Electrode paste for power storage device

Abstract

To provide an electrode paste for a power storage device, capable of manufacturing a power storage device with low internal resistance.SOLUTION: In an aspect of the present disclosure, an electrode paste for a power storage device is formed by mixing an active material, a carbon material based conductive material, fluorine resin, a nonaqueous solvent, and polyvinyl acetal resin, in which the mixing amount of the polyvinyl acetal resin relative to the total mass of the fluorine resin and the polyvinyl acetal resin is 1 mass% or more and 4 mass% or less. The acetalization ratio of the polyvinyl acetal resin is preferably 5% or more and 55% or less. The carbon material based conductive material is preferably carbon nanotube and more preferably a double layer carbon nanotube.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an electrode paste for an electricity storage device, a manufacturing method thereof, and a positive electrode for an electricity storage device manufactured using the electrode paste for an electricity storage device. [Background technology]

[0002] The demand for energy storage devices has been growing in recent years due to factors such as the widespread use of smartphones, zero-emission regulations in the automotive market, and the expanded use of natural energy. Therefore, energy storage devices are expected to be small, lightweight, and have large capacities, and there is a growing demand for higher output and higher energy density in automobiles and other applications. To meet these demands, the development of energy storage devices such as lithium-ion secondary batteries, alkali-ion secondary batteries, electric double-layer capacitors, and lithium-ion capacitors is progressing.

[0003] The composite layer of the electrode of such an electricity storage device is generally produced by applying a paste containing an active material and the like onto a metal foil and then drying the coating by applying hot air.

[0004] In recent years, amid growing demand for reducing the cost of storage batteries, there has been a demand for shortening the drying time of the coating film to increase the production efficiency of storage batteries. However, if the drying conditions (e.g., increasing the hot air temperature) are significantly changed to meet such demands, the difference between the evaporation rate of the solvent on the coating film surface and the diffusion rate of the solvent from deeper within the coating film increases, resulting in segregation (migration) of the carbon material and binder resin components in the resulting composite layer to the surface of the composite layer, which can lead to a decrease in electrode performance, specifically an increase in the internal resistance of a battery using the electrode. Therefore, for example, Patent Document 1 has proposed a method for efficiently drying an electrode layer without degrading electrode performance by drying an electrode layer (coating film) formed by applying a solvent-containing electrode slurry to an electrode foil in an electrode drying device consisting of multiple drying furnaces connected in multiple stages, and controlling the drying temperature so as to reduce the concentration difference between the solvent concentration remaining in the electrode layer and the solvent concentration in the atmosphere inside each drying furnace. Patent Document 2 discloses a resin composition containing a polyvinyl acetal resin as a dispersant for a fibrous carbon material. Patent Document 3 discloses a composition for a secondary battery electrode containing a polyvinyl acetal resin as a binder (binding agent), and when the composition for a secondary battery electrode contains polyvinylidene fluoride as a binder, the weight ratio of the polyvinyl acetal resin to polyvinylidene fluoride is 0.5:9.5 to 7:3. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-88048 [Patent Document 2] WO2022 / 210177 publication [Patent Document 3] WO2019 / 065869 publication Summary of the Invention [Problem to be solved by the invention]

[0006] However, the above method requires a large investment in equipment and is not necessarily advantageous in terms of cost. Therefore, the present invention provides an electrode paste for an electricity storage device that is less likely to cause migration even when the coating film of the electrode paste is dried at high temperatures, and as a result, it is possible to produce an electricity storage device with low internal resistance. [Means for solving the problem]

[0007] In one aspect, the present disclosure provides a battery comprising an active material, a carbonaceous conductive material, a fluororesin, a non-aqueous solvent, and a polyvinyl acetal resin, The present invention relates to an electrode paste for an electricity storage device, wherein the content of the polyvinyl acetal resin relative to the total mass of the fluororesin and the polyvinyl acetal resin is 1% by mass or more and 4% by mass or less.

[0008] In one aspect, the present disclosure relates to a method for producing a positive electrode for an electricity storage device, the method comprising applying the electrode paste for an electricity storage device of the present invention to a current collector to form a coating film, and then drying the coating film in an atmosphere at 60°C or higher. [Effects of the Invention]

[0009] According to one aspect of the present disclosure, it is possible to provide an electrode paste for an electricity storage device that makes it possible to produce an electricity storage device with low internal resistance even when the drying temperature of the coating film of the electrode paste is increased. As described above, conventionally, complicated drying conditions have been employed to dry the applied electrode paste (coating film), thereby preventing the internal resistance of the electricity storage device from increasing. However, the present disclosure makes it possible to produce an electricity storage device with low internal resistance without employing complicated drying conditions, even when the coating film is dried at a high temperature. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 shows the 1H-NMR spectrum of a polyvinyl acetal resin. DETAILED DESCRIPTION OF THE INVENTION

[0011] Although the details of the mechanism by which the effects of the present disclosure are manifested are not clear, it is presumed as follows. In the electrode paste for an electrical storage device of the present disclosure (hereinafter sometimes abbreviated as "the electrode paste of the present disclosure"), polyvinyl acetal resin is blended as a dispersant to disperse a carbon material-based electrically conductive material (hereinafter sometimes abbreviated as "the electrically conductive material"). The inclusion of this dispersant allows the electrically conductive material to be uniformly dispersed in the electrode paste. Furthermore, the inclusion of a predetermined amount of the dispersant strengthens the interaction between the electrically conductive material and the active material, suppressing migration that may occur during drying of a coating film made of the electrode paste, ultimately enabling the production of an electrical storage device with low internal resistance. More specifically, an amount of polyvinyl acetal resin that does not adversely affect the conductivity of the composite layer adsorbs not only the electrically conductive material but also the active material, which has a specific gravity much greater than that of the electrically conductive material, thereby strengthening the interaction between the electrically conductive material and the active material. In this way, the electrode paste of the present disclosure can suppress migration in the composite layer formed using the paste, and ultimately enables the production of an electricity storage device with low internal resistance. However, the present disclosure should not be interpreted as being limited to these mechanisms.

