Method for manufacturing positive electrode for lithium ion secondary battery and method for manufacturing lithium ion secondary battery

Abstract

To provide a method for manufacturing a positive electrode for a lithium ion secondary battery that can reduce the DC resistance of the battery.SOLUTION: A method for manufacturing a positive electrode for a lithium-ion secondary battery includes the steps of: obtaining a positive electrode having an active material layer containing a positive electrode active material, a solid electrolyte, and a binder; measuring a first diffusion completion time of the positive electrode using Method 1; and selecting a positive electrode whose first diffusion completion time is 25 seconds or longer and for which the first diffusion is not completed. (Method 1) A test piece of 2 cm (length)×2 cm (width) is cut out from the positive electrode active material layer. Next, 20 μL of butyl butyrate is dropped from a position 1 cm vertically away from the surface of the test piece toward a center of the test piece under conditions of a temperature of 25°C, a dew point temperature of -70°C, and atmospheric pressure. After the drop, the time until the butyl butyrate diffuses over the entire surface is measured, and this time is defined as the first diffusion completion time. When the butyl butyrate has not diffused over the entire surface even when 180 seconds passed after the drop, it is determined that "the first diffusion is not completed".SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing a positive electrode for a lithium ion secondary battery and a method for producing a lithium ion secondary battery. [Background technology]

[0002] Lithium ion secondary batteries are known as batteries with high energy density.

[0003] Patent Document 1 describes a method for producing a positive electrode-solid electrolyte composite for an all-solid-state energy storage element, which aims to enable bonding at a relatively low temperature to suppress the generation of a high-resistance reaction layer at the interface, and to maximize the bonding area by increasing the adhesion between the plate-shaped positive electrode and the plate-shaped solid electrolyte at the interface, and includes the steps of: laminating a plate-shaped positive electrode made of a ceramic sintered body containing a positive electrode active material and a plate-shaped solid electrolyte made of a ceramic sintered body having ion conductivity to obtain a laminate; and simultaneously applying heat and pressure to the laminate to integrate the positive electrode and the solid electrolyte by a solid-phase reaction. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-243111 Summary of the Invention [Problem to be solved by the invention]

[0005] The present invention provides a method for producing a positive electrode for a lithium ion secondary battery, which can reduce the DC resistance of the battery. [Means for solving the problem]

[0006] The present inventors have found that, in a positive electrode for a lithium ion secondary battery having a positive electrode active material layer including a positive electrode active material, a solid electrolyte, and a binder, there is a correlation between the density of the positive electrode active material layer and the DC resistance of the battery. As a result of further intensive research based on the above findings, the present inventors have found that the DC resistance of the battery can be reduced by setting the time measured for the positive electrode by a predetermined method within a predetermined range, and have completed the present invention.

[0007] That is, according to the present invention, there are provided a method for producing a positive electrode for a lithium ion secondary battery and a method for producing a lithium ion secondary battery, which will be described below.

[0008] [1] forming a positive electrode active material layer from a mixture containing a positive electrode active material, a solid electrolyte, and a binder, thereby obtaining a positive electrode for a lithium ion secondary battery having the positive electrode active material layer; a measuring step of measuring a first diffusion completion time for the positive electrode for a lithium ion secondary battery by the following method 1; a sorting step of sorting the positive electrode for a lithium ion secondary battery, the positive electrode for which the first diffusion completion time is 25 seconds or more, or the positive electrode for which the first diffusion is not completed when the following method 1 is performed; A method for producing a positive electrode for a lithium ion secondary battery, comprising: (Method 1) A test piece measuring 2 cm (length) × 2 cm (width) is cut from the positive electrode active material layer. Next, under conditions of a temperature of 25°C, a dew point temperature of −70°C, and atmospheric pressure, 20 μL of butyl butyrate is dropped from a position 1 cm away from the surface of the test piece in a vertical direction toward the center of the test piece. The time from the drop until the butyl butyrate is diffused over the entire surface is measured, and this time is designated as the first diffusion completion time. If the butyl butyrate has not diffused over the entire surface even after 180 seconds have passed since the drop, it is determined that "the first diffusion is not completed." [2] The method for producing a positive electrode for a lithium ion secondary battery according to [1], wherein in the selection step, the positive electrode for a lithium ion secondary battery in which the first diffusion is not completed when the method 1 is carried out is selected. [3] The measuring step includes a step of measuring a second diffusion completion time for the positive electrode for a lithium ion secondary battery by the following method 2, The selection step includes a step of selecting the positive electrode for a lithium ion secondary battery for which the second diffusion completion time is 1 second or more, or for which the second diffusion is not completed when the following method 2 is performed: The method for producing a positive electrode for a lithium ion secondary battery according to [1] or [2]. (Method 2) A test piece measuring 2 cm (length) × 2 cm (width) is cut out from the positive electrode active material layer. Then, under the conditions of a temperature of 25°C, a dew point temperature of -70°C, and atmospheric pressure, 20 μL of butyl butyrate is dropped from a position 1 cm away from the surface of the test piece in the vertical direction toward the center of the test piece. After dropping, the area where the butyl butyrate has spread is 2 cm. 2 The time until the area where the butyl butyrate has diffused is 2 cm or more is measured, and this time is defined as the second diffusion completion time. 2 If the number is not more than this, it is determined that "the second diffusion is not completed." [4] the measuring step includes measuring a diffusion stoppage time for the positive electrode for a lithium ion secondary battery by the following method 3, the selection step includes a step of selecting the lithium ion secondary battery positive electrode having the diffusion stop time of 25 seconds or more. The method for producing a positive electrode for a lithium ion secondary battery according to any one of [1] to [3]. (Method 3) A 2 cm (length) × 2 cm (width) test piece was cut from the positive electrode active material layer. Next, under conditions of a temperature of 25°C, a dew point temperature of -70°C, and atmospheric pressure, 20 μL of butyl butyrate was dropped from a position 1 cm away from the surface of the test piece in a vertical direction toward the center of the test piece. The time from the drop until the diffusion of the butyl butyrate on the surface stopped was measured, and this time was defined as the diffusion stop time. [5] The method for producing a positive electrode for a lithium ion secondary battery according to any one of [1] to [4], wherein the positive electrode active material contains a lithium composite oxide. [6] [5] The method for producing a positive electrode for a lithium ion secondary battery according to [5], wherein the lithium composite oxide contains one or more composite oxides selected from the group consisting of lithium-nickel-cobalt-manganese composite oxides, lithium-nickel composite oxides, lithium-cobalt composite oxides, and lithium-nickel-aluminum composite oxides. [7] The method for producing a positive electrode for a lithium ion secondary battery according to [5], wherein the lithium composite oxide contains a lithium-nickel-cobalt-manganese composite oxide. [8] The method for producing a positive electrode for a lithium ion secondary battery according to any one of [1] to [7], wherein the positive electrode active material layer has a porosity of 12.0% or less. [9] The method for producing a positive electrode for a lithium ion secondary battery according to any one of [1] to [8], wherein a content of the positive electrode active material in the positive electrode active material layer is 50% by mass or more and 90% by mass or less, when the entire positive electrode active material layer is taken as 100% by mass.

[10] The method for producing a positive electrode for a lithium ion secondary battery according to any one of [1] to [9], wherein the solid electrolyte contains a sulfide-based solid electrolyte.

[11] The method for producing a positive electrode for a lithium ion secondary battery according to any one of [1] to

[10] , wherein a content of the solid electrolyte in the positive electrode active material layer is 5% by mass or more and 40% by mass or less, when the entire positive electrode active material layer is taken as 100% by mass.

[12] The method for producing a positive electrode for a lithium ion secondary battery according to any one of [1] to

[11] , wherein the binder contains one or more binders selected from the group consisting of fluorine-based binders and rubber-based binders.

