Positive electrode for lithium ion secondary battery, lithium ion secondary battery, and lithium ion secondary battery module

Abstract

To provide a positive electrode for a lithium ion secondary battery that can reduce the DC resistance of the battery.SOLUTION: A positive electrode for a lithium ion secondary battery includes a positive electrode active material layer containing a positive electrode active material, a solid electrolyte, and a binder, in which a first diffusion completion time measured by following Method 1 is 25 seconds or longer, or a first diffusion is not completed when following Method 1 is performed. (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 the center of the test piece under conditions of a temperature of 25°C, a dew point temperature of -70°C, and atmospheric pressure. The time from the drop 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 does not diffuse over the entire surface even after 180 seconds have passed since 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 positive electrode for a lithium ion secondary battery, a lithium ion secondary battery, and a lithium ion secondary battery module. [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 positive electrode for a lithium ion secondary battery that 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 positive electrode for a lithium ion secondary battery, a lithium ion secondary battery, and a lithium ion secondary battery module, which are shown below.

[0008] [1] A positive electrode for a lithium ion secondary battery, comprising a positive electrode active material layer including a positive electrode active material, a solid electrolyte, and a binder, A positive electrode for a lithium ion secondary battery, in which the first diffusion completion time measured by the following method 1 is 25 seconds or more, or 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 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 positive electrode for a lithium ion secondary battery according to [1], wherein the first diffusion is not completed when the method 1 is carried out. [3] The positive electrode for a lithium ion secondary battery according to [1] or [2], wherein the time required for the second diffusion to be completed as measured by the following method 2 is 1 second or longer, or the second diffusion is not completed when the following method 2 is performed. (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 positive electrode for a lithium ion secondary battery according to any one of [1] to [3], wherein the diffusion stop time measured by the following method 3 is 25 seconds or more. (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 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 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 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 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 positive electrode for a lithium ion secondary battery according to any one of [1] to [8], wherein the 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 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 positive electrode for a lithium ion secondary battery according to any one of [1] to

[10] , wherein the 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 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 positive electrode for a lithium ion secondary battery according to any one of [1] to

[12] , wherein the 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 positive electrode for a lithium ion secondary battery according to any one of [1] to

[13] , wherein the positive electrode active material layer further contains a conductive additive.

[15]

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

[16] , further comprising a positive electrode current collector.

[18] [1] to

[17] , and a positive electrode for a lithium ion secondary battery. a solid electrolyte layer; a negative electrode; A lithium-ion secondary battery comprising the above in this order.

[19]

[18] A lithium ion secondary battery module including the lithium ion secondary battery according to

[18] . [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a positive electrode for a lithium ion secondary battery that can reduce the DC resistance of the battery. [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] (Positive electrode for lithium-ion secondary batteries) The positive electrode for a lithium ion secondary battery according to this embodiment includes a positive electrode active material layer containing a positive electrode active material, a solid electrolyte, and a binder. In the positive electrode for a lithium ion secondary battery according to this embodiment, the first diffusion completion time measured by the following method 1 is 25 seconds or more, or the first diffusion is not completed when the following method 1 is performed. (Method 1) A test piece measuring 2 cm (length) x 2 cm (width) is cut from the positive electrode active material layer. Next, under 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 vertically away from the surface of the test piece toward the center of the test piece. The time from dropping until the butyl butyrate diffuses over the entire surface is measured, and this time is defined as the first diffusion completion time. If the butyl butyrate has not diffused over the entire surface within 180 seconds after dropping, it is determined that "the first diffusion is not complete."

[0013] Hereinafter, the positive electrode for a lithium ion secondary battery according to this embodiment will be referred to as the "positive electrode."

