Method for manufacturing self-supporting membrane electrode and method for manufacturing battery

Abstract

To manufacture a self-supporting membrane electrode using a fibrous binder such as PTFE, and has reduced electrode resistance.SOLUTION: This method includes the steps of mixing a first mixture of an active material and a first polytetrafluoroethylene powder fiberized to an aspect ratio of 10 or more, and a second mixture of an active material and a second polytetrafluoroethylene powder having an aspect ratio of 1.3 or less, in a mixing ratio of the first polytetrafluoroethylene powder to the second polytetrafluoroethylene powder in a range of 30:70 to 70:30 by mass, and rolling the mixture of the first mixture and the second mixture.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present disclosure relates to a method for manufacturing a free-standing membrane electrode and a method for manufacturing a battery. [Background technology]

[0002] Secondary batteries, such as lithium-ion secondary batteries and solid-state batteries, include positive and negative electrodes. Positive and negative electrodes are typically fabricated by applying a slurry containing an active material and a binder to a current collector foil and drying the slurry. Meanwhile, Patent Document 1 discloses a dry method for producing electrodes without using a solvent, in which fibrous polytetrafluoroethylene (PTFE) powder having an aspect ratio of 1.5 or more and an active material are mixed and rolled to produce an electrode composite sheet, and the resulting electrode composite sheet is then bonded to a core material to produce an electrode. Patent Document 1 also discloses a similar method for producing an electrode by using fibrous PTFE powder having an aspect ratio of 5 or more together with the above-mentioned fibrous PTFE having an aspect ratio of 1.5 or more.

[0003] Patent Document 2 discloses a method for producing a free-standing ultrafibrillated electrode film by forming a mixture containing dried carbon particles and dried ultrafibrillated binder (e.g., PTFE) particles, ultrafibrillating the binder in the mixture to form an ultrafibrillated matrix, and then calendering the mixture. However, the production methods disclosed in Patent Documents 1 and 2 sometimes result in a decrease in electronic conductivity as a result of the active material being covered with the fiberized binder such as PTFE.

[0004] Furthermore, Patent Document 3 discloses a self-supporting dry electrode film containing a dry binder including a fibrous binder and a particulate non-fibrous binder, and an active material. Patent Document 3 lists PTFE and the like as examples of the fibrous binder, and cellulose and sodium carboxymethyl cellulose and the like as examples of the particulate non-fibrous binder. According to Patent Document 3, the particle size (D 50) is believed to result in a higher bonding strength between the dry binder and the active material, resulting in a self-supporting dry electrode film with excellent tensile strength. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2022 / 024520 [Patent Document 2] Japanese Patent Publication No. 2022-3694 [Patent Document 3] International Publication No. 2019 / 191397 Summary of the Invention [Problem to be solved by the invention]

[0006] However, when a free-standing electrode membrane (also called a free-standing membrane electrode) that does not require a substrate is produced using a fiberized binder such as PTFE, there is a problem in that the electronic conductivity between the active materials that make up the electrode decreases. Therefore, an object of the present disclosure is to provide a method for manufacturing a self-supporting membrane electrode using a fibrous binder such as PTFE, which has reduced electrode resistance, and a method for manufacturing a battery. [Means for solving the problem]

