Electrode sheet manufacturing method

A method includes performing roll pressing such that a first electrode mixture layer and a second electrode mixture layer are compressed in a thickness direction of the first electrode mixture layer and the second electrode mixture layer by passing an electrode sheet through a gap between a seventh roll and an eighth roll facing each other and rotating. When performing the roll pressing, T7 as a surface temperature of the seventh roll disposed on the first electrode mixture layer side and contacting the first electrode mixture layer and T8 as a surface temperature of the eighth roll disposed on the second electrode mixture layer side and contacting the second electrode mixture layer satisfy a relationship of T7<T8.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2017-147391 filed on Jul. 31, 2017 including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND

1. Technical Field

The disclosure relates to a method of manufacturing an electrode sheet that constitutes a battery. More specifically, the disclosure relates to a method of manufacturing an electrode sheet having a first electrode mixture layer on a first surface of a collector foil and a second electrode mixture layer on a second surface of the collector foil.

2. Description of Related Art

As an electrode sheet (positive electrode sheet or negative electrode sheet), there is known an electrode sheet having a first electrode mixture layer on a first surface of a collector foil and a second electrode mixture layer on a second surface of the collector foil. As a method of manufacturing an electrode sheet having such a structure, there is known a method as disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2013-052353 (JP 2013-052353 A). Specifically, first, an electrode mixture containing a plurality of wet granules obtained by mixing and granulating an electrode active material, a binder, and a solvent is produced. Then, in a first film forming process, while the electrode mixture is formed into a film shape by passing the electrode mixture through a gap between a first roll and a second roll facing each other and rotating, the film-shaped electrode mixture is adhered to a surface of the second roll. Further, while a second surface of a collector foil conveyed by a third roll facing the second roll and rotating is brought in contact with a surface of the third roll, the collector foil is passed through a gap between the second roll and the third roll. Consequently, the film-shaped electrode mixture adhering to the surface of the second roll is pressed against a first surface of the collector foil so as to be transferred to the first surface of the collector foil, thereby producing a first electrode mixture layer coated collector foil in which a first electrode mixture layer is formed on the first surface of the collector foil.

Further, in a second film forming process, while the electrode mixture is formed into a film shape by passing the electrode mixture through a gap between a fourth roll and a fifth roll facing each other and rotating, the film-shaped electrode mixture is adhered to a surface of the fifth roll. Further, while the first electrode mixture layer of the first electrode mixture layer coated collector foil conveyed by a sixth roll facing the fifth roll and rotating is brought in contact with a surface of the sixth roll, the first electrode mixture layer coated collector foil is passed through a gap between the fifth roll and the sixth roll. Consequently, the film-shaped electrode mixture adhering to the surface of the fifth roll is pressed against the second surface of the collector foil so as to be transferred to the second surface of the collector foil, thereby producing an electrode sheet in which a second electrode mixture layer is formed on the second surface of the first electrode mixture layer coated collector foil.

SUMMARY

In the meantime, in the electrode sheet produced as described above, there are cases where the density of the first electrode mixture layer formed on the first surface of the collector foil becomes higher than the density of the second electrode mixture layer formed on the second surface of the collector foil so that the large density difference occurs between the first and second electrode mixture layers. This is because since the first electrode mixture layer is formed by compression of the electrode mixture between the second roll and the third roll in the first film forming process and is further compressed between the fifth roll and the sixth roll when the second electrode mixture layer is formed in the second film forming process, the compressibility of the first electrode mixture layer becomes higher (the porosity of the first electrode mixture layer becomes lower) than that of the second electrode mixture layer. Therefore, in the electrode sheet produced as described above, the density of the first electrode mixture layer becomes higher than the density of the second electrode mixture layer. There is a tendency that the greater the density difference between a first electrode mixture layer and a second electrode mixture layer in an electrode sheet, the faster the deterioration of a battery using such an electrode sheet. Therefore, the density difference between the first electrode mixture layer and the second electrode mixture layer is required to be reduced in the electrode sheet produced as described above.

The disclosure provides an electrode sheet manufacturing method that can reduce the density difference between a first electrode mixture layer and a second electrode mixture layer.

An aspect of the disclosure relates to a method of manufacturing an electrode sheet including a first electrode mixture layer on a first surface of a collector foil having the first surface and a second surface, and a second electrode mixture layer on the second surface. The method includes: performing first film formation such that while an electrode mixture containing a plurality of wet granules obtained by mixing and granulating an electrode active material, a binder, and a solvent is formed into a film shape by passing the electrode mixture through a gap between a first roll and a second roll facing each other and rotating, the electrode mixture formed into the film shape is adhered to a surface of the second roll, and that while the second surface of the collector foil conveyed by a third roll facing the second roll and rotating is brought in contact with a surface of the third roll, the collector foil is passed through a gap between the second roll and the third roll to cause the film-shaped electrode mixture adhering to the surface of the second roll to be pressed against and transferred to the first surface of the collector foil so as to produce a first electrode mixture layer coated collector foil in which the first electrode mixture layer is formed on the first surface of the collector foil; performing second film formation such that while the electrode mixture is formed into a film shape by passing the electrode mixture through a gap between a fourth roll and a fifth roll facing each other and rotating, the electrode mixture formed into the film shape is adhered to a surface of the fifth roll, and that while the first electrode mixture layer of the first electrode mixture layer coated collector foil conveyed by a sixth roll facing the fifth roll and rotating is brought in contact with a surface of the sixth roll, the first electrode mixture layer coated collector foil is passed through a gap between the fifth roll and the sixth roll to cause the film-shaped electrode mixture adhering to the surface of the fifth roll to be pressed against and transferred to the second surface of the first electrode mixture layer coated collector foil so as to produce the electrode sheet in which the second electrode mixture layer is formed on the second surface of the first electrode mixture layer coated collector foil; and performing roll pressing such that the first electrode mixture layer and the second electrode mixture layer are compressed in a thickness direction of the first electrode mixture layer and the second electrode mixture layer by passing the electrode sheet through a gap between a seventh roll and an eighth roll facing each other and rotating, wherein when performing the roll pressing, T7as a surface temperature of the seventh roll disposed on a first electrode mixture layer side and contacting the first electrode mixture layer and T8as a surface temperature of the eighth roll disposed on a second electrode mixture layer side and contacting the second electrode mixture layer satisfy a relationship of T7<T8.

In the above-described manufacturing method, the electrode sheet (before the roll pressing) is produced by performing the first film formation and the second film formation described above. In the electrode sheet thus produced in the related art, there are cases where the density of the first electrode mixture layer formed on the first surface of the collector foil becomes higher than the density of the second electrode mixture layer formed on the second surface of the collector foil so that the large density difference occurs between the first and second electrode mixture layers.

In this regard, the above-described manufacturing method includes performing the roll pressing such that the first electrode mixture layer and the second electrode mixture layer are compressed in the thickness direction of the first electrode mixture layer and the second electrode mixture layer by passing the electrode sheet through the gap between the seventh roll and the eighth roll facing each other and rotating after the second film formation. Further, when performing the roll pressing, T7as the surface temperature of the seventh roll disposed on the first electrode mixture layer side and contacting the first electrode mixture layer and T8as the surface temperature of the eighth roll disposed on the second electrode mixture layer side and contacting the second electrode mixture layer satisfy the relationship of T7<T8.

