JP7350802B2 - Method for manufacturing electrodes for secondary batteries - Google Patents

Description

The present invention relates to a method for manufacturing an electrode for a secondary battery.

Electrodes used in secondary batteries such as lithium ion secondary batteries include a conductive current collector (current collector foil) and a composite material layer containing an active material etc. held on the current collector. ing. A lithium ion secondary battery is a nonaqueous secondary battery that can be charged and discharged by moving lithium ions in an electrolyte between a positive electrode and a negative electrode that insert and release lithium ions.

For the purpose of increasing the capacity and output of such secondary batteries, fine processing may be applied to the composite material layer. For example, a technique is known in which the utilization efficiency of the electrolyte solution is improved by providing micropores extending in the thickness direction in the composite material layer.

Patent Document 1 discloses that a current collector has a mixture layer containing an active material on at least one side, and the mixture layer has a thickness of 70 to 400 μm and contains a resin that dissolves in a non-aqueous electrolyte. In the thickness direction of the mixture layer, a large number of micropores having openings with a pore diameter of 0.3 to 150 μm are formed on the surface opposite to the current collector. An electrode for a non-aqueous secondary battery and a non-aqueous secondary battery having the electrode are disclosed.

Japanese Patent Application Publication No. 2012-190625

The non-aqueous secondary battery described in Patent Document 1 is characterized by a large capacity during high-output discharge and excellent load characteristics even if the electrode mixture layer (mixture material layer) is thick. However, in the technique described in Patent Document 1, fine pores are formed by pressing a coating film in which an electrode material and a solvent are mixed after drying. In the composite material layer of the electrode manufactured in this way, the active material in the processed portion where the micropores are formed may be damaged due to pressing. In this case, when the secondary battery is repeatedly charged and discharged during use, there is a problem in that substances derived from charge carriers are precipitated, resulting in a decrease in battery capacity.

The present invention was made to solve such problems, and an object of the present invention is to provide a method for manufacturing an electrode for a secondary battery with a high capacity retention rate.

A method for manufacturing an electrode for a secondary battery according to an embodiment includes rolling clay-like granules obtained by mixing at least an active material and a solvent through a pair of rolls, and rolling the rolled granules. forming a granule film on the surface of the current collector foil by transferring the granule to the surface of the current collector foil, and forming holes in the granule film extending in the thickness direction from the surface of the granule film. forming a pore-forming film by applying a liquid slurry obtained by mixing at least an active material and a solvent onto the surface of the pore-forming film; The method includes a step of drying the formed film and the slurry film to form a composite material layer.

According to the present invention, it is possible to provide a method for manufacturing an electrode for a secondary battery with a high capacity retention rate.

1 is a flowchart showing a method for manufacturing an electrode for a secondary battery according to Embodiment 1. FIG. 2 is a partial cross-sectional view showing step S1 of the flowchart shown in FIG. 1. FIG. 2 is a partial cross-sectional view showing step S2 of the flowchart shown in FIG. 1. FIG. 2 is a partial cross-sectional view showing step S3 of the flowchart shown in FIG. 1. FIG. 2 is a partial cross-sectional view showing step S4 of the flowchart shown in FIG. 1. FIG. It is a table showing the manufacturing conditions and characteristic evaluation results of the electrode.

Embodiment 1
Embodiments of the present invention will be described below with reference to the drawings. However, the present invention is not limited to the following embodiments. Further, in order to clarify the explanation, the following description and drawings are simplified as appropriate. What is shown in the figure is only a part of the whole, and many other components not shown are actually included. Furthermore, in the following description, the same or equivalent elements are given the same reference numerals, and redundant description will be omitted.

Hereinafter, as one of the preferred embodiments of the method for manufacturing an electrode for a secondary battery according to the present embodiment, a method for manufacturing an electrode for a lithium ion secondary battery will be specifically described. Lithium-ion secondary batteries are secondary batteries that use lithium ions as charge carriers, and charge and discharge are achieved by the transfer of charge between the positive electrode (positive electrode plate) and the negative electrode (negative electrode plate). It's a battery.

First, an outline of the electrode plate 100 manufactured by the method for manufacturing an electrode for a secondary battery according to this embodiment will be explained. The electrode plate 100 includes a current collector foil 11 and a composite material layer 50 formed on the surface of the current collector foil 11 (see FIG. 5).

The current collector foil 11 is formed into a plate shape or a foil shape, and is made of a metal with good conductivity. In the case of a positive electrode plate, examples of the metal constituting the current collector foil 11 include aluminum, aluminum alloy, nickel, titanium, stainless steel, and the like. In the case of a negative electrode plate, examples of the metal constituting the current collector foil 11 include copper, copper alloy, nickel, titanium, stainless steel, and the like.

The composite material layer 50 is formed on at least one surface of the current collector foil 11 except for the edge along one edge in the width direction. Further, the electrode plate 100 has an exposed portion at the edge of the current collector foil 11 where the composite material layer 50 is not formed and the current collector foil 11 is exposed. The composite material layer 50 includes at least an active material and is held by the current collector foil 11. In addition to the active material, the composite material layer 50 may contain additives such as a conductive material, a binder, and a thickener, if necessary.

(active material)
The active material used in the positive electrode plate is a material capable of inserting and releasing lithium ions, and any active material that can be used in a lithium ion secondary battery can be used without particular limitation. Examples of active materials include lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), lithium manganate (LiMn ₂ O ₄ ), lithium iron phosphate (LiFePO ₄ ), lithium nickel cobalt aluminate (NCA), Nickel cobalt lithium manganate (NCM) and the like can be used alone or in combination. Further, other metal elements may be added to the active material.

The active material used in the negative electrode plate is a material that can absorb and release lithium ions, and for example, carbon materials such as natural graphite, artificial graphite, hard carbon, soft carbon, and activated carbon can be used.

