Secondary battery electrode and method of manufacturing the same

Abstract

This secondary battery electrode is provided with a core material configured from a metal foil that softens at less than or equal to 200°C, and an electrode mixture sheet which is bonded on the surface of the core material. The electrode mixture sheet includes an active material and a fibrous binder, and the active material bites into the core material, with the maximum bite depth at least 30% of the thickness of the core material. The fibrous binder may for example have polytetrafluoroethylene as the main component.

Description

[Technical Field]

[0001] The present disclosure relates to an electrode for a secondary battery and a method for manufacturing the same, and more particularly 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 technology]

[0002] Electrodes for nonaqueous 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, and the like is applied to the surface of a metal foil core material. This method requires a drying step to volatilize and remove the solvent contained in the coating film, and there is also the issue of the binder easily migrating during drying of the coating film. When binder migration occurs, the amount of binder becomes greater on the surface side of the coating film (electrode mixture layer) than on the core material side, resulting in a bias in the distribution of the binder in the thickness direction of the electrode mixture layer.

[0003] In recent years, a dry method has also been proposed in which an electrode mixture sheet is produced by rolling an electrode mixture into a sheet, and the sheet is then attached to a core material to produce an electrode (see, for example, Patent Document 1). Patent Document 1 describes the use of a composite particle powder containing an electrode active material and a binder in the production of an electrode mixture sheet. Patent Document 1 lists fluidized bed granulation and spray-drying granulation as methods for obtaining the composite particle powder. In both methods, a slurry is produced by dispersing or dissolving predetermined raw materials in a solvent, and the slurry is dried to produce a composite particle powder, which is then compressed to form a compacted powder layer.

[0004] Furthermore, Patent Document 2 discloses a method for manufacturing an electrode by a dry method, in which a hot-melt resin that is solid at room temperature, softens when heated, and solidifies again when cooled is used as a binder to bond an electrode mixture and a core material. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-77560 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-65478 Summary of the Invention

[0006] When a slurry containing a binder is used to produce an electrode, the powdery binder is dissolved in the solvent of the slurry, causing the binder to exhibit binding properties and ensuring adhesion within the composite layer and between the composite layer and the core material. However, the solvent needs to be dried from the slurry, making it difficult to reduce the labor required for the process and equipment. In both the fluidized bed granulation method and the spray drying granulation method disclosed in Patent Document 1, the solvent needs to be dried from the slurry.

[0007] On the other hand, in the production of electrodes using a dry method that does not use a solvent, it is not easy to firmly bond the electrode mixture sheet to the core material, and there is a problem that the electrode mixture sheet is prone to peeling. This is because the process does not go through a slurry containing a binder, and the adhesiveness obtained by dissolving the binder in a solvent cannot be utilized. Note that with the technology of Patent Document 2, the molten hot-melt resin widely covers the surface of the active material particles, which is thought to significantly increase the electrode plate penetration resistance.

[0008] The electrode for a secondary battery according to the present disclosure comprises a core material made of a metal foil that softens at 200°C or less, and an electrode composite sheet bonded to the surface of the core material, the electrode composite sheet including an active material and a fibrous binder, the active material embedded in the core material with a maximum embedding depth of 30% or more of the thickness of the core material.

[0009] The method for manufacturing an electrode for a secondary battery according to the present disclosure comprises mixing an active material and a fibrous binder without using a solvent to prepare an electrode mixture having a solids concentration of substantially 100%, rolling the electrode mixture into a sheet to prepare an electrode mixture sheet, placing the electrode mixture sheet on the surface of a core material made of a metal foil that softens at 200°C or less, and hot-pressing a laminate of the electrode mixture sheet and the core material at a temperature equal to or lower than the melting point of the fibrous binder.

[0010] According to one aspect of the present disclosure, it is possible to provide an electrode for a secondary battery having low electrode plate penetration resistance and high peel strength of an electrode mixture sheet. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram showing a manufacturing process of an electrode according to an embodiment. [Figure 2] FIG. 2 is a diagram showing a manufacturing process of an electrode according to an embodiment. [Figure 3] FIG. 3 is a cross-sectional view of an electrode according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of a secondary battery electrode and a manufacturing method thereof according to the present disclosure will be described in detail. The embodiments described below are merely examples, and the present disclosure is not limited to the following embodiments. Furthermore, the drawings referred to in the description of the embodiments are schematic, and the dimensional ratios of the components depicted in the drawings should be determined in consideration of the following description.