[13] The method for producing a positive electrode for a lithium ion secondary battery according to any one of [1] to

[12] , wherein a content of the binder in the positive electrode active material layer is more than 0 mass % and 10 mass % or less, when the entire positive electrode active material layer is taken as 100 mass %.

[14] The method for producing a positive electrode for a lithium ion secondary battery according to any one of [1] to

[13] , wherein the mixture further contains a conductive additive.

[15]

[14] The method for producing a positive electrode for a lithium ion secondary battery according to

[14] , wherein the conductive additive comprises one or more selected from the group consisting of carbon black, activated carbon, graphite, mesoporous carbon, fullerenes, carbon nanotubes, carbon nanohorns, carbon nanofibers, and carbon brushes.

[16] The method for producing a positive electrode for a lithium ion secondary battery according to

[14] or

[15] , wherein the content of the conductive additive in the positive electrode active material layer is more than 0 mass % and 10 mass % or less, when the entire positive electrode active material layer is taken as 100 mass %.

[17] The method for producing a positive electrode for a lithium ion secondary battery according to any one of [1] to

[16] , wherein the step of obtaining the positive electrode for a lithium ion secondary battery includes a step of applying the mixture to a support to obtain the positive electrode active material layer.

[18] The method for producing a positive electrode for a lithium ion secondary battery according to any one of [1] to

[17] , comprising a step of applying the positive electrode active material layer to a surface of a positive electrode current collector.

[19] The method for producing a positive electrode for a lithium ion secondary battery according to

[17] , wherein the support includes a positive electrode current collector.

[20] A method for producing a lithium ion secondary battery, comprising a step of producing a lithium ion secondary battery using a positive electrode for a lithium ion secondary battery produced by the method for producing a positive electrode for a lithium ion secondary battery according to any one of [1] to

[19] , a solid electrolyte layer containing a solid electrolyte, and a negative electrode. [Effects of the Invention]

[0009] According to the present invention, a method for producing a positive electrode for a lithium ion secondary battery that can reduce the DC resistance of the battery can be provided. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a cross-sectional view schematically illustrating an example of a lithium-ion secondary battery according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that the drawings are schematic diagrams and do not correspond to actual dimensional proportions. In this specification, "A to B" indicating a numerical range means A or more and B or less unless otherwise specified.

[0012] (Method of manufacturing a positive electrode for a lithium-ion secondary battery) The method for producing a positive electrode for a lithium ion secondary battery according to this embodiment includes the steps of: obtaining a positive electrode for a lithium ion secondary battery including a positive electrode active material layer by forming a positive electrode active material layer from a mixture including a positive electrode active material, a solid electrolyte, and a binder; measuring a first diffusion completion time for the positive electrode for the lithium ion secondary battery by the following method 1; and selecting a positive electrode for a lithium ion secondary battery for which the first diffusion completion time is 25 seconds or longer or for which the first diffusion is not completed when the following method 1 is performed. (Method 1) A test piece measuring 2 cm (length) × 2 cm (width) is cut from the positive electrode active material layer. Next, under conditions of a temperature of 25°C, a dew point temperature of -70°C, and atmospheric pressure, 20 μL of butyl butyrate is dropped from a position 1 cm away from the surface of the test piece in a vertical direction toward the center of the test piece. The time from the drop until the butyl butyrate is diffused over the entire surface is measured, and this time is defined as the first diffusion completion time. If the butyl butyrate is not diffused over the entire surface even after 180 seconds have passed since the drop, it is determined that "the first diffusion is not completed."

[0013] That is, the sorting process according to this embodiment is a process for sorting out positive electrodes for lithium ion secondary batteries whose first diffusion completion time is equal to or longer than a first specified time, or whose first diffusion is not completed when method 1 is performed, and the first specified time is 25 seconds.

[0014] Hereinafter, the positive electrode for a lithium ion secondary battery according to this embodiment will be referred to as the "positive electrode."

[0015] The inventors have found that the first diffusion completion time is an index of the density of the positive electrode active material layer. More specifically, they have found that the denser the positive electrode active material layer, the longer the first diffusion completion time. Furthermore, the inventors have found that when the density of the positive electrode active material layer is set to a predetermined value or more, the ionic conductivity of the positive electrode active material layer is improved, thereby reducing the DC resistance of the battery. Therefore, it is believed that when the first diffusion completion time is set to a predetermined value or more, the positive electrode active material layer has a desirable density, thereby reducing the DC resistance of the battery. In the selection process according to this embodiment, by selecting positive electrodes whose first diffusion completion time is equal to or longer than the first specified time or whose first diffusion is not completed when Method 1 is performed, a positive electrode capable of reducing the DC resistance of the battery can be manufactured.

[0016] In this embodiment, the first specified time is determined taking into consideration both the ability to produce a positive electrode that can reduce the DC resistance of the battery and the ability to shorten the time required for the sorting step. Specifically, the first specified time is preferably 25 seconds or more and 180 seconds or less, more preferably 25 seconds or more and 150 seconds or less, even more preferably 25 seconds or more and 120 seconds or less, even more preferably 25 seconds or more and 90 seconds or less, and even more preferably 25 seconds or more and 60 seconds or less.

[0017] In the selection step, it is preferable to select positive electrodes for which the first diffusion is not completed when method 1 is carried out. This makes it possible to manufacture positive electrodes that can further reduce the DC resistance of the battery.

[0018] In the method for producing a positive electrode according to the present embodiment, the measuring step preferably includes a step of measuring the second diffusion completion time for the positive electrode by the following method 2, and the sorting step preferably includes a step of sorting out positive electrodes whose second diffusion completion time is equal to or longer than a second specified time, or whose second diffusion is not completed when the following method 2 is performed. (Method 2) A test piece measuring 2 cm (length) x 2 cm (width) is cut out from the positive electrode active material layer. Next, under the conditions of temperature: 25°C, dew point temperature: -70°C, and pressure: atmospheric pressure, 20 μL of butyl butyrate is dropped from a position 1 cm away from the surface of the test piece in a vertical direction toward the center of the test piece. After dropping, the area where the butyl butyrate has spread is 2 cm. 2 The time until the area where butyl butyrate has diffused is 2cm2 or more is measured, and this time is defined as the second diffusion completion time. 2 If the number is not more than this, it is determined that "the second diffusion is not completed."

[0019] The inventors have found that the second diffusion completion time is an index of the density of the positive electrode active material layer. More specifically, they have found that the denser the positive electrode active material layer, the longer the second diffusion completion time. Furthermore, the inventors have found that when the density of the positive electrode active material layer is set to a predetermined value or more, the ionic conductivity of the positive electrode active material layer is improved, thereby further reducing the DC resistance of the battery. Therefore, it is believed that when the second diffusion completion time is set to a predetermined value or more, the positive electrode active material layer has a desirable density, thereby further reducing the DC resistance of the battery. In the selection process according to this embodiment, by selecting positive electrodes whose second diffusion completion time is equal to or longer than the second specified time or in which the second diffusion is not completed when Method 2 is performed, a positive electrode with a reduced DC resistance of the battery can be manufactured.

[0020] In this embodiment, the second specified time is determined taking into consideration both the ability to produce a positive electrode that can further reduce the DC resistance of the battery and the ability to shorten the time required for the selection step. Specifically, the second specified time is preferably 1 second or more and 120 seconds or less, more preferably 1 second or more and 90 seconds or less, even more preferably 1 second or more and 60 seconds or less, and even more preferably 1 second or more and 30 seconds or less. The second specified time may be, for example, 1 second, 3 seconds, 5 seconds, 10 seconds, or 15 seconds. For example, when the second specified time is 1 second, the selection step according to this embodiment includes a step of selecting positive electrodes whose second diffusion completion time is 1 second or more or for which the second diffusion is not completed when Method 2 is performed.

