PTFE powder, method for manufacturing an electrode, and electrode

Abstract

Provided is a PTFE powder that is in a dry state, wherein the percentage of fibrous particles which have an aspect ratio of 1.5 or higher with respect to all particles is 20-60%, and the average major axis size of the fibrous particles is 1-20 μm. A PTFE powder according to another embodiment of the present invention is in a dry state, wherein the percentage of fibrous particles which have an aspect ratio of 5 or higher with respect to all particles is 60% or more, and the average minor axis size of the fibrous particles is 1-20 μm.

Description

Technical Field

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

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, and the coating film is dried and compressed. In this case, there is a problem that migration in which the binder moves during drying of the coating film is likely to occur. 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 has been studied in which an electrode mixture sheet is produced by rolling an electrode mixture into a sheet shape and the sheet is bonded to a core material to manufacture an electrode. Patent Document 1 discloses an electrode film (electrode mixture) in which an active material, a particulate binder, and a conductive material are mixed using a mill, and then a large shearing force is applied to this mixture at a high pressure for a long time to fibrillate the binder.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

[0005] In the production of electrodes by a dry method without using a solvent, for example, the degree of fibrillation of the binder and the mixing state of the materials constituting the electrode mixture greatly affect the formability and breaking strength of the electrode mixture sheet. As a result of investigations by the present inventors, as disclosed in Patent Document 1, it has been found that when a large shearing force is applied to the electrode mixture and processed for a long time, the formability of the electrode mixture sheet deteriorates and the breaking strength significantly decreases.

[0006] The PTFE powder according to one aspect of the present disclosure is in a dry state, the proportion of fibrous particles having an aspect ratio of 1.5 or more with respect to all particles is 20% to 60%, and the average major axis size of the fibrous particles is 1 μm to 20 μm.

[0007] The PTFE powder according to another aspect of the present disclosure is in a dry state, the proportion of fibrous particles having an aspect ratio of 5 or more with respect to all particles is 60% or more, and the average minor axis size of the fibrous particles is 1 μm to 20 μm.

[0008] The method for manufacturing an electrode according to one aspect of the present disclosure includes a mixing step of mixing the above PTFE powder, an active material, and a conductive material to produce an electrode mixture having a solid content concentration of substantially 100%, a rolling step of producing an electrode mixture sheet by rolling the electrode mixture into a sheet shape, and a bonding step of producing an electrode by bonding the electrode mixture sheet to a core material.

[0009] In the electrode according to one aspect of the present disclosure, an electrode mixture containing the above PTFE powder, an active material, and a conductive material is laminated on the surface of the core material, the coverage rate of the conductive material on the surface of the active material is 10% to 60%, and when the electrode mixture is divided into three equal parts in the thickness direction to form 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 (c - a) / (a + b + c) ≤ ±10%.

[0010] According to one aspect of the present disclosure, an electrode composite material sheet excellent in formability and having a high breaking strength can be produced. Further, since the active material contained in the electrode composite material sheet according to the present disclosure has a high coating rate of the conductive material, the battery characteristics of the electrode can be improved by using the electrode composite material sheet.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0012] Hereinafter, embodiments of a method for manufacturing an electrode and an electrode composite material 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. Further, the drawings referred to in the description of the embodiments are schematically drawn, and the dimensional ratios of the components depicted in the drawings should be determined in consideration of the following description.

[0013] [PTFE (Polytetrafluoroethylene) Powder] PTFE powder is contained in the electrode composite material as a binder, and can also be referred to as PTFE powder for batteries. The PTFE powder may be for a positive electrode. The PTFE powder is a powder in a dry state, not a powder dispersed in a dispersion such as water. Thereby, the electrode composite material can be produced by a dry method described later.

