JP7772037B2 - Method for manufacturing electrode active material layer and method for manufacturing battery - Google Patents

Description

This disclosure relates to a method for manufacturing an electrode active material layer and a method for manufacturing a battery.

The electrode active material layer of a lithium-ion battery can be produced by applying an electrode mixture slurry containing an electrode active material and a binder resin, etc., to a current collector foil and then drying it. There is a demand for higher-capacity lithium-ion batteries so that they can be used in battery electric vehicles (BEVs) and plug-in hybrid vehicles (PHEVs). To achieve this higher capacity, efforts are being made to thicken the electrode active material layer.

For example, Patent Document 1 discloses a method for producing an electrode by applying a slurry that forms an electrode composite layer onto a web-shaped current collector foil, and then transporting the slurry-coated current collector foil in one direction in a drying furnace while applying hot air to the coating to dry the coating.

JP 2013-084383 A

When a thick layer of electrode active material is produced by applying a thick layer of slurry containing the electrode active material to a current collector foil and then drying it, cracks tend to form in the electrode active material layer after drying.

The present disclosure aims to provide a method for manufacturing an electrode active material layer that is less susceptible to cracking due to drying, and a method for manufacturing a battery that includes such an electrode active material layer.

The present inventors have found that the above problems can be solved by the following means.
<Aspect 1>
coating a current collector foil with a slurry containing an electrode active material and a solvent; and drying the slurry while transporting the current collector foil coated with the slurry to obtain an electrode active material layer,
In the drying step, drying is performed at an evaporation rate that satisfies the following relational expressions (1) and (2):
Manufacturing method of electrode active material layer:
Relational formula (1): evaporation rate of the solvent (mg/cm ² ·sec)≦−2.46 × tap density of the electrode active material (g/cc) + 5.04
Relational formula (2): evaporation rate of the solvent (mg/cm ² ·sec)≦1.20.
<Aspect 2>
2. The method of claim 1, wherein the electrode active material has a tap density of 1.60 g/cc or greater and 2.00 g/cc or less.
<Aspect 3>
3. The method of claim 1 or 2, wherein the heat source in the drying step is hot air or a laser.
<Aspect 4>
Aspect 4. The method of any one of aspects 1 to 3, wherein the electrode active material layer has a thickness of 150 μm or more.
Aspect 5
Producing an electrode active material layer by the method according to any one of aspects 1 to 4;
How batteries are manufactured.

This disclosure provides a method for manufacturing an electrode active material layer that is less susceptible to cracking due to drying, and a method for manufacturing a battery that includes such an electrode active material layer.

FIG. 1 is a graph showing the relationship between the tap density of the electrode active material and the evaporation rate of the solvent, based on the data of Examples 2 to 5.

Embodiments of the present disclosure are described in detail below. Note that the present disclosure is not limited to the following embodiments, and various modifications can be made within the scope of the disclosure.

<<Method for manufacturing electrode active material layer>>
A method for producing an electrode active material layer according to the present disclosure includes applying a slurry containing an electrode active material and a solvent to a current collector foil, and drying the slurry while transporting the current collector foil to obtain an electrode active material layer. In the drying step, the method according to the present disclosure performs drying at an evaporation rate that satisfies the following relational expressions (1) and (2):
Relational formula (1): evaporation rate of the solvent (mg/cm ² ·sec)≦−2.46 × tap density of the electrode active material (g/cc) + 5.04
Relational formula (2): evaporation rate of the solvent (mg/cm ² ·sec)≦1.20

The present inventors believe that one of the causes of cracking of the electrode active material layer that occurs during drying is the aggregation of components in the coating film that occurs as the drying of the slurry containing the electrode active material progresses. Specifically, without intending to be bound by any theory, this is presumed to be as follows: As the drying of the coating film progresses, the components in the coating film aggregate. As a result, the remaining solvent evaporates from the gaps between the aggregated components, which is likely to cause bumping, and as a result, it is thought that the electrode active material layer is more likely to crack.

In response to this, the present inventors have discovered that cracking of the electrode active material layer due to drying is less likely to occur if the coating film is dried under conditions where the tap density of the electrode active material and the solvent evaporation rate of the slurry satisfy a specific relationship. Specifically, without intending to be bound by any theory, it is believed that drying the coating film under conditions where relationship formulas (1) and (2) are satisfied makes it easier for gaps to form between the components in the coating film even as drying progresses, and the solvent evaporates efficiently from these gaps, preventing bumping. As a result, it is believed that cracking of the electrode active material layer due to drying of the coating film is less likely to occur.

