JP7576741B2 - Method for manufacturing positive electrode for lithium ion secondary battery - Google Patents

Description

The present invention relates to a method for producing a positive electrode for a lithium ion secondary battery, a positive electrode for a lithium ion secondary battery, and a lithium ion secondary battery.

A lithium-ion secondary battery is a secondary battery that can be charged and discharged by the movement of lithium ions in an electrolyte between a positive electrode and a negative electrode, which store and release lithium ions.

Patent Document 1 discloses a technology relating to a manufacturing method for electrodes for lithium ion secondary batteries that can omit or simplify the rolling process. The technology disclosed in Patent Document 1 makes it possible to omit or simplify the rolling process by using a negative electrode active material powder and a positive electrode conductive agent powder in which a polymer compound component is chemically covalently bonded to the particle surface.

JP 2002-100360 A

The positive electrode of a lithium-ion secondary battery comprises a positive electrode current collector and a positive electrode mixture layer formed on the positive electrode current collector. The positive electrode mixture layer contains a positive electrode active material and a conductive material, and a conductive path is formed in the positive electrode mixture layer using the conductive material.

Here, in order to form a sufficient conductive path in the positive electrode mixture layer, it is necessary to increase the density of the positive electrode mixture layer using a pressing process. However, when the density of the positive electrode mixture layer is increased using a pressing process, the porosity of the positive electrode mixture layer decreases, resulting in a problem of reduced output of the lithium-ion secondary battery.

In view of the above problems, the object of the present invention is to provide a method for manufacturing a positive electrode for a lithium ion secondary battery that can form a sufficient conductive path in the positive electrode mixture layer and increase the porosity of the positive electrode mixture layer, a positive electrode for a lithium ion secondary battery, and a lithium ion secondary battery.

The method for producing a positive electrode for a lithium ion secondary battery according to one aspect of the present invention is a method for producing a positive electrode for a lithium ion secondary battery comprising a positive electrode current collector and a positive electrode composite layer formed on the positive electrode current collector, and includes the steps of preparing a positive electrode paste containing at least a positive electrode active material, a conductive material, and a solvent, applying the positive electrode paste to the positive electrode current collector, and drying the applied positive electrode paste to form the positive electrode composite layer on the positive electrode current collector. The conductive material contained in the positive electrode paste has an aspect ratio of 80 or more, and is characterized in that a pressing step is not included after drying the positive electrode paste.

The positive electrode for a lithium-ion secondary battery according to one aspect of the present invention comprises a positive electrode current collector and a positive electrode composite layer formed on the positive electrode current collector, the positive electrode composite layer containing at least a positive electrode active material and a conductive material, and the conductive material has an aspect ratio of 80 or more.

The lithium ion secondary battery according to one aspect of the present invention is a lithium ion secondary battery having the above-mentioned positive electrode for lithium ion secondary batteries.

The present invention provides a method for producing a positive electrode for a lithium ion secondary battery that can form a sufficient conductive path in the positive electrode mixture layer and increase the porosity of the positive electrode mixture layer, a positive electrode for a lithium ion secondary battery, and a lithium ion secondary battery.

FIG. 2 is a cross-sectional view for explaining a configuration example of a positive electrode for a lithium ion secondary battery according to an embodiment. FIG. 2 is a schematic diagram for explaining details of a positive electrode mixture layer included in the positive electrode for a lithium ion secondary battery according to the embodiment. 2 is a flowchart for explaining a method for producing a positive electrode for a lithium ion secondary battery according to an embodiment. FIG. 4 is a cross-sectional view for illustrating another configuration example of the positive electrode for a lithium ion secondary battery according to the embodiment. 1 is a graph showing the relationship between the porosity and resistance value of a positive electrode mixture layer.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
Fig. 1 is a cross-sectional view for explaining a configuration example of a positive electrode for a lithium ion secondary battery according to the present embodiment. Fig. 2 is a schematic diagram for explaining details of a positive electrode mixture layer included in the positive electrode for a lithium ion secondary battery according to the present embodiment. As shown in Fig. 1, the positive electrode for a lithium ion secondary battery 1 according to the present embodiment includes a positive electrode current collector 10 and a positive electrode mixture layer 11 formed on the positive electrode current collector 10.