[0014] As an example of an embodiment, the PTFE powder contains fibrous particles with an aspect ratio of 1.5 or more at a ratio of 20% to 60% with respect to all particles. Further, the average major axis size of the fibrous particles is 1 μm to 20 μm (hereinafter, the fibrous particles of this shape are referred to as fibrous particles A). By using this PTFE powder, an electrode composite sheet with good moldability and high breaking strength can be produced. The ratio of the fibrous particles A to all particles can be calculated as follows. Note that the ratio of the fibrous particles B to all particles, which will be described later, can also be measured by the same method. (1) Image the PTFE powder containing the fibrous particles A with a scanning electron microscope (SEM). The imaging magnification can be, for example, 300 to 1000 times. (2) Import the imaged image into a computer and divide all the particles into fibrous particles A and particles with an aspect ratio of less than 1.5 using image analysis software such as ImageJ. (3) Divide the number of the fibrous particles A by the total number of all particles, that is, the sum of the number of the fibrous particles A and the number of particles with an aspect ratio of less than 1.5, to calculate the ratio of the fibrous particles A to all particles.

[0015] The average major axis size of the fibrous particles A can be calculated by analyzing the SEM image with image analysis software in the same way as calculating the ratio of the fibrous particles A to all particles described above, measuring the major axis sizes (major axis lengths) of 100 fibrous particles with an aspect ratio of 1.5 or more, and averaging the measured values. Note that the average minor axis size of the fibrous particles B, which will be described later, can also be measured by the same method.

[0016] 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 by analyzing the SEM image with image analysis software in the same way as calculating the average major axis size described above, measuring the aspect ratios (major axis / minor axis) of 100 fibrous particles with an aspect ratio of 1.5 or more, and averaging the measured values. Note that the average aspect ratio of the fibrous particles B, which will be described later, can also be measured by the same method.

[0017] The PTFE powder containing fibrous particles A can be produced by fibrillating a PTFE raw material (PTFE particles) belonging to a fine powder capable of fibrillation (fibering) with a dry grinder such as a jet mill grinder. The PTFE raw material may be secondary particles. The average particle size 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 size of the PTFE raw material can be determined by observing the particles of the PTFE raw material with an SEM. Specifically, after identifying the outer shapes of 100 randomly selected particles, the major axis (longest diameter) of each of the 100 particles is determined, and the average value thereof is taken as the average particle size of the PTFE raw material. When producing a PTFE powder containing fibrous particles A with a jet mill grinder, the ratio of 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.

[0018] As another example of the embodiment, the PTFE powder contains fibrous particles having an aspect ratio of 5 or more at a ratio of 60% or more based on all particles. The PTFE powder preferably contains fibrous particles having an aspect ratio of 5 or more at a ratio of 80% or more based on all particles. Further, the average minor axis size of the fibrous particles is 1 μm to 20 μm (hereinafter, the fibrous particles of this shape are referred to as fibrous particles B). By using this PTFE powder, an electrode binder sheet with good moldability and high breaking strength can be produced.

[0019] The PTFE powder containing fibrous particles B can be produced by fibrillating a PTFE raw material (PTFE particles) belonging to a fine powder capable of fibrillation (fibering) with a dry grinder such as an air jet mill. The same PTFE raw material as that used in the case of producing the PTFE powder containing the above-mentioned fibrous particles A can be used. When producing a PTFE powder containing fibrous particles B with an air jet mill, the ratio of fibrous particles B to all particles can be adjusted to 60% or more by appropriately adjusting the supply rate of the PTFE raw material, the rotation speed and gap of the blade, etc.

[0020] 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 meter. The fact that the median diameter of the PTFE powder containing fibrous particles A and / or B is 2 μm to 20 μm means that the PTFE powder containing fibrous particles A and / or B is in a size micronized with respect to the PTFE particles of the PTFE raw material.

[0021] [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 an aqueous electrolyte or energy storage devices such as capacitors. Hereinafter, an electrode for a non-aqueous electrolyte secondary battery (particularly when applied to a positive electrode) will be described as an example.

[0022] FIG. 1 is a cross-sectional view of an electrode which is an example of an embodiment. The electrode 10 includes a core material 11 and an electrode mixture layer 12 provided on the surface of the core material 11. As shown in FIG. 1, the electrode 10 may include the electrode mixture layer 12 on both sides of the core material 11. The electrode 10 may be a long electrode constituting a wound electrode body, or may be a rectangular electrode constituting a laminated electrode body. Note that the electrode 10 can be applied to the positive electrode, negative electrode, or both of a non-aqueous electrolyte secondary battery.