[0014] In the positive electrode according to this embodiment, the first diffusion completion time is 25 seconds or more, or the first diffusion is not completed when Method 1 is performed. In the positive electrode according to this embodiment, the first diffusion completion time is preferably 30 seconds or more, or the first diffusion is not completed when Method 1 is performed. In the positive electrode according to this embodiment, the first diffusion completion time is more preferably 35 seconds or more, or the first diffusion is not completed when Method 1 is performed. In the positive electrode according to this embodiment, the first diffusion is even more preferably not completed when Method 1 is performed. This further reduces the DC resistance of the battery. The upper limit of the first diffusion completion time is not particularly limited, and may be, for example, 180 seconds or less, 150 seconds or less, 120 seconds or less, 90 seconds or less, or 60 seconds or less. Moreover, from the viewpoint of further reducing the DC resistance of the battery, the first diffusion completion 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 30 seconds or more and 90 seconds or less, and even more preferably 35 seconds or more and 60 seconds or less.

[0015] The mechanism by which the DC resistance of the battery can be reduced by setting the first diffusion completion time within the above range is not entirely clear, but according to the studies of the present inventors, it is presumed as follows.

[0016] 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, the inventors have found that the denser the positive electrode active material layer, the longer the first diffusion completion time. The inventors have also found that when the density of the positive electrode active material layer is set to a predetermined value or more, the ionic conductivity in 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.

[0017] In this embodiment, for example, the first diffusion completion time can be set within the above range by controlling the following manufacturing conditions. (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

[0018] In the positive electrode according to this embodiment, the second diffusion completion time measured by the following method 2 is preferably 1 second or longer, or the 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] In the positive electrode according to this embodiment, the second diffusion completion time is preferably 1 second or longer, or the second diffusion is not completed when Method 2 is performed. In the positive electrode according to this embodiment, the second diffusion completion time is more preferably 3 seconds or longer, or the second diffusion is not completed when Method 2 is performed. In the positive electrode according to this embodiment, the second diffusion completion time is even more preferably 5 seconds or longer, or the second diffusion is not completed when Method 2 is performed. In the positive electrode according to this embodiment, the second diffusion completion time is even more preferably 10 seconds or longer, or the second diffusion is not completed when Method 2 is performed. In the positive electrode according to this embodiment, the second diffusion completion time is even more preferably 15 seconds or longer, or the second diffusion is not completed when Method 2 is performed. In the positive electrode according to this embodiment, the second diffusion is even more preferably not completed when Method 2 is performed. This further reduces the DC resistance of the battery.

[0020] The upper limit of the second diffusion completion time is not particularly limited, but may be, for example, 120 seconds or less, 90 seconds or less, 60 seconds or less, or 30 seconds or less. Furthermore, from the viewpoint of further reducing the DC resistance of the battery, the second diffusion completion time is preferably 3 seconds or more and 120 seconds or less, more preferably 5 seconds or more and 90 seconds or less, even more preferably 10 seconds or more and 60 seconds or less, and even more preferably 15 seconds or more and 30 seconds or less.

[0021] The mechanism by which the DC resistance of the battery can be further reduced by setting the second diffusion completion time within the above range is not entirely clear, but according to the studies of the present inventors, it is presumed as follows.

[0022] 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, they have found that when the density of the positive electrode active material layer is set to a predetermined value or more, the ionic conductivity in the positive electrode active material layer is improved, thereby enabling the DC resistance of the battery to be further reduced. 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 enabling the DC resistance of the battery to be further reduced.

[0023] In this embodiment, for example, the second diffusion completion time can be set within the above range by controlling the following manufacturing conditions. (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

[0024] The positive electrode according to this embodiment preferably has a diffusion stop time of 25 seconds or more as measured by the following method 3. (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.

[0025] 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.

[0026] The diffusion stop time is preferably 25 seconds or more, more preferably 30 seconds or more, even more preferably 35 seconds or more, and even more preferably 40 seconds or more. This allows the DC resistance of the battery to be further reduced. The upper limit of the diffusion stop time is not particularly limited, but may be, for example, 3600 seconds or less, 1800 seconds or less, 1200 seconds or less, 600 seconds or less, 300 seconds or less, 150 seconds or less, 100 seconds or less, or 80 seconds or less. Furthermore, from the viewpoint of further reducing the DC resistance of the battery, the diffusion stop time is preferably from 25 seconds to 3600 seconds, more preferably from 25 seconds to 1800 seconds, even more preferably from 25 seconds to 1200 seconds, even more preferably from 25 seconds to 600 seconds, even more preferably from 30 seconds to 300 seconds, even more preferably from 30 seconds to 150 seconds, even more preferably from 35 seconds to 100 seconds, and even more preferably from 40 seconds to 80 seconds.