[0012] <Electrode paste for energy storage devices> [Active material] The active material contained in the electrode paste of the present disclosure is preferably used for a positive electrode. The positive electrode active material is not particularly limited as long as it is an inorganic compound, and for example, a compound having an olivine structure or a lithium transition metal composite oxide can be used. Examples of compounds having an olivine structure include compounds represented by the general formula LixM1sPO4 (where M1 is a 3d transition metal, 0≦x≦2, 0.8≦s≦1.2). Compounds having an olivine structure may be used after being coated with amorphous carbon or the like. Examples of lithium transition metal composite oxides include lithium manganese oxides having a spinel structure and lithium manganese oxides having a layered structure and the general formula LixMO 2-Examples include lithium transition metal composite oxides represented by x and δ (where M is a transition metal, 0.4≦x≦1.2, 0≦δ≦0.5). The transition metal M may include Co, Ni, or Mn. The lithium transition metal composite oxide may further contain one or more elements selected from Al, Mn, Fe, Ni, Co, Cr, Ti, Zn, P, and B.

[0013] [Carbon-based conductive materials] In one or more embodiments, the conductive material contained in the electrode paste of the present disclosure may be carbon nanotubes (hereinafter sometimes referred to as "CNTs"), carbon black, graphite, graphene, etc., and among these, from the viewpoint of realizing high conductivity, at least one selected from carbon black and carbon nanotubes is preferred, with carbon nanotubes being more preferred. The conductive material may be one type or a combination of two or more types.

[0014] The average diameter of CNTs that can be used as a conductive material is not particularly limited, but from the viewpoint of improving the dispersibility of CNTs, it is preferably 2 nm or more, more preferably 3 nm or more, and even more preferably 5 nm or more, and from the viewpoint of improving the conductivity, it is preferably 100 nm or less, more preferably 70 nm or less, and even more preferably 50 nm or less. In the present disclosure, the average diameter of CNTs can be measured using a scanning electron microscope (SEM) or an atomic force microscope (AFM).

[0015] CNTs with two or more different diameters may be used as the conductive material to achieve both electrical conductivity and dispersibility. When using CNTs with two or more different diameters, the average diameter of the relatively thin CNTs is preferably 2 nm or more, more preferably 3 nm or more, and even more preferably 5 nm or more from the viewpoint of dispersibility, and is preferably less than 30 nm, more preferably 25 nm or less, and even more preferably 20 nm or less from the viewpoint of electrical conductivity. The average diameter of the relatively thick CNTs is preferably 30 nm or more, more preferably 35 nm or more, and even more preferably 40 nm or more from the viewpoint of dispersibility, and is preferably 100 nm or less, more preferably 70 nm or less, and even more preferably 50 nm or less from the viewpoint of improved electrical conductivity.

[0016] The average length of the CNTs is measured using a scanning electron microscope (SEM) or an atomic force microscope (AFM). In the present disclosure, the average length is not particularly limited, but from the viewpoint of improving electrical conductivity, it is preferably 2 μm or more, more preferably 5 μm or more, even more preferably 10 μm or more, and even more preferably 30 μm or more, and from the viewpoint of improving dispersibility, it is preferably 800 μm or less, more preferably 600 μm or less, and even more preferably 200 μm or less.

[0017] In this disclosure, CNT refers to a whole that includes multiple CNTs. The form of the CNTs used in preparing the electrode paste of this disclosure is not particularly limited, but for example, multiple CNTs may be independent, multiple CNTs may be bundled or entangled, or a mixture of these forms may be used. CNTs may have various numbers of walls or diameters. CNTs may contain impurities (e.g., catalysts and amorphous carbon) derived from the CNT production process.

[0018] The impurity content in CNTs is measured by methods such as thermogravimetric analysis, and in the present disclosure, the lower the better. From the viewpoint of achieving a high concentration of useful CNT content, the impurity content in CNTs is preferably 40% by mass or less, more preferably 30% by mass or less, even more preferably 20% by mass or less, even more preferably 10% by mass or less, and even more preferably substantially 0% by mass.

[0019] In one or more embodiments, CNTs that can be used as a conductive material have a cylindrical shape formed by winding a single graphite sheet. Those wound in one layer are called single-walled carbon nanotubes (SWCNTs), those wound in two layers are called double-walled carbon nanotubes (DWCNTs), and those wound in three or more layers are called multi-walled carbon nanotubes (MWCNTs). In the present disclosure, single-walled, double-walled, or multi-walled CNTs, or mixtures thereof, can be used depending on the properties required for the composite layer formed using the electrode paste. However, two or more walls are preferred for ease of dispersion. Among these, the use of double-walled CNTs is particularly preferred from the perspective of reducing the internal resistance of the battery with a small additive amount. The number of turns (number of layers) of the CNTs can be measured by observation with a transmission electron microscope (TEM).

