JP7838538B2 - Method for manufacturing an electrode active material layer, and method for manufacturing a battery - Google Patents

Description

This disclosure relates to a method for manufacturing an electrode active material layer and a method for manufacturing a battery.

The electrode active material layer of a lithium-ion battery can be manufactured by applying an electrode mixture slurry containing the electrode active material and binder to a current collector and drying it. During the drying process, the binder may migrate to the slurry surface, reducing the amount of binder near the current collector. As a result, the adhesive strength between the electrode active material layer and the current collector decreases. Technologies have been developed to improve this adhesive strength.

For example, Patent Document 1 discloses a negative electrode comprising a current collector and a negative electrode active material layer located on at least one side of the current collector, containing artificial graphite, single-walled carbon nanotubes (SWCNTs), and a binder polymer. The negative electrode active material layer comprises a lower region facing the current collector and an upper region facing the lower region and extending to the surface of the negative electrode active material layer. The content of the single-walled carbon nanotubes is 0.003 to 0.07 parts by weight based on 100 parts by weight of the lower region, the average diameter of the single-walled carbon nanotubes is 0.5 to 15 nm, and the upper region does not contain single-walled carbon nanotubes (SWCNTs).

Patent Document 1 discloses that, in order to manufacture such a negative electrode, a slurry for the lower layer is produced by mixing artificial graphite as the negative electrode active material, single-walled carbon nanotubes (SWCNTs), styrene-butadiene rubber (SBR) as a binder polymer, and water, and an upper layer slurry is produced in the same manner as the lower layer slurry, except that it does not contain SWCNTs. Patent Document 1 further discloses that the lower layer slurry is coated on one side of a copper (Cu) clad thin film used as the negative electrode current collector, the upper layer slurry is coated on top of this slurry, and both slurries are dried.

Special Publication No. 2023-522659

This disclosure aims to provide a method for manufacturing an electrode active material layer that allows for simpler and more uniform dispersion of a binder within the electrode active material layer, in order to suppress a decrease in the adhesive strength between the electrode active material layer and the current collector, and a method for manufacturing a battery containing the electrode active material layer manufactured in this way.

The Disclosing Party has found that the above-mentioned problems can be solved by the following means.
<Aspect 1>
A method for producing an electrode active material layer, including the following steps:
A preliminary slurry is obtained by mixing a fibrous material and a binder in a dispersion medium.
The obtained preliminary slurry and electrode active material are mixed to obtain an electrode mixture slurry, and the obtained electrode mixture slurry is applied to a substrate, and the dispersion medium is dried and removed.
<Aspect 2>
The fibrous material is a carbon nanotube.
The method according to embodiment 1.
<Aspect 3>
The binder is styrene-butadiene rubber.
The method according to embodiment 2.
<Aspect 4>
The heat source in the drying process is hot air or a laser.
The method according to any one of embodiments 1 to 3.
<Aspect 5>
The method described in any one of embodiments 1 to 4 is used to produce an electrode active material layer,
Battery manufacturing method.

According to this disclosure, a method for manufacturing an electrode active material layer that allows for more easily and uniformly dispersing a binder within the electrode active material layer in order to suppress a decrease in the adhesive strength between the electrode active material layer and the current collector, and a method for manufacturing a battery containing the electrode active material layer manufactured in this way can be provided.

Figure 1(a) is a schematic diagram showing an electrode mixture slurry prepared by the method according to the embodiment of this disclosure, and Figure 1(b) is a schematic diagram showing an electrode mixture slurry prepared by the method according to the comparative example of this disclosure. Figure 2(a) is an SEM image of an electrode active material layer manufactured by the method according to the embodiment of this disclosure, and Figure 2(b) is an SEM image of an electrode active material layer manufactured by the method according to the comparative example of this disclosure. Figure 3(a) is a cross-sectional SEM image of an electrode active material layer manufactured by the method according to the embodiment of this disclosure, and Figure 3(b) is a cross-sectional SEM image of an electrode active material layer manufactured by the method according to the comparative example of this disclosure.

