Positive electrode for power storage device, and method for producing slurry for positive electrode

Abstract

A positive electrode (21) comprises: a positive electrode current collector (21a); and a positive electrode active material layer (21b) which is formed on the positive electrode current collector (21a) and has a thickness of 200 μm or more. The positive electrode active material layer (21b) contains: first active material particles having a particle diameter of 5-15 μm; second active material particles having a particle diameter of 0.7-3 μm; a water-dispersible binding agent; a dispersing agent; and single-walled carbon nanotubes. A plurality of aggregated parts, in which the water-dispersible binding agent is aggregated, are dispersed in the positive electrode active material layer (21b). In a cross section parallel to the thickness direction of the positive electrode active material layer (21b), the ratio (S1 / S2) of the average area (S1) of the aggregated parts in a first region positioned on the positive electrode current collector (21a) side with respect to the center position in the thickness direction to the average area (S2) of the aggregated parts in a second region positioned on the side opposite to the positive electrode current collector (21a) with respect to the center position is 0.9-1.1.

Description

Method for manufacturing a positive electrode for an energy storage device and a slurry for the positive electrode.

[0001] This disclosure relates to a positive electrode for an energy storage device and a method for manufacturing a slurry for a positive electrode.

[0002] Patent Document 1 discloses a flat-type energy storage device constructed by stacking a plurality of energy storage cells in series. The energy storage cell comprises a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode. The positive electrode has a positive electrode active material layer formed on one side of a foil-shaped positive electrode current collector. The negative electrode has a negative electrode active material layer formed on one side of a foil-shaped negative electrode current collector, and the negative electrode active material layer is arranged to face the positive electrode active material layer of the positive electrode. The energy storage device disclosed in Patent Document 1 is formed by stacking a plurality of the above-mentioned energy storage cells such that the positive electrode current collector and the negative electrode current collector are in contact with each other.

[0003] Patent Document 2 discloses a manufacturing method for continuously manufacturing electrodes using rolls. The electrode manufacturing method disclosed in Patent Document 2 includes a coating step and a drying step. The coating step is a step of coating an electrode mixture containing an active material and a solvent onto a current collector fed from a feed roll. The drying step is a step of drying the coating layer of the electrode mixture applied to the current collector after the coating step. After the drying step, the current collector is wound onto a winding roll.

[0004] JP2017-16825A JP11-102696A

[0005] Methods to increase the energy density of a storage cell include increasing the basis weight of the active material layer and increasing the amount of active material within the active material layer. One method to increase the basis weight of the active material layer is to form a thicker active material layer. One method to increase the amount of active material within the active material layer is to increase the density of the active material layer by using a mixture of active material particles with large particle sizes and active material particles with small particle sizes as the active material.

[0006] The inventors investigated a positive electrode that simultaneously employs a configuration in which the active material layer is formed to a thickness of 200 μm or more, and a configuration in which active material particles of different particle sizes are used. As a result, cracks sometimes occurred in the active material layer during the manufacturing of the positive electrode. These cracks in the active material layer are thought to be due to a decrease in the flexibility of the active material layer as the thickness and density of the active material layer increase. In other words, when a bending load is applied to the active material layer during the manufacturing of the active material, the active material layer cannot withstand the bending load and cracks occur in the active material layer.

[0007] The embodiments of the method for manufacturing the positive electrode and the slurry for the positive electrode for the energy storage device of this disclosure are described below. [Aspect 1] A positive electrode for an energy storage device comprising a current collector having a first surface and a positive electrode active material layer formed on the first surface of the current collector and having a thickness of 200 μm or more, wherein the positive electrode active material layer contains first active material particles having a particle diameter of 5 μm or more and 15 μm or less, second active material particles having a particle diameter of 0.7 μm or more and 3 μm or less, a water-dispersible binder, a dispersant, and single-walled carbon nanotubes, wherein a plurality of aggregated portions formed by the aggregation of the water-dispersible binder are dispersed in the positive electrode active material layer, and the ratio (S1 / S2) of the average area (S1) of the aggregated portions in a first region located on the current collector side of the center position in the thickness direction to the average area (S2) of the aggregated portions in a second region located on the opposite side of the current collector from the center position in a cross section parallel to the thickness direction of the positive electrode active material layer is 0.9 or more and 1.1 or less.

[0008] [Aspect 2] The average area of ​​the aggregated portion in the entire cross-section is 0.02 μm ２ 0.05 μm or more ２ The positive electrode for the energy storage device described in Embodiment 1 below.

[0009] [Aspect 3] A positive electrode for an energy storage device according to Aspect 1 or Aspect 2, wherein the ratio (A1 / A2) of the content of the first active material particles (A1) to the content of the second active material particles (A2) in the positive electrode active material layer is 2.4 or more and 9.0 or less.

[0010] [Aspect 4] A positive electrode for an energy storage device according to any one of aspects 1 to 3, wherein the total content of the first active material particles and the second active material particles in the positive electrode active material layer is 97.0% by mass or more and 99.0% by mass or less, the content of the water-dispersible binder in the positive electrode active material layer is 0.8% by mass or more and 2.0% by mass or less, the content of the dispersant in the positive electrode active material layer is 0.2% by mass or more and 0.9% by mass or less, and the content of the single-walled carbon nanotubes in the positive electrode active material layer is 0.01% by mass or more and 0.1% by mass or less.

[0011] [Aspect 5] The positive electrode for an energy storage device according to any one of aspects 1 to 4, wherein the first active material particles and the second active material particles are particles mainly composed of a polyanionic compound having an olivine-type structure, the water-dispersible binder is a compound having an aromatic ring, and the dispersant is at least one selected from carboxymethylcellulose and a salt of carboxymethylcellulose.

[0012] [Aspect 6] A method for manufacturing a cathode slurry used in the manufacture of a cathode for an energy storage device, comprising a current collector having a first surface, and a positive electrode active material layer formed on the first surface of the current collector and having a thickness of 200 μm or more, wherein the positive electrode active material layer contains positive electrode active material particles, a water-dispersible binder, a dispersant, and single-walled carbon nanotubes, comprising the steps of preparing a first slurry by mixing a third active material particle as the positive electrode active material particle having an average particle diameter (D50) of 0.7 μm or more and 3 μm or less, the single-walled carbon nanotubes, the dispersant, and an aqueous solvent, and the first slurry, the positive electrode active material particle having an average particle diameter (D50) of 5 μm or more and 15 μm or less A method for producing a cathode slurry, comprising the steps of preparing a second slurry by mixing a fourth active material particle, the single-walled carbon nanotube, and an aqueous solvent, wherein when the amount of the third active material particle mixed in the step of preparing the first slurry is A3, and the amount of the fourth active material particle mixed in the step of preparing the second slurry is A4, the ratio (C2 / C1) of the amount of the single-walled carbon nanotube mixed in the step of preparing the first slurry (C1) to the amount of the single-walled carbon nanotube mixed in the step of preparing the second slurry (C2) is 0.9 times or more and 1.1 times or less of the ratio (A4 / A3).

[0013] [Aspect 7] A method for producing a cathode slurry according to aspect 6, further comprising the step of preparing a second slurry, followed by the step of mixing the second slurry and the water-dispersible binder to prepare a third slurry.

[0014] [Aspect 8] A method for producing a cathode slurry according to aspect 6 or aspect 7, wherein the solid content concentration of the first slurry is higher than that of the second slurry.

[0015] According to the present invention, cracking of the positive electrode active material layer caused by bending load can be suppressed.

[0016] Figure 1 is a cross-sectional view of the positive electrode of the embodiment. Figure 2 is a cross-sectional view of a modified bipolar electrode.