[0023] For the core material 11, a metal foil, a film having a metal layer formed on its surface, or the like can be used. The thickness of the core material 11 is, for example, 5 μm to 20 μm. In the case of a positive electrode, a metal foil mainly composed of aluminum can be used for the core material 11. In the case of a negative electrode, a metal foil mainly composed of copper can be used. In this specification, the main component means the constituent component having the highest mass ratio. The core material 11 may be an aluminum foil substantially 100% aluminum, or may be a copper foil substantially 100% copper.

[0024] The electrode composite material 12 includes PTFE powder, an active material, and a conductive material. The thickness of the electrode composite material 12 is, for example, 30 μm to 120 μm, preferably 50 μm to 100 μm. In addition to the PTFE powder, the electrode composite material 12 may contain a binder such as non-fibrillated polyvinylidene fluoride (PVdF). The electrode composite material 12 is mainly composed of an active material. The content of the active material is preferably 85% by mass to 99% by mass, more preferably 90% by mass to 99% by mass, based on the mass of the electrode composite material 12.

[0025] Generally, a lithium transition metal composite oxide is used as the active material of the positive electrode (positive electrode active material). Examples of the metal elements contained in the lithium transition metal composite oxide include Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, W, etc. Among them, it is preferable to contain at least one of Ni, Co, and Mn. As the active material of the negative electrode (negative electrode active material), for example, carbon-based active materials such as natural graphite such as flaky graphite, massive graphite, and earthy graphite, artificial massive graphite (MAG), and graphitized mesophase carbon microbeads (MCMB) are used. In addition, an Si-based active material that alloyizes with lithium may be used as the negative electrode active material.

[0026] Examples of the conductive material contained in the electrode composite material 12 include carbon materials such as carbon black (CB), acetylene black (AB), ketjen black, and graphite. The content of the conductive material is, for example, 0.5% by mass to 5.0% by mass based on the mass of the electrode composite material 12.

[0027] The coverage rate of the conductive material on the surface of the active material is preferably 10% to 60%, more preferably 20% to 60%. Since the coverage rate of the conductive material is sufficiently high, the battery characteristics of the electrode can be improved. As described later, by relatively shortening the mixing time of the PTFE powder, the active material, and the conductive material, the coverage rate of the conductive material can be increased.

[0028] The content rate of the PTFE powder is, for example, 0.5 mass% to 5.0 mass% with respect to the mass of the electrode mixture 12. The PTFE powder adheres to the particle surface of the active material and is intertwined with the active material. In other words, the positive electrode active material is held by the PTFE powder present in a network form. By containing a predetermined amount of fibrous particles A and / or fibrous particles B in the PTFE powder, it becomes possible to produce an electrode mixture sheet having good moldability and high breaking strength.

[0029] When the electrode mixture 12 is divided into three equal parts in the thickness direction and the first region, the second region, and the third region are defined from the side of the core material 11, 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 (c - a) / (a + b + c) ≤ ±10%, and preferably satisfy (c - a) / (a + b + c) ≤ ±5%. That is, the difference between the content (a) of the PTFE powder in the first region near the surface and the content (c) of the PTFE powder in the third region near the core material 11 is within the range of ±10%, preferably within the range of ±5% with respect to the total content (a + b + c) of the PTFE powder. By doing so, the PTFE powder can be present substantially uniformly throughout the whole without being ubiquitous in a part of the electrode mixture 12.