The embodiments of this disclosure will be described in detail below. However, this disclosure is not limited to the embodiments described below, and can be implemented in various modified forms within the scope of the essence of the disclosure.

Method for manufacturing electrode active material layer
A method for producing an electrode active material layer according to this disclosure includes mixing a fibrous material and a binder in a dispersion medium to obtain a preliminary slurry, mixing the obtained preliminary slurry and an electrode active material to obtain an electrode composite slurry, and applying the obtained electrode composite slurry to a substrate and drying off the dispersion medium.

As described above, the electrode active material layer of a lithium-ion battery can be manufactured by applying an electrode mixture slurry containing the electrode active material and binder to a current collector and drying it. During the drying process, the binder may migrate to the slurry surface, potentially reducing the amount of binder near the current collector.

In this regard, the Disclosing Parties have found that by mixing fibrous material and a binder in a dispersion medium to prepare a preliminary slurry, and then mixing this preliminary slurry with electrode active material to prepare an electrode composite slurry, the binder can be uniformly dispersed in the resulting electrode active material layer. While not intended to be bound by any particular theory, it is believed that the binder becomes entangled and trapped by the fibrous material in the preliminary slurry, thereby suppressing the binder's migration to the slurry surface during the drying of the subsequently prepared electrode composite slurry. As a result, the binder can be uniformly dispersed in the electrode active material layer.

In other words, as shown in Figure 1(a), in the electrode mixture slurry 1 prepared after preparing a preliminary slurry, the binder 20 is entangled and captured by the fibrous material 10, thus suppressing the movement of the binder 20 to the slurry surface. In contrast, as shown in Figure 1(b), in the electrode mixture slurry 1 prepared without preparing a preliminary slurry, the binder 20 is not sufficiently captured by the fibrous material 10, thus failing to suppress the movement of the binder 20 to the slurry surface.

<Preliminary slurry preparation process>
The method of this disclosure includes mixing a fibrous material and a binder in a dispersion medium to obtain a preliminary slurry.

The fibrous material is not particularly limited as long as it has the function of entangling and capturing the binder. For example, the fibrous material may be, but is not limited to, materials such as carbon nanotubes (CNTs) or carbon nanofibers (CNFs), or combinations thereof. The carbon nanotubes may be single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), or combinations thereof.

The average diameter of the fibrous material may be 0.1 nm or more, 1.0 nm or more, 5.0 nm or more, 10 nm or more, or 15 nm or more, and may be 100 nm or less, 75 nm or less, 50 nm or less, 30 nm or less, or 25 nm or less.

The fibrous material content may be greater than 0 parts by mass, 0.001 parts by mass or more, 0.003 parts by mass or more, or 0.005 parts by mass or more, per 100 parts by mass of electrode composite material, and may be 1.0 parts by mass or less, 0.1 parts by mass or less, or 0.01 parts by mass or less.

Furthermore, in this disclosure, "electrode mixture" means a composition that can constitute an electrode active material layer, either in its own form or by further containing other components.

The binder is not particularly limited as long as it is one that is commonly used as a binder for electrode active material layers. For example, it may be, but is not limited to, materials such as polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HEP), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyacrylic acid, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, polyvinylpyrrolidone, carboxymethylcellulose (CMC), hydroxypropylcellulose, regenerated cellulose, polyethylene, polypropylene, starch, butadiene rubber (BR), styrene- butadiene rubber (SBR), fluororubber, or combinations thereof. The binder may be in the form of colloidal particles.

The binder can be a material with high affinity for the fibrous material. For example, if the fibrous material is formed by a six-membered ring network such as CNTs, using a binder with aromatic rings such as SBR is thought to allow the fibrous material to capture the binder more efficiently through π-π interactions.