The primary particles of the active material are particulate. The average particle diameter (D50) of the active material (primary particles) is, for example, 0.1 μm to 50 μm, and preferably 1 μm to 20 μm from the viewpoint of fluidity during granulation. The average particle diameter (D50) of the active material can be determined, for example, from the results of particle size distribution measurement using a laser diffraction scattering method.

(conductive material)
The conductive material has a function of transmitting electricity generated in the active material to the current collecting foil 11. As the conductive material, for example, carbon materials such as various carbon blacks (eg, acetylene black), activated carbon, graphite, and carbon fibers can be used.

(binder)
The binder binds the electrode materials constituting the composite material layer 50 to each other to form the composite material layer 50, and binds the formed composite material layer 50 onto the surface of the current collector foil 11. . The binder having such a function is not particularly limited as long as it is a material that can be dissolved or dispersed in the solvent described below. As the binder, for example, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), polyethylene oxide (PEO), styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), etc. can be used.

(Mixture layer)
The proportion of the active material in the entire composite material layer 50 is, for example, 70% by mass to 99% by mass, and preferably 85% by mass to 95% by mass, from the viewpoint of realizing high energy density. Further, the proportion of the conductive material in the entire composite material layer 50 is, for example, 0.5% by mass to 20% by mass, preferably 2% by mass, from the viewpoint of achieving both output characteristics and energy density at a high level. % to 10% by mass.

The proportion of the binder in the entire composite material layer 50 is, for example, 0.5% by mass to 10% by mass, preferably 1% by mass to 5% by mass, from the viewpoint of peel strength and film formability. If the proportion of the binder is too small, the amount of binder near the current collector foil 11 may be insufficient, and the peel strength between the composite material layer 50 and the current collector foil 11 may tend to decrease. In addition, the film formability of the composite material layer 50 is reduced, and the formation of scratches and streaks may cause lithium to easily precipitate during charging and discharging, leading to a decrease in battery performance. On the other hand, if the proportion of the binder is too large, the binder covers the surface of the positive electrode active material, thereby hindering the movement of lithium ions, which may tend to increase the internal resistance of the battery.

The composite material layer 50 has a large number of micropores 51 formed so as to extend from the surface of the composite material layer 50 in the thickness direction. An example of the shape of the micropores 51 is a conical shape that tapers from the surface of the composite material layer 50 toward the current collector foil 11, but is not limited thereto.

It is preferable that at least some of the micropores 51 have a depth of 50% or more of the thickness of the composite material layer 50. The ratio of the total area of the openings of the micropores 51 to the surface area of the composite material layer 50 is preferably, for example, 1% to 10%. The distance between the centers of adjacent micropores 51 is preferably 0.5 to 500 μm. In the electrode plate 100 provided with the micropores 51, the electrolyte penetrates deep into the electrode plate 100, which increases the chances of contact between the active material and the lithium ions as charge carriers, increasing the efficiency of electrolyte use. improves. This has the effect of suppressing the increase in diffusion resistance and improving output performance.

Further, the composite material layer 50 includes a first composite material layer 24 formed on the surface of the current collector foil 11 and a second composite material layer 42 formed on the surface of the first composite material layer 24. , has. In this embodiment, the first composite material layer 24 is formed using clay-like granules having a relatively high solid content. Further, the second composite material layer 42 is formed using a liquid slurry having a relatively low solid content. Both the granules and the slurry are compositions for forming a composite material layer for forming the composite material layer 50.

The thickness of the composite material layer 50 is, for example, 70 to 400 μm. The density of the composite material layer 50 is, for example, 0.9 g/cm ³ to 1.5 g/cm ³ . Of the composite material layers 50, the first composite material layer 24 has a density of, for example, 1.0 g/cm ³ to 1.8 g/cm ³ . Further, among the composite material layers 50, the density of the second composite material layer 42 is, for example, 0.8 g/cm ³ to 1.2 g/cm ³ . The density of the second mixture layer 42 is preferably lower than the density of the first mixture layer 24.

Next, a method for manufacturing an electrode for a secondary battery according to this embodiment will be described with reference to FIG. FIG. 1 is a flowchart showing a method for manufacturing an electrode for a secondary battery according to the first embodiment. The method for manufacturing an electrode for a secondary battery according to this embodiment is applicable to both the positive electrode plate and the negative electrode plate 100.

As shown in FIG. 1, the method for manufacturing an electrode for a secondary battery according to the present embodiment includes steps S1 to S4. In step S1, clay-like granules obtained by mixing at least an active material and a solvent are rolled through a pair of rolls, and the rolled granules are transferred to the surface of the current collector foil 11 and collected. A granule film 21 is formed on the surface of the electric foil 11. In step S2, holes 22 extending in the thickness direction from the surface of the granule film 21 are provided in the granule film 21 to form a hole forming film 23. In step S3, a liquid slurry obtained by mixing at least an active material and a solvent is applied onto the surface of the hole-forming film 23 to form a slurry film 41. In step S4, the hole forming film 23 and the slurry film 41 are dried to form the composite material layer 50.

Each of the above steps will be explained in detail with reference to FIGS. 2 to 5. FIG. 2 is a partial cross-sectional view showing step S1 of the flowchart shown in FIG. FIG. 3 is a partial cross-sectional view showing step S2 of the flowchart shown in FIG. FIG. 4 is a partial cross-sectional view showing step S3 of the flowchart shown in FIG. FIG. 5 is a partial cross-sectional view showing step S4 of the flowchart shown in FIG. Note that FIGS. 2 to 5 show a part of a cross section taken along a direction perpendicular to the surface of the current collector foil 11.

As shown in FIG. 1, in step S1, a granule film 21 is formed on the surface of the current collector foil 11. As shown in FIG. In this embodiment, a coating method is employed in which a wet granule is passed between a pair of rolls and rolled, and the rolled granule is transferred onto the surface of a base material (current collector foil 11 in this case) to form a film. The granule film 21 is formed using the method. The granules contain an electrode material and a solvent for forming the composite material layer 50 (first composite material layer 24), and have a clay-like form with a relatively high solid content and viscosity.

