Electrodes for secondary batteries and methods for manufacturing the same

Abstract

This electrode for secondary batteries is provided with a core material and an electrode mixture sheet that is bonded to the surface of the core material. The electrode mixture sheet contains an active material, a fibrous first binder and a particulate second binder; and the second binder is mainly composed of a polyvinylidene fluoride, while having a volume-based median diameter of 50 μm or less. The first binder is mainly composed, for example, of a polytetrafluoroethylene.

Description

Technical Field

[0001] The present disclosure relates to an electrode for a secondary battery and a method for manufacturing the same, and particularly relates to an electrode suitable for a non-aqueous electrolyte secondary battery such as a lithium ion battery and a method for manufacturing the same.

Background Art

[0002] Electrodes of non-aqueous electrolyte secondary batteries such as lithium ion batteries are generally manufactured by a wet method in which an electrode mixture slurry containing an active material, a binder, etc. is applied to the surface of a core material which is a metal foil. In this case, a drying process for volatilizing and removing the solvent contained in the coating film is required, and there is a problem that migration in which the binder moves during drying of the coating film easily occurs. When migration of the binder occurs, the amount of the binder becomes larger on the surface side than on the core material side of the coating film (electrode mixture layer), and a bias occurs in the distribution of the binder in the thickness direction of the electrode mixture layer.

[0003] In recent years, a dry method for manufacturing an electrode using a powdery electrode mixture without using a solvent has also been proposed. For example, Patent Document 1 describes a method for manufacturing an electrode by supplying and depositing a powder of an electrode mixture on the surface of a core material and applying pressure in the thickness direction while heating this deposited layer. Also, a method in which an electrode mixture sheet is formed by rolling an electrode mixture and then this electrode mixture sheet is bonded to a core material has been proposed.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

[0005] When using a slurry containing a binder during electrode fabrication, the powdered binder dissolves in the solvent of the slurry, causing it to exhibit its binding properties and ensuring adhesion within the asphalt layer and between the asphalt layer and the core material. However, as mentioned above, it is necessary to remove the solvent contained in the coating film by evaporation, making it difficult to reduce labor in the process and equipment.

[0006] On the other hand, in the manufacture of electrodes using a dry method without solvents, it is not easy to firmly bond the electrode composite sheet to the core material, and there is a problem that the electrode composite sheet is prone to peeling. This is because, since the process does not go through a slurry containing a binder, the adhesive properties obtained when the binder dissolves in the solvent cannot be utilized.

[0007] The electrode for a secondary battery according to this disclosure comprises a core material and an electrode composite sheet bonded to the surface of the core material, wherein the electrode composite sheet contains an active material, a fibrous first binder and a particulate second binder, and the second binder is mainly composed of polyvinylidene fluoride and has a volume-based median diameter of 50 μm or less.

[0008] The present disclosure relates to a method for manufacturing an electrode for a secondary battery, characterized by mixing an active material, a fibrous first binder, and a particulate second binder mainly composed of polyvinylidene fluoride with a volume-based median diameter of 50 μm or less without using a solvent to produce an electrode composite material with a solid content of substantially 100%, rolling the electrode composite material to form a sheet to produce an electrode composite material sheet, and heat-pressing a laminate of the electrode composite material sheet and a core material to bond the electrode composite material sheet to the surface of the core material.

[0009] According to one aspect of this disclosure, it is possible to provide an electrode for a secondary battery manufactured by a dry process, wherein the bonding force of the electrode composite sheet to the core material is strong and the peel strength of the electrode composite sheet is high. [Brief explanation of the drawing]

[0010] [Figure 1] Figure 1 shows the manufacturing process of an electrode, which is an example of an embodiment. [Figure 2] Figure 2 shows the manufacturing process of an electrode, which is an example of an embodiment. [Figure 3] Figure 3 is a cross-sectional view of an electrode, which is an example of an embodiment. [Figure 4] Figure 4 is a cross-sectional view of an electrode, which is another example of an embodiment. [Modes for carrying out the invention]

[0011] The embodiments of the secondary battery electrode and its manufacturing method described herein will be explained in detail below. 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 depicted in the drawings should be determined by referring to the following description.

