JP2023161721A - Manufacturing method of electrode - Google Patents

Abstract

To provide a manufacturing method of an electrode with reduced resistance.SOLUTION: A manufacturing method of an electrode includes the steps of forming an active material layer containing an active material and a binder, removing the binder on the surface of the active material layer by irradiating the surface of the active material layer with a pulsed laser having a pulse width of 100 ns or less.SELECTED DRAWING: Figure 2

Description

The present disclosure relates to a method of manufacturing an electrode.

Patent Document 1 (Japanese Unexamined Patent Publication No. 2011-171020) discloses a method of forming a region with less binder by irradiating the surface of an electrode layer of a nonaqueous electrolyte secondary battery with a continuous wave (CW) laser. ing.

Japanese Patent Application Publication No. 2011-171020

A non-aqueous electrolyte secondary battery as disclosed in Patent Document 1 avoids difficulty in penetrating the non-aqueous electrolyte due to swelling of the binder by irradiating the surface of the electrode layer with a CW laser, thereby reducing battery capacity. can be suppressed. However, when irradiated with a CW laser, the binder may melt and the resistance may increase.

Accordingly, it is an object of the present disclosure to provide a method for manufacturing electrodes with reduced resistance.

[1] The present disclosure includes a step of forming an active material layer containing an active material and a binder;
The method for manufacturing an electrode includes the step of removing the binder on the surface of the active material layer by irradiating the surface of the active material layer with a pulsed laser having a pulse width of 100 ns or less.

According to the manufacturing method [1] above, an electrode with reduced resistance can be manufactured.

That is, by using a pulsed laser with a pulse width of 100 ns or less, the binder present on the surface of the active material layer can be removed without dissolving it. In addition, since the binder is generally a resistive component, by removing the binder from the surface of the active material layer, the surface area of the active material in contact with the electrolyte increases, and the diffusion of lithium ions is promoted, resulting in resistance. can be reduced.

FIG. 1 is a schematic flowchart of the method for manufacturing an electrode in this embodiment. FIG. 2 is a schematic cross-sectional view of the active material layer in this embodiment. FIG. 3 is a schematic cross-sectional view illustrating the distribution of the binder in the active material layer in this embodiment. FIG. 4 shows the reference electrode No. of this embodiment. 1 and no. 4 is a SEM image showing the distribution of binder in the cross section of the electrode.

An embodiment of the present disclosure will be described below. However, the present disclosure is not limited thereto. Note that in this specification, the "positive electrode" and the "negative electrode" are collectively referred to as "electrode."

FIG. 1 is a flowchart outlining the method for manufacturing an electrode according to this embodiment. Hereinafter, "the method for manufacturing an electrode in this embodiment" may be abbreviated as "this manufacturing method". This manufacturing method includes at least forming an active material layer (a) and irradiating with a pulsed laser (b).

Note that the electrode manufactured in this embodiment is, for example, a sheet-shaped electrode (electrode sheet) for a lithium ion secondary battery. The electrode may be either a positive electrode or a negative electrode.

<<Formation of active material layer (a)>>
FIG. 2 is a schematic cross-sectional view of the active material layer in this embodiment. This manufacturing method includes a step of forming an active material layer 12 containing an active material 2 and a binder 1. Active material layer 12 may be formed by any method. For example, a slurry containing active material 2 and binder 1 may be made. For example, a slurry may be made by mixing the active material 2, the binder 1, the conductive material 3, and a solvent.

For example, the active material layer 12 may be formed by applying slurry to the surface of the current collector foil 11 using a die coater.

This manufacturing method may include drying the active material layer 12. For example, the active material layer 12 may be dried using a hot air drying oven. This manufacturing method may include compressing the active material layer 12. For example, the active material layer 12 may be compressed by roll pressing.

Active material layer 12 may have any thickness. The active material layer 12 may have a thickness of, for example, 5 to 1000 μm, 10 to 500 μm, or 50 to 250 μm. good.

