Electrode and method for manufacturing electrode

Abstract

This electrode is obtained by superposing an electrode mixture material on the surface of a core material, said electrode mixture material containing a coated active material, wherein a conductive material adheres to the surface of an active material, and a PTFE powder. With respect to this electrode, the coverage of the surface of the coated active material by the conductive material is from 10% to 60%; the mixture material resistance of the electrode mixture material is 20 Ωcm or less; in cases where the electrode mixture material is divided into three equal parts in the thickness direction, namely into a first region, a second region and a third region from the core material side, the content (a) of the PTFE powder in the first region, the content (b) of the PTFE powder in the second region and the content (c) of the PTFE powder in the third region satisfy -10% ≤ (c – a) / (a + b + c) ≤ 10%.

Description

[Technical Field]

[0001] The present disclosure relates to electrodes and methods for manufacturing electrodes. [Background technology]

[0002] Electrodes for nonaqueous electrolyte secondary batteries such as lithium-ion batteries are generally produced 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, and the resulting coating is then dried and compressed. This method presents a problem of the tendency for the binder to migrate during drying. When binder migration occurs, the amount of binder is 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 been studied in which an electrode mixture is rolled into a sheet to produce an electrode mixture sheet, and the sheet is then attached to a core material to produce an electrode. Patent Document 1 discloses an electrode film (electrode mixture) produced by mixing an active material, a particulate binder, and a conductive material using a mill, and then treating the mixture under high pressure and large shearing force for a long period of time to fibrillate the binder. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2019-512872 Summary of the Invention

[0005] As a result of investigations by the present inventors, it has been found that, as disclosed in Patent Document 1, when a large shear force is applied to an electrode mixture for a long period of time, the conductive material adheres to the binder instead of the active material, resulting in a significant decrease in the mixture resistance, and as a result, a decrease in the discharge capacity of the battery.

[0006] An electrode according to one embodiment of the present disclosure is an electrode in which an electrode composite including a coated active material having a conductive material adhered to the surface of the active material and PTFE powder is laminated on the surface of a core material, wherein the coverage of the conductive material on the surface of the coated active material is 10% to 60%, the composite resistance of the electrode composite is 20 Ωcm or less, and when the electrode composite is divided into three equal parts in the thickness direction into a first region, a second region, and a third region from the core material side, the PTFE powder content (a) in the first region, the PTFE powder content (b) in the second region, and the PTFE powder content (c) in the third region satisfy -10%≦(ca) / (a+b+c)≦10%.

[0007] A method for manufacturing an electrode according to one embodiment of the present disclosure includes a mixing step of mixing a coated active material having a conductive material adhered to the surface of the active material with PTFE powder to produce electrode composite particles having a solid content of substantially 100%, a rolling step of rolling the electrode composite particles to form them into a sheet to produce an electrode composite sheet, and a laminating step of laminating the electrode composite sheet to a core material to produce an electrode.

[0008] According to one aspect of the present disclosure, an electrode with low composite resistance can be obtained, and the discharge capacity of a battery can be increased. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a cross-sectional view of an electrode according to an embodiment. [Figure 2] FIG. 2 is a diagram showing an outline of a mechanofusion reactor, which is an apparatus for producing a coated active material in one example of the embodiment. [Figure 3] FIG. 3(a) is a diagram showing a mixing step in the manufacturing process of an electrode according to an embodiment, and FIG. 3(b) is a diagram showing a rolling step. [Figure 4] The figure shows a bonding step in the manufacturing process of an electrode according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of an electrode and an electrode manufacturing method 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.

[0011] [electrode] The 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 batteries containing aqueous electrolytes or power storage devices such as capacitors. The following description will be given taking as an example an electrode for a non-aqueous electrolyte secondary battery (particularly when applied to a positive electrode).

[0012] FIG. 1 is a cross-sectional view of an electrode according to an embodiment. Electrode 10 includes a core material 11 and an electrode composite material 12 provided on the surface of core material 11. As shown in FIG. 1, electrode 10 may include electrode composite materials 12 on both sides of core material 11. Electrode 10 may be a long electrode constituting a wound electrode body, or a rectangular electrode constituting a stacked electrode body. Electrode 10 may be used as a positive electrode, a negative electrode, or both of a nonaqueous electrolyte secondary battery.