[0013] The secondary battery electrode according to the present 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. Note that the following description will be given taking a positive electrode for a non-aqueous electrolyte secondary battery as an example.

[0014] 1 and 2 are diagrams schematically illustrating a manufacturing process for a positive electrode 10 according to an embodiment, and FIG. 3 is a cross-sectional view of the positive electrode 10. As shown in FIG. 1(a), in the manufacturing process for the positive electrode 10, a positive electrode active material 21 (see FIG. 3) and a binder are dry-mixed without using a solvent to produce a positive electrode composite 20 having a solids concentration of substantially 100%. Note that dry mixing refers to a method in which positive electrode active material 21 particles and binder particles are mixed without using a solvent, so that the solids concentration of the positive electrode active material and the binder is substantially 100%. During dry mixing, a conductive material other than the positive electrode active material and the binder may also be added. Even when materials other than the positive electrode active material and the binder are added, the solids concentration during dry mixing is substantially 100%. The positive electrode composite 20 obtained by dry mixing includes the positive electrode active material 21 and a fibrous binder 22 (see FIG. 3).

[0015] Next, as shown in FIG. 1( b), the positive electrode mixture 20 is rolled and formed into a sheet to produce a positive electrode mixture sheet 12. Then, as shown in FIG. 2, the positive electrode mixture sheet 12 is placed on the surface of the core material 11, and the laminate of the core material 11 and the positive electrode mixture sheet 12 is heat-pressed at a temperature equal to or lower than the melting point of the fibrous binder 22. Through the above steps, a positive electrode 10 is produced in which the positive electrode mixture sheet 12 is bonded to the surface of the core material 11. In the positive electrode mixture sheet 12, the positive electrode active material 21 is held by the fibrous binder 22 present in a mesh pattern, and the bond between the positive electrode mixture sheet 12 and the core material 11 can be achieved by the anchor effect of the core material 11 on the positive electrode mixture sheet 12, without relying on the melting of the fibrous binder 22.

[0016] In the manufacturing process of the positive electrode 10, the fibrous binder 22 is preferably obtained by applying a shear force to the binder particles to fibrillate them.

[0017] [Positive electrode] As shown in FIG. 3, positive electrode 10 includes core material 11 and positive electrode composite sheet 12 bonded to the surface of core material 11. Positive electrode composite sheet 12 is preferably provided on both sides of core material 11. Positive electrode composite sheet 12 also includes fibrous binder 22 as a binder. Positive electrode 10 may be a long electrode plate constituting a wound electrode body, or may be a rectangular electrode plate constituting a stacked electrode body. Positive electrode 10 is manufactured by bonding positive electrode composite sheet 12 to core material 11 and then cutting it to a predetermined shape and dimensions.

[0018] A metal foil that softens at 200°C or less is used for the core material 11. If a core material that does not soften at 200°C or less is used, it is difficult to achieve both low electrode plate penetration resistance and high peel strength. In this specification, core material 11 that softens at 200°C or less refers to a core material in which metal grains grow with increasing temperature, and whose elongation rate when heated at 170°C is double that at room temperature, and whose tensile strength when heated at 170°C is 80% or less of that at room temperature.

[0019] The metal foil constituting the core material 11 is, for example, a metal foil containing at least aluminum, and is preferably an aluminum alloy foil containing aluminum as 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. Among these, an aluminum alloy foil containing 1.2 to 1.7 mass% iron (the aluminum content is, for example, 98.3 to 98.8 mass%) is preferred.

[0020] The thickness of core material 11 is preferably 5 to 20 μm, more preferably 8 to 15 μm. The softening temperature of core material 11 is preferably 130 to 200°C, more preferably 150 to 190°C. The softening temperature of core material is the temperature at which metal grains grow due to a temperature increase, the temperature at which the elongation of the core material becomes twice the elongation at room temperature, and the temperature at which the tensile strength of the core material becomes 80% or less of the tensile strength at room temperature. If the softening temperature of core material 11 is within this range, the core material is included in a metal foil that softens at 200°C or less, and this makes it easy to soften core material 11 in the heat pressing step without melting fibrous binder 22. As will be described in detail later, softening core material 11 promotes penetration of positive electrode active material 21 into core material 11, thereby firmly bonding positive electrode composite sheet 12 to core material 11.