[0017] The following describes one embodiment of the positive electrode of the present invention. <Positive Electrode> The positive electrode of this embodiment is used as an electrode in an energy storage device. The energy storage device is, for example, a secondary battery such as a nickel-metal hydride secondary battery or a lithium-ion secondary battery. The energy storage device may also be an electric double-layer capacitor. The following description will focus on the case where the positive electrode is in a lithium-ion secondary battery.

[0018] As shown in Figure 1, the positive electrode 21 comprises a positive electrode current collector 21a having a first surface 21a1 and a positive electrode active material layer 21b formed on the first surface 21a1 of the positive electrode current collector 21a. In this embodiment, the positive electrode current collector 21a corresponds to the current collector described in the claims.

[0019] The positive electrode current collector 21a is a chemically inert electrical conductor that allows current to continue flowing through the positive electrode active material layer 21b during the discharge or charging of the lithium-ion secondary battery. The positive electrode current collector 21a has a shape that can be curved, for example, a foil shape. The thickness of the foil-shaped positive electrode current collector 21a is, for example, 5 μm or more, preferably 10 μm or more. The thickness of the foil-shaped positive electrode current collector 21a is, for example, 100 μm or less, preferably 60 μm or less.

[0020] An example of a positive electrode current collector 21a is an aluminum current collector in which the surface that becomes the first surface 21a1 is made of aluminum. The aluminum current collector may be a single object made entirely of aluminum, or it may be a composite having a part made of aluminum and a part made of a material other than aluminum. An example of the single object is aluminum foil. An example of the composite is a multilayer structure in which the layer making up the first surface 21a1 is an aluminum layer, or a substrate in which the surface including the first surface 21a1 is coated with an aluminum film.

[0021] The positive electrode current collector 21a may be formed of a material other than aluminum. Examples of the material other than aluminum include metal materials, conductive resin materials, and conductive inorganic materials. Examples of the metal materials include copper, nickel, titanium, stainless steel (such as SUS304, SUS301, SUS304, etc. defined in JIS G 4305:2015). Examples of the conductive resin materials include resin obtained by adding a conductive filler to a conductive polymer material or a non-conductive polymer material as needed.

[0022] The first surface 21a1 of the positive electrode current collector 21a may be coated with a known protective layer such as a carbon coating layer. The first surface 21a1 of the positive electrode current collector 21a may be processed by a known method such as plating treatment.

[0023] The thickness of the positive electrode active material layer 21b is 200 μm or more. Preferably, the thickness of the positive electrode active material layer 21b is 240 μm or more. By increasing the thickness of the positive electrode active material layer 21b, the energy density of the lithium ion secondary battery using the positive electrode 21 can be effectively increased. By increasing the energy density of the lithium ion secondary battery, battery characteristics such as battery capacity and long-term output can be enhanced.

[0024] The thickness of the positive electrode active material layer 21b is, for example, 600 μm or less, preferably 530 μm or less, and more preferably 470 μm or less. The positive electrode active material layer 21b may be subjected to pressing treatment or may not be subjected to pressing treatment. Even when the pressing treatment is not performed, the thickness of the positive electrode active material layer 21b is preferably 227 μm or more.

[0025] The basis weight of the positive electrode active material layer 21b is, for example, 50 mg / cm ２ or more. Preferably, the basis weight of the positive electrode active material layer 21b is 60 mg / cm ２ or more, and more preferably 70 mg / cm ２ or more. The basis weight of the positive electrode active material layer 21b is, for example, 90 mg / cm ２ or less, preferably 80 mg / cm ２ or less.

[0026] The density of the positive electrode active material layer 21b is, for example, 1.5 g / cm ３ or more, preferably 1.6 g / cm ３ or more, more preferably 1.7 g / cm ３ or more. Also, the density of the positive electrode active material layer 21b is, for example, 2.3 g / cm ３ or less.

[0027] The porosity of the positive electrode active material layer 21b is, for example, 48% or less, preferably 45% or less, more preferably 43% or less. Also, the porosity of the positive electrode active material layer 21b is, for example, 38% or more, preferably 40% or more.

[0028] The positive electrode active material layer 21b contains a positive electrode active material, single-walled carbon nanotubes, a dispersant, and a water-dispersible binder. The positive electrode active material can occlude and release charge carriers such as lithium ions. The positive electrode active material is not particularly limited, and those that can be used as the positive electrode active material of a lithium ion secondary battery are adopted. Examples of the positive electrode active material include polyanion-based compounds having an olivine-type structure, lithium composite metal oxides having a layered rock salt structure, and metal oxides having a spinel structure. The positive electrode active material may be used alone or in combination of plural kinds.

[0029] The positive electrode active material is preferably a polyanion-based compound having an olivine-type structure (hereinafter referred to as an olivine-type active material). Examples of the olivine-type active material include compounds represented by the general formula LiM ｈ PO ４ . In the general formula LiM ｈ PO ４ , M is at least one element selected from the group consisting of Mn, Fe, Co, Ni, Cu, Mg, Zn, V, Ca, Sr, Ba, Ti, Al, Si, B, Te, Mo, and h is a numerical value satisfying 0 < h < 2. Specific examples of the olivine-type active material include, for example, olivine-type lithium iron phosphate (LiFePO ４ ) and olivine-type lithium manganese iron phosphate (LiMnFePO ４ ).

[0030] The positive electrode active material includes first active material particles and second active material particles having different particle sizes. Preferably, the first active material particles and the second active material particles have the same main component. Having the same main component means that the components constituting 95% or more by mass are the same. The first active material particles and the second active material particles may have different main components.

[0031] The first active material particles have a particle diameter of 5 μm or more and 15 μm or less, and are larger in size than the second active material particles. The second active material particles have a particle diameter of 0.7 μm or more and 3 μm or less, and are smaller in size than the second active material particles. The particle diameters of the first and second active material particles can be measured using an electron microscope, such as a scanning electron microscope (SEM) and a transmission electron microscope (TEM). In one example, the cross-section of the active material layer obtained by cutting the positive electrode active material layer 21b is observed using an electron microscope, and the major axis of the particle cross-section is estimated to be the particle diameter of each individual active material particle appearing in the cross-section. The orientation of the cross-section of the active material layer is not particularly limited. In one example, the cross-section of the active material layer is a cross-section parallel to the thickness direction of the positive electrode active material layer 21b (the evaluation cross-section described later).

[0032] Preferably, the first active material particles and the second active material particles are particles that each independently have a carbon coating on their surface. In this case, hydrophobic interactions act between the particle surface of the first active material particles and the single-walled carbon nanotubes, or between the particle surface of the second active material particles and the single-walled carbon nanotubes, within the positive electrode active material layer 21b. As a result, when a bending load is applied to the positive electrode active material layer 21b, the positive electrode active material and the single-walled carbon nanotubes can work together. A preferred example of positive electrode active material particles having a carbon coating is a particle having a core composed of granules of olivine-type active material and a carbon coating formed on the surface of the core.

[0033] The total content of first active material particles and second active material particles in the positive electrode active material layer 21b is, for example, 97% by mass or more, preferably 98% by mass or more, and more preferably 98.5% by mass or more. The total content of first active material particles and second active material particles in the positive electrode active material layer 21b is, for example, 99% by mass or less, and preferably 98.8% by mass or less. By increasing the content of positive electrode active material, the energy density of the lithium-ion secondary battery using the positive electrode 21 can be effectively increased. By increasing the energy density of the lithium-ion secondary battery, battery characteristics such as battery capacity and long-term output are improved.

[0034] In one example, the content of first active material particles in the positive electrode active material layer 21b is greater than the content of second active material particles. In this case, the ratio (A1 / A2) of the content of first active material particles (A1) to the content of second active material particles (A2) in the positive electrode active material layer 21b is, for example, 2.4 or more and 9.0 or less, preferably 3.0 or more and 5.7 or less.