[0013] The core material 11 may be a metal foil or a film with a metal layer formed on its surface. The thickness of the core material 11 is, for example, 5 μm to 20 μm. In the case of a positive electrode, the core material 11 may be a metal foil containing aluminum as its main component. In the case of a negative electrode, the core material 11 may be a metal foil containing copper as its main component. In this specification, the term "main component" refers to the component with the highest mass ratio. The core material 11 may be an aluminum foil that is substantially 100% aluminum, or a copper foil that is substantially 100% copper.

[0014] The electrode mixture 12 includes a coated active material and PTFE powder. The thickness of the electrode mixture 12 is, for example, 30 μm to 120 μm, and preferably 50 μm to 100 μm. In addition to the PTFE powder, the electrode mixture 12 may also include a binder such as non-fibrillating polyvinylidene fluoride (PVdF).

[0015] The coated active material is an active material having a conductive material attached to its surface. The coverage of the conductive material on the surface of the coated active material is preferably 10% to 60%, and more preferably 20% to 60%. Since the coverage of the conductive material is sufficiently high, the battery characteristics of the electrode can be improved. It is preferable that the surface of the coated active material has irregularities, and the conductive material penetrates into the recesses of these irregularities and adheres to them. This makes it difficult for the conductive material on the surface of the coated active material to be removed by the PTFE powder during the mixing process of the coated active material and the PTFE powder, as described below. As described below, the coverage of the conductive material can be increased by relatively shortening the time for mixing the PTFE powder with the active material and the conductive material. If the mixing process is performed for a short time, the increase or decrease in the conductive material before and after the mixing process is within ±5%.

[0016] The coated active material can be produced by dry-mixing an active material and a conductive material. Examples of dry-mixing methods include mechanofusion. Mechanofusion is a dry processing method carried out in a mechanofusion reactor 15, as shown in FIG. 2, which has a cylindrical chamber 16 that rotates at high speed and is equipped with a compression tool 17 inside. The rotation speed is typically greater than 1000 rpm. The conductive material and active material are placed in the chamber 16, and the chamber 16 is rotated, causing the particles to be pressed against each other and against the walls of the chamber 16. The use of the compression tool 17 and the centrifugal force generated by the high-speed rotation promotes adhesion bonding between the conductive material and the active material. Examples of the mechanofusion reactor 15 include the "Nobilta" (registered trademark) pulverizer or the "Mechanofusion" (registered trademark) pulverizer manufactured by Hosokawa Micron Corporation (Japan), the "Hybridizer" (trademark) pulverizer manufactured by Nara Machinery Works, Ltd., the "Balance Gran" manufactured by Freund-Turbo Corporation, and the "COMPOSI" manufactured by Nippon Coke & Engineering Co., Ltd.

[0017] Lithium transition metal composite oxides are generally used for the positive electrode active material (positive electrode active material). Metal elements contained in the 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, at least one of Ni, Co, and Mn is preferably contained. Carbon-based active materials, such as natural graphite (e.g., flake graphite, lump graphite, and amorphous graphite), and artificial graphite (e.g., massive artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB)), are used for the negative electrode active material. Alternatively, Si-based active materials that can be alloyed with lithium may be used for the negative electrode active material. The active material is the main component of the electrode mixture 12, and the content of the active material in the electrode mixture 12 is preferably 85% to 99% by mass, and more preferably 90% to 99% by mass.

[0018] The positive electrode active material is, for example, a secondary particle formed by the aggregation of multiple primary particles. This creates irregularities on the surface of the positive electrode active material, allowing the conductive material to penetrate and adhere to the recesses of these irregularities as described above. The particle size of the primary particles constituting the secondary particles is, for example, 0.05 μm to 1 μm. The particle size of the primary particles is measured as the diameter of the circumscribed circle in a particle image observed with a scanning electron microscope (SEM). The positive electrode active material is a particle having a volume-based median diameter (D50) of, for example, 3 μm to 30 μm, preferably 5 μm to 25 μm, and particularly preferably 7 μm to 15 μm. D50 refers to the particle size at which the cumulative frequency of the smallest particle size in the volume-based particle size distribution is 50%, and is also referred to as the median diameter. The particle size distribution of the positive electrode active material can be measured using a laser diffraction particle size distribution analyzer (e.g., MT3000II, manufactured by Microtrack Bell Corporation) using water as a dispersion medium.

