JP7410074B2 - 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 are made of a conductive current collector foil (current collector) and a composite material layer containing an active material and a binder held on the current collector foil. , is equipped with. In such a secondary battery, a conductive material may be used in order to form a sufficient conductive path in the composite material layer. For example, a carbon material is preferably used as the conductive material. The electrode materials such as the active material, the binder, and the conductive material are coated on the current collector foil in the form of a liquid or fluid slurry containing an appropriate solvent, or a wet granule. By drying this, a composite material layer can be formed on the current collector foil.

In particular, in secondary batteries that require high output performance, primary particles of a finely divided active material tend to aggregate and easily form secondary particles. Therefore, in order to suppress the increase in ion diffusion resistance in the composite material layer, it is desirable that the active material is well dispersed in the form of primary particles. Furthermore, when a carbon material with high cohesive force is used as the conductive material, by appropriately dispersing the conductive material in the composite material layer, a good conductive path can be formed and the conductivity of the electrode can be improved. In this way, the state of dispersion of the electrode material in the composite material layer can affect battery performance.

Patent Document 1 describes a step S1 of mixing carbon nanoparticles and a dispersion medium to obtain a first mixture, a step S2 of mixing the first mixture and carbon black particles to obtain a second mixture, and a step S2 of mixing the first mixture and carbon black particles to obtain a second mixture. Step S3 of mixing the material particles and kneading them at a shear stress τ = 200 N/m2 or ^more to obtain a third mixture; Step S4 of forming a particle aggregate of wet particles; A method for manufacturing an electrode plate is disclosed, which includes a step S5 of forming an active material layer on the active material layer, and a step S6 of drying the undried active material film to form an active material layer.

Japanese Patent Application Publication No. 2020-140793

In Patent Document 1, in forming an electrode plate, a particle aggregate (granule) of wet particles containing an electrode material is rolled through a gap between a pair of rolls and formed into a film (sheet), which is then used as a current collector foil. A coating method is used in which the granules are transferred onto the current collector foil to form a film. Since the granules used in this coating method are produced using a smaller amount of solvent than the above-described slurry, there is an advantage that the drying time of the electrode is shortened.

On the other hand, when producing granules with a high solid content, it may be difficult to mix electrode materials uniformly. Therefore, in Patent Document 1, the dispersibility of the electrode material in the composite layer is improved by applying high shear stress to a mixture containing a dispersion medium and having a relatively high solid content and mixing the electrode material. It is being considered to do so. However, this method of applying high shear stress has a problem in that productivity decreases.

The present invention was made to solve these problems, and provides a method for manufacturing electrodes for secondary batteries that has excellent battery performance by dispersing electrode materials well while ensuring productivity. The purpose is to

A method for manufacturing an electrode for a secondary battery according to one embodiment is a method for producing an electrode for a secondary battery, which is synthesized using a magnetic metal catalyst, and powdered while applying a vertically downward magnetic field to a conductive carbon material containing the magnetic metal catalyst. a binder liquid in which a binder is dissolved or dispersed in a solvent while applying a vertically downward magnetic field to the mixed powder; dropwise to obtain a granule, and rolling the granule to form a composite layer on the current collector foil, wherein the bulk density of the carbon material is smaller than the bulk density of the active material. .

According to the present invention, it is possible to provide an electrode for a secondary battery and a method for manufacturing the electrode for a secondary battery that have excellent battery performance by dispersing electrode materials well while ensuring productivity.

1 is a flowchart showing a method for manufacturing an electrode for a secondary battery according to Embodiment 1. FIG. FIG. 3 is a diagram showing manufacturing conditions and performance evaluation results of an 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.

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. A lithium ion secondary battery is a secondary battery that uses lithium ions as electrolyte ions and is charged and discharged by the transfer of charge associated with lithium ions between the positive electrode (positive electrode plate) and the negative electrode (negative electrode plate). .

First, an outline of an electrode plate manufactured by the method for manufacturing an electrode for a secondary battery according to this embodiment will be explained. The electrode plate includes a current collector foil and a composite material layer formed on the current collector foil. The current collector foil 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, examples of the metal constituting the current collector foil include aluminum, aluminum alloy, nickel, titanium, stainless steel, and the like. In the case of a negative electrode, examples of the metal constituting the current collector foil include copper, copper alloy, nickel, titanium, stainless steel, and the like.

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

In this embodiment, wet granules having a relatively high solids content are rolled between a pair of rolls, and the rolled granules are transferred onto the current collector foil. A case will be described in which an electrode plate is formed using a coating method in which granules are formed into a film. The granule is a material containing an active material, a binder, a conductive agent, and a solvent, and is used to form a composite material layer.