[0021] In the method for producing a positive electrode according to the present embodiment, the measuring step preferably includes a step of measuring a diffusion stop time of a lithium ion secondary battery positive electrode by the following method 3, and the sorting step preferably includes a step of sorting a lithium ion secondary battery positive electrode having a diffusion stop time of not less than a third specified time. (Method 3) A 2 cm (length) x 2 cm (width) test piece was cut from the positive electrode active material layer. Next, under the conditions of a temperature of 25°C, a dew point temperature of -70°C, and atmospheric pressure, 20 μL of butyl butyrate was dropped from a position 1 cm vertically away from the surface of the test piece toward the center of the test piece. The time from the drop until the diffusion of butyl butyrate on the surface stopped was measured, and this time was defined as the diffusion stop time.

[0022] In Method 3, if no diffusion of butyl butyrate occurs for another 60 seconds after a predetermined time has elapsed since the dropping, that predetermined time is considered the diffusion stop time. Whether or not diffusion has not occurred can be determined by continuously taking images of the surface of the test piece and comparing the image of the surface of the test piece taken a predetermined time after the dropping of butyl butyrate with the image of the surface of the test piece taken another 60 seconds later.

[0023] In this embodiment, the third specified time is determined taking into consideration both the ability to produce a positive electrode that can further reduce the DC resistance of the battery and the ability to shorten the time required for the sorting step. Specifically, the third specified time is preferably 25 seconds or more and 3600 seconds or less, more preferably 25 seconds or more and 1800 seconds or less, even more preferably 25 seconds or more and 1200 seconds or less, even more preferably 25 seconds or more and 600 seconds or less, even more preferably 30 seconds or more and 300 seconds or less, even more preferably 30 seconds or more and 150 seconds or less, even more preferably 35 seconds or more and 100 seconds or less, and even more preferably 40 seconds or more and 80 seconds or less. The third specified time may be, for example, 25 seconds, 30 seconds, 35 seconds, or 40 seconds. For example, when the third specified time is 25 seconds, the sorting step according to this embodiment includes a step of selecting positive electrodes having a diffusion stop time of 25 seconds or more.

[0024] The method for manufacturing a positive electrode according to this embodiment includes a step of forming a positive electrode active material layer from a mixture containing a positive electrode active material, a solid electrolyte, and a binder, thereby obtaining a positive electrode having a positive electrode active material layer. The step of obtaining a positive electrode will be described below.

[0025] The step of obtaining a positive electrode preferably includes a step of applying the mixture to a support to obtain a positive electrode active material layer. More specifically, the step of obtaining a positive electrode preferably includes a step (A) of mixing a positive electrode active material, a solid electrolyte, and a binder, and a step (B) of applying the mixture obtained in the step (A) to a support.

[0026] The step of obtaining a positive electrode preferably includes step (C) of drying the mixture applied to the support in step (B) to obtain a positive electrode precursor, and step (D) of compressing the obtained positive electrode precursor.

[0027] (Process (A)) In step (A), the positive electrode active material, solid electrolyte, and binder are mixed. Mixing can be performed using, for example, a mixer. The stirring speed of the mixer is preferably 500 rpm to 3500 rpm, more preferably 1000 rpm to 3000 rpm, and even more preferably 1500 rpm to 2500 rpm. Mixing is preferably performed while adding the solvent dropwise. This allows for a more uniform mixture to be obtained. The solvent preferably includes one or more solvents selected from the group consisting of acetonitrile, xylene, dimethoxyethane, dimethyl carbonate, triethylamine, and other tertiary amine solvents, heptane, hexane, tetrahydrofuran, toluene, N-methylpyrrolidone, as well as ether solvents, thiol solvents, and butyl butyrate, and all of these solvents are preferably dehydrated. When adding the solvent dropwise, the solids concentration of the mixture obtained in step (A) is preferably 40% to 80% by mass, more preferably 45% to 75% by mass, and even more preferably 50% to 70% by mass.

[0028] (Process (B)) In step (B), the mixture obtained in step (A) is applied to a support. The support preferably includes a positive electrode current collector. This eliminates the need to apply a positive electrode active material layer to the surface of the positive electrode current collector in a subsequent step, thereby simplifying the positive electrode manufacturing process.

[0029] (Process (C)) In step (C), the mixture applied to the support in step (B) is dried to obtain a positive electrode precursor. For example, the mixture is dried for 30 minutes on a hot plate at 80°C under an argon atmosphere, and then dried in a vacuum dryer at 100°C. Drying is preferably performed under an inert gas atmosphere (e.g., argon or helium) or under vacuum. This prevents moisture from being adsorbed onto the solid electrolyte.

[0030] (Process (D)) In step (D), the obtained positive electrode precursor is compressed. Compression can be performed using, for example, a vacuum laminator. Specifically, the obtained positive electrode precursor is vacuum-sealed using the vacuum laminator and held at room temperature (25°C) under a pressure of 300 MPa for 1 minute. Next, the positive electrode precursor is removed from the vacuum laminator and compressed by cold isostatic pressing (CIP).

[0031] The method for producing a positive electrode according to this embodiment may include a step of applying a positive electrode active material layer to the surface of a positive electrode current collector, if necessary.

[0032] Furthermore, the step of obtaining the positive electrode is preferably carried out in a low moisture environment under dew point control in order to prevent moisture from being adsorbed onto the solid electrolyte.

[0033] As described above, in the step of obtaining the positive electrode, for example, by controlling the following production conditions, the first diffusion completion time, the second diffusion completion time, and the diffusion stop time can be set within predetermined ranges. (A) Types and compounding ratios of the positive electrode active material, solid electrolyte, and binder contained in the positive electrode active material layer (B) Use of conductive additives, types and compounding ratios of conductive additives used (C) Preparation conditions of the slurry for forming the positive electrode active material layer (use of solvent, type and blending ratio of the solvent used, stirring speed) (D) Drying conditions of the slurry (temperature, time, atmosphere) (E) Pressing conditions for the positive electrode active material layer

[0034] (Cathode active material layer) Next, each component contained in the positive electrode active material layer formed by the manufacturing method according to this embodiment will be described.

[0035] (Cathode active material) The positive electrode active material according to this embodiment may be, for example, a lithium composite oxide; a transition metal sulfide such as TiS2, FeS, or MoS2; MnO, VO5, or VO 13The positive electrode active material may contain one or more elements selected from the group consisting of transition metal oxides such as TiO2, and olivine-type lithium phosphate. The olivine-type lithium phosphate contains, for example, one or more elements selected from the group consisting of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B, Nb, and Fe, as well as lithium, phosphorus, and oxygen. Some elements in these compounds may be partially substituted with other elements to improve their properties. Furthermore, multiple types of positive electrode active materials may be used in combination.

[0036] The positive electrode active material preferably contains a lithium composite oxide, which can improve the capacity of the lithium ion secondary battery.

[0037] The lithium composite oxide preferably contains one or more composite oxides selected from the group consisting of lithium-nickel composite oxide, lithium-cobalt composite oxide, lithium-manganese composite oxide, lithium-nickel-cobalt composite oxide, lithium-nickel-manganese composite oxide, lithium-nickel-aluminum composite oxide, lithium-nickel-cobalt-aluminum composite oxide, lithium-nickel-cobalt-manganese composite oxide, lithium-nickel-manganese-aluminum composite oxide, and lithium-nickel-cobalt-manganese-aluminum composite oxide, more preferably contains one or more composite oxides selected from the group consisting of lithium-nickel-cobalt-manganese composite oxide, lithium-nickel composite oxide, lithium-cobalt-manganese composite oxide, and lithium-nickel-aluminum composite oxide, and even more preferably contains lithium-nickel-cobalt-manganese composite oxide, which can improve the capacity of the lithium ion secondary battery.