[0019] Examples of the conductive material contained in the electrode mixture 12 include carbon materials such as carbon black (CB), acetylene black (AB), ketjen black, carbon nanotubes (CNT), and graphite. The particle diameter of the conductive material is, for example, 0.01 μm to 0.1 μm. This allows the conductive material to penetrate and adhere to recesses on the surface of the positive electrode active material. The content of the conductive material in the electrode mixture 12 is, for example, 0.5% to 5.0% by mass.

[0020] The PTFE powder is contained as a binder in the electrode mixture 12. The PTFE powder is a dry powder, not a powder dispersed in a dispersion such as water. This allows the electrode mixture to be produced by a dry method, which will be described later.

[0021] The content of PTFE powder in electrode mixture 12 is, for example, 0.5% by mass to 5.0% by mass. The PTFE powder adheres to the particle surfaces of the coated active material and is entangled with the coated active material. In other words, the coated active material is held in place by the PTFE powder present in a network shape.

[0022] In one embodiment, the PTFE powder contains fibrous particles with an aspect ratio of 1.5 or more at a ratio of 20% to 60% of the total particles. The average major axis size of the fibrous particles is 1 μm to 20 μm (hereinafter, fibrous particles of this shape will be referred to as fibrous particles A). By using this PTFE powder, an electrode mixture sheet with good moldability and high breaking strength can be produced. The ratio of fibrous particles A to the total particles can be calculated as follows. The ratio of fibrous particles B to the total particles, which will be described later, can also be measured in a similar manner. (1) The PTFE powder containing the fibrous particles A is photographed with a scanning electron microscope (SEM). The photographing magnification can be, for example, 300 to 1000 times. (2) The captured image is imported into a computer, and all particles are separated into fibrous particles A and particles with an aspect ratio of less than 1.5 using image analysis software such as ImageJ. (3) The number of fibrous particles A is divided by the number of all particles, i.e., the sum of the number of fibrous particles A and particles with an aspect ratio of less than 1.5, to calculate the ratio of fibrous particles A to all particles.

[0023] The average major axis size of fibrous particles A can be calculated by analyzing SEM images with image analysis software, measuring the major axis size (major axis length) of 100 fibrous particles with an aspect ratio of 1.5 or more, and averaging the measured values, in the same way as in the calculation of the ratio of fibrous particles A to all particles described above. The average minor axis size of fibrous particles B, which will be described later, can also be measured in the same way.

[0024] The average aspect ratio of the fibrous particles A may be 2 to 20. The average aspect ratio of the fibrous particles A can be calculated, similarly to the calculation of the average major axis size, by analyzing SEM images with image analysis software, measuring the aspect ratios (major axis / minor axis) of 100 fibrous particles having an aspect ratio of 1.5 or more, and averaging the measured values. The average aspect ratio of the fibrous particles B, which will be described later, can also be measured in a similar manner.

[0025] PTFE powder containing fibrous particles A can be produced by fibrillating a PTFE raw material (PTFE particles), which is a fibrillable fine powder, using a dry grinder such as a jet mill grinder. The PTFE raw material may be secondary particles. The average particle diameter of the PTFE raw material is, for example, 100 μm to 700 μm, preferably 100 μm to 500 μm, and more preferably 100 μm to 400 μm. The average particle diameter of the PTFE raw material can be determined by observing the PTFE raw material particles with an SEM. Specifically, the external shapes of 100 randomly selected particles are identified, and the major axis (longest diameter) of each of the 100 particles is determined, and the average value is taken as the average particle diameter of the PTFE raw material. When producing PTFE powder containing fibrous particles A using a jet mill grinder, the ratio of the fibrous particles A to all particles can be adjusted to 20% to 60% by appropriately adjusting the supply rate of the PTFE raw material, the grinding pressure, etc.

