Method for manufacturing electrode film, electrode film, and battery

Abstract

To provide a method for manufacturing an electrode film excellent in pulling strength, an electrode film excellent in pulling strength, and a battery including the electrode film excellent in pulling strength.SOLUTION: A method for manufacturing an electrode film includes the steps of: attaching polyvinylidene fluoride to active material particles; mixing the active material particle to which polyvinylidene fluoride is attached and polytetrafluoroethylene to obtain a mixture; and converting polytetrafluoroethylene in the mixture into fibers. The step of conversion into fibers is executed at a temperature of 50°C or more, and the mixture does not include a solvent.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a method for manufacturing an electrode film, the electrode film, and a battery.

Background Art

[0002] For batteries such as lithium-ion secondary batteries, electrodes in which active material particles are fixed to the surface of a current collector such as a metal foil with a binder are used. As a method for manufacturing an electrode, there are a method of applying a composition prepared by mixing active material particles and a binder with a solvent to the surface of a current collector (also referred to as a wet method), and a method of fixing active material particles to a current collector with a binder without using a solvent (also referred to as a dry method).

[0003] As a method for manufacturing an electrode by a dry method, a method of using a resin having a property of being fibrillated (fibered) when a shearing force is applied as a binder has been proposed. For example, Patent Document 1 describes a method of manufacturing an electrode film by fibrillating PTFE in a mixture containing active material particles and polytetrafluoroethylene (PTFE), and then integrating this electrode film with a current collector to manufacture an electrode.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] Patent Document 1 describes that a calendering process for manufacturing an electrode film containing fibrillated PTFE may be performed at a temperature of 10°C to 300°C, but the relationship between the physical properties of the electrode film and the temperature conditions of the calendering process has not been studied. An object to be solved by one embodiment of the present disclosure is to provide a method for manufacturing an electrode film excellent in tensile strength, an electrode film excellent in tensile strength, and a battery including the electrode film excellent in tensile strength.

Means for Solving the Problems

[0006] Means for solving the above problems include the following embodiments. <1>A step of attaching polyvinylidene fluoride to active material particles, A step of mixing the active material particles to which polyvinylidene fluoride is attached and polytetrafluoroethylene to obtain a mixture, A step of fibrillating polytetrafluoroethylene in the mixture, and The fibrillating step is carried out at a temperature of 50°C or higher, The mixture does not contain a solvent, a method for manufacturing an electrode film. <2>The fibrillating step is carried out at a temperature of 100°C or higher, the method for manufacturing an electrode film according to <1>. <3>The fibrillating step is carried out at a temperature of 200°C or lower, the method for manufacturing an electrode film according to <1> or <2>. <4>The fibrillating step is carried out at a temperature of 180°C or lower, the method for manufacturing an electrode film according to any one of <1> to <3>. <5>The fibrillating step includes forming the mixture into a film shape, the method for manufacturing an electrode film according to any one of <1> to <4>. <6>An electrode film including active material particles, polyvinylidene fluoride attached to the active material particles, and fibrous polytetrafluoroethylene. <7>The number of fibrous polytetrafluoroethylene intersecting a straight line having a length of 20 μm arranged at an arbitrary position in an image of a cross section of the electrode film is 5 or more, the electrode film according to <6>. <8>The number of fibrous polytetrafluoroethylene intersecting the straight line is 20 or more, the electrode film according to <7>. <9>An electrode film according to any one of <6> to <8>, wherein at least a part of fibrous polytetrafluoroethylene is attached to the active material particles. <10>An electrode film according to any one of <6> to <9>, wherein the coverage rate of the surface of the active material particles with polyvinylidene fluoride is 5% or more. <11>A battery including the electrode film according to any one of <6> to <10>.