In this way, by setting T8as the surface temperature of the eighth roll brought in contact with the second electrode mixture layer having the relatively low density to be higher than T7as the surface temperature of the seventh roll brought in contact with the first electrode mixture layer having the relatively high density, the compression can be performed while the temperature of the second electrode mixture layer is made higher than the temperature of the first electrode mixture layer. Consequently, in the roll pressing, the compression can be performed while the second electrode mixture layer is made softer than the first electrode mixture layer, and accordingly, the second electrode mixture layer is compressed more easily than the first electrode mixture layer. As a result, in the electrode sheet after performing the roll pressing, the density difference between the first electrode mixture layer and the second electrode mixture layer is reduced compared to the electrode sheet before performing the roll pressing. As described above, according to the above-described electrode sheet manufacturing method, it is possible to reduce the density difference between the first electrode mixture layer and the second electrode mixture layer.

The first film formation can be performed using, for example, a roll film forming apparatus having the first roll, the second roll, and the third roll. The second film formation can be performed using, for example, a roll film forming apparatus having the fourth roll, the fifth roll, and the sixth roll. The roll film forming apparatus for use in performing the first film formation and the roll film forming apparatus for use in performing the second film formation may be the same roll film forming apparatus. In this case, the first roll and the fourth roll are the same roll, the second roll and the fifth roll are the same roll, and the third roll and the sixth roll are the same roll. However, in the second film formation, the gap between the second roll and the third roll (the gap between the fifth roll and the sixth roll) is set to be greater than the gap therebetween in the first film formation. This is because the thickness of the first electrode mixture layer coated collector foil that is passed through the gap between the fifth roll and the sixth roll in the second film formation is greater than the thickness of the collector foil that is passed through the gap between the second roll and the third roll in the first film formation.

The wet granules are each a substance (granular body) in which the solvent and particles of the electrode active material are collected (joined) together in the state where the solvent and the particles of the electrode active material are held (absorbed) by the binder. In the above-described manufacturing method, first drying for drying the first electrode mixture layer formed on the first surface may be provided after the first film formation and before the second film formation. Further, second drying for drying the second electrode mixture layer formed on the second surface may be provided after the second film formation and before the roll pressing.

In the electrode sheet manufacturing method according to the aspect of the disclosure, when performing the roll pressing, T7as the surface temperature of the seventh roll may be maintained at a temperature lower than (Ts-50)° C. being a temperature 50° C. lower than Ts as a softening temperature of the binder contained in the electrode mixture, and T8as the surface temperature of the eighth roll may be maintained at a temperature equal to or higher than (Ts-50)° C.

In the above-described manufacturing method, when performing the roll pressing, T7as the surface temperature of the seventh roll is maintained at the temperature lower than (Ts-50)° C. being the temperature 50° C. lower than Ts as the softening temperature of the binder contained in the electrode mixture, and further, T8as the surface temperature of the eighth roll is maintained at the temperature equal to or higher than (Ts-50)° C. That is, the roll pressing is performed in the state where T7as the surface temperature of the seventh roll brought in contact with the first electrode mixture layer having the relatively high density is maintained at the temperature lower than (Ts-50)° C., and T8as the surface temperature of the eighth roll brought in contact with the second electrode mixture layer having the relatively low density is maintained at the temperature equal to or higher than (Ts-50)° C.

By maintaining T8as the surface temperature of the eighth roll brought in contact with the second electrode mixture layer at the temperature equal to or higher than (Ts-50)° C., it is possible to soften the binder contained in the second electrode mixture layer so that the compressibility of the second electrode mixture layer can be increased. On the other hand, by maintaining T7as the surface temperature of the seventh roll brought in contact with the first electrode mixture layer at the temperature lower than (Ts-50)° C., it is possible to suppress softening of the binder contained in the first electrode mixture layer so that the compressibility of the first electrode mixture layer by the roll pressing can be made lower than the compressibility of the second electrode mixture layer by the roll pressing. As a result, in the electrode sheet after performing the roll pressing, the density difference between the first electrode mixture layer and the second electrode mixture layer is reduced compared to the electrode sheet before performing the roll pressing.

As the binder contained in the electrode mixture, polyvinylidene difluoride (PVdF), for example, can be cited. Ts as the softening temperature of PVdF is 150° C. Therefore, when PVdF is used as the binder, T7as the surface temperature of the seventh roll is maintained at a temperature lower than (Ts-50)=150-50=100° C., and T8as the surface temperature of the eighth roll is maintained at a temperature equal to or higher than (Ts-50)=100° C.

In the electrode sheet manufacturing method according to the aspect of the disclosure, when performing the roll pressing, T7as the surface temperature of the seventh roll may be maintained at a temperature in a range from 15° C. to 35° C.

In the above-described manufacturing method, when performing the roll pressing, T7as the surface temperature of the seventh roll is maintained at the temperature in the range from 15° C. to 35° C. (i.e. normal temperature). In this way, by maintaining T7as the surface temperature of the seventh roll brought in contact with the first electrode mixture layer at the temperature in the range from 15° C. to 35° C., it is possible to suppress softening of the binder contained in the first electrode mixture layer at the time of the roll pressing so that the compressibility of the first electrode mixture layer by the roll pressing can be made lower than the compressibility of the second electrode mixture layer by the roll pressing. Consequently, in the electrode sheet after performing the roll pressing, the density difference between the first electrode mixture layer and the second electrode mixture layer can be reduced compared to the electrode sheet before performing the roll pressing.

In the electrode sheet manufacturing method according to the aspect of the disclosure, the seventh roll and the eighth roll may have a diameter of 300 mm or more.

In the above-described manufacturing method, the roll pressing is performed using the seventh roll and the eighth roll having the diameter (outer diameter) of 300 mm or more. By performing the roll pressing such that T7as the surface temperature of the seventh roll and T8as the surface temperature of the eighth roll, having the diameter of 300 mm or more, satisfy the relationship of T7<T8, the compressibility of the second electrode mixture layer by this roll pressing can be effectively increased. Consequently, it is possible to further reduce the density difference between the first electrode mixture layer and the second electrode mixture layer. Specifically, when the electrode sheet is passed through the gap between the seventh roll and the eighth roll so as to be pressed, the greater the diameter of the seventh roll and the eighth roll, the easier the first electrode mixture layer and the second electrode mixture layer are compressed in the thickness direction thereof and the more difficult the first electrode mixture layer and the second electrode mixture layer extend in the longitudinal direction thereof. This is because the greater the diameter of the seventh roll and the eighth roll, the more difficult the forces applied from the seventh roll and the eighth roll to the first electrode mixture layer and the second electrode mixture layer are applied in the rotational directions of the seventh roll and the eighth roll at the position of the gap between the seventh roll and the eighth roll (i.e. the longitudinal direction of the first electrode mixture layer and the second electrode mixture layer) and the easier the forces applied from the seventh roll and the eighth roll to the first electrode mixture layer and the second electrode mixture layer are applied in the thickness direction of the first electrode mixture layer and the second electrode mixture layer. Therefore, by performing the roll pressing using the seventh roll and the eighth roll having the diameter of 300 mm or more in the state where the second electrode mixture layer is more easily deformed than the first electrode mixture layer by causing T7as the surface temperature of the seventh roll and T8as the surface temperature of the eighth roll to satisfy the relationship of T7<T8, the compressibility of the second electrode mixture layer by the roll pressing can be effectively further increased compared to the compressibility of the first electrode mixture layer by the roll pressing. Consequently, it is possible to further reduce the density difference between the first electrode mixture layer and the second electrode mixture layer. Although the upper limit value of the diameter of the seventh roll and the eighth roll is not particularly limited, since a press apparatus that performs the roll pressing is preferably small, the diameter of the seventh roll and the eighth roll is preferably set to 500 mm or less.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, an embodiment embodying the disclosure will be described in detail with reference to the drawings. In this embodiment, the disclosure is applied to the manufacture of a positive electrode sheet (electrode sheet) of a lithium-ion secondary battery. In this embodiment, as materials of a positive electrode mixture (electrode mixture) for forming a positive electrode mixture layer (electrode mixture layer) of the positive electrode sheet, a positive electrode active material (electrode active material), a conductive material, a binder, and a solvent are used.