[0035] The content of first active material particles in the positive electrode active material layer 21b is, for example, 68.47% by mass or more and 89.10% by mass or less, preferably 72.75% by mass or more and 84.22% by mass or less. The content of second active material particles in the positive electrode active material layer 21b is, for example, 9.70% by mass or more and 29.11% by mass or less, preferably 14.47% by mass or more and 24.75% by mass or less.

[0036] The positive electrode active material contained in the positive electrode active material layer 21b may include active material particles other than the first active material particles and the second active material particles. These other active material particles are active material particles with a particle diameter of less than 0.7 μm, active material particles with a particle diameter greater than 3 μm and less than 5 μm, and active material particles with a particle diameter greater than 15 μm.

[0037] Other active material particles may or may not have a carbon coating on their surface. Other active material particles may have the same main component as one or both of the first and second active material particles, or they may have different main components. In one example, the first active material particles, the second active material particles, and the other active material particles are all particles with the same main component.

[0038] The positive electrode active material contained in the positive electrode active material layer 21b has a bimodal particle size distribution. More specifically, in a graph with particle size on the horizontal axis and volume ratio on the vertical axis, the positive electrode active material contained in the positive electrode active material layer 21b has a first main peak located in the range of 0.7 μm to 3 μm and a second main peak located in the range of 5 μm to 15 μm. In other words, the positive electrode active material contained in the positive electrode active material layer 21b has a unimodal distribution in the particle size distribution range of less than 4 μm with a first main peak in the range of 0.7 μm to 3 μm, and a unimodal distribution in the particle size distribution range of 4 μm or more with a second main peak in the range of 5 μm to 15 μm.

[0039] The first main peak is formed primarily by second active material particles, and the second main peak is formed primarily by first active material particles. Note that having a peak within a specific range means that the peak's apex is located within that specific range.

[0040] The particle size distribution of the positive electrode active material can be determined, for example, from the particle cross-sections of the active material particles that appear in the cross-section of the active material layer obtained by cutting the positive electrode active material layer 21b. The cross-section of the active material layer obtained by cutting the positive electrode active material layer 21b is observed using an electron microscope, and the major axis and area of ​​the particle cross-section of each active material particle appearing in the cross-section are determined. The major axis of the particle cross-section is then estimated to be the particle size of the active material particle. In addition, the ratio of the area of ​​the particle cross-section of each active material particle to the total area of ​​the particle cross-sections appearing in the cross-section of the active material layer is estimated to be the volume ratio of each active material particle. The particle size distribution is then determined from the estimated particle size and volume ratio of each active material particle.

[0041] Carbon nanotubes can be broadly classified into two types: single-walled carbon nanotubes and multi-walled carbon nanotubes. Single-walled carbon nanotubes are cylindrical structures formed by seamlessly winding a single sheet of graphene. In contrast, multi-walled carbon nanotubes are composite structures in which multiple single-walled carbon nanotubes of different diameters are housed within a single single-walled carbon nanotube. Therefore, single-walled carbon nanotubes are more flexible than multi-walled carbon nanotubes.

[0042] The positive electrode active material layer 21b contains single-walled carbon nanotubes. Within the positive electrode active material layer 21b, the single-walled carbon nanotubes may exist in bundles of several tens of nanotubes, for example.

[0043] The fiber length and fiber diameter of the single-walled carbon nanotubes contained in the positive electrode active material layer 21b are not particularly limited. For example, the fiber length of the single-walled carbon nanotubes is 5 μm to 1000 μm. For example, the fiber diameter of the carbon nanotubes is 1 nm to 20 nm. The fiber length and fiber diameter of the single-walled carbon nanotubes can be measured using an electron microscope such as a SEM and a TEM.

[0044] The content of single-walled carbon nanotubes in the positive electrode active material layer 21b is, for example, 0.01% by mass or more, preferably 0.03% by mass or more. The content of single-walled carbon nanotubes in the positive electrode active material layer 21b is, for example, 0.1% by mass or less, preferably 0.07% by mass or less.

[0045] The dispersant is, for example, a water-soluble dispersant. A water-soluble dispersant is a dispersant that is soluble in an aqueous solvent and has the effect of promoting the dispersion of first active material particles and second active material particles in an aqueous solvent.

[0046] Examples of water-soluble dispersants include carboxymethylcellulose and salts of carboxymethylcellulose. Examples of carboxymethylcellulose salts include sodium salt, lithium salt, and ammonium salt. The dispersant contained in the positive electrode active material layer 21b may be one type or two or more types.

[0047] The dispersant content in the positive electrode active material layer 21b is, for example, 0.9% by mass or less, preferably 0.6% by mass or less. The dispersant content in the positive electrode active material layer 21b is, for example, 0.2% by mass or more.

[0048] A water-dispersible binder is a binder that can be dispersed in an aqueous solvent. Examples of water-dispersible binders include various resin materials, such as acrylic resins like poly(meth)acrylic acid and styrene-butadiene rubber. The water-dispersible binder contained in the positive electrode active material layer 21b may be one type or two or more types.

[0049] The water-dispersible binder is preferably a compound having an aromatic ring. In this case, a π-π interaction acts between the aromatic ring of the water-dispersible binder and the single-walled carbon nanotube within the positive electrode active material layer 21b. As a result, when a bending load is applied to the positive electrode active material layer 21b, the water-dispersible binder and the single-walled carbon nanotube can work together. Examples of compounds having an aromatic ring include styrene-butadiene rubber.

[0050] The content of the water-dispersible binder in the positive electrode active material layer 21b is, for example, 0.8% by mass or more, preferably 0.9% by mass or more, and more preferably 1.0% by mass or more. By increasing the content of the water-dispersible binder, the binding properties within the positive electrode active material layer 21b can be improved. The content of the water-dispersible binder in the positive electrode active material layer 21b is, for example, 2.0% by mass or less, preferably 1.5% by mass or less, and more preferably 1.3% by mass or less. The water-dispersible binder is a component that does not contribute to charging and discharging within the positive electrode active material layer 21b. By reducing the content of components that do not contribute to charging and discharging, the energy density of the lithium-ion secondary battery can be increased. By increasing the energy density of the lithium-ion secondary battery, battery characteristics such as battery capacity and long-term output can be improved.

[0051] The positive electrode active material layer 21b may contain other components as needed, in addition to the positive electrode active material, single-walled carbon nanotubes, dispersant, and water-dispersible binder. Examples of other components include conductive additives, electrolytes (polymer matrix, ion-conducting polymer, electrolyte solution, etc.), and electrolyte support salts (lithium salts) to enhance ion conductivity. The types and contents of other components are not particularly limited, and conventionally known knowledge regarding lithium-ion secondary batteries may be referenced as appropriate.

[0052] Examples of conductive additives include acetylene black, carbon black, and graphite. The content of the conductive additive in the positive electrode active material layer 21b is, for example, an amount such that the total amount with the single-walled carbon nanotubes is 1.0% by mass or less, preferably 0.5% by mass or less, and more preferably 0.1% by mass or less.

[0053] <Distribution of each component within the positive electrode active material layer> Next, the distribution of each component within the positive electrode active material layer 21b will be explained. Within the positive electrode active material layer 21b, the first active material particles and the second active material particles are stacked in the thickness direction. The first active material particles and the second active material particles are densely packed together due to the mixing of the relatively larger first active material particles and the relatively smaller second active material particles.

[0054] In the gaps between the first and second active material particles, aggregated portions of a water-dispersible binder are formed. These aggregated portions are dispersed in multiple locations within the positive electrode active material layer 21b. The dispersant is thinly distributed on the surfaces of the first and second active material particles, particularly on the surface of the second active material particles.

[0055] Here, the aggregated portions of the water-dispersible binder are uniformly dispersed within the positive electrode active material layer 21b. In other words, the individual aggregated portions do not form large clumps, but are finely dispersed as small clumps within the positive electrode active material layer 21b.

