PTFE binder dispersion, electrode slurry, and battery electrode

Abstract

[Problem] The present invention provides a PTFE binder dispersion excellent in output characteristics when made into an electrode, wherein polytetrafluoroethylene is used as a binder. The present invention also provides: electrode slurry which uses said PTFE binder dispersion; and a battery electrode which uses said electrode slurry. [Solution] A PTFE binder dispersion 30 in which the number average molecular weight Mn of PTFE is 10000-35000, D / P is 0.1%-10%, the amount of PTFE blended is 10-70 wt% of the total amount, the cumulant average particle diameter is 0.3 μm or more and less than 3.0 μm, and the shear viscosity ratio (38.3s-1 viscosity / 3.83s-1 viscosity) is 0.1-0.6, and an electrode slurry 40 using same are excellent in coatability. A battery electrode 80 using the PTFE binder dispersion 30 or electrode slurry 40 has high output characteristics.

Description

PTFE binder dispersion, electrode slurry, and battery electrodes

[0001] The present invention relates to a PTFE (polytetrafluoroethylene) binder dispersion for manufacturing electrodes for secondary batteries, an electrode slurry using this PTFE binder dispersion, and electrodes for batteries.

[0002] In recent years, with the spread of portable electronic devices and environmentally friendly electric vehicles, the market for rechargeable secondary batteries, particularly lithium-ion batteries, has been attracting attention. These lithium-ion batteries consist of a negative electrode and a positive electrode containing electrode active materials that allow lithium ions to move in and out reversibly, and an electrolyte that transports lithium ions. Generally, the positive and negative electrodes are made by dissolving and dispersing electrode active materials, conductive additives, and binders in water or a solvent to form an electrode slurry, which is then applied to a current collector such as copper foil or aluminum foil and dried. However, conventional binders have components similar to adhesives and tend to densely fill the gaps between the electrode active materials, which can hinder the movement of lithium ions through the electrolyte.

[0003] Here, the present inventors have made the inventions described in [Patent Document 1], [Patent Document 2], and [Patent Document 3] below, relating to a dispersion suitable for materials such as coating solutions and plating solutions, by dispersing PTFE or a fluororesin in a solvent.

[0004] Patent No. 7078441 Patent No. 7038508 Patent No. 6033939

[0005] As mentioned above, conventional binders can fill the gaps between electrode active materials, potentially hindering the movement of lithium ions. In this respect, the PTFE described in [Patent Document 1], [Patent Document 2], and [Patent Document 3] is in powder form and does not unnecessarily block the gaps between electrode active materials, thus improving the electrical characteristics of secondary batteries.

[0006] The present invention has been made in view of the above circumstances, and an object thereof is to provide a PTFE binder dispersion excellent in output characteristics when polarized, using polytetrafluoroethylene as a binder. Another object is to provide an electrode slurry using this PTFE binder dispersion and a battery electrode using this electrode slurry.

[0007] The present invention relates to a PTFE binder dispersion 30 containing (1) a micropowder 32 of polytetrafluoroethylene, a dispersant 34, and a solvent 36. The micropowder 32 of polytetrafluoroethylene has a number average molecular weight Mn of 10,000 or more and 35,000 or less, the ratio D / P of the amount of the dispersant to the amount of polytetrafluoroethylene is 0.1% or more and 10% or less, the blending amount of the polytetrafluoroethylene 32 is 10 wt% or more and 70 wt% or less of the whole, the cumulant average particle diameter by the dynamic light scattering method is 0.3 μm or more and less than 3.0 μm, and the shear rate is 38.3 s －１ viscosity and shear rate 3.83 s －１ ratio of viscosities (38.3 s －１ viscosity / 3.83 s －１ viscosity) is 0.1 or more and 0.6 or less. By providing the PTFE binder dispersion 30 characterized by this, the above problems are solved. (2) By providing the PTFE binder dispersion 30 according to (1) above, characterized in that (38.3 s －１ viscosity / 3.83 s －１ viscosity) is 0.2 or more and 0.55 or less, the above problems are solved. (3) By providing the PTFE binder dispersion 30 according to (1) above, characterized in that (38.3 s －１ viscosity / 3.83 s －１ viscosity) is 0.3 or more and 0.5 or less, the above problems are solved. (4) By providing the PTFE binder dispersion 30 according to (1) above, characterized in that D / P is 0.1% or more and 5% or less, and (38.3 s －１ viscosity / 3.83 s －１ viscosity) is 0.2 or more and 0.55 or less, the above problems are solved. (5) By providing the PTFE binder dispersion 30 according to (1) above, characterized in that D / P is 0.1% or more and 5% or less, and (38.3 s －１ viscosity / 3.83 s －１The above problem is solved by providing the PTFE binder dispersion 30 described in (1) above, characterized in that its viscosity is 0.3 or more and 0.5 or less. (6) The above problem is solved by providing an electrode slurry 40 comprising the PTFE binder dispersion 30 described in any of (1) to (5) above and an electrode active material 42 for a positive electrode or a negative electrode. (7) The above problem is solved by providing a battery electrode 80 in which an electrode mixture sheet 50, which is a sheet of the electrode slurry 40 described in (6) above, is formed into a film on a current collector 52.