The binder content may be 10 parts by mass or more, 15 parts by mass or more, 25 parts by mass or more, or 30 parts by mass or more, per 100 parts by mass of electrode composite material, and may be 50 parts by mass or less, 40 parts by mass or less, 35 parts by mass or less, or 33 parts by mass or less.

The mass ratio of fibrous material to binder in the electrode composite may be 1:3000 to 1:6000, 1:3500 to 1:5500, or 1:4000 to 1:5000.

The dispersion medium is not particularly limited as long as it can disperse the fibrous material and the binder. For example, the dispersion medium may be water, and organic solvents such as N-methylpyrrolidone and acetone, but is not limited to these.

The method for mixing the fibrous material and the binder in the dispersion medium is not particularly limited, but one example is mixing them in a kneader. In this case, the shear rate may be greater than 0 rpm, 1 rpm or more, 3 rpm or more, or 5 rpm or more, and may be 300 rpm or less, 200 rpm or less, 100 rpm or less, or 50 rpm or less. The kneading time may be greater than 0 minutes, 1 minute or more, 3 minutes or more, or 5 minutes or more, and may be 60 minutes or less, 30 minutes or less, 20 minutes or less, or 10 minutes or less.

<Electrode mixture slurry preparation process>
The method disclosed herein includes mixing the obtained preliminary slurry and electrode active material to obtain an electrode composite slurry.

In this disclosure, "electrode mixture slurry" means a slurry that includes a dispersion medium in addition to the "electrode mixture," and which can be applied and dried to form an electrode active material layer.

The electrode active material is not particularly limited as long as it can be used as an electrode active material in a battery. Therefore, the electrode active material may be either a positive electrode active material or a negative electrode active material. From known electrode active materials, two materials with different potentials for intercalating and releasing predetermined ions (charge/discharge potentials) can be selected, and the material exhibiting a noble potential can be used as the positive electrode active material, and the material exhibiting a base potential can be used as the negative electrode active material.

Any known active material can be used as the positive electrode active material. For example, when constructing a lithium-ion battery, various lithium-containing composite oxides such as lithium cobaltate, lithium nickelate, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , lithium manganeseate, and spinel-based lithium compounds can be used as the positive electrode active material. In addition, lithium iron phosphate (LFP) can be used as an olivine-type positive electrode active material. The positive electrode active material may be in particulate form, for example, and its size is not particularly limited.

Any known active material can be used as the negative electrode active material. For example, when constructing a lithium-ion battery, silicon-based active materials such as silicon, silicon alloys, and silicon oxide; carbon-based active materials such as graphite and hard carbon; various oxide-based active materials such as lithium titanate; metallic lithium and lithium alloys can be used as the negative electrode active material. The negative electrode active material may be in particulate form, for example, and its size is not particularly limited.

Furthermore, in this disclosure, the electrode active material is used particularly as the negative electrode active material.

The content of the electrode active material may be 50 parts by mass or more, 55 parts by mass or more, 60 parts by mass or more, or 65 parts by mass or more, per 100 parts by mass of the electrode mixture, and may be 90 parts by mass or less, 85 parts by mass or less, 80 parts by mass or less, or 75 parts by mass or less.

<Drying process>
The method disclosed herein includes applying the obtained electrode mixture slurry to a substrate and then drying and removing the dispersion medium.

The substrate is not particularly limited, but it may be a current collector foil that constitutes the electrode together with the electrode active material layer.

The current collector foil is not particularly limited as long as it can be used as a current collector foil in a battery. For example, in the case of a lithium-ion battery, aluminum foil or copper foil may be used.

The method for applying the electrode mixture slurry to the substrate is not particularly limited, but a blade coating method is an example.

The method for drying and removing the dispersion medium is not particularly limited. For example, the heat source in the drying process may be hot air or a laser. The drying temperature and drying time can be determined as needed, depending on the boiling point of the dispersion medium used.

One method for confirming that the binder is entangled and trapped in the fibrous material in the electrode active material layer manufactured by the method disclosed herein is to acquire an image of the electrode active material layer using a scanning electron microscope (SEM) and observe it.