The positive electrode collector 10 is made of a metal foil or a metal plate. For example, aluminum or an alloy mainly composed of aluminum can be used for the positive electrode collector 10.

As shown in FIG. 2, the positive electrode composite layer 11 contains at least a positive electrode active material 15 and a conductive material 16.

The positive electrode active material 15 is a material capable of absorbing and releasing lithium ions, and may be, for example, lithium cobalt oxide (LiCoO ₂ ), lithium manganese oxide (LiMn ₂ O ₄ ), lithium nickel oxide (LiNiO ₂ ), lithium aluminate (LiAlO ₂ ), or the like. For example, an NCA-based material in which lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), and lithium aluminate (LiAlO ₂ ) are mixed in any ratio may be used. As an example, LiNi _0.8 Co _0.15 Al _0.05 O ₂ may be used. Also, a material in which LiCoO ₂ , LiMn ₂ O ₄ , and LiNiO ₂ are mixed in any ratio may be used. For example, lithium nickel cobalt manganese oxide (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ), which is a mixture of these materials in equal proportions, may be used.

The particle size of the positive electrode active material is, for example, 3 to 15 μm. In the present invention, the particle size of each material is the median diameter D50, which is a value measured using a laser diffraction/scattering method. The positive electrode active material is not limited to these materials, and any material that can absorb and release lithium ions may be used.

Conductive material 16 is a material for forming a conductive path in positive electrode composite layer 11, and is adjusted so that the aspect ratio is 80 or more. In this embodiment, the aspect ratio of conductive material 16 is preferably 100 or more. An aspect ratio exceeding 1000 is also not preferable because the conductive material 16 does not disperse when made into a paste. As shown in FIG. 2, in the positive electrode according to this embodiment, a part of the surface of positive electrode active material 15 is covered with conductive material 16.
Note that the aspect ratio referred to here is the aspect ratio after the positive electrode mixture layer has been applied and dried. There are cases in which the conductive material is cut when dispersed in a solvent using the positive electrode paste. In such cases, the conductive material is selected taking this into consideration.

The conductive material 16 may also have functional groups on its surface. The amount of functional groups contained in the conductive material 16 may be 0.1% by mass or more and 30% by mass or less, preferably 1% by mass or more and 5% by mass or less, relative to the mass of the conductive material 16.

When the conductive material 16 has a functional group on its surface, the functional groups on the conductive material 16 attract each other, and the conductive materials 16 are electrically connected to each other (see reference numeral 18 in FIG. 2). Thus, a conductive path can be efficiently formed in the positive electrode composite layer 11. That is, when the conductive material 16 has a functional group on its surface, the conductive materials 16 (which may be the conductive material 16 and the positive electrode active material 15) in the positive electrode paste come into contact with each other through weak bonds such as hydrogen bonds in a fluid state immediately after coating in the coating process described below (see step S2 in FIG. 3), and a dense conductive network (conductive path) is formed.

As the functional group, a functional group containing oxygen, such as O-C=O, O=C-N, or O-H, is preferable. Since oxygen has a high electronegativity, it easily forms hydrogen bonds with the functional group of the conductive material. Therefore, the conductive materials 16 (or the conductive material 16 and the positive electrode active material 15) bond to each other through hydrogen bonds.

The conductive material 13 may be, for example, a carbon nanotube. When forming functional groups on the surface of the conductive material 16, an acid treatment, a base treatment, or the like may be performed.

In this embodiment, the amount of conductive material 16 relative to the positive electrode active material 15 can be 0.1% by mass or more and 10% by mass or less, and preferably 0.1% by mass or more and 2% by mass or less.

The positive electrode mixture layer 11 may further contain a dispersant and a binder.

Examples of dispersants include polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyvinylpyrrolidone (PVP), polyacrylate, polymethacrylate, polyoxyethylene alkyl ether, polyalkylene polyamine, and benzimidazole.

For example, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), polyacrylic acid, polyacrylate, etc. can be used for the binder 14.

In the present embodiment, the density of positive electrode mixture layer 11 is preferably 1.2 to 2.4 g/cm ^3. The porosity of positive electrode mixture layer 11 is preferably 50% or more and 70% or less. The film thickness of positive electrode mixture layer 11 is preferably 20 to 150 μm.