[0012] The secondary battery electrode according to this disclosure is suitable for non-aqueous electrolyte secondary batteries such as lithium-ion batteries, but can also be applied to aqueous batteries containing aqueous electrolytes. In the following explanation, a positive electrode for a non-aqueous electrolyte secondary battery will be used as an example.

[0013] Figures 1 and 2 schematically show the manufacturing process of a positive electrode 10, which is an example of an embodiment, and Figure 3 is a cross-sectional view of the positive electrode 10. As shown in Figure 1(a), in the manufacturing process of the positive electrode 10, the positive electrode active material 21 (see Figure 3) and the binder are dry-mixed without the use of a solvent to produce a positive electrode composite material 20 with a solid content concentration of substantially 100%. Dry mixing is a method of mixing the positive electrode active material 21 and the binder without the use of a solvent in a state where the solid content concentration is substantially 100%. Conductive materials other than the positive electrode active material and binder can also be added during dry mixing. Even when materials other than the positive electrode active material and binder are added, the solid content concentration in dry mixing is substantially 100%.

[0014] Next, as shown in Figure 1(b), a positive electrode composite sheet 12 is produced by rolling the positive electrode composite material 20 into a sheet. Then, as shown in Figure 2, the laminate of the core material 11 and the positive electrode composite sheet 12 is heat-pressed to bond the positive electrode composite sheet 12 to the surface of the core material 11. Through these steps, a positive electrode 10 is manufactured in which a positive electrode composite layer consisting of the positive electrode composite sheet 12 is provided on the surface of the core material 11. As will be described in more detail later, the positive electrode composite sheet 12 includes a fibrous first binder 22 and a particulate second binder 23.

[0015] [Positive electrode] As shown in Figure 3, the positive electrode 10 comprises a core material 11 and a positive electrode composite sheet 12 bonded to the surface of the core material 11. Preferably, the positive electrode composite sheet 12 is provided on both sides of the core material 11. The positive electrode composite sheet 12 also contains a positive electrode active material 21, a fibrous first binder 22, and a particulate second binder 23. The positive electrode 10 may be a long electrode plate constituting a wound electrode body, or a rectangular electrode plate constituting a laminated electrode body. The positive electrode 10 is manufactured by bonding the positive electrode composite sheet 12 to the core material 11 and then cutting it to a predetermined shape and dimensions.

[0016] For example, a metal foil with a thickness of 5 to 20 μm is used for the core material 11. An example of a metal foil constituting the core material 11 is an aluminum-containing metal foil, preferably an aluminum alloy foil in which aluminum is the main component (the component with the highest mass ratio) and at least one metal selected from iron, manganese, copper, magnesium, zirconium, silicon, chromium, titanium, and nickel.

[0017] The positive electrode composite sheet 12 is provided on the surface of the core material 11 to form the composite layer of the positive electrode 10. As described above, the positive electrode composite sheet 12 includes a fibrous first binder 22 and a particulate second binder 23 as binders, and has a thickness of, for example, 30 to 120 μm, preferably 50 to 100 μm. By using the fibrous first binder 22, it becomes possible to roll the positive electrode composite 20 into a sheet. Furthermore, by using the second binder 23 in combination, the bonding strength of the positive electrode composite sheet 12 to the core material 11 is improved.

[0018] The positive electrode composite sheet 12 preferably contains a conductive material 24 to improve its electronic conductivity. Examples of conductive material 24 include carbon materials such as carbon black, acetylene black, Ketjen black, and graphite. The content of the conductive material 24 is, for example, 0.5 to 5% by mass relative to the mass of the positive electrode composite sheet 12. An example of the volume-based median diameter (D50) of the conductive material 24 is 0.05 to 1 μm.