[0021] Positive electrode mixture sheet 12 is provided on the surface of core material 11 and constitutes a mixture layer of positive electrode 10. Positive electrode mixture sheet 12 includes positive electrode active material 21 and fibrous binder 22, and has a thickness of, for example, 30 to 120 μm, preferably 50 to 100 μm. By using fibrous binder 22, it is possible to form positive electrode mixture 20 into a sheet by rolling. Note that positive electrode mixture sheet 12 may include a non-fibrillated binder in addition to fibrous binder 22.

[0022] To improve electronic conductivity, the positive electrode mixture sheet 12 preferably contains a conductive material 23. Examples of the conductive material 23 include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. The content of the conductive material 23 is, for example, 0.5 to 5.0 mass % with respect to the mass of the positive electrode mixture sheet 12. The volume-based median diameter (D50) of the conductive material 23 is, for example, 0.05 to 1 μm.

[0023] The positive electrode mixture sheet 12 is composed primarily of a positive electrode active material 21. The content of the positive electrode active material 21 is preferably 85 to 99 mass %, and more preferably 90 to 98 mass %, relative to the mass of the positive electrode mixture 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 23 is measured using a laser diffraction particle size distribution analyzer (LA-920, manufactured by Horiba, Ltd.) with water as the dispersion medium.

[0024] Generally, a lithium transition metal composite oxide is used for the positive electrode active material 21. 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, 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.

[0025] Fibrous binder 22 is attached to the particle surfaces of positive electrode active material 21 and is entangled with positive electrode active material 21. In other words, positive electrode active material 21 is held by fibrous binder 22 present in a mesh-like pattern. It is preferable that conductive material 23 is attached to the particle surfaces of positive electrode active material 21 without being taken up by fibrous binder 22. The content of fibrous binder 22 is, for example, 0.05 to 5.0 mass % with respect to the mass of positive electrode mixture sheet 12.

[0026] The fibrous binder 22 has a melting point higher than the softening temperature of the core material 11. The melting point of the fibrous binder 22 is preferably at least 50°C higher than the softening temperature of the core material 11, and more preferably at least 100°C higher. The fibrous binder 22 is made of, for example, a resin containing polytetrafluoroethylene (PTFE) as a main component and having a melting point of 300°C or higher. PTFE is easily fibrillated and has good adhesion to the positive electrode active material 21, making it suitable as a binder for the positive electrode mixture sheet 12. The fibrous binder 22 may be composed essentially of PTFE alone, or may contain other components to the extent that the object of the present disclosure is not impaired.

[0027] As described above, the positive electrode mixture sheet 12 may contain a non-fibrillating (non-fibrous) binder. An example of a non-fibrillating binder is polyvinylidene fluoride (PVdF). The melting point of PVdF is lower than that of PTFE, typically about 160°C. When fibrous PTFE and non-fibrous PVdF are used as binders, it is preferable to add more PTFE than PVdF. Specifically, the mass ratio is preferably PTFE = 1:0 ≦ PVdF ≦ 0.25, and more preferably PTFE = 1:0 ≦ PVdF ≦ 0.20.

[0028] When PVdF is contained in the positive electrode mixture sheet 12, the PVdF may melt depending on the temperature at which the laminate of the core material 11 and the positive electrode mixture sheet 12 is heat-pressed. Melting of PVdF contributes to strengthening the bonds between particles contained in the positive electrode mixture sheet 12 and between the positive electrode mixture sheet 12 and the core material 11. However, the positive electrode active material may be coated with molten PVdF, which may increase the plate penetration resistance. However, the particles contained in the positive electrode mixture sheet 12 are basically held in place by the fibrous binder 22 present in a mesh-like structure, and the bond between the positive electrode mixture sheet 12 and the core material 11 is achieved by the anchor effect of the core material 11 on the positive electrode mixture sheet 12. Therefore, the positive electrode mixture sheet 12 only needs to contain a small amount of PVdF compared to the fibrous binder 22, and an increase in the plate penetration resistance can be suppressed.

[0029] Positive electrode mixture sheet 12 may contain substantially only fibrous binder 22 (for example, fibrous PTFE) as the binder.

[0030] It is preferable that positive electrode mixture sheet 12 does not contain a large amount of binder that melts and covers the particle surfaces of positive electrode active material 21. Fibrous binder 22 exists in a fibrous state that has not undergone a melting process, and even when a binder such as PVdF is used in combination, only a small amount of PVdF is contained compared to fibrous binder 22. For this reason, the binder does not impede contact between positive electrode active material 21 and the electrolyte, and the plate penetration resistance can be kept low.