Advantages of the Invention

[0007] According to one embodiment of the present disclosure, there is provided a method for manufacturing an electrode film excellent in tensile strength, an electrode film excellent in tensile strength, and a battery including the electrode film excellent in tensile strength.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Modes for Carrying Out the Invention

[0009] In the present disclosure, a numerical range indicated by using "~" means a range including the numerical values described before and after "~" as the minimum value and the maximum value, respectively. In the numerical ranges described stepwise in the present disclosure, the upper limit value or the lower limit value described in a certain numerical range may be replaced with the upper limit value or the lower limit value of the numerical range described in other stepwise descriptions. In the numerical ranges described in the present disclosure, the upper limit value or the lower limit value described in a certain numerical range may be replaced with the value shown in the examples. In the present disclosure, the term "step" includes not only an independent step but also this term even when it cannot be clearly distinguished from other steps as long as the intended purpose of the step is achieved. In the present disclosure, a combination of two or more preferred embodiments is a more preferred embodiment. In the present disclosure, unless otherwise specified, when there are multiple substances corresponding to each component, the amount of each component means the total amount of the multiple substances.

[0010] <Method for manufacturing an electrode> The method for manufacturing the electrode film of the present disclosure is as follows. A step of attaching polyvinylidene fluoride (hereinafter also referred to as PVdF) to the active material particles. A step of mixing the active material particles to which PVdF is attached with PTFE to obtain a mixture. A step of fibrillating PTFE in the mixture, and includes: The fibrillating step is carried out at a temperature of 50 °C or higher. The mixture does not contain a solvent, and it is a method for manufacturing an electrode film.

[0011] In the present disclosure, the "electrode film" means a film-shaped object that contains a substance that functions as an active material of an electrode and is in a self-supporting state (that is, it can maintain its shape without a support). The electrode film is used, for example, as an active material layer disposed on one or both sides of a current collector. The method of the present disclosure does not use a solvent when manufacturing the electrode film. Therefore, the method of the present disclosure is excellent in affinity for living bodies and the environment. Furthermore, the method of the present disclosure can omit the step of volatilizing the solvent after film formation, and is excellent in manufacturing efficiency.

[0012] The electrode film manufactured by the method of the present disclosure exhibits excellent tensile strength compared to the electrode film obtained by carrying out the fibrillation of PTFE at a temperature below 50 °C. For example, according to the method of the present disclosure, it is also possible to manufacture an electrode film having a tensile strength of 0.5 MPa or more. When the tensile strength of the electrode film is 0.5 MPa or more, it is advantageous in that damage such as cracks is less likely to occur during the manufacture of the electrode film. The reason why the electrode film manufactured by the method of the present disclosure exhibits excellent tensile strength can be considered, for example, as follows. When the fibrillation of PTFE is carried out at a temperature of 50 °C or higher, the fibrillation of PTFE is promoted. Furthermore, the PVdF attached to the active material particles melts or softens, and the fibrillated PTFE adheres to the active material particles through the melted or softened PVdF. As a result, it is considered that the tensile strength of the obtained electrode film is improved.

[0013] Hereinafter, the step of attaching PVdF to the active material particles is also referred to as "Step 1", the step of mixing the active material particles to which PVdF is attached and PTFE to obtain a mixture is also referred to as "Step 2", and the step of fibrillating PTFE in the mixture is also referred to as "Step 3".

[0014] (Step 1) In Step 1, PVdF is attached to the active material particles. The method of attaching PVdF to the active material particles is not particularly limited and can be carried out by a known method. For example, PVdF may be attached to the active material particles by applying a shearing force to a mixture containing the active material particles and PVdF using a device such as a mixer, blender, or mill.

[0015] From the viewpoint of effectively attaching the fibrillated PTFE to the active material particles, the amount of PVdF with respect to the active material particles is preferably 1% by mass or more, more preferably 2% by mass or more, and still more preferably 3% by mass or more with respect to 100 parts by mass of the active material particles. From the viewpoint of maintaining good electrode performance, the amount of PVdF with respect to the active material particles is preferably 10% by mass or less, more preferably 8% by mass or less, and still more preferably 6% by mass or less with respect to 100 parts by mass of the active material particles.

[0016] PVdF may adhere to the entire surface of the active material particles or may adhere to a part of the surface of the active material particles. From the viewpoint of effectively attaching the fibrillated PTFE to the active material particles, the coverage rate of the surface of the active material particles by PVdF is preferably 5% or more, more preferably 10% or more, and still more preferably 15% or more. From the perspective of maintaining good electrode performance, the coverage rate of the surface of the active material particles by PVdF is preferably 60% or less, more preferably 50% or less, and even more preferably 40% or less.

