Manufacturing method for electrode and manufacturing apparatus for electrode

Abstract

To provide a manufacturing method for an electrode with high adhesion between a current collector and an active material layer.SOLUTION: A manufacturing method for an electrode for a secondary battery includes an unevenness forming step for forming unevenness on the surface of a metal foil which is a current collector, and an active material layer forming step for forming an active material layer by placing coated powder particles, formed by coating the surface of the active material powder particles with a non-aqueous binder, on the surface of the metal foil.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method and an apparatus for manufacturing an electrode. [Background technology]

[0002] Metal foils are used as current collectors in secondary batteries such as lithium-ion batteries, electric double-layer capacitors, etc. For example, in the case of lithium-ion batteries, aluminum foil is generally used as the current collector for the positive electrode, and copper foil is generally used as the current collector for the negative electrode.

[0003] The positive electrode of a lithium-ion secondary battery is produced by, for example, dispersing and mixing a positive electrode active material powder such as lithium cobalt oxide, a binder such as polyvinylidene fluoride, and a conductive additive such as carbon black in an organic solvent such as N-methylpyrrolidone to prepare a paste, which is then applied to both sides of an aluminum foil approximately 15 μm thick to form a coating layer, and then drying the coating layer to evaporate and remove the organic solvent in the coating layer. After drying, a pressure bonding process is performed, if necessary, to increase the density within the coating layer. In this way, a positive electrode having a positive electrode active material-containing layer on the surface of an aluminum foil as a current collector is produced.

[0004] In such an electrode manufacturing process, if the electrode active material layer peels off from the current collector, it will result in a decrease in yield, and if it peels off after being incorporated into a secondary battery or an electric double layer capacitor, the characteristics of these devices, such as their lifespan, will deteriorate. Therefore, it is important that the current collector and the electrode active material layer are sufficiently adhered to each other.

[0005] A method for improving the adhesion between a current collector and an electrode active material-containing layer is known, which involves roughening the surface of an aluminum foil. For example, Patent Document 1 discloses an aluminum foil for a current collector, in which the roughness of at least one surface is, according to JIS B 0601:1994, an average roughness Ra of 0.3 μm to 1.5 μm and a maximum height Ry of 0.5 μm to 5.0 μm. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 11-162470 [Patent Document 2] International Publication No. 2013 / 128685 Summary of the Invention [Problem to be solved by the invention]

[0007] However, when a conventional metal foil is used as a current collector, the adhesion between the current collector and the electrode active material layer is still insufficient, and further improvement is currently required. In particular, when a powdered electrode active material is used, there is a problem that the adhesion between the current collector and the electrode active material-containing layer is likely to decrease.

[0008] The present invention has been made in view of the above points, and an object of the present invention is to provide a method for producing an electrode in which the adhesion between the current collector and the active material layer is high. [Means for solving the problem]

[0009] The method for manufacturing an electrode of the present invention is a method for manufacturing an electrode for a secondary battery, and includes an unevenness forming step of forming unevenness on the surface of a metal foil that serves as a current collector, and an active material layer forming step of placing coated powder particles, in which the surface of active material powder particles is coated with a non-aqueous binder, on the surface of the metal foil to form an active material layer.

[0010] In the active material layer formation process, at least one of a conductive additive and a liquid may be placed on the surface of the metal foil along with the coated powder. The liquid is used to create a wet powder by adding it to the coated powder. Depending on the ratio of powder to liquid, the wet powder can be classified into four regions: the "pendular region," the "funicular region," the "capillary region," and the "slurry region," depending on the liquid content.

[0011] In the unevenness forming step, it is preferable to form unevenness on both sides of the metal foil.

[0012] In the unevenness forming step, the metal foil may be pressed against a transfer member having unevenness formed on its surface to transfer the unevenness.

[0013] In the unevenness forming step, unevenness may be formed on the surface of the metal foil by non-contact physical processing such as plasma processing, sputtering processing, and electric discharge processing. Alternatively, contact physical processing such as sandblasting may be performed.

[0014] In the unevenness forming step, the unevenness may be formed by scraping off the surface of the metal foil.