[0031] 3, in the positive electrode 10, the positive electrode active material 21 penetrates into the core material 11, with a maximum penetration depth D of 6.0 μm or more. Here, the penetration depth of the positive electrode active material 21 refers to the length along the thickness direction of the core material 11 from the surface of the core material 11 to the part where the positive electrode active material 21 penetrates most deeply. The penetration depth of the positive electrode active material 21 can be measured by observing a cross section of the positive electrode 10 using a scanning electron microscope (SEM).

[0032] As described above, the maximum penetration depth D of the positive electrode active material 21 is at least 6.0 μm, and preferably 7.0 μm or more. The maximum penetration depth D is, for example, preferably 30% or more of the thickness of the core material 11, and more preferably 40% or more. From the viewpoint of suppressing foil breakage, the upper limit of the maximum penetration depth D is preferably 55% or less of the thickness of the core material 11. It is preferable that most of the positive electrode active material 21 in contact with the core material 11 is embedded in the core material 11. For example, 50% or more of the positive electrode active material 21 in contact with the core material 11 is embedded in the core material 11.

[0033] The maximum penetration depth was evaluated by observing the cross section of the positive electrode using an SEM over a 0.25 mm range along the surface of the core material, and measuring the length along the thickness direction of the core material from the surface to the deepest part of the positive electrode active material that had penetrated the deepest. This length was taken as the maximum penetration depth. Alternatively, the cross section of the positive electrode can be observed over four 0.06 mm ranges, and measuring the length along the thickness direction of the core material from the surface to the deepest part of the positive electrode active material that had penetrated the deepest. This length was also taken as the maximum penetration depth.

[0034] The maximum penetration depth D of the positive electrode active material 21 can be controlled, for example, by the softening temperature of the core material 11 and the heating temperature and pressing pressure in the heat-pressing process. The lower the softening temperature of the core material 11 and the higher the heating temperature and pressing pressure, the larger the maximum penetration depth D. In the positive electrode 10, the positive electrode active material 21 penetrates deeply into the core material 11, and a large amount of the positive electrode active material 21 penetrates into the core material 11, resulting in a strong anchor effect and firmly bonding the positive electrode mixture sheet 12 to the core material 11. This allows for high peel strength of the positive electrode mixture sheet 12. However, if the heating temperature in the heat-pressing process is too high, the fibrous binder 22 will melt, increasing the plate penetration resistance.

[0035] [Negative electrode] The negative electrode includes 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 for the core material of the negative electrode. The negative electrode may be a conventionally known electrode plate manufactured by a wet method, or an electrode plate including a negative electrode composite sheet manufactured by a dry method. The negative electrode may have a configuration similar to that of the positive electrode 10 described above, including a core material made of metal foil that softens at 200°C or less and a negative electrode composite sheet bonded to the surface of the core material.

[0036] The negative electrode active material may be a carbon-based active material, such as natural graphite (e.g., flake graphite, lump graphite, or amorphous graphite), or artificial graphite (e.g., massive artificial graphite (MAG) or graphitized mesophase carbon microbeads (MCMB)). Alternatively, the negative electrode active material may be a Si-based active material that alloys with lithium. Since the carbon-based active material has higher electronic conductivity than the positive electrode active material 21, the negative electrode may not contain the conductive material 23.

[0037] [Nonaqueous electrolyte secondary battery] A nonaqueous electrolyte secondary battery, an example of an embodiment, includes an electrode assembly in which the above-described positive electrode 10 and negative electrode are stacked with a separator interposed therebetween, a nonaqueous electrolyte, and an exterior housing that houses these. The electrode assembly may be either a wound type electrode assembly or a laminated type electrode assembly. Examples of the exterior housing include a cylindrical exterior can, a rectangular exterior can, a coin-shaped exterior can, and an exterior housing made of an aluminum laminate sheet.

[0038] The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent include esters, ethers, nitriles, amides, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted compound in which at least a portion of the hydrogen atoms in these solvents are substituted with halogen atoms such as fluorine. The electrolyte salt may be, for example, a lithium salt such as LiPF6. The electrolyte is not limited to a liquid electrolyte, but may also be a solid electrolyte.