[0007] The present disclosure, which achieves the above-mentioned objectives, includes the following. <1> a step of mixing a first mixture of an active material and a first polytetrafluoroethylene powder fiberized to an aspect ratio of 10 or more with a second mixture of an active material and a second polytetrafluoroethylene powder having an aspect ratio of 1.3 or less, in a mixing ratio of the first polytetrafluoroethylene powder to the second polytetrafluoroethylene powder in a range of 30:70 to 70:30 by mass; rolling the mixture of the first mixture and the second mixture; A method for manufacturing a free-standing membrane electrode, comprising: <2> The active material includes graphite. <1> A method for producing the self-supporting membrane electrode according to claim 1. <3> applying shear stress to a mixture containing the active material and polytetrafluoroethylene to prepare the first mixture; <1> or <2> A method for producing the self-supporting membrane electrode according to claim 1. <4> preparing the second mixture by mixing the active material with polytetrafluoroethylene; <1> ~ <3> 10. A method for producing a free-standing membrane electrode according to any one of claims 1 to 9. <5> The rolling step is carried out by a dry rolling method. <1> ~ <4> 10. A method for producing a free-standing membrane electrode according to any one of claims 1 to 9. <6> <1> ~ <5> a step of producing a free-standing membrane electrode by the method for producing a free-standing membrane electrode according to any one of the methods; a step of bonding the produced free-standing membrane electrode and a current collecting foil to form an electrode layer; A method for manufacturing a battery comprising: <7> The electrode layer is a negative electrode layer. <6> A method for manufacturing the battery described in claim 1. [Effects of the Invention]

[0008] According to the present disclosure, it is possible to provide a method for manufacturing a free-standing membrane electrode and a method for manufacturing a battery, which are free-standing membrane electrodes using polytetrafluoroethylene as a fiberizing binder and have reduced electrode resistance. [Brief explanation of the drawings]

[0009] [Figure 1] 2 is an electron microscope photograph of a mixture of fiberized PTFE (first PTFE powder) and an active material prepared in an example. [Figure 2] 1 is an electron microscope photograph of a mixture of non-fibrillated PTFE (second PTFE powder) and an active material prepared in an example. [Figure 3] FIG. 3 is a characteristic diagram showing the relationship between the mass ratio of fiberized PTFE (first PTFE powder) and the powder resistance and tensile strength. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present disclosure will be described. The description is for illustrating the embodiments and is not intended to limit the scope of the present disclosure.

[0011] In this specification, a numerical range indicated using "to" indicates a range that includes the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In the present specification, the upper or lower limit of one numerical range may be replaced with the upper or lower limit of another numerical range. In addition, in the present specification, the upper or lower limit of a numerical range may be replaced with a value shown in the examples.

[0012] In this specification, the term "process" includes not only an independent process but also a process that cannot be clearly distinguished from other processes, as long as the intended purpose of the process is achieved.

[0013] In this specification, when an embodiment is described with reference to drawings, the configuration of the embodiment is not limited to the configuration shown in the drawings. Furthermore, the sizes of components in each drawing are conceptual, and the relative size relationships between components are not limited to these.

[0014] In this specification, each component may contain a plurality of corresponding substances. When referring to the amount of each component in the composition in this embodiment, if a plurality of substances corresponding to each component are present in the composition, the amount refers to the total amount of the plurality of substances present in the composition unless otherwise specified.

[0015] [Manufacturing method of freestanding membrane electrodes] The method for manufacturing a free-standing membrane electrode of the present disclosure includes the steps of: mixing a first mixture of an active material and a first polytetrafluoroethylene powder (first PTFE powder) that has been fiberized to an aspect ratio of 10 or more, and a second mixture of an active material and a second polytetrafluoroethylene powder (second PTFE powder) that has an aspect ratio of 1.3 or less, in a mixing ratio of the first polytetrafluoroethylene powder to the second polytetrafluoroethylene powder in a range of 30:70 to 70:30 by mass; and rolling the mixture of the first mixture and the second mixture.

[0016] According to the manufacturing method of the self-supporting membrane electrode of the present disclosure, a self-supporting membrane electrode using polytetrafluoroethylene (hereinafter, PTFE) as a fiberized binder can be manufactured, and the electrode resistance can be reduced. Here, a self-supporting membrane refers to a membrane that exists independently without a base material, substrate, or other scaffolding. A self-supporting membrane electrode refers to an electrode containing an active material manufactured as a self-supporting membrane. Note that the self-supporting membrane electrode may be a negative electrode containing a negative electrode active material or a positive electrode containing a positive electrode active material. Furthermore, the self-supporting membrane electrode becomes an electrode layer (negative electrode layer or positive electrode layer) that constitutes a battery by being bonded to a current collector foil (negative electrode current collector foil or positive electrode current collector foil).