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 a positive electrode and a negative electrode.

As shown in FIG. 1, the method for manufacturing an electrode for a secondary battery according to this embodiment includes steps S1 to S3. Step S1 is a conductive carbon material that is synthesized using a magnetic metal catalyst and is mixed with a powdery active material while applying a vertically downward magnetic field to the conductive carbon material containing the magnetic metal catalyst to form a mixed powder. This is a powder mixing process to obtain Step S2 is a granulation step in which a binder liquid in which a binder is dissolved or dispersed in a solvent is dropped to obtain granules while applying a vertically downward magnetic field to the mixed powder. be. Step S3 is a composite material layer forming step of rolling the granules to form a composite material layer on the current collector foil.

Note that in steps S1 and S2, the same stirring device may be used for stirring, or different stirring devices may be used for stirring. From the viewpoint of productivity, it is preferable to use the same stirring device.

The stirring device for stirring is not particularly limited. The stirring device may include, for example, a container and a rotating stirrer (stirring blades and crushing blades), and can crush, mix, and granulate powder by rotating the stirrer. good. Examples of such stirring devices include mechanical stirring devices such as a planetary mixer, a food processor, a homomixer, a Henschel mixer, a super mixer, a propeller stirrer, and a high-speed mixer.

Each of the above steps will be explained in detail. First, in the powder mixing step of step S1, an active material and a carbon material that is a conductive material are uniformly mixed to obtain a mixture.

[Active material]
The active material used in the positive electrode is a material capable of intercalating and deintercalating lithium ions, and any active material that can be used in lithium ion secondary batteries 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 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 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. In this embodiment, the active material is mixed with a carbon material as a conductive material in powder form. The bulk density of the active material is, for example, 0.5 g/cm ³ to 2.5 g/cm ³

[Conductive material]
The conductive material has a function of transmitting electricity generated in the active material to the current collecting foil. By mixing a conductive material into the composite material layer, conductive paths can be formed between the active material particles and between the current collector foil and the active material, thereby improving the conductivity of the entire electrode.

In this embodiment, the electrically conductive material contains a soft magnetic material that can become magnetic by becoming magnetic upon contact with or proximity to a magnet. Examples of the conductive material containing a soft magnetic substance include a carbon material with good conductivity synthesized using a magnetic metal catalyst exhibiting soft magnetism. Specific examples of such carbon materials are carbon nanofibers (CNF) and carbon nanotubes (CNT). In addition, in the following description, CNF and CNT may be collectively referred to as a fibrous carbon material.

Further, the magnetic metal catalyst is a growth catalyst that serves as a growth starting point for the fibrous carbon material. Such magnetic metal catalysts are metal nanoparticles made of transition metals such as iron, nickel, and cobalt alone, or an alloy containing at least one of these metals. The magnetic metal catalyst may be a metal oxide containing at least one of these. When the magnetic metal catalyst is composed of an alloy, it is preferably an alloy containing at least 50% by mass of at least one of iron, nickel, and cobalt. Depending on the composition of the magnetic metal catalyst, it is possible to adjust the magnetic properties of the fibrous carbon material containing the magnetic metal catalyst.

The particle size of the magnetic metal catalyst is, for example, 1 nm to 500 nm, preferably 10 nm to 200 nm. If the particle size of the magnetic metal catalyst is too small, there is a risk that the fibrous carbon material will not grow sufficiently. Furthermore, if the particle size of the magnetic metal catalyst is too large, it becomes difficult to support the magnetic metal catalyst on the fibrous carbon material.

Fibrous carbon materials can be synthesized using catalytic vapor phase growth. When synthesizing a fibrous carbon material by a catalytic vapor phase growth method, a magnetic metal catalyst dispersed in a solvent is coated on a base material and dried to support the magnetic metal catalyst on the base material. This base material is placed in a reaction furnace of a CVD (Chemical Vapor Deposition) device and heated to a required temperature. A hydrocarbon gas serving as a carbon source is introduced into this reactor together with a carrier gas, and heated and maintained at a reaction temperature of several hundred degrees Celsius to several thousand degrees Celsius. By heating, the hydrocarbon gas undergoes benzene ring condensation, and a fibrous carbon material grows in a vapor phase using the magnetic metal catalyst as a core.

The magnetic metal catalyst remains in the fibrous carbon material taken out from the base material. In this way, since the growth catalyst for the fibrous carbon material synthesized using the catalytic vapor phase growth method is a soft magnetic magnetic metal catalyst, the soft magnetic magnetic metal catalyst is retained at the growth starting point. A fibrous carbon material can be obtained. The carbon material containing the magnetic metal catalyst thus obtained can be attracted by a magnet. In this embodiment, a carbon material containing the above magnetic metal catalyst can be used as the conductive material.

