Protective layer slurry and method for producing protective layer slurry

Abstract

A protective layer slurry according to one embodiment of the present disclosure is used for producing a protective layer which is formed on a surface of an electrode current collector, said protective layer slurry comprising: ceramic particles; normal polyvinylidene fluoride; acid-modified polyvinylidene fluoride; and a dispersion medium. The content of the normal polyvinylidene fluoride is 10-50 mass% with respect to the total mass of the acid-modified polyvinylidene fluoride and the normal polyvinylidene fluoride. The protective layer slurry is obtained by adding the normal polyvinylidene fluoride to a liquid mixture in which the ceramic particles have been mixed into the dispersion medium, and then further adding the acid-modified polyvinylidene fluoride.

Description

Protective layer slurry and method for producing the same

[0001] The present disclosure relates to a protective layer slurry and a method for producing the same, and more particularly to a protective layer slurry having few aggregates and high adhesion and a method for producing the same.

[0002] In recent years, non-aqueous electrolyte secondary batteries have been increasingly used as power sources for electric vehicles and energy storage devices for utilizing natural energy. The electrodes of non-aqueous electrolyte secondary batteries generally have an electrode current collector and an electrode mixture layer formed on the surface of the electrode current collector. From the viewpoint of safety, an insulating protective layer may be provided on the surface of the electrode current collector (see, for example, Patent Documents 1 and 2).

[0003] Japanese Patent Application Laid-Open No. 2013-45659

[0004] As a method for providing a protective layer, a method of applying a liquid protective layer slurry containing ceramic particles and a binder such as polyvinylidene fluoride to a predetermined position on the current collector and drying it is widely used. As a result of intensive studies by the present inventors, it has been found that when acid-modified polyvinylidene fluoride having a stronger adhesive force than ordinary polyvinylidene fluoride is used as a binder, the ceramic particles aggregate in the protective layer slurry. If the ceramic particles aggregate, the uniformity of the protective layer may be reduced.

[0005] As a result of intensive studies to solve the above problems, the present inventors have found that by using acid-modified polyvinylidene fluoride and ordinary polyvinylidene fluoride in a predetermined ratio and adjusting the timing of adding acid-modified polyvinylidene fluoride and ordinary polyvinylidene fluoride, a protective layer slurry having good dispersibility of ceramic particles and high adhesion can be obtained.

[0006] An object of the present disclosure is to provide a protective layer slurry in which the dispersibility of ceramic particles and high adhesion are compatible.

[0007] A protective layer slurry, according to one aspect of the present disclosure, is used in the manufacture of a protective layer formed on the surface of an electrode current collector, and comprises ceramic particles, ordinary polyvinylidene fluoride, acid-modified polyvinylidene fluoride, and a dispersion medium, wherein the content of ordinary polyvinylidene fluoride is 10% by mass or more and 50% by mass or less relative to the total mass of acid-modified polyvinylidene fluoride and ordinary polyvinylidene fluoride, and is obtained by adding ordinary polyvinylidene fluoride to a mixture of ceramic particles mixed in the dispersion medium, and further adding acid-modified polyvinylidene fluoride.

[0008] A method for producing a protective layer slurry, as described in this disclosure, is characterized by comprising the steps of: adding acid-modified polyvinylidene fluoride to a mixture of ceramic particles in a dispersion medium to pre-disperse the ceramic particles; and, after pre-dispersion, adding ordinary polyvinylidene fluoride to fully disperse the ceramic particles.

[0009] By using the protective layer slurry according to this disclosure, a protective layer can be obtained that has good dispersibility of ceramic particles and high adhesion.

[0010] This is a cross-sectional view of an electrode prepared using a protective layer slurry, which is an example of an embodiment. This is a flowchart showing the processing procedure in the method for manufacturing a protective layer slurry, which is an example of an embodiment. This is a flowchart showing the processing procedure in the method for manufacturing a protective layer slurry in Comparative Examples 1 to 3. This is a flowchart showing the processing procedure in the method for manufacturing a protective layer slurry in Comparative Example 4.

[0011] The following describes in detail embodiments of the protective layer slurry and the method for manufacturing the protective layer slurry according to this disclosure. The embodiments described below are merely examples, and this disclosure is not limited to these embodiments. Furthermore, the drawings referenced in the description of the embodiments are schematic, and the dimensional ratios of the components drawn in the drawings should be determined by referring to the following description.