Next, a method for manufacturing a positive electrode for a lithium ion secondary battery according to the present embodiment will be described. FIG. 3 is a flowchart for explaining the method for manufacturing a positive electrode for a lithium ion secondary battery according to the present embodiment.

When manufacturing a positive electrode for a lithium ion secondary battery, a positive electrode paste is first prepared (step S1). Specifically, a positive electrode paste containing at least a positive electrode active material, a conductive material, and a solvent is prepared. The above-mentioned materials can be used for the positive electrode active material and the conductive material. For example, isopropyl alcohol, N-methylpyrrolidone, water, etc. can be used for the solvent. The positive electrode paste may further contain a dispersant and a binder. The above-mentioned materials can be used for the dispersant and the binder. In this embodiment, the aspect ratio of the conductive material contained in the positive electrode paste is set to 80 or more.

Next, the positive electrode paste prepared in step S1 is applied onto the positive electrode current collector 10 (step S2). The above-mentioned materials can be used for the positive electrode current collector 10. The viscosity of the positive electrode paste is preferably 500 to 30,000 mps. If the viscosity of the positive electrode paste is too low, it becomes difficult to apply the positive electrode paste onto the positive electrode current collector 10. If the viscosity of the positive electrode paste is too high, the fluidity of the conductive material in the positive electrode paste becomes poor when the positive electrode paste is applied onto the positive electrode current collector 10, preventing the formation of a conductive path.

Then, the positive electrode paste applied to the positive electrode current collector 10 is dried to form a positive electrode composite layer 11 on the positive electrode current collector 10 (step S3). The drying temperature at this time is 100°C to 200°C. Through these steps, a positive electrode 1 for a lithium ion secondary battery as shown in FIG. 1 can be manufactured. In this embodiment, it is preferable that the porosity of the positive electrode composite layer of the manufactured positive electrode is 50% or more and 70% or less. The porosity of the positive electrode composite layer 11 can be adjusted by changing the material used for the positive electrode paste, the drying speed when drying the positive electrode paste in step S3, etc.

This embodiment is characterized in that it does not include a pressing step after drying the positive electrode paste in step S3. In the present invention, the pressing step is a step of applying an external force to the positive electrode composite layer to compress the positive electrode composite layer, and is a step of reducing the porosity of the positive electrode composite layer by 5% or more. In other words, even if an external force is applied to the positive electrode composite layer, if the reduction in the porosity of the positive electrode composite layer is less than 5%, it is not included in the pressing step of the present invention. For example, a step of applying an external force to the positive electrode composite layer to adjust the shape of the positive electrode composite layer after forming the positive electrode composite layer (the reduction in the porosity of the positive electrode composite layer is less than 5%) is not included in the pressing step.

As described above, in the present embodiment, the conductive material 16 contained in the positive electrode composite layer 11 has an aspect ratio of 80 or more. When a conductive material with an aspect ratio of 80 or more is used in this manner, a dense conductive network (conductive path) can be formed in the positive electrode composite layer 11 (see FIG. 2).

On the other hand, the inventors have found that when a conductive material with an aspect ratio of 80 or more is used, the conductive path breaks when an external force is applied to the positive electrode mixture layer by the pressing process, and the electronic resistance of the positive electrode mixture layer increases. This is thought to be because, in a fluid paste state, the contact state between the conductive materials and between the conductive material and the active material is good, and pressing destroys this good state. For this reason, in the invention according to this embodiment, the pressing process of pressing the positive electrode mixture layer after drying the positive electrode paste in step S3 is omitted. By omitting the pressing process in this way, it is possible to prevent the conductive network (conductive path) in the positive electrode mixture layer from breaking, and the conductive path in the positive electrode mixture layer can be made dense. Therefore, it is possible to improve the electronic conductivity in the positive electrode mixture layer. Therefore, it is possible to suppress an increase in the resistance of the positive electrode for a lithium ion secondary battery. In other words, it is possible to suppress an increase in the internal resistance of the lithium ion secondary battery.