The granules that serve as the material for the granule film 21 are made by adding a small amount of solvent to a powder containing an active material and other solid components such as a conductive material, a binder, and a thickener as necessary. It is prepared by keeping it in a wet state and granulating it. For example, a stirring granulation device such as a planetary mixer can be used to prepare the granules. The thus obtained granules are spherical secondary particles in which the solvent wets between particles of the conductive material and the active material, and the particles are bonded to each other.

For coating the granules, a roll coater having three rolls, for example, can be used as a coating device. The three rolls are a first roll, a second roll, and a third roll. A predetermined gap is formed between a pair of horizontally adjacent first rolls and second rolls. The outer circumferential surface of the first roll and the outer circumferential surface of the second roll are opposed to each other. The first roll and the second roll rotate in opposite directions. A third roll that conveys the current collector foil 11 is arranged next to the second roll. The outer circumferential surface of the second roll and the outer circumferential surface of the third roll are opposed to each other. The second roll and the third roll rotate in opposite directions. Note that the third roll may be placed at any position that allows the molded body to be transferred to the surface of the current collector foil 11.

Further, partition walls are provided at both ends of the first roll and the second roll in the width direction. The partition walls hold the granule powder on the first roll and the second roll, and the distance between the partition walls defines the width of the granule film 21 formed on the surface of the current collector foil 11. Has a function.

When applying the granules onto the surface of the current collector foil 11, first, the granules are supplied between the first roll and the second roll. In this state, when the first roll, second roll, and third roll are rotated in opposite directions, the granules are sent to the gap and compressed between the first roll and the second roll, forming a sheet. is formed into. That is, the granules are rolled by the first roll and the second roll, thereby forming a sheet-like molded body (rolled granules).

At this time, if the circumferential speed of the second roll is made larger than the circumferential speed of the first roll, and the circumferential speed of the third roll is made larger than the circumferential speed of the second roll, the molded body remains attached to the surface of the second roll and Up to 3 rolls are transported. The molded body conveyed to the third roll is continuously transferred onto the surface of the current collector foil 11 while being compressed between the second roll and the third roll. Thereby, the first laminate L1 in which the granule film 21 is formed on the surface of the current collector foil 11 can be formed. Note that the thickness of the molded body is preferably adjusted to an appropriate thickness so as not to cause scratches or cracks.

Here, the density of the first composite material layer 24 is the thickness of the molded body and can also be adjusted by the degree of compression of the granules. The degree of compression of the granules is adjusted by the gap between the first roll and the second roll. The smaller the gap, the thinner the molded body becomes, and the higher the density of the first composite material layer 24 can be formed. On the other hand, if the density of the composite material layer 50 as a whole is too high, the voids within the composite material layer 50 will decrease, making it difficult for the composite material layer 50 to penetrate and retain the electrolytic solution. If the amount of electrolyte retained in the composite material layer 50 decreases, the capacity retention rate may decrease, which is not preferable. It is preferable to adjust the coating amount of the granules so that the coating weight is, for example, 5.0 mg/cm ² to 20.0 mg/cm ² per one side of the current collector foil 11.

The granules have, for example, an average particle diameter (D50) of 100 μm to 4 mm, preferably 200 μm to 1 mm. The average particle diameter (D50) of the granules can be adjusted depending on the conditions at the time of granulation. Note that the average particle diameter (D50) of the granules can be determined, for example, from the results of particle size distribution measurement using a laser diffraction scattering method. In addition, the solid content of the granules obtained by removing the solvent from the entire granules is, for example, 60% to 95% by mass, preferably 70% to 80% by mass, from the viewpoint of film-forming properties. be. The solid content of the granules can be adjusted by adjusting the amounts of the active material, other solid components, and solvent.

As the solvent, non-aqueous solvents and aqueous solvents can be used without particular limitation as long as the binder can be uniformly dissolved or dispersed. The solvent is appropriately selected depending on the binder used. Examples of solvents include non-aqueous solvents such as N-methyl-2-pyrrolidone (NMP), methyl ethyl ketone (MEK), dimethyl formamide (DMF), toluene, mixed solvents of non-aqueous solvents, water, and water-based solvents. An aqueous solvent such as a mixed solvent can be used.

The ratio of the solvent to the entire granule is adjusted by adjusting the amount of solvent added so that the solid content of the granule falls within the above range. If the proportion of the solvent is too small or too large, film-forming properties will deteriorate. If the proportion of the solvent is too large, the drying time when drying the granule film 21 becomes long, resulting in a decrease in productivity.

Subsequently, as shown in FIG. 3, in step S2, holes 22 are formed in the granule film 21 formed in step S1. For example, a mold 30 having a rectangular base 31 and a large number of protrusions 32 protruding from the surface of the base 31 can be used to form the hole 22 . The mold 30 has a flat surface other than the protrusions 32 on the side where the protrusions 32 are provided. The shape of the convex portion 32 is appropriately designed according to the desired shape of the micropore 51, and has, for example, a conical shape that tapers toward the tip.

The mold 30 is arranged such that the tip of the convex portion 32 faces the surface of the first laminate L1 (the surface of the granule film 21). Then, the mold 30 is lowered toward the first laminate L1, and the convex portions 32 are inserted into the granule film 21. At this time, it is preferable to press the surface of the granule film 21 using the mold 30. Since the granules are clay-like with a high solid content, when the convex portions 32 of the mold 30 are inserted into the granule membrane 21, the granules membrane 21 is deformed. Further, even after the convex portion 32 of the mold 30 is removed from the granule membrane 21, the given deformation can be maintained. Therefore, the granule membrane 21 can be formed into a desired shape.