[0020] Examples of CNTs that can be used as conductive materials include Nanocyl's NC-7000 (mean diameter: 9.5 nm), NX7100 (10 nm), Cnano's FT6100 (9 nm), FT-6110 (9 nm), FT-6120 (9 nm), FT-7000 (9 nm), FT-7010 (9 nm), FT-7320 (9 nm), FT-9000 (12.5 nm), FT-9100 (12.5 nm), FT-9110 (12.5 nm), FT-9200 (19 nm), FT-9220 (19 nm), FT-2020 (4 nm), and Cabot Performance. Examples of such nanotubes include HCNTs4 (4.5 nm), CNTs5 (7.5 nm), HCNTs5 (7.5 nm), GCNTs5 (7.5 nm), HCNTs10 (15 nm), CNTs20 (25 nm), and CNTs40 (40 nm) from Materials (Shenzhen), CTUBE170 (13.5 nm), CTUBE199 (8 nm), and CTUBE298 (10 nm) from Korea CNT Company, K-Nanos100P (11.5 nm) from Kumho, CP-1001M (12.5 nm) and BT-1003M (12.5 nm) from LG Chem, and 3003 (10 nm) and 3021 (20 nm) from Nano Tech Port. Examples of CNTs used in combination of two types include Cabot Performance Materials (Shenzhen) CNTs40 (40 nm) and HCNTs4 (4.5 nm) or HCNTs5 (7.5 nm), CNTs40 (40 nm) and GCNTs5 (7.5 nm), CNTs40 (40 nm) and Cnano's FT-7010 (9 nm), CNTs40 (40 nm) and FT-9100 (12.5 nm), and CNTs40 (40 nm) and LG Chem's BT-1003M (12.5 nm).

[0021] In the powder X-ray diffraction spectrum of CNT obtained by irradiating with Cu KAl line (λ=0.15418 nm), the half width of the peak having an apex within the diffraction angle 2θ (θ is the Bragg angle) region of 23° to 27° (25°±2°) is preferably 1° or more, more preferably 2° or more, and even more preferably 3° or more, from the viewpoint of facilitating dispersion of the CNT, and the half width of the peak is preferably 10° or less, more preferably 8° or less, and even more preferably 5° or less, from the viewpoint of increasing the conductivity of the CNT.

[0022] The peak intensity of the D band of the CNTs (I D ) versus G-band peak intensity (I G ) ratio (I G / I D ) is preferably 0.5 or more, more preferably 30 or more, and even more preferably 40 or more from the viewpoint of increasing the conductivity of the CNTs, and is preferably 70 or less, more preferably 65 or less, and even more preferably 60 or less from the viewpoint of facilitating the dispersion of the CNTs.

[0023] [CNT slurry] When the conductive material contained in the electrode paste of the present disclosure is CNT, from the viewpoint of maximizing the conductive performance thereof, when preparing the electrode paste of the present disclosure, the CNT is preferably mixed with the other components contained in the electrode paste of the present disclosure in the form of a CNT slurry containing CNT, polyvinyl acetal resin, and a non-aqueous solvent.

[0024] In one or more embodiments, the CNT slurry can be prepared by mixing CNTs, a polyvinyl acetal resin, and a non-aqueous solvent using a mixer / disperser.

[0025] Examples of the mixer / disperser include at least one selected from ultrasonic homogenizers, vibration mills, jet mills, ball mills, bead mills, sand mills, roll mills, homogenizers, high-pressure homogenizers, ultrasonic devices, attritors, dissolvers, and paint shakers. Some of the components constituting the CNT slurry can be mixed first and then mixed with the remainder, or each component can be added in multiple batches rather than all at once. The polyvinyl acetal resin can be dissolved in a non-aqueous solvent and then mixed with other components such as CNTs. The CNTs may be dry or contain a solvent just before being mixed with the other components.

[0026] The amount of CNTs in the CNT slurry is preferably 0.05% by mass or more, more preferably 0.1% by mass or more, and even more preferably 1% by mass or more, from the viewpoint of improving the convenience of adjusting the concentration of the electrode paste of the present disclosure, and is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 3% by mass or less, from the viewpoint of giving the electrode paste of the present disclosure an easy-to-handle viscosity.

[0027] The polyvinyl acetal resin is preferably contained in the CNT slurry because it has the ability to disperse CNTs in a non-aqueous solvent. The amount of polyvinyl acetal resin in the CNT slurry is preferably 0.01% by mass or more, more preferably 0.05% by mass or more, even more preferably 0.08% by mass or more, and even more preferably 0.15% by mass or more from the viewpoint of improving CNT dispersibility, and is preferably 2.0% by mass or less, more preferably 1.5% by mass or less, even more preferably 1.2% by mass or less, and even more preferably 1.0% by mass or less from the viewpoint of reducing the internal resistance of the electricity storage device.

[0028] The amount of polyvinyl acetal resin relative to the CNTs in the CNT slurry (amount of polyvinyl acetal resin / amount of CNTs × 100) is preferably 5% by mass or more, more preferably 10% by mass or more, and even more preferably 15% by mass or more, from the viewpoint of improving the dispersibility of the CNTs. Furthermore, from the viewpoint of reducing the internal resistance of the electricity storage device of the present disclosure, it is preferably 300% by mass or less, more preferably 250% by mass or less, and even more preferably 200% by mass or less.

[0029] Various carbon blacks can be used as conductive materials, including furnace black, channel black, thermal black, acetylene black (AB), and ketjen black. Conventional oxidation-treated carbon black and hollow carbon can also be used. Carbon oxidation involves treating carbon at high temperatures in air or secondary treatment with nitric acid, nitrogen dioxide, ozone, or the like, to directly introduce (covalently bond) oxygen-containing polar functional groups, such as phenol groups, quinone groups, carboxy groups, and carbonyl groups, onto the carbon surface. This oxidation treatment is commonly used to improve carbon dispersibility. However, the greater the amount of functional groups introduced, the lower the carbon's conductivity generally becomes. Therefore, it is preferable to use carbon that has not been subjected to oxidation treatment.

[0030] The larger the specific surface area of ​​carbon black that can be used as a conductive material, the more contact points there are between carbon black particles, which is advantageous for reducing the internal resistance of an electricity storage device. Specifically, the specific surface area (BET) calculated from the amount of nitrogen adsorption is preferably 20 m 2 / g or more 1500m 2 It is desirable to use one with a density of 1 / g or less.

[0031] From the viewpoint of conductivity, the primary particle size (diameter) of carbon black that can be used as a conductive material is preferably 5 to 1000 nm. In the present disclosure, the primary particle size of carbon black is the average particle size measured using an electron microscope or the like.