[0017] In the present disclosure, the coverage rate of the surface of the active material particles by PVdF is measured by an image analysis method. Examples of the image analysis method include a method of performing elemental mapping by EDX (energy dispersive X-ray spectroscopy). Specifically, the active material particles with PVdF attached to the surface are observed with an SEM (scanning electron microscope), and F mapping is performed by EDX. The region X corresponding to the active material particles and the region Y in the region X where the element (F) contained in PVdF exists are binarized, and the coverage rate is calculated by the following formula. Coverage rate (%) = (Area of Y / Area of X) × 100

[0018] The type of the active material particles used in the production of the electrode film may be a negative electrode active material used for the negative electrode or a positive electrode active material used for the positive electrode.

[0019] Specific examples of the negative electrode active material include carbon materials such as graphite, soft carbon, and hard carbon, and silicon.

[0020] Examples of the positive electrode active material include lithium transition metal composite oxides. Examples of the lithium transition metal composite oxide include layered lithium transition metal composite oxides, spinel-type lithium transition metal composite oxides, olivine-type lithium transition metal composite oxides, and the like. Specific examples of the layered lithium transition metal composite oxide include compounds represented by LiMO2 (M is at least one transition metal selected from the group consisting of Ni, Co, and Mn), and compounds obtained by adding different elements to this compound. Examples of the different elements include Al, Mg, La, Ti, Zn, B, W, Fe, Cr, V, Ru, Cu, Cd, Ag, Y, Sc, Ga, In, As, Sb, Pt, Au, Si, and the like. Specific examples of the spinel-type lithium transition metal composite oxide include LiMn2O4. Specific examples of the olivine-type lithium transition metal composite oxide include LiMPO4 (where M is Fe, Co, Ni, or Mn). The positive electrode active material contained in the positive electrode material may be a single type or two or more types.

[0021] Among the lithium transition metal composite oxides, a layered lithium transition metal composite oxide containing at least one selected from Ni, Co, and Mn as the transition metal is more preferable, a layered lithium transition metal composite oxide containing Ni as the transition metal and at least one selected from Co and Mn is even more preferable, and a layered lithium transition metal composite oxide containing Ni, Co, and Mn as the transition metals respectively (NCM, nickel cobalt manganese oxide) is even more preferable.

[0022] The active material particles used in the production of the electrode film may be a single type or two or more types. The volume average particle diameter of the active material particles is not particularly limited and can be selected, for example, from the range of 5 μm to 30 μm. In the present disclosure, the volume average particle diameter of the particles is the value (D50) when the cumulative from the small diameter side reaches 50% in the volume-based particle size distribution measured by the laser diffraction / scattering method.

[0023] In Step 1, a conductive assistant may be attached to the active material particles together with PVdF. When the active material particles are positive electrode active material particles, it is preferable to attach a conductive assistant to the active material particles. Specific examples of the conductive assistant include carbon materials such as carbon black (acetylene black, thermal black, furnace black, etc.), carbon nanotubes, and graphite.

[0024] The state in which the conductive assistant is attached to the active material particles can be obtained, for example, by applying a shearing force to a mixture containing the active material particles, the conductive assistant, and PVdF using a device such as a mixer, blender, or mill.

[0025] (Step 2) In Step 2, the active material particles to which PVdF is attached and PTFE are mixed to obtain a mixture. The method of mixing the active material particles to which PVdF is attached and PTFE is not particularly limited and can be carried out using known means.

[0026] From the viewpoint of maintaining the strength of the electrode film, the amount of PTFE with respect to 100 parts by mass of the active material particles is preferably 1% by mass or more, more preferably 2% by mass or more, and even more preferably 3% by mass or more. From the viewpoint of maintaining good electrode performance, the amount of PTFE with respect to 100 parts by mass of the active material particles is preferably 10% by mass or less, more preferably 8% by mass or less, and even more preferably 6% by mass or less.