After the pressing process, the contact points between the positive electrode active material and the conductive material are separated, causing the electronic resistance of the positive electrode mixture layer to increase. However, if the positive electrode mixture layer is pressed further to increase the density of the positive electrode mixture layer, the contact points between the positive electrode active material and the conductive material gradually increase, causing the electronic resistance of the positive electrode mixture layer to decrease. However, even in this case, the electronic resistance of the positive electrode mixture layer is not as low as when the pressing process is omitted. When a conductive material with an aspect ratio of less than 80 is used, the resistance when not pressed is higher than the resistance when pressed, so the above phenomenon is considered to be unique to conductive materials with an aspect ratio of 80 or more.

In addition, in the present embodiment, the pressing process for pressing the positive electrode composite layer is omitted, so the porosity of the positive electrode composite layer can be increased. Therefore, the output of the lithium ion secondary battery can be increased.

As described above, in the present embodiment, the conductive material contained in the positive electrode composite layer has an aspect ratio of 80 or more, and the pressing step of pressing the positive electrode composite layer is omitted. Therefore, sufficient conductive paths are formed in the positive electrode composite layer, and the porosity of the positive electrode composite layer can be increased.

In addition, in the present embodiment, the pressing process for pressing the positive electrode composite layer can be omitted, simplifying the manufacturing process for the positive electrode for lithium ion secondary batteries.

Next, another example of the configuration of the positive electrode for a lithium ion secondary battery according to this embodiment will be described. As described above, if the pressing process for pressing the positive electrode composite layer 11 is omitted, there is a risk that the resistance between the positive electrode current collector 10 and the positive electrode composite layer 11 will increase. In such a case, it is preferable to form a conductive path between the positive electrode current collector 10 and the positive electrode composite layer 11 to reduce the resistance.

An example of a method for forming a conductive path between the positive electrode collector 10 and the positive electrode mixture layer 11 is to form irregularities on the surface of the positive electrode collector 10 on the side of the positive electrode mixture layer 11. When irregularities are formed on the surface of the positive electrode collector 10 in this manner, the contact area between the positive electrode collector 10 and the positive electrode mixture layer 11 increases, so that the resistance between the positive electrode collector 10 and the positive electrode mixture layer 11 can be reduced. Specifically, the surface roughness of the positive electrode collector 10 is preferably 100 nm or more.
The unevenness on the surface can be formed by physically roughening the surface with sandpaper or the like, or by increasing the surface roughness of the press roll during rolling.

Also, as in the positive electrode 2 for a lithium ion secondary battery shown in FIG. 4, the positive electrode composite layer 20 may be configured as a two-layer structure of a first positive electrode composite layer 11 (corresponding to the positive electrode composite layer 11 in FIG. 1) and a second positive electrode composite layer 21. That is, the second positive electrode composite layer 21 may be formed between the positive electrode current collector 10 and the first positive electrode composite layer 11. In this case, the amount of conductive material contained in the second positive electrode composite layer 21 may be greater than the amount of conductive material contained in the first positive electrode composite layer 11. In this way, when the amount of conductive material of the second positive electrode composite layer 21 on the side in contact with the positive electrode current collector 10 is increased, the resistance between the second positive electrode composite layer 21 and the positive electrode current collector 10 can be reduced, and as a result, the resistance between the positive electrode current collector 10 and the positive electrode composite layer 20 can be reduced. As an example, the proportion of conductive material in the second positive electrode composite layer 21 may be 10% or more, and the proportion of conductive material in the first positive electrode composite layer 11 may be less than 10%.

The conductive material of the second positive electrode composite layer 21 may be a material different from the conductive material of the first positive electrode composite layer 11. For example, acetylene black (AB) or a graphite-based material may be used as the conductive material of the second positive electrode composite layer 21.

In addition, when manufacturing the positive electrode 2 for a lithium ion secondary battery shown in FIG. 4, the positive electrode paste may be applied to the positive electrode collector 10 and dried to form the second positive electrode composite layer 21, and then the second positive electrode composite layer 21 may be pressed. By pressing the second positive electrode composite layer 21 in this manner, the density of the second positive electrode composite layer 21 can be increased and the conductivity can be improved. In addition, the resistance between the positive electrode collector 10 and the second positive electrode composite layer 21 can be reduced. Then, the first positive electrode composite layer 11 is formed on the second positive electrode composite layer 21 using the above method (without the pressing process). That is, when manufacturing the positive electrode 2 for a lithium ion secondary battery shown in FIG. 4, only the second positive electrode composite layer 21 may be pressed, and the first positive electrode composite layer 11 may not be pressed.