[0017] An electrode including a freestanding membrane electrode manufactured by the method for manufacturing a freestanding membrane electrode of the present disclosure is an excellent electrode with low electrode resistance. Furthermore, the method for manufacturing a freestanding membrane electrode of the present disclosure also improves the strength of the freestanding membrane electrode, making it possible to achieve both reduced electrode resistance and strength. The electrode resistance can be evaluated by measuring the resistance value using an electrode resistance measurement system in a state where the freestanding membrane electrode and current collecting foil are bonded together. The electrode resistance measurement system used can be the RM2610 manufactured by Hioki E.E. Corporation.

[0018] The first PTFE powder used in the method for producing a free-standing membrane electrode of the present disclosure is prepared by applying shear stress to the raw material PTFE powder having fiber-forming ability to promote fiberization, thereby providing an aspect ratio of at least 10. That is, in the method for producing a free-standing membrane electrode of the present disclosure, the first mixture can be prepared by applying shear stress to a mixture containing an active material and PTFE to promote fiberization of the PTFE.

[0019] The method for fiberizing PTFE is not particularly limited, and a process of applying shear stress as described above may be carried out. For example, a method of treating a mixture containing an active material and PTFE in a kneader may be used. A kneader equipped with a container and a stirring blade disposed in the container may be used as the kneader. A kneader such as a kneader can apply shear stress to the PTFE contained in the mixture, for example, by the stirring blade. Alternatively, a dry grinder such as a jet mill grinder may be used to apply shear stress to the PTFE contained in the mixture.

[0020] The first mixture includes an active material and a first PTFE powder, and may include a PTFE powder having an aspect ratio of less than 10, for example.

[0021] The second PTFE powder used in the method for producing a free-standing membrane electrode of the present disclosure differs from the first PTFE powder in that it does not promote fibrillation relative to the raw material PTFE powder having fibrillation ability and has an aspect ratio of 1.3 or less. For example, in the method for producing a free-standing membrane electrode of the present disclosure, the second mixture can be prepared by mixing an active material and PTFE.

[0022] The second mixture includes an active material and a second PTFE powder, and may include, for example, a PTFE powder having an aspect ratio of greater than 1.3.

[0023] Here, the aspect ratio defining the first PTFE powder and the second PTFE powder is a value defined as the ratio of the longest distance between two points (length of the major axis) for any PTFE powder observed in an image to the shortest distance between two points (length of the minor axis) perpendicular to the major axis for the same PTFE powder. In other words, the aspect ratio can be calculated as [aspect ratio] = [length of the major axis] / [length of the minor axis]. Images used to calculate the aspect ratio are not particularly limited, but a scanning electron microscope (SEM) can be used.

[0024] The proportion (by number) of the first PTFE powder or the second PTFE powder contained in the total PTFE powder can be calculated, for example, based on an image captured by a scanning electron microscope (SEM). That is, the aspect ratios of all PTFE powder particles contained in the entire or partial region of the PTFE powder image are measured, and the proportion of the first PTFE powder particles having an aspect ratio of 10 or more can be calculated by dividing the number of PTFE powder particles having an aspect ratio of 10 or more by the total number of particles. Similarly, the aspect ratios of the PTFE powder particles contained in the entire or partial image of the PTFE powder can be measured, and the proportion of the second PTFE powder particles having an aspect ratio of 1.3 or less can be calculated.

[0025] In the method for producing a free-standing membrane electrode of the present disclosure, the first mixture and the second mixture are mixed so that the mixing ratio of the first PTFE powder to the second PTFE powder is in the range of 30:70 to 70:30 by mass. By mixing the first PTFE powder to the second PTFE powder in the range of 30:70 to 70:30 by mass, the electrode resistance of the produced free-standing membrane electrode can be kept low. Furthermore, it is preferable to mix the first PTFE powder to the second PTFE powder in the range of 50:50 to 70:30 by mass. In this case, the electrode resistance of the produced free-standing membrane electrode can be kept low, and the tensile strength of the produced free-standing membrane electrode can be increased.

