Electrode layer forming method

Abstract

In this electrode layer forming method, a second liquid having a lower binder component concentration than a first liquid is applied to a surface of a current collector foil to form a liquid layer on the surface, a powder of an active material is supplied onto the liquid layer to form a powder layer on the surface, and the first liquid is applied onto the powder layer.

Description

Electrode Layer Forming Method

[0001] The present invention relates to an electrode layer forming method.

[0002] Patent Document 1 discloses a method of forming a powder layer on a current collector, applying an ink containing a binder, and drying the same to form an active material layer.

[0003] International Publication No. 2025 / 009096

[0004] Incidentally, when a binder solution is applied to the surface of a powder layer to form an electrode layer, air remaining in the lower portion of the powder layer prevents the binder liquid from penetrating between the powders or between the powders and the surface of the current collector foil, which may cause a risk of reducing the peel strength of the electrode layer.

[0005] An object of the present invention is to provide an electrode layer forming method capable of improving peel strength.

[0006] An electrode layer forming method according to one aspect of the present invention includes: applying a second liquid having a binder component concentration lower than that of a first liquid to a surface of a current collector foil to form a liquid layer on the surface; supplying active material powder onto the liquid layer to form a powder layer on the surface; and applying the first liquid onto the powder layer.

[0007] According to the above electrode layer forming method, the peel strength can be improved.

[0008] FIG. 1 is a schematic diagram illustrating the electrode layer forming method according to the embodiment.

[0009] Hereinafter, the electrode layer forming method according to the embodiment will be described with reference to the drawings. In addition, configurations having the same functions as those of the configurations already described are denoted by the same reference numerals, and descriptions thereof are omitted.

[0010] The electrode layer forming method according to the embodiment will be described. As exemplified in FIG. 1, the electrode layer is formed on the surface 1A of the current collector foil 1. The electrode layer is, for example, a positive electrode layer or a negative electrode layer used as an electrode of a secondary battery. The current collector foil 1 is a thin-film-shaped current collector, and known current collectors such as aluminum foil, aluminum alloy foil, copper foil, and nickel foil can be used.

[0011] The electrode layer formation method comprises a first step S1, a second step S2, and a fourth step S4. Furthermore, as illustrated in Figure 1, the electrode layer formation method according to this embodiment may further comprise a third step S3 and a fifth step S5. The current collector foil 1 is appropriately transported, for example, by a transport device from left to right in Figure 1.

[0012] In the first step S1, the second liquid L2 is applied to the surface 1A of the current collector foil 1. This forms a liquid layer 2 on the surface 1A of the current collector foil 1. As illustrated in Figure 1, the second liquid L2 is applied to the surface 1A of the current collector foil 1 by a coating device 11 positioned above the current collector foil 1. This allows a liquid layer 2 having a predetermined thickness to be formed on the surface 1A of the current collector foil 1 in the horizontal direction (width direction and transport direction of the current collector foil 1). That is, the coating device 11 can apply the second liquid L2 in a predetermined pattern to a predetermined area on the surface 1A of the current collector foil 1. This forms a liquid layer 2 having a predetermined thickness on the surface 1A of the current collector foil 1. The thickness of the liquid layer 2 according to this embodiment is approximately 20 μm.

[0013] The second liquid L2 has a relatively lower binder component concentration than the first liquid L1 (hereinafter referred to as binder solution L1), which is a binder solution described later. Binder solution L1 contains a binder and a solvent. The type of binder is not particularly limited, but known materials such as polyvinylidene fluoride (PVdF) and a composite of polyvinylidene fluoride and hexafluoropropylene (HFP) can be used. The solvent is not particularly limited, but known materials such as N-methylpyrrolidone (NMP) and acetonitrile can be used. Furthermore, the structure, arrangement, and coating method of the coating apparatus 11 are not particularly limited.

[0014] In the second step S2, active material powder P is supplied onto the liquid layer 2 to form a powder layer 3. As illustrated in Figure 1, after the first step S1, active material powder P is supplied downward from a supply unit 12 located above the current collector foil 1 and deposited onto the liquid layer 2. This allows a powder layer 3 to be formed on the surface 1A of the current collector foil 1. By pre-forming the liquid layer 2 on the surface of the current collector foil 1, the powder layer 3 can be formed on the liquid layer 2. Therefore, the second liquid L2 permeates into the lower part of the powder layer 3 and the gap between the powder layer 3 and the current collector foil 1. In other words, compared to the case where the powder layer 3 is formed without pre-forming the liquid layer 2 on the surface of the current collector foil 1, it is possible to suppress the retention of air in the lower part of the powder layer 3 and the gap between the powder layer 3 and the current collector foil 1. The powder layer 3 includes the layer of liquid layer 2 and active material powder P formed on the surface 1A of the current collector foil 1. Furthermore, the means for supplying the active material powder P downward from the supply unit 12 are not particularly limited, and known methods such as dropping or projection can be selected.