[0027] The PTFE mixed with the active material particles in Step 2 may be in a particulate form. The mixture obtained in Step 2 may or may not contain a solvent. From the viewpoint of workability, it is preferable that the mixture obtained in Step 2 does not contain a solvent. In Step 2, at least a part of the PTFE may be fibrillated. In this case, a granule in which at least a part of the active material particles is bound with the fibrillated PTFE may be produced.

[0028] The mixture obtained in Step 2 may contain a resin other than PVDF and PTFE. Specific examples of the binder other than PVDF and PTFE include polyethylene, polypropylene, polyethylene terephthalate, cellulose, nitrocellulose, carboxymethyl cellulose, polyethylene oxide, polyepichlorohydrin, polyacrylonitrile, styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber (NBR), polyacrylate, polymethacrylate, and the like.

[0029] When the mixture contains resins other than PVDF and PTFE, the amount may be 20 parts by mass or less, 10 parts by mass or less, or 5% by mass or less based on 100 parts by mass in total of PVDF and PTFE.

[0030] (Step 3) In Step 3, the PTFE in the mixture obtained in Step 2 is fibrillated. The PTFE in the mixture obtained in Step 2 is generally in a state where PTFE particles form aggregates, and when a shearing force is applied, the aggregates are broken and part of the particles changes into a fibrous shape. In the present disclosure, in addition to PTFE that is completely in the shape of fibers, a portion where part of the PTFE particles has changed into a fibrous shape is also defined as "fibrillated PTFE". The method for fibrillating PTFE is not particularly limited and can be carried out using known means capable of applying a shearing force to PTFE.

[0031] Step 3 may include forming the mixture into a film shape. Examples of the method for forming the mixture into a film shape while fibrillating PTFE include rolling treatment using a roll press machine.

[0032] In Step 3, the fibrillation of PTFE is carried out at a temperature of 50°C or higher. The above temperature is the temperature of the apparatus used for fibrillating PTFE. For example, when fibrillating PTFE using a roll press machine, the above temperature is the surface temperature of the roll.

[0033] The fibrillation of PTFE is preferably carried out at a temperature of 100°C or higher, more preferably at a temperature of 130°C or higher, and even more preferably at a temperature of 150°C or higher. As shown in the examples described later, by carrying out the fibrillation of PTFE at a temperature of 50°C or higher, the tensile strength of the electrode film is improved. The reason for this is, for example, that the fibrillation of PTFE is promoted and the softening or melting of PVdF occurs, and it is considered that at least part of the fibrillated PTFE adheres to the active material particles by the softened or melted PVdF.

[0034] The fibrillation of PTFE is preferably carried out at a temperature of 200°C or lower, more preferably at a temperature of 180°C or lower, and even more preferably at a temperature of 160°C or lower. As shown in the examples described below, by carrying out the fibrillation of PTFE at a temperature of 200°C or lower, the tensile strength of the electrode film is favorably maintained. For example, it is considered that appropriate softening or melting of PVdF occurs, so that the fibrillated PTFE adheres to the active material particles appropriately, and the stretchability of the obtained electrode film is favorably maintained.

[0035] When the mixture is formed into a film in Step 3, the thickness of the formed body is not particularly limited and can be adjusted according to the desired thickness of the electrode film. For example, the thickness of the formed body (i.e., the thickness of the electrode film) can be selected from the range of 50 μm to 300 μm.

[0036] From the viewpoint of increasing the tensile strength of the electrode film, the fibrillated PTFE obtained in Step 3 is preferably present at a high density in the electrode film. For example, when a straight line with a length of 20 μm is arranged at an arbitrary position in an image of the cross section of the electrode film, it is preferable that the number of fibrous PTFE intersecting the straight line is 5 or more. The number of fibrous PTFE intersecting the straight line with a length of 20 μm is more preferably 10 or more, and even more preferably 20 or more. The image of the cross section of the electrode film preferably includes at least one region where the number of fibrous PTFE intersecting a straight line with a length of 20 μm is 5 or more, 10 or more, or 20 or more. The dimensions of the image of the cross section of the electrode film are preferably 40 μm or more in the width direction and preferably 40 μm or more in the thickness direction. The image of the cross section of the electrode film is obtained using known means such as a transmission electron microscope (SEM). The magnification of the image of the cross section of the electrode film is not particularly limited as long as the fibrillated PTFE can be sufficiently observed.

