JP7698979B2 - Method for manufacturing positive electrode of lithium ion secondary battery and method for manufacturing lithium ion secondary battery - Google Patents

Description

The present invention relates to a method for manufacturing a positive electrode of a lithium ion secondary battery, and a method for manufacturing a lithium ion secondary battery, and more specifically, to a method for manufacturing a positive electrode of a lithium ion secondary battery with low reaction resistance, and a method for manufacturing a lithium ion secondary battery.

2. Description of the Related Art In recent years, lithium ion secondary batteries have come to be widely used as a power source in electrically powered vehicles, such as electric automobiles or hybrid vehicles having a motor and an engine as a drive source for the vehicle.
In such a lithium-ion secondary battery, a material (active material) capable of reversibly absorbing and releasing lithium ions (Li ions) is provided in the positive and negative electrodes, and charging and discharging are performed by the movement of Li ions between the positive and negative electrodes. However, since such an active material itself has low electronic conductivity, the positive electrode is usually formed by mixing the active material with a conductive material such as carbon powder to form a composite material. In this case, in order to improve the output of the positive electrode, it is desirable to increase the reaction area and reduce the resistance.

The positive electrode plate of a lithium-ion secondary battery comprises a current collector and an active material layer that is held by the current collector and contains active material particles and a conductive material. The active material particles are classified into primary particles that are generated by firing and secondary particles that are formed by agglomeration of these particles. The secondary particles have a shell and a hollow portion formed inside the shell. In general, the smaller the particle diameter, the larger the specific surface area that contributes to the reaction. Therefore, it is considered to crush the secondary particles into primary particles to increase the specific surface area. However, if the secondary particles are crushed in advance to form primary particles, the active material particles tend to aggregate. If the particles aggregate, the viscosity increases and the solid content decreases, which causes poor drying, making handling extremely difficult in the composite preparation process. Therefore, if the particles are made into primary particles, the desired positive electrode plate cannot be easily produced.

Therefore, the invention described in Patent Document 1 is characterized in that the conductive material contained in the active material layer is disposed both in the hollow portions of the secondary particles of the active material particles and between the active material particles outside the secondary particles, with the secondary particles remaining intact.

In addition, in the energy storage element of the invention described in Patent Document 2, the first electrode plate includes a current collector and a composite layer laminated on the current collector. The composite layer contains at least one of a binder and a conductive assistant, primary particles of an active material, and secondary particles formed by aggregating a plurality of the primary particles to have a hollow region inside. At least one of the binder and the conductive assistant is partly disposed in the hollow region.

In this way, secondary particles of active material with hollow spaces are generated from multiple primary particles of active material. Such secondary particles are suitable for absorbing and releasing lithium due to the crystal structure of the active material. Through holes are provided in these secondary particles, and conductive material is placed inside to reduce the resistance of the positive electrode.

In the inventions described in Patent Documents 1 and 2, a conductive material is placed inside the secondary particles of the active material to reduce resistance.

JP 2013-45761 A JP 2014-150052 A

In the inventions described in Patent Documents 1 and 2, a conductive material is disposed within the secondary particles of the active material to reduce the resistance, but it is desirable to reduce the resistance further.
The problem to be solved by the method for producing a positive electrode of a lithium ion secondary battery and the method for producing a lithium ion secondary battery of the present invention is to further reduce the resistance of the positive electrode.

In order to solve the above problems, the method for manufacturing a positive electrode of a lithium ion secondary battery of the present invention is a method for manufacturing a positive electrode of a lithium ion secondary battery comprising a current collector, positive electrode active material particles held on the current collector, a conductive material, and a positive electrode mixture layer containing a binder, and the method comprises an impregnation step of disposing the fibrous conductive material together with the binder inside and outside secondary particles of a porous positive electrode active material having a shell portion with through holes, the shell portion being formed by aggregating primary particles into a hollow shell shape, and a pressing step of crushing at least a portion of the secondary particles.

The average diameter of the fibrous conductive material is preferably 60% or less of the average diameter of the through holes of the secondary particles.
In the pressing step, it is preferable that 50% or more of the positive electrode active material particles are converted into primary particles.

In the impregnation step, it is preferable that the binder containing the fibrous conductive material is impregnated into 85% or more of the secondary particles together with the fibrous conductive material.
The through holes of the secondary particles preferably have a diameter of 50 nm or more.

The diameter of the fibrous conductive material is preferably 1 nm to 100 nm.
The length of the fibrous conductive material is preferably 100 nm to 1000 nm.
The positive electrode active material is preferably composed of a lithium transition metal oxide.

The fibrous conductive material is preferably made of carbon nanotubes.
The method for producing a lithium ion secondary battery may include a method for producing the positive electrode of the lithium ion secondary battery.

The method for manufacturing the positive electrode of a lithium ion secondary battery of the present invention can suppress the reaction resistance of the positive electrode and improve the input/output characteristics of the lithium ion secondary battery.