The primary particles of the conductive material made of a carbon material containing a magnetic metal catalyst have, for example, an average fiber length of 1 μm to 100 μm from the viewpoint of conductivity, mechanical properties, and dispersibility. If the fiber length is too short, electrical conductivity and mechanical properties tend to be insufficient. On the other hand, if the fiber length is too large, agglomerates (clumps) are likely to form. The average fiber length of the conductive material can be measured using, for example, an electron microscope. In this embodiment, the conductive material is mixed with the active material in powder form. The bulk density of the conductive material is, for example, 0.01 g/cm ³ to 0.15 g/cm ³

Here, particulate carbon materials such as fibrous carbon materials and carbon black have small diameters and low bulk density, so they cannot enter into minute gaps between active material particles or between active material and current collector foil. , is effective for reducing the internal resistance of the battery. In particular, in an electrode using a fibrous carbon material, a conductive path can be formed more efficiently with a smaller amount than when using a particulate carbon material, and the amount of conductive material contained in the electrode can be reduced.

On the other hand, since the fibrous carbon material has a particularly small diameter, its cohesive force is very strong, and it is difficult to disperse it well in the composite material layer. In a secondary battery equipped with an electrode containing lumps of conductive material, a good conductive path is not formed and the electronic resistance is extremely low in the vicinity of the lumps, so charging and discharging is preferentially carried out in these parts. It can happen. This is not preferable because local deterioration of the secondary battery progresses.

Therefore, in this embodiment, it is preferable to crush the conductive material in advance before mixing it with the active material. By mixing the conductive material deagglomerated by crushing with other electrode materials, the dispersibility of the conductive material in the composite material layer can be improved. For crushing the conductive material, a stirring device equipped with a stirrer having a function of crushing blades can be used. The rotation speed of the stirrer when crushing the conductive material is, for example, 1000 rpm to 5000 rpm. The stirring time is, for example, 1 second to 60 seconds. It is preferable to appropriately adjust the rotational speed of the stirrer and the stirring time so as to break up the agglomeration of the conductive material and avoid cutting the fibers of the conductive material.

By the way, when a conductive material that originally has a small diameter is crushed, the difference in bulk density between the conductive material and the active material increases as the crushing progresses. Therefore, if powders having significantly different bulk densities are simply dry mixed, they will separate from each other and result in a non-uniform mixed state.

Since the bulk density of the conductive material made of the fibrous carbon material is smaller than the bulk density of the active material, when the two are dry mixed, the conductive material tends to float upward and the active material tends to sink downward. As described above, if an electrode is manufactured with the conductive material and the active material separated in the vertical direction in the container in a non-uniform mixed state, the conductive material and the active material will be unevenly distributed in the composite layer due to poor dispersion. If the dispersion state of the electrode material is unbalanced, it may become a factor that increases the internal resistance due to a decrease in conductivity, a decrease in the reaction area of the active material that can be involved in battery reactions, and the like.

Therefore, one of the features of the method for manufacturing an electrode for a secondary battery according to the present embodiment is that it is possible to form a composite material layer in which even a very small diameter fibrous carbon material can be dispersed well after being crushed. has. In particular, since many of the active materials used in the positive electrode have low conductivity, the positive electrode may contain a larger amount of carbon material as a conductive material than the negative electrode. The larger the amount of conductive material mixed, the more likely it is to aggregate, so the method for manufacturing an electrode for a secondary battery according to this embodiment is suitably used for a positive electrode.

In this embodiment, a magnetic field generator is placed around a container of a stirring device used for mixing electrode materials. Although a permanent magnet, an electromagnet, etc. can be used for the magnetic field generator, in this embodiment, an electromagnet is preferable. The magnetic field generator may be arranged such that the conductive material receives a downward magnetic force in the vertical direction within the container. For example, a magnetic field generator is placed on the bottom side (below) of the container of the stirring device. The strength of the magnetic field generated by the magnetic field generator is, for example, in the range of 1000G to 20000G, and is preferably adjusted as appropriate depending on the size of the container, the conductive material used, its amount, etc.

In the powder mixing process, when mixing the conductive material and the active material, the conductive material and the active material are placed in a container of a stirring device equipped with a stirrer that functions as a stirring blade, and a magnetic field generator is used to mix the conductive material and active material. Dry mix the two by stirring while applying a vertically downward magnetic field. In the powder mixing step, the rotation speed of the stirrer is, for example, 1000 rpm to 5000 rpm. The stirring time is, for example, 1 second to 60 seconds. In addition, the method of applying the magnetic field in the powder mixing process may be such that the magnetic field is continuously applied while the electromagnet is energized. Similarly to the granulation process described later, the magnetic field may be applied by turning the electromagnet on and off. The magnetic field may be applied intermittently by switching.

