JP7658932B2 - Electrode manufacturing method - Google Patents

Description

This disclosure relates to a method for manufacturing an electrode.

JP 2018-192380 A (Patent Document 1) discloses an electrostatic powder coating device.

JP 2018-192380 A

It has been proposed to manufacture electrodes using electrostatic coating technology. For example, an electric field is formed. One end of the electric field is the workpiece (substrate). Powder paint is sprayed into the electric field. The powder paint contains an active material. Electrostatic force acts on the powder paint. The electrostatic force causes the powder paint to adhere to the substrate. Hereinafter, the amount of powder paint adhered per unit area (mass of the coating film) is also referred to as "basis weight".

Currently, electrodes manufactured using electrostatic coating technology tend to have a large variation in coating weight across the surface of the coating. This variation is particularly noticeable in electrodes with a large surface area.

The purpose of this disclosure is to reduce the variation in coating weight across the surface of the coating.

The technical configuration and effects of the present disclosure are explained below. However, the mechanism of action in this specification includes assumptions. The mechanism of action does not limit the technical scope of the present disclosure.

The method for producing the electrode includes the following steps (a) to (d).
(a) A positive voltage is applied to the substrate.
(b) A negative voltage is applied to the screen to form an electric field between the substrate and the screen.
(c) The powder coating is introduced into the electric field through the screen.
(d) The powder coating is applied to a substrate to form a coating film.
The powder coating contains an active material. The powder coating is given a negative charge by contacting with the screen. The powder coating flies in the electric field due to electrostatic force, and the powder coating reaches the substrate. The powder coating flies vertically upward or vertically downward and crosses the direction.
The execution time of the above (d) is determined to be equal to or longer than the saturation time. The saturation time indicates the time for the coating film to reach a saturation weight under a given positive voltage. The saturation weight is a function of the positive voltage applied to the substrate.

It is believed that it is difficult for the flow speed of powder paint flying in an electric field to be uniform within the surface. Furthermore, the shape of the electrodes that form the electric field can cause variation in the electric field strength within the surface. As a result, it is believed that variation occurs in the electrostatic force (adhesion force) within the surface of the coating film. It is believed that variation in adhesion force within the surface causes variation in the coating weight.

In the manufacturing method disclosed herein, the powder paint flies vertically upward or crosses the vertical downward direction. Therefore, gravity (mg) acts in a direction that pulls the powder paint away from the substrate. "m" represents the mass of the powder paint, and "g" represents the gravitational acceleration. The electrostatic force (adhesion force) acting on the powder paint is expressed by the following formula (1).

F=k×q ₁ q ₂ /r ² (1)
"F" denotes the electrostatic force.
"k" denotes a proportionality constant.
"q ₁ " represents the amount of electricity imparted to the powder coating.
"q ₂ " represents the amount of electricity imparted to the substrate.
"r" indicates the distance between the substrate and the powder coating.

In the initial stage of film formation, the electrostatic force (F) is greater than the gravity (mg). Therefore, the powder coating can adhere to the substrate.

As the coating becomes thicker, the distance (r) between the coating surface and the substrate becomes longer. In other words, the electrostatic force (F) acting on the powder paint on the coating surface gradually becomes smaller. Eventually, the electrostatic force (F) and gravity (mg) are balanced on the coating surface. At this point, the coating is considered to have reached its "saturation weight". Even if coating is continued beyond the saturation weight, gravity is greater than the electrostatic force, making it difficult for the powder paint to adhere to the coating and causing it to fall off.

When the coating reaches the saturation weight, it is believed that the distance (r) between the surface of the coating and the substrate will be approximately constant at each point on the surface of the coating. In other words, it is believed that the thickness of the coating (amount of powder paint applied) will be uniform. Therefore, it is believed that by forming the coating so that it reaches the saturation weight, the variation in the coating weight can be reduced.

Here, the basis weight saturation amount is determined by the positive voltage applied to the substrate. It is considered that the positive voltage determines the quantity of electricity ( _q2 ) in the above formula (1). By adjusting the magnitude of the positive voltage, the basis weight saturation amount can be adjusted to a desired value.