(active material)
The active material 2 may be a positive electrode active material or a negative electrode active material. The active material 2 may be in the form of particles, or may be porous active material particles formed by aggregation of primary particles made of the active material 2.

Examples of the positive electrode active material include lithium-containing metal oxides, lithium-containing phosphates, and the like. Examples of lithium-containing metal oxides include LiCoO ₂ , LiNiO ₂ , and compounds represented by the general formula _LiNia Co _b O ₂ (wherein a+b=1, 0<a<1, 0<b<1). LiMnO ₂ , LiMn ₂ O ₄ , general formula _LiNia Co _b Mn _c O ₂ (wherein a+b+c=1, 0<a<1, 0<b<1, 0<c<1) ), LiFePO ₄ and the like. Here, examples of the compound represented by the general formula _LiNia Co _b Mn _c O ₂ include LiNi _1/3 Co _1/3 Mn _1/3 O ₂ and the like. Examples of the lithium-containing phosphate include LiFePO ₄ and the like.

The average particle size of the positive electrode active material may be, for example, about 1 to 25 μm. Note that the "average particle size" here means the particle size (D50) at an integrated value of 50% in a volume-based particle size distribution measured by a laser diffraction/scattering method.

Examples of negative electrode active materials include carbon-based negative electrode active materials such as graphite, graphitizable carbon, and non-graphitizable carbon, and alloy-based negative electrode active materials containing silicon (Si), tin (Sn), etc. Can be mentioned. The average particle diameter (D50) of the negative electrode active material particles may be, for example, about 1 to 25 μm.

(binder)
Binder 1 can bind solid materials together. The blending amount of the binder 1 may be, for example, 0.1 to 10 parts by mass based on 100 parts by mass of the active material. Binder 1 may contain any components. Examples of the binder 1 include carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and polyacrylic acid (PAA). One type of binder 1 may be used alone, or two or more types may be used in combination.

(conductive material)
The conductive material 3 can form an electron conduction path. The amount of the conductive material 3 may be, for example, 0.1 to 10 parts by mass based on 100 parts by mass of the active material 2. The conductive material 3 may contain arbitrary components. Examples of the conductive material 3 include carbon black, vapor-grown carbon fiber, carbon nanotubes, and graphene flakes. The conductive material 3 may be used alone or in combination of two or more.

(solvent)
Examples of the solvent include aqueous solvents and organic solvents. The aqueous solvent means water or a mixed solvent containing water and a polar organic solvent. For example, an appropriate dispersion medium can be selected depending on the type of active material, binder, etc.

As the aqueous solvent, water can be preferably used from the viewpoint of ease of handling. Examples of polar organic solvents that can be used in the mixed solvent include alcohols such as methanol, ethanol, and isopropyl alcohol, ketones such as acetone, and ethers such as tetrahydrofuran. Note that an aqueous solvent can be suitably used as a solvent for producing a negative electrode.

Examples of the organic solvent include N-methyl-2-pyrrolidone (NMP). Note that an organic solvent can be suitably used as a solvent for producing a positive electrode.

(Current collector foil)
The current collector foil 11 is a support for the active material layer 12. The current collector foil 11 may be, for example, in the form of a sheet or a band. As the current collector foil 11, for example, aluminum (Al) foil, Al alloy foil, copper (Cu) foil, Cu alloy foil, nickel (Ni) foil, Ni alloy foil, titanium (Ti) foil, Ti alloy foil, etc. Can be mentioned. When the electrode 10 is a positive electrode, the current collector foil 11 is, for example, Al foil. When the electrode 10 is a negative electrode, the current collector foil 11 is, for example, a Cu foil. The current collector foil 11 may have a thickness of, for example, 5 to 50 μm or 5 to 20 μm.