FIG. 2 is a schematic diagram showing the appearance of secondary particles of a positive electrode active material. FIG. 3 is an end view showing the inside of a secondary particle of a positive electrode active material. A micrograph of a conductive material made from carbon nanotubes. FIG. 4 is a schematic diagram showing secondary particles of a positive electrode active material before kneading. FIG. 4 is a schematic diagram showing a state of a positive electrode mixture before a binder containing a conductive material and a positive electrode active material are kneaded together. FIG. 4 is a schematic diagram showing the state of a positive electrode mixture after a binder containing a conductive material and a positive electrode active material are kneaded together. FIG. 4 is a schematic diagram showing the positive electrode mixture layer after a pressing step (crushing step). 3 is a flowchart showing a method for manufacturing a lithium ion secondary battery according to the present embodiment. 4 is a flowchart of a positive electrode manufacturing process according to the present embodiment. 1 is a comparison table showing a comparison between the positive electrode of Example 1 of the present embodiment and the positive electrode of Comparative Example 1, which is a conventional technique, in Experiment 1. 1 is a micrograph showing the appearance of secondary particles of the active material of Comparative Example 1. 1 is a micrograph showing a cross section of a secondary particle of the active material of Comparative Example 1. 4 is a micrograph showing the appearance of secondary particles of the active material of the present embodiment in Example 1. 4 is a micrograph showing a cross section of a secondary particle of Example 1 of the active material of the present embodiment. 1 is a micrograph showing a state in which secondary particles of the active material of Comparative Example 1 are not crushed. 1 is a micrograph showing a state in which secondary particles of the active material of Example 1 of the present embodiment are crushed. 1 is a table showing conditions of Example 2, Comparative Example 2, and Comparative Example 3 in Experiment 2. 4 is a micrograph of a cross section of a positive electrode plate in Example 2. 13 is a micrograph of a cross section of a positive electrode plate in Comparative Example 2. 13 is a micrograph of a cross section of a positive electrode plate in Comparative Example 3. 13 is a micrograph showing an enlarged portion of a cross section of a positive electrode plate in Example 2. 13 is a micrograph showing an enlarged portion of a cross section of a positive electrode plate in Comparative Example 2. 13 is a micrograph showing an enlarged portion of a cross section of a positive electrode plate in Comparative Example 3. Graph comparing input/output characteristics in Experiment 2.

(Principle and Overview of the Embodiment)
The present invention will be described with reference to the drawings by way of one embodiment of a method for manufacturing a lithium ion secondary battery. In the method for manufacturing a lithium ion secondary battery of this embodiment, the purpose is to manufacture a positive electrode plate 2 that increases the reaction area and reduces resistance in order to improve the output of the positive electrode in the positive electrode manufacturing process. For this purpose, not only is the surface area of the particles of the positive electrode active material itself increased, but also a conductive material is appropriately arranged. The specific configuration will be described below.

<Lithium-ion secondary battery>
In the lithium ion secondary battery of this embodiment, carbon or graphite capable of inserting and desorbing lithium is used for the negative electrode, and various lithium composite oxides are used for the positive electrode.

When charging, the lithium atoms in the positive electrode crystals are released into the electrolyte as lithium ions, and at the same time, the lithium ions in the electrolyte penetrate into the negative electrode crystals, and when discharging, the lithium atoms in the negative electrode return to the positive electrode.

<Materials for Positive Electrode Active Material Particles 3>
The positive electrode active material is generally synthesized by mixing a lithium salt powder (LiOH, _Li2Co3 , _etc. ) with a transition metal oxide powder and baking the mixture.

Specific examples of the positive electrode active material include lithium cobalt oxide (LiCoO ₂ ). Further, lithium nickel oxide (LiNiO ₂ ). Further, lithium manganese oxide (LiMn ₂ O ₄ ). Further, lithium nickel cobalt manganese oxide (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ). Further, lithium iron phosphate (LiFePO ₄ ) and the like are also included. By firing, first, small-sized primary particles are generated, and then a plurality of primary particles are aggregated to generate secondary particles.

<Manufacturing process of positive electrode plate 2>
In general, in the manufacturing process of a positive electrode plate 2 (see FIG. 18 ), a paste-like positive electrode mixture is prepared by mixing granular positive electrode active material particles 3, which are the raw material of the positive electrode plate 2, with a conductive material 33 and a binder 34, the viscosity of which is adjusted by a solvent. The paste-like positive electrode mixture is applied in a layer form to a current collector (positive electrode substrate) 5 made of, for example, an Al foil. The applied positive electrode mixture layer 4 is dried in a drying furnace and shaped to a predetermined thickness in a pressing process. The positive electrode plate 2 is then cut into a predetermined shape to complete the positive electrode plate 2.

<Configuration of secondary particles 32>
FIG. 1 is a schematic diagram showing the appearance of a secondary particle 32 of a positive electrode active material particle 3. FIG. 2 is an end view showing the inside of a secondary particle 32 of a positive electrode active material. In the positive electrode active material particle 3 synthesized by the above-mentioned method, the primary particles 31 grow to reflect the crystal form of the active material. The average diameter is, for example, 0.5 to 3 μm. The primary particles 31 aggregate to become secondary particles 32, and the particle size becomes larger. The size of the secondary particles 32 is preferably, for example, an average diameter of 10 to 100 μm. Although the shapes of the primary particles 31 and secondary particles 32 are not necessarily regular, the average diameters can be 50% integrated values measured by a laser diffraction type particle size distribution measurement method using the MIE scattering theory.

The fired secondary particles 32 may have a shell portion 32a, a hollow portion 32b formed inside the shell portion 32a, and a through hole 32c that penetrates the shell portion 32a. There are porous particles with many through holes 32c and porous particles with few through holes 32c, but in this embodiment, porous particles with many through holes 32c are preferred.

<Average diameter W of through holes 32c>
The average diameter W of the through holes 32c is a 50% average particle diameter by SEM (scanning electron microscope photograph) observation, and varies from large to small. For the purpose of this embodiment, the fibrous conductive material 33 (see FIG. 3) is disposed inside the through holes 32c of the secondary particles 32 of the positive electrode active material together with the resin of the binder 34 through the through holes 32c. That is, the diameter of the through holes 32c that receives the fibrous conductive material 33 with the average diameter D is required. The condition for the average diameter W of the through holes 32c is that the fibrous conductive material 33 can be disposed inside the through holes 32c of the secondary particles 32 of the positive electrode active material through the through holes 32c. For this reason, the average diameter D of the fibrous conductive material 33 is desirably 60% or less of the average diameter W of the through holes 32c of the secondary particles 32. In other words, the average diameter W of the through holes 32c of the secondary particles 32 is desirably 167% or more of the average diameter D of the fibrous conductive material 33.