[0056] The state in which aggregated portions are finely dispersed within the positive electrode active material layer 21b can be determined based on the state of aggregated portions appearing in a cross-section parallel to the thickness direction of the positive electrode active material layer 21b (hereinafter sometimes referred to as the evaluation cross-section). The position of the evaluation cross-section in the positive electrode active material layer 21b is not particularly limited, as long as the cross-section is parallel to the thickness direction of the positive electrode active material layer 21b.

[0057] More specifically, with respect to the positive electrode active material layer 21b, the region located on the positive electrode current collector 21a side of the center in the thickness direction is defined as the first region, and the region located on the opposite side of the positive electrode current collector from the center is defined as the second region. In the evaluation cross-section of the positive electrode active material layer 21b, the average area of ​​the multiple aggregated portions located in the first region is defined as the average area (S1), and the average area of ​​the multiple aggregated portions located in the second region is defined as the average area (S2).

[0058] In this case, the ratio (S1 / S2) of the average area (S1) of the aggregated portion located in the first region to the average area (S2) of the multiple aggregated portions located in the second region is 0.9 or more and 1.1 or less. Preferably, the ratio (S1 / S2) is 0.95 or more and 1.05 or less. A ratio (S1 / S2) close to "1" means that the distribution of aggregated portions in the first region and the distribution of aggregated portions in the second region are similar. Therefore, when the ratio (S1 / S2) is within the above range, it can be said that the aggregated portions are uniformly dispersed in the positive electrode active material layer 21b at a certain level or higher.

[0059] The average area of ​​the aggregated portions located in the first region R1 and the second region R2, that is, the average area of ​​the aggregated portions in the entire evaluation cross-section of the positive electrode active material layer 21b, is, for example, 0.02 μm. ２ 0.05 μm or more ２ The following applies. Furthermore, the average area (S1) of the aggregated portion located in the first region is, for example, 0.02 μm. ２ 0.05 μm or more ２ The following applies: The average area (S2) of the multiple aggregates located in the second region is, for example, 0.02 μm. ２ 0.05 μm or more ２ The following applies:

[0060] Furthermore, the evaluation cross section is divided into multiple equal regions in the thickness direction of the positive electrode active material layer 21b (for example, into 4, 8, or 12 equal parts), and the average area (Smax) of the region with the largest average area is taken as the average area (Smax), and the average area (Smin) of the region with the smallest average area is taken as the average area (Smin). In this case, the ratio of the average area (Smax) to the average area (Smin) (Smax / Smin) is, for example, 1.0 or more and 1.25 or less, preferably 1.0 or more and 1.2 or less, and more preferably 1.0 or more and 1.3 or less.

[0061] The area of ​​individual aggregates appearing in the evaluation cross-section can be measured by analyzing the evaluation cross-section with a scanning electron microscope (SEM). For example, an evaluation cross-section parallel to the thickness direction is formed by cutting the positive electrode active material layer 21b in the thickness direction, and an SEM image of the evaluation cross-section is obtained. In the SEM image, for each of the multiple aggregates formed between the first active material particles and the second active material particles, a continuously distributed aggregate is treated as a single aggregate, and its area is measured. In the SEM image, the first and second active material particles and the aggregates formed between the first and second active material particles appear differently and can therefore be distinguished.

[0062] <Method for Manufacturing the Positive Electrode> Next, the method for manufacturing the positive electrode 21 will be described. The method for manufacturing the positive electrode 21 includes a coating step of applying a positive electrode mixture to the positive electrode current collector 21a, and a drying step of drying the positive electrode slurry applied to the positive electrode current collector 21a.

[0063] The positive electrode slurry is a mixture that solidifies to form the positive electrode active material layer 21b. The positive electrode slurry contains the positive electrode active material, single-walled carbon nanotubes, a dispersant, a water-dispersible binder, and an aqueous solvent, and may contain other components as needed. The components contained in the positive electrode slurry are the same as those described in the positive electrode active material layer section above. The solid content concentration of the positive electrode slurry is, for example, 70% by mass or more and 85% by mass or less. A detailed method for manufacturing the positive electrode slurry will be described later.

[0064] The coating process involves applying a positive electrode slurry to the first surface 21a1 of the positive electrode current collector 21a to form a coated layer of positive electrode slurry. Examples of methods for applying the positive electrode slurry include the roll method, die coating method, reverse roll method, doctor blade method, knife method, gravure method, dip method, and squeeze method. The positive electrode slurry may be applied continuously or intermittently to the first surface 21a1 of the positive electrode current collector 21a. The thickness of the coated layer of positive electrode slurry is adjusted so that the final thickness of the positive electrode active material layer 21b is 200 μm or more. The length and width of the coated layer of positive electrode slurry are appropriately set according to the size of the lithium-ion secondary battery.

[0065] The drying process involves drying the coating layer formed on the positive electrode current collector 21a to remove the aqueous solvent and solidify the coating layer. The coating layer solidified through the drying process becomes the positive electrode active material layer 21b. Examples of methods for drying the coating layer include natural drying, low-temperature air, hot air, vacuum, infrared radiation, far-infrared radiation, electron beams, and microwaves. Two or more of these drying methods may be combined. The drying temperature is, for example, 20 degrees Celsius or more and 120 degrees Celsius or less, preferably 40 degrees Celsius or more and 100 degrees Celsius or less.

[0066] Furthermore, in order to increase the electrode density, a compression step may be performed after the drying step to compress the positive electrode active material layer 21b. This compression step corresponds to the pressing process described above. Examples of methods for compressing the positive electrode active material layer 21b include the die press method and the calender press method. The pressing pressure is, for example, 0.1 t / cm. ２ More than 10t / cm ２ The following is preferred, preferably 0.5 t / cm ２ 5.0t / cm or more ２ The following applies:

[0067] When a compression process is performed, the thickness of the positive electrode active material layer 21b mixed in the compression process, i.e., the thickness of the positive electrode active material layer 21b before the pressing process, is, for example, 227 μm or more, preferably 273 μm or more. The thickness of the positive electrode active material layer 21b before the pressing process is, for example, 600 μm or less, preferably 533 μm or less. The density of the positive electrode active material layer 21b before the pressing process is, for example, 1.5 g / cm³.３ 2.2g / cm or more ３ The following applies:

[0068] A winding process may be performed between each of the above processes, and at least one timing after all the processes have been completed, to wind the positive electrode current collector 21a into a roll. Alternatively, a drying process may be performed again after the compression process.

[0069] Here, the method for manufacturing the positive electrode includes a bending step after the drying step in which the positive electrode current collector 21a on which the positive electrode active material layer 21b is formed is curved. The bending refers to a state in which at least a part of the positive electrode active material layer 21b is curved to a radius of curvature of a certain value or less, for example, 35 mm or less.

[0070] Examples of bending processes include a winding process in which a positive electrode current collector 21a, on which a positive electrode active material layer 21b is formed, is wound around a core material, and a bending process in which the positive electrode current collector 21a is curved along the circumferential surface of a cylindrical body. An example of a winding process is the winding process described above. An example of a bending process is when a positive electrode current collector 21a, on which a positive electrode active material layer 21b is formed, is transported by rollers, and the positive electrode current collector 21a is curved along the circumferential surface of a cylindrical body such as a guide roller or a tension roller. The bending process may be performed once or multiple times.

[0071] <Method for producing positive electrode slurry> The method for producing a positive electrode slurry includes a step (I) for preparing a first slurry and a step (II) for preparing a second slurry. Step (I) for preparing the first slurry is a step of preparing the first slurry by mixing third active material particles, single-walled carbon nanotubes, a dispersant, and an aqueous solvent. Step (II) for preparing the second slurry is a step of preparing the second slurry by mixing the first slurry, fourth active material particles, single-walled carbon nanotubes, and an aqueous solvent. The method for producing a positive electrode slurry further includes a step (III) for preparing a third slurry by mixing the second slurry and an aqueous dispersible binder.