In this embodiment, a lithium transition metal composite oxide (specifically, LiNi1/3Mn1/3Co1/3O2) in the form of powder is used as the positive electrode active material. Acetylene black powder is used as the conductive material. Polyvinylidene difluoride (PVdF) is used as the binder. N-methylpyrrolidone (NMP) is used as the solvent.

In this embodiment, in an electrode mixture producing process, a large number of wet granules16are produced by mixing and granulating the materials described above, so that a positive electrode mixture6(electrode mixture) containing the large number of wet granules16is produced. Then, in a first film forming process, the positive electrode mixture6is adhered (coated) in a film shape on a first surface7bof a collector foil7, thereby producing a first positive electrode mixture layer coated collector foil41(first electrode mixture layer coated collector foil) in which a first positive electrode mixture layer18b(first electrode mixture layer) is formed on the first surface7bof the collector foil7. Then, in a first drying process, the first positive electrode mixture layer18b(first electrode mixture layer) on the first surface7bof the collector foil7is dried.

Then, in a second film forming process, the positive electrode mixture6is adhered (coated) in a film shape on a second surface7cof the first positive electrode mixture layer coated collector foil41, thereby producing a positive electrode sheet19(electrode sheet) in which a second positive electrode mixture layer18c(second electrode mixture layer) is formed on the second surface7c. Then, in a second drying process, the second positive electrode mixture layer18c(second electrode mixture layer) on the second surface7cis dried. Thereafter, in a roll pressing process, the first positive electrode mixture layer18band the second positive electrode mixture layer18cof the positive electrode sheet19are compressed in the thickness direction thereof. In this way, the positive electrode sheet19(electrode sheet) in which the density of the first positive electrode mixture layer18band the second positive electrode mixture layer18cis increased is obtained.

Herein, a method of manufacturing the electrode sheet (positive electrode sheet19) according to this embodiment will be described in detail.FIG. 1is a schematic diagram of a first roll film forming apparatus20according to the embodiment.FIG. 2is a perspective view of the first roll film forming apparatus20.FIG. 3is a flowchart showing the flow of the method of manufacturing the electrode sheet (positive electrode sheet19) according to the embodiment.FIG. 4is a schematic diagram of the first roll film forming apparatus20and a first drying apparatus50according to the embodiment.FIG. 5is a schematic diagram of a second roll film forming apparatus30according to the embodiment.FIG. 6is a perspective view of the second roll film forming apparatus30.FIG. 7is a schematic diagram of the second roll film forming apparatus30and a second drying apparatus70according to the embodiment.FIG. 8is a schematic diagram of a press apparatus60according to the embodiment.

As shown inFIG. 3, first, at step S1(electrode mixture producing process), the large number of wet granules16are produced by mixing and granulating the positive electrode active material (LiNi1/3Mn1/3Co1/3O2), the conductive material (acetylene black), the binder (PVdF), and the solvent (NMP), so that the positive electrode mixture6containing the large number of wet granules16is produced. In this embodiment, the positive electrode active material, the conductive material, the binder, and the solvent are put into a known stirring granulator (not shown) and stirred so that the materials are mixed (dispersed) and granulated into the large number of wet granules16. In this way, the positive electrode mixture6containing the large number of wet granules16is obtained.

The wet granules16are each a substance (granular body) in which NMP as the solvent and particles of the positive electrode active material are collected (joined) together in the state where NMP and the particles of the positive electrode active material are held (absorbed) by the binder. The positive electrode mixture6is an aggregate of the wet granules16described above.

Then, at step S2(first film forming process), the positive electrode mixture6produced at step S1(electrode mixture producing process) is formed into a film shape, and the film-shaped positive electrode mixture6is adhered to the first surface7bof the collector foil7, thereby producing the first positive electrode mixture layer coated collector foil41(first electrode mixture layer coated collector foil) in which the first positive electrode mixture layer18b(first electrode mixture layer) is formed on the first surface7bof the collector foil7. In this embodiment, step S2(first film forming process) is performed using the first roll film forming apparatus20shown inFIGS. 1 and 2.

As shown inFIGS. 1 and 2, the first roll film forming apparatus20includes three rolls, i.e. a first roll1, a second roll2, and a third roll3. The first roll1and the second roll2are disposed side by side in the horizontal direction (right-left direction inFIG. 1). On the other hand, the second roll2and the third roll3are disposed side by side in the vertical direction (up-down direction inFIG. 1). The first roll1and the second roll2face (confront) each other with a slight interval therebetween. Likewise, the second roll2and the third roll3face (confront) each other with a slight interval therebetween. Further, on the upper side of the facing portion between the first roll1and the second roll2, partition plates4,5are disposed to be spaced apart from each other in the width direction of the roll (axial direction of the roll, i.e. a direction perpendicular to the sheet surface inFIG. 1).

The rotational directions of the three rolls1to3are set such that, as indicated by arrows inFIGS. 1 and 2, the rotational directions of the adjacent (facing) two rolls are opposite to each other, i.e. the facing two rolls rotate in the forward direction relative to each other. At the facing portion between the first roll1and the second roll2, the surfaces of these rolls are configured to move downward by the rotation. At the facing portion between the second roll2and the third roll3, the surfaces of these rolls are configured to move rightward by the rotation. Regarding the rotational speed, the moving speed of the surface of the roll by the rotation is set to be the lowest at the first roll1, the highest at the third roll3, and between them at the second roll2.

In the first roll film forming apparatus20thus configured, the positive electrode mixture6produced at step S1(electrode mixture producing process) is put into an accommodation space between the partition plates4,5located above the facing portion between the first roll1and the second roll2. The collector foil7is wound on the third roll3. At step S2(first film forming process), the collector foil7is wound on the third roll3such that the second surface7cof the collector foil7is brought in contact with the surface of the third roll3(in other words, the first surface7bof the collector foil7faces radially outward of the third roll3).

The collector foil7is a metal foil (aluminum foil) having the first surface7b(front surface) and the second surface7c(back surface). With the rotation of the third roll3, the collector foil7passes through the facing portion (gap) between the second roll2and the third roll3so as to be conveyed from the lower right to the upper right of the third roll3. In the state where the collector foil7passes through the facing portion (gap) between the second roll2and the third roll3, a slight gap is provided between the second roll2and the collector foil7. That is, the gap between the second roll2and the third roll3(gap in the state where the collector foil7is not present) is slightly greater than the thickness of the collector foil7.

At step S2(first film forming process), while the positive electrode mixture6is formed into a film shape by passing the positive electrode mixture6through the gap between the first roll1and the second roll2facing each other and rotating, the film-shaped positive electrode mixture6is adhered to the surface of the second roll2. At the same time, while the second surface7cof the collector foil7conveyed by the third roll3facing the second roll2and rotating is brought in contact with the surface of the third roll3, the collector foil7is passed through the gap between the second roll2and the third roll3. Consequently, the film-shaped positive electrode mixture6adhering to the surface of the second roll2is pressed against the first surface7bof the collector foil7so as to be transferred to the first surface7b. When passing through the gap between the second roll2and the third roll3(i.e. when transferred to the first surface7bof the collector foil7), the film-shaped positive electrode mixture6is compressed in its thickness direction so that its density is increased.

