Cathode material, all-solid-state battery, and method for producing cathode material

Abstract

To provide: a cathode material capable of improving battery characteristics of an all-solid-state battery; an all-solid-state battery; and a method for producing the cathode material.SOLUTION: The cathode material for an all-solid-state battery according to the present invention comprises a lithium transition metal oxide, a solid electrolyte, and carbon nanotubes, where at least one of the carbon nanotubes has a length of at least 100 μm.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] Embodiments of the present invention relate to a positive electrode material, an all-solid-state battery, and a method for manufacturing a positive electrode material.

Background Art

[0002] Currently, lithium-ion secondary batteries using a liquid electrolyte are widely used. However, when using a liquid electrolyte, if deformation or an external impact occurs, there is a risk of short circuit, leading to overheating or explosion. In order to improve the safety of lithium-ion secondary batteries, in recent years, all-solid-state batteries that do not use an organic electrolyte solution have attracted attention as next-generation batteries, and various research and developments have been carried out.

[0003] Carbon nanotubes are known as a conductive material added to the electrode material of an all-solid-state battery. Conventionally, carbon nanotubes used as a conductive material for all-solid-state batteries have been exclusively those with a length of 10 μm or less, and at most several tens of μm (see, for example, Patent Documents 1 to 3).

[0004] Exceptionally, Patent Document 4 describes a positive electrode for an all-solid-state battery having a molded body of a positive electrode mixture containing a positive electrode active material, a sulfide-based solid electrolyte, and a conductive auxiliary agent, the conductive auxiliary agent containing fibrous carbon and granular carbon, and the thickness of the molded body of the positive electrode mixture being 250 μm or more, and it is described that the fiber length of the fibrous carbon is preferably 3 to 600 μm. However, Patent Document 4 does not particularly describe the reason why the numerical range is preferable, and as an example actually demonstrating the effect, only an example using fibrous carbon VGCF (vapor-grown carbon fiber) manufactured by Showa Denko KK is described. The fiber length of VGCF is about 10 μm, and Patent Document 4 does not describe specific experimental results regarding carbon nanotubes with a length exceeding 10 μm.

Prior Art Documents

Patent Documents

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2020-507893 [Patent Document 2] Japanese Patent Application Laid-Open No. 2022-529987 [Patent Document 3] Japanese Patent Application Laid-Open No. 2023-132317 [Patent Document 4] Japanese Patent Application Laid-Open No. 2021-144906 [Summary of the Invention] [Problems to be Solved by the Invention]

[0006] Even referring to the above patent documents, the reason why carbon nanotubes with lengths exceeding several tens of micrometers have been avoided is not clear. However, the inventor has found that in the process of making the present invention, carbon nanotubes with a length of about 50 μm tend to aggregate with each other and are difficult to disperse sufficiently in the positive electrode. As a result of such properties, the inventor infers that when carbon nanotubes with lengths exceeding several tens of micrometers are used as a conductive material, the battery performance is not considered to be improved much.

[0007] By adding conventionally used carbon nanotubes with lengths of several tens of micrometers or less, battery characteristics such as the charge and discharge capacity and rate characteristics of all-solid-state batteries are improved to some extent. However, the need for higher-performance batteries is increasing day by day, and further improvement of battery characteristics has been desired.

[0008] The problem to be solved by the present invention is to provide a positive electrode material, an all-solid-state battery, and a method for manufacturing the positive electrode material that can improve the battery characteristics of all-solid-state batteries. [Means for Solving the Problems]

[0009] The inventor conceived that when carbon nanotubes with a length of 100 μm or more are used as a conductive material, the battery characteristics may be improved by efficiently forming an electron conduction path between the electrode active materials. Therefore, the inventor examined the influence on battery characteristics by using carbon nanotubes of various lengths exceeding 50 μm, which had been conventionally avoided, and found that the aggregation tendency observed in carbon nanotubes with a length of about 50 μm was suppressed by using carbon nanotubes with a length of 100 μm or more as a conductive material. As a result, the inventor found long carbon nanotubes suitable for practical use and completed the present invention.

[0010] The present invention may include the following aspects. [1] A positive electrode material for an all-solid-state battery, comprising a lithium transition metal oxide, a solid electrolyte, and a carbon nanotube, wherein one or more of the carbon nanotubes have a length of 100 μm or more. [2] The positive electrode material according to [1], wherein one or more of the carbon nanotubes have a length of 100 μm or more and 500 μm or less. [3] The positive electrode material according to [1], wherein one or more of the carbon nanotubes have a length of 100 μm or more and 200 μm or less. [4] Positive electrode active material particles having an average particle diameter of 1 μm to 20 μm and containing a lithium transition metal oxide, a solid electrolyte, and a carbon nanotube that contacts a plurality of positive electrode active material particles to form an electron conduction path between the positive electrode active material particles. [5] The positive electrode material according to any one of [1] to [4], wherein the content of the carbon nanotube in the positive electrode material is 0.01 mass% or more and 10 mass% or less. [6] The positive electrode material according to any one of [1] to [5], wherein the carbon nanotube is a multi-walled carbon nanotube. [7] The carbon nanotube is the positive electrode material according to any one of [1] to [6], which forms an electron conduction path between a plurality of particles not in contact with each other. [8] The carbon nanotube is the positive electrode material according to any one of [1] to [7], which is dispersed without substantially forming aggregates or bundles. [9] The lithium transition metal oxide is the positive electrode material according to any one of [1] to [8], which contains 50 mol% or more of nickel based on the total amount of the transition metal.

[10] The solid electrolyte is a sulfide-based solid electrolyte, and is the positive electrode material according to any one of [1] to [9].

[11] The carbon nanotube is the positive electrode material according to any one of [1] to

[10] , and has an average diameter in the range of 2 nm to 20 nm.

[12] The carbon nanotube is the positive electrode material according to any one of [1] to

[11] , and has an aspect ratio in the range of 10,000 to 100,000.

[13] A positive electrode including the positive electrode material according to any one of [1] to

[12] , A negative electrode, A solid electrolyte layer between the positive electrode and the negative electrode, And a solid-state battery comprising the same.

[14] A method for manufacturing a positive electrode material for a solid-state battery, including a step of mixing a lithium transition metal oxide, a solid electrolyte, and a carbon nanotube having a length of 100 μm or more.

Advantages of the Invention

[0011] According to the present invention, it is possible to provide a positive electrode material, a solid-state battery, and a method for manufacturing a positive electrode material that can improve the battery characteristics of the solid-state battery.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Mode for Carrying Out the Invention

[0013] Hereinafter, the positive electrode material, all-solid-state battery, and method for manufacturing the positive electrode material of the embodiment will be described. Note that the following embodiments show one aspect of the present invention, do not limit the present invention, and can be arbitrarily changed within the scope of the technical idea of the present invention. In addition, each configuration and each feature of the embodiment can be arbitrarily combined.

[0014] Hereinafter, a singular expression is used in the sense of "one or more" unless the context clearly indicates only the singular.

[0015] In this specification, when a part such as a layer, film, region, or plate is described as being "on" another part, this includes not only the case where it is "directly on" the other part but also the case where there is another part in between. Conversely, when a part such as a layer, film, region, or plate is described as being "under" another part, this includes not only the case where it is "directly under" the other part but also the case where there is another part in between. Also, in this specification, being "disposed on" can include not only the upper part but also the case of being disposed in the lower part.