[0072] In step (I), a first slurry is prepared by mixing and kneading the third active material particles, single-walled carbon nanotubes, a dispersant, and an aqueous solvent. The details of the third active material particles are the same as those described for the first and second active material particles, except for their particle size. The average particle size (D50) of the third active material particles is 0.7 μm or more and 3 μm or less. The particle size (D90) of the third active material particles is, for example, 9.5 μm or less. The relatively small second active material particles contained in the positive electrode active material layer 21b described above are composed entirely or mostly of the third active material particles. The average particle size (D50) and particle size (D90) of the third active material particles can be measured using a laser diffraction particle size analyzer. Details of the mixing amounts in step (I) will be described later.

[0073] The single-walled carbon nanotubes mixed in step (I) are a portion of the single-walled carbon nanotubes contained in the positive electrode active material layer 21b described above. Details of the amount of single-walled carbon nanotubes mixed in step (I) will be described later.

[0074] The dispersant mixed in step (I) is the dispersant contained in the positive electrode active material layer 21b described above. Details of the amount of dispersant mixed in step (I) will be described later. The aqueous solvent mixed in step (I) is water, or a mixed solvent of water and a non-aqueous solvent. The water is not particularly limited, but for example, ion-exchanged water, which is water treated with an ion exchange resin, and ultrapure water, which is water treated by a reverse osmosis membrane water purification system, are preferred. Examples of non-aqueous solvents constituting the mixed solvent include solvents that are miscible with water, such as lower alcohols, acetone, tetrahydrofuran, ethylene glycol, N-methyl-2-pyrrolidone, dimethylformamide, dimethylacetamide, acetonitrile, and dimethyl sulfoxide. These non-aqueous solvents can be used alone or in combination of two or more. The volume percentage of water in the mixed solvent is preferably 50% by volume or more and 99.9% by volume or less, and more preferably 60% by volume or more and 99% by volume or less.

[0075] The amount of aqueous solvent mixed in step (I) is, for example, an amount such that the solid content concentration of the first slurry is 75% by mass or more, preferably 78% by mass or more. Alternatively, the above amount is, for example, an amount such that the solid content concentration of the first slurry is 85% by mass or less, preferably 82% by mass or less.

[0076] It is preferable to have a high solid content concentration in the first slurry. By increasing the solid content concentration of the first slurry, the second active material particles, which are smaller in size, can be suitably dispersed in the first slurry. The solid content concentration of the first slurry is, for example, higher than the solid content concentration of the second slurry. Note that the above solid content concentration refers to the concentration of the components in the first slurry excluding the aqueous solvent.

[0077] The specific mixing method in step (I) is not particularly limited as long as it can uniformly mix each component contained in the first slurry, and conventionally known mixing methods used in the production of cathode slurries can be applied. Examples of the above mixing methods include mixing by hand using a stirring rod, and mixing by mechanical stirring using conventional mixers such as planetary mixers, homomixers, homodispersers, Henschel mixers, Banbury mixers, ribbon mixers, V-type mixers, and orbital mixers, as well as ultrasonic dispersers.

[0078] Furthermore, it is preferable to perform the mixing in step (I) more vigorously than the mixing in step (II), which will be described later. For example, when the mixing in step (I) and the mixing in step (II) are performed using the same apparatus, the rotations per unit time in the mixing in step (I) should be greater than those in the mixing in step (I). This allows for the optimal dispersion of the smaller second active material particles in the first slurry.

[0079] In step (II), the second slurry is prepared by mixing and kneading the first slurry, the fourth active material particles, single-walled carbon nanotubes, and an aqueous solvent. The details of the fourth active material particles are the same as those described for the first and second active material particles, except for their particle size. The average particle size (D50) of the fourth active material particles is 5 μm or more and 15 μm or less. The particle size (D10) of the fourth active material particles is, for example, 2 μm or more. The relatively large first active material particles contained in the positive electrode active material layer 21b described above are composed entirely or mostly of fourth active material particles. The average particle size (D50) and particle size (D10) of the fourth active material particles can be measured using a laser diffraction particle size analyzer. Details of the amount of fourth active material particles mixed in step (II) will be described later.

[0080] The single-walled carbon nanotubes mixed in step (II) are a portion of the single-walled carbon nanotubes contained in the cathode slurry. Details of the proportions of single-walled carbon nanotubes mixed in step (II) will be described later.

[0081] The aqueous solvent mixed in step (II) is water, or a mixed solvent of water and a non-aqueous solvent. The details of the water and mixed solvent constituting the aqueous solvent are the same as those of the aqueous solvent mixed in step (I). In one example, the composition of the aqueous solvent mixed in step (II) is the same as the composition of the aqueous solvent mixed in step (I).

[0082] The amount of aqueous solvent mixed in step (II) is, for example, an amount such that the solid content concentration of the second slurry is 70% by mass or more and 78% by mass or less, preferably 72% by mass or more and 76% by mass or less. The above solid content concentration refers to the concentration of the components in the second slurry excluding the aqueous solvent.

[0083] The specific mixing method in step (II) is not particularly limited, and conventionally known mixing methods used in the production of cathode slurries can be applied, similar to the mixing method in step (I). In step (III), the second slurry and a water-dispersible binder are mixed and kneaded to prepare the third slurry.

[0084] The water-dispersible binder mixed in step (III) is the water-dispersible binder contained in the positive electrode active material layer 21b described above. Details of the amount of water-dispersible binder mixed in step (III) will be described later.

[0085] The specific mixing method in step (III) is not particularly limited, and a conventionally known mixing method used for the manufacture of positive electrode slurry can be applied, similar to the mixing method in step (I). When manufacturing a positive electrode slurry containing other components, the other components may be mixed in any of steps (I) to (III). Furthermore, a step (IV) may be provided after step (III) in which some or all of the other components are mixed into the third slurry prepared in step (III). If the method for manufacturing a positive electrode slurry does not include step (IV), the slurry prepared in step (III) becomes the positive electrode slurry. Furthermore, if the method for manufacturing a positive electrode slurry includes step (IV), the slurry prepared in step (IV) becomes the positive electrode slurry.

[0086] The manufactured cathode slurry contains third active material particles, fourth active material particles, single-walled carbon nanotubes, a dispersant, a water-dispersible binder, and an aqueous solvent. Below, the proportions of each component will be described using relative values ​​when the total mass of the main components—the third active material particles, fourth active material particles, single-walled carbon nanotubes, dispersant, and water-dispersible binder contained in the cathode slurry—is set to 100 parts by mass.

[0087] The amount of third active material particles mixed in step (I) is, for example, 9.70 parts by mass or more and 29.11 parts by mass or less, preferably 14.47 parts by mass or more and 24.75 parts by mass or less.

[0088] The amount of fourth active material particles mixed in step (II) is, for example, 68.47 parts by mass or more and 89.10 parts by mass or less, preferably 72.75 parts by mass or more and 84.22 parts by mass or less. The ratio (A4 / A3) of the amount of third active material particles mixed in step (I) (A3) to the amount of fourth active material particles mixed in step (II) (A4) is, for example, 2.4 or more and 9.0 or less, preferably 3.0 or more and 5.7 or less.

[0089] (Amount of single-walled carbon nanotubes) The total amount of single-walled carbon nanotubes mixed in steps (I) and (II) is, for example, 0.01 parts by mass or more and 0.1 parts by mass or less, preferably 0.03 parts by mass or more and 0.07 parts by mass or less. In step (I), a portion of the total amount of single-walled carbon nanotubes is mixed, and in step (II), the remaining portion of the total amount of single-walled carbon nanotubes is mixed.