More specifically, first, the positive electrode mixture6produced at step Si (electrode mixture producing process) is put into the accommodation space between the partition plates4,5of the first roll film forming apparatus20. The input positive electrode mixture6is supplied into the gap at the facing portion between the first roll1and the second roll2facing each other and rotating and is, by the rotation of the first roll1and the second roll2, passed through the gap between the first and second rolls1,2so as to be formed into a film shape (seeFIGS. 1 and 2). In this event, since the rotational speed of the second roll2is higher than the rotational speed of the first roll1, the positive electrode mixture6(wet granules16) is stretched more on the surface of the second roll2than on the surface of the first roll1, and therefore, the liquid bridge area becomes greater on the surface of the second roll2than on the surface of the first roll1so that the positive electrode mixture6is carried (adhered) on the surface of the second roll2.

Then, the film-shaped positive electrode mixture6carried (adhered) on the surface of the second roll2(hereinafter referred to as a “first film-shaped positive electrode mixture8”) is conveyed with the rotation of the second roll2(seeFIGS. 1 and 2). Then, the first surface7bof the collector foil7and the first film-shaped positive electrode mixture8meet each other at the facing portion between the second roll2and the third roll3so that the collector foil7and the first film-shaped positive electrode mixture8are sandwiched between the second roll2and the third roll3. The gap dimension (minimum gap dimension) at the facing portion between the second roll2and the third roll3is set to be smaller than the sum of the thickness of the collector foil7and the thickness of the first film-shaped positive electrode mixture8.

Therefore, when the collector foil7and the first film-shaped positive electrode mixture8are sandwiched between the second roll2and the third roll3, the pressing load is applied to the first film-shaped positive electrode mixture8from the surface of the second roll2toward the first surface7bof the collector foil7. Consequently, the first film-shaped positive electrode mixture8adhering to the surface of the second roll2is pressed against the first surface7bof the collector foil7so that the first film-shaped positive electrode mixture8is transferred (adhered) from the second roll2to the first surface7bof the collector foil7that is moving with the rotation of the third roll3. In this event, the first film-shaped positive electrode mixture8is compressed in its thickness direction so as to be the first positive electrode mixture layer18b, thereby obtaining the first positive electrode mixture layer coated collector foil41(first electrode mixture layer coated collector foil) in which the first positive electrode mixture layer18bis formed on the first surface7bof the collector foil7.

Then, at step S3(first drying process), the first positive electrode mixture layer coated collector foil41(first positive electrode mixture layer18b) is dried. Specifically, as shown inFIG. 4, the first positive electrode mixture layer18bformed on the first surface7bof the collector foil7is dried by passing the first positive electrode mixture layer coated collector foil41through the inside of the first drying apparatus50(drying furnace). Consequently, the solvent absorbed (retained) in the first positive electrode mixture layer18b(wet granules16) is removed (evaporated).

Then, at step S4(second film forming process), the positive electrode mixture6is adhered (coated) in a film shape on the second surface7cof the first positive electrode mixture layer coated collector foil41, thereby producing the positive electrode sheet19(electrode sheet) in which the second positive electrode mixture layer18c(second electrode mixture layer) is formed on the second surface7cof the collector foil7. In this embodiment, step S4(second film forming process) is performed using the second roll film forming apparatus30shown inFIGS. 5 and 6. Details will be described below.

As shown inFIGS. 5 and 6, the second roll film forming apparatus30includes three rolls, i.e. a fourth roll31, a fifth roll32, and a sixth roll33. The fourth roll31and the fifth roll32are disposed side by side in the horizontal direction (right-left direction inFIG. 5). On the other hand, the fifth roll32and the sixth roll33are disposed side by side in the vertical direction (up-down direction inFIG. 5). The fourth roll31and the fifth roll32face (confront) each other with a slight interval therebetween. Likewise, the fifth roll32and the sixth roll33face (confront) each other with a slight interval therebetween. Further, on the upper side of the facing portion between the fourth roll31and the fifth roll32, partition plates4,5are disposed to be spaced apart from each other in the width direction of the roll (axial direction of the roll, i.e. a direction perpendicular to the sheet surface inFIG. 5).

The second roll film forming apparatus30differs from the first roll film forming apparatus20in that the gap dimension at the facing portion between the fifth roll32and the sixth roll33is slightly greater than the gap dimension at the facing portion between the second roll2and the third roll3. The other configuration is the same as the first roll film forming apparatus20. Therefore, the fourth roll31is equivalent to the first roll1, the fifth roll32is equivalent to the second roll2, and the sixth roll33is equivalent to the third roll3.

In the second roll film forming apparatus30thus configured, the positive electrode mixture6produced at step S1(electrode mixture producing process) is put into an accommodation space between the partition plates 4, 5. The first positive electrode mixture layer coated collector foil41treated at step S3(first drying process) is wound on the sixth roll33. Specifically, the first positive electrode mixture layer coated collector foil41is wound on the sixth roll33such that the first positive electrode mixture layer18bof the first positive electrode mixture layer coated collector foil41is brought in contact with the surface of the sixth roll33(in other words, the second surface7cof the collector foil7faces radially outward of the sixth roll33).

With the rotation of the sixth roll33, the first positive electrode mixture layer coated collector foil41passes through the facing portion (gap) between the fifth roll32and the sixth roll33so as to be conveyed from the lower right to the upper right of the sixth roll33. In the state where the first positive electrode mixture layer coated collector foil41passes through the facing portion (gap) between the fifth roll32and the sixth roll33, a slight gap is provided between the fifth roll32and the first positive electrode mixture layer coated collector foil41. That is, the gap between the fifth roll32and the sixth roll33(gap in the state where the first positive electrode mixture layer coated collector foil41is not present) is slightly greater than the thickness of the first positive electrode mixture layer coated collector foil41.

At step S4(second film forming process), while the positive electrode mixture6is formed into a film shape by passing the positive electrode mixture6through the gap between the fourth roll31and the fifth roll32facing each other and rotating, the film-shaped positive electrode mixture6is adhered to the surface of the fifth roll32. At the same time, while the first positive electrode mixture layer18bof the first positive electrode mixture layer coated collector foil41conveyed by the sixth roll33facing the fifth roll32and rotating is brought in contact with the surface of the sixth roll33, the first positive electrode mixture layer coated collector foil41is passed through the gap between the fifth roll32and the sixth roll33. Consequently, the film-shaped positive electrode mixture6adhering to the surface of the fifth roll32is pressed against the second surface7cof the first positive electrode mixture layer coated collector foil41so as to be transferred to the second surface7c. When passing through the gap between the fifth roll32and the sixth roll33(i.e. when transferred to the second surface7cof the collector foil7), the film-shaped positive electrode mixture6is compressed in its thickness direction so that its density is increased.

More specifically, first, the positive electrode mixture6produced at step S1(electrode mixture producing process) is put into the accommodation space between the partition plates4,5of the second roll film forming apparatus30. The input positive electrode mixture6is supplied into the gap at the facing portion between the fourth roll31and the fifth roll32facing each other and rotating and is, by the rotation of the fourth roll31and the fifth roll32, passed through the gap between the fourth and fifth rolls31,32so as to be formed into a film shape and carried (adhered) on the surface of the fifth roll32(seeFIGS. 5 and 6).