<Slurry coating process>
The method of the present disclosure includes applying a slurry containing an electrode active material and a solvent to a current collector foil.

There are no particular restrictions on the current collecting foil, as long as it can be used as a current collecting foil for a battery. For example, when constructing a lithium-ion battery, aluminum foil, copper foil, etc. can be used.

There are no particular restrictions on the electrode active material, as long as it can be used as an electrode active material in a battery. Therefore, the electrode active material may be either a positive electrode active material or a negative electrode active material. Two known active materials with different potentials (charge/discharge potentials) at which they absorb and release specific ions can be selected, and the material exhibiting a more noble potential can be used as the positive electrode active material, while the material exhibiting a more base potential can be used as the negative electrode active material.

Known active materials may be used as the positive electrode active material. For example, when configuring a lithium ion battery, various lithium-containing composite oxides such as lithium cobalt oxide, lithium nickel oxide, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , lithium manganate, and spinel-based lithium compounds may be used as the positive electrode active material. Furthermore, lithium iron phosphate (LFP) may be used as an olivine-type positive electrode active material. The positive electrode active material may be, for example, particulate, and the size thereof is not particularly limited.

Any known active material may be used as the negative electrode active material. For example, when constructing a lithium-ion battery, the negative electrode active material may be silicon-based active materials such as silicon, silicon alloys, and silicon oxide; carbon-based active materials such as graphite and hard carbon; various oxide-based active materials such as lithium titanate; metallic lithium, lithium alloys, etc. The negative electrode active material may be, for example, in particulate form, and there is no particular limit to its size.

In the context of this disclosure, the slurry may be an electrode mixture slurry. In the context of this disclosure, "electrode mixture slurry" refers to a slurry that contains an "electrode mixture" and a dispersion medium, and that can be applied and dried to form an electrode active material layer. Furthermore, the "electrode mixture" optionally contains a solid electrolyte, a conductive additive, and a binder in addition to the electrode active material.

The material for the solid electrolyte is not particularly limited, and any material that can be used as a solid electrolyte in a lithium-ion battery can be used. For example, the solid electrolyte may be a sulfide solid electrolyte.

Examples of sulfide solid electrolytes include, but are not limited to, sulfide amorphous solid electrolytes, sulfide crystalline solid electrolytes, and argyrodite-type solid electrolytes. Specific examples of sulfide solid electrolytes include Li ₂ S—P ₂ S ₅ systems (Li ₇ P ₃ S ₁₁ , Li ₃ PS ₄ , Li ₈ P ₂ S ₉ , etc.), Li ₂ S—SiS ₂ , LiI—Li ₂ S—SiS ₂ , LiI—Li ₂ S—P ₂ S ₅ , LiI—LiBr—Li ₂ S—P ₂ S ₅ , Li ₂ S—P ₂ S ₅ —GeS ₂ (Li ₁₃ GeP ₃ S ₁₆ , Li ₁₀ GeP ₂ S _{12 ,} etc.), LiI—Li ₂ S—P ₂ O ₅ , LiI—Li ₃ PO ₄ —P ₂ S ₅ , Li _7-x PS _6-x Cl _x, etc.; or combinations thereof.

The sulfide solid electrolyte may be glass or crystallized glass (glass ceramic).

The conductive additive is not particularly limited. For example, the conductive additive may be VGCF (Vapor Grown Carbon Fiber), acetylene black (AB), ketjen black (KB), carbon nanotubes (CNT), carbon nanofibers (CNF), etc., but is not limited to these.

The binder is not particularly limited. For example, the binder may be a material such as polyvinylidene fluoride (PVdF), butadiene rubber (BR), or styrene butadiene rubber (SBR), or a combination thereof, but is not limited to these.

There are no particular restrictions on the solvent, as long as it can disperse the electrode active material and does not significantly impair the properties of the electrode active material when dispersed. Examples of solvents include water and N-methyl-2-pyrrolidone (NMP).

The method for applying the slurry is not particularly limited, but blade application is an example.

<Slurry drying process>
The method of the present disclosure includes drying the slurry while transporting the current collector foil coated with the slurry to obtain an electrode active material layer.