[0026] Next, in the method for producing a free-standing membrane electrode of the present disclosure, the mixture of the first mixture and the second mixture is rolled. By rolling the mixture of the first mixture and the second mixture, some of the non-fibrillated second PTFE particles contained in the second mixture are fibrillated, and are formed into a free-standing membrane together with the first PTFE particles. In this process, in the method for producing a free-standing membrane electrode of the present disclosure, the fibrillated PTFE does not form a structure that covers the active material, thereby maintaining electronic conductivity between the active materials. Therefore, the method for producing a free-standing membrane electrode of the present disclosure can produce a free-standing membrane electrode that contains PTFE and has low electrode resistance.

[0027] In particular, in the method for producing a free-standing membrane electrode of the present disclosure, it is preferable to use a dry rolling method when rolling the mixture of the first mixture and the second mixture. The dry rolling method is a method in which the mixture is rolled in a dry, non-wet state. The dry rolling method can further promote the fiberization of some of the non-fiberized second PTFE particles contained in the second mixture, thereby producing a free-standing membrane electrode with excellent strength. Furthermore, as a method for rolling the mixture of the first mixture and the second mixture, for example, a method using a roll press can be mentioned. The roll press can process the mixture into a membrane by putting the mixture between a pair of rollers and driving the pair of rollers at a predetermined pressure and rotation speed.

[0028] [Battery manufacturing method] A battery can be manufactured using the free-standing membrane electrode manufactured by the method for manufacturing a free-standing membrane electrode of the present disclosure. The method for manufacturing a battery of the present disclosure includes the steps of manufacturing a free-standing membrane electrode by the method for manufacturing a free-standing membrane electrode described above and bonding the manufactured free-standing membrane electrode to a current collecting foil to form an electrode layer. Here, if the free-standing membrane electrode contains a negative electrode active material, the free-standing membrane electrode can be used to manufacture the negative electrode layer, and if the free-standing membrane electrode contains a positive electrode active material, the free-standing membrane electrode can be used to manufacture the positive electrode layer. In other words, the method for manufacturing a battery of the present disclosure may be used to manufacture a battery in which both the negative electrode layer and the positive electrode layer include the free-standing membrane electrode described above, or may be used to manufacture a battery in which one of the negative electrode layer and the positive electrode layer includes the free-standing membrane electrode described above.

[0029] The positive current collector foil constituting the positive electrode layer is not particularly limited and may be in the form of a foil, plate, mesh, punching metal, or foam. Examples of the metal constituting the positive current collector foil include Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, stainless steel, and the like. In particular, from the viewpoint of ensuring oxidation resistance, etc., the positive current collector foil may contain Al.

[0030] The positive electrode active material is not particularly limited, and conventionally known materials can be appropriately used. Examples of the positive electrode active material include LiCoO2, LiNiO2, LiMnO2, LiMn2O4, Li(NiCoMn)O2, Li(NiCoAl)O2, and LiFePO4. Note that the positive electrode active material particles may be Hi-Nickel (positive electrode active material with a high Ni ratio) or a ternary positive electrode active material.

[0031] The negative current collector foil constituting the negative electrode layer is not particularly limited and may be in the form of a foil, plate, mesh, punching metal, or foam. Examples of the metal constituting the negative current collector foil include Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, stainless steel, and the like. In particular, from the viewpoints of ensuring reduction resistance and being difficult to alloy with lithium, the negative current collector foil may contain at least one metal selected from the group consisting of Cu, Ni, and stainless steel.

[0032] The negative electrode active material is not particularly limited, and conventionally known materials can be appropriately used. Examples of the negative electrode active material include graphite, Si, SiOx (0 < x < 2), and Li4Ti5O 12 can be mentioned.

[0033] Moreover, the manufacturing method of the battery of the present disclosure can be applied to any battery manufacturing method as long as it is a battery provided with a positive electrode layer and a negative electrode layer. As an embodiment of the battery, a lithium ion secondary battery can be mentioned. The lithium ion secondary battery 1 shown as this embodiment includes a positive electrode layer, a negative electrode layer, and an electrolyte layer disposed between the positive electrode layer and the negative electrode layer.