[0032] Examples of carbon black that can be used as a conductive material include Toka Black #4300, #4400, #4500, #5500, etc. (furnace black, manufactured by Tokai Carbon Co., Ltd.), Printex L, etc. (furnace black, manufactured by Degussa), Raven 7000, 5750, 5250, 5000ULTRA III, 5000ULTRA, etc., Conductex SC ULTRA, and Conductex 975. ULTRA etc. (Columbian furnace black), #2350, #2400B, #30050B, #3030B, #3230B, #3350B, #3400B, #5400B etc. (Mitsubishi Chemical furnace black), MONARCH1400, 1300, 900, VulcanXC-72R, BlackPearls2000 etc. (Cabot furnace black), Ensaco2 Examples of suitable acetylene black include, but are not limited to, acetylene black, such as 50G, Ensaco260G, Ensaco350G, and SuperP-Li (manufactured by TIMCAL), Ketjen Black EC-300J and EC-600JD (manufactured by Akzo Chemicals), Denka Black, Denka Black HS-100, FX-35, Li-100, Li-250, Li-400, and Li-435 (manufactured by Denka Co., Ltd.).

[0033] [Fluorine resin] The electrode paste of the present disclosure contains a fluororesin. Fluororesin is a resin containing fluorine, has excellent heat resistance, chemical resistance, and adhesiveness, and functions as a binder resin. The fluororesin contained in the electrode paste of the present disclosure may have a structure in which hydrogen atoms in polyethylene are substituted with fluorine or trifluoromethyl. Examples of fluororesins include homopolymers such as polytetrafluoroethylene (PTFE), polyvinyl fluoride (PVF), polyvinylidene fluoride (PVDF), and polychlorotrifluoroethylene (PCTFE); and copolymers such as perfluoroalkoxyalkane (PFA), perfluoroethylenepropene copolymer (FEP), ethylene-tetrafluoroethylene copolymer (ETFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), and tetrafluoroethylene-perfluorodioxol copolymer (TPE / PDD). These fluororesins may be used alone or in combination of two or more. Among fluororesins, polytetrafluoroethylene (PTFE), polyvinyl fluoride (PVF), polyvinylidene fluoride (PVDF), resins containing these structural units, modified products thereof, or combinations thereof are preferred from the viewpoint of adhesion between the electrode composite layer for an electricity storage device and the current collector. The fluororesin contained in the electrode paste of the present disclosure is preferably a polyvinylidene fluoride-based resin, such as a homopolymer of polyvinylidene fluoride or a copolymer of vinylidene fluoride with hexafluoropropylene or tetrafluoroethylene. The polyvinylidene fluoride-based resin may be modified, for example, by introducing an acidic group such as a carboxyl group.

[0034] The weight-average molecular weight (Mw) of the fluororesin is preferably 100,0000 or more, more preferably 200,000 or more, and even more preferably 500,000 or more from the viewpoint of adhesion of the electrode mixture layer for an electricity storage device to the current collector, and is preferably 5,000,000 or less, more preferably 3,000,000 or less, and even more preferably 1,500,000 or less from the viewpoint of suitable viscosity. The glass transition point of the fluororesin is preferably 20° C. or less, more preferably 10° C. or less, and even more preferably 0° C. or less from the viewpoint of film-forming properties.

[0035] Commercially available polyvinylidene fluoride and modified products thereof include, for example, KF Polymer series manufactured by Kureha Corporation (W#7300, W#7200, W#1700, W#1300, W#1100, W#9700, W#9300, W#9100, L#7305, L#7208, L#1710, L#1320, L#1120) and Solvay's Solef series (6008, 6010, 6012, 1015, 6020, 5130, 9007, 460, 41308, 11010, 21510, 31508, 60512) (all trade names).

[0036] [Non-aqueous solvent] The non-aqueous solvent contained in the electrode paste of the present disclosure is not particularly limited as long as it can dissolve the polyvinyl acetal resin of the present disclosure, and examples thereof include cyclohexane, methyl ethyl ketone, methyl isopropyl ketone, tetrahydrofuran, toluene, isopropyl alcohol, N-methylpyrrolidone, ethanol, etc. Among these, N-methylpyrrolidone (NMP), which can dissolve fluororesin, is preferably used.

[0037] [Polyvinyl acetal resin] The polyvinyl acetal resin contained in the electrode paste of the present disclosure is preferably a polymer compound containing at least a constituent unit represented by the following formula (1) containing an acetal group, a constituent unit represented by the following formula (2) containing a hydroxyl group, and a constituent unit represented by the following formula (3) containing an acetyl group. [ka]

[0038] R in formula (1) is an alkyl group having 1 to 20 carbon atoms, and examples of the alkyl group include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, pentyl, hexyl, heptyl, 2-ethylhexyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, and octadecyl groups. Among these, methyl and n-propyl groups are preferred due to their high reactivity, and methyl is more preferred.

[0039] The molar fraction a in formula (1) is 5 to 55 mol%, preferably 15 to 50 mol%, more preferably 25 to 45 mol%. The molar fraction b in formula (2) is 45 to 95 mol%, preferably 50 to 85 mol%, more preferably 55 to 75 mol%. The molar fraction c in formula (3) is 0 to 30 mol%, preferably 0 to 10 mol%, more preferably 0 to 5 mol%.

[0040] The acetalization rate of the polyvinyl acetal resin is preferably 5% or more, more preferably 8% or more, even more preferably 15% or more, and even more preferably 25% or more from the viewpoint of reducing DCR, and is preferably 55% or less, more preferably 50% or less, even more preferably 45% or less, even more preferably 40% or less, and even more preferably 35% from the viewpoint of suppressing migration. In the present disclosure, the acetalization rate is a numerical value expressed as a percentage, which is obtained by dividing the molar fraction a of the structural unit represented by formula (1) in the polyvinyl acetal resin by the sum of the molar fraction a and the molar fraction b of the structural unit represented by formula (2) (a / (a+b)).