The solvent evaporation rate can be adjusted by controlling the drying conditions during the drying process.

In the method of the present disclosure, the heat source in the drying step may be hot air or a laser. When the solvent is water and hot air is used, the drying conditions may be 5 m/s or more, 10 m/s or more, or 15 m/s or more, and 40 m/s or less, 35 m/s or less, or 30 m/s or less at 90°C, 3 m/s or more, 8 m/s or more, or 10 m/s or more, and 30 m/s or less, 25 m/s or less, or 20 m/s or less at 120°C, and 1 m/s or more, 5 m/s or more, or 10 m/s or more, and 20 m/s or less, 17 m/s or less, or 15 m/s or less at 150°C. When the solvent is water and a laser is used, the drying conditions may be such that the energy density is 0.5 W/ ^cm2 or more, 1.0 W/ ^cm2 or more, 1.5 W/ ^cm2 or more, 2.0 W/ ^cm2 or more, or 2.5 W/ ^cm2 or more, and 5.0 W/ ^cm2 or less, 4.5 W/ ^cm2 or less, 4.0 W/ ^cm2 or less, or 3.5 W/ ^cm2 or less.

One example of a method for transporting the current collecting foil is to use a transport roller.

In the method of the present disclosure, in the drying step, drying is performed at an evaporation rate that satisfies the following relational expressions (1) and (2).
Relational formula (1): Evaporation rate of solvent (mg/cm ² ·sec)≦−2.46 × Tap density of electrode active material (g/cc) + 5.04
Relational formula (2): Evaporation rate of solvent (mg/cm ² ·sec)≦1.20

As described above, one method for adjusting the solvent evaporation rate is to control the drying conditions. The solvent evaporation rate may be greater than 0 mg/cm ² ·sec, 0.10 mg/cm ² ·sec or more, 0.20 mg/cm ² ·sec or more, 0.30 mg/cm ² ·sec or more, or 0.40 mg/cm ² ·sec or more, and may be 1.00 mg/cm ² ·sec or less, 0.90 mg/cm ² ·sec or less, 0.80 mg/cm ² ·sec or less, 0.70 mg/cm ² ·sec or less, or 0.60 mg/cm ² ·sec or less. The solvent evaporation rate can be calculated from the weight change of the solvent evaporation amount using a weighing scale. A personal electronic balance from A&D Corporation can be used as the weighing scale.

The tap density of the electrode active material in the method of the present disclosure may be 1.60 g/cc or more and 2.00 g/cc or less. The tap density of the electrode active material may be 1.65 g/cc or more, and may be 1.90 g/cc or less, 1.80 g/cc or less, or 1.75 g/cc or less.

The tap density of the electrode active material can be measured using a common tapping-type density measuring device, such as a Tap Denser from Seishin Enterprise Co., Ltd., according to the method specified in JIS K1469:2003.

In the method of the present disclosure, when the slurry contains components other than the electrode active material, the electrode active material may be the main component of the electrode mixture. This prevents the solvent in the slurry from becoming less volatile due to components other than the electrode active material in the electrode mixture. Here, in the method of the present disclosure, "main component" means a component whose mass proportion in the electrode mixture is 50% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, or 90% by mass or more.

For example, if the electrode active material is not the main component of the electrode mixture, the value of the electrode mixture may be used as the tap density. This allows the ease of volatilization of the solvent in the slurry to be determined taking into account the influence of components other than the electrode active material in the electrode mixture.

The thickness of the electrode active material layer produced by the method of the present disclosure may be 150 μm or more, 200 μm or more, or 250 μm or more, and may be 500 μm or less, 450 μm or less, 400 μm or less, or 350 μm or less. Having the thickness of the electrode active material layer within the above range is preferable from the perspective of increasing the capacity of this electrode active material layer for use in, for example, BEVs and PHEVs.

<Battery manufacturing method>
The disclosed method of fabricating a battery includes fabricating an electrode active material layer.

For the method of manufacturing the electrode active material layer, please refer to the above description of the method of manufacturing the electrode active material layer disclosed herein.

The disclosed method for manufacturing a battery may include, in addition to manufacturing an electrode active material layer, arranging an anode current collector layer, an anode active material layer, an electrolyte layer, a cathode active material layer, and a cathode current collector layer in this order. In this case, the electrode active material layer manufactured by the disclosed method can be used for either or both of the anode active material layer and the cathode active material layer.