[0008] The PTFE binder dispersion 30 according to the present invention uses polytetrafluoroethylene, and an electrode slurry 40, a battery electrode sheet 50, and a battery electrode 80 with excellent coating properties and output characteristics can be obtained. Furthermore, by using the PTFE binder dispersion 30 with polytetrafluoroethylene as the binder, a secondary battery with excellent output characteristics can be manufactured using the electrode slurry 40 and battery electrode 80 according to the present invention.

[0009] This is a diagram illustrating the manufacturing process for the PTFE binder dispersion, electrode slurry, and battery electrode according to the present invention. This is a table showing the experimental results for the PTFE binder dispersion according to the present invention.

[0010] The PTFE binder dispersion 30, electrode slurry 40, and battery electrode 80 according to the present invention will be described based on the drawings. Here, Figure 1 is a diagram of the manufacturing process of the PTFE binder dispersion 30, electrode slurry 40, and battery electrode 80 according to the present invention. First, the PTFE binder dispersion 30 according to the present invention is mainly composed of PTFE (polytetrafluoroethylene) micropowder 32, a dispersant 34, and an aqueous or oily solvent 36, of which the PTFE 32 has a number average molecular weight Mn of 10,000 or more and 35,000 or less. The amount of PTFE is 10 wt% or more and 70 wt% or less of the total, and the ratio of the amount of dispersant to the amount of PTFE (D / P (%) = (amount of dispersant) / (amount of PTFE) × 100) is 0.1% or more and 10% or less. Furthermore, the cumulant average particle size of the PTFE binder dispersion 30, determined by dynamic light scattering, was set to 0.3 μm or more and less than 3.0 μm, and the shear rate was 38.3 s. －１ Viscosity and shear rate 3.83 s－１ This is the viscosity ratio (38.3s). －１ Viscosity / 3.83s －１ The viscosity shall be between 0.1 and 0.6.

[0011] Furthermore, PTFE32 is a fluororesin composed of fluorine atoms and carbon atoms, and is a useful material used in various products due to its excellent water repellency, oil repellency, electrical properties, heat resistance, electrical insulation, low dielectric properties, low friction properties, non-stick properties, weather resistance, and flame retardancy. In this invention, as mentioned above, the PTFE32 used has a number average molecular weight Mn of 10,000 to 35,000. Here, the number average molecular weight Mn refers to the number average molecular weight obtained by the DSC method (Differential Scanning Calorimetry), and is calculated based on the following formula using the heat of crystallization ΔHc (cal / g) obtained when PTFE is heated to 350°C at a heating rate of 10°C / min and cooled at 10°C / min, and represents the simple average of the molecular weight of one polymer chain contained in the polymer. Mn = 2.1 × 10 １０ ×ΔHc －５．１６ Furthermore, PTFE32 is generally obtained by an emulsion polymerization method, and if the number average molecular weight Mn meets the above range, it can be used as is in the present invention. If the number average molecular weight Mn is greater than the above range, the PTFE particles may be irradiated with radiation such as gamma rays to cleave them, bringing the number average molecular weight Mn within the above range for use in the present invention. However, PTFE with a number average molecular weight Mn of less than 10,000 is difficult to obtain and leads to increased material costs, and if the number average molecular weight Mn exceeds 35,000, the dispersibility is poor and it becomes difficult to form a homogeneous sheet.