[0038] The lithium-nickel-cobalt-manganese composite oxide preferably contains a composite oxide represented by the following formula (1), which can improve the capacity of the lithium ion secondary battery. Li a Ni b Coc Mn d M e O2(1) In the above formula (1), M is one or more selected from the group consisting of Al, Mg, Na, Co, K, W, Cu, Fe, Ba, V, Cr, Ti, Zr, Zn, In, Ta, Y, In, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo; 0.5 ≦ a ≦ 1.5, 0.6 ≦ b < 1.0, 0 < c ≦ 0.2, 0 < d ≦ 0.2, 0 ≦ e < 1.0.

[0039] In the composite oxide of the above formula (1), 0.5 ≦ a ≦ 1.5, preferably 0.6 ≦ a ≦ 1.4, more preferably 0.7 ≦ a ≦ 1.3, still more preferably 0.8 ≦ a ≦ 1.2, still more preferably 0.9 ≦ a ≦ 1.1, and still more preferably a = 1.0.

[0040] In the composite oxide of the above formula (1), 0.6 ≦ b < 1.0. From the viewpoint of improving the capacity of the lithium ion secondary battery, preferably 0.65 ≦ b < 1.0, more preferably 0.7 ≦ b < 1.0, still more preferably 0.75 ≦ b < 1.0, and still more preferably 0.8 ≦ b ≦ 0.95.

[0041] In the composite oxide of the above formula (1), 0 < c ≦ 0.2, preferably 0.01 ≦ c ≦ 0.15, more preferably 0.02 ≦ c ≦ 0.15, and still more preferably 0.03 ≦ c ≦ 0.1.

[0042] In the composite oxide of the above formula (1), 0 < d ≦ 0.2, preferably 0.01 ≦ d ≦ 0.15, more preferably 0.02 ≦ d ≦ 0.15, and still more preferably 0.02 ≦ d ≦ 0.1.

[0043] In the composite oxide of the above formula (1), 0 ≦ e < 1.0, preferably 0 ≦ e ≦ 0.5, more preferably 0 ≦ e ≦ 0.3, still more preferably 0 ≦ e ≦ 0.2, still more preferably 0 ≦ e ≦ .1, and still more preferably e = 0.

[0044] The lithium-nickel-cobalt-manganese composite oxide is preferably LiNi 0.8 Mn 0.1 Co 0.1 O2 (NMC811) and LiNi 0.6 Mn 0.2 Co 0.2 O2 (NMC622), etc. This can improve the capacity of the lithium ion secondary battery.

[0045] The content of the positive electrode active material in the positive electrode active material layer, when the entire positive electrode active material layer is taken as 100% by mass, is preferably 50% by mass to 90% by mass, more preferably 55% by mass to 90% by mass, even more preferably 60% by mass to 90% by mass, even more preferably 65% ​​by mass to 90% by mass, even more preferably 70% by mass to 88% by mass, and even more preferably 75% by mass to 85% by mass. By setting the content of the positive electrode active material in the positive electrode active material layer within the above range, the capacity of the lithium ion secondary battery can be improved.

[0046] The average particle diameter d of the positive electrode active material according to this embodiment in the volume-based particle size distribution measured by the laser diffraction scattering particle size distribution measurement method 50 is preferably 0.1 μm or more and 30 μm or less, more preferably 0.5 μm or more and 20 μm or less, even more preferably 1 μm or more and 15 μm or less, and even more preferably 5 μm or more and 12 μm or less. 50 By setting the range above, the DC resistance of the battery can be further reduced.

[0047] (solid electrolyte) The solid electrolyte according to this embodiment includes, for example, one or more selected from the group consisting of a sulfide-based solid electrolyte, an oxide-based solid electrolyte, a polymer-based solid electrolyte, and a halide-based solid electrolyte. The solid electrolyte preferably includes one or more selected from the group consisting of a sulfide-based solid electrolyte, an oxide-based solid electrolyte, and a polymer-based solid electrolyte, and more preferably includes a sulfide-based solid electrolyte. This allows the DC resistance of the battery to be further reduced.

[0048] Examples of sulfide-based solid electrolytes include Li2S-P2S5 materials, Li2S-SiS2 materials, Li2S-GeS2 materials, Li2S-Al2S3 materials, Li2S-SiS2-Li3PO4 materials, Li2S-P2S5-GeS2 materials, Li2S-Li2O-P2S5-SiS2 materials, Li2S-GeS2-P2S5-SiS2 materials, Li2S-SnS2-P2S5-SiS2 materials, Li2S-P2S5-Li3N materials, and Li2S 2+X -P4S3 material, Li2S-P2S5-P4S3 material, LiPO4-Li2S-SiS material, Li3PS4, Li3PO4-Li2S-Si2S material, Li3PO4-Li2S-SiS2 material, LiI-L i2S-B2S3 material, LiI-Li2S-SiS2 material, LiI-Li2S-P2S5 material, LiI-Li2S-P2O5 material, LiI-Li3PO4-P2S5Li2S-P2S5-LiCl material, Li 7-x PS 6-x Cl x (where 0≦x≦2), Li 7-x PS 6-x Br x (where 0≦x≦2), Li 7-x PS 6-x I x (where 0≦x≦2), Li 10 GeP2S 12 , and Li 3.25 Ge 0.25 P 0.75 S4, more preferably Li 7-x PS 6-x Cl x (where 0≦x≦2), Li 7-x PS 6-x Br x (where 0≦x≦2) and Li 7-x PS 6-x I x(where 0≦x≦2), more preferably it contains one or two or more selected from the group consisting of Li6PS5Cl, Li6PS5Br and Li6PS5I, even more preferably it contains one or two or more selected from the group consisting of Li6PS5Cl and Li6PS5Br, and even more preferably it contains Li6PS5Cl.

[0049] Examples of oxide-based solid electrolytes include NASICON-type solid electrolyte materials such as LiTi2(PO4)3, LiZr2(PO4)3, and LiGe2(PO4)3; 0.5+x Li 0.5-3x ) Perovskite-type solid electrolyte materials such as TiO3; one or more selected from the group consisting of Li2O-P2O5 materials, Li2O-P2O5-Li3N materials, etc.

[0050] The polymer-based solid electrolyte includes one or more selected from the group consisting of polyether-based electrolyte materials such as polyethylene oxide, polypropylene oxide, ethylene oxide-propylene copolymer, and dimethylsiloxane-ethylene oxide copolymer; gel polymer electrolyte materials such as polyacrylonitrile, polyvinylidene fluoride, and vinylidene fluoride-hexafluoropropylene polymer; and polymer solid electrolyte materials using hyperbranched polymers.

[0051] Halide-based solid electrolytes include, for example, Li 6-3x Y xOne or more selected from the group consisting of X6 (where 0 < x < 2), Li3MX6 (where M is one or more selected from the group consisting of Al, Ga, and In), LiMX4 (where M is one or more selected from the group consisting of Al, Ga, and In), Li2MgX4, and Li2FeX4. Here, the element X is one or more selected from the group consisting of F, Cl, Br, and I. More specifically, the halide-based solid electrolyte contains, for example, one or more selected from the group consisting of Li3YF6, Li3YCl6, Li3YBr6, and Li3YI6. These compounds may have some elements partially substituted with other elements to improve their properties.

[0052] When the total of the positive electrode active material layer is 100% by mass, the content of the solid electrolyte in the positive electrode active material layer is preferably 5% by mass or more and 40% by mass or less, more preferably 10% by mass or more and 35% by mass or less, still more preferably 12% by mass or more and 30% by mass or less, and still more preferably 15% by mass or more and 25% by mass or less. By setting the content of the solid electrolyte in the positive electrode active material layer within the above range, the capacity of the lithium-ion secondary battery can be improved.

[0053] With respect to 100 parts by mass of the positive electrode active material contained in the positive electrode active material layer, the solid electrolyte contained in the positive electrode active material layer is preferably 5 parts by mass or more and 50 parts by mass or less, more preferably 10 parts by mass or more and 45 parts by mass or less, still more preferably 15 parts by mass or more and 40 parts by mass or less, and still more preferably 20 parts by mass or more and 35 parts by mass or less. Thereby, the capacity of the lithium-ion secondary battery can be improved.