[0016] In this specification, the average particle diameter (D 50 ) can be defined as the particle diameter corresponding to 50% of the volume cumulative amount in the particle size distribution curve of the particles. The average particle diameter (D 50 ) can be measured, for example, using the laser diffraction method. The laser diffraction method can generally measure particle diameters in the range from the submicron region to about several millimeters and can obtain highly reproducible and highly resolved results.

[0017] In this specification, "single-walled carbon nanotube" (SWCNT) means a carbon nanotube in which the tubular wall composed of carbon atoms is composed of a single atomic layer (i.e., one layer of graphene sheet). "Multi-walled carbon nanotube" (MWCNT) means a carbon nanotube in which the tubular wall composed of carbon atoms is composed of a plurality of atomic layers (i.e., a plurality of layers of graphene sheets).

[0018] <Positive electrode material> The positive electrode material according to this embodiment includes a lithium transition metal oxide, a solid electrolyte, and carbon nanotubes, and one or more of the carbon nanotubes have a length of 100 μm or more.

[0019] FIG. 1 is a schematic diagram showing the microscopic structure of the positive electrode material. The positive electrode material includes particles 10 of a lithium transition metal oxide, particles 12 of a solid electrolyte, and carbon nanotubes 14. In FIG. 1, the solid electrolyte particles 12 are smaller than the particles 10 of the lithium transition metal oxide and are scattered around the particles 10 of the lithium transition metal oxide. During the charge and discharge process, lithium ions move between the particles 10 of the lithium transition metal oxide and the solid electrolyte particles 12 that are in contact with each other. The carbon nanotubes 14 are intertwined with the particles 10 of the lithium transition metal oxide. Some of the carbon nanotubes 14 are in contact with a plurality of particles 10 of the lithium transition metal oxide. During the charge and discharge process, the carbon nanotubes 14 can form an electron conduction path from the particles 10 of the lithium transition metal oxide to another particle 10 or a current collector. Electron conduction occurs in conjunction with lithium ion conduction.

[0020] [Lithium transition metal oxide] The lithium transition metal oxide functions as a positive electrode active material that stores and releases lithium ions at the positive electrode in cooperation with the negative electrode active material described below during charge and discharge.

[0021] The lithium transition metal oxide may be one commonly used as a positive electrode active material in the relevant technical field. The lithium transition metal oxide may be any compound capable of reversible insertion (intercalation) and desorption (deintercalation) of lithium, and its type is not particularly limited. Specific examples include, for example, lithium metal composite oxides containing one or more metals such as cobalt, manganese, nickel, copper, vanadium, aluminum, etc. and lithium. More specifically, such lithium metal composite oxides include lithium-manganese-based oxides (for example, LiMnO2, LiMnO3, LiMn2O3, LiMn2O4, etc.); lithium-cobalt-based oxides (for example, LiCoO2, etc.); lithium-nickel-based oxides (for example, LiNiO2, etc.); lithium-copper-based oxides (for example, Li2CuO2, etc.); lithium-vanadium-based oxides (for example, LiV3O8, etc.); lithium-nickel-manganese-based oxides (for example, LiNi 1-z Mn z O2 (0 < z < 1), LiMn 2-z Ni z O4 (0 < z < 2), etc.); lithium-nickel-cobalt-based oxides (for example, LiNi 1-y Co y O2 (0 < y < 1), etc.); lithium-manganese-cobalt-based oxides (for example, LiCo 1-z Mn z O2 (0 < z < 1), LiMn 2-y Co y O4 (0 < y < 2), etc.); lithium-nickel-manganese-cobalt-based oxides (for example, Li(Ni x Co y Mn z )O2 (0 < x < 1, 0 < y < 1, 0 < z < 1, x + y + z = 1), Li(Ni x Co y Mn z )O4 (0 < x < 2, 0 < y < 2, 0 < z < 2, x + y + z = 2), etc.); lithium-nickel-cobalt-metal (M) oxides (for example, Li(Ni x Co y Mn z M w)O2 (M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, 0 < x < 1, 0 < y < 1, 0 < z < 1, 0 < w < 1, x + y + z + w = 1), etc.); Li-excess solid solution cathode (for example, pLi2MnO3-(1-p)Li(Ni x Co y Mn z )O2 (0 < x < 1, 0 < y < 1, 0 < z < 1, x + y + z = 1, 0 < p < 1); Examples include compounds in which the transition metal elements in these compounds are partially substituted with one or more other metal elements. The positive electrode active material layer can contain any one or two or more of these compounds. However, it is not limited to only these.

[0022] In particular, in lithium transition metal oxides with a high nickel content that are effective for increasing the battery capacity, the lithium transition metal oxide preferably contains 50 mol% or more of nickel based on the total amount of transition metals. Examples of such lithium transition metal oxides include Li a NiO2 (0.5 ≦ a ≦ 1.5); Li a (Ni x Co y Mn z )O2 (0.5 ≦ a ≦ 1.5, 0.5 ≦ x < 1, 0 < y < 0.5, 0 < z < 0.5, x + y + z = 1); Li a (Ni x Co y Mn z )O2 (0.7 ≦ x < 1, 0 < y < 0.3, 0 < z < 0.3, x + y + z = 1); Li a (Ni x Co y Mn z )O2 (0.8 ≦ x < 1, 0 < y < 0.2, 0 < z < 0.2, x + y + z = 1); Li a (Ni x Co y Mn z )O2 (0.9 ≦ x < 1, 0 < y < 0.1, 0 < z < 0.1, x + y + z = 1); Li a Ni 1-y Co y O2 (0.5 ≦ a ≦ 1.5, 0 < y ≦ 0.5); Li a Ni 1-z Mnz O2 (0.5 ≤ a ≤ 1.5, 0 < z ≤ 0.5); Li a (Ni x Co y Mn z )O4 (0.5 ≤ a ≤ 1.5, 1 ≤ x < 2, 0 < y < 1, 0 < z < 1, x + y + z = 2); Li a (Ni x Co y M w )O2 (M is one or more elements selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, Mo, Zr, Zn, Ga, and In, 0.5 ≤ a ≤ 1.5, 0.5 ≤ x < 1, 0 < y < 0.5, 0 < w < 0.5, x + y + w = 1); Li a (Ni x Co y Mn z M w )O2 (M is one or more elements selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, Mo, Zr, Zn, Ga, and In, 0.5 ≤ a ≤ 1.5, 0.5 ≤ x < 1, 0 < y < 0.5, 0 < z < 0.5, 0 < w < 0.5, x + y + z + w = 1); Compounds in which at least part of the transition metal atoms in these compounds are substituted with one or more other metal elements (for example, one or more of Al, Fe, V, Cr, Ti, Ta, Mg, Mo, Zr, Zn, Ga, and In); Compounds in which the oxygen atoms in these compounds are partially substituted with one or more other non-metal elements (for example, one or more of P, F, S, and N), etc. are mentioned. Preferably, the lithium transition metal oxide is Li a Ni x M yO2 (M is one or more metal elements other than Ni, for example, one or more elements selected from the group consisting of Al, Fe, Co, Mn, V, Cr, Ti, Ta, Mg, Mo, Zr, Zn, Ga, and In, 0 < a ≤ 1.05, x + y = 1), and the value of x may be, for example, 0.5 or more, 0.6 or more, 0.7 or more, 0.8 or more, and 0.9 or more, and may be, for example, 1 or less, 0.9 or less, 0.8 or less, 0.7 or less, or 0.6 or less. The positive electrode active material can include one or more of the above, but is not limited thereto. Also, even within the same particle, there may be a concentration distribution of substitution between the inside and the surface layer. It may also be coated on the surface of the particle. For example, there are surfaces coated with metal oxides, lithium transition metal oxides, polymers, etc., but it is not limited thereto.