[0019] The positive electrode composite sheet 12 is mainly composed of positive electrode active material 21. The content of positive electrode active material 21 is preferably 85 to 99% by mass, and more preferably 90 to 98% by mass, relative to the mass of the positive electrode composite sheet 12. The D50 of the positive electrode active material 21 is, for example, 1 to 30 μm, preferably 2 to 15 μm, and more preferably 3 to 15 μm. The D50 of the positive electrode active material 21 and the conductive material 24 is measured using a laser diffraction particle size distribution analyzer (Horiba, Ltd., LA-920) with water as the dispersion medium.

[0020] Generally, lithium transition metal composite oxides are used as the positive electrode active material 21. Examples of metal elements contained in lithium transition metal composite oxides include Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, and W. Among these, it is preferable to contain at least one of Ni, Co, and Mn. Examples of suitable composite oxides include lithium transition metal composite oxides containing Ni, Co, and Mn, and lithium transition metal composite oxides containing Ni, Co, and Al.

[0021] The first binder 22 adheres to the surface of the particles of the positive electrode active material 21 and is intertwined with the positive electrode active material 21. In other words, the positive electrode active material 21 is held by the first binder 22 existing in a network shape, and the sheet shape is maintained. The first binder 22 is mainly composed of, for example, polytetrafluoroethylene (PTFE), and is obtained by fibrillating PTFE particles. The first binder 22 may be composed of only PTFE, or may contain resin components other than PTFE as long as the object of the present disclosure is not impaired. The content of the first binder 22 is, for example, 0.05 to 10% by mass, preferably 0.1 to 8% by mass, and more preferably 0.2 to 5% by mass based on the mass of the positive electrode mixture sheet 12.

[0022] The second binder 23 is mainly composed of polyvinylidene fluoride (PVdF) and has a median diameter (D50) of 50 μm or less on a volume basis. The D50 of the second binder 23 is measured using a laser diffraction particle size distribution measuring device with water as a dispersion medium, similar to the positive electrode active material 21 and the like. PVdF is not fibrillated in the production process of the positive electrode mixture 20, and the state of the particles is maintained. The second binder 23 may be composed of only PVdF, or may contain resin components other than PVdF as long as the object of the present disclosure is not impaired. Hereinafter, the second binder 23 will be described as being PVdF particles.

[0023] As described above, PVdF particles with a D50 of 50 μm or less are used for the second binder 23. By using the first binder 22 and the second binder 23 with a D50 of 50 μm or less in combination, the bonding strength of the positive electrode mixture sheet 12 to the core material 11 is specifically improved, and the peel strength of the positive electrode mixture sheet 12 is significantly improved. Note that even if PVdF particles with a D50 exceeding 50 μm are used, the effect of improving the bonding strength is hardly obtained, and the peel strength hardly changes compared to the case where only the first binder 22 is used as the binder.

[0024] The D50 of the second binder 23 is more preferably 40 μm or less, and particularly preferably 30 μm or less. The lower limit value of D50 is not particularly limited, but is preferably 0.1 μm, more preferably 0.5 μm, and particularly preferably 1 μm. An example of a suitable range of D50 of the second binder 23 is 0.5 to 40 μm, or 1 to 30 μm. In this case, the bonding strength of the positive electrode composite sheet 12 to the core material 11 is more significantly improved. The D50 of the second binder 23 can be controlled to the target range, for example, by adjusting the polymerization conditions of PVdF or by grinding the polymerized PVdF.

[0025] The content of the second binder 23 is preferably less than the content of the first binder 22. As a specific example, in terms of mass ratio, the first binder 22: the second binder 23 = 1.5:1 to 10:1, or 2:1 to 8:1, or 3:1 to 7:1. The content of the second binder 23 is, for example, 0.1 to 5% by mass based on the mass of the positive electrode composite sheet 12, preferably 0.2 to 4% by mass, and more preferably 0.5 to 3% by mass. In this case, while suppressing an increase in the plate resistance, the peel strength of the positive electrode composite sheet 12 can be efficiently improved.