[0037] The electrode film obtained by the method of the present disclosure may be integrated with a current collector. The method of integrating the electrode with the current collector is not particularly limited and can be carried out using known means. For example, the electrode film and the current collector may be pressure-bonded using a roll press, a flat press, or the like. The material of the current collector is not particularly limited and can be selected from known materials such as aluminum, copper, nickel, titanium, and stainless steel.

[0038] <Electrode film> The electrode film of the present disclosure is an electrode film containing active material particles, PVdF adhering to the active material particles, and fibrous PTFE.

[0039] The electrode film of the present disclosure can be manufactured, for example, by the method for manufacturing the electrode film described above. For details and preferred embodiments of the electrode film of the present disclosure and each material contained in the electrode film, reference can be made to the details and preferred embodiments of the electrode film manufactured by the method for manufacturing the electrode film described above or each material contained in the electrode film.

[0040] It is preferable that at least a part of the fibrous polytetrafluoroethylene adheres to the active material particles in the electrode film.

[0041] The tensile strength of the electrode film is not particularly limited and can be selected according to the type of battery to which the electrode film is applied, etc. For example, the tensile strength of the electrode film may be 0.4 MPa or more, 0.5 MPa or more, or 0.6 MPa or more. When the tensile strength of the electrode film is 0.5 MPa or more, damage such as cracks is less likely to occur during the manufacture of the electrode film. The tensile strength of the electrode film is measured by the method described in the examples.

[0042] <Battery> The battery of the present disclosure includes the electrode film of the present disclosure described above. The battery of the present disclosure includes, for example, a positive electrode and a negative electrode, and an electrode body having a laminated structure including a separator disposed between the positive electrode and the negative electrode as needed. An example of the laminated structure of the electrode body is schematically shown in FIG. 1. The laminated structure 100 of the electrode body shown in FIG. 1 includes a positive electrode 10, a negative electrode 20, and a separator 30 disposed between the positive electrode 10 and the negative electrode 20. The positive electrode 10 includes a positive electrode active material layer 10A and a positive electrode current collector 10B. The negative electrode 20 includes a negative electrode active material layer 20A and a negative electrode current collector 20B.

[0043] The electrode film is included in the battery as an active material layer of the electrode. The electrode film may be included in the battery as either one of the positive electrode active material layer or the negative electrode active material layer, or may be included in the battery as both the positive electrode active material layer and the negative electrode active material layer.

[0044] The type of the battery of the present disclosure is not particularly limited, and can be selected from batteries such as lithium ion secondary batteries (including liquid batteries and all-solid-state batteries), lead-acid batteries, nickel-hydrogen storage batteries, nickel-cadmium storage batteries, nickel-iron storage batteries, nickel-zinc storage batteries, silver oxide-zinc storage batteries, cobalt titanium lithium secondary batteries, and sodium ion secondary batteries.

[0045] When the battery includes a separator, the type of the separator is not particularly limited, and known separators can be used. Specifically, examples of the separator include nonwoven fabrics, cloths, and microporous films mainly composed of polyolefins such as polyethylene and polypropylene. The thickness of the separator is not particularly limited, and for example, it may be selected from the range of 5 μm to 50 μm.

Examples

[0046] Hereinafter, the present disclosure will be described in more detail with reference to examples, but the invention of the present disclosure is not limited to these examples.

[0047] (Fabrication of Electrode Film) Graphite particles as the active material particles (volume average particle diameter: 20 μm, 92.2 parts by mass) and PVdF (4.8 parts by mass) were put into an MP mixer (Nippon Coke & Engineering Co., Ltd.), and a compounding treatment was carried out to adhere PVdF to the surface of the graphite particles under the conditions of 10,000 rpm and 2 minutes.

[0048] After the compounding treatment, PTFE (3 parts by mass) was further added to the MP mixer and mixed under the conditions of 300 rpm and 180 seconds. Then, it was further mixed under the conditions of 5,000 rpm and 500 seconds so that a granulated body in a state where the graphite particles were bound with fibrillated PTFE was formed.