One method for confirming that the binder is uniformly dispersed in the electrode active material layer manufactured by the method disclosed herein is to obtain a cross-sectional SEM image of the electrode active material layer and observe the degree of binder dispersion.

Furthermore, the degree of binder dispersion in the electrode active material layer can be quantified, for example, by the following method. Specifically, the acquired cross-sectional SEM image is first imported into image processing software (Image J) and quantified. The digital image data is in the form of a matrix corresponding to the cross-sectional SEM image of the electrode active material layer, with a numerical value assigned to each pixel. This numerical value is represented in 8-bit format with 2 to the power of 8, or 256 gradations, and takes integer values from 0 to 255, but is binarized to be represented as 0 and 1. Binarization is performed by counting the numerical values displayed from 0 to 255, and setting the top 10% of the values to "1" and the remaining values to "0". Next, the binarized numerical values are added in each row to calculate the total value for each row. Furthermore, the total values for each row are added for the row corresponding to the upper half of the electrode active material layer in the thickness direction. This is called the "total value for the upper half of the electrode active material layer". Similarly, the "total value for the lower half of the electrode active material layer" is calculated. Based on these, the degree of dispersion of the binder in the electrode active material layer can be quantified using the following formula, and this can be called the "binder dispersion index":
The variance index of the binder = total value in the upper half of the electrode active material layer ÷ total value in the lower half of the electrode active material layer ... (1).

The binder's variance index may be 1.00 or greater, 1.05 or greater, 1.10 or greater, or 1.15 or greater, and may be less than 1.90, 1.80 or less, 1.70 or less, 1.60 or less, 1.50 or less, 1.40 or less, 1.30 or less, or 1.20 or less.

Furthermore, a smaller dispersion index value for the binder indicates that the binder is more uniformly dispersed within the electrode active material layer.

Battery manufacturing method
The method for manufacturing a battery according to this disclosure includes manufacturing an electrode active material layer by the method of this disclosure. For a method for manufacturing an electrode active material layer, refer to the above description of the method for manufacturing an electrode active material layer according to this disclosure.

The method for manufacturing a battery according to this disclosure may include, in addition to manufacturing the electrode active material layer, arranging the negative electrode current collector layer, negative electrode active material layer, electrolyte layer, positive electrode active material layer, and positive electrode current collector layer in this order. In this case, either or both of the negative electrode active material layer and the positive electrode active material layer may be electrode active material layers manufactured by the method of this disclosure.

Example 1
<Formation of electrode active material layer>
(Preliminary slurry preparation process)
0.01 parts by mass of single-walled carbon nanotubes (SWCNTs) as a fibrous material (average diameter 20 nm), a 3% aqueous solution of styrene-butadiene rubber (SBR) binder (45 parts by mass as SBR), and water as a dispersion medium were placed in a kneader and kneaded at a shear rate of 10 rpm for 30 minutes. This yielded a preliminary slurry.

(Electrode mixture slurry preparation process)
96.6 parts by mass of graphite as the negative electrode active material, 0.4 parts by mass of carboxymethylcellulose (CMC) as a dispersant, and water as a dispersion medium were placed in a kneader separate from the one used to prepare the preliminary slurry, and kneaded at a shear rate of 10 rpm for 30 minutes. This mixture was then kneaded with the preliminary slurry to obtain the electrode mixture slurry.

(drying process)
The obtained electrode mixture slurry was applied to a substrate, and the dispersion medium in the slurry was dried and removed by laser irradiation to form the electrode active material layer of Example 1.

Comparative Example 1
Without preparing a preliminary slurry, the electrode active material layer was formed as follows. First, 96.6 parts by mass of graphite as the negative electrode active material and 0.4 parts by mass of CMC as a dispersant were kneaded to obtain a mixture. Next, 0.01 parts by mass of SWCNT as a fibrous material and water as a dispersion medium were kneaded together with the mixture to obtain a slurry. Furthermore, a 3% aqueous solution of SBR-based binder (45 parts by mass as SBR) and water as a dispersion medium were kneaded together with the slurry to obtain an electrode composite slurry. The kneading conditions were the same as in Example 1. The dispersion medium of the obtained electrode composite slurry was dried in the same manner as in Example 1 to form the electrode active material layer of Comparative Example 1.