[0012] [Electrodes] The electrodes according to this disclosure are electrodes used in non-aqueous electrolyte secondary batteries, such as lithium-ion secondary batteries. In this specification, an electrode refers to either or both of two electrodes with different polarities, namely a positive electrode and a negative electrode. A non-aqueous electrolyte secondary battery contains, for example, an electrode body and a non-aqueous electrolyte in an outer casing. The outer casing may be made of metal in the shape of a circle, square, coin, etc., or it may be made of a laminate sheet containing a metal layer and a resin layer. The electrode body may be a wound type in which a positive electrode and a negative electrode are wound around each other with a separator, or a laminate type in which multiple positive electrodes and multiple negative electrodes are stacked alternately one by one with a separator. The non-aqueous electrolyte contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. As the non-aqueous solvent, for example, esters, ethers, nitriles, amides, and mixed solvents of two or more of these can be used. The non-aqueous solvent may contain halogen-substituted products in which at least a portion of the hydrogen in these solvents is replaced with halogen atoms such as fluorine. As the electrolyte salt, for example, LiPF ６ Lithium salts such as the following are used.

[0013] Figure 1 is a cross-sectional view of an electrode 10 made using a protective layer slurry, which is an example of an embodiment. The electrode 10 includes an electrode current collector 11, a protective layer 12 formed on the surface of the electrode current collector 11, and an electrode mixture layer 13 formed on the surface of the protective layer 12. As shown in Figure 1, there may be areas on the surface of the protective layer 12 where the electrode mixture layer 13 is not formed. The electrode 10 may have the protective layer 12 and the electrode mixture layer 13 on both sides of the electrode current collector 11. The electrode 10 may be a long electrode constituting a wound electrode body, or a rectangular electrode constituting a laminated electrode body. The electrode 10 can be applied to a positive electrode, a negative electrode, or both. In the following description, a positive electrode in which the protective layer and the positive electrode mixture layer are formed on the surface of the positive electrode current collector will be used as an example, but the protective layer may be provided on the surface of the negative electrode current collector.

[0014] The electrode current collector 11 can be made of metal foil or a film with a metal layer formed on its surface. The thickness of the electrode current collector 11 is, for example, 5 to 20 μm. In the case of a positive electrode, the electrode current collector 11 can be made of metal foil mainly composed of aluminum. In the case of a negative electrode, metal foil mainly composed of copper can be used. In this specification, "main component" means the component with the highest mass ratio. The electrode current collector 11 may be aluminum foil that is substantially 100% aluminum, or copper foil that is substantially 100% copper.

[0015] The protective layer 12 includes, for example, ceramic particles and polyvinylidene fluoride as a binder. As will be described later, the polyvinylidene fluoride contained in the protective layer 12 includes ordinary polyvinylidene fluoride and acid-modified polyvinylidene fluoride. The protective layer 12 may also further contain a conductive agent. Examples of conductive agents included in the protective layer include acetylene black (AB), carbon black (CB) such as Ketjenblack, carbon nanotubes (CNT), graphene, and carbon-based particles such as graphite. These may be used individually or in combination of two or more types.

[0016] The electrode mixture layer 13 includes, for example, an active material, a conductive agent, a binder, etc. An example of the positive electrode active material (positive electrode active material) included in the electrode mixture layer 13 is a lithium transition metal composite oxide. Examples of metal elements contained in the lithium transition metal composite oxide include Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, W, etc. Among these, it is preferable to contain at least one of Ni, Co, and Mn. Examples of the negative electrode active material (negative electrode active material) included in the electrode mixture layer 13 include carbon-based active materials such as natural graphite such as flake graphite, lump graphite, and clay-like graphite, artificial graphite such as lump graphite (MAG) and graphitized mesophase carbon microbeads (MCMB), and Si-based active materials that alloy with lithium. An example of a Si-based active material is SiO ｘ A Si-containing compound represented by (0.5 ≤ x ≤ 1.6) (hereinafter referred to as SiO), or Li ２ｙ SiO（２＋ｙ） An example is a Si-containing compound (hereinafter referred to as LSX) in which fine Si particles are dispersed in a lithium silicate phase represented by (0 < y < 2). The active material is the main component of the electrode mixture layer 13, and the content of the active material in the electrode mixture layer 13 is preferably 85 to 99% by mass, and more preferably 90 to 99% by mass.