[0015] The electrode manufacturing apparatus of the present invention is an apparatus for manufacturing an electrode for a secondary battery, and includes an irregularity forming section for forming irregularities on the surface of a metal foil serving as a current collector, and an active material layer forming section for placing coated powder particles, in which the surface of active material powder particles is coated with a non-aqueous binder, on the surface of the metal foil to form an active material layer. [Effects of the Invention]

[0016] The active material layer is formed by forming irregularities on the surface of the metal foil that serves as the current collector, and placing coated powder particles in which the surface of active material powder particles is coated with a non-aqueous binder on the surface of the metal foil, thereby increasing the adhesion between the current collector and the active material layer. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 2 is a schematic diagram showing an electrode manufacturing apparatus according to an embodiment. [Figure 2] FIG. 2 is a schematic cross-sectional view showing an example of a transfer roll. [Figure 3] FIG. 10 is a schematic cross-sectional view showing another example of a transfer roll. [Figure 4] FIG. 10 is a schematic cross-sectional view showing another example of a transfer roll. [Figure 5]FIG. 10 is a schematic diagram showing an electrode manufacturing apparatus according to another embodiment. [Figure 6] FIG. 10 is a schematic diagram showing an electrode manufacturing apparatus according to another embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0018] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The following description of the preferred embodiments is merely exemplary in nature and is not intended to limit the present invention, its applications, or its uses. In the following drawings, for the sake of simplicity, components having substantially the same functions are designated by the same reference numerals.

[0019] (Embodiment 1) In the manufacturing of the electrode according to the first embodiment, the manufacturing of a positive electrode will be described, but a negative electrode can also be manufactured in a similar manner. That is, in this embodiment, a positive electrode is manufactured by placing a coated powder in which a powder of the active material of the positive electrode is coated with a non-aqueous binder on a metal foil having an uneven surface. Similarly, a negative electrode can be manufactured by placing a coated powder in which a powder of the active material of the negative electrode is coated with a non-aqueous binder on a metal foil having an uneven surface. The same metal foil and binder may be used for the positive electrode and the negative electrode, or different metal foils and binders may be used.

[0020] The secondary battery to which the positive electrode of this embodiment is applied includes a lithium ion secondary battery, a lithium sulfur secondary battery, etc. In addition, the positive electrode active material is, for example, LiCoO2, LiNiO2, LiNi 0.8 Co 0.2 The positive electrode active material preferably includes lithium-containing composite oxides such as LiMnO2 and LiMnO4. The positive electrode active material preferably includes two or more positive electrode active materials having different particle sizes. The positive electrode active material having a small particle size can be formed, for example, by a flux method.

[0021] Active materials are generally distributed as powders, but in this embodiment, they are referred to as granular powders, as explained below. In powder engineering, a collection of solid particles is called a powder, and typically refers to a collection of particles in which interparticle interactions, such as adhesive forces, are greater than the gravity of a single particle. When such interactions are smaller than gravity, they are called granular, and when such granular materials are intentionally included, they are called granular. However, the distinction between these is not necessarily clearly defined. Although powders are solid, they have fluid-like properties, meaning they flow in response to external forces, and are often treated as a material form distinct from solids, liquids, and gases.

[0022] In the manufacturing of the positive electrode according to this embodiment, a coating step is first performed in which the surface of the powder of the positive electrode active material is coated with a non-aqueous binder. Specifically, the non-aqueous binder is made of a fluorine-based resin that is substantially insoluble in aqueous solvents such as water but soluble in certain non-aqueous solvents such as organic solvents. Representative examples include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluorine-based rubber, etc. Note that "substantially insoluble in aqueous solvents" refers to the fact that when the non-aqueous binder is added to an aqueous solvent (liquid), the solubility of the non-aqueous binder in the liquid is less than 1 g / 1 L at room temperature. Since a solvent is something that dissolves a solute, in this embodiment, a solvent that does not dissolve a solute will simply be referred to as a liquid.