With such a configuration, a downward magnetic field can be applied to the conductive material, so that the conductive material that tends to float upward can be mixed with the active material while being pulled downward. As a result, a mixed powder in which the conductive material and the active material are uniformly mixed is obtained.

Subsequently, in the granulation process of step S2, a binder liquid in which a binder is dissolved or dispersed in a solvent is prepared.

[Binder]
The binder binds the materials constituting the composite material layer to each other to form a composite material layer, and binds the formed composite material layer onto the current collector foil. 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.

[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.

Note that by adding the binder in liquid form, a decrease in binding strength can be suppressed. For example, if a binder is added in the form of powder, the binding strength may be insufficient due to powder remaining due to insufficient dissolution of the binder. Insufficient binding strength may cause a decrease in film formability or a decrease in adhesion between the composite material layer and the current collector foil.

Subsequently, the prepared binder liquid is dropped onto the mixed powder obtained in the powder mixing step to granulate it. This granulation step is performed in the same container as the powder mixing step using a stirring device equipped with a stirring bar having the function of stirring blades. In the granulation step, it is preferable to granulate the mixed powder by dropping the binder liquid while stirring the mixed powder by rotating a stirrer. At this time, the rotation speed of the stirrer is, for example, 1000 rpm to 5000 rpm. The stirring time is, for example, 10 seconds to 600 seconds. It is preferable to adjust the rotation speed of the stirrer and the stirring time as appropriate so that the granules have a desired particle size.

In the granulation process, the binder liquid is dropped while applying a vertically downward magnetic field to the mixed powder in the container using a magnetic field generator. By applying a magnetic field to the mixed powder in the container, the conductive material contained in the mixed powder is attracted by the magnet and moves downward. This improves the uniformity of the mixed state of the electrode materials. By dropping the binder liquid while stirring the mixed powder, the uniformity of the mixed state of the electrode materials is further improved.

Furthermore, in the granulation step, it is preferable that the magnetic field is applied intermittently to the mixed powder. The intermittent application of the magnetic field can be switched between applying and stopping the magnetic field by switching ON/OFF of energization to the electromagnet.

By intermittently applying a magnetic field to the mixed powder in the container, the conductive material contained in the mixed powder is attracted to the magnet and moves downward while the magnetic field is applied, and when the magnetic field is stopped, it moves downward. moves upwards. Therefore, in order to suppress the uneven distribution of the conductive material, it is preferable to frequently switch between applying and stopping the magnetic field by the magnetic field generator. In this way, the magnetic force acts as a driving force for the conductive material and promotes vertical movement of the conductive material, thereby improving the dispersibility of the electrode materials by mixing them more uniformly.

As a result, the binder liquid wets between the particles of the conductive material and the active material, and a spherical granule in which the particles are bonded to each other is obtained. The granules contain an active material, a conductive material, a binder, and a solvent. The granules have a clay-like form with a high solids content and have no fluidity when at rest. The solid content of the granules can be adjusted by the blending amounts of the conductive material, active material, and binder, and the concentration of the binder liquid.

The granules granulated in the granulation step have, for example, an average particle diameter (D50) of 100 μm to 4 mm, preferably 200 μm to 2 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 granule obtained by removing the solvent from the entire granule is, from the viewpoint of film-forming properties, for example, 60% by mass to 95% by mass, preferably 70% by mass to 90% by mass. .

The ratio of the solvent to the entire granule is adjusted by adjusting the amount of the solvent in the binder liquid 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 high, it will take time to dry the electrode plate, resulting in decreased productivity. Moreover, if the proportion of the solvent is too small, the load of stirring performed in the granulation process will become large.

The ratio of the binder to the total solid content of the granules 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 may be insufficient, and the peel strength between the composite material layer and the current collector foil may tend to decrease. In addition, the film formability of the composite material layer 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, inhibiting the inflow and outflow of charge carriers, and the internal resistance of the battery may tend to increase.

The ratio of the active material to the total solid content of the granules is, for example, 70% by mass to 99% by mass, preferably 85% by mass to 95% by mass, from the viewpoint of realizing high energy density.

The proportion of the conductive material to the total solid content of the granules is, for example, 0.1% by mass to 15% by mass, preferably 2% by mass to 5% by mass. When a fibrous carbon material is used as a conductive material, high conductivity can be ensured even if the amount used is small. If the amount of conductive material used is reduced, the amount of active material can be increased, thereby achieving higher energy density.