[0017] Examples of conductive agents included in the electrode mixture layer 13 include AB, CB such as Ketjenblack, CNTs, graphene, and other carbon-based particles. Examples of binders included in the electrode mixture layer 13 include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide, acrylic resin, polyolefin, styrene-butadiene rubber (SBR) or its modified forms, and carboxymethylcellulose (CMC).

[0018] The electrode 10 can be manufactured, for example, by applying a protective layer slurry (described later) to the surface of the electrode current collector 11, drying it to form a protective layer 12, then applying an electrode slurry containing an active material, a conductive agent, a binder, etc., to the surface of the protective layer 12, drying it to form an electrode mixture layer 13, and then rolling the protective layer 12 and the electrode mixture layer 13.

[0019] [Protective Layer Slurry] The protective layer slurry is used in the production of the protective layer 12. The protective layer slurry contains ceramic particles, ordinary polyvinylidene fluoride, acid-modified polyvinylidene fluoride, and a dispersion medium. The content of ceramic particles in the protective layer slurry is, for example, 50% by mass or more and 90% by mass or less, relative to the total mass of solids contained in the protective layer slurry. The sum of the content of ordinary polyvinylidene fluoride and the content of acid-modified polyvinylidene fluoride in the protective layer slurry is, for example, 10% by mass or more and 50% by mass or less, relative to the total mass of solids contained in the protective layer slurry. An example of a dispersion medium is N-methyl-2-pyrrolidone (NMP).

[0020] Examples of ceramic particles include alumina, zirconia, titania, and silica. The ceramic particles may include first ceramic particles with a D50 of 0.2 μm to 0.6 μm in the particle size distribution measured by laser diffraction, and second ceramic particles with a D50 of 1 μm to 3 μm in the particle size distribution measured by laser diffraction. This improves the linearity of the edge portion of the protective layer slurry when the protective layer slurry is applied to the surface of the electrode current collector, and improves the positional accuracy of the protective layer 12. D50 refers to the particle size at which the cumulative frequency of the smallest particle size accounts for 50% in the volume-based particle size distribution. The particle size distribution of the ceramic particles can be measured using a laser diffraction type particle size distribution analyzer (for example, MT3000II manufactured by Microtrac-Bell Co., Ltd.).

[0021] The content of the second ceramic particles in the protective layer slurry is, for example, 70% by mass or more and 80% by mass or less, relative to the total mass of the first ceramic particles and the second ceramic particles in the protective layer slurry.

[0022] In this specification, "acid-modified polyvinylidene fluoride" refers to a copolymer of vinylidene fluoride monomer and a monomer having an acidic functional group such as a carboxyl group, epoxy group, hydroxyl group, or carbonyl group. Furthermore, "ordinary polyvinylidene fluoride" refers to a homopolymer of vinylidene fluoride monomer, or a copolymer of vinylidene fluoride monomer and a monomer that does not have the above-mentioned acidic functional groups.

[0023] Acid-modified polyvinylidene fluoride exhibits stronger binding force to electrode current collectors compared to ordinary polyvinylidene fluoride. However, our investigations have revealed that ceramic particles tend to aggregate in a dispersion medium containing dissolved acid-modified polyvinylidene fluoride. On the other hand, in a dispersion medium containing dissolved ordinary polyvinylidene fluoride, ceramic particles can be dispersed almost uniformly without aggregation.

[0024] The typical polyvinylidene fluoride content is 10% to 50% by mass relative to the total mass of acid-modified polyvinylidene fluoride and typical polyvinylidene fluoride. By preparing the protective layer slurry using a process described later, while maintaining a typical polyvinylidene fluoride content of 10% to 50% by mass, a protective layer slurry can be obtained that achieves both high dispersibility of ceramic particles and high binding strength. If the typical polyvinylidene fluoride content is less than 10% by mass, the dispersibility of ceramic particles decreases. If the typical polyvinylidene fluoride content exceeds 30% by mass, the binding strength of the protective layer slurry decreases.

[0025] The weight-average molecular weight of typical polyvinylidene fluoride is, for example, between 200,000 and 400,000, while the weight-average molecular weight of acid-modified polyvinylidene fluoride is, for example, between 800,000 and 1,200,000. By setting the weight-average molecular weight of typical polyvinylidene fluoride within the above range, the dispersibility of ceramic particles in the protective layer slurry is further improved.