[0012] Furthermore, the dispersant 34 used in the present invention is not particularly limited as long as it can uniformly and stably disperse the PTFE 32 micropowder, and any well-known dispersant 34 can be used. For example, when the solvent 36 is aqueous, acrylic copolymers, fluorine copolymers, polyester copolymers, acetylene copolymers, silicon copolymers, and others can be used. In particular, it is preferable to use a nonionic dispersant 34 from the viewpoint of suppressing adverse effects on other materials constituting the battery electrode 80. Also, when the solvent 36 is oily, in addition to well-known oily dispersions, for example, a ternary polymer composed of vinyl butyral / vinyl acetate / vinyl alcohol having butyral groups, acetyl groups, and hydroxyl groups, obtained by reacting polyvinyl alcohol (PVA) with butyraldehyde (BA), can be used.

[0013] The amount of these dispersants 34 is formulated such that the ratio D / P of the amount of dispersant 34 to the amount of PTFE is between 0.1% and 10%. If the amount of dispersant 34 is less than 0.1% D / P, the dispersibility is insufficient, while if it exceeds 10%, the viscosity of the PTFE binder dispersion 30 increases, the number of aggregated particles increases, and the coating properties deteriorate.

[0014] Furthermore, there are no particular limitations on the solvent 36 used in the present invention, and as an aqueous solvent, for example, ion-exchanged water, distilled water, purified water, pure water, ultrapure water, tap water, etc. can be used. Furthermore, examples of oily solvents include acetone, methyl ethyl ketone, hexane, heptane, octane, 2-heptanone, cycloheptanone, cyclohexanone, cyclohexane, methylcyclohexane, ethylcyclohexane, methyl-n-pentyl ketone, methyl isobutyl ketone, methyl isopentyl ketone, ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, ethylene glycol monoacetate, ethylene glycol monomethyl ether acetate, ethylene glycol monoethyl ether acetate, diethylene glycol monoacetate, diethylene glycol diethyl ether, propylene glycol monoacetate, dipropylene glycol monoacetate, propylene glycol diacetate, propylene glycol monomethyl ether acetate, propylene glycol monoethyl ether acetate, cyclohexyl acetate, ethyl 3-ethoxypropionate, dioxane, methyl lactate, ethyl lactate, methyl acetate L, ethyl acetate, butyl acetate, methyl pyruvate, ethyl pyruvate, methyl methoxypropionate, ethyl ethoxypropionate, anisole, ethyl benzyl ether, cresyl methyl ether, diphenyl ether, dibenzyl ether, phenetol, butylphenyl ether, benzene, ethylbenzene, diethylbenzene, pentylbenzene, isopropylbenzene, toluene, xylene, cymene, mesitylene, methanol, ethanol, isopropanol, butanol, dihydroterpineol, methyl monoglycidyl ether, ethyl monoglycidyl ether, butyl monoglycidyl ether, phenyl monoglycidyl ether, methyl diglycidyl ether, ethyl diglycidyl ether, butyl diglycidyl ether, phenyl diglycidyl ether, methylphenol monoglycidyl ether, ethylphenol monoglycidyl ether, butylphenol monoglycidyl ether, mineral spirits, 2-hydroxyethyl acrylate, tetrahydrofurfuryl acrylate, 4-vinylpyridine,2-Ethylhexyl acrylate, 2-Hydroxyethyl methacrylate, Hydroxypropyl methacrylate, Glycidyl methacrylate, Neopentyl glycol diacrylate, Hexanediol diacrylate, Trimethylolpropane triacrylate, Methacrylate, Methyl methacrylate, Styrene, Perfluorocarbon, Hydrofluoroether, Hydrochlorofluorocarbon, Hydrofluorocarbon, Perfluoropolyether, Dimethylimidazoline, Tetrahydrofuran, Pyridine, Formamide, Acetanilide, Dioxy Solane, o-cresol, m-cresol, p-cresol, phenol, N-methyl-2-pyrrolidone, N-acetyl-2-pyrrolidone, N,N-dimethylformamide, N,N-diethylformamide, N,N-dimethylacetamide, N,N-diethylacetamide, 1,3-dimethyl-2-imidazolidinone, dimethyl sulfoxide, diethyl sulfoxide, dimethyl sulfone, diethyl sulfone, γ-butyrolactone, sulfolane, halogenated phenols, various silicone oils, etc., or mixtures of two or more of these solvents can be used.