[0023] In particular, in terms of improving the capacity characteristics and stability of the battery, Li a NiO2, Li a (Ni 0.5 Mn y Co z )O2 (y + z = 0.5), Li a (Ni 0.6 Mn y Co z )O2 (y + z = 0.4), Li a (Ni 0.7 Mn y Co z )O2 (y + z = 0.3), Li a (Ni 0.8 Mn y Co z )O2 (y + z = 0.2), Li a (Ni 0.8 Co y Mn z Al w )O2 (y + z + w = 0.2), Li a (Ni 0.85 Co y Mn z )O2 (y + z = 0.15), Li a (Ni 0.85 Co y Mn z Al w)O2 (y + z + w = 0.15), Li a (Ni 0.9 Co y Mn z )O2 (y + z = 0.1), Li a (Ni 0.9 Co y Mn z Al w )O2 (y + z + w = 0.1), Li a (Ni 0.9 Co y Mn z )O2 (y + z = 0.1), Li a (Ni 0.95 Co y Mn z Al w )O2 (y + z + w = 0.05), etc. are preferred. Here, the value of a can all be, for example, 0.5 ≦ a ≦ 1.5, and preferably 1.0 ≦ a ≦ 1.5.

[0024] More specifically, LiNiO2, Li(Ni 0.5 Mn 0.3 Co 0。2 )O2, Li(Ni 0.6 Mn 0.2 Co 0.2 )O2, Li(Ni 0.7 Mn 0.15 Co 0.15 )O2, Li(Ni 0.8 Mn 0.1 Co 0.1 )O2, Li(Ni 0.8 Co 0.15 Al 0.05 )O2, Li(Ni 0.8 Co 0.1 Mn 0.05 Al 0.05 )O2, Li(Ni 0.85 Co 0.10 Mn 0.05 )O2, Li(Ni 0.85 Co 0.10 Mn 0.03 Al 0.02 )O2, Li(Ni 0.9 Co 0.05 Mn 0.05 )O 2、 Li(Ni 0.9 Co 0.05Al 0.05 )O2, Li(Ni 0.95 Co 0.03 Mn 0.02 )O2, Li(Ni 0.95 Co 0.03 Al 0.02 )O2, etc. are preferable.

[0025] The particle size of the lithium transition metal oxide is, for example, 10 nm to 20 μm, 50 nm to 18 μm, 100 nm to 15 μm, 200 nm to 13 μm, 500 nm to 12 μm, or 1 μm to 10 μm. If the particle size of the lithium transition metal oxide is 10 nm or more, surface deterioration of the particles can be suppressed. If the particle size of the lithium transition metal oxide is 20 μm or less, the diffusion path of lithium does not become excessively long, and the electron conduction path by the carbon nanotubes described later can be efficiently formed.

[0026] Preferably, a coating containing a metal oxide is formed on the particle surface of the lithium transition metal oxide. The metal oxide may be at least one selected from the group consisting of LiNbO2, LiNbO3, LiCoO2, and Li2TiO3. More preferably, a coating containing LiNbO2 is formed on the particle surface of the lithium transition metal oxide. Such a coating can reduce the internal resistance of the positive electrode.

[0027] [Solid electrolyte] The solid electrolyte assists the transfer of lithium ions between the positive electrode active material and the negative electrode active material. The particles of the solid electrolyte can come into contact with the positive electrode active material particles, the negative electrode active material particles, or other solid electrolyte particles to mediate the transfer of lithium ions.

[0028] In order for the solid electrolyte to mainly play a role of transferring lithium ions in the electrode, any material with high ionic conductivity (for example, ionic conductivity of 10 -5 s / m or more, preferably 10 -4 s / m or more) can be used, and it is not limited to specific components.

[0029] The solid electrolyte may be one or more selected from the group consisting of, for example, sulfide-based solid electrolytes, oxide-based solid electrolytes, and polymer-based solid electrolytes. The polymer-based solid electrolyte may be a polymer solid electrolyte formed by adding a polymer resin to a solvated lithium salt, or a polymer gel electrolyte in which an organic electrolyte solution containing an organic solvent and a lithium salt, an ionic liquid, a monomer or an oligomer, etc. is contained in a polymer resin. On the other hand, the sulfide-based solid electrolyte has high ionic conductivity, and the oxide-based solid electrolyte is excellent in electrochemical stability. Therefore, an appropriate solid electrolyte component can be selected and used according to the characteristics of the solid electrolyte and the purpose of use of the battery. Preferably, the solid electrolyte is a sulfide-based solid electrolyte.

[0030] The sulfide-based solid electrolyte contains sulfur (S) and has ionic conductivity of a metal belonging to Group 1 or Group 2 of the periodic table. The sulfide-based solid electrolyte may include, for example, Li-P-S-based glass or Li-P-S-based glass ceramics. The sulfide-based solid electrolytes include glassy solid electrolytes, crystalline solid electrolytes, and glass-ceramic solid electrolytes. Specifically, Li2S-P2S5, Li2S-LiI-P2S5, Li2S-LiI-Li2O-P2S5, Li2S-LiBr-P2S5, Li2S-Li2O-P2S5, Li2S-Li3PO4-P2S5, Li2S-P2S5-P2O5, Li2S-P2S5-SiS2, Li2S-P2S5-SnS, Li2S-P2S5-Al2S3, Li2S-GeS2, Li2S-GeS2-ZnS, argyrodite-based solid electrolytes (such as Li6PS5X (X: Cl, Br, I), etc.), LGPS-based solid electrolytes (Li 10 GeP2S 12 、Li 3.25 Ge 0.25 P 0.75 S4, etc.), Li7P3S 11 and the like, but are not limited thereto. For example, the solid electrolyte may include one or more selected from the group consisting of the above compounds.

[0031] The oxide-based solid electrolyte contains oxygen (O) and has ionic conductivity of a metal belonging to Group 1 or Group 2 of the periodic table. Examples of the oxide-based solid electrolyte include LLTO-based compounds, Li6La2CaTa2O 12 , Li6La2ANb2O 12 (A: Ca or Sr), Li2Nd3TeSbO 12 , Li3BO 2.5 N 0.5 , Li9SiAlO8, LAGP-based compounds, LATP-based compounds, Li 1+x Ti 2-x Al x Si y (PO4) 3-y (where 0 ≤ x ≤ 1, 0 ≤ y ≤ 1), LiAl x Zr 2-x (PO4)3 (where 0 ≤ x ≤ 1 and 0 ≤ y ≤ 1), LiTi x Zr 2-x (PO4)3 (0 ≤ x ≤ 1, 0 ≤ y ≤ 1), LISICON-based compounds, LIPON-based compounds, perovskite-based compounds, NASICON-based compounds, LLZO-based compounds, etc., but are not limited thereto. For example, the solid electrolyte may include one or more selected from the group consisting of the above compounds.