In this way, the granules (granule membrane 21) have better shape retention than liquid slurry, for example, because the solid content ratio is the above-mentioned value. Moreover, by adjusting the thickness of the granule film 21 along with the pressing by the mold 30, the first composite material layer 24 can be consolidated.

After pressing is performed until the first laminate L1 has a desired thickness, the mold 30 is raised to remove the convex portions 32 from the granule film 21. In this way, the second laminate L2 including the current collector foil 11 and the hole forming film 23 can be formed. The hole-forming film 23 is provided with holes 22 extending from the surface in the thickness direction.

The granules in a wet state have a certain degree of flexibility. Therefore, when pressing the wet granule film 21, for example, compared to pressing after drying the granule film 21, the stress acting on the electrode material is reduced and the electrode material is not damaged. Hard to accept.

Therefore, damage to the active material can be suppressed by pressing the granule film 21 made of wet granules for the purpose of processing the holes 22 and compacting them. As another method for recessing the holes 22 in the granule membrane 21, laser processing can be used, but the method using the mold 30 is more preferable since the holes 22 can be processed and pressed at the same time. preferable.

Subsequently, as shown in FIG. 4, in step S3, a slurry film 41 is formed using a slurry method. The slurry method is, for example, a coating method in which slurry is discharged from a die head of a die coater to coat the slurry onto the surface of the base material (here, the second laminate L2). The slurry is a liquid composition for forming a composite material layer that contains an electrode material and a solvent for forming the composite material layer 50 (second composite material layer 42) and has a relatively low solid content and viscosity.

The slurry that is the material for the slurry film 41 is produced by adding a solvent to powder containing an active material and other solid components such as a conductive material, a binder, and a thickener as necessary, and then kneading the powder. Prepare. The slurry may be prepared by adding a solvent to the granules granulated in step S1. For example, a kneader such as a planetary mixer can be used to prepare the slurry.

In addition to the die coater described above, a slit coater, comma coater, gravure coater, blade coater, etc. can be used for coating the slurry, but a die coater is preferably used because it can form a uniform coating film with high precision.

The die coater mainly has a die head that discharges slurry toward the opposing base material (here, the second laminate L2) via a predetermined gap and applies the slurry to the surface of the base material. The second laminate L2 as a base material is attached to a conveyance roller and conveyed by rotation of the conveyance roller. The second laminate L2 and the die head are arranged such that the hole forming film 23 of the second laminate L2 faces the discharge port of the die head.

When applying the slurry onto the surface of the second laminate L2 (on the surface of the hole forming film 23), the slurry is applied from the discharge port of the die head to the second laminate L2 transported by the transport roller. By discharging the slurry film 41, the slurry film 41 is coated on the surface of the hole forming film 23. It is preferable to adjust the coating amount of the slurry so that the coating weight is, for example, 2.0 mg/cm ² to 5.0 mg/cm ² per one side of the current collector foil 11.

Since the liquid slurry has fluidity, it can also enter the inside of the hole 22 . Therefore, the slurry film 41 is formed following the surface shape of the hole forming film 23. In this way, the third laminate L3 including the current collector foil 11, the hole forming film 23, and the slurry film 41 can be formed.

From the viewpoint of drying efficiency, the solid content of the slurry is, for example, preferably 40% by mass or more, and more preferably 50% by mass or more. By increasing the solid content of the slurry, the drying time when drying the slurry film 41 is shortened. From the viewpoint of productivity, the solid content of the slurry is preferably 70% by mass or less, more preferably 60% by mass or less.

Further, it is preferable that the viscosity of the slurry is as high as possible within a range that does not impair coating properties. With such a configuration, the slurry film 41 is held uniformly on the surface of the hole-forming film 23, and a reaction interface that contributes to the battery chemical reaction between the composite material layer 50 and the electrolyte solution is easily formed.

The viscosity of the slurry is, for example, preferably 3000 mPa·s or more, more preferably 5000 mPa·s or more. On the other hand, if the viscosity is too high, coating properties may be impaired and streaks or thickness unevenness may occur in the slurry film 41, which is not preferable. Therefore, the viscosity of the slurry is preferably 20,000 mPa·s or less, and more preferably 17,000 mPa·s or less, for example. The solid content and viscosity of the slurry can be adjusted by adjusting the amounts of the active material, other solid components, and solvent. The ratio of the solvent to the entire slurry is appropriately adjusted so that the solid content or viscosity of the slurry falls within the above range.

Subsequently, as shown in FIG. 5, in step S4, the third laminate L3 formed in step S3 is dried. A drying oven that generates hot air can be used to dry the third laminate L3. Note that the drying method is not limited to this.

The third laminate L3 is placed in a drying oven and subjected to hot air drying to remove the solvent contained in the hole forming film 23 and the slurry film 41, thereby drying them. The pore-forming film 23 is dried to form the first composite layer 24 of the composite material layer 50, and the slurry film 41 is dried to form the second composite layer 42 of the composite material layer 50. In this way, a composite material layer 50 in which a large number of micropores 51 are provided is formed.

By the above manufacturing method, it is possible to manufacture the electrode plate 100 having a configuration in which the composite material layer 50 is held on the surface of the current collector foil 11. Further, after drying, pressing may be performed using a flat plate press or the like to adjust the thickness of the entire electrode plate 100.

Examples and comparative examples will be described below with reference to FIG. 6. Note that the examples do not limit the present invention. FIG. 6 is a table showing electrode manufacturing conditions and characteristic evaluation results. Negative electrode plates of Examples 1 to 3 and Comparative Examples 1 to 6 were manufactured based on the manufacturing conditions shown in FIG. Then, evaluation lithium ion secondary batteries (evaluation batteries) were constructed using each of the manufactured negative electrode plates, and battery characteristics were evaluated (cost, lithium precipitation resistance).