Furthermore, the time required to reach the saturation weight (saturation time) can be determined, for example, through preliminary experiments. By setting the film formation time to be equal to or longer than the saturation time, electrodes with small variation in weight can be efficiently manufactured.

Below, an embodiment of the present disclosure (hereinafter may be abbreviated as "this embodiment") and an example of the present disclosure (hereinafter may be abbreviated as "this example") are described. However, this embodiment and this example do not limit the technical scope of the present disclosure.

FIG. 1 is a schematic flow chart of a method for producing an electrode in this embodiment. FIG. 2 is a conceptual diagram showing a method for producing an electrode in this embodiment. FIG. 3 is a conceptual diagram showing a method for producing an electrode according to the present embodiment. FIG. 4 is a graph showing an example of the relationship between the basis weight saturation amount and the voltage of the substrate. FIG. 5 is a graph showing an example of the relationship between the coating weight and the film formation time. FIG. 6 is a schematic cross-sectional view showing a film forming apparatus in this embodiment. FIG. 7 is a graph showing the relationship between the basis weight saturation amount and the voltage of the substrate in this embodiment.

<Definition of terms>
In this specification, the words "comprise,""include,""have," and variations thereof (e.g., "consisting of") are open-ended. The open-ended form may or may not include additional elements in addition to the required elements. The word "consisting of" is closed-ended. However, even in the closed form, additional elements that are normally associated impurities or unrelated to the disclosed technology are not excluded. The word "consisting essentially of" is semi-closed. In the semi-closed form, the addition of elements that do not substantially affect the basic and novel characteristics of the disclosed technology is permitted.

In this specification, the order of execution of multiple steps, actions, operations, etc. included in various methods is not limited to the order described unless otherwise specified. For example, multiple steps may proceed simultaneously. For example, multiple steps may occur before or after each other.

In this specification, expressions such as "may" and "may" are used in the permissive sense of "possibly" rather than the obligatory sense of "must."

In this specification, elements expressed in the singular form include the plural form unless otherwise specified. For example, a "particle" can mean not only "one particle" but also "an aggregate of particles (powder, powder, particle group)."

In this specification, unless otherwise specified, a numerical range such as "m to n%" includes an upper limit and a lower limit. That is, "m to n%" indicates a numerical range of "m% or more and n% or less." Furthermore, "m% or more and n% or less" includes "more than m% and less than n%." Furthermore, a numerical value arbitrarily selected from within the numerical range may be set as a new upper limit or lower limit. For example, a new numerical range may be set by arbitrarily combining a numerical value within the numerical range with a numerical value described in another part of this specification, in a table, in a figure, etc.

Geometric terms in this specification (e.g., "parallel," "perpendicular," "orthogonal," etc.) should not be interpreted in a strict sense. For example, "parallel" may deviate slightly from the strict meaning of "parallel." Geometric terms in this specification may include, for example, tolerances, errors, etc. in design, operation, manufacturing, etc. The dimensional relationships in each figure may not match the actual dimensional relationships. In order to facilitate understanding of the disclosed technology, the dimensional relationships (length, width, thickness, etc.) in each figure may be changed. Furthermore, some configurations may be omitted.

In this specification, when a compound is expressed by a stoichiometric composition formula (e.g., "LiCoO ₂ ", etc.), the stoichiometric composition formula is merely a representative example of the compound. The compound may have a non-stoichiometric composition. For example, when lithium cobalt oxide is expressed as "LiCoO ₂ ", unless otherwise specified, the lithium cobalt oxide is not limited to a composition ratio of "Li/Co/O = 1/1/2" and may contain Li, Co and O in any composition ratio. Furthermore, doping, substitution, etc. with trace elements may also be allowed.

In this specification, "D50" is defined as the particle size at which the cumulative frequency of the smaller particle sizes reaches 50% in a volume-based particle size distribution. D50 can be measured using a laser diffraction particle size distribution analyzer.

In this specification, "melting point" refers to the peak top temperature of the melting peak (endothermic peak) in a DSC (Differential Scanning Calorimetry) curve. The DSC curve can be measured in accordance with "JIS K 7121." "Near the melting point" can refer to, for example, a range of the melting point ±20°C.