Next, in the mixture layer forming step of step S3, the granules are rolled between a pair of rotating rolls to form a sheet-like molded body, and this is transferred onto a current collector foil to form the granules. Coat on the current collector foil. A roll coater having three rolls can be used for coating the granules.

The three rolls are a first roll, a second roll, and a third roll. A gap of a predetermined width 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 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.

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 wet powder of the granules on the first roll and the second roll, and have the function of defining the width of the composite material layer formed on the current collector foil depending on the distance between the partition walls.

In addition, a drive device, a granule supply device, a current collector foil supply device, and a control device are provided. The drive device is a device for rotating the three rolls and adjusting the gap between the rolls. The granule supply device is a device for supplying granules to the gap between the first roll and the second roll. The current collector foil supply device is a device for supplying current collector foil to the third roll. A control device controls the operation of these devices.

In coating the granules on the current collector foil, 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. molded. That is, the granules are rolled by being crushed and stretched by the first roll and the second roll, and a sheet-like molded product is formed.

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 while being compressed between the second roll and the third roll. Thereby, the granules can be formed into a film on the current collector foil. That is, an undried mixture layer is formed on the current collector foil. Note that it is preferable to select an optimal thickness for the molded body so that no scratches or cracks occur. Thereafter, the undried mixture layer is dried together with the current collector foil using a drying oven or the like.

By the method for manufacturing an electrode for a secondary battery according to this embodiment, it is possible to manufacture an electrode having a configuration in which a mixture layer is held on a current collector foil. According to such a configuration, even when a granule with a relatively high solid content is used as a material for forming the composite material layer, the electrode material can be maintained in a good dispersion state in the composite material layer of the manufactured electrode plate. can be secured.

Here, a slurry coating method is known in which a liquid slurry with a relatively low solid content is prepared as a material for forming the composite material layer, and the slurry is coated onto the current collector foil using a die coater or the like. On the other hand, since the granules used in this embodiment can be produced using a smaller amount of solvent than slurry, the drying time of the mixture layer can be shortened. Moreover, when such a granule is used, a high-density mixture layer can be formed, so there is no need to perform pressing for the purpose of increasing the density after drying. Therefore, by using granules as a material for forming the composite material layer, it is possible to efficiently form a high-density composite material layer that can contribute to higher capacity and higher energy density.

An example will be described below with reference to FIG. 2. Note that the examples do not limit the present invention. FIG. 2 is a diagram showing electrode manufacturing conditions and performance evaluation results. Positive electrode plates of Examples 1 to 11 and Comparative Examples 1 to 3 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 positive electrode plates, and battery performance was evaluated.

<Positive electrode>
The positive electrode plates of Examples 1 to 11 and Comparative Examples 1 to 3 were manufactured using a common material and by a common manufacturing method except for step S2 of the flow shown in FIG. The manufacturing conditions shown in FIG. 2 indicate the conditions regarding the rotation speed of the stirrer, the method of applying the magnetic field, and the method of dropping the binder liquid in the granulation process of step S2.

Each item of the manufacturing conditions shown in FIG. 2 will be explained.
[Rotation speed (rpm)]
The rotation speed is the rotation speed of a stirrer included in the stirring device. When dropping the binder liquid onto the mixed powder, the granules were granulated using one of the following stirring conditions: no stirring, 1000 rpm, and 3000 rpm. The higher the rotation speed, the faster the stirring can be performed.

[How to apply magnetic field]
When dropping the binder onto the mixed powder, the method of applying the magnetic field is to apply no magnetic field, to apply the magnetic field continuously, or to apply the magnetic field, depending on the ON/OFF switching status of the energization to the electromagnet. There are three methods: intermittent application. When no magnetic field was applied, the electromagnet was turned off and no magnetic field was generated. When applying a magnetic field continuously, the electromagnet was energized and the magnetic field was continuously applied. When applying a magnetic field intermittently, the magnetic field was applied and stopped intermittently by repeating ON/OFF of electricity to the electromagnet at 1 minute intervals.

[How to drop the binder liquid]
The method of dropping the binder liquid is classified into three methods depending on the timing of dropping the binder liquid: continuous, while the magnetic field is stopped, and while the magnetic field is applied. When the binder liquid was continuously dropped, the binder liquid was continuously dropped for 30 seconds regardless of whether the magnetic field was applied or stopped. When dropping the binder liquid while the magnetic field is stopped, the magnetic field is applied intermittently; the binder liquid is dropped while the magnetic field is stopped, and the binder liquid is dropped while the magnetic field is being applied. did not do so. When the binder liquid was dropped while the magnetic field was stopped, the dropping time of the binder liquid was adjusted so that the total dropping time while the magnetic field was stopped was 30 seconds. When dropping the binder liquid while a magnetic field is applied, the magnetic field is applied intermittently, and the binder liquid is dropped while the magnetic field is applied, and the binder liquid is dropped while the magnetic field is stopped. did not do so. When the binder liquid was dropped while the magnetic field was being applied, the dropping time of the binder liquid was adjusted so that the total dropping time while the magnetic field was being applied was 30 seconds.