[0041] The amount of acetal groups and the amount of hydroxyl groups used to determine the acetalization rate can be measured by NMR.

[0042] From the viewpoint of dispersibility of the carbonaceous conductive material, the average degree of polymerization of the polyvinyl acetal resin is preferably 100 or more, more preferably 150 or more, and even more preferably 200 or more, and from the viewpoint of solubility in non-aqueous solvents, it is preferably 800 or less, more preferably 600 or less, and even more preferably 500 or less. The average degree of polymerization can be measured, for example, by gel permeation chromatography (GPC).

[0043] The polyvinyl acetal resin can be synthesized by a known method, and the synthesized product can be used alone or in combination of two or more kinds.

[0044] Additionally, the hydroxyl groups may be adjusted by chemical modification methods such as acylation and urethanization reactions, as long as the effects of the present disclosure are not impaired.

[0045] The polyvinyl acetal resin may further contain a structural unit having an acid-modified group, provided that the effects of the present disclosure are not impaired. Examples of the acid-modified group include a carboxyl group, a sulfonic acid group, a maleic acid group, a sulfinic acid group, a sulfenic acid group, a phosphoric acid group, a phosphonic acid group, an amino group, an amide group, and salts thereof. Among these, the acid-modified group is preferably a carboxyl group, a sulfonic acid group, or an amide group.

[0046] [Positive electrode paste] In one aspect, the present disclosure relates to an electrode paste, preferably a positive electrode paste, comprising an active material, a carbonaceous conductive material, a fluororesin, a non-aqueous solvent, and a polyvinyl acetal resin. In this aspect, the non-aqueous solvent is preferably NMP.

[0047] The amount of the positive electrode active material in the positive electrode paste of the present disclosure is not particularly limited as long as it can be adjusted according to the viscosity of the positive electrode paste suitable for application to a current collector. From the viewpoints of energy density and stability of the positive electrode paste, however, the amount is preferably 40% by mass or more, more preferably 50% by mass or more, even more preferably 60% by mass or more, and is preferably 90% by mass or less, more preferably 85% by mass or less, even more preferably 80% by mass or less.

[0048] The amount of conductive material blended in the positive electrode paste of the present disclosure is preferably 0.005% by mass or more, more preferably 0.01% by mass or more, and even more preferably 0.015% by mass or more, from the viewpoint of the conductivity of a composite layer obtained by applying the positive electrode paste to a current collector such as aluminum foil and drying it; and from the viewpoint of maintaining a high energy density of the composite layer, the amount is preferably 5% by mass or less, more preferably 0.6% by mass or less, even more preferably 0.2% by mass or less, and even more preferably 0.06% or less.

[0049] The amount of fluororesin in the positive electrode paste of the present disclosure is preferably 0.5% by mass or more, more preferably 0.8% by mass or more, and even more preferably 1% by mass or more, from the viewpoint of enhancing adhesion to the current collector of a composite layer obtained by coating the positive electrode paste on a current collector such as aluminum foil and drying it. Also, from the viewpoint of reducing the internal resistance of the electricity storage device, the amount is preferably 5% by mass or less, more preferably 3% by mass or less, and even more preferably 2% by mass or less.

[0050] The amount of non-aqueous solvent in the positive electrode paste of the present disclosure is preferably 10% by mass or more, more preferably 15% by mass or more, and even more preferably 20% by mass or more, from the viewpoint of ease of application of the positive electrode paste to a current collector, and is preferably 40% by mass or less, more preferably 45% by mass or less, and even more preferably 30% by mass or less, from the viewpoint of reducing the internal resistance of the electricity storage device.

[0051] The blending amount of the polyvinyl acetal resin of the present disclosure in the positive electrode paste of the present disclosure is preferably 0.01% by mass or more, more preferably 0.02% by mass or more, and even more preferably 0.03% by mass or more, from the viewpoint of viscosity reduction of the positive electrode paste, and is preferably 0.08% by mass or less, more preferably 0.07% by mass or less, and even more preferably 0.06% by mass or less, from the viewpoint of reducing the internal resistance of the electricity storage device.

[0052] In the positive electrode paste of the present disclosure, the amount of polyvinyl acetal resin relative to the total mass of the fluororesin and polyvinyl acetal resin (amount of polyvinyl acetal resin ÷ (amount of polyvinyl acetal resin + amount of fluororesin) × 100) is preferably 1 mass% or more, more preferably 1.5 mass% or more, and even more preferably 2 mass% or more from the viewpoint of suppressing migration, and is preferably 4 mass% or less, more preferably 3.5 mass% or less, and even more preferably 3 mass% or less from the viewpoint of reducing the internal resistance of the electricity storage device.

[0053] [Positive electrode paste manufacturing method] In one or more embodiments, the positive electrode paste of the present disclosure can be prepared by mixing an active material, a carbonaceous conductive material, a fluororesin, a non-aqueous solvent, a polyvinyl acetal resin, and the like, and stirring the mixture until homogeneous. The components may be mixed in any order. The method for producing the positive electrode paste preferably includes a step of mixing the active material, the CNT slurry, the fluororesin, and, if necessary, an additional non-aqueous solvent. A planetary mixer, a bead mill, a jet mill, or the like can be used to mix and stir these components, or these can be used in combination.

[0054] The positive electrode paste of the present disclosure can be prepared by premixing some of the components used in preparing the positive electrode paste and then mixing the premixed components with the remaining components. Alternatively, each component may be added in multiple batches rather than all at once. This reduces the mechanical load on the stirring device.