Then, the film-shaped positive electrode mixture6carried (adhered) on the surface of the fifth roll32(hereinafter referred to as a “second film-shaped positive electrode mixture48”) is conveyed with the rotation of the fifth roll32(seeFIGS. 5 and 6). Then, the second surface7cof the first positive electrode mixture layer coated collector foil41and the second film-shaped positive electrode mixture48meet each other at the facing portion between the fifth roll32and the sixth roll33so that the first positive electrode mixture layer coated collector foil41and the second film-shaped positive electrode mixture48are sandwiched between the fifth roll32and the sixth roll33. The gap dimension (minimum gap dimension) at the facing portion between the fifth roll32and the sixth roll33is set to be smaller than the sum of the thickness of the first positive electrode mixture layer coated collector foil41and the thickness of the second film-shaped positive electrode mixture48.

Therefore, when the first positive electrode mixture layer coated collector foil41and the second film-shaped positive electrode mixture48are sandwiched between the fifth roll32and the sixth roll33, the pressing load is applied to the second film-shaped positive electrode mixture48from the surface of the fifth roll32toward the second surface7cof the first positive electrode mixture layer coated collector foil41. Consequently, the second film-shaped positive electrode mixture48adhering to the surface of the fifth roll32is pressed against the second surface7cof the first positive electrode mixture layer coated collector foil41so that the second film-shaped positive electrode mixture48is transferred (adhered) from the fifth roll32to the second surface7cof the first positive electrode mixture layer coated collector foil41that is moving with the rotation of the sixth roll33. In this event, the second film-shaped positive electrode mixture48is compressed in its thickness direction so as to be the second positive electrode mixture layer18cso that the second positive electrode mixture layer18cis formed on the second surface7cof the collector foil7, thereby obtaining the positive electrode sheet19.

Then, at step S5(second drying process), the positive electrode sheet19(second positive electrode mixture layer18c) is dried. Specifically, as shown inFIG. 7, the second positive electrode mixture layer18cformed on the second surface7cof the collector foil7is dried by passing the positive electrode sheet19through the inside of the second drying apparatus70(drying furnace). Consequently, the solvent absorbed (retained) in the second positive electrode mixture layer18c(wet granules16) is removed (evaporated).

In the meantime, in the positive electrode sheet19produced as described above, there are cases where the density of the first positive electrode mixture layer18bformed on the first surface7bof the collector foil7becomes higher than the density of the second positive electrode mixture layer18cformed on the second surface7cof the collector foil7so that the large density difference occurs between the first and second electrode mixture layers18b,18c. This is because since the first positive electrode mixture layer18bis formed by compression of the positive electrode mixture6between the second roll2and the third roll3in the first film forming process (step S2) and is further compressed between the fifth roll32and the sixth roll33when the second positive electrode mixture layer18cis formed in the second film forming process (step S4), the compressibility of the first positive electrode mixture layer18bbecomes higher (the porosity of the first positive electrode mixture layer18bbecomes lower) than that of the second positive electrode mixture layer18c. Therefore, in the positive electrode sheet19produced as described above, there is a tendency that the density of the first positive electrode mixture layer18bbecomes higher than the density of the second positive electrode mixture layer18c.

In this regard, the manufacturing method of this embodiment includes step S6(roll pressing process) in which, after step S5(second drying process), the first positive electrode mixture layer18band the second positive electrode mixture layer18care compressed in the thickness direction thereof by passing the positive electrode sheet19through a gap between a seventh roll61and an eighth roll62facing each other and rotating (seeFIG. 3).

Specifically, at step S6(roll pressing process), the first positive electrode mixture layer18band the second positive electrode mixture layer18cof the positive electrode sheet19are compressed using the press apparatus60having a pair of rolls (seventh roll61and eighth roll62) (seeFIG. 8). The gap dimension (minimum gap dimension) at the facing portion between the seventh roll61and the eighth roll62is set to be smaller than the thickness of the positive electrode sheet19produced at step S5(second drying process).

As shown inFIG. 8, the press apparatus60of this embodiment includes the seventh roll61, the eighth roll62, a first temperature sensor141for measuring a temperature T7of the surface of the seventh roll61(the surface contacting the first positive electrode mixture layer18b), and a second temperature sensor142for measuring a temperature T8of the surface of the eighth roll62(the surface contacting the second positive electrode mixture layer18c). The press apparatus60further includes a first chiller110for maintaining the temperature T7of the surface of the seventh roll61(the surface contacting the first positive electrode mixture layer18b) at a first temperature, a second chiller120for maintaining the temperature T8of the surface of the eighth roll62(the surface contacting the second positive electrode mixture layer18c) at a second temperature (higher than the first temperature), and a heater130for heating the surface of the eighth roll62.

The first chiller110includes a circulation path112in which a heat medium is circulated, and a first body portion111that controls the temperature of the heat medium. Based on the surface temperature T7of the seventh roll61measured by the first temperature sensor141, the first body portion111adjusts the temperature of the heat medium circulating in the circulation path112so that the surface temperature T7of the seventh roll61is maintained at the first temperature. Consequently, in the press apparatus60of this embodiment, the surface temperature T7of the seventh roll61can be maintained at the first temperature.

The second chiller120includes a circulation path122in which a heat medium is circulated, and a second body portion121that controls the temperature of the heat medium. Based on the surface temperature T8of the eighth roll62measured by the second temperature sensor142, the second body portion121adjusts the temperature of the heat medium circulating in the circulation path122so that the surface temperature T8of the eighth roll62is maintained at the second temperature. The heater130is an IH heater and includes heat generating portions132,133and a controller131. Based on the surface temperature T8of the eighth roll62measured by the second temperature sensor142, the controller131adjusts the calorific values in the heat generating portions132,133so that the surface temperature T8of the eighth roll62is maintained at the second temperature.

Consequently, in the press apparatus60of this embodiment, the surface temperature T8of the eighth roll62can be maintained at the second temperature higher than the first temperature.

Using the press apparatus60thus configured, in the roll pressing process (step S6) of this embodiment, the first positive electrode mixture layer18band the second positive electrode mixture layer18cof the positive electrode sheet19are compressed so that the surface temperature T7of the seventh roll61disposed on the first positive electrode mixture layer18bside and contacting the first positive electrode mixture layer18band the surface temperature T8of the eighth roll62disposed on the second positive electrode mixture layer18cside and contacting the second positive electrode mixture layer18csatisfy a relationship of T7<T8.

In this way, by setting the surface temperature T8of the eighth roll62brought in contact with the second positive electrode mixture layer18chaving the relatively low density to be higher than the surface temperature T7of the seventh roll61brought in contact with the first positive electrode mixture layer18bhaving the relatively high density, the compression can be performed while the temperature of the second positive electrode mixture layer18cis made higher than the temperature of the first positive electrode mixture layer18b. Consequently, in the roll pressing process (step S6), the compression can be performed while the second positive electrode mixture layer18cis made softer than the first positive electrode mixture layer18b, and accordingly, the second positive electrode mixture layer18cis compressed more easily than the first positive electrode mixture layer18b. As a result, in the positive electrode sheet19after performing the roll pressing process (step S6), the density difference between the first positive electrode mixture layer18band the second positive electrode mixture layer18cis reduced compared to the positive electrode sheet19before performing the roll pressing process.

Specifically, in the roll pressing process (step S6) of this embodiment, the surface temperature T7of the seventh roll61is maintained at a temperature lower than (Ts-50)° C. being a temperature 50° C. lower than a softening temperature Ts of the binder contained in the positive electrode mixture6. On the other hand, the surface temperature T8of the eighth roll62is maintained at a temperature equal to or higher than (Ts-50)° C. That is, the roll pressing process is performed in the state where the surface temperature T7of the seventh roll61brought in contact with the first positive electrode mixture layer18bhaving the relatively high density is maintained at the first temperature lower than (Ts-50)° C., and the surface temperature T8of the eighth roll62brought in contact with the second positive electrode mixture layer18chaving the relatively low density is maintained at the second temperature equal to or higher than (Ts-50)° C.