[0027] In this embodiment, for example, the diffusion stop time can be set within the above range by controlling the following manufacturing conditions. (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

[0028] The positive electrode according to this embodiment includes a positive electrode active material layer containing a positive electrode active material, a solid electrolyte, and a binder. Each component contained in the positive electrode active material layer according to this embodiment will be described below.

[0029] (Cathode active material) The positive electrode active material layer according to this embodiment includes a positive electrode active material, such as a lithium composite oxide; a transition metal sulfide such as TiS, FeS, or MoS; MnO, VO, or VO. 13 The 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.

[0030] The positive electrode active material preferably contains a lithium composite oxide, which can improve the capacity of the lithium ion secondary battery.

[0031] 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.

[0032] 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 Co c 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, and 0.5≦a≦1.5, 0.6≦b<1.0, 0 <c≦0.2、0<d≦0.2、0≦e<1.0である。

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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 ≤ 0.1, and still more preferably e = 0.

[0038] The lithium-nickel-cobalt-manganese composite oxide preferably contains one or more selected from the group consisting of LiNi 0.8 Mn 0.1 Co 0.1 O2 (NMC811) and LiNi 0.6 Mn 0.2 Co 0.2 O2 (NMC622). Thereby, the capacity of the lithium-ion secondary battery can be improved.

[0039] 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.

[0040] 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.

[0041] (solid electrolyte) The positive electrode active material layer according to this embodiment includes a solid electrolyte. The solid electrolyte includes, for example, one or more solid electrolytes selected from the group consisting of sulfide-based solid electrolytes, oxide-based solid electrolytes, polymer-based solid electrolytes, and halide-based solid electrolytes. The solid electrolyte preferably includes one or more solid electrolytes selected from the group consisting of sulfide-based solid electrolytes, oxide-based solid electrolytes, and polymer-based solid electrolytes, and more preferably includes a sulfide-based solid electrolyte. This can further reduce the DC resistance of the battery.

[0042] 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.

[0043] 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; including one or more selected from the group consisting of Li2O-P2O5 materials, Li2O-P2O5-Li3N materials, etc.

[0044] Polymer solid electrolytes include, for example, polyether-based electrolyte materials such as polyethylene oxide, polypropylene oxide, ethylene oxide-propylene copolymer, dimethylsiloxane-ethylene oxide copolymer, etc.; gel polymer electrolyte materials such as polyacrylonitrile, polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene polymer, etc.; and one or more selected from the group consisting of polymer solid electrolyte materials using highly branched polymers.

[0045] Halide-based solid electrolytes include, for example, Li 6-3x Y x 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, halide-based solid electrolytes include, 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.

[0046] 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 even 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.

[0047] The amount of 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, even more preferably 15 parts by mass or more and 40 parts by mass or less, and even more preferably 20 parts by mass or more and 35 parts by mass or less, relative to 100 parts by mass of the positive electrode active material contained in the positive electrode active material layer, thereby improving the capacity of the lithium ion secondary battery.

[0048] The average particle diameter d of the solid electrolyte 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 0.01 μm or more and 30 μm or less, more preferably 0.1 μm or more and 20 μm or less, even more preferably 0.2 μm or more and 10 μm or less, and even more preferably 0.5 μm or more and 5 μm or less. 50 By setting the range above, the DC resistance of the battery can be further reduced.

[0049] (binder) The positive electrode active material layer according to this embodiment contains a binder, which preferably contains one or more binders selected from the group consisting of fluorine-based binders and rubber-based binders.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] (Conductive additive) The positive electrode active material layer according to this embodiment preferably further contains a conductive additive, which can improve the conductivity of the positive electrode and further reduce the DC resistance of the battery.

[0057] 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.

[0058] 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.

[0059] 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.