《Pulsed laser irradiation (b)》
This manufacturing method includes a step of removing the binder 1 on the surface of the active material layer 12 by irradiating the surface of the active material layer 12 with a pulsed laser having a pulse width of 100 ns or less. The entire surface of the active material layer 12 is irradiated with a pulsed laser whose pulse width is 100 ns or less. Specifically, the surface of the active material layer 12 opposite to the surface in contact with the current collector foil 11 is irradiated with a pulsed laser having a pulse width of 100 ns or less. Note that in this embodiment, "removing the binder 1 on the surface of the active material layer 12" does not mean completely removing the binder 1 from the surface of the active material layer 12; This means reducing the amount of binder 1 on the surface of the surface.

The pulse width of the pulsed laser is 100 ns or less. When the pulse width of the pulsed laser is 100 ns or less, the binder 1 present on the surface of the active material layer 12 can be removed without being dissolved, and the resistance can be reduced. The pulse width of the pulsed laser is preferably 1 ns or less, more preferably 0.01 ns or less, and even more preferably 0.001 ns or less. The pulse width of the pulsed laser only needs to exceed 0 ns, and may be 0.0001 ns or more.

The output of the pulse laser is, for example, 0.5W or more and 1000W or less. The wavelength of the pulsed laser is, for example, 200 nm or more and 1600 nm or less.

(Distribution of binder in active material layer)
The amount of binder 1 on the surface of active material layer 12 is smaller than the amount of binder 1 on other surfaces than the surface of active material layer 12. Here, the distribution of the binder 1 in the thickness direction of the active material layer 12 is determined, for example, by the following procedure.

First, a sample for cross-sectional observation is cut out from the electrode 10, and the cross-section is cleaned using a cross-section polisher (CP) or the like. Next, the binder is modified with a predetermined element or compound. For example, in the case of a binder containing fluorine (F) such as PVDF, mapping of F may be performed. When the binder does not contain F, an element serving as an index is appropriately selected. For example, in the case of a binder containing a carbon-carbon double bond such as SBR, the double bond can be modified with bromine (Br) or the like.

After modifying the binder 1, the cross section is analyzed using a SEM (Scanning Electron Microscope)-EPMA (Electron Probe Micro Analyzer) or SEM-EDX (Energy Dispersive X-ray Spectrometry) to perform predetermined elemental mapping. For example, with reference to FIG. 3, in the thickness direction of the cross section of the active material layer 12, a region 5 μm from the surface of the active material layer 12 is the first region 4 (the solid rectangular part in FIG. 3), and the active material layer A region of 5 μm located on the current collector foil side beyond 5 μm from the surface of 12 is defined as a second region 5 (the area surrounded by a dotted square in FIG. 3). Then, by dividing the integrated value of the detection intensity of the predetermined element in the first region 4 by the integrated value of the detection intensity of the predetermined element in the second region 5, the The distribution of the binder 1 (size relationship of contained elements) can be determined. Hereinafter, the value obtained by dividing the integrated value of the detection intensity of a predetermined element in the first region 4 by the integrated value of the detection intensity of a predetermined element in the second region 5 will be referred to as MI.

If Binder 1 is uniformly distributed, MI will be a value close to 1.0. In this embodiment, since the binder 1 on the surface of the active material layer 12 is removed, the MI is less than 1.0. Further, from the viewpoint of reducing resistance, MI is preferably 0.67 or less, more preferably 0.5 or less.

In this way, electrode 10 can be manufactured. The electrode 10 may be cut into a predetermined size using, for example, a slitter.

《Lithium ion secondary battery》
The electrode sheet obtained by the manufacturing method of the present disclosure can be used, for example, as an electrode of a lithium ion secondary battery (nonaqueous electrolyte secondary battery). The lithium ion secondary battery can be used as a power source for, for example, a hybrid vehicle (HV), an electric vehicle (EV), a plug-in hybrid vehicle (PHV), and the like. However, the electrode obtained by the manufacturing method of the present disclosure is not limited to such in-vehicle use, but can be applied to all kinds of uses.

For example, the lithium ion secondary battery may include a case. The case may be, for example, a metal container, a pouch made of a metal foil laminate film, or the like. The case houses an electrode group and an electrolyte.