[0090] The amounts of single-walled carbon nanotubes mixed in step (I) and in step (II) are adjusted based on the ratio (A4 / A3) of the amount of third active material particles (A3) mixed in step (I) and the amount of fourth active material particles (A4) mixed in step (II). In other words, the ratio (C2 / C1) is defined as the ratio of the amount of single-walled carbon nanotubes (C1) mixed in step (I) to the amount of single-walled carbon nanotubes (C2) mixed in step (II). The amounts of single-walled carbon nanotubes (C1) mixed in step (I) and in step (II) (C2) are adjusted so that the ratio (C2 / C1) is between 0.9 and 1.1 times the ratio (A4 / A3).

[0091] In step (I), the entire amount of dispersant contained in the positive electrode slurry is incorporated. In this case, the amount of dispersant mixed in step (I) is, for example, 0.2 parts by mass or more and 0.9 parts by mass or less, preferably 0.3 parts by mass or more and 0.6 parts by mass or less. The total amount of dispersant contained in the positive electrode slurry is adjusted based on the total amount of third active material particles and fourth active material particles. Therefore, when the entire amount of dispersant contained in the positive electrode slurry is incorporated into the first slurry, the first slurry will contain an excess amount of dispersant relative to the third active material particles. This allows for the optimal dispersion of the smaller second active material particles in the first slurry.

[0092] Furthermore, the dispersant may be divided and blended in step (I) and in any of the other steps. In this case as well, it is preferable that 80% by mass or more of the total amount of dispersant contained in the cathode slurry is blended in step (I) in order to suitably disperse the second active material particles. The amount of dispersant blended in step (I) is, for example, 0.015 times or more the amount of third active material particles (A3) blended in step (I).

[0093] In step (III), the entire amount of the water-dispersible binder contained in the positive electrode slurry is incorporated. In this case, the amount of water-dispersible binder mixed in step (III) is, for example, 0.8 parts by mass or more and 2.0 parts by mass or less, preferably 0.9 parts by mass or more and 1.5 parts by mass or less. Alternatively, the water-dispersible binder may be incorporated in two separate steps: one in step (III) and the other in any of the other steps.

[0094] Furthermore, the form in which each component is blended in steps (I) to (IV) is not particularly limited and can be appropriately selected based on the properties of each component. For example, they may be blended in their individual form, or in the form of a mixture such as a solution or dispersion. The same solvents exemplified for aqueous solvents can be used as the solvents that make up the mixture.

[0095] For example, since single-walled carbon nanotubes have strong cohesive properties, it is preferable to incorporate them in the form of a paste-like dispersion. By incorporating them in the form of a dispersion, aggregation of single-walled carbon nanotubes in the first or second slurry can be suppressed. When single-walled carbon nanotubes are incorporated in the form of a dispersion, the amount of single-walled carbon nanotubes to be incorporated is equal to the amount of solids in the dispersion.

[0096] <Lithium-ion secondary battery> An example of a lithium-ion secondary battery using the positive electrode 21 described above is a stacked energy storage device in which multiple energy storage cells are stacked. The basic configuration of the above energy storage device will be described below.

[0097] The energy storage cell of the above-described energy storage device comprises a negative electrode having a negative electrode active material layer formed on a foil-shaped negative electrode current collector, a positive electrode which is an electrode for the positive electrode of a secondary battery, a separator, a spacer, and an electrolyte. The negative electrode and positive electrode are arranged so that the negative electrode active material layer and the positive electrode active material layer face each other with the separator in between. The spacer is arranged to surround the positive electrode active material layer and the negative electrode active material layer and is bonded to the positive electrode current collector and the negative electrode current collector. Inside the energy storage cell, a sealed space is formed surrounded by the spacer, the positive electrode, and the negative electrode. The separator and the electrolyte are housed in the sealed space.

[0098] The above-described energy storage device comprises a cell stack in which a plurality of energy storage cells are stacked in a stacking direction, and a pair of current-carrying elements that sandwich the cell stack in the stacking direction of the cell stack. The cell stack has a structure in which a plurality of energy storage cells are directly or indirectly stacked on top of each other so that a negative electrode current collector and a positive electrode current collector are electrically connected. As a result, the plurality of energy storage cells constituting the cell stack are connected in series. The pair of current-carrying elements consists of a positive electrode current-carrying plate electrically connected to the positive electrode current collector and a negative electrode current-carrying plate electrically connected to the negative electrode current collector. The above-described energy storage device is charged and discharged through terminals provided on the positive electrode current-carrying plate and the negative electrode current-carrying plate, respectively.

[0099] <Effects> According to this embodiment, the following effects can be obtained. (1) The positive electrode 21 comprises a positive electrode current collector 21a and a positive electrode active material layer 21b formed on the positive electrode current collector 21a and having a thickness of 200 μm or more. The positive electrode active material layer 21b contains first active material particles with a particle diameter of 5 μm or more and 15 μm or less, second active material particles with a particle diameter of 0.7 μm or more and 3 μm or less, a water-dispersible binder, a dispersant, and single-walled carbon nanotubes. Multiple aggregated portions formed by the aggregation of the water-dispersible binder are dispersed in the positive electrode active material layer 21b. In a cross-section parallel to the thickness direction of the positive electrode active material layer 21b, the ratio (S1 / S2) of the average area of ​​the aggregated portion in the first region located on the positive electrode current collector 21a side of the center in the thickness direction (S1) to the average area of ​​the aggregated portion in the second region located on the opposite side of the positive electrode current collector 21a from the center (S2) is 0.9 or more and 1.1 or less.

[0100] According to the above configuration, the flexibility of the positive electrode active material layer 21b is improved because the water-dispersible binder is uniformly dispersed within the positive electrode active material layer 21b. As a result, cracking of the positive electrode active material layer 21b caused by bending loads can be suppressed during the manufacturing process of the positive electrode 21.

[0101] (2) A method for producing a cathode slurry containing cathode active material particles, a water-dispersible binder, a dispersant, and single-walled carbon nanotubes includes the steps of: preparing a first slurry by mixing third active material particles having an average particle size (D50) of 0.7 μm or more and 3 μm or less, single-walled carbon nanotubes, a dispersant, and an aqueous solvent; and preparing a second slurry by mixing the first slurry, fourth active material particles having an average particle size (D50) of 5 μm or more and 15 μm or less, single-walled carbon nanotubes, and an aqueous solvent. When the amount of third active material particles mixed in the process of preparing the first slurry is A3, and the amount of fourth active material particles mixed in the process of preparing the second slurry is A4, the ratio (C2 / C1) of the amount of single-walled carbon nanotubes mixed in the process of preparing the first slurry (C1) to the amount of single-walled carbon nanotubes mixed in the process of preparing the second slurry (C2) is between 0.9 and 1.1 times the ratio (A4 / A3).

[0102] According to the above configuration, a positive electrode slurry suitable for forming a positive electrode active material layer 21b containing positive electrode active material particles of different particle sizes and having a thickness of 350 μm or more can be obtained. In other words, by forming the positive electrode active material layer 21b using this positive electrode slurry, a water-dispersible binder can be uniformly dispersed within the positive electrode active material layer 21b.

[0103] <Examples of Modifications> This embodiment can be implemented with the following modifications. This embodiment and the following examples of modifications can be combined with each other to the extent that they do not contradict each other technically.

[0104] ○The positive electrode 21 may constitute a bipolar electrode. An example of a bipolar electrode is shown in Figure 2. The bipolar electrode 100 shown in Figure 2 comprises a bipolar current collector 101, a positive electrode active material layer 102, and a negative electrode active material layer 103. In this case, the bipolar current collector 101 corresponds to the current collector described in the claims. The bipolar current collector 101 has a first surface 101b and a second surface 101a facing the opposite side of the first surface 101b. The first surface 101b and the second surface 101a are surfaces perpendicular to the thickness direction of the bipolar current collector 101.