In addition, in the following explanation, among the composite material layers obtained under the manufacturing conditions of each example, the one placed on the surface of the current collector foil will be referred to as the first layer, and the one placed on the surface of the first layer will be referred to as the first layer. will be described as the second layer. In addition, the operation of drying the first layer will be referred to as intermediate drying, the operation of pressing using a mold will be referred to as intermediate pressing, and the operation of pressing using a flat plate press will be referred to as final pressing.

[Negative electrode]
Natural graphite (C, average particle size 8 μm) was used as the negative electrode active material. CMC was used as the thickener. SBR was used as a binder. Ion-exchanged water was used as a solvent. Copper foil (thickness: 10 μm) was used as the current collector foil.

Weigh the negative electrode active material, thickener, and binder so that the mass ratio of the total solid content is C:CMC:SBR=98:1:1, and add the required amount of solvent to these electrode materials. The mixture was added and mixed to prepare granules and/or slurry. The total thickness of the negative electrode plate manufactured using the prepared granules and/or slurry was approximately 150 μm. Note that the granules and slurries used in the Examples and Comparative Examples described below have a negative electrode active material, a thickener, and a binder mixed in a predetermined ratio, so the solid content and viscosity are are both adjusted by the content of the solvent.

(Example 1)
A negative electrode plate 100 (electrode plate 100) according to Example 1 was manufactured according to the flow shown in FIG. In step S1, a negative electrode active material, a thickener, a binder, and a solvent are put into a stirring granulator and mixed and granulated to form granules with a solid content of 80% by mass. body was prepared. Next, a first layer is formed on the surface of the current collector foil 11 by transferring the rolled granules onto the surface of the current collector foil 11 using a roll coater equipped with three rolls as a coating device. A granule film 21 was formed to obtain a first laminate L1. Then, the first laminate L1 was cut into approximately 30 cm square pieces. The coating amount of the granules was adjusted so that the basis weight was 15 mg/cm ² on one side of the current collector foil 11 and 30 mg/cm ² on both sides.

Subsequently, in step S2, a conical hole 22 was formed in the cut out granule film 21 of the first laminate L1 using a mold 30. At this time, intermediate pressing was performed to adjust the thickness of the granule film 21 by pressing the surface of the granule film 21 using the mold 30. As a result, a second laminate L2 in which the first hole forming film 23 was formed on the surface of the current collector foil 11 was obtained. In the second laminate L2, a plurality of holes 22 were arranged in a grid pattern along the longitudinal direction and the width direction. The depth of the hole 22 was 50 μm, and the diameter of the opening of the hole 22 on the surface was 25 μm. Further, the distance between the centers of the holes 22 adjacent to each other in the same direction was 100 μm.

Subsequently, in step S3, the negative electrode active material, thickener, binder, and required amount of solvent are put into a kneader, and by kneading these, a material with a viscosity of 15000 mPa・s is obtained. Got slurry. The slurry was applied to the surface of the hole-forming film 23 using a die coater. Thereby, a second layer of slurry film 41 was formed on the surface of the hole-forming film 23, and a third laminate L3 was obtained. The coating amount of the slurry was adjusted so that the coating weight was 2 mg/cm ² on one side of the current collector foil 11 and 4 mg/cm ² on both sides.

Subsequently, in step S4, the third laminate L3 was placed in a drying oven and subjected to hot air drying (90° C., 5 minutes) for final drying to obtain the negative electrode composite material layer 50. In this way, a negative electrode plate 100 in which the negative electrode composite material layer 50 was held on both sides of the current collector foil 11 was manufactured.

The negative electrode plate 100 manufactured under the manufacturing conditions of Example 1 includes a negative electrode composite layer 50 having a first layer formed using granules and a second layer formed using slurry. This negative electrode composite material layer 50 is provided with micropores 51 . Further, the processing and pressing of the holes 22 are performed in a wet state before the applied composition for forming a composite material layer (granules) is dried.

(Example 2)
In step S2, the intermediate press was omitted. Further, in step S4, final pressing was performed. In Example 2, the negative electrode plate 100 was manufactured in the same manner as in Example 1 except for these points.

The negative electrode plate 100 manufactured under the manufacturing conditions of Example 2 includes a negative electrode composite layer 50 having a first layer formed using granules and a second layer formed using slurry. This negative electrode composite material layer 50 is provided with micropores 51 . Further, the processing of the holes 22 is performed in a wet state before drying the applied composition for forming a composite material layer (granules). On the other hand, pressing is performed after drying the applied composition for forming a composite material layer (granules and slurry).

(Example 3)
In step S2, the intermediate press was omitted. Further, in step S3, a second layer of slurry film 41 was formed using a slurry having a viscosity of 5000 mPa·s. Furthermore, in step S4, final pressing was performed. In Example 3, the negative electrode plate 100 was manufactured in the same manner as in Example 1 except for these points.

The negative electrode plate 100 manufactured under the manufacturing conditions of Example 3 includes a negative electrode composite layer 50 having a first layer formed using granules and a second layer formed using slurry. This negative electrode composite material layer 50 is provided with micropores 51 . Further, the processing of the holes 22 is performed in a wet state before drying the applied composition for forming a composite material layer (granules). On the other hand, pressing is performed after drying the applied composition for forming a composite material layer (granules and slurry). The second layer is formed using a low viscosity slurry.

(Comparative example 1)
The negative electrode active material, thickener, binder, and solvent were put into a kneader and kneaded to obtain a slurry with a viscosity of 3000 mPa·s. Next, a coating film was formed on the surface of the current collector foil by applying the slurry to the surface of the current collector foil using a die coater. After final drying by placing this in a drying oven and drying with hot air (80° C., 10 minutes), a final press was applied to produce a negative electrode plate cut into approximately 30 cm squares. The coating amount of the slurry was adjusted so that the coating weight was 17 mg/cm ² on one side of the current collector foil and 34 mg/cm ² on both sides.