By maintaining the surface temperature T8of the eighth roll62brought in contact with the second positive electrode mixture layer18cat the second temperature equal to or higher than (Ts-50)° C., it is possible to soften the binder contained in the second positive electrode mixture layer18cso that the compressibility of the second positive electrode mixture layer18ccan be increased. On the other hand, by maintaining the surface temperature T7of the seventh roll61brought in contact with the first positive electrode mixture layer18bat the first temperature lower than (Ts-50)° C., it is possible to suppress softening of the binder contained in the first positive electrode mixture layer18bso that the compressibility of the first positive electrode mixture layer18bby the roll pressing process can be made lower than the compressibility of the second positive electrode mixture layer18cby the roll pressing process. As a result, in the positive electrode sheet19after performing the roll pressing process (step S6), the density difference between the first positive electrode mixture layer18band the second positive electrode mixture layer18cis reduced compared to the positive electrode sheet19before performing the roll pressing process.

In this embodiment, polyvinylidene difluoride (PVdF) is used as the binder. The softening temperature Ts of PVdF is 150° C. Therefore, in the roll pressing process (step S6) of this embodiment, temperature adjustment is performed by the first chiller110so that the surface temperature T7of the seventh roll61is maintained at the first temperature lower than (Ts-50)=150-50=100° C., and temperature adjustment is performed by the second chiller 120 and the heater 130 so that the surface temperature T8of the eighth roll62is maintained at the second temperature equal to or higher than (Ts-50)=100° C.

More specifically, in the roll pressing process (step S6) of this embodiment, the surface temperature T7of the seventh roll61is maintained at a temperature in a range from 15° C. to 35° C. (i.e. normal temperature) by the first chiller110. In this way, by maintaining the surface temperature T7of the seventh roll61brought in contact with the first positive electrode mixture layer18bat the temperature in the range from 15° C. to 35° C., it is possible to suppress softening of the binder (PVdF) contained in the first positive electrode mixture layer18bat the time of roll pressing so that the compressibility of the first positive electrode mixture layer18bby the roll pressing process (step S6) can be made lower than the compressibility of the second positive electrode mixture layer18cby the roll pressing process (step S6). Consequently, in the positive electrode sheet19after performing the roll pressing process (step S6), the density difference between the first positive electrode mixture layer18band the second positive electrode mixture layer18ccan be reduced compared to the positive electrode sheet19before performing the roll pressing process.

The positive electrode sheet19produced as described above can be used, for example, as a positive electrode sheet of a lithium-ion secondary battery. Specifically, for example, the positive electrode sheet19is combined with a negative electrode sheet and a separator and forms an electrode body. Then, terminal members are attached to the electrode body, and then the electrode body and an electrolyte solution are housed in a battery case. In this way, the lithium-ion secondary battery is completed.

EXAMPLES 1 TO 7 AND COMPARATIVE EXAMPLE 1

In Examples 1 to 7 and Comparative Example 1, the positive electrode sheets19were produced by changing the surface temperature T7of the seventh roll61and the surface temperature T8of the eighth roll62at step S6(roll pressing process). The other configurations were the same as each other.

Specifically, in Example 1, at step S6(roll pressing process), the first positive electrode mixture layer18band the second positive electrode mixture layer18cof the positive electrode sheet19were compressed while maintaining the surface temperature T7of the seventh roll61at 25° C. and maintaining the surface temperature T8of the eighth roll62at 100° C. In Example 2, at step S6(roll pressing process), the first positive electrode mixture layer18band the second positive electrode mixture layer18cof the positive electrode sheet19were compressed while maintaining the surface temperature T7of the seventh roll61at 25° C. and maintaining the surface temperature T8of the eighth roll62at 150° C.

In Example 3, at step S6(roll pressing process), the first positive electrode mixture layer18band the second positive electrode mixture layer18cof the positive electrode sheet19were compressed while maintaining the surface temperature T7of the seventh roll61at 25° C. and maintaining the surface temperature T8of the eighth roll62at 200° C. In Example 4, at step S6(roll pressing process), the first positive electrode mixture layer18band the second positive electrode mixture layer18cof the positive electrode sheet19were compressed while maintaining the surface temperature T7of the seventh roll61at 35° C. and maintaining the surface temperature T8of the eighth roll62at 200° C.

In Example 5, at step S6(roll pressing process), the first positive electrode mixture layer18band the second positive electrode mixture layer18cof the positive electrode sheet19were compressed while maintaining the surface temperature T7of the seventh roll61at 15° C. and maintaining the surface temperature T8of the eighth roll62at 200° C. In Example 6, at step S6(roll pressing process), the first positive electrode mixture layer18band the second positive electrode mixture layer18cof the positive electrode sheet19were compressed while maintaining the surface temperature T7of the seventh roll61at 35° C. and maintaining the surface temperature T8of the eighth roll62at 100° C.

In Example 7, at step S6(roll pressing process), the first positive electrode mixture layer18band the second positive electrode mixture layer18cof the positive electrode sheet19were compressed while maintaining the surface temperature T7of the seventh roll61at 15° C. and maintaining the surface temperature T8of the eighth roll62at 100° C. In Comparative Example 1, at step S6(roll pressing process), the first positive electrode mixture layer18band the second positive electrode mixture layer18cof the positive electrode sheet19were compressed while maintaining the surface temperature T7of the seventh roll61at 25° C. and maintaining the surface temperature T8of the eighth roll62also at 25° C.

Thereafter, the density difference (g/cc) between the first positive electrode mixture layer18band the second positive electrode mixture layer18cwas examined for the positive electrode sheet19of each of Examples 1 to 7 and Comparative Example 1. The results are shown in Table 1. In each of Examples 1 to 7 and Comparative Example 1, in the positive electrode sheet19before performing the processing of step S6(roll pressing process), the density of the first positive electrode mixture layer18bwas higher than the density of the second positive electrode mixture layer18c, and the density difference (g/cc) between the first positive electrode mixture layer18band the second positive electrode mixture layer18cwas 0.85 (g/cc). In each of Examples 1 to 7 and Comparative Example 1, the processing of step S6(roll pressing process) was performed using the seventh roll61and the eighth roll62having a diameter of 390 mm.

As shown in Table 1, in the positive electrode sheet of Comparative Example 1, the density difference between the first positive electrode mixture layer18band the second positive electrode mixture layer18cwas 0.7 (g/cc). On the other hand, in the positive electrode sheet19of each of Examples 1 to 7, the density difference between the first positive electrode mixture layer18band the second positive electrode mixture layer18cwas equal to or less than 0.09 (g/cc).

As described above, in Comparative Example 1, the processing of step S6(roll pressing process) was performed by setting the surface temperature T7of the seventh roll61and the surface temperature T8of the eighth roll62to the same temperature (specifically, 25° C.). On the other hand, in each of Examples 1 to 7, the processing of step S6(roll pressing process) was performed by setting the surface temperature T8of the eighth roll62to be higher than the surface temperature T7of the seventh roll61(satisfying the relationship of T7<T8).

From these results, it can be said that when the processing of step S6(roll pressing process) is performed by setting the surface temperature T8of the eighth roll62to be higher than the surface temperature T7of the seventh roll61(satisfying the relationship of T7<T8), it is possible to reduce the density difference between the first positive electrode mixture layer18band the second positive electrode mixture layer18ccompared to the case where T7=T8. The reason can be considered as follows.