[0105] The bipolar current collector 101 is a laminate formed by integrally joining a positive electrode current collector 104 and a negative electrode current collector 105 in the thickness direction. The second surface 101a of the bipolar current collector 101 is formed by the positive electrode current collector 104. The first surface 101b of the bipolar current collector 101 is formed by the negative electrode current collector 105. The positive electrode active material layer 102 is provided on the second surface 101a of the bipolar current collector 101. The negative electrode active material layer 103 is provided on the first surface 101b of the bipolar current collector 101.

[0106] ○Instead of step (III) of mixing the second slurry with the water-dispersible binder, the water-dispersible binder may be mixed in either step (I) or step (II), or both. Step (III) of mixing the water-dispersible binder with the second slurry may be omitted. In this case, the manufactured slurry for the positive electrode of the secondary battery will be used after mixing it with the water-dispersible binder at the time of use.

[0107] The following describes an example that further elaborates on the above embodiment. <Example 1> (Preparation of cathode slurry) A cathode slurry with a solid content of 74% was prepared, containing third active material particles, fourth active material particles, a dispersant, single-walled carbon nanotubes, and a water-dispersible binder, with each solid content in the proportions shown in Table 1. As shown in Table 1, the mixing ratio of the third active material particles to the fourth active material particles (fourth active material particles / third active material particles) is 4 (= 78.6 / 19.65). Details of each component contained in the cathode slurry are as follows.

[0108] Third active material particle: LiFePO4 with an average particle diameter (D50) of 1.3 μm and a particle diameter (D90) of 2.8 μm. ４ The particles of the fourth active material: LiFePO4 with an average particle diameter (D50) of 7 μm and a particle diameter (D10) of 3 μm. ４ The particles.

[0109] Dispersant: Carboxymethylcellulose (CMC) Water-dispersible binder: Styrene-butadiene rubber (SBR) Single-walled carbon nanotubes (CNTs): Paste of single-walled carbon nanotubes First, in step (I), the entire amount of the third active material particles, the entire amount of CMC, a portion of the CNTs, and water were mixed and kneaded using a planetary mixer to obtain the first slurry. The amount of water added in step (I) was such that the solid content concentration of the first slurry was 80% by mass.

[0110] Next, in step (II), the first slurry, the entire amount of the fourth active material particles, the remaining CNTs, and water were mixed and kneaded using a planetary mixer at 16 rpm for 40 minutes to obtain the second slurry. The amount of water added in step (II) was such that the solid content concentration of the second slurry was 75.6% by mass.

[0111] Here, the amount of CNTs used in process (I) was set to be 20% by mass relative to the total amount of CNTs (solids). The amount of CNTs used in process (II) was set to be 80% by mass relative to the total amount of CNTs (solids). Therefore, the ratio of the amount of CNTs used in process (I) to the amount of CNTs used in process (II) is 4 (= 80 / 20). This ratio is equal to the ratio of the amount of third active material particles used in process (I) to the amount of fourth active material particles used in process (II). In processes (I) and (II), the same solids content paste of CNTs was used.

[0112] Next, in step (III), the second slurry and the entire amount of SBR were mixed and kneaded using a planetary mixer at 16 rpm for 20 minutes to obtain the cathode slurry of Example 1.

[0113] (Preparation of the positive electrode) The positive electrode of Example 1 was prepared using the positive electrode slurry of Example 1. First, the positive electrode slurry was applied to one side of a 50 μm thick aluminum foil, which would serve as the positive electrode current collector, using an applicator. Then, the coated positive electrode slurry was heated on a 50°C hot plate for 12 minutes to dry, thereby forming a positive electrode active material layer. The thickness of the positive electrode active material layer after drying was 350 μm. The obtained electrode was used as the positive electrode of Example 1.

[0114] <Example 2> Example 2 is an example in which the mixing ratios of the third active material particles, fourth active material particles, dispersant, single-walled carbon nanotubes, and water-dispersible binder were changed compared to Example 1. A cathode slurry with a solid content of 74% was prepared, containing the third active material particles, fourth active material particles, dispersant, single-walled carbon nanotubes, and water-dispersible binder in the mixing ratios shown in Table 1 for each solid content. As shown in Table 1, the mixing ratio of the third active material particles to the fourth active material particles (fourth active material particles / third active material particles) is 4 (= 78.2 / 19.55). The details of each component contained in the cathode slurry are the same as in Example 1. Using the prepared cathode slurry, the cathode slurry of Example 2 and the cathode of Example 2 were prepared by the same process as in Example 1.

[0115]

[0116] <Comparative Example 1> (Preparation of positive electrode slurry) The positive electrode slurry of Comparative Example 1 has the same composition as the positive electrode slurry of Example 1, but the order in which the components are mixed is different from that of Example 1.

[0117] First, in step (Ia), the entire amount of the third active material particles, the entire amount of the fourth active material particles, and the entire amount of CMC were mixed and kneaded using a planetary mixer at 16 rpm for 40 minutes to obtain a powder mixture.

[0118] Next, in step (IIa), the powder mixture, the total amount of CNTs, and water were mixed and kneaded using a planetary mixer to obtain an intermediate slurry. The amount of water added in step (IIa) was such that the solid content concentration of the intermediate slurry was 76% by mass.

[0119] Next, in step (IIIa), the intermediate slurry and the total amount of SBR were mixed and kneaded using a planetary mixer to obtain the cathode slurry of Comparative Example 1. (Preparation of the cathode) The cathode of Comparative Example 1 was prepared in the same manner as in Example 1.

[0120] <Analysis of the cross-section of the positive electrode active material layer> The positive electrode of each example was cut into 5 mm x 5 mm sections and used as a test sample. One of the cross-sections of the positive electrode active material layer formed by cutting the test sample was used as the evaluation cross-section. The evaluation cross-section was polished by CP processing (ion milling). The polished evaluation cross-section was observed using SEM. For SEM observation, the observation field was set to include the entire thickness direction of the evaluation cross-section, and eight SEM images were obtained, shifted by 50 μm at intervals in the thickness direction of the evaluation cross-section.

[0121] The conditions for SEM observation are as follows: Acceleration voltage: 3kV Detector: ESB Magnification: 2000x Pixel count: 4096×3072 Aperture diameter: 60μm Field of view: 8 For each of the obtained SEM images, LiFePO4 ４ The dispersion state of aggregated regions between particles was observed. In the SEM image, LiFePO ４ The components other than the particles are LiFePO ４ It can be distinguished from other particles because they look different. LiFePO ４ Of the components other than the particles, CMC is water-soluble, therefore LiFePO ４ It is thinly distributed around the particles. Also, CNTs are present in trace amounts compared to other components. Therefore, compared to other components, LiFePO ４ The aggregated regions between the particles can be considered to be composed of SBR. In addition, energy-dispersive X-ray analysis was performed separately on the evaluation cross-section to confirm that the aggregated regions visible in the resulting SEM image are carbon-containing components.

[0122] For all aggregates visible in the obtained SEM images, continuously distributed aggregates were treated as a single aggregate, and the area of ​​each aggregate was measured. The area of ​​each aggregate was measured by counting the number of pixels of each aggregate obtained through image processing. Then, for the evaluation cross section, eight regions (R1 to R8) were set by dividing the active material layer into eight equal parts from the positive electrode current collector side in the thickness direction, and the average area of ​​the individual aggregates located in each region was calculated. The results are shown in Tables 2 and 3.

[0123] Furthermore, the average area of ​​individual aggregates located in regions R1 to R4 (hereinafter referred to as average area (S1)) and the average area of ​​individual aggregates located in regions R5 to R8 (hereinafter referred to as average area (S2)) were calculated. The results are shown in Tables 2 and 3. Regions R1 to R4 correspond to the first region described above, and regions R5 to R8 correspond to the second region described above.