"evaluation"
<Evaluation using SEM images>
Scanning electron microscope (SEM) images of the electrode active material layer in each case were acquired, and the degree of binder capture by the fibrous material was evaluated.

<Evaluation using cross-sectional SEM images>
Cross-sectional SEM images of the electrode active material layer were obtained for each example, and the degree of binder dispersion in the electrode active material layer was observed.

Furthermore, the degree of binder dispersion in the electrode active material layer was quantified using the following method. Specifically, the acquired cross-sectional SEM images were first imported into image processing software (Image J) and quantified. The digital image data was in the form of a matrix corresponding to the cross-sectional SEM image of the electrode active material layer, with a numerical value assigned to each pixel. This numerical value was represented in 8-bit format with 2 to the power of 8 = 256 gradations, taking integer values from 0 to 255, but was binarized to represent 0 and 1. Binarization was performed by counting the numerical values displayed from 0 to 255, and setting the top 10% of the values to "1" and the remaining values to "0". Next, the binarized numerical values were added in each row to calculate the total value for each row. Furthermore, the total values for each row were added for the row corresponding to the upper half in the thickness direction of the electrode active material layer. This was defined as the "total value in the upper half of the electrode active material layer". Similarly, the "total value in the lower half of the electrode active material layer" was calculated. Based on these, the degree of dispersion of the binder in the electrode active material layer was quantified using the following formula, and this was defined as the binder dispersion index:
The variance index of the binder = total value in the upper half of the electrode active material layer ÷ total value in the lower half of the electrode active material layer ... (1).

"result"
<Evaluation results using SEM images>
SEM images of the electrode active material layers in each example are shown in Figure 2. As shown in Figure 2(a), in the electrode active material layer formed by the method of the example, it was confirmed that the SBR, acting as a binder, was trapped in the CNT, which acted as a fibrous material. In contrast, as shown in Figure 2(b), in the electrode active material layer formed by the method of the comparative example, it was not confirmed that the SBR, acting as a binder, was trapped in the CNT, which acted as a fibrous material.

<Evaluation results using cross-sectional SEM images>
Figure 3 shows cross-sectional SEM images of the electrode active material layers in each example. As shown in Figure 3(a), in the electrode active material layer formed by the method of the example, it was confirmed that the SBR, acting as a binder, was uniformly dispersed throughout the electrode active material layer. In contrast, as shown in Figure 3(b), in the electrode active material layer formed by the method of the comparative example, it was confirmed that the SBR, acting as a binder, was unevenly distributed towards the surface of the electrode active material layer.

Furthermore, the dispersion index of the binder calculated using the above formula (1) was 1.17 for the electrode active material layer of Example 1, and 1.90 for the electrode active material layer of Comparative Example 1.

1. Electrode mixture slurry 10. Fibrous material 20. Binder 30. Electrode active material 40. Dispersion medium 50. Base material.

Claims (5)

A method for producing an electrode active material layer, including the following steps:
A preliminary slurry is obtained by mixing a fibrous material and a binder in a dispersion medium, where the mass ratio of the fibrous material to the binder is 1:3000 to 1:6000.
The obtained preliminary slurry and electrode active material are mixed to obtain an electrode mixture slurry, and the obtained electrode mixture slurry is applied to a substrate, and the dispersion medium is dried and removed. The fibrous material is a carbon nanotube.
The method according to claim 1. The binder is styrene-butadiene rubber.
The method according to claim 2. The method according to claim 1, wherein the heat source in the drying step is hot air or a laser. A method for manufacturing a battery, comprising manufacturing an electrode active material layer by the method described in any one of claims 1 to 4.