<Conductive material 33>
FIG. 3 is a micrograph of the conductive material 33 made of carbon nanotubes. The conductive material 33 of the positive electrode plate 2 of this embodiment is made of carbon nanotubes. Carbon nanotubes are substances in which a six-membered ring network (graphene sheet) made of carbon is in the form of a single or multiple coaxial tube, and are molecules in which carbon with a layered structure is in the form of a cylinder. They are characterized by having very high electrical conductivity, thermal conductivity, and heat resistance. In this embodiment, shapes such as single layer, multiple layer, open end, and closed end are not acceptable. The conductive material 33 made of carbon nanotubes can add high electrical conductivity even if the resin used for the binder 34 is a material that does not normally conduct electricity. This conductive material 33 improves the conduction between the current collector 5, the positive electrode active material particles 3, and the electrolyte, and the resistance of the positive electrode plate 2 can be kept low.

<Average length L of conductive material 33>
For this purpose, it is desirable that the carbon tubes contact each other to form a conductive network. When the carbon nanotubes are long, they exhibit electrical and thermal conductivity even in small quantities, and also have high strength. From this point of view, it is desirable that the average length L of the fibrous conductive material 33 is long.

On the other hand, in this embodiment, the carbon nanotubes are placed inside the secondary particles 32 of the positive electrode active material together with the resin of the binder 34 via the through holes 32c. Therefore, it is not necessarily better to make them longer. If the average length L of the fibrous conductive material 33 is too long, aggregation will occur due to intermolecular forces and hydrogen bonds between the carbon nanotubes, making it difficult for them to penetrate inside the secondary particles 32.

In order to achieve the objective of distributing conductive material 33 evenly throughout positive electrode mixture layer 4 in this embodiment, it is not necessary to make the lengths uniform, and it is also preferable for carbon nanotubes of various lengths to be present.
Therefore, in this embodiment, the average length L of the fibrous conductive material 33 is set to, for example, 100 to 1000 nm.

<Average diameter D of conductive material 33>
The average diameter D of the conductive material 33 must be such that it can be disposed inside the through-holes 32c of the secondary particles 32 of the positive electrode active material. For this reason, it is desirable that the average diameter D of the fibrous conductive material 33 be 60% or less of the average diameter W of the through-holes 32c of the secondary particles 32.

On the other hand, if the electrode is too thin, the contact area with the active material is small, and sufficient conductivity may not be ensured.
In addition, since the purpose of the average diameter D is to be disposed inside the through-holes 32c of the secondary particles 32 of the positive electrode active material, a uniform thickness is not necessarily required, and it is desirable to set the average diameter D by actually conducting tests in relation to the through-holes 32c of the secondary particles 32 of the positive electrode active material.

In this embodiment, the average diameter D of the fibrous conductive material 33 is, for example, 1 nm to 100 nm.
<Binder 34>
The binder 34 is formed by integrating the positive electrode active material particles 3 and the conductive material 33 in a kneading process (FIG. 9: S12), applying the positive electrode composite layer 4 to the current collector 5 (S13), and fixing the positive electrode composite layer 4 in a drying process (S14). In this embodiment, the binder is a concept including a solvent for adjusting viscosity and additives for adjusting other characteristics. The binder is then shaped in a pressing process (S15). Examples of the material of the binder 34 include PVdF (polyvinylidene fluoride). In this embodiment, the material of the binder is not particularly limited. In the experimental example described below, a cold embedding resin made of epoxy resin "Specifix-20 (registered trademark)" made by Struers is used for testing because of its ease of handling. It has been confirmed that there is no difference in characteristics between the product and the experimental results even if the type of resin is changed in this way. In this way, the binder 34 itself is not limited in this embodiment.

The purpose of this embodiment is to place the fibrous conductive material 33, together with the resin of the binder 34, inside the secondary particles 32 of the positive electrode active material through the through holes 32c. Therefore, the binder 34 before hardening is required to have a degree of fluidity that allows it to be impregnated into the secondary particles 32. The degree of fluidity is appropriately adjusted depending on the characteristics of the secondary particles 32 and the characteristics of the conductive material 33.

<Impregnation rate [%]>
Here, in this embodiment, the "impregnation rate [%]" refers to the density of the image of the secondary particles 32 becoming denser when the binder 34 containing the fibrous conductive material 33 penetrates into the interior of the secondary particles 32 in a micrograph taken after the kneading step (S12) which is the impregnation step. Thus, the respective shadings were visually observed to calculate the impregnation rate [%]. In other words, the impregnation rate [%] is the ratio of the number of secondary particles 32 in which the binder 34 containing the conductive material 33 has entered the hollow portion 32b to the total number of secondary particles. In this embodiment, the impregnation rate [%] is set to be 85% or more.

<Positive electrode mixture>
The positive electrode mixture is composed of positive electrode active material particles 3, conductive material 33, binder 34 containing a solvent, and necessary additives. An example of the distribution is 75 to 99% of the positive electrode active material particles 3 (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ), 0.3 to 15% of the conductive material 33 (carbon nanotubes), and 0.3 to 15% of the binder 34 (PVdF). These materials are kneaded to generate a paste of the positive electrode mixture. The viscosity in this embodiment is approximately 1000 to 5000 Pa·s.

<Positive electrode mixture layer 4>
The obtained positive electrode mixture is applied to a current collector 5 made of Al foil (S13), and then passed through a drying step (S14) and a pressing step (S15) to adjust the thickness and flatten the surface, completing a positive electrode mixture layer 4. At this time, some of the secondary particles 32 are crushed to become primary particles 31, which will be described in detail later.