[0055] The solids concentration of the positive electrode paste of the present disclosure and the blending amounts of each component can be adjusted depending on the viscosity of the positive electrode paste suitable for application to a current collector. From the viewpoint of drying properties, a small amount of organic solvent is preferable, but from the viewpoints of uniformity and surface smoothness of the positive electrode composite layer, it is preferable that the viscosity of the positive electrode paste is not too high. On the other hand, from the viewpoints of inhibiting drying and obtaining a sufficient film thickness of the positive electrode composite layer, it is preferable that the viscosity of the positive electrode paste is not too low.

[0056] The positive electrode paste of the present disclosure is preferably able to be adjusted to a high concentration from the viewpoint of production efficiency, but a significant increase in viscosity is undesirable from the viewpoint of workability. The additives allow the high concentration to be maintained while maintaining a preferred viscosity range.

[0057] The positive electrode paste of the present disclosure may further contain other components, such as antioxidants, neutralizing agents, antifoaming agents, preservatives, dehydrating agents, rust inhibitors, and plasticizers, as long as the effects of the present disclosure are not impaired.

[0058] [Method of manufacturing positive electrodes for energy storage devices] A positive electrode for an electricity storage device is produced, for example, by applying the positive electrode paste of the present disclosure to a current collector such as aluminum foil and drying the applied paste. To increase the density of the positive electrode composite layer, compaction can be performed using a press. A die head, a cone reverse roll, a direct roll, a gravure roll, or the like can be used to apply the positive electrode paste. Drying after application can be performed using heating, airflow, infrared irradiation, or a combination thereof. Drying after application is performed at a temperature at which the non-aqueous solvent in the positive electrode paste can no longer be present in the positive electrode paste after the drying time has elapsed. The drying temperature is not particularly limited as long as it is below the thermal decomposition temperature of the fluororesin or polyvinyl acetal resin in the drying environment (atmospheric pressure or vacuum). However, from the viewpoint of increasing the drying rate, the drying temperature is preferably 60°C or higher, more preferably 100°C or higher, and even more preferably 120°C or higher. Furthermore, from the viewpoint of reducing the internal resistance of the electricity storage device, the drying temperature is preferably 200°C or lower, more preferably 170°C or lower, and even more preferably 150°C or lower. The drying time is preferably 10 minutes to 24 hours. The positive electrode mixture layer can be pressed using a roll press or the like. After pressing, the positive electrode may be processed to a size suitable for incorporation into an electricity storage device, and then re-dried under the above conditions.

[0059] [Electricity storage device and its manufacturing method] In one aspect, the present disclosure relates to an electricity storage device including a positive electrode for an electricity storage device obtained by the method for producing a positive electrode for an electricity storage device of the present disclosure, and a method for producing the same. In one or more embodiments, examples of the power storage device include a lithium ion secondary battery, a lithium-air secondary battery, a sodium ion battery, a sodium-sulfur secondary battery, a sodium-nickel chloride secondary battery, an organic radical battery, a zinc-air secondary battery, and an all-solid-state battery.

[0060] The method for producing an electricity storage device according to the present disclosure includes the same steps as those for producing known electricity storage devices, except that the positive electrode according to the present disclosure is used as the positive electrode for the electricity storage device. The method for producing an electricity storage device according to the present disclosure includes, for example, a step of overlapping two electrodes (a positive electrode and a negative electrode) with a separator interposed therebetween and winding or stacking them into a battery shape, and a step of placing the resulting wound body or stack into a battery container or laminate container, injecting an electrolyte into the container, and sealing the container. [Example]

[0061] Examples and comparative examples of the present disclosure will be shown below, but the present disclosure is not limited thereto.

[0062] 1.Measuring methods for each parameter [CNT slurry viscosity] The viscosity of the CNT slurry (25°C) was measured as follows: An Anton Paar MCR302 rheometer was equipped with a parallel plate PP50, and the shear rate was set to 0.1 s -1 After increasing it to 1000s-1 (outward), from 1000s-1 to 0.1s -1 Return to (return route), return shear rate 1s -1 The viscosity was measured at 100°C, and the results are shown in Table 2.

[0063] [Measurement of acetalization rate] Polyvinyl acetal resin heavy DMSO solution 1H-NMR (nuclear magnetic resonance spectrum) was measured, and the value obtained by the following calculation formula using the integrated values ​​of the peaks appearing around 2.0 ppm (peak a), 3.8 ppm (peak b), 4.2 ppm (peak b'), 4.7 ppm (peak c), and 4.9 ppm (peak c') was used to determine the acetalization rate. 1 The H-NMR spectrum is shown in Figure 2. Acetalization rate (%) = ((area of ​​peak c + area of ​​peak c') × 2) / ((area of ​​peak b + area of ​​peak b') - (area of ​​peak a / 3)) × 100

[0064] [Color density measurement] The positive electrode paste was applied to a 20 μm thick aluminum foil (current collector) with a positive electrode capacity of 2 mAh / cm 2 The electrode material (positive electrode material) was fabricated by drying the electrode material under two drying conditions: 1 hour at 80°C and 1 hour at 140°C in a blower dryer. The color density of the electrode surface of this positive electrode material was measured using a color densitometer (eXact Standard, manufactured by X-Rite). The color density results are shown in Table 4. Since CNTs are black in color, when migration occurs, the CNT concentration on the electrode surface and in its vicinity increases, resulting in a high color concentration.