[0026] In another embodiment, the PTFE powder contains fibrous particles having an aspect ratio of 5 or more at a ratio of 60% or more of all particles. The PTFE powder preferably contains fibrous particles having an aspect ratio of 5 or more at a ratio of 80% or more of all particles. The average minor axis size of the fibrous particles is 1 μm to 20 μm (hereinafter, fibrous particles having this shape will be referred to as fibrous particles B). By using this PTFE powder, an electrode mixture sheet having good moldability and high breaking strength can be produced.

[0027] PTFE powder containing fibrous particles B can be produced by fibrillating a PTFE raw material (PTFE particles), which is a fine powder that can be fibrillated, using a dry mill such as an airflow mill. The same PTFE raw material can be used as when producing the PTFE powder containing fibrous particles A. When producing PTFE powder containing fibrous particles B using an airflow mill, the ratio of fibrous particles B to all particles can be adjusted to 60% or more by appropriately adjusting the feed rate of the PTFE raw material, the blade rotation speed, gap, etc.

[0028] The median diameter of the PTFE powder containing fibrous particles A and / or B is preferably 2 μm to 20 μm. The median diameter can be measured with a particle size distribution analyzer. The median diameter of the PTFE powder containing fibrous particles A and / or B being 2 μm to 20 μm means that the PTFE powder containing fibrous particles A and / or B has a finer size than the PTFE particles of the PTFE raw material.

[0029] The composite resistance of the electrode composite 12 is 20 Ωcm or less. This allows the discharge capacity of the battery to be increased. The composite resistance is the resistance of only the electrode composite 12. The composite resistance can be measured, for example, using an electrode resistance measurement system manufactured by Hioki E.E. Corporation as follows. (1) Prepare an electrode 10 cut to a size of 20 mm x 50 mm. (2) The thickness of the electrode composite material 12 and the thickness of the core material 11 are measured and input as measurement parameters into the measurement conditions. (3) Select the appropriate mains current and voltage range. (4) The electrode 10 is set in a predetermined position, the probe is brought into contact, and the composite resistance is measured.

[0030] When the electrode mixture 12 is divided into three equal parts in the thickness direction, namely, a first region, a second region, and a third region from the core material 11 side, the PTFE powder content (a) in the first region, the PTFE powder content (b) in the second region, and the PTFE powder content (c) in the third region satisfy −10%≦(ca) / (a+b+c)≦10%, and more preferably −5%≦(ca) / (a+b+c)≦5%. That is, by making the difference between the PTFE powder content (a) in the first region near the surface and the PTFE powder content (c) in the third region near the core material 11 within a range of ±10%, preferably ±5%, of the total PTFE powder content (a+b+c), the PTFE powder can be distributed substantially uniformly throughout the electrode mixture 12, without being distributed unevenly in some parts.

[0031] The electrode mixture 12 is preferably a mixture in which the coated active material and the PTFE powder are uniformly dispersed. Furthermore, the electrode mixture 12 preferably has minimal particle cracking of the active material, and most of the conductive material adheres to the particle surfaces of the active material, forming conductive paths between the particles. In other words, the electrode mixture 12 must be prepared so that particle cracking of the active material is suppressed while the conductive material is not incorporated into the PTFE powder, preventing a decrease in the amount of conductive material adhering to the particle surfaces of the active material. The manufacturing method described below makes it possible to prepare a high-quality electrode mixture 12 that satisfies these conditions.

[0032] [Electrode manufacturing method] The method for manufacturing the electrode 10 will be described in more detail below. While the method for manufacturing a positive electrode will be exemplified below, this 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.

[0033] 3 and 4 are diagrams schematically illustrating the manufacturing process of an electrode 10, which is an example of an embodiment. The manufacturing method of the electrode 10 includes a mixing step shown in FIG. 3(a), a rolling step shown in FIG. 3(b), and a laminating step shown in FIG. 4. In the mixing step, a coated active material and a PTFE powder are mixed to prepare electrode mixture particles 12a having a solid content concentration of substantially 100%. In the rolling step, the electrode mixture particles 12a are rolled and formed into a sheet to prepare an electrode mixture sheet. In the laminating step, the electrode mixture sheet is laminated to a core material to prepare an electrode.