[0023] The coating process includes dissolving a non-aqueous binder powder in a non-aqueous solvent, dispersing a positive electrode active material powder in the non-aqueous solvent, and heating and evaporating the non-aqueous solvent to coat the surfaces of the positive electrode active material powder with the non-aqueous binder. The coating process produces coated powder particles. The non-aqueous solvent may be dried by vacuum drying at room temperature without heating. While the non-aqueous binder may coat at least a portion of the surface of the positive electrode active material powder, it is preferable that the entire surface be coated. Coating the surface of the positive electrode active material powder with the non-aqueous binder prevents lithium ions in the positive electrode active material from leaching into the liquid even when the coated powder comes into contact with an aqueous liquid, thereby preventing a decrease in electrode capacity. Coating the entire surface with the non-aqueous binder significantly suppresses the decrease in electrode capacity. The non-aqueous solvent is made of a material in which the powder of the non-aqueous binder can be dissolved, and includes an organic solvent such as N-methyl-2-pyrrolidone (NMP).

[0024] Next, a wet powder preparation step is carried out in which a liquid in which the non-aqueous binder is substantially insoluble (i.e., the solubility of the non-aqueous binder is less than 1 g / 1 L at room temperature) is added to the coated powder to prepare a wet powder. In this step, a conductive additive or an additive that can be generally added to electrodes may also be added to prepare the wet powder. Such a liquid may be an aqueous liquid such as water. The wet powder will be described below.

[0025] The state of solid particles, liquid, and gas in wet granular materials (packing state) can be classified into four regions: the "pendular region," the "funicular region," the "capillary region," and the "slurry region."

[0026] The "pendular region" is a state in which the solid phase is continuous, meaning that solid particles are in contact with each other, and the gas phase is also continuous, but the liquid phase is small, forming a discontinuous phase. It is a state in which a liquid (liquid phase) exists discontinuously, bridging the active material particle granules (solid phase), and the active material particle granules can exist in a connected (continuous) state. The liquid content is relatively low, and many of the voids (gas phase) in the wet granules exist continuously, forming interconnected pores that lead to the outside. Liquid is present in patches on the surface of the granules (solid phase), giving the granules a "dry" appearance. A characteristic of the pendular region is that, when observed under an electron microscope (SEM), no continuous solvent layer is observed across the entire outer surface of the wet granules.

[0027] The "funicular region" is a state in which the liquid content in wet granular materials is relatively higher than in the pendular region, and liquid exists continuously around the granular materials (solid phase). However, because the amount of liquid is still small, the granular materials exist in a connected (continuous) state. Among the voids (gas phase) present in wet granular materials, the proportion of interconnected pores that lead to the outside decreases slightly, and the proportion of discontinuous, isolated pores increases. Funicular I is a packed state in which the solid phase is continuous, in the sense that the solid particles are in contact with each other, and the liquid and gas phases are also continuous. Furthermore, a state in which the amount of liquid increases and the number of interconnected pores leading to the gas outside becomes very few is called Funicular II. Both states can be considered as states in which the liquid evenly coats the solid.

[0028] In the "capillary region," the liquid content in the wet powder / granules increases to the point where the liquid is close to saturation, separating the solid particles with the liquid, making the solid phase discontinuous. The gas phase exists as small, isolated voids or is almost non-existent, so only the liquid phase is continuous. In other words, the voids in the powder / granule layer are almost entirely filled with liquid.

[0029] The "slurry region" contains a larger proportion of liquid than the capillary region, there is almost no gas phase, and the powder or granules in the solid phase are suspended in the liquid.

[0030] Although the positive electrode may be formed using wet granular material in the slurry region, it is preferable to use wet granular material in any of the pendular, funicular, and capillary regions, since less energy and time are required to remove the liquid.

[0031] FIG. 1 shows an electrode manufacturing apparatus according to the first embodiment. Metal foil 4, which is a current collector, is supplied from the bottom of the figure, sandwiched between a transfer roll (transfer member) 10 with a textured surface and a pressure roll 12, and then changes direction to the right side of the figure. Sandwiched between the transfer roll 10 and the pressure roll 12, textures are formed on the surface of the metal foil 4 (texture formation process), resulting in metal foil 4a with a textured surface. The transfer roll 10 and the pressure roll 12 constitute the texture formation unit of the electrode manufacturing apparatus of this embodiment. In the case of a positive electrode, aluminum foil is generally used as the metal foil 4.