<Features of the manufacturing method of the lithium-ion secondary battery according to this embodiment>
The method for producing a lithium ion secondary battery according to this embodiment is characterized in that it includes a kneading step (S12) as an impregnation step and a pressing step (S15) as a crushing step.

The "impregnation process" is a process in which a fibrous conductive material 33 is placed together with a binder 34 in the hollow portion 32b of a secondary particle 32 of a positive electrode active material having a shell portion 32a with hollow or porous through holes 32c formed in a spherical shell shape by an aggregation of primary particles 31.

The "crushing step" is a step in which the positive electrode plate 2 is pressed from both sides with press rolls (not shown) to crush the secondary particles 32 into primary particles.
<Kneading process (impregnation process) (S12)>
Fig. 4 is a schematic diagram showing the positive electrode active material before the kneading process. In the figure, for the purpose of explanation, only an end view of secondary particles 32 is shown. In reality, primary particles that did not grow into secondary particles 32 and primary particles 31 formed by cracking secondary particles 32 are also included, but are omitted. In addition, they are shown only as schematics and do not reflect the actual size. The same applies to Figs. 5 to 7. Fig. 4 shows the state before kneading, i.e., before mixing with binder 34 containing conductive material 33.

FIG. 5 is a schematic diagram showing the state of a paste-like positive electrode mixture obtained by mixing a binder 34 containing a conductive material 33 and secondary particles 32 which are positive electrode active material particles 3 but before kneading.
In this state, the binder 34 containing the conductive material 33 is present around the secondary particles 32 , but the binder 34 containing the conductive material 33 has not yet penetrated into the hollow portions 32 b of the secondary particles 32 .

Figure 6 is a schematic diagram showing the state of a paste-like positive electrode mixture after kneading a binder 34 containing a conductive material 33 and secondary particles 32, which are positive electrode active material particles 3. The binder 34 containing the conductive material 33 has a viscosity adjusted by an appropriate solvent, and has a viscosity sufficient to penetrate into the through holes 32c of the secondary particles 32. In addition, since the conductive material 33 is composed of fibrous and flexible carbon nanotubes, it enters the hollow portion 32b from the through holes 32c of the secondary particles 32 together with the penetration of PVdF (polyvinylidene fluoride) of the viscosity-adjusted fluid. In this case, the average length L of the conductive material 33 varies even if it is constant. A conductive material 33 with a short length entirely enters the hollow portion 32b of the secondary particles 32. On the other hand, a conductive material 33 with a long length may have one end in the hollow portion 32b and the other end protruding from the outside of the secondary particles 32. In such a case, it is preferable because it serves to provide electrical conductivity between the inside and outside of the secondary particles 32. In this way, the variation in the length of the conductive material 33 serves its respective role, and a conductive material network is constructed by the conductive material 33 in the positive electrode mixture layer 4.

<Positive electrode mixture layer 4>
FIG. 7 is a schematic diagram showing the positive electrode mixture layer 4 after the pressing step (S15). In the kneading step (S12), the conductive material 33 is arranged in the hollow portion 32b of the secondary particle 32 so that the impregnation rate [%] is 85% or more. In this way, the conductive material 33 is sufficiently arranged inside and outside the secondary particle 32 together with the binder 34. In this state, in the pressing step (S15), the positive electrode plate 2 is pressed from both sides by a press roll not shown. The general purpose of the pressing step (S15) is to set the thickness of the positive electrode mixture layer 4 of the positive electrode plate 2 to a set value and to form the surface of the positive electrode plate 2 flat. In this embodiment, in addition to these purposes, the secondary particles 32 of the positive electrode active material particles 3 are crushed to form primary particles.

In detail, in the pressing step (S15), 50% or more of the positive electrode active material particles 3 of the secondary particles 32 are crushed to "convert into primary particles." Note that "convert into primary particles" in this embodiment does not necessarily mean decomposing into single positive electrode active material primary particles 31, but includes crushing to decompose into a mass of multiple primary particles 31 so that the hollow portions 32b of the secondary particles 32 are exposed.

<Manufacturing process of lithium ion secondary battery>
8 is a flow chart showing the manufacturing process of the lithium ion secondary battery of this embodiment. The manufacturing process of the lithium ion secondary battery of this embodiment will be outlined with reference to FIG.

<Source process (S1)>
In this embodiment, first, a source process (S1) is performed. Here, the source process is a process of preparing elements of a lithium ion secondary battery. Specifically, it is a process of preparing a positive electrode plate 2, a negative electrode plate (not shown), and a separator that constitute the battery elements of a lithium ion secondary battery.

The manufacturing process for the lithium ion secondary battery of this embodiment is characterized by the creation of the positive electrode plate 2. Therefore, with reference to FIG. 9, the "positive electrode plate manufacturing process" that constitutes part of the source process (S1) will be described.

<Positive electrode plate manufacturing process>
The positive electrode plate manufacturing process includes a secondary particle manufacturing process (S11), a kneading process (S12), a coating process (S13), a drying process (S14), a pressing process (S15), and a cutting process (S16).

<Secondary particle manufacturing process (S11)>
In the secondary particle production step (S11), raw materials are mixed and sintered to generate secondary particles 32 via primary particles 31, but detailed procedures are omitted since known techniques can be used. In the secondary particle production step (S11), particles are produced that have a shell portion 32a formed in a spherical shell shape by the aggregation of primary particles 31, a hollow portion 32b inside the shell portion 32a, and many through holes 32c. The average diameter of the through holes 32c is 10 to 100 μm, and the average diameter W of the predetermined through holes is 50 nm or more.