[0030] The breaking peripheral speed ratio of the electrode composite material 12 preferably satisfies 8 or more. The breaking peripheral speed ratio of the electrode composite material 12 is the breaking peripheral speed ratio during the formation of the test sheet for measuring the breaking peripheral speed ratio. The breaking peripheral speed ratio can be measured by changing the peripheral speed ratio of a pair of forming rolls when forming the test sheet from the electrode composite material particles and specifying the peripheral speed ratio when the sheet breaks. "The breaking peripheral speed ratio of the electrode composite material 12 preferably satisfies 8 or more" means that "the breaking peripheral speed ratio during the formation of the test sheet for measuring the breaking peripheral speed ratio preferably satisfies 8 or more". When the breaking peripheral speed ratio of the electrode composite material 12 satisfies 8 or more, it can be determined that the fibrillation of the PTFE powder contained in the electrode composite material 12 is appropriately performed. The electrode composite material 12 that satisfies this condition has excellent formability, and from the electrode composite material 12 that satisfies this condition, an electrode composite material 12 with high breaking strength can be produced. The breaking peripheral speed ratio is more preferably 9 or more, and even more preferably 10 or more.

[0031] The method for measuring the breaking peripheral speed ratio of the electrode composite material 12 is as follows. The breaking peripheral speed ratio can be measured using the same apparatus as the apparatus for rolling the electrode composite material particles 12a shown in Fig. 2(b) and forming them into a sheet-like electrode composite material sheet 12b as described later. As shown in Fig. 2(b), the electrode composite material particles 12a are rolled using two rolls to form the electrode composite material 12 into a sheet shape. When forming the electrode composite material into a sheet shape, the linear pressure of the two constant-pressure rolls is set to about 0.03 t / cm, the gap is set to 0 μm, the peripheral speed of one roll is fixed at 5 m / min, and the electrode composite material is formed into a sheet while changing the peripheral speed ratio one by one from 1 to 10. The smallest peripheral speed ratio among the peripheral speed ratios at which breakage is observed in the sheet is taken as the breaking peripheral speed ratio. Note that the sheet of the electrode composite material obtained during the measurement of the breaking peripheral speed ratio of this electrode composite material corresponds to the aforementioned test sheet. In this specification, the test sheet can also be read as the electrode composite material sheet.

[0032] The peripheral speed ratio is the ratio representing the peripheral speed of one roll with respect to that of the other roll when the peripheral speed of one roll is taken as 1. The breaking peripheral speed ratio is the peripheral speed ratio at the time of breakage of the test sheet, which is expressed as a ratio of the peripheral speed of one roll taken as 1 to the peripheral speed of the other roll when breakage occurs in the test sheet. The greater the peripheral speed ratio of the two rolls, the greater the shearing force in the lateral direction and the easier it is for the test sheet to break. Therefore, it can be evaluated that the higher the value of the peripheral speed ratio at the time of breakage, the higher the breaking strength of the sheet.

[0033] The electrode composite material 12 is preferably a mixture in which the active material, PTFE powder, and conductive material are uniformly dispersed. The breaking peripheral speed ratio of the electrode composite material 12 serves as an index indicating the dispersibility of the constituent materials. Further, in the electrode composite material 12, it is preferable that there is little cracking of the active material particles and that most of the conductive material adheres to the surface of the active material particles to form a conductive path between the particles. That is, it is necessary to produce the electrode composite material 12 while suppressing cracking of the active material particles and preventing the amount of the conductive material incorporated into the PTFE powder and adhering to the surface of the active material particles from decreasing. According to the manufacturing method described later, it is possible to produce a high-quality electrode composite material 12 that satisfies such conditions.

[0034] [Manufacturing Method of Electrode] Hereinafter, the manufacturing method of the electrode 10 will be described in further detail. Hereinafter, the manufacturing method of the positive electrode will be exemplified, but this manufacturing method can be similarly applied to the manufacturing of the negative electrode. In the case of the negative electrode, a negative electrode active material is used instead of the positive electrode active material. Also, it may not be necessary to add a conductive material.

[0035] FIG. 2 and FIG. 3 are diagrams schematically showing the manufacturing process of the electrode 10 which is an example of the embodiment. The manufacturing method of the electrode 10 includes a mixing step shown in FIG. 2(a), a rolling step shown in FIG. 2(b), and a laminating step shown in FIG. 3. In the mixing step, PTFE powder, an active material, and a conductive material are mixed to produce electrode composite material particles 12a having a solid content concentration of substantially 100%. In the rolling step, the electrode composite material particles 12a are rolled and formed into a sheet shape to produce an electrode composite material sheet. In the laminating step, an electrode is produced by laminating the electrode composite material sheet on a core material.