Specifically, by setting the surface temperature T8of the eighth roll62brought in contact with the second positive electrode mixture layer18chaving the relatively low density to be higher than the surface temperature T7of the seventh roll61brought in contact with the first positive electrode mixture layer18bhaving the relatively high density, the compression can be performed while the temperature of the second positive electrode mixture layer18cis made higher than the temperature of the first positive electrode mixture layer18b. Consequently, in the roll pressing process (step S6), the compression can be performed while the second positive electrode mixture layer18cis made softer than the first positive electrode mixture layer18b, and therefore, the second positive electrode mixture layer18cis compressed more easily than the first positive electrode mixture layer18b.

In particular, in each of Examples 1 to 7, the surface temperature T7of the seventh roll61was maintained at the temperature lower than (Ts-50)=100° C. being the temperature 50° C. lower than the softening temperature Ts (=150° C.) of the binder (PVdF) contained in the positive electrode mixture6. On the other hand, the surface temperature T8of the eighth roll62was maintained at the temperature equal to or higher than (Ts-50)=100° C. That is, the roll pressing process was performed in the state where the surface temperature T7of the seventh roll61brought in contact with the first positive electrode mixture layer18bhaving the relatively high density was maintained at the first temperature lower than (Ts-50)° C., and the surface temperature T8of the eighth roll62brought in contact with the second positive electrode mixture layer18chaving the relatively low density was maintained at the second temperature equal to or higher than (Ts-50)° C.

In this way, by maintaining the surface temperature T8of the eighth roll62brought in contact with the second positive electrode mixture layer18cat the second temperature equal to or higher than (Ts-50)° C., it is possible to soften the binder contained in the second positive electrode mixture layer18cso that the compressibility of the second positive electrode mixture layer18ccan be increased. On the other hand, by maintaining the surface temperature T7of the seventh roll61brought in contact with the first positive electrode mixture layer18bat the first temperature lower than (Ts-50)° C., it is possible to suppress softening of the binder contained in the first positive electrode mixture layer18bso that the compressibility of the first positive electrode mixture layer18bby the roll pressing process can be made lower than the compressibility of the second positive electrode mixture layer18cby the roll pressing process.

More specifically, in each of Examples 1 to 7, the surface temperature T7of the seventh roll61was maintained at the temperature in the range from 15° C. to 35° C. (i.e. normal temperature). In this way, by maintaining the surface temperature T7of the seventh roll61brought in contact with the first positive electrode mixture layer18bat the temperature in the range from 15° C. to 35° C., it is possible to suppress softening of the binder (PVdF) contained in the first positive electrode mixture layer18bat the time of roll pressing so that the compressibility of the first positive electrode mixture layer18bby the roll pressing process (step S6) can be made sufficiently lower than the compressibility of the second positive electrode mixture layer18cby the roll pressing process (step S6). In other words, the compressibility of the second positive electrode mixture layer18cby the roll pressing process (step S6) can be made sufficiently higher than the compressibility of the first positive electrode mixture layer18bby the roll pressing process (step S6). Accordingly, it can be concluded that, in each of Examples 1 to 7, the density difference between the first positive electrode mixture layer18band the second positive electrode mixture layer18cin the positive electrode sheet19was made extremely small by performing the roll pressing process (step S6).

EXAMPLES 8 TO 10 AND COMPARATIVE EXAMPLE 2

In Examples 8 to 10 and Comparative Example 2, the positive electrode sheets19were produced by changing the surface temperature T7of the seventh roll61and the surface temperature T8of the eighth roll62at step S6(roll pressing process). The other configurations were the same as each other. Differently from Examples 1 to 7 and Comparative Example 1, in each of Examples 8 to 10 and Comparative Example 2, the processing of step S6(roll pressing process) was performed using the seventh roll61and the eighth roll62having a diameter of 150 mm.

Specifically, in Example 8, at step S6(roll pressing process), the first positive electrode mixture layer18band the second positive electrode mixture layer18cof the positive electrode sheet19were compressed while maintaining the surface temperature T7of the seventh roll61at 25° C. and maintaining the surface temperature T8of the eighth roll62at 100° C. In Example 9, at step S6(roll pressing process), the first positive electrode mixture layer18band the second positive electrode mixture layer18cof the positive electrode sheet19were compressed while maintaining the surface temperature T7of the seventh roll61at 25° C. and maintaining the surface temperature T8of the eighth roll62at 150° C.

In Example 10, at step S6(roll pressing process), the first positive electrode mixture layer18band the second positive electrode mixture layer18cof the positive electrode sheet19were compressed while maintaining the surface temperature T7of the seventh roll61at 25° C. and maintaining the surface temperature T8of the eighth roll62at 200° C. In Comparative Example 2, at step S6(roll pressing process), the first positive electrode mixture layer18band the second positive electrode mixture layer18cof the positive electrode sheet19were compressed while maintaining the surface temperature T7of the seventh roll61at 25° C. and maintaining the surface temperature T8of the eighth roll62also at 25° C.

Thereafter, the density difference (g/cc) between the first positive electrode mixture layer18band the second positive electrode mixture layer18cwas examined for the positive electrode sheet19of each of Examples 8 to 10 and Comparative Example 2. The results are shown in Table 2. In each of Examples 8 to 10 and Comparative Example 2, in the positive electrode sheet19before performing the processing of step S6(roll pressing process), the density of the first positive electrode mixture layer18bwas higher than the density of the second positive electrode mixture layer18c, and the density difference (g/cc) between the first positive electrode mixture layer18band the second positive electrode mixture layer18cwas 0.85 (g/cc).

As shown in Table 2, in the positive electrode sheet of Comparative Example 2, the density difference between the first positive electrode mixture layer18band the second positive electrode mixture layer18cwas 0.68 (g/cc). On the other hand, in the positive electrode sheet19of each of Examples 8 to 10, the density difference between the first positive electrode mixture layer18band the second positive electrode mixture layer18cwas equal to or less than 0.65 (g/cc) and thus was smaller than that of Comparative Example 2.

As described above, in Comparative Example 2, the processing of step S6(roll pressing process) was performed by setting the surface temperature T7of the seventh roll61and the surface temperature T8of the eighth roll62to the same temperature (specifically, 25° C.). On the other hand, in each of Examples 8 to 10, the processing of step S6(roll pressing process) was performed by setting the surface temperature T8of the eighth roll62to be higher than the surface temperature T7of the seventh roll61(satisfying the relationship of T7<T8).

Also from these results, it can be said that when the processing of step S6(roll pressing process) is performed by setting the surface temperature T8of the eighth roll62to be higher than the surface temperature T7of the seventh roll61(satisfying the relationship of T7<T8), it is possible to reduce the density difference between the first positive electrode mixture layer18band the second positive electrode mixture layer18ccompared to the case where T7=T8. Further, it can be said that the surface temperature T7of the seventh roll61is preferably set to be lower than (Ts-50)° C. being the temperature 50° C. lower than the softening temperature Ts of the binder (PVdF) contained in the positive electrode mixture6, while the surface temperature T8of the eighth roll62is preferably set to be equal to or higher than (Ts-50)° C. More specifically, it can be said that the surface temperature T7of the seventh roll61is preferably set to the temperature in the range from 15° C. to 35° C.

EXAMPLES 11 TO 13 AND COMPARATIVE EXAMPLE 3

In Examples 11 to 13 and Comparative Example 3, the positive electrode sheets19were produced by changing the surface temperature T7of the seventh roll61and the surface temperature T8of the eighth roll62at step S6(roll pressing process). The other configurations were the same as each other. Differently from Examples 1 to 7 and Comparative Example 1, in each of Examples 11 to 13 and Comparative Example 3, the processing of step S6(roll pressing process) was performed using the seventh roll61and the eighth roll62having a diameter of 300 mm.