<Kneading process (impregnation process) (S12)>
In the kneading step (S12) of this embodiment, a paste-like positive electrode mixture obtained by mixing the positive electrode active material particles 3 including the secondary particles 32, the fibrous conductive material 33, and the binder 34 with adjusted viscosity is kneaded by a kneader. In this embodiment, this kneading causes the binder 34 accompanied by the fibrous conductive material 33 to penetrate into the hollow portion 32b through the through-holes 32c of the secondary particles 32. At this time, the carbon nanotubes, which are the fibrous conductive material 33, are disposed inside and outside the secondary particles 32 together with the binder 34. The conditions such as the viscosity of the positive electrode mixture, the kneading speed, and the time are ultimately optimized by measuring the reaction resistance of the positive electrode plate 2.

<Coating process (S13)>
A paste-like positive electrode mixture in which carbon nanotubes, which are fibrous conductive materials 33, are arranged inside and outside secondary particles 32 is applied to a current collector 5 made of Al foil, which becomes a positive electrode substrate. Here, the current collector 5 is cut into a rectangle. A coating device (not shown) is provided with a nozzle corresponding to the positive electrode mixture layer 4 along the width direction (direction perpendicular to the longitudinal direction) of the current collector 5. The nozzle applies the paste-like positive electrode mixture by scanning the positive electrode mixture from one end to the other end of the longitudinal direction of the current collector 5 relative to the current collector 5. The relative movement may be performed by moving either the stage on which the current collector 5 is placed or the nozzle itself. The coating device adjusts the applied paste-like positive electrode mixture to a predetermined thickness using a roller, a doctor blade, or the like.

<Drying step (S14)>
After the coating step (S13) is completed, the solvent in the binder 34 is evaporated using, for example, an infrared drying device, and the binder 34 is hardened to a certain hardness.

<Pressing process (crushing process) (S15)>
The pressing step is performed by a pressing device (not shown). The pressing device includes at least one press roll, and performs pressing so as to move relatively from one end to the other end in the longitudinal direction of the positive electrode mixture layer 4 formed on the current collector 5 and scan it. The press roll contacts the positive electrode mixture layer 4 at a position closer to the current collector 5 than the surface of the positive electrode mixture layer 4 formed in the coating step (S13). That is, the gap (the distance between a pair of press rolls, or the distance between the press roll and the stage facing the press roll) is shorter than the thickness of the positive electrode plate 2 by a predetermined length. That is, the pressing device works to crush the positive electrode mixture layer 4.

The purpose of the pressing step (S15) is the same as in the conventional method in that the thickness of positive electrode mixture layer 4 is set to a specified value and the surface of positive electrode mixture layer 4 is shaped to be flat.
In addition to the above, the pressing step (S15) of the present embodiment also functions as a crushing step as follows: Binder 34 of positive electrode mixture layer 4 has some plasticity even after drying and is plastically deformed, but secondary particles 32 have almost no plasticity and are crushed and divided by the compressive force.

The state of positive electrode mixture layer 4 before the pressing step (S15) in FIG. 6 is changed to the state shown in FIG.
In this way, the secondary particles 32 are crushed, and thus the secondary particles 32 are broken down into smaller particles, resulting in the formation of primary particles. Note that this "primary particles" does not mean that the secondary particles 32 become completely primary particles, but rather means that the large secondary particles 32 become small fragments 32d, and the particle size approaches the size of the primary particles 31.

In other words, as the secondary particles 32 are crushed and divided, the surface area of the positive electrode active material of the secondary particles 32 itself increases, and the reaction area of the positive electrode active material increases. In addition, the wall surface of the hollow portion 32b, which was not previously exposed to the outside, is also exposed.

Furthermore, in the vicinity of the wall surface of the hollow portion 32b, there is a large amount of fibrous conductive material 33. These fibrous conductive materials 33 are also exposed when the secondary particles 32 are crushed.
In particular, since the pressing step (S15) is performed after the drying step (S14), the conductive material 33 is less likely to flow. Therefore, even if the secondary particles 32 become primary particles, they do not aggregate. Furthermore, even after the pressing step (S15), the conductive material 33 remains present near the wall surface of the hollow portion 32b. Therefore, these factors work together to ensure a conductive network from the hollow portion 32b, and the effective specific surface area that contributes to the reaction of the secondary particles 32 is increased.

<Cutting step (S16)>
After the pressing step (S15) is completed, the positive electrode plate 2 is cut to a size according to the specifications of the battery to be assembled as necessary, and the positive electrode plate 2 is completed.

<Assembly of lithium-ion secondary battery>
In the source step (S1), although the explanation has been omitted, a negative electrode plate manufacturing step and a separator manufacturing step are carried out simultaneously with the positive electrode plate manufacturing step shown in FIG.

After these steps are completed in the source process (S1), the cell battery assembly process is performed. Here, the winding process (S2) is performed first. In the winding process (S2), the positive electrode plate 2 and the negative electrode plate prepared in the source process (S1) are stacked with a separator in between and wound in that state. The wound positive electrode plate 2, the negative electrode plate, and the separator are shaped to accommodate them in a battery case, and a flattening press process (S3) is performed to make the plates closer together. The electrode plate group consisting of the positive electrode plate 2, the negative electrode plate, and the separator shaped in the flattening press process has an Al current collector 5 without the positive electrode composite layer 4 at one end, and a Cu current collector without the negative electrode composite layer protrudes at the other end (not shown). These current collectors are pressure welded, and in terminal welding (S4), the positive electrode internal terminal and the negative electrode internal terminal are welded, respectively. External terminals are attached to these internal terminals via the lid of the battery case. The wound and flattened electrode plate group and the positive and negative terminals welded thereto are then inserted into the battery case in the case insertion process (S5). In the can sealing process (S6), the battery case and the lid are sealed by laser welding or the like. At this stage, the electrolyte injection port of the lid is open. After that, in the cell drying process (S7), any moisture remaining in the battery case is sufficiently dried, and then in the electrolyte injection and sealing process (S8), non-aqueous electrolyte is injected through the electrolyte injection port. After the electrolyte injection is completed, the electrolyte injection process is sealed. This completes the assembly of the cell battery. After the assembly of the cell battery is completed, the activation process (S9) is performed for the purpose of forming a SEI (Solid Electrolyte Interphase) coating, etc. Here, the cell battery is chemically activated by performing initial charging, aging, etc. Then, in the inspection process (S10), cell voltage, battery resistance, etc. are inspected, and those that demonstrate the required performance become the product. For lithium-ion secondary batteries for vehicles, about 6 to 12 cell batteries are stacked to form a battery module, and several battery modules are housed in a container and equipped with a control device and various sensors to form a vehicle battery pack.