[0015] Furthermore, in the PTFE binder dispersion 30 according to the present invention, the amount of PTFE 32 is set to 10 wt% or more and 70 wt% or less of the total dispersion, as described above. If this amount is less than 10 wt%, in addition to poor production efficiency, it becomes difficult to increase the solid content of the electrode slurry, and it takes time to remove the solvent 36 in the coating process. If it exceeds 70 wt%, the viscosity is high and sufficient dispersion cannot be achieved, and problems occur during coating.

[0016] Furthermore, in the PTFE binder dispersion 30 according to the present invention, the cumulant average particle diameter measured by dynamic light scattering is set to 0.3 μm or more and less than 3.0 μm. Here, the cumulant average particle diameter refers to the average particle diameter obtained by cumulant analysis of the scattering intensity distribution of PTFE particles in the PTFE binder dispersion 30 measured by dynamic light scattering. If the cumulant average particle diameter is less than 0.3 μm, deterioration of electrical properties (output properties) is observed, and if the cumulant average particle diameter is 3.0 μm or more, an increase in aggregated particles and deterioration of coating properties are observed.

[0017] Furthermore, in the PTFE binder dispersion 30 according to the present invention, the shear rate is 3.83 s. －１ Viscosity and shear rate in 38.3 s －１ This is the ratio to viscosity in (38.3 s). －１ Viscosity / 3.83s －１ The viscosity is preferably in the range of 0.1 to 0.6, more preferably in the range of 0.2 to 0.55, and most preferably in the range of 0.3 to 0.5. Furthermore, in this case, further improvement in properties can be observed by setting the D / P in the range of 0.1% to 5%. －１ Viscosity / 3.83s －１ If the viscosity falls outside the range of 0.1 to 0.6, the number of aggregated particles increases, and the coating properties deteriorate.

[0018] Next, the results of verification experiments of the PTFE binder dispersion 30 according to the present invention are shown. First, purified water was used as the aqueous solvent 36, and PTFE 32 and an aqueous acrylic block copolymer dispersant 34 were mixed in the D / P ratio shown in Figure 2(a), and the mixture was performed using a homomixer (AS ONE HM-300) at 10,000 rpm for 5 minutes. This yielded the aqueous PTFE binder dispersions 30 of Examples 1 to 7. The PTFE binder dispersions of Comparative Examples 1 to 7 were also obtained. Furthermore, N-methyl-2-pyrrolidone was used as the oily solvent 36, and PTFE 32 and a polyvinyl alcohol resin-based dispersant 34 were mixed in the D / P ratio shown in Figure 2(b), and the mixture was performed using a homomixer at 10,000 rpm for 5 minutes. This yielded the oily PTFE binder dispersions 30 of Examples 8 to 13. Furthermore, PTFE binder dispersions of Comparative Examples 8 to 13 were obtained (binder dispersion mixing step S102).

[0019] In this process, the cumulant average particle size of the PTFE particles in the prepared PTFE binder dispersion 30 was obtained using a particle size analyzer (Otsuka Electronics Co., Ltd.: FPAR-1000).

[0020] Furthermore, using a cone-plate viscometer (Toki Sangyo Co., Ltd.: E-type rotational viscometer TV-25 (rotor: 1°34′ × R24 mm)), the shear rate at 25°C was 3.83 s. －１ Viscosity and shear rate of 38.3 s －１ The viscosity was measured for each, and the viscosity ratio (38.3 s) was determined. －１ Viscosity / 3.83s －１ The viscosity was calculated.

[0021] Furthermore, these PTFE binder dispersions 30 were placed on a grind gauge, and the number of aggregated particles in the range of 20 μm to 100 μm was visually counted. In the evaluation of the number of aggregated particles in Figure 2, ◎ indicates 5 or fewer aggregated particles, ○ indicates 6 to 20 particles, △ indicates 21 to 50 particles, and × indicates 51 or more particles.