[0039] [Electrode manufacturing method] The method for manufacturing the positive electrode 10 will be described in more detail below. Although the method for manufacturing the positive electrode 10 will be exemplified below, this manufacturing method can also be applied to the manufacture of a negative electrode. In the case of a negative electrode, a negative electrode active material is used instead of a positive electrode active material.

[0040] As shown in FIG. 1(a), in the manufacturing process of a positive electrode 10, binder particles are placed in a mixer 40 and fibrillated by applying shear force. In this embodiment, a positive electrode active material 21, binder particles, and a conductive material 23 are placed in the mixer 40, and these materials are mixed while fibrillating the binder particles to produce a positive electrode composite 20 (hereinafter, this process will be referred to as the "first process"). Next, as shown in FIG. 1(b), the positive electrode composite 20 is rolled and formed into a sheet to produce a positive electrode composite sheet 12 (hereinafter, this process will be referred to as the "second process"). This manufacturing process is a dry process for manufacturing a positive electrode 10 using a positive electrode composite 20 with a solids concentration of substantially 100%.

[0041] The binder particles used in the first step are preferably polytetrafluoroethylene (PTFE) particles having a melting point of 300°C or higher. Non-fibrillating PVdF or the like may be added together with the PTFE particles. The mixer 40 may be, for example, a conventionally known mechanical stirring mixer. Specific examples of suitable mixers 40 include devices capable of applying mechanical shearing force, such as cutter mills, pin mills, bead mills, microparticle composite devices (devices in which shearing force is generated between a specially shaped rotor rotating at high speed inside a tank and an impact plate), granulators, and kneaders such as twin-screw extrusion kneaders and planetary mixers. Among these, cutter mills, microparticle composite devices, granulators, and twin-screw extrusion kneaders are preferred.

[0042] In the second step, the positive electrode mixture 20 is rolled using two rolls 30 to form a sheet. The two rolls 30 are arranged with a predetermined gap between them and rotate in the same direction. The positive electrode mixture 20 is supplied to the gap between the two rolls 30, whereby the positive electrode mixture 20 is compressed by the two rolls 30 and stretched into a sheet. The obtained positive electrode mixture sheet 12 may be passed through the gap between the two rolls 30 multiple times, or may be stretched one or more times using other rolls with different roll diameters, peripheral speeds, gaps, etc. Alternatively, the rolls may be heated to heat-press the positive electrode mixture sheet 12.

[0043] The thickness of the positive electrode mixture sheet 12 can be controlled by, for example, the gap between the two rolls 30, the peripheral speed, the number of stretching treatments, etc. In the second step, the positive electrode mixture 20 is preferably formed into a sheet using two rolls 30 having a peripheral speed ratio that differs by at least two times. By making the peripheral speed ratio of the two rolls 30 different, for example, it becomes easier to thin the positive electrode mixture sheet 12, improving productivity. The peripheral speed ratio of the two rolls 30 is preferably 2.5 times or more, and may be 3 times or more. The peripheral speed ratio of the two rolls 30 is, for example, 1:3.

[0044] Next, as shown in FIG. 2, positive electrode composite sheet 12 is bonded to core material 11, thereby obtaining positive electrode 10 in which a composite layer made of positive electrode composite sheet 12 is provided on the surface of core material 11 (hereinafter, this step will be referred to as "third step"). FIG. 2 shows a state in which positive electrode composite sheet 12 is bonded to only one surface of core material 11, but positive electrode composite sheet 12 is preferably bonded to both surfaces of core material 11. Two positive electrode composite sheets 12 may be bonded to both surfaces of core material 11 at the same time, or one sheet may be bonded to one surface of core material 11 and then the other sheet may be bonded to the other surface.

[0045] In the third step, positive electrode composite sheet 12 is placed on the surface of core material 11, and two rolls 31 are used to heat-press the laminate of core material 11 and positive electrode composite sheet 12 at a temperature equal to or lower than the melting point of the binder, thereby bonding positive electrode composite sheet 12 to the surface of core material 11. The two rolls 31 are, for example, arranged with a predetermined gap between them and rotate in the same direction at the same peripheral speed. At least one of the two rolls 31 is heated to a predetermined temperature by a heater. The press linear pressure is, for example, 0.2 [t / cm] to 5.0 [t / cm].