The electrode group includes a positive electrode, a negative electrode, and a separator. A separator is placed between the positive electrode and the negative electrode. An electrolyte exists in the voids within the electrode group.

(Separator)
The separator is porous. The separator is electrically insulating. The separator may be, for example, a porous film made of polyethylene (PE), polypropylene (PP), or the like. The separator may have a thickness of, for example, 10 μm or more and 50 μm or less.

The separator may have a single layer structure. The separator may be formed only from a porous film made of PE, for example. The separator may have a multilayer structure. The separator may be formed, for example, by laminating a porous film made of PP, a porous film made of PE, and a porous film made of PP in this order.

(electrolyte)
The electrolyte includes a solvent and a supporting electrolyte. The solvent is aprotic. The solvent may contain optional ingredients. The solvent may be, for example, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), diethyl carbonate (DEC), or the like. One type of solvent may be used alone, or two or more types of solvents may be used in combination.

The supporting electrolyte is dissolved in a solvent. The supporting electrolyte may be, for example, LiPF ₆ , LiBF ₄ , LiN(FSO ₂ ) ₂ or the like. One type of supporting electrolyte may be used alone, or two or more types of supporting electrolytes may be used in combination. The supporting electrolyte may have a molar concentration of, for example, 0.5 mol/L or more and 2 mol/L or less.

The electrolyte may further include optional additives. The electrolytic solution may contain, for example, an additive in a mass fraction of 0.1% or more and 5% or less. The additive may be, for example, vinylene carbonate (VC), lithium difluorophosphate (LiPO ₂ F ₂ ), lithium fluorosulfonate (FSO ₃ Li), lithium bisoxalatoborate (LiBOB), and the like. One type of additive may be used alone, or two or more types of additives may be used in combination.

The present embodiment will be described below using Examples, but the present embodiment is not limited thereto.

[No. 1]
<Manufacture of positive electrode>
No. Electrode No. 1 (positive electrode) was manufactured as follows.

<<Formation of active material layer (a)>>
The following materials were prepared.

Positive electrode active material: NCM (nickel cobalt lithium manganate)
Binder: PVDF
Conductive material: AB
Solvent: NMP
Positive electrode current collector: Al foil A positive electrode slurry was prepared by mixing a positive electrode active material, a binder, a conductive material, and a solvent. A positive electrode active material layer was formed by applying the positive electrode slurry to the surface of the positive electrode current collector and drying it.

《Pulsed laser irradiation (b)》
The entire surface of the positive electrode active material layer formed above was irradiated with a pulsed laser having a pulse width of 0.0004 ns, an output of 25/W, and a wavelength of 1064 nm. As a result, No. One positive electrode was manufactured.

[No. 2-5]
No. 1 except that the pulse widths were changed to the values listed in Table 1. A positive electrode was manufactured in the same manner as in Example 1.

[No. 6]
No. 1 was irradiated with a CW laser with an output of 25/W and a wavelength of 1064 nm. A positive electrode was manufactured in the same manner as in Example 1.

<Manufacture of lithium ion secondary batteries>
(Manufacture of negative electrode)
The following materials were prepared.

Negative electrode active material: Natural graphite Binder: PVDF
Conductive material: AB
Solvent: Water Negative electrode current collector foil: Cu foil A negative electrode slurry was prepared by mixing a negative electrode active material, a binder, a conductive material, and a solvent. A negative electrode active material layer was formed by applying the negative electrode slurry to the surface of the negative electrode current collector and drying it. A negative electrode was manufactured by compressing the negative electrode active material layer.

(Separator)
A separator (porous membrane) having a thickness of 15 μm was prepared. This separator has a three-layer structure. The three-layer structure is constructed by laminating a porous layer made of PP, a porous layer made of PE, and a porous layer made of PP in this order.

(Preparation of electrolyte)
A mixed solvent was prepared by mixing EC, DMC and EMC. An electrolytic solution having the following composition was prepared by dissolving LiPF ₆ in a mixed solvent.