[0065] The resistance to migration was evaluated using the degree of change in color density calculated by the following formula as an index. If the degree of change in color density was 1.02 or less, it was determined that migration was unlikely to occur. From the viewpoint of the resistance to migration, the degree of change in color density is preferably 1.01 or less, and more preferably 1.00 or less. Color density change rate = (color density of product dried at 140°C) ÷ (color density of product dried at 80°C)

[0066] [Measurement of DC resistance (DCR)] The positive electrode material is punched and pressed to a diameter of 13 mm to a density of 3 g / cm 3A positive electrode of 19 mm in diameter and a coin-shaped lithium metal of 15 mm in diameter and 0.5 mm in thickness were placed on the positive electrode to prepare a 2032-type coin cell (a test half cell). The electrolyte used was 1M LiPF6EC / DEC (volume ratio = 3 / 7). The test half cell was kept at a constant temperature of 30°C in a thermostatic chamber and subjected to a three-cycle charge-discharge test under the following charge-discharge conditions: (Charge / discharge conditions) 30℃, 0.2C, charge 4.45V CC / CV 1 / 10C cutoff, discharge CC 3.0V cutoff. Next, the battery was charged at 0.2 C for 2.5 hours, and the direct current resistance (DCR) was calculated from the voltage drop over 10 seconds during discharge at 0.2 C to 8 C. The results are shown in Table 4.

[0067] From the viewpoint of enabling rapid charging and discharging of the test half cell, the internal resistance (DCR) of the test half cell is preferably 15.60 Ω or less, more preferably 15.40 Ω or less, and even more preferably 15.00 Ω or less, regardless of whether a positive electrode prepared at a drying temperature of 80°C (80°C dried product) or a positive electrode prepared at a drying temperature of 140°C (140°C dried product) is incorporated.

[0068] The resistance to migration was evaluated using the DCR change rate calculated by the following formula as an index. If the DCR change rate was 1.05 or less, it was determined that migration was unlikely to occur. From the viewpoint of the resistance to migration, the DCR change rate is preferably 1.03 or less, more preferably 1.01 or less. DCR change rate = (DCR of product dried at 140°C) ÷ (DCR of product dried at 80°C)

[0069] 2. Production of polyvinyl acetal resin (1) Production Example 1 (Production of Polyvinyl Acetal Resin 1 (Acetalization Rate 10%)) 30 g of polyvinyl alcohol resin (saponification degree: 98 mol%, average polymerization degree: 300) was added to 150 g of ion-exchanged water, and the resulting mixture was stirred at 90°C for 4 hours to dissolve the polyvinyl alcohol resin in the ion-exchanged water. The resulting solution was cooled to 40°C, and 3 g of 2N hydrochloric acid was added to it. The liquid temperature was then lowered to 5°C, and 1.5 g of acetaldehyde cooled to 5°C was added to carry out an acetalization reaction for 2 hours. The liquid temperature was then adjusted to 60°C and maintained for 5 hours to terminate the reaction. Polyvinyl acetal resin 1 was obtained by neutralization, water washing, and drying using standard methods. The acetal group content, hydroxyl group content, and acetyl group content of the resulting polyvinyl acetal resin 1 were measured using 1H-NMR (nuclear magnetic resonance spectroscopy), and the results are shown in Table 1. 1H-NMR measurements were performed using heavy DMSO as the solvent. Polyvinyl acetal resin 1 contains 10 mol % of the constitutional unit represented by formula (1), 88 mol % of the constitutional unit represented by formula (2), and 2 mol % of the constitutional unit represented by formula (3).

[0070] [Table 1]

[0071] (2) Production Example 2 (Production of Polyvinyl Acetal Resin 2 (Acetalization Rate 20%)) Polyvinyl acetal resin 2 shown in Table 1 was synthesized in the same manner as in Production Example 1 above, except that the amount of acetaldehyde used was 3.0 g.

[0072] (3) Production Example 3 (Production of Polyvinyl Acetal Resin 3 (Acetalization Rate 30%)) Polyvinyl acetal resin 3 shown in Table 1 was synthesized in the same manner as in Production Example 1 above, except that the amount of acetaldehyde used was 4.5 g.

[0073] (4) Production Example 4 (Production of Polyvinyl Acetal Resin 4 (Acetalization Rate 40%)) Polyvinyl acetal resin 4 shown in Table 1 was synthesized in the same manner as in Production Example 1 above, except that the amount of acetaldehyde used was 6.0 g.

[0074] (5) Production Example 5 (Production of Polyvinyl Acetal Resin 5 (Acetalization Rate 50%)) Polyvinyl acetal resin 5 shown in Table 1 was synthesized in the same manner as in Production Example 1 above, except that the amount of acetaldehyde used was 7.5 g.

[0075] 3. Preparation of CNT Slurry (1) Preparation of CNT slurry 1 Polyvinyl acetal resin 1 was dissolved in NMP to prepare a 1 mass% NMP solution of polyvinyl acetal resin 1. Carbon nanotubes (BT-1003M manufactured by LG Chem, average diameter 12.5 nm, length 10-70 μm) were used as a carbonaceous conductive material. The 1 mass% NMP solution of polyvinyl acetal resin 1 and NMP (non-aqueous solvent) were mixed so that the polyvinyl acetal resin 1, CNTs, and non-aqueous solvent were mixed in the proportions shown in Table 2. Next, while stirring this mixture with a stirrer, it was dispersed for 20 minutes using an ultrasonic homogenizer (US-300AT manufactured by Nippon Seiki Seisakusho) with the longitudinal amplitude of the ultrasonic probe set to 30%, to obtain CNT slurry 1.

[0076] [Table 2]

[0077] (2) Preparation of CNT slurries 2 to 5 As shown in Table 2, CNT slurries 2 to 5 were produced in the same manner as CNT slurry 1, except that polyvinyl acetal resin 1 was replaced with polyvinyl acetal resins 2 to 5, respectively.

[0078] (3) Preparation of CNT slurry 6 CNT slurry 6 was obtained by the same production method as CNT slurry 6, except that polyvinyl acetal resin 3 was used instead of polyvinyl acetal resin 1.