[0026] [Method for producing protective layer slurry] As shown in Figure 2, the protective layer slurry is obtained by adding ordinary polyvinylidene fluoride to a mixture of ceramic particles in a dispersion medium, and then adding acid-modified polyvinylidene fluoride. In other words, the method for producing the protective layer slurry includes the steps of adding acid-modified polyvinylidene fluoride to a mixture of ceramic particles in a dispersion medium to pre-disperse the ceramic particles, and, after pre-dispersion, adding ordinary polyvinylidene fluoride to fully disperse the ceramic particles.

[0027] For this dispersion, a commercially available bead mill may be used, for example. Furthermore, when stirring the protective layer slurry using a bead mill, ultrasonic waves may be applied, for example. This makes the ceramic particles easier to disperse. Stirring with a bead mill can be performed using, for example, alumina beads with a diameter of 600 μm to 1200 μm, at a peripheral speed of 8 m / sec to 12 m / sec, and a packing density of 50% to 80%. Note that the apparatus for this dispersion is not limited to a bead mill; for example, a commercially available cutter mill, pin mill, kneader, planetary mixer, etc., may also be used. Pre-dispersion can be performed more simply than this dispersion. Pre-dispersion can be performed manually without any special equipment, or with a simple agitator such as a disperser.

[0028] The present disclosure will be further illustrated below with reference to examples, but the present disclosure is not limited to these examples.

[0029] <Example 1> [Preparation of protective layer slurry] A protective layer slurry was prepared according to pattern 1 shown in Figure 2. As ceramic particles, a mixture of ceramic particles with a D50 of 2.2 μm and ceramic particles with a D50 of 0.4 μm was used in a mass ratio of 75:25. First, a mixture was prepared by mixing these ceramic particles with N-methyl-2-pyrrolidone (NMP) as a dispersion medium in a mass ratio of 80:20. Next, 10 parts by mass of ordinary polyvinylidene fluoride was added to 100 parts by mass of ceramic particles. The weight-average molecular weight of ordinary polyvinylidene fluoride was 280,000. After that, the mixture was manually stirred until the ordinary polyvinylidene fluoride dissolved (pre-dispersion). Next, 10 parts by mass of acid-modified polyvinylidene fluoride was added to this mixture to 100 parts by mass of ceramic particles. The weight-average molecular weight of acid-modified polyvinylidene fluoride was 1 million. Finally, the mixture was stirred using a bead mill (WAB Corporation, Dyno-Mill) while applying ultrasound to perform the final dispersion. Alumina beads with a diameter of 1000 μm were used at a peripheral speed of 10 m / sec and a packing density of 70%.

[0030] [Preparation of test specimens] A protective layer slurry was applied to one side of aluminum foil, the coating was dried, and then it was rolled using a rolling mill and cut to prepare test specimens with a width of 15 mm and a length of 80 mm.

[0031] [Evaluation of Dispersibility] The particle size distribution of ceramic particles in the protective layer slurry was measured using a laser diffraction particle size distribution analyzer (Microtrac-Bell Co., Ltd., MT3000II). The D90 of the protective layer slurry was calculated from the obtained particle size distribution to evaluate its dispersibility. D90 refers to the particle size at which the cumulative frequency of the smallest particle size accounts for 90% in the volume-based particle size distribution.

[0032] [Evaluation of Peel Strength] Double-sided tape (manufactured by Nitto Denko Corporation) was attached to the protective layer on the surface of the test specimen, and it was fixed to a stainless steel substrate with a smooth surface. The stainless steel substrate to which the test specimen was fixed was set up so that it was horizontal. One end of the protective layer in the longitudinal direction of the test specimen was fixed to a movable fixture of a tensile testing machine (product name: Tensilon Universal Testing Machine RTC1210, manufactured by A&D Co., Ltd.), and it was set to peel the protective layer in a direction of 90° to the substrate surface of the stainless steel substrate. The movable fixture was then moved to peel the aluminum foil of the test specimen from the protective layer at a speed of 20 mm / min. At that time, the tensile direction was always maintained at 90° to the substrate surface of the stainless steel substrate to which the test specimen was fixed. The stable tensile strength value when the test specimen was peeled off by 30 mm or more was read and was taken as the peel strength (N / m) of the protective layer from the aluminum foil.