[0124]

[0125]

[0126] As shown in Tables 2 and 3, in Comparative Example 1, the average area of ​​the aggregated portion in each region R1 to R8 was 0.044 to 0.093 μm. ２ It changes within this range. In Comparative Example 1, the ratio (S1 / S2) of the average area S1 of the aggregated portion in the first region to the average area S2 of the aggregated portion in the second region was approximately 1.3 (= 0.081 / 0.064), which was outside the range of 0.9 to 1.1.

[0127] On the other hand, in Example 1, the average area of ​​the aggregated portion in each region R1 to R8 was 0.030 to 0.034 μm. ２ It changes within this range. In addition, in Example 1, the ratio (S1 / S2) of the average area S1 of the aggregated parts in the first region to the average area S2 of the aggregated parts in the second region was 1.0 (= 0.32 / 0.32), which was within the range of 0.9 to 1.1. From these results, it can be seen that in Example 1, compared to Comparative Example 1, the individual aggregated parts are finer and more uniformly dispersed.

[0128] In Example 2, the average area of ​​the aggregated portion in each region R1 to R8 was 0.038 to 0.048 μm. ２ It changes within this range. In Example 2, the ratio (S1 / S2) of the average area S1 of the aggregated portion in the first region to the average area S2 of the aggregated portion in the second region was 1.0 (= 0.43 / 0.42), which was within the range of 0.9 to 1.1. Thus, it was confirmed that the same results as in Example 1 could be obtained in Example 2, in which the blending ratio was changed.

[0129] Furthermore, the ratio (R3 / R4) corresponding to the ratio (Smax / Smin) of Comparative Example 1 was approximately 2.1 (= 0.093 / 0.044). In contrast, the ratio (R1 / R2) corresponding to the ratio (Smax / Smin) of Example 1 was approximately 1.1 (= 0.034 / 0.030), showing a significant difference between Example 1 and Comparative Example 1.

[0130] The ratio (R2 / R1) corresponding to the ratio (Smax / Smin) in Example 2 was approximately 1.3 (= 0.048 / 0.038). Thus, it was confirmed that the same results as in Example 1 could be obtained in Example 2, even with a changed blending ratio.

[0131] <Bending Test> The positive electrode of each example was cut into 5 cm x 10 cm pieces and used as a test sample. With the side of the test sample where the positive electrode active material layer is formed facing outwards, the test sample was wrapped around half the circumference of the outer surface of a 70 mm diameter cylinder, and the surface condition of the positive electrode active material layer was visually evaluated. As a result, no cracks were found on the surface of the positive electrode active material layer of the positive electrodes of Example 1 and Example 2. On the other hand, cracks were found on the surface of the positive electrode active material layer of the positive electrode of Comparative Example 1.

[0132] As shown by the analysis of the cross-section of the positive electrode active material layer described above, in Examples 1 and 2, the aggregated portion is uniformly dispersed within the positive electrode active material layer compared to Comparative Example 1. It is believed that the uniform dispersion of the aggregated portion, i.e., the water-dispersible binder, within the positive electrode active material layer suppressed the decrease in the flexibility of the active material layer that resulted from simultaneously employing a configuration that forms a thick active material layer and a configuration that uses active material particles of different particle sizes.

[0133] Although detailed experimental data is omitted, a positive electrode with a 150 μm thick positive electrode active material layer was prepared using the positive electrode slurry of Comparative Example 1, and the above bending test was performed. As a result, no cracks were observed on the surface of the positive electrode active material layer. Furthermore, a positive electrode with a 350 μm thick positive electrode active material layer was prepared using the positive electrode slurry obtained by the same method as in Comparative Example 1, except that the third active material particles were changed to the same amount of fourth active material particles, and the above bending test was performed. As a result, no cracks were observed on the surface of the positive electrode active material layer.

[0134] 21... Positive electrode 21a... Positive electrode current collector 21a1... First surface 21b... Positive electrode active material layer

Claims

1. A positive electrode for an energy storage device comprising a current collector having a first surface and a positive electrode active material layer formed on the first surface of the current collector and having a thickness of 200 μm or more, wherein the positive electrode active material layer contains first active material particles having a particle diameter of 5 μm or more and 15 μm or less, second active material particles having a particle diameter of 0.7 μm or more and 3 μm or less, a water-dispersible binder, a dispersant, and single-walled carbon nanotubes, wherein a plurality of aggregated portions formed by the aggregation of the water-dispersible binder are dispersed in the positive electrode active material layer, and the ratio (S1 / S2) of the average area (S1) of the aggregated portions in a first region located on the current collector side of the center position in the thickness direction to the average area (S2) of the aggregated portions in a second region located on the opposite side of the current collector from the center position in a cross section parallel to the thickness direction of the positive electrode active material layer is 0.9 or more and 1.1 or less.

2. The average area of ​​the aggregated portion in the entire cross-section is 0.02 μm². ２ 0.05 μm or more ２ The positive electrode for an energy storage device according to claim 1, which is as follows:

3. The positive electrode for an energy storage device according to claim 1 or claim 2, wherein the ratio (A1 / A2) of the content of the first active material particles (A1) to the content of the second active material particles (A2) in the positive electrode active material layer is 2.4 or more and 9.0 or less.

4. The positive electrode for an energy storage device according to any one of claims 1 to 3, wherein the total content of the first active material particles and the second active material particles in the positive electrode active material layer is 97.0% by mass or more and 99.0% by mass or less, the content of the water-dispersible binder in the positive electrode active material layer is 0.8% by mass or more and 2.0% by mass or less, the content of the dispersant in the positive electrode active material layer is 0.2% by mass or more and 0.9% by mass or less, and the content of the single-walled carbon nanotubes in the positive electrode active material layer is 0.01% by mass or more and 0.1% by mass or less.

5. The positive electrode for an energy storage device according to any one of claims 1 to 4, wherein the first active material particles and the second active material particles are particles mainly composed of a polyanionic compound having an olivine-type structure, the water-dispersible binder is a compound having an aromatic ring, and the dispersant is at least one selected from carboxymethylcellulose and a salt of carboxymethylcellulose.

6. A method for manufacturing a cathode slurry used in the manufacture of a cathode for an energy storage device, comprising a current collector having a first surface, and a positive electrode active material layer formed on the first surface of the current collector and having a thickness of 200 μm or more, wherein the positive electrode active material layer contains positive electrode active material particles, a water-dispersible binder, a dispersant, and single-walled carbon nanotubes, comprising the steps of: preparing a first slurry by mixing third active material particles as positive electrode active material particles having an average particle diameter (D50) of 0.7 μm or more and 3 μm or less, the single-walled carbon nanotubes, the dispersant, and an aqueous solvent; and preparing a second slurry by mixing the first slurry, fourth active material particles as positive electrode active material particles having an average particle diameter (D50) of 5 μm or more and 15 μm or less, the single-walled carbon nanotubes, and an aqueous solvent. A method for producing a cathode slurry, wherein when the amount of the third active material particles mixed in the step of preparing the first slurry is A3, and the amount of the fourth active material particles mixed in the step of preparing the second slurry is A4, the ratio (C2 / C1) of the amount of the single-walled carbon nanotubes mixed in the step of preparing the first slurry (C1) to the amount of the single-walled carbon nanotubes mixed in the step of preparing the second slurry (C2) is 0.9 times or more and 1.1 times or less of the ratio (A4 / A3).

7. A method for producing a cathode slurry according to claim 6, comprising the step of preparing a third slurry by mixing the second slurry and the water-dispersible binder after the step of preparing the second slurry.

8. The method for producing a cathode slurry according to claim 6 or claim 7, wherein the solid content concentration of the first slurry is higher than that of the second slurry.

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