[0079] (4) Preparation of CNT slurry 7 An 8% NMP solution of polyvinylidene fluoride resin (resin manufactured by Kureha) was dissolved in NMP to prepare a 1% by mass NMP solution of polyvinylidene fluoride resin. Carbon nanotubes (BT-1003M manufactured by LG Chem, average diameter 12.5 nm, length 10-70 μm) as a carbonaceous conductive material were mixed with a 1 mass % NMP solution of the polyvinylidene fluoride resin and NMP (a non-aqueous solvent). Next, while stirring the resulting mixture with a stirrer, dispersion was carried out for 20 minutes using an ultrasonic homogenizer (US-300AT manufactured by Nippon Seiki Seisakusho) with the ultrasonic probe's vertical amplitude set to 30%, to obtain CNT slurry 7.

[0080] [Example 1] As shown in Table 3, 15.4 g of active material (NCM523, manufactured by Nippon Chemical Industry Co., Ltd.), 2.6 g of CNT slurry 1, and 3.92 g of an 8 mass % PVDF NMP solution (KF polymer L#7208, weight average molecular weight (Mw): 630,000, glass transition temperature −35°C, manufactured by Kureha Corporation) were each weighed out, mixed, and stirred for 5 minutes with a planetary centrifugal mixer (AR-100, manufactured by Thinky Corporation) to obtain a positive electrode paste with a solids concentration of 71.9 mass %. The mass ratio of the active material, CNT, PVDF, NMP, and polyvinyl acetal resin 1 in the obtained positive electrode paste was 70.25:0.18:1.43:28.10:0.04. The obtained positive electrode paste was coated on a current collector by the method described above in [Measurement of color density] and dried, and the composition ratio of the positive electrode material in the obtained positive electrode mixture layer is shown in Table 4. Table 4 also shows the color density and degree of change in color density on the electrode surface after drying when the coating drying temperature was 80°C and 140°C. Furthermore, Table 4 also shows the DCR and degree of change in DCR measured by the method described above in [Measurement of direct current resistance (DCR)].

[0081] [Examples 2 to 7, Comparative Examples 1 and 2] Positive electrode pastes of Examples 2 to 7 and Comparative Examples 1 and 2 having the compositions shown in Table 3 were prepared in the same manner as in Example 1, and the color density and DCR of the electrode surface were measured.

[0082] Comparative Example 3 As shown in Table 3, 15.4 g of active material (NCM523, manufactured by Nippon Chemical Industry Co., Ltd.), 2.6 g of CNT slurry 3, 3.15 g of an NMP solution of 10 mass % polyvinyl acetal resin 3, and 0.77 g of NMP as a non-aqueous solvent were weighed out, mixed, and stirred for 5 minutes with a planetary centrifugal mixer (AR-100, manufactured by Thinky Corporation) to obtain a positive electrode paste with a solids concentration of 71.9 mass %. The mass ratio of the active material, CNT, PVDF, NMP, and polyvinyl acetal resin 3 in the obtained positive electrode paste was 70.16:0.18:0:28.18:1.48. The obtained positive electrode paste was coated on a current collector by the method described above in [Measurement of color density] and dried, and the composition ratio of the positive electrode material in the obtained positive electrode mixture layer is shown in Table 4. Table 4 also shows the color density and degree of change in color density on the electrode surface after drying when the coating drying temperature was 80°C and 140°C. Furthermore, Table 4 also shows the DCR and degree of change in DCR measured by the method described above in [Measurement of direct current resistance (DCR)].

[0083] [Table 3]

[0084] [Table 4]

[0085] From the results shown in Table 4, the positive electrode pastes of Examples 1 to 7 all had a color density change rate of 1.02 or less, making migration unlikely to occur, and the DCR of the batteries including electrodes formed using the positive electrode pastes of Examples 1 to 7 was also low, at 15.60Ω or less, for both the 80°C dried product and the 140°C dried product. In Comparative Example 1, the positive electrode paste did not contain polyacetal resin, and the dispersibility of CNTs was poor, resulting in a significantly high DCR value. In Comparative Examples 2 and 3, it is presumed that the conductivity was reduced because the blending amount of polyvinyl acetal resin relative to the total mass of the fluororesin and polyvinyl acetal resin was too large.

[0086] From the above, it was confirmed that when the blending amount of polyvinyl acetal resin relative to the total mass of fluororesin and polyvinyl acetal resin is 1 mass% or more and 4 mass% or less, it is possible to produce an electricity storage device with an internal resistance regardless of the drying temperature of the coating film. [Industrial Applicability]

[0087] The positive electrode paste of the present disclosure allows the coating to be dried at a high temperature, thereby shortening the electrode manufacturing time and, as a result, reducing the manufacturing costs of the electrode and the electricity storage device.

Claims

1. The battery is made by blending an active material, a carbonaceous conductive material, a fluororesin, a non-aqueous solvent, and a polyvinyl acetal resin, an electrode paste for an electricity storage device, wherein the blending amount of the polyvinyl acetal resin relative to the total mass of the fluororesin and the polyvinyl acetal resin is 1 mass % or more and 4 mass % or less.

2. 2. The electrode paste for a storage battery device according to claim 1, wherein the polyvinyl acetal resin has an acetalization rate of 5% or more and 55% or less.

3. The electrode paste for a storage battery device according to claim 1 , wherein the carbonaceous conductive material is a carbon nanotube.

4. 2. The electrode paste for a storage battery device according to claim 1, wherein the carbonaceous conductive material is a double-walled carbon nanotube.

5. 2. The electrode paste for a storage battery device according to claim 1, wherein the carbon material-based conductive material is a carbon nanotube having an X-ray diffraction peak at half width of 5° or more and 10° or less at 2θ of 25°±2° and a G / D ratio of 40 or more and 70 or less.

6. 2 . The electrode paste for a storage battery device according to claim 1 , wherein the blending amount of the carbon material-based conductive material is 0.005% by mass or more and 5% by mass or less.

7. 7. A method for producing a positive electrode for an electricity storage device, comprising: applying the electrode paste for an electricity storage device according to claim 1 to a current collector to form a coating film, and then drying the coating film in an atmosphere at 60°C or higher.

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