[0026] In the positive electrode 10, the positive electrode active material 21 may penetrate into the core material 11. The maximum penetration depth D of the positive electrode active material 21 is, for example, 30% or more of the thickness of the core material 11, and a specific example is 6 μm or more. Here, the penetration depth D of the positive electrode active material 21 means the length along the thickness direction of the core material 11 from the surface of the core material 11 to the deepest part where the positive electrode active material 21 penetrates. The penetration depth D can be measured by observing the cross section of the positive electrode 10 using SEM. The maximum penetration depth D can be controlled, for example, by the softening temperature of the core material 11, the heating temperature and the pressing pressure in the heat pressing process described below.

[0027] In the positive electrode 10, when the positive electrode composite sheet 12 is divided into two equal parts in the thickness direction and defined as the first region and the second region from the core material 11 side, it is preferable that the difference (ab) between the content of the second binder 23 in the first region (a) and the content of the second binder 23 in the second region (b) is within ±5%. The content (a) and (b) may be substantially the same.

[0028] In other words, the second binder 23 is evenly distributed throughout the entire positive electrode composite sheet 12, without being concentrated in a specific area. Such a uniform distribution of the binder can be achieved by a dry process that does not cause binder migration. Similarly, for the first binder 22, it is preferable that the difference (AB) between the content of the first binder 22 in the first region (A) and the content of the first binder 22 in the second region (B) is within the range of ±5%.

[0029] The concentration distribution of PVdF particles in the positive electrode composite sheet 12 is measured by the following method. (1) The positive electrode is immersed in an alkaline solution to convert the PVdF in the positive electrode composite sheet into polyene. (2) The PVdF in the cathode composite sheet treated in (1) is stained with bromine (Br). (3) The cross-section of the positive electrode containing the stained PVdF is measured using an electron beam microanalyzer (EPMA) to determine the concentration distribution of Br, which is then taken as the concentration distribution of PVdF.

[0030] Figure 4 is a cross-sectional view of a positive electrode composite sheet 13, which is another example of the embodiment. 4 As shown, the positive electrode composite sheet 13 has a multilayer structure including a first sheet 14 containing a positive electrode active material 21, a first binder 22, a second binder 23, and a conductive material 24, and a second sheet 15 containing the positive electrode active material 21, the first binder 22, and the conductive material 24, but substantially excluding the second binder 23. The first sheet 14 and the second sheet 15 are arranged in that order from the core material 11 side. The positive electrode composite sheet 13 makes it possible to efficiently improve the peel strength of the positive electrode composite sheet 13 while suppressing an increase in electrode plate resistance.

[0031] The first sheet 14 and the second sheet 15 are preferably manufactured by a dry process and are bonded together in the hot pressing process described later or before the hot pressing to form the positive electrode composite sheet 13. The thickness of each sheet is not particularly limited, and each sheet may have substantially the same thickness. Alternatively, the thickness of the first sheet 14 may be thinner than that of the second sheet 15. Similarly, the thickness of the second sheet 15 may be thinner than that of the first sheet 14. An example of the thickness of each sheet is 30 to 60 μm. The content of each component in the first sheet 14 is, for example, the same as in the case of the positive electrode composite sheet 12. Since the second sheet 15 substantially does not contain the second binder 23, the content of at least one of the positive electrode active material 21, the first binder 22, and the conductive material 24 can be increased compared to the first sheet 14.

[0032] [Negative electrode] The negative electrode comprises a core material made of metal foil and a negative electrode composite layer provided on the surface of the core material. Copper foil is generally used as the core material of the negative electrode. The negative electrode may use a conventionally known electrode plate manufactured by a wet process, or an electrode plate equipped with a negative electrode composite sheet manufactured by a dry process. The negative electrode may comprise a negative electrode composite sheet containing a fibrous first binder and a particulate second binder, and may have the same configuration as the positive electrode 10 described above.