The negative electrode plate manufactured under the manufacturing conditions of Comparative Example 1 includes a negative electrode composite material layer having only the first layer formed using slurry, and this negative electrode composite material layer is not provided with micropores. . Further, the pressing is performed after drying the applied composition for forming a composite material layer (slurry).

(Comparative example 2)
A slurry having a viscosity of 3000 mPa·s was obtained in the same manner as in Comparative Example 1. Next, a coating film was formed on the surface of the current collector foil by applying the slurry to the surface of the current collector foil using a die coater. The mold was lowered toward the coating film on the surface of the current collector foil, the protrusions of the mold were inserted into the coating film, and then the protrusions of the mold were removed. However, it was not possible to confirm that holes were formed in the coating film. Note that the basis weight of the slurry is the same as that of Comparative Example 1.

As shown in Comparative Example 2, since the liquid slurry contains a large amount of solvent and has a low solid content, it is difficult to process the holes. For example, even if the coating film made of slurry is deformed by inserting the protrusions of a mold into it, the coating film cannot maintain its deformation after the protrusions of the mold are removed. Therefore, the slurry is considered to have low shape retention. Therefore, under the manufacturing conditions of Comparative Example 2, it was not possible to manufacture a negative electrode plate.

(Comparative example 3)
A slurry having a viscosity of 3000 mPa·s was obtained in the same manner as in Comparative Example 1. Next, a first layer coating film was formed on the surface of the current collector foil by applying the slurry to the surface of the current collector foil using a die coater. This was placed in a drying oven and dried with hot air (80° C., 10 minutes) to perform intermediate drying to obtain a dried slurry.

After forming holes using a mold in the dry coating film of the dried slurry body cut into approximately 30 cm squares, the intermediate press was omitted, and the second layer of slurry was prepared in the same manner as in Example 1. A film was formed to obtain a laminate. Then, a negative electrode plate was manufactured by subjecting the laminate to final pressing. The coating weight of the first layer of the slurry was adjusted so that it was 15 mg/cm ² on one side of the current collector foil and 30 mg/cm ² on both sides, and the coating weight of the second layer was as follows. It is the same as 1.

The negative electrode plate manufactured under the manufacturing conditions of Comparative Example 3 includes a negative electrode composite layer having a first layer and a second layer both formed using slurry, and this negative electrode composite layer has micropores. It is provided. However, pressing is performed after drying the applied composition for forming a composite material layer (slurry).

(Comparative example 4)
A negative electrode plate was manufactured in the same manner as Comparative Example 3 except that the processing of the holes was omitted. The negative electrode plate manufactured under the manufacturing conditions of Comparative Example 4 includes a negative electrode composite layer having a first layer and a second layer both formed using slurry, and this negative electrode composite layer has micropores. Not provided. Further, the pressing is performed after drying the applied composition for forming a composite material layer (slurry).

(Comparative example 5)
Steps S2 and S3 in the flowchart of FIG. 1 are omitted, and a granule film is formed on the surface of the current collector foil by hot air drying (80°C, 5 minutes) according to steps S1 and S4, and then the final pressing is performed. Then, a negative electrode plate cut out into approximately 30 cm squares was manufactured. The coating amount of the granules was adjusted so that the basis weight was 17 mg/cm ² on one side of the current collector foil and 34 mg/cm ² on both sides.

The negative electrode plate manufactured under the manufacturing conditions of Comparative Example 5 includes a negative electrode composite layer having only the first layer formed using granules, and this negative electrode composite layer is provided with micropores. Not yet. Further, the pressing is performed after drying the applied composition for forming a composite material layer (granules).

(Comparative example 6)
After forming a granule film on the surface of the current collector foil according to step S1 of the flowchart in FIG. 1, it is placed in a drying oven and subjected to hot air drying (90°C, 5 minutes) for intermediate drying. A dry product was obtained. After forming holes using a mold in the dried granule membrane of the dry body cut into approximately 30 cm squares, a second layer of slurry membrane was formed using a slurry with a viscosity of 15,000 mPa・s. A laminate was obtained. At this time, the intermediate press was omitted. Then, the laminate was subjected to final pressing to produce a negative electrode plate. Note that the basis weight of the granules and slurry was the same as in Example 1.

The negative electrode plate manufactured under the manufacturing conditions of Comparative Example 6 includes a negative electrode composite layer having a first layer formed using granules and a second layer formed using slurry. , this negative electrode composite material layer is provided with micropores. However, pressing is performed after drying the applied composition for forming a composite material layer (granules and slurry).

[Positive electrode]
NCM powder (average particle size: 5 μm) was used as the positive electrode active material. Acetylene black (AB) powder was used as the conductive material. PVdF was used as the binder. NMP was used as a solvent. Aluminum foil (thickness: 15 μm) was used as the current collector foil. Note that no micropores were provided in the positive electrode composite material layer of the positive electrode plate.

Weigh the positive electrode active material, conductive material, and binder so that the mass ratio of the total solid content is NCM:AB:PVdF=95:4:1, add a solvent to these electrode materials, and knead. In this way, a positive electrode slurry was prepared. Then, using a die coater, the positive electrode slurry was applied to both sides of the current collector foil, and after placing it in a drying oven and drying it with hot air (120°C, 10 minutes), the thickness of the entire positive electrode plate was approximately 160 μm. It was pressed so that In this way, a positive electrode plate having positive electrode composite material layers on both sides of the current collector foil was produced. The coating amount of the positive electrode slurry was adjusted so that the basis weight was 19 mg/cm ² on one side of the current collector foil and 38 mg/cm ² on both sides.

[evaluation]
Each negative electrode plate, a positive electrode plate, and a separator, which is a polyolefin resin sheet, were laminated in an insulating state to form an electrode body. Furthermore, each evaluation battery was constructed by housing the electrode body and the non-aqueous electrolyte in a battery case serving as an exterior body. For each constructed evaluation battery, a relative cost evaluation and an evaluation of lithium precipitation resistance (Li precipitation resistance) were performed based on the measurement results of capacity retention rate.