[0015] The third step S3 will be described later. In the fourth step S4, a binder layer 4 is formed by applying a binder solution L1 onto the powder layer 3. As illustrated in Figure 1, the binder solution L1 is applied onto the powder layer 3 (which may be a flattened powder layer 3A described later) by a coating device 14 positioned above the current collector foil 1. The binder layer 4 includes the powder layer 3 (i.e., the layer of liquid layer 2 and active material powder P formed on the surface 1A of the current collector foil 1, or the flattened powder layer 3A described later) and the liquid layer of binder solution L1. As mentioned above, the binder solution L1 has a relatively higher binder component concentration than the second liquid L2. Furthermore, the structure, arrangement, and coating method of the coating device 14 are not particularly limited.

[0016] As illustrated in Figure 1, the electrode layer formation method according to the embodiment may include a third step S3 after the second step S2 and before the fourth step S4. In the third step S3, the surface of the powder layer 3 is flattened while applying pressure to the powder layer 3 with a leveling jig 13 such as a squeegee or roller. As a result, a flattened powder layer 3A is formed on the surface 1A of the current collector foil 1.

[0017] As illustrated in Figure 1, the electrode layer formation method according to this embodiment may include a fifth step S5 in which drying is performed after the fourth step S4. As illustrated in Figure 1, the drying apparatus 15 can be used to dry the binder layer 4 and promote the evaporation of the solvent contained in the binder layer 4. As a result, a part of the binder layer 4 hardens, and a layer 5 is formed in which voids are formed within the binder layer 4 during the solvent evaporation process. The drying apparatus 15 may be, for example, a lamp or a hot plate, and their arrangement is not limited to the arrangement shown in Figure 1.

[0018] The second liquid L2 does not necessarily have to contain a binder component. That is, it may contain only the solvent component of the binder solution L1. In that case, since the liquid layer 2 formed by the second liquid L2 does not contain a binder component, when the binder solution L1 is applied in the fourth step S4, the binder component will diffuse into the liquid layer 2 more quickly.

[0019] As illustrated in Figure 1, the liquid layer 2 is formed to be thinner than the powder layer 3. The thickness of the powder layer 3 according to this embodiment is 180 to 230 μm. Therefore, the surface of the powder layer 3 is kept dry. As a result, when the third step S3 is performed, it is possible to suppress the adhesion of the active material powder P near the surface of the powder layer 3 to the leveling jig 13, and a smoother surface of the flattened powder layer 3A can be formed.

[0020] The coating apparatus 11 according to this embodiment is a mist generator (hereinafter referred to as mist generator 11). That is, the second liquid L2 is sprayed onto the surface of the current collector foil 1 by the mist generator 11. Therefore, the liquid layer 2 is formed by spraying the second liquid L2 by the mist generator 11. The coating apparatus 14 according to this embodiment is an inkjet device. The droplets of the second liquid L2 sprayed by the mist generator 11 are relatively smaller than the droplets of the binder solution L1 produced by the inkjet device. Therefore, the amount of second liquid L2 applied by the mist generator 11 can be precisely controlled, and the liquid layer 2 can be formed with higher precision.

[0021] In the above example, when the electrode layer is used as the positive electrode layer, the active material powder P can be, for example, layered rock salt type active materials such as LiCoO2, LiMnO2, LiNiO2, LiVO2, Li(Ni-Mn-Co)O2, spinel type active materials such as LiMn2O4, LiNi0.5Mn1.5O4, olivine type active materials such as LiFePO4, LiMnPO4, or Si-containing active materials such as Li2FeSiO4, Li2MnSiO4. In addition, other oxide active materials such as Li4Ti5O12 can be used. Other positive electrode active materials may also be used. When the electrode layer is used as the negative electrode layer, the active material powder P can be, for example, carbon materials, metal oxides, and metal active materials. The negative electrode active material may be a metal containing Li, and such a negative electrode active material is not particularly limited as long as it contains Li.

[0022] The active material powder P described above may contain a conductive additive. This allows for the formation of an electrode layer to which the conductive additive has been added. Examples of conductive additives include, but are not limited to, metals such as aluminum, stainless steel, silver, gold, copper, and titanium, alloys or metal oxides containing these metals, carbon fibers, carbon nanotubes, carbon nanofibers, and carbon black (such as acetylene black). Two or more of these conductive additives may be included.