[0032] The polymer solid electrolyte may include, for example, polyether-based polymers, polycarbonate-based polymers, acrylate-based polymers, polysiloxane-based polymers, phosphagen-based polymers, polyethylene derivatives, alkylene oxide derivatives such as PEO (polyethylene oxide) and PPO (polypropylene oxide), phosphate ester polymers, polyaditation lysine, polyester sulfide, polyvinyl alcohol, polyvinylidene fluoride, polymers containing ionic dissociation groups, etc. For example, the solid electrolyte may include one or more selected from the group consisting of the above compounds.

[0033] The content of the solid electrolyte may be, for example, 1% by mass to 70% by mass, 5% by mass to 50% by mass, 10% by mass to 45% by mass, or 20% by mass to 40% based on the total weight of the positive electrode active material. If the content of the solid electrolyte particles is 1% by mass or more, the particles of the solid electrolyte can sufficiently form a lithium ion conduction path by surrounding the surface of the particles of the lithium transition metal oxide. If the content of the solid electrolyte particles is 50% by mass or less, it is possible to suppress the solid electrolyte particles from aggregating away from the lithium transition metal oxide.

[0034] The particle size of the solid electrolyte is, for example, 10 nm to 10 μm, 50 nm to 5 μm, 100 nm to 1 μm, or 200 nm to 500 nm. If the particle size of the solid electrolyte is within the above range, lithium ion transfer with the active material can be smoothly performed at an appropriate electrode density and tap density.

[0035] [Carbon nanotube] The carbon nanotubes mainly come into contact with the surface of the particles of the positive electrode active material and the positive electrode current collector, and contribute to forming a conductive network between the particles of the positive electrode active material or between the positive electrode active material particles and the positive electrode current collector. The carbon nanotubes are not in a bundled state, and most of them exist one by one and are mainly arranged on the surface of the positive electrode active material.

[0036] Among the carbon nanotubes included in the positive electrode material, at least one or more carbon nanotubes have a length of 100 μm or more. The length of the carbon nanotube may be, for example, 100 μm to 500 μm, 100 μm to 450 μm, 100 μm to 400 μm, 100 μm to 350 μm, 100 μm to 300 μm, 100 μm to 250 μm, or 100 μm to 200 μm. The length of the carbon nanotube may be 110 μm or more, 120 μm or more, 130 μm or more, 140 μm or more, 150 μm or more, 180 μm or more, 200 μm or more, 210 μm or more, 230 μm or more, or 250 μm or more. From the results of the following examples and comparative examples, it is presumed that carbon nanotubes with a length of 100 μm or more can efficiently form an electron conduction path between active material particles or between an active material particle and a current collector without excessive aggregation between the carbon nanotubes. On the other hand, carbon nanotubes with a length of 500 μm or less can suppress the aggregation of extremely long carbon nanotubes adhering to each other and forming bundles.

[0037] Preferably, the average length of the carbon nanotubes in the positive electrode material is 100 μm or more, and may be 100 μm to 500 μm, 100 μm to 450 μm, 100 μm to 400 μm, 100 μm to 350 μm, 100 μm to 300 μm, 100 μm to 250 μm, or 100 μm to 200 μm. The average length of the carbon nanotubes may be 110 μm or more, 120 μm or more, 130 μm or more, 140 μm or more, 150 μm or more, 180 μm or more, 200 μm or more, 210 μm or more, 230 μm or more, or 250 μm or more. As described above, carbon nanotubes with a length of 100 μm or more can efficiently form an electron conduction path between active material particles or between an active material particle and a current collector without excessive aggregation between the carbon nanotubes during electrode formation. Also, an efficient conductive network can be formed even with a small content of the conductive material. Here, the "average length" means the average value of the lengths of the top 100 carbon nanotubes and the bottom 100 carbon nanotubes observed by SEM.

[0038] Preferably, the carbon nanotubes are dispersed without substantially forming aggregates or bundles. For example, when observed by SEM, it is preferable that 50% (number basis) or more, 60% or more, 70% or more, 80% or more, 90% or more, or 100% of the carbon nanotubes in the positive electrode material are not in contact with two or more other carbon nanotubes. Alternatively, when observed by SEM, it is preferable that 50% (number basis) or more, 60% or more, 70% or more, 80% or more, 90% or more, or 100% of the carbon nanotubes in the positive electrode material are not in contact with other carbon nanotubes at two or more points. Alternatively, when observed by SEM, it is preferable that 50% (number basis) or more, 60% or more, 70% or more, 80% or more, 90% or more, or 100% of the carbon nanotubes in the positive electrode material are not in contact with other carbon nanotubes.

[0039] The average diameter of the carbon nanotubes is not particularly limited, and may be, for example, 1 nm or more and 3 μm or less, 5 nm or more and 1 μm or less, 10 nm or more and 500 nm or less, or 20 nm or more and 100 nm or less. In a specific embodiment, the average diameter of the carbon nanotubes may be 2 nm or more, 5 nm or more, 7 nm or more, or 10 nm or more. In another specific embodiment, the average diameter of the carbon nanotubes may be 20 nm or less, 15 nm or less, or 10 nm or less. When the above range is satisfied, the carbon nanotubes are likely to be dispersed in the positive electrode without excessive aggregation. Here, the "average diameter" means the average value of the diameters of the 100 carbon nanotubes with the largest diameters and the 100 carbon nanotubes with the smallest diameters observed by SEM. On the other hand, in a specific embodiment of the present invention, the carbon nanotubes according to the present invention may have an aspect ratio in the range of 10,000 to 100,000. The above aspect ratio may be 10,000 or more, 12,000 or more, 13,000 or more, 15,000 or more, 20,000 or more, 30,000 or more, 50,000 or more, or 70,000 or more. The above aspect ratio may be 100,000 or less, 90,000 or less, 85,000 or less, 80,000 or less, 75,000 or less, 70,000 or less, 50,000 or less, 40,000 or less, or 30,000 or less. The diameter and length of the aforementioned carbon nanotubes can be appropriately adjusted within the range that satisfies the above aspect ratio. In one embodiment, the above term "aspect ratio" may mean the ratio of length to diameter (length / diameter). In another embodiment, the above aspect ratio may mean the ratio of the average length to the average diameter (average length / average diameter).

[0040] The carbon nanotubes may be single-walled carbon nanotubes or multi-walled carbon nanotubes. Multi-walled carbon nanotubes are easier to manufacture than single-walled carbon nanotubes and are excellent in terms of cost.

[0041] The content of the carbon nanotubes is, based on the total mass of the positive electrode material, for example, 0.01% by mass or more and 10% by mass or less, 0.05% by mass or more and 8% by mass or less, 0.1% by mass or more and 6% by mass or less, 0.2% by mass or more and 5% by mass or less, 0.5% by mass or more and 4% by mass or less, or 1% by mass or more and 3% by mass or less. When the above range is satisfied, the dispersibility and stability of the carbon nanotubes can be maintained while efficiently forming a conductive network in the positive electrode, so that the charge and discharge characteristics of the battery can be improved.