[0022] Next, an electrode active material 42, a conductive assistant 44, and a solvent 36 were added to these PTFE binder dispersions 30 and mixed using a planetary mixer under the condition of a revolution speed of 70 rpm to prepare an electrode slurry 40 (electrode slurry mixing step S104). At this time, the compounding amounts of the PTFE binder dispersion 30 and the solvent 36 were adjusted so that the amount of PTFE 32 was 2.5 wt% of the entire electrode slurry 40. Further, in the electrode slurry 40 of the negative electrode, an aqueous PTFE binder dispersion 30 and an aqueous solvent 36 (purified water) were used, 45 wt% of artificial graphite and 5 wt% of silicon monoxide were used as the electrode active material 42, and the conductive assistant 44 was 2 wt%. Further, in the electrode slurry 40 of the positive electrode, an oily PTFE binder dispersion 30 and an oily solvent 36 (N-methyl-2-pyrrolidone) were used, 60 wt% of NCM811 (nickel 0.8, cobalt 0.1, manganese 0.1, O ２ ), 60 wt% was used, and the conductive assistant 44 was 3 wt%.

[0023] Next, the electrode slurry 40 of the negative electrode prepared as described above was coated on a copper foil as a current collector 52 of the negative electrode with a knife coater at a basis weight of 80 g / m ２ , a width of 200 mm, and a coating thickness of 100 µm, and this was dried in a continuous drying furnace to obtain a battery electrode sheet 50 for the negative electrode. Similarly, the electrode slurry 40 of the positive electrode was coated on an aluminum foil as a current collector 52 of the positive electrode at a basis weight of 80 g / m ２ , a width of 200 mm, and a coating thickness of 100 µm, and this was dried in a continuous drying furnace to obtain a battery electrode sheet 50 for the positive electrode (electrode sheet coating step S106).

[0024] Also, at this time, defects (film formation defects, cracks, holes, streaks, etc.) exceeding an area of 10 mm ２ on the surface of the battery electrode sheet 50 were visually counted to evaluate the coating property of the electrode slurry 40. At this time, large defects were counted as multiple, such as 2 defects exceeding an area of 20 mm ２ and 3 defects exceeding an area of 30 mm ２ . In the evaluation of the coating property in FIG. 2, the ◎ mark indicates 0 to 2 defects, the ○ mark indicates 3 to 5 defects, the △ mark indicates 6 to 10 defects, and the × mark indicates 11 or more defects.

[0025] Next, the battery electrode sheet 50 obtained above is punched out into the required shape, and the electrode density is set to 2.1 g / cm³ for the negative electrode. ３ The positive electrode has a concentration of 3.0 g / cm³. ３ The battery electrodes 80 were fabricated by pressing them with a press machine (electrode molding process S108). For testing purposes, a circular electrode with a diameter of 2 cm was used. Then, in order to individually evaluate the output characteristics of these positive and negative battery electrodes 80, lithium metal was used as the counter electrode, and three test lithium batteries consisting of a single-electrode coin-type cell were fabricated under each condition.

[0026] Next, the test lithium battery underwent its initial charge-discharge cycle at a C-rate of 0.2C. Afterward, the discharge capacity was measured when the C-rate was changed to 5C. The ratio (%) of the 5C discharge capacity to the 0.2C discharge capacity was calculated, and the output characteristics of the battery electrode 80 were evaluated. Measurements were performed on three test lithium batteries. Those with an average value of 90% or higher are indicated with a ◎ in Figure 2, those between 89% and 80% with a ○, those between 79% and 60% with a △, and those below 60% with a ×.

[0027] These experimental results are shown in Figure 2. From Figure 2, Comparative Examples 7 and 13, which used PTFE 32 with a number-average molecular weight Mn exceeding 35,000 (180,000), showed significantly poor dispersibility, resulting in poor results in terms of aggregated particle count, coating properties, and output characteristics. Next, regarding the amount of PTFE blended, Comparative Examples 5 and 11, which used a blending amount exceeding 70 wt% (72 wt%), had poor fluidity of the PTFE binder dispersion 30, making it impossible to measure the shear viscosity, and also showed poor results in terms of aggregated particle count, coating properties, and output characteristics. Furthermore, Comparative Examples 1 and 8, which used a PTFE blending amount less than 10 wt% (8 wt%), showed good output characteristics, but poor results in terms of aggregated particle count and coating properties. Next, regarding the cumulant particle size, Comparative Examples 2 and 9, with particle sizes exceeding 3 μm (3.51 μm and 3.45 μm), showed a large number of aggregated particles, possibly due to the large particle size, resulting in poor coating properties and output characteristics. Furthermore, Comparative Examples 3 and 10, with particle sizes less than 0.3 μm (0.28 μm and 0.29 μm), showed poor output characteristics, as well as inferior aggregate particle count and coating properties. Next, the shear viscosity ratio (38.3 s) －１ Viscosity / 3.83s －１In Comparative Example 4 where the viscosity was 0.09, which is less than 0.1, both the number of aggregated particles and the coating property were poor. Also, for Comparative Examples 6 and 12 where the shear viscosity ratio (38.3 s －１ viscosity / 3.83 s[[ID=!]] －１ viscosity) was 0.64, which exceeded 0.6, the results were inferior in terms of the number of aggregated particles, coating property, and output characteristics.