[0033] The negative electrode active material can be a carbon-based active material such as natural graphite including flake graphite, lump graphite, or clay-like graphite, or artificial graphite including lump graphite (MAG) or graphitized mesophase carbon microbeads (MCMB). Alternatively, a Si-based active material alloyed with lithium may also be used as the negative electrode active material. Since carbon-based active materials have higher electronic conductivity than the positive electrode active material 21, the negative electrode does not necessarily need to contain a conductive material 24.

[0034] [Nonaqueous electrolyte secondary battery] An example of an embodiment of a non-aqueous electrolyte secondary battery comprises an electrode body in which the positive electrode 10 and negative electrode described above are stacked with a separator in between, a non-aqueous electrolyte, and an outer casing that houses these. The electrode body may be either a wound electrode body or a stacked electrode body. Examples of the outer casing include a cylindrical outer casing, a rectangular outer casing, a coin-shaped outer casing, and an outer casing made of an aluminum laminate sheet.

[0035] A non-aqueous electrolyte comprises a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of non-aqueous solvents include esters, ethers, nitriles, amides, and mixtures of two or more of these. The non-aqueous solvent may also contain halogen-substituted solvents in which at least some of the hydrogen atoms in the solvent are replaced with halogen atoms such as fluorine. Examples of lithium salts, such as LiPF6, are used as the electrolyte salt. The electrolyte is not limited to a liquid electrolyte but may also be a solid electrolyte.

[0036] [Manufacturing method for positive electrode] The manufacturing method for the positive electrode 10 will be described in detail below. The following example illustrates a manufacturing method for the positive electrode 10 including a conductive material, but this manufacturing method can also be applied to the negative electrode. In the case of the negative electrode, a negative electrode active material is used instead of the positive electrode active material, and the conductive material does not need to be added to the composite sheet.

[0037] As shown in Figure 1(a), in the manufacturing process of the electrode 10, firstly, positive electrode active material 21, PTFE particles, PVdF particles (second binder 23), and conductive material 24 are put into a mixer 40, and these materials are mixed while fibrillating the PTFE particles to produce electrode composite material 20 (hereinafter, this process will be referred to as the "first process"). Next, as shown in Figure 1(b), electrode composite material sheet 12 is produced by rolling the electrode composite material 20 into a sheet (hereinafter, this process will be referred to as the "second process"). This manufacturing process is a dry method for producing the electrode 10 using electrode composite material 20 with a solid content concentration of substantially 100%.

[0038] For the PTFE particles, for example, particles with a D50 of 5 to 100 μm are used. In this case, the mixing process can be carried out in a short time with relatively low shear force, resulting in less particle cracking of the positive electrode active material 21, good dispersibility of the constituent materials, and a positive electrode composite sheet 12 with high fracture strength. The PTFE particles are fibrillated in the first step to become a fibrous first binder 22. For the second binder 23, PVdF particles with a D50 of 50 μm or less, preferably 0.5 to 40 μm or 1 to 30 μm, are used. The PVdF is not fibrillated in the first step and maintains its particle shape and D50 before mixing.

[0039] While conventionally known devices can be used for the mixer 40, it is preferable to use a mechanically agitated mixer. Specific examples of suitable mixers 40 include devices capable of applying mechanical shear force, such as cutter mills, pin mills, bead mills, microparticle compounding devices (devices that generate shear force between a specially shaped rotor and impact plate that rotates at high speed inside a tank), granulators, twin-screw extruders, and planetary mixers. Among these, cutter mills, microparticle compounding devices, granulators, or twin-screw extruders are preferred.

[0040] As shown in Figure 1(b), in the second step, the electrode composite 20 is rolled using two rolls 30 to form a sheet. The two rolls 30 are positioned with a predetermined gap between them and rotate in the same direction. The electrode composite 20 is supplied into the gap between the two rolls 30 and compressed and stretched into a sheet by the two rolls 30. The two rolls 30 have, for example, the same roll diameter. The resulting electrode composite sheet 12 may be passed through the gap between the two rolls 30 multiple times, or it may be stretched one or more times using other rolls with different roll diameters, peripheral speeds, gaps, etc. Alternatively, the rolls may be heated and the electrode composite sheet 12 may be hot-pressed.