[0042] When the content of the carbon nanotubes is based on the total mass of the lithium transition metal oxide, for example, it is 0.01% by mass or more and 20% by mass or less, 0.05% by mass or more and 15% by mass or less, 0.1% by mass or more and 12% by mass or less, 0.2% by mass or more and 10% by mass or less, 0.5% by mass or more and 8% by mass or less, or 1% by mass or more and 5% by mass or less. When the above range is satisfied, since a sufficient amount of carbon nanotubes exists with respect to the lithium transition metal oxide, a conductive network in the positive electrode can be efficiently formed.

[0043] The positive electrode material for an all-solid-state battery according to another embodiment includes positive electrode active material particles having an average particle size of 1 μm to 20 μm and containing a lithium transition metal oxide, a solid electrolyte, and carbon nanotubes that come into contact with a plurality of the positive electrode active material particles to form an electron conduction path between the positive electrode active material particles.

[0044] Since the carbon nanotubes have high electron conductivity, an electron conduction path between the active material particles is formed by coming into contact with a plurality of the positive electrode active material particles. The carbon nanotubes have a length that can come into contact with a plurality of the positive electrode active material particles having an average particle size of 1 μm to 20 μm. For example, when observed by SEM, it is preferable that 50% (number-based) or more, 60% or more, 70% or more, 80% or more, 90% or more, or 100% of the carbon nanotubes in the positive electrode material are in contact with a plurality of the positive electrode active material particles.

[0045] When viewed from the perspective of the positive electrode active material particles, it is preferable that an electron conduction path is formed with other active material particles. For example, when observed by SEM, it is preferable that 50% (number basis) or more, 60% or more, 70% or more, 80% or more, 90% or more, or 100% of the positive electrode active material particles in the positive electrode material are in contact with carbon nanotubes. Also, when observed by SEM, it is preferable that 50% (number basis) or more, 60% or more, 70% or more, 80% or more, 90% or more, or 100% of the positive electrode active material particles in the positive electrode material are in contact with carbon nanotubes and the carbon nanotubes are also in contact with other positive electrode active material particles.

[0046] From another perspective, it is preferable that the length of at least one carbon nanotube is greater than the average particle diameter of the positive electrode active material particles. For example, the length of at least one carbon nanotube is 5 times or more, 10 times or more, 20 times or more, or 30 times or more the average particle diameter of the positive electrode active material particles. Preferably, the average length of the carbon nanotubes is greater than the average particle diameter of the positive electrode active material particles, for example, 5 times or more, 10 times or more, 20 times or more, or 30 times or more the average particle diameter of the positive electrode active material particles.

[0047] As described above, carbon nanotubes can form an electron conduction path between a plurality of particles that are not in contact with each other. Thereby, the electron conduction in the positive electrode can be smoothed, so that the active material particles in the positive electrode can be uniformly involved in the charge and discharge reaction. As a result, the battery capacity can be improved. Also, since carbon nanotubes have excellent electron conductivity, energy losses such as heat generation under high current can be suppressed. As a result, the rate characteristics (especially high-speed charge and discharge characteristics) can be improved.

[0048] Note that the carbon nanotubes do not necessarily have to be in direct contact with the positive electrode active material. For example, when the surface of the positive electrode active material particles is coated with a conductive material such as carbon, the carbon nanotubes contact the positive electrode active material particles through the coating, and even in this case, an electron conduction path is formed. Therefore, the contact between the carbon nanotubes and the positive electrode active material particles does not have to be direct as long as an electron conduction path through which the carbon nanotubes mediate the electron conduction between the positive electrode active material particles is formed, and it may be indirect contact through another configuration. Further, the carbon nanotubes may support the positive electrode active material particles by adsorption or the like, but they do not have to support the positive electrode active material particles just by being in contact. For example, the carbon nanotubes and the positive electrode active material particles do not have to be adsorbed.

[0049] <Method for manufacturing positive electrode material> The method for manufacturing a positive electrode material for an all-solid-state battery according to the present embodiment includes a step of mixing a lithium transition metal oxide, a solid electrolyte, and carbon nanotubes having a length of 100 μm or more.

[0050] The lithium transition metal oxide, the solid electrolyte, and the carbon nanotubes are the same as those described above. By mixing these, an electron conduction path through the carbon nanotubes can be formed between the particles of the lithium transition metal oxide.

[0051] The above mixing step can be carried out using a dry mixer, a stirrer, a shaker such as an orbital shaker, a mortar mixer, a milling machine such as a planetary ball mill, etc., which are generally used for mixing powders, but is not limited thereto. The mixing method is not limited to dry mixing, and may be wet mixing in which the lithium transition metal oxide, the solid electrolyte, and the carbon nanotubes are mixed in an arbitrary liquid medium. The order of adding the lithium transition metal oxide, the solid electrolyte, and the carbon nanotubes is not particularly limited. Further, other additives may be added.

[0052] <All-solid-state battery> The all-solid-state battery according to this embodiment includes a positive electrode containing the above positive electrode material, a negative electrode, and a solid electrolyte layer between the positive electrode and the negative electrode.

[0053] FIG. 2 is a schematic diagram showing the structure of the all-solid-state battery 100 according to the embodiment. As shown in FIG. 2, the all-solid-state battery 100 includes a positive electrode 110, a solid electrolyte layer 120, and a negative electrode 130 in this order. The positive electrode 110 includes a positive electrode current collector 112 and a positive electrode material layer 114. The negative electrode 130 includes a negative electrode material layer 132 and a negative electrode current collector 134. However, for example, when the negative electrode is formed of metallic lithium alone, the negative electrode material layer 132 and the negative electrode current collector 134 are provided integrally. As a whole, the all-solid-state battery 100 is formed by laminating a positive electrode current collector 112, a positive electrode material layer 114, a solid electrolyte layer 120, a negative electrode material layer 132, and a negative electrode current collector 134 in this order.

[0054] [Positive electrode 110] In the all-solid-state battery 100 according to the embodiment, the positive electrode 110 includes a positive electrode current collector 112 and a positive electrode active material layer 114 formed on one or both surfaces of the positive electrode current collector 112. The positive electrode active material layer 114 may be formed over the entire surface of the positive electrode current collector 112, or may be formed only on a part thereof.

[0055] (Positive electrode current collector 112) The positive electrode current collector 112 used for the positive electrode 110 is not particularly limited as long as it does not induce a chemical change in the battery and has conductivity. For example, as the positive electrode current collector 112, stainless steel; aluminum; nickel; titanium; fired carbon; those obtained by surface treatment with carbon, nickel, titanium, silver, etc. on the surface of aluminum or stainless steel can be used.

[0056] The positive electrode current collector 112 can have a thickness of, for example, 3 μm or more and 500 μm or less. Fine irregularities can also be formed on the surface of the positive electrode current collector 112 to increase the adhesion to the positive electrode active material. The positive electrode current collector 112 can have various forms such as a film, a sheet, a foil, a net, a porous body, a foam, a nonwoven fabric body, etc.