(Operation of the embodiment)
Next, the operation of this embodiment will be described with reference to an experimental example.
<Experiment 1>
FIG. 10 is a comparison table showing a comparison between the positive electrode plate 2 of Example 1 of this embodiment and the positive electrode plate 2 of Comparative Example 1, which is a conventional technology, in Experiment 1. In Experiment 1, the input and output of the positive electrode plate 2 was compared by changing the conditions of the conventional positive electrode plate 2 and the positive electrode plate 2 of this embodiment. The binder used in the experiment was Struers' cold embedding resin "Specifix-20 (registered trademark)" made of epoxy resin, for ease of handling, etc. It has been confirmed that this resin can produce results equivalent to those of PVdF (polyvinylidene fluoride) used in products.

<Particle shape>
Fig. 11 is a micrograph showing the appearance of secondary particles 32 of the active material of Comparative Example 1. Fig. 12 is a micrograph showing a cross section of a secondary particle of the active material of Comparative Example 1.

Comparative example 1 of the prior art shows the prior art, in which the particle shape of the target secondary particles 32 is porous, but the average diameter W of the through holes 32c is small, at 25 nm, less than 50 nm.

Figure 13 is a micrograph showing the appearance of secondary particles 32 of the active material of Example 1 of this embodiment. Figure 14 is a micrograph showing a cross section of a secondary particle of the active material of Example 1 of this embodiment.

On the other hand, in Example 1 of this embodiment, the secondary particles 32 have a porous particle shape, and the average diameter W of the through holes 32c is 200 nm, which is large and is 50 nm or more.
When kneaded with the binder 34, the impregnation rate was 54.8%, which is less than 85%, in Comparative Example 1, whereas the impregnation rate was 90%, which is 85% or more, in Example 1.

Although we will not go into detail here, we conducted numerous experiments under different conditions and confirmed that the relationship between the average diameter W of the through holes 32c and the impregnation rate [%] is such that when the average diameter W of the through holes 32c is 50 nm or greater, the impregnation rate [%] is approximately 85% or greater.

<Condition of mixture>
In Comparative Example 1 of the prior art, the average diameter D of the granular acetylene black, which is the conductive material 33, is 200 nm, which is more than 30 nm, and the average length L is also 200 nm, which is less than 500 nm. The conductive material 33 was present only outside the secondary particles 32.

On the other hand, in Example 1 of this embodiment, the average diameter D of the fibrous carbon nanotubes that are the conductive material 33 is 20 nm (10% of the through hole 32c), which is 30 nm or less, and the average length L is 700 nm, which is 500 nm or more and 1000 nm or less. The conductive material 33 also penetrates into the hollow portion 32b from the through hole 32c of the secondary particle 32, and is present not only outside but also inside the secondary particle 32.

<Positive electrode plate 2 after pressing>
15 is a micrograph showing a state in which secondary particles 32 of the active material of Comparative Example 1 are not crushed. If secondary particles 32 are not crushed, hollow portions 32b will have regions that cannot contact conductive material 33 and therefore cannot contribute to the reaction.

Even if the secondary particles 32 are crushed in the pressing step (S15), there is no conductive material 33 near the wall surface of the hollow portion 32b of the crushed secondary particles 32. Therefore, even if the wall surface of the hollow portion 32b is exposed, that area will not be able to contribute to the reaction.

Figure 16 is a micrograph showing the state in which the secondary particles of the active material of Example 1 of this embodiment are crushed. In Example 1 of this embodiment, the secondary particles 32 are crushed in the pressing step (S15). Furthermore, the conductive material 33 has originally infiltrated and is present in the hollow portion 32b. Therefore, the conductive material 33 is also present near the wall surface of the hollow portion 32b of the crushed secondary particle 32. Therefore, if the wall surface of the hollow portion 32b is exposed, that area can contribute to the reaction. Furthermore, the conductive material 33 arranged inside and outside the secondary particles 32 effectively forms a conductive network.

<Results>
In Comparative Example 1, there are regions in the positive electrode active material particles 3 that cannot come into contact with the conductive material 33 and are not connected to the conductive network, and thus cannot contribute to the reaction. As a result, the input and output of the positive electrode plate 2 is insufficient.

On the other hand, in Example 1 of this embodiment, the secondary particles 32 are converted into primary particles, increasing the surface area of the positive electrode active material. At the same time, the conductive material 33 is also present near the wall surface of the hollow portion 32b of the crushed secondary particles 32. Therefore, the conductive material 33 arranged inside and outside the secondary particles 32 effectively forms a conductive network. As a result, the input and output of the positive electrode plate 2 is improved.