[0041] The thickness of the electrode composite sheet 12 can be controlled, for example, by the gap between the two rolls 30, the peripheral speed, the number of stretching cycles, etc. In the second step, it is preferable to form the electrode composite 20 into a sheet using two rolls 30 with a peripheral speed ratio that differs by more than twice. By making the peripheral speed ratios of the two rolls 30 different, for example, it becomes easier to thin the electrode composite sheet 12 and productivity is improved. The peripheral speed ratio of the two rolls 30 is preferably 2.5 times or more, and may also be 3 times or more. The peripheral speed ratio of the two rolls 30 is, for example, 1:3.

[0042] Next, as shown in Figure 2, the electrode composite sheet 12 is bonded to the core material 11 to obtain an electrode 10 having a composite layer made of the electrode composite sheet 12 on the surface of the core material 11 (hereinafter, this step will be referred to as the "third step"). Figure 2 shows a state in which the electrode composite sheet 12 is bonded to only one side of the core material 11, but it is preferable that the electrode composite sheet 12 be bonded to both sides of the core material 11. Two electrode composite sheets 12 may be bonded to both sides of the core material 11 simultaneously, or one may be bonded to one side of the core material 11 and then the other may be bonded to the other side.

[0043] In the third step, the electrode composite sheet 12 is bonded to the surface of the core material 11 using two rolls 31. The two rolls 31, for example, have the same roll diameter, are positioned with a predetermined gap between them, and rotate in the same direction at the same peripheral speed. Preferably, the two rolls 31 are heated to a predetermined temperature and subjected to a predetermined pressure.

[0044] As shown in the embodiment described below, the electrode 10 manufactured through the above process has a positive electrode composite sheet 12 with high peel strength that is firmly bonded to the core material 11.

[0045] <Examples> The present disclosure will be further illustrated by the following examples, but the present disclosure is not limited to these examples.

[0046] <Example 1> [Preparation of positive electrode composite material] The positive electrode active material, PTFE particles with a D50 of 10 μm, PVdF particles with a D50 of 25 μm, and acetylene black were mixed in a mass ratio of 100:4:0.8:0.9 using a mixer (Wonder Crusher, manufactured by Osaka Chemical). This mixing process fibrillated the PTFE particles, resulting in a positive electrode mixture in which the active material, fibrous PTFE, PVdF particles, and acetylene black were uniformly dispersed. The resulting positive electrode mixture had a solid content concentration of 100%.

[0047] [Preparation of positive electrode composite sheet] The obtained cathode composite material was passed between two rolls and rolled to produce a cathode composite material sheet. The peripheral speed ratio of the two rolls was set to 1:3, and the sheet was stretched multiple times to adjust the thickness of the cathode composite material sheet to 130 μm.

[0048] [Fabrication of the positive electrode] The obtained cathode composite sheet was placed on the surface of the core material, and the laminate of the cathode composite sheet and the core material was hot-pressed (press pressure: 0.2 [t / cm]) using two rolls heated to 200°C. This hot-pressing resulted in a cathode in which the cathode composite sheet was firmly bonded to the surface of the core material. An aluminum alloy foil with a thickness of 15 μm was used as the core material.

[0049] The peel strength of the resulting cathode composite sheet was evaluated using the method described below, and the evaluation results, along with the D50 of the PVdF particles (second binder) used, are shown in Table 1.

[0050] [Evaluation of peel strength] (1) Fix the positive electrode to the base with the core material side facing the base side. (2) Peel off a portion of the positive electrode composite sheet from the fixed positive electrode and bend it at a 90° angle relative to the core material. (3) Using a universal testing machine, the positive electrode composite sheet bent at a 90° angle was pulled, and the force required to peel the sheet was measured. This force was defined as the peel strength.

[0051] <Example 2> The cathode was prepared in the same manner as in Example 1, except that PVdF particles with a D50 of 10 μm were used as the second binder, and the peel strength was evaluated as described above.