[0057] (Positive electrode active material layer 114) The positive electrode active material layer 114 can contain the above-mentioned positive electrode material. In addition to the above-mentioned positive electrode material, the positive electrode active material layer 114 may contain another conductive material, a binder, any additives, etc.

[0058] The thickness of the positive electrode active material layer 114 may be, for example, 1 μm or more and 500 μm or less, 5 μm or more and 250 μm or less, 10 μm or more and 200 μm or less, 20 μm or more and 150 μm or less, or 50 μm or more and 100 μm or less.

[0059] (Conductive material) The positive electrode active material layer 114 may further contain a conductive material different from the above-mentioned carbon nanotubes. The conductive material is not particularly limited as long as it has conductivity without inducing chemical changes. For example, graphite such as natural graphite and artificial graphite; carbon blacks such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black; conductive fibers such as carbon fibers like VGCF (vapor grown carbon fiber) and metal fibers; metal powders such as carbon fluoride, aluminum, nickel powder; conductive whiskers such as zinc oxide, potassium titanate; conductive metal oxides such as titanium oxide; and one or more mixtures selected from conductive materials such as polyphenylene derivatives. The positive electrode active material layer 114 may contain carbon nanotubes with a length of less than 100 μm as the conductive material.

[0060] (Binder) The positive electrode active material layer 114 can further contain a binder. The binder is for ensuring the adhesive force between the positive electrode active material particles or between the positive electrode active material particles and the current collector. As the binder, a general binder used in the technical field can be used, and its type is not particularly limited. Examples of the binder include vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof. These materials may be used alone or in a mixture of two or more.

[0061] The content of the binder may be, for example, 10% by mass or less, preferably 0.1% by mass to 5% by mass, based on the total mass of the positive electrode active material layer 114. When the content of the binder satisfies the above range, excellent electrode adhesion can be realized while minimizing the increase in electrode resistance.

[0062] [Negative electrode 130] The negative electrode 130 may be composed of lithium metal alone, or may include a negative electrode current collector 134 and a negative electrode active material layer 132 formed on one or both surfaces of the negative electrode current collector 134. The negative electrode active material layer 132 may be formed over the entire surface of the negative electrode current collector 134 or only on a part thereof.

[0063] (Negative electrode current collector 134) The negative electrode current collector 134 used for the negative electrode is not particularly limited as long as it does not induce chemical changes in the battery and has conductivity. For example, as the negative electrode current collector 134, copper; stainless steel; aluminum; nickel; titanium; fired carbon; those obtained by surface treatment with carbon, nickel, titanium, silver, etc. on the surface of copper or stainless steel; aluminum-cadmium alloy, etc. can be used.

[0064] The negative electrode current collector 134 can have a thickness of 3 μm or more and 500 μm or less. Fine irregularities can also be formed on the surface of the negative electrode current collector 134 to enhance the adhesive force with the negative electrode active material. The negative electrode current collector 134 can have various forms such as a film, sheet, foil, net, porous body, foam, non-woven fabric body, etc.

[0065] (Negative electrode active material layer 132) The negative electrode active material layer 132 contains a negative electrode active material and a solid electrolyte. The negative electrode active material layer 132 may contain a conductive material, a binder, and other optional additives as necessary. Since the solid electrolyte, conductive material, and binder are the same as those described for the positive electrode active material layer 114, they will not be described here. Note that the negative electrode active material layer 132 may contain carbon nanotubes with a length of 100 μm or more, similar to the positive electrode active material layer 114.

[0066] (Negative electrode active material) Examples of the negative electrode active material include lithium metal; lithium alloy; lithium metal composite oxide; lithium-containing titanium composite oxide (LTO); carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic compounds capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloy, Sn alloy, and Al alloy; SiO v (0 < v < 2), SnO2, vanadium oxide, lithium vanadium oxide, Li x Fe2O3(0 < x ≤ 1), Li xMetal oxides capable of doping and undoping lithium, such as WO2(0 < x ≤ 1); composites containing the metallic compound and the carbonaceous material, such as Si-C composites and Sn-C composites, etc. These may be used alone or as a mixture of two or more.

[0067] The negative electrode active material may be contained in an amount of 70% by mass to 100% by mass, preferably 80% by mass to 99% by mass, based on the total mass of the negative electrode active material layer 132. When the content of the electrode active material satisfies the above range, excellent energy density, electrode adhesion, and electrical conductivity can be realized.

[0068] [Solid electrolyte layer 120] The solid electrolyte layer 120 is disposed between the positive electrode 110 and the negative electrode 130 and mediates the transfer of lithium ions between the positive electrode active material and the negative electrode active material. The solid electrolyte layer 120 also functions as a separator layer that prevents short circuit between the electrodes by physically separating the positive electrode 110 and the negative electrode 130.

[0069] The solid electrolyte layer 120 is, for example, a layer formed of the same material as the solid electrolyte contained in the positive electrode material. However, the solid electrolyte layer 120 may further contain another solid electrolyte or additive, or may be formed of another solid electrolyte.

[0070] [Manufacturing method of all-solid-state battery] The manufacturing method of the all-solid-state battery 100 is not particularly limited. For example, the all-solid-state battery 100 may be manufactured by sequentially placing the constituent materials of the battery in a cylindrical mold and pressing them. Alternatively, after separately molding each layer of the all-solid-state battery 100, the all-solid-state battery 100 may be manufactured by laminating and pressing them. Other arbitrary methods are available.

[0071] [Effect] To explain the technical effects of the above embodiments, first as a premise, the significance of carbon nanotubes in conventional all-solid-state batteries will be explained. All-solid-state batteries have advantages such as reduced leakage and ignition risks of electrolytes, while also having disadvantages due to being solid. For example, for the smooth movement of lithium ions and electrons in the charge and discharge reactions of all-solid-state batteries, it is important to bring the active material particles into contact with the solid electrolyte particles and suppress the resistance at their interfaces. For this reason, a coating with high ion conductivity such as LiNbO2 is often provided on the positive electrode surface. However, when particles are separated from each other and isolated due to volume expansion during charge and discharge, etc., deterioration of battery characteristics occurs, and conventional general surface coating techniques cannot maintain contact between particles.

[0072] On the other hand, in order to impart conductivity to the electrode material, it is common to add a conductive material containing carbon nanotubes such as VGCF having a high conductivity. However, in order to ensure the conductivity of the entire electrode with VGCF, which is a needle-shaped fiber with a length of several μm, it is necessary to introduce a large amount of conductive material. Also, when the conductive material is insufficient and the active material particles are isolated, adverse effects such as promotion of the decomposition of the solid electrolyte can occur.

[0073] In contrast, carbon nanotubes with a length of 100 μm or more have not been used as conductive materials for solid electrolytes in the past. It is presumed that this is because those skilled in the art recognized that long carbon nanotubes could have an adverse effect on battery characteristics. According to Comparative Example 2 described later, when carbon nanotubes with a length of 50 μm were used, it was confirmed that the carbon nanotubes aggregated and the battery capacity was inferior to the case of using VGCF. Thus, since even 50-μm carbon nanotubes deteriorated battery characteristics in the past, it can be said that those skilled in the art had no motivation to use carbon nanotubes longer than that.