[0036] The manufacturing method of the electrode 10 is a dry process for manufacturing the electrode 10 using an electrode mixture 12 with a solid content concentration of substantially 100%. The dry process is a process of mixing active material particles and binder particles without using a solvent, that is, mixing in a state where the solid content concentration of the active material and the binder is substantially 100%. The manufacturing method of the electrode 10 according to the present disclosure does not require the use of a solvent like the conventional manufacturing method of the electrode 10. Not requiring the use of a solvent not only means that it is unnecessary as a mere raw material, but also means that the drying process of the solvent is unnecessary, and it is intended that exhaust facilities related to the drying process can be made unnecessary.

[0037] In the mixing step, raw materials such as PTFE powder, active material, and conductive material are mixed by a mixer 20 to produce electrode mixture particles 12a. In the mixing step, by using PTFE powder containing a predetermined amount of fibrous particles A and / or fibrous particles B and performing a short-time mixing process, the coating rate of the conductive material on the surface of the active material can be increased while improving the dispersibility of the constituent materials, and the moldability and breaking strength of the electrode mixture 12 can be improved. The breaking peripheral speed ratio of the electrode mixture 12 is preferably 8 or more. The breaking peripheral speed ratio of the electrode mixture 12 is an index indicating the dispersibility of the constituent materials, and thus the moldability and breaking strength of the electrode mixture 12. When a long-time mixing process is performed, the conductive material is incorporated into the binder, and the coating rate of the conductive material on the surface of the active material is less than 10%. Also, by using PTFE powder containing a predetermined amount of fibrous particles A and / or fibrous particles B, the dispersibility of the constituent materials can be improved even in a short-time mixing process, so that cracking of the active material during the mixing process can be suppressed. Note that, among the active materials contained in the electrode, those cracked by the mixing process and those cracked in the rolling step described later may be included.

[0038] In the mixing step, before introducing the active material and the conductive material into the mixer 20, the conductive material may be preliminarily adhered to the surface of the active material by a method such as the mechanofusion method. By preliminarily adhering the conductive material to the surface of the active material, the mixing process in the mixer 40 can be set to a short time for obtaining a state in which raw materials other than the conductive material are dispersed. The mechanofusion method is a dry treatment method performed in a mechanofusion reaction device having a cylindrical chamber that is internally provided with a compression tool and a blade and rotates at high speed. The rotation speed is usually faster than 1000 rpm. By putting the conductive material and the active material into the chamber and rotating the chamber, the particles are pressed against each other and against the chamber wall. When a compression tool is used and centrifugal force is generated by high-speed rotation, the adhesion bond between the conductive material and the active material is promoted. Examples of the mechanofusion reaction device include the "Nobiruter" (registered trademark) crusher or the "Mechanofusion" (registered trademark) crusher manufactured by Hosokawa Micron Corporation (Japan), the "Hybridizer" (trademark) crusher manufactured by Nara Machinery Co., Ltd., the "Balance Gran" manufactured by Freund Turbo Co., Ltd., the "COMPOSI" manufactured by Nippon Coke Industry Co., Ltd., and the like.

[0039] As the mixer 40, for example, a conventionally well-known mechanical stirring mixer can be used. Specific examples of suitable mixers 40 include cutter mills, pin mills, bead mills, fine particle composite devices (devices in which shear force is generated between a rotor having a special shape that rotates at high speed inside a tank and a collision plate), granulators, kneaders such as twin-screw extrusion kneaders and planetary mixers, etc. Cutter mills, fine particle composite devices, granulators, and twin-screw extrusion kneaders are preferred. Thereby, while mixing the raw materials, the PTFE powder can be further fibrillated. The processing time (the time for applying shear force to the material) in the mixing step is preferably within several minutes, and can be, for example, 0.5 minutes to 4 minutes. Since the PTFE powder already contains a predetermined amount of fibrillated fibrous particles A and / or fibrous particles B, if the processing time is 0.5 minutes or more, the PTFE powder can be adhered to the surface of the active material particles and intertwined with the active material. If the processing time is too long, the amount of conductive material incorporated into the PTFE powder increases. In this case, it has an adverse effect on battery characteristics such as a significant decrease in the conductivity of the electrode composite sheet and an increase in resistance. Also, as the processing time becomes longer, the fibrillation of PTFE progresses. Therefore, if the fibrillation progresses excessively, the breaking strength of the sheet decreases.