Next, the positive electrode material and manufacturing equipment will be explained.
[Positive electrode material]
NCM powder represented by LiNi _1/3 Co _1/3 Mn _1/3 O ₂ was used as the positive electrode active material. Carbon nanotube (CNT, average fiber length 1 μm to 100 μm) powder was used as the conductive material. A binder liquid was prepared using PVdF as a binder and NMP as a solvent. The proportion of the binder in the binder liquid was 10% by mass. Aluminum foil (thickness: 15 μm) was used as the current collector foil. Note that the bulk density of the positive electrode active material was 1.5 g/cm ³ , and the bulk density of the conductive material was 0.1 g/cm ³ .

[Device]
A series of manufacturing steps were performed using the same stirring device. The container of the stirring device had a bottomed cylindrical shape with an open upper end, and its dimensions were 260 mm in diameter and 60 mm in height. An electromagnet was used as the magnetic field generator.

Next, the manufacturing method of each of the positive electrode plates of Examples 1 to 11 and Comparative Examples 1 to 3 will be described in detail.
(Example 1)
The positive electrode plate of Example 1 was manufactured according to the flow shown in FIG. In step S1, the conductive material was placed in a container of a stirring device, and the conductive material was crushed by stirring for 60 seconds at a stirring bar rotation speed of 3000 rpm. Subsequently, the positive electrode active material was placed in the container, and a mixed powder was obtained by stirring the positive electrode active material and the conductive material for 20 seconds at a stirring bar rotation speed of 3000 rpm while continuously applying a 5000 G magnetic field. .

Next, in step S2, while applying a magnetic field of 5000 G to the obtained mixed powder, the binder liquid was dropped into the container so that the total dropping time was 30 seconds. At this time, stirring was not performed. Furthermore, the method of applying the magnetic field was continuous. Further, the method of dropping the binder liquid was continuous. The amount of the binder liquid dropped was such that the mass ratio of the positive electrode active material, conductive material, and binder in the total solid content of the granules was NCM:AB:PVdF=93:5:2. . Thereby, the granules were granulated. The solid content of the granules thus obtained was approximately 85% by mass.

Thereafter, in step S3, the obtained granules were rolled and transferred onto a current collector foil using a roll coater, and then heated and dried to form a mixture layer. In this way, a positive electrode plate in which the mixture layer was held on both sides of the current collector foil was manufactured.

In addition, the gap between the first roll and the second roll and the supply amount of the granules were adjusted so that the granules had a basis weight of 10 mg/cm ² on one side of the current collector foil and 20 mg/cm ² on both sides. The total thickness of the positive electrode thus produced was approximately 80 μm.

(Example 2)
A positive electrode plate was manufactured in the same manner as in Example 1, except that the magnetic field was applied intermittently in step S2.

(Example 3)
The method of applying the magnetic field in step S2 was intermittent. Furthermore, the method for dropping the binder was to drop the binder liquid while applying a magnetic field. A positive electrode plate was manufactured in the same manner as in Example 1 except for these.

(Example 4)
A positive electrode plate was produced in the same manner as in Example 1, except that the rotation speed of the stirrer in step S2 was set to 1000 rpm, and the magnetic field was applied and the binder liquid was dropped while stirring the mixed powder.

(Example 5)
The rotation speed of the stirrer in step S2 was set to 1000 rpm, and while stirring the mixed powder, a magnetic field was applied and the binder liquid was dropped. Furthermore, the method of applying the magnetic field was intermittent. A positive electrode plate was manufactured in the same manner as in Example 1 except for these.

(Example 6)
The rotation speed of the stirrer in step S2 was set to 1000 rpm, and while stirring the mixed powder, a magnetic field was applied and the binder liquid was dropped. Furthermore, the method of applying the magnetic field was intermittent. Furthermore, the method for dropping the binder liquid was to drop the binder liquid while the magnetic field was stopped. A positive electrode plate was manufactured in the same manner as in Example 1 except for these.

(Example 7)
The rotation speed of the stirrer in step S2 was set to 1000 rpm, and while stirring the mixed powder, a magnetic field was applied and the binder liquid was dropped. Furthermore, the method of applying the magnetic field was intermittent. Furthermore, the method for dropping the binder liquid was to drop the binder liquid while applying a magnetic field. A positive electrode plate was manufactured in the same manner as in Example 1 except for these.