[0023] (1) The electrode layer formation method according to the embodiment involves applying a second liquid L2, which has a lower binder component concentration than the first liquid L1, to the surface of the current collector foil 1 to form a liquid layer 2 on the surface, supplying active material powder P onto the liquid layer 2 to form a powder layer 3 on the surface, and applying the first liquid L1 onto the powder layer 3. By pre-forming a liquid layer 2 with the second liquid L2 on the surface of the current collector foil 1, a powder layer 3 with the active material powder P can be formed on the liquid layer 2. As a result, the second liquid L2 penetrates into the lower part of the powder layer 3 and into the gap between the powder layer 3 and the current collector foil 1. In other words, compared to the case in which the powder layer 3 is formed without pre-forming a liquid layer 2 on the surface of the current collector foil 1, the above method can suppress the retention of air in the lower part of the powder layer 3 and into the gap between the powder layer 3 and the current collector foil 1. Therefore, when the first liquid L1 is applied to the powder layer 3, the binder component of the first liquid L1 diffuses into the liquid layer 2, and the binder component diffuses into the lower part of the powder layer 3 and the gap between the powder layer 3 and the current collector foil 1. This improves the peel strength of the electrode layer formed on the current collector foil 1. The peel strength can be evaluated by measuring the force required to peel off the formed electrode layer in a predetermined direction.

[0024] (2) In this embodiment, the second liquid L2 does not contain a binder component. As a result, when the first liquid, the binder solution L1, is applied, the binder component of the binder solution L1 diffuses more quickly due to the liquid layer 2 formed by the second liquid L2. This makes it possible to further improve the peel strength of the electrode layer formed on the current collector foil 1.

[0025] (3) Furthermore, in this embodiment, the liquid layer 2 is formed thinner than the powder layer 3. As a result, the surface of the powder layer 3 remains dry. Therefore, the surface of the powder layer 3 can be formed more smoothly. For example, when the leveling process, which is the third step S3, is performed afterward, it is possible to suppress the adhesion of the active material powder P near the surface of the powder layer 3 to the leveling jig 13. In addition, it is possible to suppress the occurrence of irregularities and cracks on the surface of the powder layer 3, thereby suppressing the unevenness of the current flow and forming an electrode layer with high safety performance.

[0026] (4) In the embodiment, a drying step is provided after applying the first liquid L1. This dries the binder layer 4 and promotes the evaporation of the solvent contained in the binder layer 4. As a result, a portion of the binder layer 4 hardens, and a layer 5 is formed in which voids are formed within the binder layer 4 during the solvent evaporation process. Therefore, an electrode layer with high ionic conductivity can be formed.

[0027] (5) Furthermore, in this embodiment, the liquid layer 2 is formed by spraying the second liquid L2 with the mist generator 11. This allows for precise control of the amount of the second liquid L2 applied, enabling the formation of the liquid layer 2 with greater precision. As a result, the surface of the powder layer 3 can be kept smoother. For example, if the leveling process, which is the third step S3, is performed afterward, the adhesion of the active material powder P near the surface of the powder layer 3 to the leveling jig 13 can be further suppressed. In addition, the occurrence of irregularities and cracks on the surface of the powder layer 3 can be further suppressed, the unevenness of the current flow can be suppressed, and an electrode layer with high safety performance can be formed.

[0028] The embodiments described above are merely illustrative examples provided to facilitate understanding of the invention. The technical scope of the invention is not limited to the specific technical matters disclosed in the embodiments above, but also includes various modifications, changes, and alternative technologies that can be easily derived therefrom.

[0029] 1 Current collector foil 2 Liquid layer 3 Powder layer 11 Mist generator L1 First liquid L2 Second liquid P Active material powder

Claims

1. An electrode layer formation method comprising: applying a second liquid having a lower binder component concentration than a first liquid to the surface of a current collector foil to form a liquid layer on the surface; supplying active material powder onto the liquid layer to form a powder layer on the surface; and applying the first liquid onto the powder layer.

2. The electrode layer forming method according to claim 1, wherein the second liquid does not contain a binder component.

3. The electrode layer forming method according to claim 1 or 2, wherein the liquid layer is formed to be thinner than the powder layer.

4. The electrode layer forming method according to claim 3, further comprising a drying step after applying the first liquid.

5. The electrode layer forming method according to claim 4, wherein the liquid layer is formed by spraying the second liquid with a mist generating device.

Images