In this specification, "electrode" is a general term for a positive electrode and a negative electrode. The electrode may be a positive electrode or a negative electrode. The electrode may be for a lithium ion battery, for example. The lithium ion battery may be, for example, a liquid battery or an all-solid-state battery. However, the electrode may be applied to any battery system. In this embodiment, an application to a lithium ion battery is described as an example.

As used herein, "aerosol" refers to a dispersion system in which at least one of a solid and a liquid is dispersed in a gas. An aerosol may also be referred to as, for example, a mist or mist. The appearance of an aerosol may be described as, for example, cloud-like or plume-like. An aerosol may also be referred to as "cloud powder."

<Electrode manufacturing method>
1 is a schematic flow chart of a method for manufacturing an electrode in this embodiment. Hereinafter, the "method for manufacturing an electrode in this embodiment" may be abbreviated as "this manufacturing method". This manufacturing method includes "(a) setting the voltage of the substrate", "(b) setting the voltage of the screen", "(c) charging", and "(d) film formation". This manufacturing method may further include, for example, "(e) fixing" and the like.

(a) Voltage setting of substrate
2 is a conceptual diagram showing a method for producing an electrode in this embodiment. This production method includes applying a positive voltage to the substrate 11. The substrate 11 may be, for example, in the form of a sheet. The substrate 11 may be a current collector. The substrate 11 may include, for example, a metal foil. The substrate 11 may include, for example, at least one selected from the group consisting of aluminum (Al), copper (Cu), nickel (Ni), chromium (Cr), titanium (Ti) and iron (Fe). The substrate 11 may include, for example, an Al foil, an Al alloy foil, a Cu foil, or the like. The substrate 11 may have a thickness of, for example, 5 to 50 μm.

(b) Screen voltage setting
This manufacturing method includes forming an electric field between the substrate 11 and the screen 122 by applying a negative voltage to the screen 122. The screen 122 is porous. The screen 122 may be, for example, a conductive mesh. The mesh size of the screen 122 can be set so that the powder paint can pass through and the frequency of contact between the powder paint and the screen 122 is appropriate. The mesh size of the screen 122 may be, for example, 30 to 300 μm.

The substrate 11 and the screen 122 are connected to a DC power supply 133. A first high-voltage power supply 131 is connected to the substrate 11. The first high-voltage power supply 131 applies a positive voltage (V _B ) to the substrate 11. In FIG. 2, a voltage of +1000 V is applied as an example. A second high-voltage power supply 132 is connected to the screen 122. The second high-voltage power supply 132 applies a negative voltage (V _S ) to the screen 122. In FIG. 2, a voltage of −3000 V is applied as an example. For example, when the distance (gap) between the substrate 11 and the screen 122 is 8 mm, the electric field strength is 500 V/mm.

The positive voltage (V _B ) may be, for example, +500 to +1500 V. The negative voltage (V _S ) may be, for example, −3500 to −2500 V. The electric field strength may be, for example, 100 to 2000 V/mm.

(c) Electric Charge
The method includes introducing the powder coating material (particles 5) into the electric field through a screen 122. In the vertical direction (Z-axis direction), the screen 122 is disposed below the substrate 11. The screen 122 may be disposed directly below the substrate 11. The powder coating material will be described below.

For example, the particles 5 may be transported to the screen 122 by air. The powder paint may form an aerosol, for example. The particles 5 are given a negative charge by contacting the screen 122. The particles 5 that have passed through the screen 122 are introduced into an electric field. An electrostatic force acts on the particles 5 that have been introduced into the electric field. The particles 5 fly due to the electrostatic force. The flight direction of the particles 5 is vertically upward or crosses the vertically downward direction. In other words, the electrostatic force includes a component in a direction against gravity (mg).