(Example 8)
A positive electrode plate was produced in the same manner as in Example 1, except that the rotation speed of the stirrer in step S2 was set to 3000 rpm, and the magnetic field was applied and the binder liquid was dropped while stirring the mixed powder.

(Example 9)
The rotational speed of the stirrer in step S2 was set to 3000 rpm, and while stirring the mixed powder, a magnetic field was applied and the binder liquid was dropped. Furthermore, the method of applying the magnetic field was intermittent. A positive electrode plate was manufactured in the same manner as in Example 1 except for these.

(Example 10)
The rotation speed of the stirrer in step S2 was set to 3000 rpm, and while stirring the mixed powder, a magnetic field was applied and the binder liquid was dropped. Furthermore, the method of applying the magnetic field was intermittent. Furthermore, the method for dropping the binder liquid was to drop the binder liquid while the magnetic field was stopped. A positive electrode plate was manufactured in the same manner as in Example 1 except for these.

(Example 11)
In step S2, the rotation speed of the stirrer was set to 3000 rpm, and while stirring the mixed powder, a magnetic field was applied and the binder liquid was dropped. Furthermore, the method of applying the magnetic field was intermittent. Furthermore, the method for dropping the binder liquid was to drop the binder liquid while applying a magnetic field. A positive electrode plate was manufactured in the same manner as in Example 1 except for these.

(Comparative example 1)
A positive electrode plate was manufactured in the same manner as in Example 1, except that no magnetic field was applied in step S2.

(Comparative example 2)
In step S2, the rotation speed of the stirrer was set to 1000 rpm, and the binder liquid was dropped while stirring the mixed powder. At this time, no magnetic field was applied. A positive electrode plate was manufactured in the same manner as in Example 1 except for these.

(Comparative example 3)
In step S2, the rotation speed of the stirrer was set to 3000 rpm, and the binder liquid was dropped while stirring the mixed powder. At this time, no magnetic field was applied. A positive electrode plate was manufactured in the same manner as in Example 1 except for these.

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

The negative electrode active material, binder, and thickener are weighed so that the mass ratio is C:SBR:CMC=98:1:1, and ion-exchanged water is added and kneaded to form the negative electrode. A slurry for forming a composite material layer was prepared. The slurry was coated on both sides of the current collector foil, and the slurry was dried to produce a negative electrode plate including a negative electrode composite layer. The coating amount of the slurry was adjusted so that the coating weight was 15 mg/cm ² on one side of the current collector foil and 30 mg/cm ² on both sides, and pressed so that the total thickness of the negative electrode was approximately 120 μm. .

<Construction of evaluation battery>
Each of the positive electrode plates and the negative electrode plate were laminated in an insulating state via two separators made of polyolefin resin sheets, and wound around a winding axis to form a wound electrode body. Furthermore, each evaluation battery was constructed by housing the electrode body and non-aqueous electrolyte in a rectangular parallelepiped battery case.

[evaluation]
For each evaluation battery obtained using the positive electrode plates of Examples 1 to 11 and Comparative Examples 1 to 3, the IV resistance was measured in order to evaluate the output performance. The measurement was performed by discharging at 1C, 3C, and 5C from a state of charge of 60% SOC (State of Charge) at 25°C, and measuring the voltage drop for 10 seconds from the start of discharge. The resistance value was calculated by dividing this voltage drop amount by the corresponding current value, and the average value of the three resistance values (average resistance value) was determined.

Furthermore, the output performance ratio (%) was calculated as a percentage of the value obtained by dividing the average resistance value of Comparative Example 1 by each average resistance value of Examples 1 to 11 and Comparative Examples 1 to 3, and the results were evaluated in Figure 2. It is shown in the column. In the evaluation column of FIG. 2, it is considered that the higher the percentage value is, the more the resistance increase is suppressed.

As can be seen from the results in FIG. 2, compared to the evaluation batteries of Comparative Examples 1 to 3, the evaluation batteries of Examples 1 to 11 had a larger output performance ratio and better battery performance. Among Examples 1 to 11, the evaluation batteries of Examples 9 to 11 had a large resistance reduction effect and particularly good output performance. The reason for this result is considered to be as follows.

In the manufacturing method of the positive electrode plate of Comparative Examples 1 to 3, when the binder liquid is dropped and granulated, a magnetic field is not applied to the mixed powder in the container, so the conductive material and Due to the difference in bulk density between the conductive material and the active material, the conductive material tends to float, reducing the uniformity of the mixed state of the electrode materials. On the other hand, in the method for manufacturing the positive electrode plate of Examples 1 to 11, granulation is performed by dropping the binder liquid while applying a vertically downward magnetic field to the mixed powder in the container. Thereby, a driving force for moving the conductive material downward can be applied to the conductive material whose bulk density is smaller than that of the active material. Furthermore, in this manufacturing method, the application of a magnetic field to the mixed powder in the container and the dropping of the binder liquid are preferably performed while stirring the mixed powder, and more preferably the stirring is performed at high speed. According to such a configuration, the electrode material is uniformly mixed in the granule, so that the dispersibility of the electrode material in the composite material layer is improved.