[0032] Examples of irregularities formed on the surface of the transfer roll 10 are shown in Figures 2 to 4. The transfer roll 10a shown in Figure 2 has a large number of hemispherical protrusions 71 formed on its surface. The height of the protrusions 71 is preferably 15 µm or more. The roll surface of the transfer roll 10a preferably has a surface roughness Ra (measurement range 48 µm x 36 µm) of 1.26 µm or more. It is preferable that at least the surface of the pressing roll 12 is made of an elastic material such as rubber. If the surface of the pressing roll 12 is made of an elastic material, recesses are formed on the side of the metal foil 4 that contacts the transfer roll 10 and protrusions are formed on the opposite side, resulting in irregularities on both sides of the metal foil 4.

[0033] Different types of transfer rolls are shown in Figures 3 and 4. The transfer rolls 10b and 10c shown in Figures 3 and 4 have different uneven shapes from those shown in Figure 2. Specifically, in Figure 3, a large number of hemispherical depressions 72 are formed on the roll surface, and in Figure 4, a large number of quadrangular pyramidal depressions 73 are formed on the roll surface. The depth of the depressions 72 and 73 is preferably 15 µm or more. Furthermore, the roll surfaces of the transfer rolls 10b and 10c preferably have a surface roughness Ra (measurement range 48 µm x 36 µm) of 1.26 µm or more.

[0034] When depressions 72 and 73 are formed on the roll surface as in the transfer rolls 10b and 10c, the metal foil 4 is pressed against the transfer rolls 10b and 10c by the pressing roll 12, forming protrusions on the transfer rolls 10b and 10c side and depressions on the pressing roll 12 side. Therefore, as with the transfer roll 10a described above, irregularities are formed on the surface of the metal foil 4.

[0035] By using the transfer roll 10 having an irregular surface formed thereon as the transfer member, it becomes easy to control the irregular shape, height of the protrusions, depth of the depressions, gradient of the irregularities, distribution of the irregularities, etc. on the surface of the metal foil 4. In other words, because a transfer technique is used, simply by controlling the surface shape of the transfer roll 10, it is possible to make uniform the irregular shape, height of the protrusions, depth of the depressions, gradient of the irregularities, distribution of the irregularities, etc. over the entire length of the metal foil 4a having an irregular surface, and it becomes easy to maintain and stabilize the quality of the metal foil 4a having an irregular surface in a constant state in the length and width directions of the metal foil 4a.

[0036] Next, the aforementioned wet powder containing a positive electrode active material is applied to both sides of the metal foil 4a with the surface having the irregularities formed thereon. As shown in FIG. 1, the wet powder is applied to the upper surface of the metal foil 4a from an application member 20 to form an upper wet powder film 2 (upper surface-side film-forming step). At this time, the lower surface of the metal foil 4a is supported by a backup roll 15. Thereafter, the wet powder is applied to the lower surface of the metal foil 4a from a lower application member 22 to form a lower wet powder film 3 (lower surface-side film-forming step). The wet powder is supplied to the application members 20 and 22 by a supply member 21.

[0037] The metal foil 4a with the wet powder / granule films 2, 3 formed on both sides is placed in a drying furnace 50 to remove the liquid (liquid removing step). The upper surface film forming step, lower surface film forming step, and liquid removing step are collectively referred to as an active material layer forming step. The electrode active material films 2a, 3a from which the liquid has been removed are then sandwiched between a transport roll 16 and a nip roll 17 and pressure is applied, which increases the adhesion to the metal foil 4a, increases the film density of the electrode active material films 2a, 3a, and improves the electrode properties of the electrode active material films 2a, 3a, thereby forming an electrode (positive electrode) 80. The metal foils 4, 4a and the electrode 80 are transported by the transport roll 16 and the nip roll 17.