[0060] (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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] (Positive electrode manufacturing method) The positive electrode according to this embodiment can be manufactured, for example, by the following method. That is, the manufacturing method of the positive electrode according to this embodiment includes, for example, a step (A) of mixing a positive electrode active material, a solid electrolyte, and a binder, a step (B) of applying the mixture obtained in step (A) to a support, a step (C) of drying the mixture applied to the support to obtain a positive electrode precursor, and a step (D) of compressing the obtained positive electrode precursor. In order to prevent water from being adsorbed by the solid electrolyte, it is preferable to manufacture the positive electrode in a low-moisture environment under dew point control. Each step will be described in more detail below.

[0065] (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.

[0066] (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.

[0067] (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.

[0068] (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).

[0069] 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.

[0070] (lithium-ion secondary battery) The positive electrode 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 be described below 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 the lithium ion secondary battery 100 is not particularly limited, and any known method can be applied. For example, the lithium ion secondary battery 100 can be manufactured by the following method.

[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] (Lithium-ion secondary battery module) The lithium ion secondary battery module of this embodiment includes the lithium ion secondary battery of this embodiment. The lithium ion secondary battery of this embodiment can reduce the DC resistance of the battery, so the lithium ion secondary battery module of this embodiment can reduce the DC resistance within the module.

[0086] The lithium-ion secondary battery module of this embodiment preferably includes two or more lithium-ion secondary batteries of this embodiment connected in series or parallel. The battery module of this embodiment more preferably includes a housing capable of accommodating two or more lithium-ion secondary batteries of this embodiment connected in series or parallel. The battery module of this embodiment further preferably includes one or more components selected from the group consisting of a protection circuit that protects the lithium-ion secondary batteries from overcurrent, a balancing circuit that equalizes the voltage between the electrodes of the lithium-ion secondary batteries, a controller that controls the lithium-ion secondary batteries, a cooler that can cool the lithium-ion secondary batteries, and a heater that can heat the lithium-ion secondary batteries.

[0087] The lithium-ion secondary battery module of this embodiment can be used in a battery system including a plurality of electrically connected battery modules and a battery control system. Examples of battery systems include battery packs, stationary storage battery systems, automotive power storage battery systems, automotive auxiliary storage battery systems, and emergency power storage battery systems.

[0088] 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]

[0089] 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.

[0090] (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 0.1 O2(NMC811, average particle size d 50 :6μm)

[0091] (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)

[0092] (binder) SBR: Styrene-butadiene rubber PVDF-HFP: Vinylidene fluoride-hexafluoropropylene copolymer

[0093] 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

[0094] 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.

[0095] (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.

[0096] 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.

[0097] (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.

[0098] 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. 2 It 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.

[0099] (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.

[0100] (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.

[0101] [Table 1] [Explanation of symbols]

[0102] 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. A positive electrode for a lithium ion secondary battery, comprising a positive electrode active material layer including a positive electrode active material, a solid electrolyte, and a binder, A positive electrode for a lithium ion secondary battery, in which the first diffusion completion time measured by the following method 1 is 25 seconds or more, or the first diffusion is not completed when the following method 1 is performed. (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 positive electrode for a lithium ion secondary battery according to claim 1, wherein the first diffusion is not completed when the method 1 is carried out.

3. 3. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the second diffusion completion time measured by the following method 2 is 1 second or more, or the second diffusion is not completed when the following method 2 is performed. (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 more 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 positive electrode for a lithium ion secondary battery according to any one of claims 1 to 3, wherein the diffusion stop time measured by the following method 3 is 25 seconds or more. (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 positive electrode for a lithium ion secondary battery according to any one of claims 1 to 4, wherein the positive electrode active material comprises a lithium composite oxide.

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

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

13. 13. The positive electrode for a lithium ion secondary battery according to claim 1, 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 positive electrode for a lithium ion secondary battery according to any one of claims 1 to 13, wherein the positive electrode active material layer further contains a conductive additive.

15. 15. The 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 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 positive electrode for a lithium ion secondary battery according to any one of claims 1 to 16, further comprising a positive electrode current collector.

18. The positive electrode for a lithium ion secondary battery according to any one of claims 1 to 17, a solid electrolyte layer; a negative electrode; A lithium-ion secondary battery comprising the above in this order.

19. A lithium ion secondary battery module comprising the lithium ion secondary battery according to claim 18.

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