Further, in the method of manufacturing the positive electrode plates of Examples 1 to 11, a preferred method of applying a magnetic field in step S2 is an intermittent method of applying a magnetic field that frequently switches application and stop of the magnetic field. In this way, it is thought that by intermittently applying a vertically downward magnetic field to the mixed powder in the container, the vertical movement of the conductive material is promoted. On the other hand, when the magnetic field is applied continuously, the conductive material is attracted in one direction (downward in this embodiment) toward the electromagnet, so compared to when the magnetic field is applied intermittently, the conductive material is attracted in one direction (downward in this embodiment). , the conductive material and the active material tend to be unevenly distributed. Therefore, by applying an intermittent magnetic field during granulation, the electrode material is mixed in a more uniform state in the resulting granules, which improves the dispersibility of the electrode material in the composite layer. Further improvement.

Further, in the method for manufacturing the positive electrode plates of Examples 1 to 11, a preferred method for dropping the binder liquid in step S2 is a method in which the binder liquid is dropped at least while a magnetic field is being applied. The most suitable method for dropping the binder liquid is a method in which the binder liquid is dropped only while the magnetic field is being applied. While applying a vertically downward magnetic field, the conductive material is drawn downward within the container. Therefore, by dropping the binder liquid at the timing when the magnetic field is being applied, direct contact between the binder liquid added from above the mixed powder and the conductive material is suppressed. When a conductive material with a small bulk density and a binder liquid come into direct contact, they tend to aggregate and form lumps, so by suppressing this, it is possible to reduce lumps of the conductive material.

As explained above, according to the method for manufacturing an electrode for a secondary battery according to the present embodiment, it is possible to uniformly dry mix the active material and the conductive material, which have a large difference in bulk density. Moreover, by wetting the binder liquid between the dispersed particles of the conductive material and the active material, it is possible to obtain a granule in which these electrode materials are uniformly mixed. By using granules in a uniformly mixed state as the material for forming the composite material layer, it is possible to form a composite material layer in which the electrode material is well dispersed.

For example, in the method disclosed in Patent Document 1, which improves the dispersibility of electrode materials by applying high shear stress to a wet mixture, the production rate is significantly reduced, resulting in a problem of low productivity. be. In contrast, in this embodiment, the dry mixability is good, so the production speed can be improved and productivity is high.

Furthermore, in this embodiment, even when granules with a relatively high solid content are used as the material for forming the composite material layer, it is possible to maintain a good dispersion state of the electrode material. Since the granules have a smaller amount of solvent than the slurry, the drying time of the electrode is shortened and the productivity is improved.

Therefore, 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 excellent battery performance by dispersing the electrode material well while ensuring productivity. I can do it.

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, in the above embodiment, a method is exemplified in which a composite material layer is formed on a current collector foil by rolling the granules through a pair of rolls and transferring the granules onto the current collector foil. The formation method is not limited to this. For example, a method may be used in which the granules are deposited on a current collector foil and then press-molded to form a composite material layer. However, from the viewpoint of production efficiency and production stability, it is preferable to apply the former method.

Claims (5)

A step of synthesizing using a magnetic metal catalyst and mixing the conductive carbon material containing the magnetic metal catalyst with a powdered active material while applying a vertically downward magnetic field to obtain a mixed powder. and,
obtaining a granule by dropping a binder liquid in which a binder is dissolved or dispersed in a solvent while applying a vertically downward magnetic field to the mixed powder;
rolling the granules to form a composite material layer on the current collector foil;
has
The granules have a solid content of 60% by mass to 95% by mass,
A method for manufacturing an electrode for a secondary battery, wherein the bulk density of the carbon material is smaller than the bulk density of the active material. applying a magnetic field intermittently to the mixed powder;
A method for manufacturing an electrode for a secondary battery according to claim 1. Dropping the binder liquid at least while applying a magnetic field;
A method for manufacturing an electrode for a secondary battery according to claim 1 or 2. The carbon material is used after being crushed in advance,
The method for manufacturing an electrode for a secondary battery according to any one of claims 1 to 3. The composite material layer is formed by rolling the granules through a pair of rotating rolls and transferring the rolled granules onto a current collector foil.
The method for manufacturing an electrode for a secondary battery according to any one of claims 1 to 4.

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