[0038] The above-described unevenness-forming step forms unevenness on the surface of the metal foil 4, producing a metal foil 4a with an uneven surface, and the active material layer-forming step forms an active material layer from the wet powder granules. As a result, the wet powder granules enter the recesses of the metal foil 4a and are arranged around the protrusions, resulting in strong adhesion between the metal foil 4a and the wet powder granules, as well as strong adhesion between the wet powder granules themselves. Therefore, the adhesion between the active material layer and the metal foil and the adhesion between the active material granules themselves are significantly improved compared to when an active material layer is formed directly on ordinary metal foil without an uneven surface.

[0039] The thickness of the metal foil 4 for the current collector is preferably 10 μm or more and 100 μm or less. When the thickness is 10 μm or more, the metal foil is less likely to break or crack when forming irregularities on the metal foil surface, which contributes to improving the adhesive strength between the active material layer and the metal foil. Furthermore, when the thickness is 100 μm or less, the foil has an appropriate volume and weight for a current collector, which contributes to the miniaturization and weight reduction of secondary batteries and electric double layer capacitors incorporating the current collector, and is also advantageous in terms of cost reduction. Therefore, the thickness of the aluminum foil for the current collector is preferably 10 μm or more and 100 μm or less, more preferably 10 μm or more and 50 μm or less, and even more preferably 10 μm or more and 30 μm or less.

[0040] In this embodiment, a wet powder is produced by applying an aqueous liquid to coated powder (active material) coated with a non-aqueous binder. This reduces the energy required for drying in terms of both the quality and quantity of the liquid, shortens the drying time, and eliminates the need for environmental measures that would be required if an organic solvent were used. While organic solvents require costs for recovery and recycling of the removed solvent, using water as the liquid eliminates the need for recovery, significantly reducing costs. Furthermore, the conventional drying ovens used for organic solvents are no longer required, significantly reducing manufacturing costs.

[0041] The smaller the amount of liquid in the wet powder / granules, the less energy is required for drying. On the other hand, if the amount of liquid is large, the state approaches the conventional slurry region, and the electrode characteristics become almost the same as those produced by conventional manufacturing methods. Therefore, if you want to reduce the drying energy and increase the production speed to lower production costs, it is preferable to use wet powder / granules in the pendular region or funicular I region. If you want to obtain electrode characteristics similar to those of electrodes produced by conventional slurry manufacturing methods, it is preferable to use wet powder / granules in the capillary region.

[0042] (Embodiment 2) The electrode manufacturing method and manufacturing apparatus according to the second embodiment differ from those of the first embodiment in that a plasma irradiation device is used as the irregularity forming unit in the irregularity forming step. Hereinafter, the present embodiment will be described with reference to FIG. 5, focusing on the differences from the first embodiment.

[0043] In this embodiment, both surfaces of the metal foil 4 serving as a current collector are irradiated with plasma by a plasma irradiation device 6, which forms minute irregularities on the surface, resulting in metal foil 4b with an irregular surface. Two plasma irradiation devices 6 are installed facing both surfaces of the metal foil 4 at a certain distance from the surface of the metal foil 4.

[0044] The metal foil 4b with the irregularities formed on its surface is guided by a guide roll 14 to change its direction of travel and is led to the same active material layer forming section as in embodiment 1, where active material layers (electrode active material films 2a, 3a) are formed on both sides.

[0045] The degree of unevenness formed on the metal foil 4 by the plasma treatment is expressed by the surface roughness Ra (measurement range 48 μm×36 μm), and the numerical value is preferably 1.26 μm or more.

[0046] The second embodiment also provides the same effects as the first embodiment.

[0047] (Embodiment 3) The electrode manufacturing method and manufacturing device according to embodiment 3 differ from embodiment 1 in that a scraping device that roughens the surface of a metal foil by scraping it off is used as the roughness forming unit in the roughness forming step. Hereinafter, this embodiment will be described with reference to Fig. 6, focusing on the differences from embodiment 1.

[0048] In this embodiment, the scraping rolls 60, 62 scrape and roughen the surface of the metal foil 4, which is the current collector, forming fine irregularities on the surface of the metal foil 4, resulting in a metal foil 4c with an irregular surface. The scraping rolls 60, 62 are rolls with numerous abrasive grains on their surfaces that function as files, and the two rolls are arranged to contact both sides of the metal foil 4 and sandwich the metal foil 4. The scraping rolls 60, 62 rotate in the opposite direction to the direction of travel of the metal foil 4 to scrape the surface of the metal foil 4. Alternatively, the surface of the metal foil 4 may be scraped by rotating in the same direction as the direction of travel of the metal foil 4 but at a slower speed than the speed at which the metal foil 4 travels.