When the particle 5 approaches the substrate 11, an image force acts on the particle 5. The image force satisfies the relationship of "q ₁ = q ₂ " in the above formula (1). By applying a positive voltage to the substrate 11, the amount of electricity in the substrate 11 increases. That is, an amount of electricity of "α×q ₁ " is imparted to the substrate 11. "α" is a value greater than 1. Therefore, an electrostatic force of "F=k×αq ₁ q ₂ /r ² " acts on the particle 5. This is expected to improve the adhesion of the powder paint. By improving the adhesion, it is expected that the basis weight saturation amount (upper limit value) will increase. In addition, it is expected that the frequency of adhesion of the powder paint to the substrate 11 will also increase. By improving the frequency of adhesion, it is expected that the film formation speed will increase.

FIG. 3 is a conceptual diagram showing a method for manufacturing an electrode in the reference embodiment. In FIG. 3, a high-voltage power supply is not connected to the substrate 11. That is, a positive voltage is not applied to the substrate 11. The substrate 11 is ground (GND, 0 V). A negative voltage of −4000 V is applied to the screen 122. In the reference embodiment, when the gap between the substrate 11 and the screen 122 is 8 mm, the electric field strength is 500 V/mm. However, in the reference embodiment, the electrostatic force acting on the particles 5 is “F=k×q ₁ q ₂ /r ² ”. The electrostatic force in the reference embodiment is “1/α” of the electrostatic force in the present manufacturing method. That is, the electrostatic force in the reference embodiment is smaller than the electrostatic force in the present manufacturing method. Therefore, in the reference embodiment, the basis weight saturation amount (upper limit value) may be smaller than that in the present manufacturing method. In the reference embodiment, the film formation speed may be lower than that in the present manufacturing method.

(d) Film formation
This manufacturing method includes forming a coating film by adhering a powder coating to a substrate 11. The film formation in this technology is similar to "development" in electrophotography. The coating film may also be called, for example, an "active material layer" or a "composite layer."

The duration of the coating (coating time) is set to be equal to or greater than the saturation time. The coating time may be equal to the saturation time. The saturation time may be determined, for example, by a preliminary experiment. The saturation time indicates the time at which the coating reaches a saturation amount per unit area under a given positive voltage. The saturation amount per unit area is a function of the positive voltage applied to the substrate 11.

Figure 4 is a graph showing an example of the relationship between the basis weight saturation amount and the voltage of the substrate. The graph in Figure 4 may be called a calibration curve. The calibration curve may be determined, for example, by regression analysis. The calibration curve may be, for example, a linear function or a quadratic or higher function. The magnitude of the positive voltage to be applied to the substrate 11 can be determined from the target basis weight value and the calibration curve.

Figure 5 is a graph showing an example of the relationship between basis weight and film formation time. For example, the basis weight may increase monotonically until it reaches the basis weight saturation amount. When it reaches the basis weight saturation amount, it becomes approximately constant. For example, when the basis weight changes within ±3% for 100 seconds or more, it can be considered that the basis weight is approximately constant. The time required for the basis weight to become approximately constant is the saturation time. By having the film formation time be equal to or longer than the saturation time, the variation in the basis weight can be reduced.

The coating film may be formed on only one side of the substrate 11, or on both sides of the substrate 11.

(e) Establishment
The method may include applying at least one of pressure and heat to the coating film to fix the coating film to the substrate 11. Fixation of the coating film is expected to improve the peel strength of the coating film.

Pressure and heat may be applied separately. Pressure and heat may be applied substantially simultaneously. For example, the coating film may be compressed by a heat roll, a heat plate, or the like. The heating temperature of the coating film may be, for example, a temperature near the melting point of the binder. The heating temperature may be, for example, 80 to 200°C. The pressure may be adjusted, for example, according to the target thickness, target density, etc. of the coating film. For example, a pressure of 50 to 200 MPa may be applied to the coating film.

The coating (after fixing) may have a thickness of, for example, 10 μm to 1 mm. The coating may have a density of, for example, 1 to 5 g/cm ³ .