[0049] The mixture containing the granulated body was rolled (linear pressure: 0.4 t / cm, without cooling water) with a roll press to fibrillate PTFE and form it into a film shape to obtain an electrode film. The rolling treatment was carried out at temperatures of 25 °C, 50 °C, 100 °C, 150 °C, 160 °C, 170 °C, 180 °C, and 200 °C, respectively.

[0050] (Measurement of Tensile Strength) Test pieces with the dimensions shown in Table 1 were prepared from the obtained electrode film. Using a texture analyzer (Eihiro Seiki Co., Ltd.), a test was carried out to pull the test piece at a speed of 2 mm / s. The tensile strength was calculated from the maximum value (F) of the load applied to the test piece by the following formula. The results are shown in Table 1. Tensile strength (MPa) = F (g) × 0.0098 / (thickness (mm) × width (mm))

[0051]

Table 1

[0052] As shown in Table 1, the electrode film obtained by carrying out the rolling treatment at a temperature of 50 °C or higher showed a higher tensile strength than the electrode film obtained by carrying out the rolling treatment at 50 °C. Among the electrode films obtained by carrying out the rolling treatment at a temperature of 50 °C or higher, the electrode film obtained by carrying out the rolling treatment at a temperature of 160 °C to 190 °C showed particularly high tensile strength.

[0053] (Electron Microscopic Observation of Electrode Film) The SEM image of the cross-section of the electrode film obtained by performing the rolling process at 150°C is shown in Fig. 2. As shown in Fig. 2, fibrous PTFE was observed in the cross-section of the electrode film obtained by performing the rolling process at 150°C, and a part of it was observed to be attached to graphite particles. Also, the coating rate of the surface of the graphite particles by PVdF measured by EDX was 10% or more. The SEM image shown in Fig. 2 included a region where 20 or more fibrillated PTFEs intersecting a straight line with a length of 20 μm were observed.

Explanation of Symbols

[0054] 10: Positive electrode 10A: Positive electrode active material layer 10B: Positive electrode current collector 20: Negative electrode 20A: Negative electrode active material layer 20B: Negative electrode current collector 30: Separator 100: Laminated structure

Claims

1. A step of attaching polyvinylidene fluoride to active material particles; A step of mixing the active material particles to which polyvinylidene fluoride is attached with polytetrafluoroethylene to obtain a mixture; A step of fibrillating polytetrafluoroethylene in the mixture, and The fibrillating step is carried out at a temperature of 50°C or higher, A method for manufacturing an electrode film, wherein the mixture does not contain a solvent.

2. The method for manufacturing an electrode film according to Claim 1, wherein the fibrillating step is carried out at a temperature of 100°C or higher.

3. The method for manufacturing an electrode film according to Claim 1 or Claim 2, wherein the fibrillating step is carried out at a temperature of 200°C or lower.

4. The method for manufacturing an electrode film according to Claim 1 or Claim 2, wherein the fibrillating step is carried out at a temperature of 180°C or lower.

5. The method for manufacturing an electrode film according to Claim 1 or Claim 2, wherein the fibrillating step includes forming the mixture into a film shape.

6. An electrode film including active material particles, polyvinylidene fluoride attached to the active material particles, and fibrous polytetrafluoroethylene.

7. The electrode film according to Claim 6, wherein the number of fibrous polytetrafluoroethylene intersecting a straight line having a length of 20 μm arranged at an arbitrary position in an image of a cross section of the electrode film is 5 or more.

8. The electrode film according to Claim 7, wherein the number of fibrous polytetrafluoroethylene intersecting the straight line is 20 or more.

9. The electrode film according to any one of Claims 6 to 8, wherein at least a part of the fibrous polytetrafluoroethylene is attached to the active material particles.

10. The electrode film according to any one of Claims 6 to 8, wherein the coverage rate of the surface of the active material particles by polyvinylidene fluoride is 5% or more.

11. A battery including the electrode film according to any one of Claims 6 to 8.

Images