[0034] <Electrolytes> The lithium ion secondary battery may have an electrolyte layer that does not contain a solid electrolyte but contains a liquid electrolyte, or that does not contain a liquid electrolyte but contains a solid electrolyte, or that contains a liquid electrolyte and a solid electrolyte. When the electrolyte layer contains a liquid electrolyte, it preferably has a separator that retains the liquid electrolyte and prevents contact between the positive electrode layer and the negative electrode layer. When the electrolyte layer contains a solid electrolyte, the electrolyte layer may optionally contain a binder or the like in addition to the solid electrolyte.

[0035] The solid electrolyte can be any solid electrolyte typically used in solid-state batteries, without any limitations. Crystalline nitrides, oxides, sulfides, and oxoacid salts, as well as amorphous glass-structured materials, can be used as such solid electrolytes. The liquid electrolyte can be any non-aqueous electrolyte typically used in non-aqueous lithium-ion secondary batteries, without any limitations. The separator used with the liquid electrolyte may be any separator typically used in non-aqueous lithium-ion secondary batteries, such as those containing resins such as polyethylene (PE), polypropylene (PP), polyester, and polyamide.

[0036] As described above, according to the battery manufacturing method of the present disclosure, since the battery is provided with electrode layers (either a negative electrode layer or a positive electrode layer) using self-supporting membrane electrodes, it is possible to manufacture a battery with low electrode resistance and excellent battery characteristics. [Example]

[0037] Hereinafter, the solid state battery according to the present disclosure will be described with reference to examples, but the technical scope of the present disclosure is not limited to the following examples.

[0038] [Example 1] <Fabrication of free-standing membrane electrodes> In this example, a negative electrode layer was fabricated using a freestanding membrane electrode. The resistance (powder resistance) of the powder contained in the mixture of the first and second mixtures, the electrode resistance of the fabricated negative electrode layer, and the tensile strength of the negative electrode layer were measured. First, graphite (manufactured by JIANGXI ZICHEN TECHNOLOGY) with a particle size (D50) of 20 μm and PTFE (manufactured by Chemours) were mixed at 97% by mass and 3% by mass, respectively. A mixer (manufactured by Nippon Coke) was used to mix the mixture at 300 rpm for 180 seconds, and then at 3,000 rpm for 8 minutes to uniformly disperse the PTFE. The resulting powder was the second mixture. Next, a portion of the obtained powder was taken out and kneaded using a batch kneader (manufactured by Nippon Spindle) at 100°C and 10 rpm for 180 seconds to fiberize the PTFE contained in the powder. The powder obtained in this manner is the first mixture.

[0039] Next, the first mixture and the second mixture were mixed using the mixer at 300 rpm for 5 minutes to obtain a mixture. Then, using a roll press (manufactured by Tester Sangyo Co., Ltd.), the obtained mixture was rolled to form a membrane under a linear pressure of 0.4 t / cm. A free-standing membrane electrode was thus produced. In Example 1, the first mixture and the second mixture were mixed in a mass ratio of 70:30. That is, in Example 1, the free-standing membrane electrode was produced so that the fiberized PTFE (first PTFE powder) accounted for 70 mass% of the total of the fiberized PTFE (first PTFE powder: aspect ratio = 10.5) and the non-fiberized PTFE (second PTFE powder: aspect ratio = 1.3).

[0040] Next, the fabricated free-standing membrane electrode was placed on a copper foil with a thickness of 8 μm, and the foil was laminated using a flat press (manufactured by AS ONE) under a load of 5 t. In this way, a negative electrode layer using the free-standing membrane electrode was fabricated.

[0041] <Powder Resistivity Measurement> The powder resistivity of the mixture obtained by mixing the first mixture and the second mixture was measured using an automatic powder resistivity measurement system, low resistance version MCP-PD600 (manufactured by Nitto Seiko Analytech).

