptable level of deformation.

Confirmation of Effect of Shortest Distance in Multi-Slot Die Coater Including 3 Die Blocks

[0116] As seen in ‘Consideration of angle of upper slot and angle of lower slot’ above, as in case 1, when the upper slot is inclined in a direction away from the intermediate die block, that is, when the angle between the upper slot and the surface of the substrate is greater than 90 degrees, it is not suitable for forming a vortex when the electrode active material slurry (an upper layer slurry) discharged from the upper slot and the electrode active material slurry (a lower layer slurry) discharged from the lower slot meet. Therefore, in the same structure as the multi-slot die coater 100 described with reference to FIG. 3, with respect to the cases where the angle between the upper slot 102 and the surface of the substrate 300, that is, the angle of the upper discharge port 102a is fixed to 90 degrees, the angle of the intermediate die block 120 is set to 10 degrees and the angle of the lower discharge port 101a is set to 80 degrees so that an angle formed by the lower slot and the upper lost is set to 10 degrees (Comparative Example 1-1), the angle of the intermediate die block 120 is set to 10 degrees and the angle of the lower discharge port 101a is set to 70 degrees so that the angle formed by the lower slot and the upper lost is set to 20 degrees (Embodiment 1-1), the angle of the intermediate die block 120 is set to 70 degrees and the angle of the lower discharge port 101a is set to 20 degrees so that the angle formed by the lower slot and the upper lost is set to 70 degrees (Embodiment 1-2), and the angle of the intermediate die block 120 is set to 75 degrees and the angle of the lower discharge port 101a is set to 15 degrees so that the angle formed by the lower slot and the upper lost is set to 75 degrees (Comparative Example 1-2), deviations of the slot gap of the lower slot were measured and shown in Table 4 below. In the respective cases, since the depth of the second manifold 132 is the same, the shortest distance P among distances from any points on the bottom surface of the second manifold 132 to any points on the second surface 124b varies depending on the angle of the intermediate die block 120, and was 9 mm, 12.7 mm, 27.6 mm, and 28.5 mm, respectively. Here, the target value of the slot gap was set to 1 mm, the discharge pressure of the slurry was 100 kPa, and a degree to which the slot gap increased in the center compared to the side in the width direction was measured and summarized. It was assumed that the multi-slot die coater used in the simulation had an overall width of 1500 cm in the TD, a total height of 190 cm from the upper surface of the upper die block to the lower surface of the lower die block, and a front and back length of 190 cm in the MD. It was assumed that the width of a manifold including the slurry in the TD was 1390 cm, and the length of a land portion which is the distance from the end of the manifold to the discharge port was 50 cm. It was assumed that the lower die block and the intermediate die block are fastened with 14 M14 bolts, and the intermediate die block and the upper die block are also fastened with 14 M14 bolts.

TABLE-US-00004 TABLE 4 Comparative Embodiment Embodiment Comparative Example 1-1 1-1 1-2 Example 1-2 Angle of upper 90 degrees 90 degrees 90 degrees 90 degrees discharge port Angle of 10 degrees 20 degrees 70 degrees 75 degrees intermediate die block Shortest 9  12.7  27.6 28.5 distance in intermediate die block (mm) Angle of lower 80 degrees 70 degrees 20 degrees 15 degrees discharge port Deviation of 15.4 8.8 8  11.7 slot gap of Lower slot (μm)

[0117] In an embodiment of the present disclosure, the acceptable level is on the basis that the deviation of the slot gap is 10 μm. In Comparative Example 1-1 and Comparative Example 1-2 of Table 4, the deviation of the slot gap are 15.4 μm and 11.7 μm, respectively, which are beyond 10 μm. However, as proposed in the present disclosure, in Embodiment 1-1 or Embodiment 1-2 in which the angle formed by the lower slot and the upper slot is 20 degrees to 70 degrees and the shortest distance P is equal to or greater than 10 mm, the deviation of the slot gap are 8.8 μm and 8 μm, respectively, which are within 10 μm. Therefore, when the angle between the upper slot 102 and the surface of the substrate 300, that is, the angle of the upper discharge port 102a is fixed to 90 degrees, the angle formed by the lower slot and the upper slot should be 20 degrees to 70 degrees and the shortest distance P should be equal to or greater than 10 mm so that the slot gap may be managed within an acceptance level of deformation.