[0040] As shown in Fig. 2(b), in the rolling step, the electrode composite particles 12a are rolled using two rolls 22 to form a sheet shape. The two rolls 22 are arranged with a predetermined gap and rotate in the same direction. The electrode composite particles 12a are supplied into the gap between the two rolls 22 and are compressed by the two rolls 22 and stretched into a sheet shape. The two rolls 22, for example, have the same roll diameter. The obtained electrode composite 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. Also, the rolls may be heated to thermally press the electrode composite particles 12a.

[0041] The thickness of the electrode composite material sheet 12b can be controlled by, for example, the gap between the two rolls 22, the peripheral speed, the number of stretching treatments, etc. In the rolling step, it is preferable to form the electrode composite material particles 12a into a sheet shape using two rolls 22 with a peripheral speed ratio different by two times or more. By varying the peripheral speed ratio of the two rolls 22, for example, the thinning of the electrode composite material sheet 12b becomes easy and the productivity is improved. The peripheral speed ratio of the two rolls 22 is more preferably 2.5 times or more, and may be 3 times or more. Note that as the peripheral speed ratio increases, the shearing force acting on the electrode composite material sheet 12b increases, so the electrode composite material sheet 12b is required to have a high breaking strength.

[0042] Next, as shown in FIG. 3, in the bonding step, by bonding the electrode composite material sheet 12b to the core material 11, an electrode 10 provided with a composite material layer made of the electrode composite material 12 on the surface of the core material 11 is obtained. In FIG. 3, a state where the electrode composite material 12 is joined only to one surface of the core material 11 is shown, but the electrode composite material 12 is preferably joined to both surfaces of the core material 11. The two electrode composite materials 12 may be joined to both surfaces of the core material 11 simultaneously, or one may be joined to one surface of the core material 11 and then the other may be joined to the other surface.

[0043] In the bonding step, two rolls 24 are used to bond the electrode composite material sheet 12b to the surface of the core material 11. The two rolls 24, for example, have the same roll diameter, are arranged with a predetermined gap, 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.

[0044] <Example> Hereinafter, the present disclosure will be further described by way of examples, but the present disclosure is not limited to these examples.

[0045] <Example 1-1> [Production of PTFE Powder] PTFE raw material (PTFE particles) with an average particle diameter of 343 μm was fed into a jet mill crusher at a feeding rate of 3 kg / h under the condition of a crushing pressure of 0.6 MPa to obtain PTFE powder. The median diameter of the obtained whole PTFE powder was 15.4 μm.

[0046] [Adhesion treatment of conductive material on the surface of the positive electrode active material] Using NOB300? Nobiruter (registered trademark) manufactured by Hosokawa Micron Corporation, 1000 g of lithium transition metal composite oxide and 10 g of acetylene black (AB) were mixed in a Nobiruter crusher for 5 minutes to produce a carbon-attached positive electrode active material.

[0047] [Production of positive electrode composite material particles] The above carbon-attached positive electrode active material and PTFE powder were put into a mixer (manufactured by Osaka Chemical, Wonder Crusher) at a mass ratio of 101:4 and mixed at room temperature for 2 minutes at the rotation speed of Memory 5. The rotation speed of the Wonder Crusher is 28000 rpm at the maximum of Memory 10. By this mixing treatment, positive electrode composite material particles in which the positive electrode active material, PTFE powder, and AB were uniformly dispersed were obtained. The obtained positive electrode composite material had a solid content concentration of 100%.

[0048] [Production of positive electrode composite material sheet] The obtained positive electrode composite material particles were passed between two rolls and rolled to produce a positive electrode composite material sheet. The peripheral speed ratio of the two rolls was set to 1:3, and the thickness of the positive electrode composite material sheet was adjusted to about 100 μm.