Powder coatings
The powder coating includes an active material. The powder coating may further include a conductive material, a solid electrolyte, a binder, and the like in addition to the active material. The active material is particulate. The active material may have a D50 of, for example, 1 to 30 μm. The active material includes a positive electrode active material or a negative electrode active material. The positive electrode active material may include, for example, at least one selected from the group consisting of Li(NiCoMn)O ₂ , Li(NiCoAl)O ₂ , and LiFePO _4. The negative electrode active material may include, for example, at least one selected from the group consisting of graphite, silicon, and silicon oxide. The amount of the binder may be, for example, 0.1 to 10 parts by mass with respect to 100 parts by mass of the active material. The binder may include, for example, polyvinylidene fluoride (PVdF), and the like. The amount of the conductive material may be, for example, 0.1 to 10 parts by mass with respect to 100 parts by mass of the active material. The conductive material may include, for example, conductive carbon. The amount of the solid electrolyte may be, for example, 1 to 100 parts by volume per 100 parts by volume of the active material. The solid electrolyte may _include , for example, _Li2S - _P2S5 .

The powder coating may contain, for example, a composite powder. A "composite powder" is an aggregate of composite particles. For example, composite particles may be formed by mixing an active material with other solid materials under conditions where a strong shear force is applied. In composite particles, for example, a binder and a conductive material may be fixed to the surface of the active material (core particle). By bonding the active material with other solid materials in advance, it is expected that the coating composition will be homogenized.

"electrode"
In general, the shape of the electrode for a battery can be roughly divided into, for example, a strip shape and a sheet shape. The strip shape electrode can be used, for example, in a wound type battery. The sheet shape electrode can be used, for example, in a stacked type battery. For example, there is a need for a large-area stacked type battery. The large-area stacked type battery includes a large-area sheet-shaped electrode. The present manufacturing method is considered to be suitable, for example, for manufacturing a large-area sheet-shaped electrode. The large-area sheet-shaped electrode may be manufactured, for example, by a batch process.

<Creating a calibration curve>
1. Powder coating preparation The following materials were prepared:
Active material: Li(NiCoMn)O ₂
Conductive material: Acetylene black Binder: PVdF

A mixing device called the "Multi-Purpose Mixer" manufactured by Nippon Coke and Engineering Co., Ltd. was prepared. The device includes a spherical tank (mixing vessel). The spherical tank's convection-promoting effect generates strong shear forces, which can compound the solid materials.

The active material, conductive material, and binder were placed in a spherical tank. The material ratio was "active material/conductive material/binder = 90/5/5 (mass ratio)". The rotation speed of the stirring blade was set to 10,000 rpm. The materials were mixed for 10 minutes. This resulted in the formation of a composite powder. The composite powder contains composite particles. Each composite particle is believed to contain substantially one grain of active material. The binder and conductive material are believed to be adhered to the surface of the active material.

A metal tray was prepared. The composite powder was spread thinly on the tray. The tray was stored in an oven to subject the composite powder to a heat treatment. The oven temperature was set at 160°C. The storage time was 30 minutes. It is believed that the binder and conductive material were fixed to the surface of the active material by the heat treatment.

6 is a schematic cross-sectional view showing a film forming apparatus in this embodiment. The film forming apparatus 100 includes an introduction section 110, a development section 120, and an electric field forming section .

The introduction section 110 includes an agitator blade 111, a perforated plate 112, and a fan 113. The perforated plate 112 is an alumina perforated plate (mesh size 10 μm, planar size 75 mm × 75 mm).

The developing section 120 includes a developing electrode 121 and a screen 122. The screen 122 is a SUS mesh (mesh size 100 μm). The gap between the developing electrode 121 and the screen 122 is 8 mm.

The electric field forming unit 130 includes a first high-voltage power supply 131, a second high-voltage power supply 132, and a DC power supply 133. The first high-voltage power supply 131 applies a positive voltage to the development electrode 121. The second high-voltage power supply 132 applies a negative voltage to the screen 122.

The substrate 11 was placed on the surface of the developing electrode 121. The substrate 11 was an Al foil (thickness 12 μm). The substrate 11 had the same potential as the developing electrode 121. The composite powder was supplied onto the porous plate 112. The composite powder was stirred up by the fan 113 supplying air to the composite powder. The air flow rate was 25 L/min. The agitating blade 111 mixed the composite powder and air to form the aerosol 7. The rotation speed of the agitating blade 111 was 120 rpm. The aerosol 7 passed through the screen 122. The charged aerosol 7 was introduced into the electric field. The aerosol 7 came into contact with the surface of the substrate 11, and the composite powder adhered to the substrate 11. This formed the coating film 12. The coating film 12 was formed until the coating film 12 reached a saturation weight. The planar size of the coating film 12 was 60 mm x 200 mm.