<Experiment 2>
17 is a table showing the conditions of Example 2, Comparative Example 2, and Comparative Example 3 in Experiment 2. The input/output characteristics were compared by changing the electrode plate state, the average diameter W of the through holes of the secondary particles 32 of the active material, the impregnation rate [%], and the average diameter D of the conductive material 33. Example 1 of Experiment 1 and Example 2 of Experiment 2 are common to each other.

Here, FIG. 18 is a micrograph of a cross section of a positive electrode plate 2 in Example 2. Example 2 shown in FIG. 18 is an example in which fibrous conductive material 33 is placed inside and outside secondary particles 32 and pressed into primary particles. FIG. 19 is a micrograph of a cross section of a positive electrode plate 2 in Comparative Example 2. Comparative Example 2 shown in FIG. 19 is an example in which fibrous conductive material 33 is placed inside and outside secondary particles 32, but is not pressed into primary particles. FIG. 20 is a micrograph of a cross section of a positive electrode plate 2 in Comparative Example 3. As shown in FIG. 20, Comparative Example 3 is an example in which some of the secondary particles 32 are made into primary particles, but the conductive material 33 is granular acetylene black rather than fibrous carbon nanotubes.

<Plate condition>
Fig. 21 is a micrograph showing an enlarged portion of the cross section of the positive electrode plate 2 in Example 2. As shown in Fig. 21, in Example 2, it can be seen that fibrous conductive material 33 is present inside and outside secondary particles 32 that have been partially converted into primary particles.

Figure 22 is a micrograph showing an enlarged portion of the cross section of the positive electrode plate 2 in Comparative Example 2. As shown in Figure 22, in Comparative Example 2, it can be seen that fibrous conductive material 33 is present inside and outside the secondary particles 32.

23 is a micrograph showing an enlarged portion of the cross section of the positive electrode plate 2 in Comparative Example 3. In Comparative Example 3, the fibrous conductive material 33 is not present inside or outside the secondary particles 32 that have been partially converted into primary particles.
<Average diameter W of through holes of secondary particles 32 of active material>
The average diameter W of the through holes of the secondary particles 32 of the active material of Example 2 is large at 200 nm, the average diameter W of the through holes of the secondary particles 32 of the active material of Comparative Example 2 is somewhat small at 100 nm, and the average diameter W of the through holes of the secondary particles 32 of the active material of Example 3 is small at 25 nm.

<Average diameter D of conductive material 33>
The average diameter D of the fibrous conductive material 33 in Example 2 is as small as 10 nm (5% of the through holes 32c), and the average diameter D of the fibrous conductive material 33 in Comparative Example 2 is also as small as 10 nm. On the other hand, the conductive material 33 in Comparative Example 3 is granular and has an average diameter D of 200 nm or more.

<Impregnation rate [%]>
The impregnation rate [%] of Example 2 is 90.0%, while it is low at 63.3% in Comparative Example 2 and 54.8% in Comparative Example 3. Particularly in Comparative Example 3, even if the binder 34 can be impregnated into the secondary particles 32, the accompanying penetration of the conductive material 33 is almost nonexistent because the average diameter D is large.

<Results>
24 is a graph comparing the input/output characteristics in Experimental Example 2. Here, the input/output characteristics of Comparative Example 2, which is the conventional technology, are set to 100%. In this case, Example 2 exhibited high input/output characteristics of approximately 135%.

In Comparative Example 2, the input/output characteristics were low, at approximately 90%. The reason for this is that the secondary particles 32 were not converted into primary particles. Also, the average diameter W of the through holes 32c of the secondary particles 32 of the positive electrode active material was small, resulting in a low impregnation rate [%].

In Comparative Example 3, even if some of the secondary particles 32 in the positive electrode active material particles 3 become primary particles, the conductive material 33 is granular rather than fibrous, so it is assumed that it is hardly able to penetrate into the hollow portion 32b and therefore difficult to form a conductive network.

In Comparative Example 2, since fibrous conductive material 33 is used, it can be said that a conductive network is easily formed between the conductive material 33. However, the fibrous conductive material 33 does not easily penetrate into the secondary particles 32. Furthermore, since the secondary particles 32 are not primary particles, the surface area is small, and the wall surface of the hollow portion 32b of the secondary particles 32 is an area that is unlikely to contribute to the reaction, which is presumably the cause of the deterioration of the input/output characteristics.

On the other hand, in Example 2 of this embodiment, the fibrous conductive material 33 easily penetrates into the secondary particles 32. Furthermore, the secondary particles 32 are converted into primary particles, increasing the surface area of the positive electrode active material. At the same time, the fibrous conductive material 33 is also present near the wall surface of the hollow portion 32b of the crushed secondary particles 32. Therefore, the conductive material 33 arranged inside and outside the secondary particles 32 easily forms an effective conductive network. As a result, the input/output characteristics of the positive electrode plate 2 are improved.

(Effects of this embodiment)
(1) In the method for manufacturing a lithium ion secondary battery according to the present embodiment, the resistance of the positive electrode plate 2 can be reduced, and the input/output characteristics of the positive electrode can be improved.

(2) In this embodiment, primary particles 31 are aggregated to form a hollow shell shape, and the porous secondary particles 32 of the positive electrode active material have a shell portion 32a with through holes 32c. A kneading process (impregnation process) (S12) is also provided in which fibrous conductive material 33 is disposed inside and outside the secondary particles 32 together with a binder 34. Therefore, the fibrous conductive material 33 can be disposed inside the secondary particles 32 as well.

(3) The secondary particles 32 are crushed into primary particles in the pressing step (S15), which is a crushing step, so that the primary particles of the active material are not aggregated and the primary particles of the active material can be placed in the positive electrode mixture layer 4 in a dispersed state.