[0028] In contrast, for the PTFE binder dispersion 30 and electrode slurry 40 according to the present invention of Examples 1 to 13, where the number average molecular weight Mn of PTFE is 10,000 or more and 35,000 or less, D / P is 0.1% or more and 10% or less, the blending amount of PTFE is 10 wt% or more and 70 wt% or less of the whole, the cumulative average particle diameter is 0.3 μm or more and less than 3.0 μm, and the shear viscosity ratio (38.3 s －１ viscosity / 3.83 s －１ viscosity) is 0.1 or more and 0.6 or less, the coating property is excellent. Also, for the battery electrode 80 using this PTFE binder dispersion 30 and electrode slurry 40, results with high output characteristics were obtained.

[0029] As described above, it can be seen that the PTFE binder dispersion 30 according to the present invention and the electrode slurry 40 using the same have excellent coating properties, and further, the battery electrode 80 using the same has high output characteristics.

[0030] Note that the members such as the dispersant 34, solvent 36, electrode active material 42, conductive assistant 44, current collector 52, etc., manufacturing method, manufacturing apparatus, manufacturing conditions, production order, etc. constituting the PTFE binder dispersion 30, electrode slurry 40, battery electrode sheet 50, and battery electrode 80 shown in this example are merely examples and are not limited thereto. Other necessary procedures, members, etc. may be added as appropriate, and it is also possible to implement with modifications within the scope not departing from the gist of the present invention.

[0031] 30 PTFE binder dispersion 34 Dispersant 36 Solvent 40 Electrode slurry 42 Electrode active material 44 Conductive assistant 50 Battery electrode sheet 52 Current collector 80 Battery electrode

Claims

1. In a PTFE binder dispersion containing polytetrafluoroethylene micropowder, a dispersant, and a solvent, the polytetrafluoroethylene micropowder has a number average molecular weight (Mn) of 10,000 to 35,000, a D / P ratio (dispersant to polytetrafluoroethylene) of 0.1% to 10%, a polytetrafluoroethylene content of 10 wt% to 70 wt% of the total, a cumulant average particle diameter of 0.3 μm to less than 3.0 μm as measured by dynamic light scattering, and a shear rate of 38.3 s. －１ Viscosity and shear rate 3.83 s －１ This is the viscosity ratio (38.3 s). －１ Viscosity / 3.83s －１ A PTFE binder dispersion characterized by having a viscosity of 0.1 or more and 0.6 or less.

2. (38.3s) －１ Viscosity / 3.83s －１ The PTFE binder dispersion according to claim 1, characterized in that its viscosity is 0.2 or more and 0.55 or less.

3. (38.3s) －１ Viscosity / 3.83s －１ The PTFE binder dispersion according to claim 1, characterized in that its viscosity is 0.3 or more and 0.5 or less.

4. The D / P is 0.1% or more and 5% or less, and (38.3 s －１ viscosity / 3.83 s －１ viscosity) is 0.2 or more and 0.55 or less. The PTFE binder dispersion according to claim 1, characterized in that.

5. D / P is between 0.1% and 5%, (38.3s) －１ Viscosity / 3.83s －１ The PTFE binder dispersion according to claim 1, characterized in that its viscosity is 0.3 or more and 0.5 or less.

6. An electrode slurry comprising a PTFE binder dispersion according to any one of claims 1 to 5, and an electrode active material for a positive or negative electrode.

7. A battery electrode in which an electrode mixture sheet, formed from the electrode slurry described in claim 6, is formed as a film on a current collector.

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