Electrolyte solvent: EC:DMC:EMC=3:4:3 (volume ratio)
_LiPF6 : 1.0mol/L
(Manufacture of lithium ion secondary batteries)
No. A laminate was formed by stacking each of the positive electrodes 1 to 6, the separator, and the negative electrode in this order. A pouch made of laminate film was prepared as the outer package. Each laminate was housed in the exterior body. An electrolyte solution was injected into each exterior body. After injecting the electrolyte, each exterior body was sealed. From the above, No. Lithium ion secondary batteries Nos. 1 to 6 were each manufactured.

<Reference>
No. The same material as the positive electrode in No. 1 was prepared. A positive electrode slurry was prepared by mixing a positive electrode active material, a conductive material, a binder, and a dispersion medium. A positive electrode active material layer was formed by applying the positive electrode slurry to the surface of the positive electrode current collector and drying it. A reference positive electrode was manufactured by compressing the positive electrode active material layer. That is, in manufacturing the reference positive electrode, no pulsed laser was irradiated. From now on, No. A reference battery was produced in the same manner as the lithium ion secondary batteries Nos. 1 to 6.

<Evaluation>
(Distribution of binder in active material layer)
By the above procedure, No. The distribution of the binder in the active material layers of Examples 1 to 6 and the reference positive electrodes was confirmed. The cross section was analyzed using SEM-EPMA and F element mapping was performed. The second region was a region 20±2.5 μm from the surface of the active material layer. MI was measured by dividing the integrated value of the detected intensity of F in the first region by the integrated value of the detected intensity of F in the second region. The results are shown in Table 1. Note that the MI of the reference positive electrode was 1.21.

(Battery resistance)
Battery resistance was measured. The battery resistance of each lithium ion secondary battery was relativeized and evaluated using the battery resistance of the lithium ion secondary battery related to the reference positive electrode as a reference (1.0). The results are shown in Table 1.

<Results>
No. 1, in which the binder on the surface of the active material layer was removed by irradiating the surface of the active material layer with a pulsed laser having a pulse width of 100 ns or less. In samples 1 to 4, reductions in MI and battery resistance were confirmed. It was also confirmed that MI and battery resistance were further reduced as the pulse width became smaller.

On the other hand, in No. 1, the binder on the surface of the active material layer was removed by irradiating the surface of the active material layer with a pulsed laser having a pulse width of 200 ns or less. In No. 5, MI was reduced, but battery resistance was not reduced. This has a pulse width of No. It is thought that the battery capacity decreased due to the active material disappearing due to the thermal effect.

In addition, No. 1 in which the surface of the active material layer was irradiated with a CW laser. In No. 6, the binder on the surface of the active material layer could not be removed, and MI and battery resistance were not reduced. This is considered to be because the binder was melted by irradiation with the CW laser and the battery resistance increased.

FIG. 4 shows a reference positive electrode, No. 1 and no. 4 is a SEM image showing the distribution of binder in the cross section of No. 4. In each image, the upper side shows the surface side of the active material layer, the lower side shows the current collector foil side of the active material layer, and the white part shows the binder. In the reference positive electrode, it was confirmed that the binder was also distributed on the surface of the active material layer. On the other hand, No. 1 and no. In No. 4, it was confirmed that the amount of binder was small on the surface of the active material layer, and that the binder on the surface of the active material layer was removed by irradiation with a pulsed laser having a pulse width of 100 ns or less.

The embodiments and examples disclosed this time should be considered to be illustrative in all respects and not restrictive. The scope of the present disclosure is indicated by the claims rather than the above description, and it is intended that all changes within the meaning and range equivalent to the claims are included.

Reference Signs List 1 binder, 2 active material, 3 conductive material, 4 first region, 5 second region, 10 electrode, 11 current collector foil, 12 active material layer.

Claims (1)

forming an active material layer containing an active material and a binder;
A method for manufacturing an electrode, comprising: removing the binder on the surface of the active material layer by irradiating the surface of the active material layer with a pulsed laser having a pulse width of 100 ns or less.

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