[0046] The heat pressing is preferably performed at a temperature equal to or lower than the melting point of the fibrous binder 22. In this case, the fibrous binder 22 does not melt, so the fibrous shape of the binder is maintained and the electrode plate penetration resistance can be kept low. The heat pressing is preferably performed at a temperature 50°C or more lower than the melting point of the fibrous binder 22, and more preferably at a temperature 100°C or more lower than the melting point of the fibrous binder 22.

[0047] Furthermore, it is preferable to use a metal foil that softens at 200°C or less and perform heat pressing at a temperature equal to or higher than the softening temperature of core material 11 and equal to or lower than the melting point of fibrous binder 22. Setting the heat pressing temperature to equal to or higher than the softening temperature of core material 11 promotes penetration of positive electrode active material 21 into core material 11, further improving the peel strength of positive electrode composite sheet 12. In this case as well, it is preferable to perform heat pressing at a temperature that is 50°C or higher than the melting point of fibrous binder 22, and more preferably at a temperature that is 100°C or higher than the melting point of fibrous binder 22.

[0048] When the fibrous binder 22 and a particulate binder such as PVdF are used in combination, the heat pressing is preferably performed at a temperature equal to or lower than the melting points of all the binders. In this case, the binders do not melt, so the shape of the binders (fibrous shape, particulate shape) is maintained, and the plate penetration resistance can be kept low.

[0049] In the third step, for example, an aluminum alloy foil containing 1.2 to 1.7 mass % iron and softening at 150 to 190°C is used as core material 11, and heat pressing is performed by setting the temperature of roll 31 to a temperature of 200°C or less. In this case, the set temperature of roll 31 is, for example, 130 to 200°C, and is preferably changed depending on the melting point of the binder contained in positive electrode mixture sheet 12. For example, the temperature of roll 31 is set to 145 to 155°C when PTFE and PVdF are used as the binder, and is set to 190 to 200°C when only PTFE is used.

[0050] The positive electrode 10 manufactured through the above steps has low electrode plate penetration resistance and high peel strength, as will be shown in the examples described later.

[0051] <Example> The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0052] Example 1 [Preparation of positive electrode mixture] The positive electrode active material, PTFE particles, and acetylene black were mixed in a mass ratio of 100:4.0:0.9 using a mixer (Osaka Chemical, Wonder Crusher). This mixing process fibrillated the PTFE particles, resulting in a positive electrode mixture in which the active material, fibrous PTFE, and acetylene black were uniformly dispersed. The resulting positive electrode mixture had a solids concentration of 100%.

[0053] [Preparation of positive electrode composite sheet] The resulting positive electrode composite was rolled between two rolls to produce a positive electrode composite sheet. The peripheral speed ratio of the two rolls was set to 1:3, and the sheet was stretched multiple times to a thickness of 130 μm.

[0054] [Preparation of positive electrode] The resulting positive electrode composite sheet was placed on the surface of a core material, and the laminate of the positive electrode composite sheet and core material was hot-pressed (pressing pressure: 2.0 [t / cm]) using two rolls heated to 150°C. This hot-pressing caused the positive electrode active material to penetrate into the core material, resulting in a positive electrode in which the positive electrode composite sheet was firmly bonded to the surface of the core material. The core material was an aluminum alloy foil with an iron content of 1.2 to 1.7 mass%, a softening temperature of 170°C, and a thickness of 15 μm.

[0055] The obtained positive electrodes were evaluated for the maximum penetration depth of the positive electrode active material into the core material, the electrode plate penetration resistance, and the peel strength of the positive electrode composite sheet by the methods described below. The evaluation results are shown in Table 1 together with the type of binder used, the temperature at which the core material softens (softening point), and the heat pressing temperature.

[0056] [Evaluation of maximum penetration depth] The cross section of the positive electrode was observed using an SEM within a range of 0.25 mm along the surface of the core material, and for the positive electrode active material that had penetrated deepest, the length along the thickness direction of the core material from the surface to the deepest part was measured, and this length was taken as the maximum penetration depth.

[0057] [Evaluation of plate penetration resistance] The positive electrode core material, with a positive electrode composite sheet bonded to only one side, was cut into a shape with tabs so that the positive electrode composite sheet was 20 mm x 20 mm. Two positive electrodes were stacked with the positive electrode composite sheets facing each other, and then sandwiched between two upper and lower Cu plates and pressurized at 5 MPa. The penetration resistance of the electrode plates was measured between the two tabs of the positive electrode and the upper and lower Cu plates using the DC four-probe method.