Specifically, in Example 11, at step S6(roll pressing process), the first positive electrode mixture layer18band the second positive electrode mixture layer18cof the positive electrode sheet19were compressed while maintaining the surface temperature T7of the seventh roll61at 25° C. and maintaining the surface temperature T8of the eighth roll62at 100° C. In Example 12, at step S6(roll pressing process), the first positive electrode mixture layer18band the second positive electrode mixture layer18cof the positive electrode sheet19were compressed while maintaining the surface temperature T7of the seventh roll61at 25° C. and maintaining the surface temperature T8of the eighth roll62at 150° C.

In Example 13, at step S6(roll pressing process), the first positive electrode mixture layer18band the second positive electrode mixture layer18cof the positive electrode sheet19were compressed while maintaining the surface temperature T7of the seventh roll61at 25° C. and maintaining the surface temperature T8of the eighth roll62at 200° C. In Comparative Example3, at step S6(roll pressing process), the first positive electrode mixture layer18band the second positive electrode mixture layer18cof the positive electrode sheet19were compressed while maintaining the surface temperature T7of the seventh roll61at 25° C. and maintaining the surface temperature T8of the eighth roll62also at 25° C.

Thereafter, the density difference (g/cc) between the first positive electrode mixture layer18band the second positive electrode mixture layer18cwas examined for the positive electrode sheet19of each of Examples 11 to 13 and Comparative Example 3. The results are shown in Table 3. In each of Examples 11 to 13 and Comparative Example 3, in the positive electrode sheet19before performing the processing of step S6(roll pressing process), the density of the first positive electrode mixture layer18bwas higher than the density of the second positive electrode mixture layer18c, and the density difference (g/cc) between the first positive electrode mixture layer18band the second positive electrode mixture layer18cwas 0.85 (g/cc).

As shown in Table 3, in the positive electrode sheet of Comparative Example 3, the density difference between the first positive electrode mixture layer18band the second positive electrode mixture layer18cwas 0.68 (g/cc). On the other hand, in the positive electrode sheet19of each of Examples 11 to 13, the density difference between the first positive electrode mixture layer18band the second positive electrode mixture layer18cwas equal to or less than 0.1 (g/cc) and thus was extremely small compared to that of Comparative Example 3.

As described above, in Comparative Example 3, the processing of step S6(roll pressing process) was performed by setting the surface temperature T7of the seventh roll61and the surface temperature T8of the eighth roll62to the same temperature (specifically, 25° C.). On the other hand, in each of Examples 11 to 13, the processing of step S6(roll pressing process) was performed by setting the surface temperature T8of the eighth roll62to be higher than the surface temperature T7of the seventh roll61(satisfying the relationship of T7<T8).

Also from these results, it can be said that when the processing of step S6(roll pressing process) is performed by setting the surface temperature T8of the eighth roll62to be higher than the surface temperature T7of the seventh roll61(satisfying the relationship of T7<T8), it is possible to reduce the density difference between the first positive electrode mixture layer18band the second positive electrode mixture layer18ccompared to the case where T7<T8. Further, it can be said that the surface temperature T7of the seventh roll61is preferably set to be lower than (Ts-50)° C. being the temperature 50° C. lower than the softening temperature Ts of the binder (PVdF) contained in the positive electrode mixture6, while the surface temperature T8of the eighth roll62is preferably set to be equal to or higher than (Ts-50)° C. More specifically, it can be said that the surface temperature T7of the seventh roll61is preferably set to the temperature in the range from 15° C. to 35° C.

Study of Diameter of Seventh Roll61and Eighth Roll62

Examples 1, 8, and 11 were the same in that step S6(roll pressing process) was performed by setting the surface temperature T7of the seventh roll61to 25° C. and setting the surface temperature T8of the eighth roll62to 100° C. (see Tables 1 to 3). However, the diameters (outer diameters) of the seventh rolls61and the eighth rolls62were different from each other. Specifically, the diameter of the seventh roll61and the eighth roll62was set to 390 mm in Example 1, was set to 150 mm in Example 8, and was set to 300 mm in Example 11.

As shown in Table 2, in Example 8 in which step S6(roll pressing process) was performed using the seventh roll61and the eighth roll62having the diameter of 150 mm, the density difference between the first positive electrode mixture layer18band the second positive electrode mixture layer18cwas 0.65 g/cc.

On the other hand, as shown in Table 1, in Example 1 in which step S6(roll pressing process) was performed using the seventh roll61and the eighth roll62having the diameter of 390 mm, it was possible to reduce the density difference between the first positive electrode mixture layer18band the second positive electrode mixture layer18cto equal to or less than 0.1 g/cc (specifically, to 0.08 g/cc). Likewise, as shown in Table 3, also in Example 11 in which step S6(roll pressing process) was performed using the seventh roll61and the eighth roll62having the diameter of 300 mm, it was possible to reduce the density difference between the first positive electrode mixture layer18band the second positive electrode mixture layer18cto equal to or less than 0.1 g/cc (specifically, to 0.09 g/cc). In this way, in Examples 1 and 11, it was possible to largely reduce the density difference between the first positive electrode mixture layer18band the second positive electrode mixture layer18ccompared to Example 8.

From these results, it can be said that the diameter of the seventh roll61and the eighth roll62is preferably set to 300 mm or more. More specifically, it can be said that, by performing the roll pressing process (step S6) such that the surface temperature T7of the seventh roll61and the surface temperature T8of the eighth roll62, having the diameter of 300 mm or more, satisfy the relationship of T7<T8, the compressibility of the second positive electrode mixture layer18cby this roll pressing can be effectively increased. It can be said that this makes it possible to further reduce the density difference between the first positive electrode mixture layer18band the second positive electrode mixture layer18c. This is also supported by comparison of the density differences in Examples 2, 9, and 12 and comparison of the density differences in Examples 3, 10, and 13.

While the disclosure has been described with reference to the embodiment (Examples 1 to 13), it goes without saying that the disclosure is not limited to the above-described embodiment and is applicable with appropriate changes within a range not departing from the gist of the disclosure. For example, in the embodiment, the method of manufacturing the positive electrode sheet has been described by way of example as a method of manufacturing an electrode sheet according to the disclosure. However, the disclosure may also be applied to a method of manufacturing a negative electrode sheet.

In the embodiment, the first drying process (step S3) for drying the first positive electrode mixture layer18bformed on the first surface7bof the collector foil7is provided after the first film forming process (step S2) and before the second film forming process (step S4). Further, the second drying process (step S5) for drying the second positive electrode mixture layer18cformed on the second surface7cof the collector foil7is provided after the second film forming process (step S4) and before the roll pressing process (step S6). However, it may be configured that the roll pressing process (step S6) is performed without performing the first drying process (step S3) and the second drying process (step S5).

Even in such a case, when the processing of step S6(roll pressing process) is performed by setting the surface temperature T8of the eighth roll62to be higher than the surface temperature T7of the seventh roll61(satisfying the relationship of T7<T8), it is possible to reduce the density difference between the first positive electrode mixture layer18band the second positive electrode mixture layer18ccompared to the case where T7<T8. Further, it can be said that the surface temperature T7of the seventh roll61is preferably set to be lower than (Ts-50)° C. being the temperature 50° C. lower than the softening temperature Ts of the binder (PVdF) contained in the positive electrode mixture6, while the surface temperature T8of the eighth roll62is preferably set to be equal to or higher than (Ts-50)° C. More specifically, it can be said that the surface temperature T7of the seventh roll61is preferably set to the temperature in the range from 15° C. to 35° C.