[0049] For the obtained positive electrode composite material particles and positive electrode composite material sheet, the breaking strength and the film-forming property (appearance) of the sheet were evaluated by the following method, and the evaluation results were shown in Table 1 together with the state of the PTFE powder.

[0050] [Evaluation of breaking strength] Separate from the production of the above-mentioned positive electrode composite material sheet, a test sheet was formed, the breaking peripheral speed ratio was measured, and the breaking strength was evaluated. When forming the test sheet by sheet-forming the positive electrode composite material particles as shown in Fig. 2(b), the linear pressure of the two constant-pressure rolls was set to approximately 0.03 t / cm, the gap was set to 0 μm, the peripheral speed of one roll was fixed at 5 m / min, the peripheral speed ratio was changed one by one from 1 to 10 for sheet-forming, and the smallest peripheral speed ratio when breakage occurred in the sheet was taken as the breaking peripheral speed ratio, and the breaking strength of the sheet was evaluated. The measurement of the breaking peripheral speed ratio was performed multiple times, and the average peripheral speed ratio of the smallest peripheral speed ratio when breakage occurred in the sheet in each measurement was taken as the breaking peripheral speed ratio.

[0051] [Evaluation of film-forming property] The positive electrode composite material sheet obtained by producing the positive electrode composite material sheet was visually observed, and the film-forming property was evaluated from two evaluation items: the presence or absence of white streaks due to poor dispersion of PTFE powder and the sheet state at the ends. Regarding the presence or absence of white streaks, ○ was used when almost no white streaks were confirmed and the PTFE powder was uniformly dispersed, and × was used when there were many white streaks and there was a lot of unevenness in the PTFE powder. "When there are many white streaks and a lot of unevenness in fibrous PTFE" means that the fibrillation of PTFE is insufficient and white streaks are observed due to the presence of large PTFE particles. Regarding the sheet state at the ends, ○ was used when the sheet was formed equally at both ends in the direction perpendicular to the stretching direction of the positive electrode composite material sheet as in the central part, and × was used when the ends were powdery and the sheet was not sufficiently formed. "When the ends are powdery and the sheet is not sufficiently formed" means that at the ends where the pressure during rolling is smaller than that in the central part, compression is insufficient and the forming is insufficient, resulting in a powdery state. The evaluation results of the presence or absence of white streaks and the sheet state at the ends are shown in "White streaks" and "Ends" in Table 1.

[0052] <Example 1-2> In the production of PTFE powder, an airflow mill was used instead of a jet mill, and cathode composite material particles and a cathode composite material sheet were produced and evaluated in the same manner as in Example 1, except that the pulverization was carried out under the conditions of a supply rate of 20 kg / h, a rotation speed of 8000 rpm, and a gap of 2 mm in a continuous process. The median diameter of the entire obtained PTFE powder was 9.3 μm.

[0053] <Comparative Example 1-1> Cathode composite material particles and a cathode composite material sheet were produced and evaluated in the same manner as in Example 1, except that untreated PTFE raw material was used without producing PTFE powder.

[0054] <Comparative Example 1-2> In the production of PTFE powder, an airflow mill was used instead of a jet mill, and cathode composite material particles and a cathode composite material sheet were produced and evaluated in the same manner as in Example 1, except that the pulverization was carried out under the conditions of a supply rate of 10 kg / h, a rotation speed of 5000 rpm, and a gap of 5 mm in a continuous process.

[0055] <Comparative Example 1-3> In the production of PTFE powder, a dry bead mill was used instead of a jet mill, and cathode composite material particles and a cathode composite material sheet were produced and evaluated in the same manner as in Example 1, except that the pulverization was carried out under the conditions of a flow rate of 0.4 L / min, a peripheral speed of 14 m / s, and a bead diameter of φ1.0 mm in a continuous process.

[0056] <Comparative Example 1-4> In the production of PTFE powder, a wet bead mill was used instead of a jet mill, ethanol was used as a solvent in a batch process, and the pulverization treatment was carried out with ZrO2 beads of φ2.0 mm for 60 minutes. Cathode composite material particles and a cathode composite material sheet were produced and evaluated in the same manner as in Example 1.