[0058] [Evaluation of peel strength] (1) Fix the positive electrode to the stand with the core material side facing the stand. (2) A portion of the positive electrode composite sheet is peeled off from the fixed positive electrode and bent at a 90° angle relative to the core material. (3) A positive electrode composite sheet bent at 90° was pulled using a universal testing machine, and the force required to peel the sheet was measured, and this force was taken as the peel strength.

[0059] <Example 2> A positive electrode was fabricated in the same manner as in Example 1, except that PVdF was used as the binder in addition to PTFE, and the performance evaluation was carried out as described above. The mass ratio of the positive electrode active material, PTFE, PVdF, and acetylene black was 100:4.0:0.8:0.9.

[0060] Example 3 A positive electrode was produced in the same manner as in Example 1 except that the heat press temperature was changed to 150°C, and the performance evaluation was carried out as described above.

[0061] <Comparative Example 1> A positive electrode was produced in the same manner as in Example 1, except that an aluminum alloy having a softening temperature of 270° C. was used as the core material, and the performance evaluation was carried out as described above.

[0062] [Table 1]

[0063] The evaluation results shown in Table 1 show that all of the positive electrodes of the examples were able to achieve both low electrode plate penetration resistance and high peel strength of the positive electrode composite sheet. It should be noted that the positive electrode of Comparative Example 1 was unable to bond the positive electrode composite sheet to the core material surface. That is, the core materials of the examples were able to achieve both the expression of the anchoring effect of the core material and the suppression of PTFE melting, whereas the positive electrode of Comparative Example 1, which was heat-pressed at the same temperature as the positive electrode of Example 1, was unable to achieve both the expression of the anchoring effect of the core material and the suppression of PTFE melting.

[0064] The positive electrodes of Examples 1 and 2 had higher peel strengths than the positive electrodes of Examples 3 and 4. This is thought to be because the heat press temperature in Examples 1 and 2 was higher than in Examples 3 and 4, which allowed the anchoring effect of the core material to the positive electrode mixture sheet to be fully exerted.

[0065] In the positive electrodes of the examples, no melting of the fibrous binder was observed. It was also confirmed that the positive electrode active material penetrated deep into the core material. In this case, a strong anchor effect is thought to be exerted, firmly bonding the positive electrode composite sheet to the core material. In particular, good results were obtained in Example 1. [Explanation of symbols]

[0066] 10 positive electrode 11 Core material 12 Positive electrode composite sheet 20 Positive electrode mixture 21 Cathode active material 22 Fibrous binder 23 Conductive materials 30,31 rolls 40 Mixer

Claims

1. A core material made of a metal foil that softens at 200°C or less; an electrode mixture sheet bonded to a surface of the core material; Equipped with the electrode mixture sheet includes an active material, a fibrous binder, and a non-fibrillating binder, and the active material, the fibrous binder, and the non-fibrillating binder are formed into a sheet shape by rolling; the active material is embedded in the core material, and the maximum embedding depth is 30% or more of the thickness of the core material; The fibrous binder maintains its fibrous shape, The non-fibrillated binder maintains its particle shape.

2. 2. The secondary battery electrode according to claim 1, wherein the fibrous binder is mainly composed of polytetrafluoroethylene.

3. 3. The electrode for a secondary battery according to claim 1, wherein the core material is an aluminum alloy foil containing 1.2 to 1.7 mass % of iron.

4. 4. The electrode for a secondary battery according to claim 1, wherein the melting point of the fibrous binder is higher than the softening point of the core material by 100° C. or more.

5. an active material, a fibrous binder, and a non-fibrillating binder are mixed without using a solvent to prepare an electrode mixture having a solids concentration of substantially 100%; The electrode mixture is rolled to form a sheet, thereby producing an electrode mixture sheet; A method for manufacturing an electrode for a secondary battery, comprising: placing the electrode mixture sheet on a surface of a core material made of a metal foil that softens at 200°C or less; and hot-pressing a laminate of the electrode mixture sheet and the core material at a temperature that is equal to or higher than the softening temperature of the core material and equal to or lower than the melting points of the fibrous binder and the non-fibrillating binder.

6. The method for manufacturing an electrode for a secondary battery according to claim 5 , wherein the heat pressing is performed at a temperature equal to or higher than the temperature at which the core material softens and lower by 50° C. or more than the melting point of the fibrous binder.

Images