[0074] In contrast, as in the examples described below, the present inventors have found that by using carbon nanotubes having a length of 100 μm or more as a conductive material, the battery capacity and rate characteristics are improved compared to Comparative Example 1 using VGCF. Carbon nanotubes having a length of 100 μm or more are considered to have eliminated the aggregation tendency observed in carbon nanotubes having a length of about 50 μm and to be widely dispersed in the positive electrode material. Due to such dispersion characteristics and the length of the carbon nanotubes themselves, it is presumed that carbon nanotubes having a length of 100 μm or more can form an electron conduction network between the active material particles in the positive electrode material and between the active material particles and the current collector.

[0075] Such characteristics of carbon nanotubes are particularly significant in all-solid-state batteries. Since liquids are fluid, it is also considered that the conductive material can be dispersed to some extent in the slurry during the process of mixing the positive electrode active material and the conductive material to form a slurry. On the other hand, in all-solid-state batteries, since the positive electrode active material and the conductive material are generally mixed in a powder state, if the carbon nanotubes contained in the conductive material have high aggregability, it is difficult to disperse the carbon nanotubes, and it may be difficult to form an electron conduction path between the active material particles far apart. However, if carbon nanotubes having a length of 100 μm or more, which have been found to be able to form an electron conduction path over a long distance due to their own length and have low aggregability, are used, all of the above problems can be solved. As a result, in the examples described below, it is considered that an all-solid-state battery with a high capacity that could not be achieved with VGCF or carbon nanotubes having a length of about 50 μm was realized.

[0076] In addition, in all-solid-state batteries, since a solid electrolyte is used instead of an electrolyte solution, it is important to bring the active material particles into contact with the solid electrolyte particles rather than when using a fluid electrolyte solution. For this reason, the positive electrode material of all-solid-state batteries is manufactured by mixing a positive electrode active material and a solid electrolyte. This ensures contact between the positive electrode active material particles and the solid electrolyte particles, but the contact points between the positive electrode active material particles and the positive electrode current collector are reduced accordingly. Therefore, the significance of long carbon nanotubes that can form an electron conduction path between the positive electrode active material particles existing away from the current collector and the current collector over a relatively long distance is very great.

[0077] Furthermore, when carbon nanotubes that are considerably longer than the active material particles are intertwined with the active material particles, it is considered that the contact between the carbon nanotubes and the active material particles is likely to be maintained even when the volume expansion and contraction of the active material particles due to charge and discharge are repeated. In this way, since long carbon nanotubes can form a so-called matrix structure that absorbs the expansion and contraction of the active material particles, it is considered preferable also from the viewpoints of the cycle characteristics and stability of the battery.

[0078] The advantages of long carbon nanotubes as described above have not been previously recognized. Rather, as described above, there was a recognition that long carbon nanotubes should be avoided in all-solid-state batteries. In view of this, it can be said that the technical significance of the present invention is extremely great.

Example

[0079] Hereinafter, examples and comparative examples will be described, but the present invention is not limited thereto. Also, the considerations described below are merely exemplary speculations for assisting the understanding of the invention and do not limit the present invention in any way.

[0080] <Example 1> [Manufacture of positive electrode material] Positive electrode active material LiNi with an average particle size of about 5 μm 0.8 Co 0.1 Mn 0.161.86 parts by mass of O2 powder coated with LiNbO2 and 36.08 parts by mass of powder of the argyrodite-based solid electrolyte Li6PS5Cl were mixed, and then 2.06 parts by mass of multi-walled carbon nanotubes (MWCNT) with an average length of about 400 μm were added and mixed in a mortar to make it uniform. Thus, a positive electrode material was obtained.

[0081] [Manufacture of All-Solid-State Battery] A positive electrode current collector (SUS plate) 112, a positive electrode material layer 114, a sulfide-based solid electrolyte separator layer 120, a negative electrode Li metal 132 (thickness 0.1 μm), and a negative electrode current collector (SUS plate) 134 were laminated and compressed in this order as shown in Figure 2, thereby fabricating an all-solid-state battery 100.

[0082] [Example 2] A positive electrode material and an all-solid-state battery including the positive electrode material were fabricated in the same manner as in Example 1, except that the average length of MWCNT was about 250 μm.

[0083] [Example 3] A positive electrode material and an all-solid-state battery including the positive electrode material were fabricated in the same manner as in Example 1, except that the average length of MWCNT was about 125 μm.

[0084] [Comparative Example 1] A positive electrode material and an all-solid-state battery including the positive electrode material were fabricated in the same manner as in Example 1, except that carbon nanofibers (product name VGCF-H, manufactured by Showa Denko KK) with an average length of less than 10 μm were used instead of MWCNT.

[0085] [Comparative Example 2] A positive electrode material and an all-solid-state battery including the positive electrode material were fabricated in the same manner as in Example 1, except that the average length of MWCNT was about 50 μm.

[0086] [Evaluation Example 1: Shape Observation] When the positive electrode materials of Examples 1 to 3 and Comparative Example 2 were observed with a scanning electron microscope (SEM), in Examples 1 to 3, as shown in FIG. 1, relatively large positive electrode active material particles 10, relatively small solid electrolyte particles 12, and carbon nanotubes 14 were randomly mixed. It was confirmed that the carbon nanotubes 14 were generally dispersed in a loose state one by one. Many carbon nanotubes 14 were in contact with the surfaces of a plurality of positive electrode active material particles 10 and a plurality of solid electrolyte particles 12. On the other hand, in Comparative Example 2, it was observed that the carbon nanotubes were aggregated with each other. In Comparative Example 2, many carbon nanotubes were in contact with the surfaces of a single positive electrode active material particle 10 and / or a single solid electrolyte particle 12.

[0087] <Evaluation Example 2: Battery Capacity and Rate Characteristics> For each all-solid-state battery of each example and comparative example, in a thermostatic bath maintained at 60°C, with a charge upper limit voltage of 4.25 V and a discharge lower limit voltage of 3 V, aging was performed by charging and discharging twice at a charge rate of 0.05 C and a discharge rate of 0.05 C in terms of 1 C = 200 mAh / g. Here, the "initial capacity" is defined by the following mathematical formula as the value obtained by dividing the discharge capacity in the first charge-discharge process by the discharge capacity (reference value) of Comparative Example 1. [Equation]

[0088] Thereafter, the charging current was fixed at 0.1 C, and a rate test was performed by changing the discharge rate in the order of 0.1 C → 0.2 C → 0.33 C → 0.5 C → 1.0 C. FIGS. 3 and 4 are graphs showing the changes in discharge capacity with respect to the discharge rate of Examples 1 to 3 and Comparative Examples 1 to 2. The vertical axis of FIG. 3 shows the measured value of the discharge capacity, and the vertical axis of FIG. 4 shows the normalized discharge capacity (this normalized discharge capacity is referred to as the "rate characteristic") with the discharge capacity at a rate of 0.1 C being 100% for each example so as to easily compare the changes in the discharge capacity of each example and comparative example.