<<Preparation of electrode active material layer>>
Example 1
Lithium iron phosphate (LFP) with a tap density of 1.70 g/cc was used as the electrode active material. An electrode mixture containing 98 parts by mass of this LFP, 0.4 parts by mass of carboxymethyl cellulose (CMC), and 1.6 parts by mass of styrene butadiene rubber (SBR) was dispersed in water as a solvent to prepare an electrode mixture slurry (solid content of the electrode mixture slurry was 75% by mass). This slurry was applied to a current collector foil. While transporting the current collector foil with a conveying roller, the slurry was dried by applying hot air at 150 °C and 15 m/sec to obtain a 300 μm-thick electrode active material layer. The solvent evaporation rate during the drying process was 0.5 mg/cm ² sec.

Example 2
An electrode active material layer was obtained in the same manner as in Example 1, except that LFP having a tap density of 1.60 g/cc was used as the electrode active material and the slurry was dried by applying hot air at 120°C and ¹⁰ m/s. The solvent evaporation rate in this drying step was 1.1 mg/cm sec.

Example 3
Except for using LFP having a tap density of 1.75 g/cc as the electrode active material, an electrode active material layer was obtained in the same manner as in Example 1. The solvent evaporation rate in this drying step was 0.74 mg/cm ² sec.

Example 4
Except for using LFP having a tap density of 1.825 g/cc as the electrode active material, an electrode active material layer was obtained in the same manner as in Example 1. The solvent evaporation rate in this drying step was 0.54 mg/cm ² ·sec.

Example 5
Except for using LFP having a tap density of 1.90 g/cc as the electrode active material, an electrode active material layer was obtained in the same manner as in Example 1. The solvent evaporation rate in this drying step was 0.365 mg/cm ² ·sec.

Comparative Example 1
An electrode active material layer was obtained in the same manner as in Example 1, except that LFP having a tap density of 1.90 g/cc was used as the electrode active material.

Comparative Example 2
An electrode active material layer was obtained in the same manner as in Example ¹ , except that the slurry was dried by irradiating it with a laser having an energy density of ³ W/cm. The solvent evaporation rate in this drying step was 1.3 mg/cm.

In each of the above examples, the tap density of the electrode active material was measured using a tapping-type density measuring device (Tap Denser, manufactured by Seishin Enterprise Co., Ltd.) according to the method specified in JIS K1469:2003. The solvent evaporation rate was calculated from the weight change due to the amount of solvent evaporated using a weighing scale (a personal balance manufactured by A&D Corporation).

"evaluation"
The electrode active material layer prepared in each example was visually inspected for cracks.

"result"
A graph showing the relationship between the tap density and the solvent evaporation rate of the electrode active material based on Examples 2 to 5 is shown in Figure 1. The linear approximation in this graph corresponds to the relational expression (1) of the present disclosure. The electrode active material layers of the Examples produced by the method of the present disclosure, which satisfy the relational expressions (1) and (2), did not exhibit cracking. In contrast, the electrode active material layers of Comparative Examples 1 and 2, which do not satisfy the relational expression (1), exhibited cracking. Furthermore, bumping of the solvent was observed in the electrode active material layer of Comparative Example 2, whose solvent evaporation rate did not satisfy the relational expression (2).

Claims (5)

a drying step of applying a slurry containing an electrode active material and a solvent to a current collector foil, and drying the slurry while transporting the current collector foil coated with the slurry to obtain an electrode active material layer,
In the drying step, drying is performed at an evaporation rate that satisfies the following relational expressions (1) and (2):
Manufacturing method of electrode active material layer:
Relational formula (1): evaporation rate of the solvent (mg/cm ² ·sec)≦−2.46 × tap density of the electrode active material (g/cc) + 5.04
In the relational expression (1), the tap density of the electrode active material is a value measured by a method specified in JIS K1469:2003 using a tapping type density measuring device,
Relational formula (2): 0.30≦ evaporation rate of the solvent (mg/cm ² ·sec)≦1.20. 2. The method of claim 1, wherein the electrode active material has a tap density of 1.60 g/cc or more and 1.90 g/cc or less. The method of claim 1, wherein the heat source in the drying step is hot air or a laser. The method of claim 1, wherein the thickness of the electrode active material layer is 150 μm or more. A method for manufacturing a battery, comprising producing an electrode active material layer by the method described in any one of claims 1 to 4.