[0054] The average particle diameter d in the volume-based particle size distribution by the laser diffraction and scattering method of the solid electrolyte according to the present embodiment 50 is preferably 0.01 μm or more and 30 μm or less, more preferably 0.1 μm or more and 20 μm or less, still more preferably 0.2 μm or more and 10 μm or less, and still more preferably 0.5 μm or more and 5 μm or less. By setting the average particle diameter d of the solid electrolyte within the above range, the DC resistance of the battery can be further reduced. 50

[0055] (binder) The binder according to the present embodiment preferably contains one or more binders selected from the group consisting of fluorine-based binders and rubber-based binders.

[0056] The fluorine-based binder preferably contains a constituent unit derived from vinylidene fluoride (VdF). More preferably, the fluorine-based binder contains a constituent unit derived from vinylidene fluoride (VdF) and one or more constituent units selected from the group consisting of a constituent unit derived from hexafluoropropylene (HFP), a constituent unit derived from trifluoropropylene (TFP), a constituent unit derived from tetrafluoroethylene (TFE), a constituent unit derived from 2,3,3,3-tetrafluoropropylene, a constituent unit derived from 1,3,3,3-tetrafluoropropylene, and a constituent unit derived from perfluoroalkyl vinyl ether (PAVE). This can further reduce the DC resistance of the battery. The fluorine-based binder more preferably contains one or more selected from the group consisting of a copolymer of vinylidene fluoride (VdF) and hexafluoropropylene (HFP) and a copolymer of vinylidene fluoride (VdF) and trifluoropropylene (TFP), and more preferably contains a copolymer of vinylidene fluoride (VdF) and hexafluoropropylene (HFP). This can further reduce the DC resistance of the battery.

[0057] The mass average molecular weight (Mw) of the fluorine-based binder is preferably 10,000 or more and 10,000,000 or less, more preferably 30,000 or more and 5,000,000 or less, even more preferably 50,000 or more and 1,000,000 or less, even more preferably 80,000 or more and 800,000 or less, and even more preferably 100,000 or more and 500,000 or less. By setting the mass average molecular weight (Mw) of the fluorine-based binder within the above range, the DC resistance of the battery can be further reduced.

[0058] When the fluorine-based binder contains a constituent unit derived from vinylidene fluoride (VdF), the content of the constituent unit derived from vinylidene fluoride (VdF) in the fluorine-based binder is preferably 20 mol% to 100 mol%, more preferably 20 mol% to 99 mol%, even more preferably 30 mol% to 95 mol%, even more preferably 40 mol% to 90 mol%, even more preferably 60 mol% to 85 mol%, and even more preferably 70 mol% to 85 mol%. By setting the content of the constituent unit derived from vinylidene fluoride (VdF) in the fluorine-based binder within the above range, the DC resistance of the battery can be further reduced.

[0059] The rubber binder preferably contains one or more selected from the group consisting of styrene-butadiene rubber, acrylate-butadiene rubber, butadiene rubber, and butylene rubber, and more preferably contains styrene-butadiene rubber, which can further reduce the DC resistance of the battery.

[0060] The content of the binder in the positive electrode active material layer, when the entire positive electrode active material layer is taken as 100% by mass, is preferably more than 0% by mass and 10% by mass or less, more preferably 0.5% by mass or more and 8% by mass or less, even more preferably 0.8% by mass or more and 7% by mass or less, even more preferably 1.0% by mass or more and 6% by mass or less, and even more preferably 1.5% by mass or more and 5% by mass or less. By setting the content of the binder in the positive electrode active material layer within the above range, the DC resistance of the battery can be further reduced.

[0061] The amount of binder contained in the positive electrode active material layer is preferably more than 0 part by mass and not more than 10 parts by mass, more preferably 0.5 parts by mass to 8 parts by mass, even more preferably 0.8 parts by mass to 7 parts by mass, even more preferably 1.0 parts by mass to 6 parts by mass, and even more preferably 1.5 parts by mass to 5 parts by mass, relative to 100 parts by mass of the total of the positive electrode active material and solid electrolyte contained in the positive electrode active material layer. This allows the DC resistance of the battery to be further reduced.

[0062] (Conductive additive) The mixture according to the present embodiment includes a positive electrode active material, a solid electrolyte, and a binder. Preferably, the mixture according to the present embodiment further includes a conductive additive. This improves the conductivity of the positive electrode, thereby further reducing the DC resistance of the battery.

[0063] The conductive additive preferably contains one or more selected from the group consisting of carbon black, activated carbon, graphite, mesoporous carbon, fullerenes, carbon nanotubes, carbon nanohorns, carbon nanofibers, and carbon brushes, and more preferably contains carbon black, which can further reduce the DC resistance of the battery.

[0064] The content of the conductive additive in the positive electrode active material layer is preferably more than 0% by mass and 10% by mass or less, more preferably 0.1% by mass or more and 8% by mass or less, even more preferably 0.2% by mass or more and 5% by mass or less, and even more preferably 0.5% by mass or more and 3% by mass or less, when the entire positive electrode active material layer is taken as 100% by mass. By setting the content of the conductive additive in the positive electrode active material layer within the above range, the DC resistance of the battery can be further reduced.

[0065] The amount of the conductive additive contained in the positive electrode active material layer is preferably more than 0 part by mass and not more than 10 parts by mass, more preferably 0.1 part by mass to 8 parts by mass, even more preferably 0.2 part by mass to 5 parts by mass, and still more preferably 0.5 part by mass to 3 parts by mass, relative to 100 parts by mass of the total of the positive electrode active material and solid electrolyte contained in the positive electrode active material layer. This allows the DC resistance of the battery to be further reduced.

[0066] (Other configurations of positive electrode) The thickness of the positive electrode active material layer according to this embodiment may be, for example, 1 μm or more and 500 μm or less, 5 μm or more and 300 μm or less, 10 μm or more and 200 μm or less, or 50 μm or more and 150 μm or less.

[0067] The porosity of the positive electrode active material layer according to this embodiment is preferably 12.0% or less, more preferably 10.0% or less, even more preferably 8.0% or less, and even more preferably 6.0% or less. By setting the porosity of the positive electrode active material layer within this range, the DC resistance of the battery can be further reduced. The lower limit of the porosity of the positive electrode active material layer is not particularly limited, but may be, for example, 0% or more, 0.5% or more, 1.0% or more, or 2.0% or more.

[0068] The porosity of the positive electrode active material layer according to this embodiment is calculated by the following method. First, a total of five test pieces measuring 2 cm (length) x 2 cm (width) are cut out from the positive electrode active material layer. Next, the surface of each test piece (the surface perpendicular to the film thickness direction of the test piece) is observed using a scanning electron microscope (SEM). Next, the shading of the SEM image in the observation area is binarized to separate the solid and void areas. Next, smoothing is performed over a width of 25 μm before and after the target point (the center point of the observation area). Smoothing is performed by fitting a cubic approximation equation using the least squares method and using the value at the target point of this cubic approximation equation. The average value of the five test pieces is used as the porosity.

[0069] The positive electrode according to this embodiment may further include a positive electrode current collector. The positive electrode current collector may include, for example, one or more selected from the group consisting of aluminum, stainless steel, nickel, titanium, and alloys thereof. The positive electrode current collector may be in the form of, for example, a foil, a flat plate, or a mesh. The thickness of the positive electrode current collector is not particularly limited, but may be, for example, 1 μm or more and 50 μm or less.

[0070] (lithium-ion secondary battery) The positive electrode manufactured by the manufacturing method according to this embodiment is suitable for use in a lithium ion secondary battery. A lithium ion secondary battery 100 according to this embodiment will now be described with reference to FIG.