Confirmation of Effect of Shortest Distance in Multi-Slot Die Coater Including 4 Die Blocks

[0118] In the same structure as the multi-slot die coater 100′ described with reference to FIG. 5, with respect to the cases where the angle between the upper slot 102 and the surface of the substrate 300, that is, the angle of the upper discharge port 102a is fixed to 90 degrees, and each of the angle of the intermediate die block 122 and the angle of the second intermediate die block 124 is set to 10 degrees (Comparative Example 2-1), and 14 degrees (Embodiment 2-1), the angle of the second intermediate die block 124 is set to 62 degrees and the angle of the first intermediate die block 122 is set to 62 degrees (Embodiment 2-2), the angle of the second intermediate die block 124 is set to 70 degrees and the angle of the first intermediate die block 122 is set to 10 degrees (Comparative Example 2-2), deviations of the slot gap of the lower slot were measured and shown in Table 5 below.

[0119] In the respective cases, since the depths of the second manifold 132 and the first manifold 112 are the same, the shortest distance P among the distances from any points on the bottom surface of the second manifold 132 to any points on the second surface 124b varies depending on the angle of the intermediate die block 120, and the shortest distance among distances from any points on the bottom surface of the first manifold 112 to the surface of the lower die block 110 varies depending on the angle of the lower discharge port (or the angle of the lower die block). In respective cases, the shortest distance P among the distances from any points on the bottom surface of the second manifold 132 to any points on the second surface 124b was 9 mm, 10.5 mm, 25.8 mm, and 27.6 mm, respectively. Also, in respective cases, the shortest distance among the distances from the bottom surface of the first manifold 112 provided in the lower die block 110 to the surface of the lower die block 110 was 27.6 mm, 25.8 mm, 10.5 mm, and 9 mm, respectively.

[0120] Here, the target value of the slot gap was set to 1 mm, the discharge pressure of the slurry was 94 kPa, and a degree to which the slot gap increased in the center compared to the side in the width direction was measured and summarized. It was assumed that the multi-slot die coater used in the simulation had an overall width of 1500 cm in the TD, a total height of 190 cm from the upper surface of the upper die block to the lower surface of the lower die block, and a front and back length of 190 cm in the MD. It was assumed that the width of a manifold including the slurry in the TD was 1390 cm, and the length of a land portion which is the distance from the end of the manifold to the discharge port was 50 cm. It was assumed that the lower die block and the first intermediate die block are fastened with 14 M14 bolts, and the second intermediate die block and the upper die block are also fastened with 14 M14 bolts.

TABLE-US-00005 TABLE 5 Comparative Embodiment Embodiment Comparative Example 2-1 2-1 2-2 Example 2-2 Angle of upper 90 degrees 90 degrees 90 degrees 90 degrees discharge port Angle of 10 degrees 14 degrees 62 degrees 70 degrees second intermediate die block Shortest 9  10.5 25.8 27.6 distance in second intermediate die block (mm) Angle of first 10 degrees 14 degrees 14degrees 10 degrees intermediate die block Angle of lower 70 degrees 62 degrees 14 degrees 10 degrees discharge port Shortest 27.6 25.8 10.5 9  distance in lower die block (mm)

[0121] In an embodiment of the present disclosure, the acceptable level is on the basis that the deviation of the slot gap is 10 μm. In Comparative Example 2-1 and Comparative Example 2-2 of Table 5, the deviation of the slot gap are 11.6 μm and 13.5 μm, respectively, which are beyond 10 μm. This is because in Comparative Example 2-1, the shortest distance P among the distances from any points on the bottom surface of the second manifold 132 to any points on the second surface 124b is 9 mm, and in Comparative Example 2-2, the shortest distance among the distances between the bottom surface of the first manifold 112 and the surface of the lower die block 110 is 9 mm, which is smaller than the shortest distance of 10 mm proposed in the present disclosure. However, as proposed in Embodiment 2-1 of the present disclosure, when the shortest distance P among the distances from any points on the bottom surface of the second manifold 132 to any points on the second surface 124b is 10.5 mm, and as proposed in Embodiment 2-2, when the shortest distance among the distances between the bottom surface of the first manifold 112 and the surface of the lower die block 110 is 10.5 mm, the deviation of the slot gap are 8.8 μm and 9.3 μm, respectively, which are within 10 μm. Therefore, no matter what any die block, the shortest distance from a bottom surface of a manifold in the die block including the manifold to a surface of the die block must be equal to or greater than 10 mm so that the slot gap may be managed within an acceptance level of deformation.

[0122] The present disclosure has been described in detail. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the disclosure, are given by way of illustration only, since various changes and modifications within the scope of the disclosure will become apparent to those skilled in the art from this detailed description.

[0123] Meanwhile, although terms indicating directions such as up, down, left, and right are used in the present specification, these terms are only for convenience of description, and it is apparent to those skilled in the art that these terms may vary depending on a position of a target object or a position of an observer.