(4) Furthermore, the pressing step (S15) is the same as the conventional pressing step for adjusting the thickness of the positive electrode composite layer 4 and shaping the surface. Therefore, even in the conventional production method, by adjusting the press gap, the manufacturing method of the lithium ion secondary battery of this embodiment can be carried out without introducing any new equipment, etc.

(5) The average diameter D of the fibrous conductive material 33 is set to 60% or less of the average diameter W of the through holes of the secondary particles 32, so that the fibrous conductive material 33 can easily enter the hollow portions 32b of the secondary particles 32 together with the binder 34.

(6) In the pressing step (S15), 50% or more of the secondary particles 32 of the positive electrode active material particles 3 are converted into primary particles, so that the surface area contributing to the reaction is increased.
(7) In the kneading step (S12) which is an impregnation step, the binder 34 containing the fibrous conductive material 33 is impregnated into 85% or more of the secondary particles 32 together with the fibrous conductive material 33. Therefore, a large amount of the fibrous conductive material 33 can be arranged in the vicinity of the hollow portion 32b, and an effective conductive network can be constructed.

(8) The through holes 32c of the secondary particles 32 have a diameter of 50 nm or more. This allows the binder 34 containing the fibrous conductive material 33 to be smoothly introduced into the hollow portions 32b of the secondary particles 32.

(9) Since the diameter of the fibrous conductive material 33 is set to be as small as 1 nm to 100 nm, the binder 34 containing the fibrous conductive material 33 can be smoothly introduced into the hollow portion 32b of the secondary particle 32.

(10) The length of the fibrous conductive material 33 is set to 100 nm to 1000 nm. This makes it possible to construct a conductive network that connects the hollow portions 32b of the secondary particles 32 to the outside.
Moreover, the time is not so long that aggregation occurs.

(11) The positive electrode active material is composed of a lithium transition metal oxide, so that a high-performance lithium ion secondary battery can be obtained.
(12) The fibrous conductive material 33 is made of carbon nanotubes. This makes it possible to construct a conductive network with extremely high conductivity and excellent input/output characteristics.

(13) When preparing a paste-like positive electrode mixture, the positive electrode active material is in the form of secondary particles 32, so the active material particles tend to aggregate. Aggregation increases the viscosity and reduces the solid content, making it less likely to cause poor drying. This allows for easy handling during the mixture preparation process.

(14) By providing a positive electrode with excellent input/output characteristics, a high-performance lithium-ion secondary battery can be produced.
(Modification)
The above embodiment can also be implemented as follows.

The particle diameters of the primary particles 31 and secondary particles 32, the average diameter W of the through holes 32c, the average diameter D of the conductive material 33, the average length L of the conductive material, the impregnation rate, and the viscosity of the binder 34 are merely examples. Therefore, it goes without saying that these can be optimized by those skilled in the art, and are not limited to these numerical ranges.

The most suitable positive electrode active material is selected depending on the purpose of the lithium ion secondary battery, and the manufacturing method thereof, such as sintering, is also optimized by those skilled in the art.
The configuration of the binder 34 is not limited to that exemplified in the embodiment, and the material, viscosity, kneading conditions, and the like can be optimized as appropriate by a person skilled in the art depending on the material, characteristics, and shape of the positive electrode active material and the material and shape of the conductive material 33.

The flowchart of the embodiment is an example, and the order and contents are not limited to those shown.
Although the embodiment has been described with reference to an in-vehicle lithium ion secondary battery, the purpose and size of the battery are not limited thereto.

The present invention should not be interpreted as being limited to the above-described embodiment, and it goes without saying that a person skilled in the art could add, delete, or replace the configuration without departing from the scope of the claims.

2: Positive electrode plate 3: Positive electrode active material particle 31: Primary particle 32: Secondary particle 32a: Shell portion 32b: Hollow portion 32c: Through hole 32d: Fragment W: Average diameter (of through hole) 33: Conductive material D: Average diameter (of conductive material) L: Average length (of conductive material) 34: Binder 4: Positive electrode mixture layer 5: Current collector S1: Source process S11: Secondary particle production process S12: Kneading process (impregnation process)
S13: Coating process S14: Drying process S15: Pressing process (crushing process)
S16...Cutting process

Claims (6)

A current collector;
A method for producing a positive electrode of a lithium ion secondary battery including a positive electrode mixture layer containing positive electrode active material particles held by the current collector, a conductive material, and a binder, comprising:
an impregnation step of disposing the fibrous conductive material together with the binder inside and outside secondary particles of a porous positive electrode active material in which primary particles are aggregated to form a hollow shell shape and have a shell portion with a through hole;
and a pressing step of crushing at least a part of the secondary particles .
The through holes of the secondary particles have a diameter of 50 nm or more,
the fibrous conductive material is made of carbon nanotubes having an average diameter of 60% or less of the average diameter of the through holes of the secondary particles,
The method for manufacturing a positive electrode of a lithium ion secondary battery is characterized in that the impregnation step is performed by impregnating a binder containing the fibrous conductive material together with the fibrous conductive material in an amount of 85% or more of the secondary particles . 2. The method for producing a positive electrode for a lithium ion secondary battery according to claim 1 , wherein the pressing step converts 50% or more of the positive electrode active material particles into primary particles. 3. The method for producing a positive electrode for a lithium ion secondary battery according to claim 1, wherein the diameter of the fibrous conductive material is 1 nm to 100 nm. The method for producing a positive electrode of a lithium ion secondary battery according to any one of claims 1 to 3 , characterized in that the length of the fibrous conductive material is 100 nm to 1000 nm. 5. The method for producing a positive electrode of a lithium ion secondary battery according to claim 1 , wherein the positive electrode active material is composed of a lithium transition metal oxide. A method for producing a lithium ion secondary battery, comprising the method for producing a positive electrode of a lithium ion secondary battery according to any one of claims 1 to 5 .