[0034] The method for manufacturing electrode 10 is a dry process for manufacturing electrode 10 using electrode mixture 12 having a solid content concentration of substantially 100%. The dry process is a process in which active material particles and binder particles are mixed without using a solvent, that is, the active material and binder are mixed in a state in which the solid content concentration of the active material and binder is substantially 100%. The method for manufacturing electrode 10 according to the present disclosure does not require the use of a solvent as in conventional methods for manufacturing electrode 10. Not requiring the use of a solvent not only means that a solvent is not required as a raw material, but also means that a solvent drying process is not required, and exhaust equipment and the like related to the drying process are also not required.

[0035] In the mixing step, raw materials such as coated active material and PTFE powder are mixed in a mixer 20 to produce electrode composite particles 12a. By using a coated active material prepared by mixing an active material and a conductive material, the time required for the mixing step to obtain an electrode composite in which the coated active material and PTFE powder are uniformly dispersed can be shortened. If the mixing process is performed for a long time, the conductive material is incorporated into the binder, and the coverage of the conductive material on the surface of the active material becomes less than 10%. By using a coated active material, the dispersibility of the constituent materials can be improved, thereby reducing the composite resistance. Furthermore, by shortening the mixing process time, cracking of the active material during the mixing process can be suppressed. The active material contained in the electrode may include cracks that occur during the mixing process and cracks that occur during the rolling step described below.

[0036] The mixer 20 can be, for example, a conventional mechanical agitator mixer. Specific examples of suitable mixers 20 include devices capable of applying mechanical shear force, such as cutter mills, pin mills, bead mills, microparticle composite devices (devices in which shear force is generated between a specially shaped rotor rotating at high speed inside a tank and an impact plate), granulators, twin-screw extrusion mixers, and planetary mixers. Cutter mills, microparticle composite devices, granulators, and twin-screw extrusion mixers are preferred. This allows the PTFE powder to be further fibrillated while mixing the raw materials. The processing time for the mixing step (the time during which shear force is applied to the materials) is preferably within a few minutes, for example, 0.5 to 4 minutes. If the processing time is too long, the amount of conductive material incorporated into the PTFE powder increases. This significantly reduces the conductivity of the electrode mixture sheet, increasing its resistance and adversely affecting battery characteristics. Furthermore, the longer the processing time, the more PTFE fibrillation progresses. Excessive fibrillation reduces the breaking strength of the sheet. Since the PTFE powder already contains a predetermined amount of fibrillated fibrous particles A and / or fibrous particles B, a treatment time of 0.5 minutes or more allows the PTFE powder to adhere to the particle surfaces of the active material and become entangled with the active material.

[0037] As shown in FIG. 3(b), in the rolling step, the electrode mixture particles 12a are rolled using two rolls 22 to form a sheet. The two rolls 22 are arranged with a predetermined gap between them and rotate in the same direction. The electrode mixture particles 12a are supplied to the gap between the two rolls 22, whereby they are compressed by the two rolls 22 and stretched into a sheet. The two rolls 22 have, for example, the same roll diameter. The obtained electrode mixture sheet 12b may be passed through the gap between the two rolls 22 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 electrode mixture particles 12a.

[0038] The thickness of the electrode mixture sheet 12b can be controlled by, for example, the gap between the two rolls 22, the peripheral speed, the number of times of stretching, etc. In the rolling step, the electrode mixture particles 12a are preferably formed into a sheet using two rolls 22 with a peripheral speed ratio that differs by two times or more. By making the peripheral speed ratio of the two rolls 22 different, for example, it becomes easier to thin the electrode mixture sheet 12b, improving productivity.

[0039] Next, as shown in Fig. 4, in the bonding step, the electrode mixture sheet 12b is bonded to the core material 11, thereby obtaining an electrode 10 in which a mixture layer made of the electrode mixture 12 is provided on the surface of the core material 11. Although Fig. 4 shows a state in which the electrode mixture 12 is bonded to only one surface of the core material 11, it is preferable that the electrode mixture 12 is bonded to both surfaces of the core material 11. Two sheets of the electrode mixture 12 may be bonded to both surfaces of the core material 11 at the same time, or one sheet may be bonded to one surface of the core material 11 and then the other sheet may be bonded to the other surface.