[0089] Table 1 below shows the manufacturing conditions, initial capacity values, and whether sufficient electron conduction paths were confirmed from SEM observations for Examples 1 to 3 and Comparative Examples 1 and 2. Here, when the carbon nanotubes in the SEM image were in contact with the surfaces of a plurality of positive electrode active material particles 10, it was determined that sufficient electron conduction paths were formed, and if not, it was determined that the formation of electron conduction paths was insufficient.

[0090]

Table 1

[0091] <Evaluation Example 3: Battery Life Characteristics> Next, the cycle characteristics of Examples 1 to 3 and Comparative Example 1 were examined. Specifically, after aging the all-solid-state batteries of Examples 1 to 3 and Comparative Example 1 by charging and discharging twice in the same manner as in Evaluation Example 2, the same charging and discharging were repeated 8 times at a constant current of 0.5C. That is, a total of 10 charging and discharging processes were repeated including the first and second charging and discharging processes. Fig. 5 is a graph showing the cycle characteristics of Examples 1 to 3 and Comparative Example 1. As shown in Fig. 5, it was confirmed that Examples 1 to 3 exhibited superior cycle characteristics compared to Comparative Example 1.

[0092] <Consideration of Evaluation Results of Examples and Comparative Examples> The evaluation results described above are considered below. However, the following consideration is a hypothesis at the current time and does not limit the present invention by theory. As described above, when comparing the initial capacities based on Comparative Example 1 using carbon nanofiber VGCF, which has been conventionally used as a conductive material, the initial capacities of Examples 1 to 3 with a carbon nanotube length of 100 μm or more were superior to those of Comparative Example 1, while the initial capacity of Comparative Example 2 with a carbon nanotube length of about 50 μm was inferior to that of Comparative Example 1.

[0093] Considering the observation results by SEM, carbon nanotubes with a length of 100 μm or more are long enough to contact multiple positive electrode active material particles, and each individual nanotube is loose and easily dispersed. Therefore, it is considered that the electron conductivity of the entire positive electrode material (especially the electron conductivity between the positive electrode active material particles and the electron conductivity between the positive electrode active material and the positive electrode current collector) is improved. As a result, firstly, the carbon nanotubes mediate the electron conduction from the positive electrode active material near the current collector to the current collector, thereby activating and stabilizing the charge and discharge reaction. Secondly, the carbon nanotubes mediate the electron conduction from the positive electrode active material existing away from the current collector to the positive electrode active material near the current collector, and as a result, an electron conduction path from the positive electrode active material existing away from the current collector to the current collector is formed. Thereby, more (ideally all) positive electrode active material particles can contribute to the charge and discharge reaction, so that each positive electrode active material particle in the positive electrode active material layer can participate in the charge and discharge reaction evenly. As a result, it is considered that the battery capacity is improved and a battery capacity close to the theoretical capacity can be obtained.

[0094] Also, as shown in FIGS. 3 and 4, the rate characteristics of Examples 1 to 3 in which the length of the carbon nanotubes was 100 μm or more were extremely excellent compared to Comparative Example 1. That is, even when high-speed discharge was performed, the battery characteristics did not decrease excessively compared to low-speed discharge, and it was confirmed that the battery had excellent high-speed discharge characteristics. By forming an electron conduction path between the positive electrode active material particles and an electron conduction path between the positive electrode active material particles and the positive electrode current collector with carbon nanotubes having a length of 100 μm or more, more positive electrode active materials can participate in the charge and discharge reaction more uniformly and efficiently. Thereby, it is considered that the capacity reduction during high-speed charge and discharge can be suppressed. In addition, since the internal resistance of the electrode is reduced by the widely dispersed carbon nanotubes, the heat loss under a high current can be suppressed, so that efficient charge and discharge is considered to be possible.

[0095] Also, as shown in FIG. 5, the cycle characteristics of Examples 1 to 3, in which the length of the carbon nanotubes was 100 μm or more, were also superior to those of Comparative Example 1. When very long carbon nanotubes are intertwined with the positive electrode active material particles, the above-described electron conduction path can be maintained even when the contraction and expansion of the positive electrode active material accompanying charge and discharge are repeated. As a result of the carbon nanotubes functioning as a conductive matrix that absorbs the contraction and expansion of the positive electrode active material in this way, it is presumed that the life characteristics of the battery are improved.

[0096] On the other hand, carbon nanotubes with a length of 50 μm tend to aggregate with each other. For this reason, the imparting of conductivity by the carbon nanotubes is limited, and the electron conduction between the active material particles and the electron conduction between the active material particles and the current collector cannot be efficiently promoted. As a result, it is considered that sufficient battery capacity and rate characteristics cannot be obtained. Note that carbon nanotubes that are shorter (for example, 10 μm or less in length) generally tend to be more dispersed without aggregating than carbon nanotubes with a length of 50 μm, but it is considered that such short carbon nanotubes cannot sufficiently form an electron conduction path between the active material particles.

Explanation of Reference Numerals

[0097] 10 Positive electrode active material particles 12 Solid electrolyte particles 14 Carbon nanotubes

Claims

1. A lithium transition metal oxide, a solid electrolyte, and carbon nanotubes, wherein one or more of the carbon nanotubes have a length of 100 μm or more, a positive electrode material for an all-solid-state battery.

2. One or more of the carbon nanotubes have a length of 100 μm or more and 500 μm or less, The positive electrode material according to Claim 1.

3. One or more of the carbon nanotubes have a length of 100 μm or more and 200 μm or less, The positive electrode material according to Claim 1.

4. Positive electrode active material particles containing a lithium transition metal oxide, having an average particle size of 1 μm to 20 μm, a solid electrolyte, and carbon nanotubes that are in contact with a plurality of positive electrode active material particles to form an electron conduction path between the positive electrode active material particles, A positive electrode material for an all-solid-state battery.

5. The content of the carbon nanotubes in the positive electrode material is 0.01% by mass or more and 10% by mass or less, The positive electrode material according to any one of Claims 1 to 4.

6. The carbon nanotubes are multi-walled carbon nanotubes, The positive electrode material according to any one of Claims 1 to 4.

7. The carbon nanotubes form an electron conduction path between a plurality of particles that are not in contact with each other, The positive electrode material according to any one of Claims 1 to 4.

8. The carbon nanotubes are substantially dispersed without forming aggregates or bundles, The positive electrode material according to any one of Claims 1 to 4.

9. The lithium transition metal oxide contains 50 mol% or more of nickel based on the total amount of transition metals, The positive electrode material according to any one of Claims 1 to 4.

10. The solid electrolyte is a sulfide-based solid electrolyte, The positive electrode material according to any one of Claims 1 to 4.

11. The carbon nanotubes have an average diameter in the range of 2 nm to 20 nm, The positive electrode material according to any one of Claims 1 to 4.

12. The carbon nanotubes have an aspect ratio in the range of 10,000 to 100,000, The positive electrode material according to any one of Claims 1 to 4.

13. A positive electrode including the positive electrode material according to any one of Claims 1 to 4, a negative electrode, and a solid electrolyte layer between the positive electrode and the negative electrode, An all-solid-state battery.

14. ​ A method for manufacturing a positive electrode material for an all-solid-state battery, the method including a step of mixing a lithium transition metal oxide, a solid electrolyte, and a carbon nanotube having a length of 100 μm or more. The method for manufacturing a positive electrode material for an all-solid-state battery.

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