[0052] <Comparative Example 1> The positive electrode was prepared in the same manner as in Example 1, except that the second binder was not used, and the peel strength was evaluated as described above.

[0053] <Comparative Example 2> The cathode was prepared in the same manner as in Example 1, except that PVdF particles with a D50 of 150 μm were used as the second binder, and the peel strength was evaluated as described above.

[0054] [Table 1]

[0055] As shown in Table 1, all of the positive electrodes in the examples had significantly higher peel strength compared to the positive electrodes in the comparative examples, indicating that the positive electrode composite sheet was strongly bonded to the core material. The positive electrode of Comparative Example 2, despite containing PVdF particles as a second binder, had a peel strength similar to that of the positive electrode of Comparative Example 1, which did not contain PVdF particles. In other words, the D50 of the second binder greatly affects the peel strength of the positive electrode composite sheet. When the particle size of the PVdF particles is large, the PVdF particles do not melt in a short-time hot pressing process and cannot exhibit sufficient adhesive strength. Even if they melt, the particle size is very large compared to the active material particles, and the amount added is small, so a sufficient bonding area with the active material particles cannot be secured, which is thought to be a factor in the lack of improvement in peel strength.

[0056] As described above, the peel strength of the positive electrode composite sheet is specifically improved only when a second binder with a D50 adjusted to a predetermined range is used together with a fibrous first binder. Table 1 shows the evaluation results using second binders with D50 of 10 μm and 25 μm, but high peel strength is obtained when D50 is 50 μm or less, preferably in the range of 1 to 30 μm. [Explanation of Symbols]

[0057] 10 positive electrode 11 Core material 12,13 Positive electrode composite sheet 14. Sheet 1 15. Second seat 20 Positive electrode composite material 21 Cathode active material 22 First binding agent 23. Second binding agent 24 Conductive materials 30, 31 rolls 40 Mixer

Claims

1. A first sheet comprising an active material, a fibrous first binder, a particulate second binder, and a conductive material, The multilayer structure includes the active material, the first binder, and the conductive material, and a second sheet which substantially does not include the second binder. The second binder is an electrode composite sheet made of polyvinylidene fluoride with a volume-based median diameter of 50 μm or less.

2. The electrode composite sheet according to claim 1, wherein when the electrode composite sheet is divided into two equal parts in the thickness direction and defined as a first region and a second region in order from the first sheet side, the difference between the content of the second binder in the first region (a) and the content of the second binder in the second region (b) is within ±5%.

3. The electrode composite sheet according to claim 1 or 2, wherein the first binder is polytetrafluoroethylene.

4. The content of the second binder is less than the content of the first binder. The first binder is included in an amount of 0.2 to 10% by mass relative to the mass of the electrode composite sheet. The electrode composite sheet according to any one of claims 1 to 3, wherein the second binder is included in an amount of 0.1 to 5% by mass relative to the mass of the electrode composite sheet.

5. A first electrode composite material with a solid content of substantially 100% is prepared by mixing an active material, a fibrous first binder, a particulate second binder which is polyvinylidene fluoride with a volume-based median diameter of 50 μm or less, and a conductive material without using a solvent. The first electrode composite material is rolled and formed into a sheet to produce a first electrode composite material sheet. The active material, the first binder, and the conductive material are mixed without the use of a solvent to prepare a second electrode composite material having a solid content concentration of substantially 100%, wherein the second binder is substantially absent. The second electrode composite material is rolled and formed into a sheet to produce a second electrode composite material sheet. A method for manufacturing electrodes for secondary batteries, comprising arranging the first and second electrode composite sheets in the order of the first electrode composite sheet and the second electrode composite sheet from the core material side, and heat-pressing the laminate of each sheet and the core material to bond the first and second electrode composite sheets to the surface of the core material.

6. An electrode composite sheet according to any one of claims 1 to 4, An electrode for a secondary battery comprising a core material, The electrode composite sheet is bonded to the surface of the core material. The electrode composite sheet is arranged in the order of the first sheet and the second sheet from the core material side, and is an electrode for a secondary battery.

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