[0040] In the laminating step, the electrode mixture sheet 12b is laminated to the surface of the core material 11 using two rolls 24. The two rolls 24 have, for example, the same roll diameter, are arranged with a predetermined gap between them, and rotate in the same direction at the same peripheral speed. The two rolls 24 are preferably heated to a predetermined temperature and apply a predetermined pressure.

[0041] <Example> The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0042] <Example> [Coating of conductive material on the surface of the positive electrode active material] A carbon-coated positive electrode active material was prepared by mixing 1000 g of lithium transition metal composite oxide and 10 g of acetylene black (AB) for 5 minutes in a Nobilta (registered trademark) grinder NOB300 manufactured by Hosokawa Micron Corporation. The carbon (conductive material) coverage in this carbon-coated positive electrode active material was 51.5%.

[0043] [Preparation of positive electrode composite particles (mixing step)] The PTFE powder used contained 33% fibrous particles with an aspect ratio of 1.5 or more relative to the total particles, and the average major axis size of the fibrous particles was 8.3 μm. The carbon-coated positive electrode active material and the PTFE powder were charged into a mixer (Wonder Crusher, manufactured by Osaka Chemical Co., Ltd.) in a mass ratio of 101:4, and mixed at room temperature for 5 minutes at a rotation speed of 5. The rotation speed of the Wonder Crusher was 28,000 rpm, the maximum rotation speed at 10. This mixing process yielded positive electrode composite particles in which the carbon-coated positive electrode active material and PTFE powder were uniformly dispersed. The resulting positive electrode composite had a solids concentration of 100%.

[0044] [Preparation of positive electrode composite sheet (rolling step)] The resulting positive electrode composite particles were 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 thickness of the positive electrode composite sheet was adjusted to approximately 100 μm.

[0045] [Positive electrode fabrication (lamination step)] The positive electrode composite sheet prepared in Example 1-1 was placed on the surface of the positive electrode core material, and the laminate of the positive electrode composite sheet and the positive electrode core material was pressed (linear pressure: 1.0 t / cm) using two rolls to obtain a positive electrode. An aluminum alloy foil was used as the core material. The composite resistance was 18.2 Ωcm.

[0046] [Preparation of non-aqueous electrolyte] A non-aqueous electrolyte solution was prepared by dissolving 1.0 mol / L of LiPF6 as an electrolyte salt in a non-aqueous solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 1:3.

[0047] [Test cell construction] An aluminum lead was attached to the positive electrode, and a nickel lead was attached to the lithium metal foil used as the negative electrode. The positive and negative electrodes were spirally wound with a polyolefin separator interposed therebetween, and then pressed radially to produce a flat wound electrode assembly. This electrode assembly was housed in an exterior body made of an aluminum laminate sheet, and after the nonaqueous electrolyte was poured into it, the opening of the exterior body was sealed to obtain a test cell for evaluation.

[0048] [Evaluation of initial discharge capacity] The test cell was charged at a constant current of 0.5 C in a temperature environment of 25°C until the battery voltage reached 4.2 V, then charged at a constant voltage until the current value reached 1 / 50 C at 4.2 V, and then discharged at a constant current of 0.5 C until the battery voltage reached 2.5 V. The discharge capacity at this time was defined as the initial discharge capacity.

[0049] <Comparative Example> A test cell was prepared and evaluated in the same manner as in the example, except that the surface of the positive electrode active material was not coated with a conductive material, and in the preparation of the positive electrode composite particles (mixing step), the lithium transition metal composite oxide, AB, and PTFE powder were charged into a mixer in a mass ratio of 100:1:4. The coating rate of carbon (conductive material) on the surface of the positive electrode active material before the mixing step was 0%. In addition, the composite resistance of the prepared positive electrode composite particles was 55.1 Ω cm.