[0049] The metal foil 4c having the irregularities formed on its surface is introduced into the same active material layer forming section as in the first embodiment, and active material layers (electrode active material films 2a, 3a) are formed on both sides.

[0050] The degree of unevenness formed on the metal foil 4 by scraping is expressed by the surface roughness Ra (measurement range 48 μm×36 μm), and the numerical value is preferably 1.26 μm or more.

[0051] Embodiment 3 also provides the same effects as embodiment 1. Note that a metal powder removal section for removing scraped metal powder may be provided between scraping rolls 60, 62 and the active material layer forming section.

[0052] (Other embodiments) The above-described embodiments are merely examples of the present invention, and the present invention is not limited to these examples. These examples may be combined with well-known, commonly used, or publicly known technologies, or may be partially replaced. Modified inventions that would be easily conceived by a person skilled in the art are also included in the present invention.

[0053] The surface of the transfer roll may be formed with not only protrusions or depressions, but also both protrusions and depressions. When both protrusions and depressions are formed, the height of the protrusions and the depth of the depressions are preferably 7.5 μm or more from a reference surface that offsets the unevenness. Furthermore, the shapes of the protrusions and depressions formed on the transfer roll are not limited to those shown in the embodiments. Furthermore, with regard to protrusions and depressions, a single transfer roll may have multiple shapes, sizes, heights, and depths, and the distribution density, distance between adjacent ones, etc. may not be constant.

[0054] The surface of the pressure roller may also be uneven, in addition to the transfer roller.

[0055] The unevenness-forming portion having the unevenness on its surface is not limited to a transfer roll. For example, it may be a planar unevenness-forming portion having unevenness formed on the surface of a flat plate. In this case, a metal foil is placed on the planar unevenness-forming portion, and the metal foil is pressed against the unevenness-forming portion from the opposite side with a pressure roll or the like to form unevenness on the metal foil surface.

[0056] The scraping device is not limited to a roll shape, and may be a flat device or the like. [Explanation of symbols]

[0057] 2a Electrode active material film 3a Electrode active material film 4 Metal foil 4a Metal foil with uneven surface 4b Metal foil with uneven surface 4c Metal foil with textured surface 6. Plasma irradiation device 10 Transfer roll (transfer member) 10a Transfer roll (transfer member) 10b Transfer roll (transfer member) 10c Transfer roll (transfer member) 20 Coating material 22 Coating material 50 Drying oven 60 scraping roll 62 scraping roll 80 electrodes

Claims

1. A method for manufacturing an electrode for a secondary battery, comprising: a step of forming irregularities on the surface of a metal foil serving as a current collector; an active material layer forming step of placing coated powder particles, in which the surface of active material powder particles is coated with a non-aqueous binder, on the surface of the metal foil to form an active material layer; A method for manufacturing an electrode, comprising:

2. The method for manufacturing an electrode according to claim 1 , wherein the unevenness is formed on both surfaces of the metal foil in the unevenness forming step.

3. The method for manufacturing an electrode according to claim 1 or 2, wherein the unevenness forming step comprises pressing the metal foil against a transfer member having unevenness formed on its surface to transfer the unevenness.

4. The method for manufacturing an electrode according to claim 1 or 2, wherein the unevenness is formed on the surface of the metal foil by non-contact physical processing in the unevenness forming step.

5. The method for manufacturing an electrode according to claim 1 or 2, wherein the unevenness is formed by scraping off the surface of the metal foil in the unevenness forming step.

6. An apparatus for manufacturing electrodes for secondary batteries, comprising: an unevenness forming portion that forms unevenness on the surface of the metal foil that is the current collector; an active material layer forming section for forming an active material layer on the surface of the metal foil by placing coated powder particles in which the surface of active material powder particles is coated with a non-aqueous binder; The electrode manufacturing apparatus comprises:

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