[0071] Fig. 1 is a cross-sectional view schematically showing an example of a lithium-ion secondary battery according to this embodiment. As shown in Fig. 1, the lithium-ion secondary battery 100 according to this embodiment includes a positive electrode 10, a solid electrolyte layer 30, and a negative electrode 20, in this order. The lithium-ion secondary battery 100 preferably further includes an outer casing 40. The outer casing 40 houses the positive electrode active material layer 11, the negative electrode active material layer 21, and the solid electrolyte layer 30.

[0072] The positive electrode 10 preferably includes a positive electrode active material layer 11, a positive electrode current collector 13, and a positive electrode terminal 15. The positive electrode active material layer 11 preferably includes a positive electrode active material, a solid electrolyte, and a binder. As shown in FIG. 1 , one end of the positive electrode terminal 15 is disposed in contact with the positive electrode current collector 13, and the other end of the positive electrode terminal 15 is disposed so as to be drawn out of the exterior body 40.

[0073] The negative electrode 20 preferably includes a negative electrode active material layer 21, a negative electrode current collector 23, and a negative electrode terminal 25. The negative electrode active material layer 21 preferably includes a negative electrode active material, a solid electrolyte, and a binder. As shown in FIG. 1 , one end of the negative electrode terminal 25 is disposed in contact with the negative electrode current collector 23, and the other end of the negative electrode terminal 25 is disposed so as to be drawn out of the exterior body 40.

[0074] The solid electrolyte layer 30 preferably includes a solid electrolyte and a binder.

[0075] (Method of manufacturing lithium-ion secondary batteries) The method for manufacturing a lithium ion secondary battery according to this embodiment includes a step of manufacturing a lithium ion secondary battery using a positive electrode for a lithium ion secondary battery manufactured by the method for manufacturing a positive electrode for a lithium ion secondary battery according to this embodiment, a solid electrolyte layer containing a solid electrolyte, and a negative electrode. An example of the method for manufacturing a lithium ion secondary battery according to this embodiment will be specifically described below.

[0076] First, the preparation of the negative electrode will be described. The negative electrode can be prepared by a known method. Regardless of the method used to prepare the negative electrode, it is preferable to prepare it in a low-moisture environment under dew point control in order to suppress adsorption of moisture into the solid electrolyte.

[0077] When using a negative electrode in which a negative electrode active material layer is formed on a negative electrode current collector, a slurry in which a negative electrode active material, a solid electrolyte, and a binder are dispersed in a dehydrated organic solvent is applied to part or all of the surface of a negative electrode current collector such as copper foil, followed by drying to obtain a negative electrode precursor sheet. The obtained negative electrode precursor sheet can be compressed using a press molding method such as a roll press, a uniaxial press, a rubber press, or an isostatic press (CIP, WIP) to obtain a negative electrode sheet. The organic solvent preferably includes one or more solvents selected from the group consisting of tertiary amine solvents such as acetonitrile, xylene, dimethoxyethane, dimethyl carbonate, and triethylamine, as well as heptane, hexane, tetrahydrofuran, toluene, and N-methylpyrrolidone, as well as ether solvents, thiol solvents, and butyl butyrate, all of which are preferably dehydrated.

[0078] The negative electrode can be obtained by placing a metallic lithium layer (negative electrode active material layer) such as lithium foil on part or all of a negative electrode current collector such as stainless steel foil, and then adhering them together by rolling or other processing.

[0079] Next, a solid electrolyte layer is formed on the surface of the negative electrode, preferably in a low-moisture environment under dew point control to prevent water from being adsorbed onto the solid electrolyte.

[0080] The solid electrolyte layer can be formed on the surface of the negative electrode by, for example, applying a slurry of a solid electrolyte dispersed in an organic solvent to the surface of a negative electrode active material layer formed on a negative electrode current collector and drying the slurry. The organic solvent may include, for example, one or more selected from the group consisting of acetonitrile, xylene, dimethoxyethane, dimethyl carbonate, triethylamine, and other tertiary amine solvents, heptane, hexane, tetrahydrofuran, toluene, N-methylpyrrolidone, as well as ether solvents, thiol solvents, and butyl butyrate, and it is preferable that all of the organic solvents have been dehydrated.

[0081] Next, the stacked negative electrode and solid electrolyte layer are compressed using a press molding method such as a vacuum laminator, roll press, uniaxial press, rubber press, or isostatic pressing (CIP, WIP) to obtain a negative electrode-solid electrolyte layer laminate. When the solid electrolyte layer is stacked together with a substrate layer such as a polyester sheet and pressurized, the substrate layer is peeled off from the solid electrolyte layer. In this case, it is preferable to use a substrate layer whose surface is coated with a release agent such as silicone to facilitate peeling of the substrate layer from the solid electrolyte layer.

[0082] Next, the positive electrode according to this embodiment is laminated on the negative electrode-solid electrolyte layer laminate to obtain an electrode laminate precursor. When a sulfide-based solid electrolyte is used as the solid electrolyte, the ionic conductivity of the solid electrolyte may decrease due to exposure to moisture. Therefore, after obtaining the electrode laminate precursor, it is preferable to compress the electrode laminate precursor to obtain an electrode laminate.

[0083] The electrode laminate precursor is preferably compressed using one or more methods selected from the group consisting of a vacuum laminator, a roll press, a uniaxial press, a rubber press, and an isostatic press (CIP method, WIP method), and more preferably compressed using a combination of a vacuum laminator and an isostatic press (CIP method).

[0084] The resulting electrode laminate is preferably quickly sealed in an exterior housing. One end of a rectangular metal plate serving as a negative electrode terminal is attached to the negative electrode current collector, and one end of a rectangular metal terminal serving as a positive electrode terminal is attached to the positive electrode current collector, and then the electrode laminate is housed in an aluminum exterior housing. A resin layer such as polyolefin is preferably formed on at least the surface of the inner surface of the exterior housing facing the electrode laminate. The resin layer is heated to melt the resin and solidify it again, and the electrode laminate is sealed in the aluminum exterior housing. At this time, the other end of the positive electrode terminal and the other end of the negative electrode terminal are positioned so as to extend outside the exterior housing. A layer of resin of the same type or a different type from the resin used in the resin layer on the interior surface of the exterior housing can be provided in the areas where the positive electrode terminal and the negative electrode terminal contact the resin layer on the interior surface of the exterior housing.

[0085] Although the embodiments of the present invention have been described above, these are merely examples of the present invention, and various other configurations may be adopted. Furthermore, the present invention is not limited to the above-described embodiments, and modifications and improvements within the scope of achieving the object of the present invention are included in the present invention. [Example]

[0086] The present embodiment will be described in detail below with reference to examples and comparative examples, but the present embodiment is not limited to the descriptions of these examples.

[0087] (1) Raw materials The materials used to prepare the positive electrode active material layer are shown below. (Cathode active material) ·Cathode active material A: LiNi 0.8 Mn 0.1 Co 0.1 O2(NMC811, average particle size d 50 :7μm) ·Cathode active material B: LiNi 0.6 Mn 0.2 Co 0.2 O2(NMC622, average particle size d 50 :10μm) ·Cathode active material C: LiNi 0.8 Mn 0.1 Co 0.1O2(NMC811, average particle size d 50 :6μm)

[0088] (solid electrolyte) ·Li6PS5Cl (average particle size d 50 :0.7μm) ·Li6PS5Br (average particle size d 50 :0.8μm) ·Li3YCl6-Li3YBr6 (average particle size d 50 :1μm)

[0089] (binder) SBR: Styrene-butadiene rubber PVDF-HFP: Vinylidene fluoride-hexafluoropropylene copolymer

[0090] The materials used to prepare the solid electrolyte layer are listed below. ·Solid electrolyte: Li6PS5Cl (average particle size d 50 : 6.5 μm), content in solid electrolyte layer: 95 mass% Binder: SBR (styrene-butadiene rubber), content in solid electrolyte layer: 5% by mass

[0091] The average particle diameter d of the positive electrode active material and the solid electrolyte 50 was measured using a laser diffraction / scattering particle size distribution analyzer (MT3000, manufactured by Microtrac), and the value at which the cumulative volume in the volume-based particle size distribution measured by the laser diffraction / scattering particle size distribution measurement method was 50% was used.