After the film was formed, the electrode 10 was sandwiched between two heat plates (flat plates). The temperature of the heat plates was 160°C. A load of 15 tf was applied to the coating film 12 by the heat plates. This caused the coating film 12 to be fixed to the substrate 11.

As shown in Table 1 below, the positive voltage of the substrate (positive voltage of the developing electrode 121) and the negative voltage of the screen 122 were changed, and the basis weight saturation amount was measured under each condition.

Figure 7 is a graph showing the relationship between the basis weight saturation amount and the voltage of the substrate in this embodiment. The basis weight saturation amount can be approximated as a linear function of the positive voltage of the substrate 11.

Example
The target basis weight was set to 104 mg/ ^cm2 . Based on Table 1 above and FIG. 7, the positive voltage of the substrate (developing electrode 121) was set to +1000 V. The negative voltage of the screen 122 was set to -3000 V. The field strength was 500 V/mm. The time to reach basis weight saturation (saturation time) was measured. The saturation time was 120 seconds.

The film formation time was set to 120 seconds. In other words, the film formation time was longer than the saturation time. After film formation, the coating film was fixed. After fixing, the basis weight was measured at 150 points on the surface of the coating film. The basis weight was measured using a benchtop density meter (manufactured by Hutec Co., Ltd.). Based on the basis weight at each point, the average basis weight (average basis weight) and the standard deviation of the basis weight were calculated.

Comparative Example
The target basis weight was set at 104 mg/ ^cm2 . The positive voltage of the substrate was set at +1500 V. The negative voltage of the screen 122 was set at -2500 V. The field strength was 500 V/mm. The saturation time was 110 seconds.

The film formation time was set to 93 seconds. In other words, the film formation time was less than the saturation time. After film formation, the average coating weight and standard deviation were measured in the same manner as in the examples.

<Evaluation Results>
In the examples, a good quality coating film was formed, and no coating defects were found in the coating film upon visual inspection.

The standard deviation of the basis weight is smaller in the Examples than in the Comparative Examples (see Table 2 above). In other words, the variation in the basis weight is smaller in the Examples than in the Comparative Examples. This is thought to be because the film formation time in the Examples is equal to or longer than the saturation time, while the film formation time in the Comparative Examples is shorter than the saturation time.

The present embodiment and the present examples are illustrative in all respects. The present embodiment and the present examples are not limiting. The technical scope of the present disclosure includes all modifications within the meaning and scope equivalent to the description of the claims. For example, it is contemplated from the beginning that any configuration may be extracted from the present embodiment and the present examples and that they may be combined in any desired manner.

5 Particles, 7 Aerosol, 10 Electrode, 11 Substrate, 12 Coating, 100 Film forming device, 110 Introduction section, 111 Stirring blade, 112 Perforated plate, 113 Fan, 120 Development section, 121 Development electrode, 122 Screen, 130 Electric field forming section, 131 First high voltage power supply, 132 Second high voltage power supply, 133 DC power supply.

Claims (1)

(a) applying a positive voltage to the substrate;
(b) forming an electric field between the substrate and the screen by applying a negative voltage to the screen;
(c) introducing a powder coating material into the electric field through the screen; and (d) depositing the powder coating material onto the substrate to form a coating film.
Including,
The powder coating contains an active material,
the powder coating material is negatively charged by contact with the screen;
In the electric field, the powder paint flies due to electrostatic force, and the powder paint reaches the substrate;
The powder coating material is sprayed in a vertically upward direction or in a vertically downward direction and crosses the vertically upward direction;
The execution time of (d) is determined to be equal to or greater than a saturation time;
the saturation time indicates the time it takes for the coating film to reach a saturation weight under the positive voltage,
the basis weight saturation amount is a function of the positive voltage applied to the substrate;
A method for manufacturing an electrode.

Images