[0033] <Example 2> Except for the addition of 20 parts by mass of acid-modified polyvinylidene fluoride in the preparation of the protective layer slurry, the protective layer slurry and test specimens were prepared and evaluated in the same manner as in Example 1.

[0034] <Comparative Example 1> Except for changing the processing procedure in the preparation of the protective layer slurry to Pattern 2 in Figure 3, and adding 20 parts by mass of ordinary PVDF to 100 parts by mass of ceramic, the protective layer slurry and test specimens were prepared and evaluated in the same manner as in Example 1.

[0035] <Comparative Example 2> In preparing the protective layer slurry, the procedure was changed to Pattern 2, and 20 parts by mass of acid-modified PVDF was added to 100 parts by mass of ceramic. Otherwise, the protective layer slurry and test specimens were prepared and evaluated in the same manner as in Example 1. In Pattern 2, only one of ordinary polyvinylidene fluoride or acid-modified polyvinylidene fluoride was added, and the timing of the addition was before pre-dispersion.

[0036] <Comparative Example 3> Except for changing the processing procedure in the preparation of the protective layer slurry to Pattern 2, and adding 30 parts by mass of ordinary PVDF to 100 parts by mass of ceramic, the protective layer slurry and test specimens were prepared and evaluated in the same manner as in Example 1.

[0037] <Comparative Example 4> In the preparation of the protective layer slurry, the protective layer slurry and test specimens were prepared and evaluated in the same manner as in Example 1, except that the processing procedure was changed to Pattern 3 in Figure 4. The timing of adding acid-modified polyvinylidene fluoride differs between Pattern 3 and Pattern 1 in Figure 2. In Pattern 3, acid-modified polyvinylidene fluoride is added at the same time as normal polyvinylidene fluoride.

[0038] Table 1 shows the evaluation results for the examples and comparative examples. Dispersibility was judged as good (○) if the D90 value in the particle size distribution diameter was 10 μm or less, and poor (×) otherwise. In addition, due to the specifications of the equipment, the peel strength could only be measured up to 100 N / m.

[0039]

[0040] The protective layer slurries of Examples 1 and 2 exhibited good dispersibility, and the peel strength of the protective layer was above the measurement limit. On the other hand, Comparative Examples 1 and 3, which did not contain acid-modified polyvinylidene fluoride, did not exhibit sufficient peel strength. Furthermore, Comparative Example 2, which contained only acid-modified polyvinylidene fluoride, and Comparative Example 4, which contained acid-modified polyvinylidene fluoride added simultaneously with ordinary polyvinylidene fluoride, showed poor dispersibility of ceramic particles in the protective layer slurry, and were unable to form a protective layer.

[0041] 10 Electrode, 11 Electrode current collector, 12 Protective layer, 13 Electrode mixture layer

Claims

1. A protective layer slurry used in the manufacture of a protective layer formed on the surface of an electrode current collector, comprising ceramic particles, ordinary polyvinylidene fluoride, acid-modified polyvinylidene fluoride, and a dispersion medium, wherein the content of ordinary polyvinylidene fluoride is 10% by mass or more and 50% by mass or less based on the total mass of the acid-modified polyvinylidene fluoride and the ordinary polyvinylidene fluoride, and the protective layer slurry is obtained by adding ordinary polyvinylidene fluoride to a mixture of the dispersion medium and the ceramic particles, and further adding the acid-modified polyvinylidene fluoride.

2. The protective layer slurry according to claim 1, wherein the weight-average molecular weight of the ordinary polyvinylidene fluoride is 200,000 or more and 400,000 or less, and the weight-average molecular weight of the acid-modified polyvinylidene fluoride is 800,000 or more and 1,200,000 or less.

3. The protective layer slurry according to claim 1 or 2, wherein the ceramic particles include first ceramic particles having a D50 of 0.2 μm or more and 0.6 μm or less in the particle size distribution determined by laser diffraction, and second ceramic particles having a D50 of 1 μm or more and 3 μm or less in the particle size distribution determined by laser diffraction.

4. A method for producing a protective layer slurry, comprising the steps of: adding acid-modified polyvinylidene fluoride to a mixture of ceramic particles in a dispersion medium to pre-disperse the ceramic particles; and, after the pre-dispersion, adding ordinary polyvinylidene fluoride to fully disperse the ceramic particles.

Images