(cost)
The cost was comprehensively evaluated based on the amount of materials used, the number of manufacturing steps, and the increase or decrease in costs related to manufacturing time, etc. The results are shown in the cost column in the table of FIG. As shown in FIG. 6, in the cost column, based on the evaluation battery manufactured under the manufacturing conditions of Comparative Example 1, batteries whose costs are lower overall than Comparative Example 1 are indicated as "〇". Those whose costs increased overall compared to Comparative Example 1 were indicated as "×".

For example, when quantifying the cost reduction rate, the reduction rate was highest in the order of Example 1, Example 2, Example 3, Comparative Example 5, Comparative Example 1, Comparative Example 4, Comparative Example 6, and Comparative Example 3. . Note that Comparative Example 2 was not evaluated because the negative electrode plate could not be manufactured.

(Lithium precipitation resistance)
By measuring the capacity retention rate, it is possible to evaluate the presence or absence of lithium precipitation (the quality of lithium precipitation resistance). The capacity retention rate was measured as follows. Note that "C" is a unit of current value, and "1C" means a current value that completely discharges the rated capacity of the battery in one hour.

First, in a temperature environment of 25°C, one cycle of charging and discharging was performed under 1C conditions, and then the initial capacity of each evaluation battery was measured. Thereafter, in a temperature environment of 0° C., each evaluation battery was repeatedly charged and discharged for 50 cycles under each condition of 5C, 6C, 7C, 8C, 9C, and 10C, and then the capacity of each evaluation battery was measured. Then, the capacity retention rate was determined using the following formula.

Capacity retention rate (%) = (capacity of evaluation battery after repeated charging and discharging/initial capacity of evaluation battery) x 100

Then, based on the evaluation battery manufactured under the manufacturing conditions of Comparative Example 1, the relative percentage of each example was calculated when the capacity retention rate of Comparative Example 1 was set as 100%, and the lithium precipitation resistance was evaluated. value. In the column of lithium precipitation resistance (%) in Figure 6, those with a larger percentage value than Comparative Example 1 are shown as "〇", and those with a smaller percentage value than Comparative Example 1 are shown as "x". .

Specific numerical values of the evaluation value of lithium precipitation resistance are as follows.
Comparative example 1: 100 (%)
Comparative example 2: N/A
Comparative example 3: 102 (%)
Comparative example 4: 102 (%)
Comparative example 5: 90 (%)
Comparative example 6: 95 (%)
Example 1: 112 (%)
Example 2: 108 (%)
Example 3: 105 (%)

The evaluation values shown here indicate that the larger the value, the smaller the capacity deterioration of the battery after charge/discharge cycles at large currents, and the better the lithium precipitation resistance.

As is clear from the results in FIG. 6, Examples 1 to 3 were evaluated as "○" in both cost and lithium precipitation resistance, and Comparative Examples 1 to 6 were evaluated in at least one of cost and lithium precipitation resistance. was "x". Compared to Comparative Examples 1 to 6, Examples 1 to 3 had a greater cost reduction effect and good lithium precipitation resistance. That is, compared to the evaluation batteries manufactured under the manufacturing conditions of Comparative Examples 1 to 6, the evaluation batteries manufactured under the manufacturing conditions of Examples 1 to 3 have superior battery characteristics. The reason for this result is considered to be as follows.

First, when compared from the viewpoint of cost, in Comparative Examples 1 to 4, the first layer, which is the main part of the negative electrode composite material layer, is formed from slurry. On the other hand, in Examples 1 to 3, the first layer, which is the main part of the negative electrode composite material layer 50, is formed of granules. Since granules can be produced using a smaller amount of solvent than slurry, the amount of materials used can be reduced. Moreover, the drying time for removing the solvent can be shortened.

Furthermore, as is clear from Comparative Examples 2 and 3, when the first layer to which the holes are processed is composed of slurry, intermediate drying is necessary. That is, since the slurry is liquid and has low shape retention, it is necessary to process the holes after drying. On the other hand, in Examples 1 to 3, the first layer in which the holes 22 are processed is composed of granules, so that intermediate drying can be omitted. That is, since the granules are clay-like with a high solid content and have good shape retention, the holes 22 can be formed in a wet state containing a solvent. In addition, in Comparative Example 6, although the first layer is formed of granules, intermediate drying is performed, so the cost increases due to the increase in the number of manufacturing steps.

For these reasons, when forming a mixture layer with micropores, it is preferable to use granules rather than slurry as the composition for forming the first mixture layer in which the holes 22 are processed. This will reduce costs and improve productivity. In particular, in Examples 1 and 2, the solid content (viscosity) of the slurry used for the second layer is higher than in Example 3, so the amount of material used is further reduced and the effect of shortening the drying time is high. Therefore, by using granules in the first layer and a slurry with a high solid content (viscosity) in the second layer, costs can be more effectively reduced and productivity can be improved.

Next, we will conduct a comparative study from the perspective of battery performance. The evaluation battery manufactured under the manufacturing conditions of Comparative Example 1 was different from Examples 1 to 3 and Comparative Examples 3 to 6 because the negative electrode composite layer was formed using only a slurry with a low solid content (viscosity). It has a negative electrode composite material layer with a lower density than that of the conventional one. In a low-density negative electrode composite material layer, the electrolytic solution is likely to be discharged from the negative electrode composite material layer as the negative electrode composite material layer expands and contracts due to repeated charging and discharging during use of the battery. When the amount of electrolyte retained in the negative electrode composite layer decreases due to accelerated discharge of the electrolyte, lithium precipitation resistance deteriorates and the capacity retention rate decreases.