Next, the lithium ion secondary battery according to this embodiment will be described.
As an example, a lithium ion secondary battery including a wound electrode body will be described below. The wound electrode body is formed by stacking a long positive electrode sheet (positive electrode) and a long negative electrode sheet (negative electrode) with a long separator interposed therebetween, winding this stack, and crushing the resulting wound body from the side direction. The positive electrode sheet can be the above-mentioned positive electrode for lithium ion secondary batteries. As with the positive electrode sheet, the negative electrode sheet can also be a negative electrode in which a negative electrode mixture layer containing a negative electrode active material is formed on both sides of a foil-shaped negative electrode current collector.

The container of the lithium ion secondary battery comprises a flat rectangular container body with an open upper end and a lid that covers the opening. Metal materials such as aluminum and steel are preferable as materials constituting the container. Alternatively, the container may be made of a molded resin material such as polyphenylene sulfide resin (PPS) or polyimide resin. A positive electrode terminal that is electrically connected to the positive electrode of the wound electrode body and a negative electrode terminal that is electrically connected to the negative electrode of the wound electrode body are provided on the upper surface of the container (i.e., the lid).

Then, a positive electrode lead terminal and a negative electrode lead terminal are provided on the exposed portions of the positive electrode sheet and the negative electrode sheet at both ends of the wound electrode body (portions without the positive electrode composite layer and the negative electrode composite layer), and are electrically connected to the positive electrode terminal and the negative electrode terminal, respectively. The wound electrode body thus produced is housed in a container body, and the opening of the container body is sealed with a lid. After that, electrolyte is poured through an injection hole provided in the lid, and the injection hole is closed with a sealing cap, thereby producing a lithium-ion secondary battery.

Next, an embodiment of the present invention will be described.
Samples according to the examples and comparative examples were prepared using the following method.

<Example>
Using the flow shown in FIG. 3, a sample according to the embodiment was prepared. First, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ was prepared as the positive electrode active material, carbon nanotubes as the conductive material, PVdF as the binder, and NMP as the solvent. The particle size (D50) of the positive electrode active material used at this time was 5 to 10 μm. In addition, the conductive material (carbon nanotubes) used had an average outer diameter of 10 to 20 nm and an aspect ratio of 100 to 200 in the paste state. In addition, 1 to 2% of functional groups (the sum of O-C=O, O=C-N, and O-H) were present on the surface of the conductive material.

Then, these raw materials were mixed to prepare a positive electrode paste. The mixing ratio at this time was 55 to 60 mass% positive electrode active material, 0.1 to 3 mass% conductive material, 0.01 to 1 mass% dispersant, 0.3 to 2 mass% binder, and 30 to 45 mass% solvent. The viscosity of the positive electrode paste was adjusted to be between 5000 and 10000 mps. The viscosity of the positive electrode paste was measured using a rheometer (MCR-302, cone plate CP-50-1 manufactured by Anton Paar) at 25°C and a shear rate of ^10-2 s ^-1 to ¹⁰³ s- ¹ .

Then, the prepared positive electrode paste was applied to an aluminum foil positive electrode current collector. The positive electrode paste was then dried for 2 minutes at a drying temperature of 120°C to form a positive electrode composite layer on the positive electrode current collector. The thickness of the positive electrode composite layer at this time was 35 to 40 μm.

Comparative Example
3 in the same manner as in the above-described examples, a positive electrode composite layer was formed on a positive electrode current collector using the flow shown in Fig. 3, and the positive electrode composite layer was then pressed to produce samples according to Comparative Examples 1 to 4. The pressing conditions for the samples according to Comparative Examples 1 to 4 were 1.8, 2.0, 2.4, and 2.8 g/cc, respectively.

<Sample evaluation>
The porosity of the positive electrode mixture layer of each sample was measured. Specifically, the volume of the material itself was calculated from the specific gravity and composition ratio of each material, and the volume of the voids was calculated by subtracting it from the volume of the actual positive electrode mixture layer. The porosity was then calculated by dividing the volume of the voids by the volume of the actual positive electrode mixture layer.