[0057]

Table 1

[0058] From the evaluation results shown in Table 1, it can be seen that all of the positive electrode composite sheets of the examples have good film-forming properties (appearance) and high breaking strength compared to the positive electrode composite sheets of the comparative examples.

[0059] <Example 2-1> [Fabrication of Positive Electrode] The positive electrode composite sheet prepared in Example 1-1 was placed on the surface of the positive electrode core material, and using two rolls, the laminate of the positive electrode composite sheet and the positive electrode core material was pressed (linear pressure: 1.0 t / cm) to obtain a positive electrode. An aluminum alloy foil was used as the core material.

[0060] <Example 2-2> A positive electrode was fabricated in the same manner as in Example 2-1, except that the PTFE powder obtained in Example 1-2 was used instead of the PTFE powder obtained in Example 1-1 in the fabrication of the positive electrode composite particles.

[0061] <Example 2-3> A positive electrode was fabricated in the same manner as in Example 2-2, except that the rotational speed of the mixer was changed to Memory 3 in the fabrication of the positive electrode composite particles.

[0062] <Example 2-4> A positive electrode was fabricated in the same manner as in Example 2-2, except that the rotational speed of the mixer was changed to Memory 1 in the fabrication of the positive electrode composite particles.

[0063] <Example 2-5> A positive electrode was fabricated in the same manner as in Example 2-2, except that the rotational speed of the mixer was changed to Memory 1 and the mixing time was changed to 1 minute in the fabrication of the positive electrode composite particles.

[0064] <Comparative Example 2-1> A positive electrode was fabricated in the same manner as in Example 2-1, except that the PTFE powder obtained in Comparative Example 1-1 was used instead of the PTFE powder obtained in Example 1-1 and the mixing time was changed to 5 minutes in the fabrication of the positive electrode composite particles.

[0065] Regarding the positive electrodes obtained in the examples and comparative examples, the content of the binder in the first region, second region, and third region in the positive electrode composite material, and the coating rate of AB on the surface of the active material were evaluated, and the evaluation results are shown in Table 2 together with the production conditions of the positive electrode composite material particles.

[0066]

Table 2

[0067] From the evaluation results shown in Table 2, it can be seen that for all the positive electrodes of the examples, the coating rate of the conductive material on the surface of the active material is 10% to 60%, and the content (a) of PTFE powder in the first region, the content (b) of PTFE powder in the second region, and the content (c) of PTFE powder in the third region satisfy (c - a) / (a + b + c) ≤ ±10%. On the other hand, for the positive electrodes of the comparative examples, the coating rate of the conductive material on the surface of the active material was less than 10%. Therefore, it is speculated that the positive electrodes of the examples can improve the battery characteristics of the electrodes more than the positive electrodes of the comparative examples because the coating rate of the conductive material is high for the positive electrode active material.

[0068] In Examples 2 - 5, the positive electrodes were produced at a lower mixing rotation speed and for a shorter mixing time compared to other examples. It can be seen that a high conductive coating rate can be achieved with a lower mixing rotation speed and a shorter mixing time. On the other hand, if the mixing time is too short compared to the binder content in the first region, second region, and third region, it may affect the dispersibility of the composite material particles.

Explanation of Symbols

[0069] 10 Electrode 11 Core Material 12 Positive Electrode Composite Material 12a Positive Electrode Composite Material Particles 12b Positive Electrode Composite Material Sheet 20 Mixer 22, 24 Roll

Claims

1. PTFE powder in a dry state, containing fibrous particles with an aspect ratio of 1.5 or more at a ratio of 20% to 60% with respect to all particles, PTFE powder wherein the average major axis size of the fibrous particles is 1 μm to 20 μm.

2. The PTFE powder according to claim 1, wherein the average aspect ratio of the fibrous particles is 2 to 20.

3. The PTFE powder according to claim 1 or 2, having a median diameter of 2 μm to 20 μm.

4. The PTFE powder according to any one of claims 1 to 3, which is a binder for a positive electrode of a non-aqueous electrolyte secondary battery.

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