[0042] <Electrode resistance measurement> The electrode resistance of the produced negative electrode layer was measured using an electrode resistance measurement system (product name: RM2610, manufactured by Hioki E.E. Corporation).

[0043] <Tensile strength measurement> The tensile strength of the produced negative electrode layer was measured using a tensile strength measurement system (manufactured by Eiko Seiki Co., Ltd.).

[0044] [Example 2] A self-supporting membrane electrode was produced in the same manner as in Example 1, except that the first mixture and the second mixture were mixed in a mass ratio of 50:50, and the powder resistance, electrode resistance, and tensile strength were measured. That is, in this example, the self-supporting membrane electrode was produced so that the fiberized PTFE (first PTFE powder) accounted for 50 mass% of the total of the fiberized PTFE (first PTFE powder) and the non-fiberized PTFE (second PTFE powder).

[0045] [Example 3] A self-supporting membrane electrode was produced in the same manner as in Example 1, except that the first mixture and the second mixture were mixed at a mass ratio of 30:70, and the powder resistance, electrode resistance, and tensile strength were measured. That is, in this example, the self-supporting membrane electrode was produced so that the fiberized PTFE (first PTFE powder) accounted for 30 mass% of the total of the fiberized PTFE (first PTFE powder) and the non-fiberized PTFE (second PTFE powder).

[0046] [Comparative Example 1] A free-standing membrane electrode was produced using the first mixture without using the second mixture, and the powder resistance, electrode resistance, and tensile strength were measured in the same manner as in Example 1. That is, in this example, the free-standing membrane electrode was produced so as to contain 100 mass % of fibrous PTFE (first PTFE powder).

[0047] [result] An electron microscope photograph of the first mixture is shown in Figure 1, and an electron microscope photograph of the second mixture is shown in Figure 2. The powder resistance, electrode resistance, and tensile strength measured in Examples 1 to 3 and Comparative Example 1 are summarized in Table 1.

[0048] [Table 1]

[0049] As shown in Figure 1, it was found that by applying shear stress to a mixture containing an active material and PTFE and kneading it, the PTFE was fibrous so as to cover the entire active material. In Comparative Example 1, which used only the first mixture in which the active material was entirely covered with fibrous PTFE, the powder resistance and electrode resistance showed high values.

[0050] In contrast, it was found that simply mixing the active material and PTFE did not result in the active material being covered with PTFE, as shown in Figure 2. It was also found that using the first mixture and the second mixture, in which the active material was not covered with PTFE, could reduce both the powder resistance and the electrode resistance, as shown in Examples 1 to 3. In particular, it was found that when the mixing ratio of fibrous PTFE to particulate PTFE powder was in the range of 50:50 to 70:30 by mass, it was possible to keep the powder resistance and the electrode resistance low, and to increase the tensile strength of the produced free-standing membrane electrode.

Claims

1. a step of mixing a first mixture of an active material and a first polytetrafluoroethylene powder fiberized to an aspect ratio of 10 or more with a second mixture of an active material and a second polytetrafluoroethylene powder having an aspect ratio of 1.3 or less, in a mixing ratio of the first polytetrafluoroethylene powder to the second polytetrafluoroethylene powder in a range of 30:70 to 70:30 by mass; rolling a mixture of the first mixture and the second mixture; A method for manufacturing a free-standing membrane electrode, comprising:

2. The method for producing a free-standing membrane electrode according to claim 1 , wherein the active material comprises graphite.

3. The method for producing a free-standing membrane electrode according to claim 1 , further comprising the step of applying shear stress to a mixture containing the active material and polytetrafluoroethylene to prepare the first mixture.

4. A step of producing a free-standing membrane electrode by the method for producing a free-standing membrane electrode according to any one of claims 1 to 3; a step of bonding the produced free-standing membrane electrode and a current collecting foil to form an electrode layer; A method for manufacturing a battery comprising:

5. The method for manufacturing a battery according to claim 4 , wherein the electrode layer is a negative electrode layer.

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