[0092] (2) Preparation of the positive electrode For Examples 1 to 5 and Comparative Example 1, a positive electrode active material layer was formed using the positive electrode active materials, solid electrolytes, and binders shown in Table 1 to fabricate positive electrodes. First, the positive electrode active material, solid electrolyte, and conductive additive (carbon black) were mixed, and then a binder was added. The mass ratio of the components was positive electrode active material: solid electrolyte: conductive additive: binder = 76.0:19.5:1.5:3.0. After adding the binder, a solvent (butyl butyrate, Tokyo Chemical Industry Co., Ltd., purity 99.9%) was added dropwise while mixing at 2000 rpm using a mixer (Thinky, product name: AR-100) to obtain a mixture with a solid content of 66% by mass. The mixture was then applied to a positive electrode current collector (aluminum foil, 13 μm thick), dried on a hot plate at 80° C. for 30 minutes under an argon atmosphere, and then dried in a vacuum dryer at 100° C. to obtain a positive electrode precursor. The resulting positive electrode precursor was then vacuum-sealed using a vacuum laminator and held at room temperature (25°C) under a pressure of 300 MPa for 1 minute, and then compressed by cold isostatic pressing (CIP) to form a positive electrode active material layer having a thickness of 100 µm after pressing, thereby obtaining a positive electrode.

[0093] For Examples 1 to 5 and Comparative Example 1, the porosity of the positive electrode active material layer was calculated by the following method. First, a total of five test pieces measuring 2 cm (length) x 2 cm (width) were cut out from the positive electrode active material layer. The surface of each test piece (the surface perpendicular to the film thickness direction of the test piece) was then observed using a scanning electron microscope (SEM). The SEM image in the observation area was then binarized to separate the solid and void areas. Smoothing was then performed over a 25 μm width around the target point (the center point of the observation area). Smoothing was performed by fitting a cubic approximation equation using the least squares method and using the value at the target point of this cubic approximation equation. The average value of the five test pieces was used as the porosity. The results are shown in Table 1.

[0094] (3) Measurement of the first diffusion completion time, the second diffusion completion time, and the diffusion stop time A total of five test pieces measuring 2 cm (length) × 2 cm (width) were cut out from the positive electrode active material layer. Next, 20 μL of butyl butyrate (Tokyo Chemical Industry Co., Ltd., purity 99.9%) was dropped from a position 1 cm vertically away from the surface of the test piece toward the center of the test piece under the conditions of a temperature of 25°C, a dew point temperature of −70°C, and atmospheric pressure. The time from the time of dropping until the butyl butyrate diffused over the entire surface was measured, and this time was defined as the time for the first diffusion to be completed. If the butyl butyrate had not diffused over the entire surface within 180 seconds after dropping, it was determined that the "first diffusion was not completed." After dripping, the area where butyl butyrate spreads is 2cm 2 The time until the area of ​​the butyl butyrate diffused was 2 cm or more (i.e., more than half of the surface area of ​​the test piece) was measured, and this time was defined as the second diffusion completion time. 2 If this is not the case, it is determined that "the second diffusion is not complete." The time from the time of dropping to the time when the diffusion of butyl butyrate on the surface stopped was measured, and this time was defined as the diffusion stop time. Note that if the butyl butyrate did not diffuse for another 60 seconds after the predetermined time had elapsed since dropping, this predetermined time was defined as the diffusion stop time. Here, the first diffusion completion time, second diffusion completion time, and diffusion stop time were averaged over five test pieces. In the above measurements, the surface of the test piece was photographed every second for one minute after the butyl butyrate was added, and then every 10 seconds after that. The photographed images were analyzed using image analysis software (ImageJ) to measure the first diffusion completion time, second diffusion completion time, and diffusion stop time. In measuring the diffusion stop time, whether or not diffusion movement had occurred was determined by comparing an image of the surface of the test piece taken a predetermined time after the butyl butyrate was dropped with an image of the surface of the test piece taken another 60 seconds later. The results are shown in Table 1.

[0095] In Example 1, the diffusion of butyl butyrate stopped 60 seconds after the drop. At this time, the area in which butyl butyrate was diffused was 2 cm. 2It was less than. In Example 2, the area where butyl butyrate spread was 2 cm 3 seconds after dropping. 2 The diffusion of butyl butyrate stopped 30 seconds after the drop. At this time, the area where butyl butyrate had diffused was 2 cm 2 More than 4cm 2 It was less than. In Example 4, the area where butyl butyrate spread 1 second after dropping was 2 cm 2 The diffusion of butyl butyrate stopped 48 seconds after the drop. At this time, the area where butyl butyrate had diffused was 2 cm 2 More than 4cm 2 It was less than. In Example 5, the area over which butyl butyrate had spread was 2 cm 15 seconds after the drop. 2 The diffusion of butyl butyrate stopped 30 seconds after the drop. At this time, the area where butyl butyrate had diffused was 2 cm 2 More than 4cm 2 It was less than. In Example 3 and Comparative Example 1, the butyl butyrate diffused over the entire surface of the test piece before the diffusion of the butyl butyrate stopped, so the time when the diffusion stopped could not be measured.

[0096] (4) Fabrication of lithium-ion secondary batteries A film (manufactured by Honjo Metals Co., Ltd.) in which a negative electrode active material layer (metallic lithium foil, thickness 20 μm) was formed on a negative electrode current collector (stainless steel foil, thickness 10 μm) was prepared as a negative electrode. Next, a slurry prepared by dispersing a solid electrolyte and a binder in xylene was applied to the surface of a polyester film mainly composed of polyester, and then dried to form a solid electrolyte layer on the polyester film. Next, the solid electrolyte layer was laminated on the negative electrode together with the polyester film so that the obtained solid electrolyte layer was in contact with the surface of the negative electrode active material layer of the negative electrode, thereby obtaining a negative electrode-solid electrolyte layer laminate. The resulting anode-solid electrolyte layer laminate was then vacuum-sealed using a vacuum laminator and held at room temperature (25°C) under a pressure of 300 MPa for 1 minute. The anode-solid electrolyte layer laminate was then removed from the vacuum laminator and compressed using an isostatic pressing method (CIP method) to obtain an anode-solid electrolyte layer laminate with a porosity of 7%. The dimensions of the anode-solid electrolyte layer laminate were 26 mm x 26 mm. The polyester film was peeled off from the solid electrolyte layer to obtain laminate (I). The positive electrode obtained in (2) above was cut into a size of 20 mm x 20 mm, and the positive electrode and the laminate (I) were laminated together so that the positive electrode active material layer was in contact with the solid electrolyte layer of the laminate (I), thereby obtaining an electrode laminate precursor. The number of layers of the positive electrode and the laminate (I) was one each. The obtained electrode laminate precursor was then vacuum-sealed using a vacuum laminator and held at room temperature (25°C) under a pressure of 300 MPa for 1 minute. The electrode laminate precursor was then removed from the vacuum laminator and compressed by cold isostatic pressing (CIP) to obtain an electrode laminate. Next, one end of the positive electrode terminal was attached to the positive electrode current collector, and one end of the negative electrode terminal was attached to the negative electrode current collector, and then the electrode stack was housed in an aluminum exterior body (manufactured by Dai Nippon Printing Co., Ltd.) to obtain a lithium ion secondary battery. At this time, the other end of the positive electrode terminal and the other end of the negative electrode terminal were arranged so as to be drawn out of the exterior body.