[0050] The initial discharge capacities of the Examples and Comparative Examples are shown in Table 1. The discharge capacities of the Comparative Examples are values ​​expressed relative to the discharge capacity of the Examples, which is set to 100. Table 1 also shows whether the surface of the positive electrode active material was coated with a conductive material, the composite resistance value, and the binder contents (a, b, c) of the first, second, and third regions in the positive electrode composite. Note that for the Comparative Examples, the binder contents (a, b, c) of the first, second, and third regions were not measured.

[0051] [Table 1]

[0052] As shown in Table 1, the test cells of the examples had larger initial discharge capacities than the test cells of the comparative examples. It is presumed that the positive electrodes of the examples had a higher coverage of the conductive material on the surface of the positive electrode active material than the positive electrodes of the comparative examples, and therefore had a lower composite resistance and a higher initial discharge capacity than the positive electrodes of the comparative examples. [Explanation of symbols]

[0053] 10 electrodes 11 Core material 12 Electrode mixture 12a Electrode mixture particles 12b Electrode mixture sheet 15 Mechanofusion Reactor 16 chambers 17 Compression Tools 20 Mixer 22,24 rolls

Claims

1. An electrode in which a coated active material in which a conductive material is attached to the surface of an active material and an electrode mixture containing PTFE powder are laminated on the surface of a core material, a coverage of the conductive material on the surface of the coated active material is 10% to 60%; The PTFE powder contains fibrous particles having an aspect ratio of 1.5 or more in a proportion of 20% to 60% of all particles, and the average major axis size of the fibrous particles is 1 μm to 20 μm; The electrode mixture has a composite resistance of 20 Ω cm or less, An electrode in which, when the electrode mixture is divided into three equal parts in the thickness direction, resulting in a first region, a second region, and a third region from the core material side, the content (a) of the PTFE powder in the first region, the content (b) of the PTFE powder in the second region, and the content (c) of the PTFE powder in the third region satisfy -10%≦(c−a) / (a+b+c)≦10%.

2. An electrode in which an electrode composite containing a coated active material having a conductive material attached to the surface of an active material and PTFE powder is laminated on the surface of a core material, a coverage of the conductive material on the surface of the coated active material is 10% to 60%; The PTFE powder contains fibrous particles having an aspect ratio of 5 or more at a rate of 60% or more relative to all particles, and the average minor axis size of the fibrous particles is 1 μm to 20 μm; The electrode mixture has a composite resistance of 20 Ω cm or less, An electrode in which, when the electrode mixture is divided into three equal parts in the thickness direction, resulting in a first region, a second region, and a third region from the core material side, the content (a) of the PTFE powder in the first region, the content (b) of the PTFE powder in the second region, and the content (c) of the PTFE powder in the third region satisfy -10%≦(c−a) / (a+b+c)≦10%.

3. 3. The electrode according to claim 1, wherein the coated active material has a surface having irregularities, and the conductive material penetrates into and adheres to the recesses of the irregularities.

4. The electrode according to any one of claims 1 to 3, wherein the active material is a positive electrode active material.

5. a mixing step of mixing a coated active material having a conductive material attached to the surface of the active material with PTFE powder to prepare electrode mixture particles having a solid content concentration of substantially 100%; a rolling step of rolling the electrode mixture particles to form them into a sheet, thereby producing an electrode mixture sheet; a laminating step of laminating the electrode mixture sheet to a core material to produce an electrode, The PTFE powder contains fibrous particles having an aspect ratio of 1.5 or more in a proportion of 20% to 60% of all particles, and the average major axis size of the fibrous particles is 1 μm to 20 μm.

6. A mixing step of mixing a coated active material having a conductive material attached to the surface of the active material with PTFE powder to prepare electrode composite particles having a solid content concentration of substantially 100%; a rolling step of rolling the electrode mixture particles to form them into a sheet, thereby producing an electrode mixture sheet; a laminating step of laminating the electrode mixture sheet to a core material to produce an electrode, The PTFE powder contains fibrous particles having an aspect ratio of 5 or more in a proportion of 60% or more of all particles, and the average minor axis size of the fibrous particles is 1 μm to 20 μm.

7. The method for producing an electrode according to claim 5 or 6, wherein the coated active material is obtained by dry mixing the active material and the conductive material.

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