On the other hand, in Comparative Examples 3 and 4, a high-density layer exists on the surface layer of the negative electrode composite material layer by providing a second layer formed of a slurry with a high solid content (viscosity). It is thought that this suppressed discharge of the electrolytic solution from the negative electrode composite material layer, which occurs due to expansion and contraction of the negative electrode composite material layer, and that the capacity retention rate (lithium precipitation resistance) was improved to some extent.

Next, Examples 1 to 3 and Comparative Examples 5 and 6, in which the first layer was formed using granules having a higher solid content than the slurry, will be compared and studied. Among these, Comparative Examples 5 and 6 had poor lithium precipitation resistance.

In Comparative Example 5, the negative electrode composite layer was composed of only the first layer of granules, and the surface of the negative electrode composite layer, which is the processed portion, was crushed due to pressing in a wet state. In this case, the reaction interface that contributes to the battery chemical reaction at the interface between the negative electrode composite material layer and the electrolytic solution is reduced, and the movement of lithium ions may be hindered. The lithium ions whose movement is prevented are deposited on the surface of the negative electrode composite material layer. Therefore, in Comparative Example 5, it is considered that the capacity retention rate decreased.

On the other hand, in order to prevent the surface of the composite material layer from being crushed by pressing, it is possible to increase the solid content of the granules, but if the solid content is increased more than necessary, the density of the entire composite material layer becomes high. If this happens too much, the amount of electrolyte retained in the composite material layer decreases, and there is a risk that the capacity retention rate will decrease.

Comparative Example 6 has fine pores, which improves the utilization efficiency of the electrolyte solution. It is thought that the electrode material was damaged due to the stress caused by the machining near the machined part. In particular, when the active material is damaged, the reaction area of the active material increases and a decomposition reaction of the electrolyte occurs, resulting in the generation of decomposed products and gases of the electrolyte. Therefore, if the active material is damaged, the movement of lithium ions is hindered and the battery expands. In Comparative Example 6, it is considered that the deterioration of lithium precipitation resistance due to damage to the active material had a large effect, and the capacity retention rate decreased.

In contrast, in Examples 1 to 3, the first layer in which the holes 22 are processed is formed of granules with good shape retention, so the holes 22 can be processed in a wet state. . Therefore, damage to the active material caused by processing the holes 22 is suppressed. Furthermore, compared to Examples 2 and 3, which were pressed in a dry state, in Example 1, which was pressed in a wet state, damage to the active material caused by pressing is suppressed. In Example 1, the processing and pressing of the hole 22 are performed simultaneously using one mold 30, so productivity is high.

Further, in Examples 1 to 3, a second layer formed of slurry was provided. With this configuration, the first layer whose surface has been crushed by pressing in a wet state is coated with a fluid slurry. Since the second layer formed of slurry is disposed on the surface layer of the negative electrode composite material layer 50, in the negative electrode plate 100 having such a configuration, the reaction interface between the negative electrode composite material layer 50 and the electrolyte is sufficiently can be formed into By making the slurry used for the second layer higher in viscosity, the slurry film 41 is held uniformly on the surface of the granule film 21, and the number of reaction interfaces increases.

Furthermore, for the second layer formed of slurry, it is preferable to press with as low a press pressure as possible, and it is more preferable not to press. According to such a configuration, unevenness at the interface between the negative electrode composite material layer 50 and the electrolytic solution and damage to the active material caused by pressing are further suppressed. For example, in Example 1 in which the second layer is not pressed, as compared to Examples 2 and 3, unevenness at the interface and damage to the active material caused by pressing are further suppressed, and the capacity retention rate is improved.

In the negative electrode plate 100 manufactured under the manufacturing conditions of Examples 1 to 3, fine holes 51 are provided in the negative electrode composite material layer 50. Therefore, in a secondary battery including such a negative electrode plate 100, the electrolytic solution penetrates deep into the negative electrode composite layer 50, so that the efficiency of using the electrolytic solution is improved. Furthermore, in the negative electrode plate 100 manufactured in this way, damage to the electrode material (particularly the active material) caused by the formation of micropores is suppressed, so that damage to the electrode material (particularly the active material) caused by the formation of micropores is suppressed. 51 functions more effectively.

For these reasons, in the evaluation secondary batteries of Examples 1 to 3, lithium precipitation is suppressed, and as a result, the capacity retention rate is improved.

As explained above, according to the method for manufacturing an electrode for a secondary battery according to the present embodiment, it is possible to manufacture an electrode for a secondary battery with a high capacity retention rate while ensuring productivity.

Note that the present invention is not limited to the above embodiments, and can be modified as appropriate without departing from the spirit. For example, the shape, size, material, etc., or manufacturing method of each member described in the above embodiments is not limited to those exemplified. Any method may be used as long as the electrode can be manufactured.

11 Current collector foil 21 Granule film 22 Hole 23 Hole forming film 24 First composite material layer 30 Mold 31 Base 32 Convex portion 41 Slurry film 42 Second composite material layer 50 Mixed material layer 51 Micropores 100 Electrode plate L1 First laminate L2 Second laminate L3 Third laminate

Claims (3)

Clay-like granules obtained by mixing at least an active material and a solvent are rolled through a pair of rolls, and the rolled granules are transferred to the surface of a current collector foil. forming a granule film thereon;
forming a hole-forming film by recessing holes in the granule film that extend in the thickness direction from the surface of the granule film;
Coating a liquid slurry obtained by mixing at least an active material and a solvent onto the surface of the pore-forming film to form a slurry film;
drying the hole-forming film and the slurry film to form a composite material layer;
A method for manufacturing an electrode for a secondary battery having the following. The hole is formed in the granule film by pressing the surface of the granule film with a mold provided with a convex portion corresponding to the shape of the hole,
The thickness of the granule film is adjusted as the surface of the granule film is pressed.
A method for manufacturing an electrode for a secondary battery according to claim 1. The viscosity of the slurry is 15000 mPa·s or more,
A method for manufacturing an electrode for a secondary battery according to claim 1 or 2.

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