The electrical resistivity (Ω·cm) (hereafter referred to as resistance value) of the positive electrode composite layer of each sample was measured using an MCP-T610 manufactured by Nitto Seiko Analytech Co., Ltd. (formerly Mitsubishi Chemical Analytech Co., Ltd.).

The relationship between the porosity and resistance of the positive electrode composite layer measured as described above is shown in Figure 5. As shown in Figure 5, the sample according to the embodiment was not subjected to pressing, and therefore showed the highest porosity (64%). Furthermore, as described above, carbon nanotubes with an aspect ratio of 100 to 200 were used as the conductive material, so the resistance of the positive electrode composite layer could be reduced even without pressing. Thus, in the sample according to the embodiment, a sufficient conductive path could be formed in the positive electrode composite layer, and the porosity of the positive electrode composite layer could be increased.

On the other hand, the samples according to Comparative Examples 1 to 4 had a lower porosity than the examples. Also, the higher the pressing pressure, the lower the porosity. In the sample according to Comparative Example 1, the resistance value of the positive electrode mixture layer was the highest. This is because the conductive path in the positive electrode mixture layer was broken by pressing the positive electrode mixture layer, and as a result, the electronic resistance of the positive electrode mixture layer increased. On the other hand, as shown in Comparative Examples 2 to 4, when the pressing pressure was gradually increased, the resistance value of the positive electrode mixture layer tended to decrease. The reason for this is that when the dried positive electrode mixture layer was pressed, the contact point between the positive electrode active material and the conductive material was separated, and the electronic resistance of the positive electrode mixture layer increased (Comparative Example 1), but then, when the density of the positive electrode mixture layer was increased by further pressing, the contact point between the positive electrode active material and the conductive material gradually increased, and the electronic resistance of the positive electrode mixture layer decreased (Comparative Examples 2 to 4). However, even in Comparative Example 4, which had the lowest resistance value of the positive electrode composite layer among the samples that underwent pressing, the resistance value of the positive electrode composite layer was higher than that of the sample in the Example in which the pressing process was omitted.

The present invention has been described above in accordance with the above embodiment, but the present invention is not limited to the configuration of the above embodiment, and of course includes various modifications, alterations, and combinations that a person skilled in the art could make within the scope of the invention of the claims of this patent application.

1 Positive electrode 10 Positive electrode current collector 11 Positive electrode mixture layer (first positive electrode mixture layer)
15 Positive electrode active material 16 Conductive material 20 Positive electrode mixture layer 21 Second positive electrode mixture layer

Claims (5)

A method for producing a positive electrode for a lithium ion secondary battery, the method comprising:
preparing a positive electrode paste containing at least a positive electrode active material, a conductive material, and a solvent;
applying the positive electrode paste onto the positive electrode current collector;
and drying the applied positive electrode paste to form the positive electrode mixture layer on the positive electrode current collector.
the conductive material contained in the positive electrode paste is a carbon nanotube having an aspect ratio of 100 or more and 200 or less ,
The conductive material has a functional group on a surface thereof,
the amount of the functional group contained in the conductive material is 1% by mass or more and 5% by mass or less,
The viscosity of the positive electrode paste is 5000 to 10000 mPa s;
The positive electrode paste is dried, and there is no pressing process.
A method for producing a positive electrode for a lithium-ion secondary battery. The method for producing a positive electrode for a lithium ion secondary battery according to claim 1 , wherein the conductive materials are bonded to each other by hydrogen bonds. The method for producing a positive electrode for a lithium ion secondary battery according to claim 1 or 2 , wherein the positive electrode mixture layer has a porosity of 50% or more and 70% or less. The method for producing a positive electrode for a lithium ion secondary battery according to any one of claims 1 to 3 , wherein the positive electrode current collector has an uneven surface on the side of the positive electrode mixture layer. The method further includes forming a second positive electrode mixture layer between the positive electrode current collector and the positive electrode mixture layer,
the amount of conductive material contained in the second positive electrode mixture layer is greater than the amount of conductive material contained in the positive electrode mixture layer;
The method for producing the positive electrode for a lithium ion secondary battery according to any one of claims 1 to 4 .

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