[0097] (5) Evaluation of battery characteristics The lithium ion secondary battery was charged by a constant current-constant voltage method under the following conditions. Rate: 0.1C Cut: 0.01C Temperature: 45℃ Confining pressure: 3MPa Next, a tester was applied to the positive and negative terminals of the charged lithium-ion secondary battery, and the DC resistance value [Ω] between the positive and negative electrodes was measured at room temperature (25°C). The results are shown in Table 1. In Table 1, the DC resistance values ​​of each example and comparative example are shown as relative values ​​to the DC resistance value of Example 1. Note that the DC resistance value of Comparative Example 1 exceeded the measurement limit.

[0098] [Table 1] [Explanation of symbols]

[0099] 10 positive electrode 11 Cathode active material layer 13 Positive electrode current collector 15 Positive terminal 20 negative electrode 21 Negative electrode active material layer 23 Negative electrode current collector 25 Negative terminal 30 Solid electrolyte layer 40 Exterior body 100 Lithium-ion secondary battery

Claims

1. forming a positive electrode active material layer from a mixture containing a positive electrode active material, a solid electrolyte, and a binder, thereby obtaining a positive electrode for a lithium ion secondary battery having the positive electrode active material layer; a measuring step of measuring a first diffusion completion time for the positive electrode for a lithium ion secondary battery by the following method 1; a screening step of screening the positive electrode for a lithium ion secondary battery, the positive electrode for which the first diffusion completion time is 25 seconds or more, or the positive electrode for which the first diffusion is not completed when the following method 1 is performed; A method for producing a positive electrode for a lithium ion secondary battery, comprising: (Method 1) A test piece measuring 2 cm (length) x 2 cm (width) is cut out from the positive electrode active material layer. Next, under conditions of a temperature of 25°C, a dew point temperature of -70°C, and atmospheric pressure, 20 μL of butyl butyrate is dropped from a position 1 cm away from the surface of the test piece in a vertical direction toward the center of the test piece. The time from the drop until the butyl butyrate diffuses over the entire surface is measured, and this time is designated as the first diffusion completion time. If the butyl butyrate does not diffuse over the entire surface even after 180 seconds have elapsed since the drop, it is determined that "the first diffusion is not completed."

2. 2. The method for producing a positive electrode for a lithium ion secondary battery according to claim 1, wherein the selection step selects the positive electrode for a lithium ion secondary battery in which the first diffusion is not completed when the method 1 is performed.

3. The measuring step includes measuring a second diffusion completion time for the positive electrode for a lithium ion secondary battery by the following method 2, the screening step includes a step of screening the lithium ion secondary battery positive electrode for which the second diffusion completion time is 1 second or more, or for which the second diffusion is not completed when the following method 2 is performed: The method for producing the positive electrode for a lithium ion secondary battery according to claim 1 or 2. (Method 2) A test piece measuring 2 cm (length) × 2 cm (width) is cut out from the positive electrode active material layer, and 20 μL of butyl butyrate is dropped from a position 1 cm away from the surface of the test piece in a vertical direction toward the center of the test piece under the conditions of a temperature of 25° C., a dew point temperature of −70° C., and atmospheric pressure. After dripping, the area where the butyl butyrate spread is 2 cm ２ The time until the area where the butyl butyrate has diffused is 2 cm or less even after 120 seconds from the time of dropping is measured, and this time is defined as the second diffusion completion time. ２ If it is not, it is determined that "the second diffusion is not completed."

4. the measuring step includes a step of measuring a diffusion stoppage time for the positive electrode for a lithium ion secondary battery by the following method 3, the selection step includes a step of selecting the positive electrode for a lithium ion secondary battery having the diffusion stop time of 25 seconds or more. The method for producing the positive electrode for a lithium ion secondary battery according to any one of claims 1 to 3. (Method 3) A test piece measuring 2 cm (length) × 2 cm (width) was cut out from the positive electrode active material layer. Next, under conditions of a temperature of 25°C, a dew point temperature of −70°C, and atmospheric pressure, 20 μL of butyl butyrate was dropped from a position 1 cm away from the surface of the test piece in a vertical direction toward the center of the test piece. The time from the drop until the diffusion of the butyl butyrate on the surface stopped was measured, and this time was defined as the diffusion stop time.

5. The method for producing a positive electrode for a lithium ion secondary battery according to any one of claims 1 to 4, wherein the positive electrode active material contains a lithium composite oxide.

6. 6. The method for producing a positive electrode for a lithium ion secondary battery according to claim 5, wherein the lithium composite oxide comprises one or more composite oxides selected from the group consisting of lithium-nickel-cobalt-manganese-based composite oxides, lithium-nickel-based composite oxides, lithium-cobalt-based composite oxides, and lithium-nickel-aluminum-based composite oxides.

7. 6. The method for producing a positive electrode for a lithium ion secondary battery according to claim 5, wherein the lithium composite oxide contains a lithium-nickel-cobalt-manganese composite oxide.

8. The method for producing a positive electrode for a lithium ion secondary battery according to any one of claims 1 to 7, wherein the positive electrode active material layer has a porosity of 12.0% or less.

9. 9. The method for producing a positive electrode for a lithium ion secondary battery according to claim 1, wherein a content of the positive electrode active material in the positive electrode active material layer is 50% by mass or more and 90% by mass or less, when the entire positive electrode active material layer is taken as 100% by mass.

10. The method for producing a positive electrode for a lithium ion secondary battery according to any one of claims 1 to 9, wherein the solid electrolyte includes a sulfide-based solid electrolyte.

11. 11. The method for producing a positive electrode for a lithium ion secondary battery according to claim 1, wherein a content of the solid electrolyte in the positive electrode active material layer is 5% by mass or more and 40% by mass or less, when the entire positive electrode active material layer is taken as 100% by mass.

12. The method for producing a positive electrode for a lithium ion secondary battery according to any one of claims 1 to 11, wherein the binder comprises one or more binders selected from the group consisting of fluorine-based binders and rubber-based binders.

13. 13. The method for producing a positive electrode for a lithium ion secondary battery according to any one of claims 1 to 12, wherein a content of the binder in the positive electrode active material layer is more than 0 mass% and 10 mass% or less, when the entire positive electrode active material layer is taken as 100 mass%.

14. The method for producing a positive electrode for a lithium ion secondary battery according to any one of claims 1 to 13, wherein the mixture further contains a conductive additive.

15. 15. The method for producing a positive electrode for a lithium ion secondary battery according to claim 14, wherein the conductive additive comprises one or more selected from the group consisting of carbon black, activated carbon, graphite, mesoporous carbon, fullerenes, carbon nanotubes, carbon nanohorns, carbon nanofibers, and carbon brushes.

16. 16. The method for producing a positive electrode for a lithium ion secondary battery according to claim 14 or 15, wherein a content of the conductive additive in the positive electrode active material layer is more than 0 mass % and 10 mass % or less, when the entire positive electrode active material layer is taken as 100 mass %.

17. The method for producing a positive electrode for a lithium ion secondary battery according to any one of claims 1 to 16, wherein the step of obtaining the positive electrode for a lithium ion secondary battery includes a step of applying the mixture to a support to obtain the positive electrode active material layer.

18. The method for producing a positive electrode for a lithium ion secondary battery according to any one of claims 1 to 17, comprising a step of applying the positive electrode active material layer to a surface of a positive electrode current collector.

19. The method for producing a positive electrode for a lithium ion secondary battery according to claim 17 , wherein the support comprises a positive electrode current collector.

20. A method for producing a lithium ion secondary battery, comprising a step of producing a lithium ion secondary battery using a positive electrode for a lithium ion secondary battery produced by the method for producing a positive electrode for a lithium ion secondary battery according to any one of claims 1 to 19, a solid electrolyte layer containing a solid electrolyte, and a negative electrode.

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