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

Abstract

Provided are: a cathode material capable of improving the battery characteristics of an all-solid-state battery; an all-solid-state battery; and a method for producing the cathode material. 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, wherein at least one carbon nanotube among the carbon nanotubes has a length of at least 100 µm.

Description

Cathode material, all-solid-state battery, and method for producing cathode material This application claims the benefit of priority based on Japanese Patent Application No. 2023-221543, filed December 27, 2023. Embodiments of the present invention relate to a cathode material, an all-solid-state battery, and a method for producing a cathode material. Currently, lithium ion secondary batteries using liquid electrolytes are widely used. However, when using liquid electrolytes, if deformation or external impact occurs, there is a risk of short circuits occurring, which may lead to overheating or explosion. In order to improve the safety of lithium ion secondary batteries, all-solid-state batteries that do not use organic electrolytes have recently been spotlighted as next-generation batteries, and various research and developments are being conducted. Carbon nanotubes are known as a conductive material added to electrode materials for all-solid-state batteries. Carbon nanotubes that have been used as conductive materials for all-solid-state batteries have been mainly those with a length of 10 ㎛ or less, and at most tens of ㎛ (see, for example, Patent Documents 1 to 3). As an exception, Patent Document 4 discloses an all-solid-state battery positive electrode having a molded body of a positive electrode mixture containing a positive electrode active material, a sulfide-based solid electrolyte, and a conductive agent, wherein the conductive agent comprises fibrous carbon and granular carbon, and the thickness of the molded body of the positive electrode mixture is 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 specifically describe the reason why the numerical range is preferable, and only describes an example using fibrous carbon VGCF (vapor-grown carbon fiber) from Showa Denko as an example that actually demonstrated the effect. The fiber length of the VGCF is about 10 µm, and Patent Document 4 does not describe specific experimental results on carbon nanotubes having a length exceeding 10 µm. [Prior art literature] [Patent Document] Patent Document 1: Japanese Patent Publication No. 2020-507893 Patent Document 2: Japanese Patent Publication No. 2022-529987 Patent Document 3: Japanese Patent Publication No. 2023-132317 Patent Document 4: Japanese Patent Publication No. 2021-144906 Even with reference to the above patent documents, it is not clear why carbon nanotubes exceeding several tens of micrometers in length have been avoided. However, the inventors of the present invention have found out in the course of the present invention that carbon nanotubes having a length of about 50 micrometers tend to aggregate with each other and are difficult to disperse sufficiently within the positive electrode. The inventors of the present invention presume that, as a result of this property, it has been recognized that when carbon nanotubes exceeding several tens of micrometers in length are used as a conductive material, battery performance is not significantly improved. By adding carbon nanotubes of a length of tens of micrometers or less, which have been used conventionally, the battery characteristics such as charge / 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 demanded. The problem that the present invention seeks to solve is to provide a cathode material capable of improving the battery characteristics of an all-solid-state battery, an all-solid-state battery, and a method for manufacturing the cathode material. The inventors of the present invention conceived that if carbon nanotubes having a length of 100 μm or more are used as a conductive material, battery characteristics may be improved by efficiently forming an electron conduction path between electrode active materials. Accordingly, the inventors of the present invention examined the influence of the use of carbon nanotubes of various lengths exceeding 50 μm, which had been conventionally avoided, on battery characteristics, and found that by using carbon nanotubes having a length of 100 μm or more as a conductive material, the aggregation tendency observed in carbon nanotubes having a length of about 50 μm is suppressed. Accordingly, the inventors of the present invention found long carbon nanotubes suitable for practical use, and completed the present invention. The present invention may include the following forms. [1] Lithium transition metal oxide, solid electrolyte, and Contains carbon nanotubes, A cathode material for an all-solid-state battery, wherein at least one of the above carbon nanotubes has a length of 100 μm or more. [2] The cathode material described in [1], wherein at least one of the carbon nanotubes has a length of 100 ㎛ or more and 500 ㎛ or less. [3] The cathode material described in [1], wherein at least one of the carbon nanotubes has a length of 100 ㎛ or more and 200 ㎛ or less. [4] Positive electrode active material particles containing lithium transition metal oxide and having an average particle size of 1 ㎛ to 20 ㎛, solid electrolyte, and An all-solid-state battery cathode material comprising carbon nanotubes that come into contact with a plurality of cathode active material particles and form an electronic conduction path between the cathode active material particles. [5] The cathode material according to any one of [1] to [4], wherein the content of the carbon nanotubes in the cathode material is 0.01 mass% or more and 10 mass% or less. [6] The cathode material according to any one of [1] to [5], wherein the carbon nanotube is a multilayer carbon nanotube. [7] The anode material according to any one of [1] to [6], wherein the carbon nanotube forms an electronic conduction path between a plurality of particles that are not in contact with each other. [8] The cathode material according to any one of [1] to [7], wherein the carbon nanotubes are substantially free of aggregates or bundles. [9] The positive electrode material according to any one of [1] to [8], wherein the lithium transition metal oxide contains nickel in an amount of 50 mol% or more based on the total amount of transition metal.

[0010] The above solid electrolyte is a cathode material according to any one of [1] to [9], which is a sulfide-based solid electrolyte.

[0011] The above carbon nanotube is a cathode material according to any one of [1] to

[0010] , having an average diameter of 2 nm to 20 nm.

[0012] The cathode material according to any one of [1] to

[0011] , wherein the carbon nanotube has an aspect ratio of 10,000 to 100,000.

[0013] An anode comprising a cathode material as described in any one of [1] to

[0012] , cathode, and An all-solid-state battery having a solid electrolyte layer between the positive electrode and the negative electrode.

[0014] A method for producing a cathode material for an all-solid-state battery, comprising the step of mixing a lithium transition metal oxide, a solid electrolyte, and carbon nanotubes having a length of 100 μm or more. According to the present invention, it is possible to provide a cathode material capable of improving the battery characteristics of an all-solid-state battery, an all-solid-state battery, and a method for manufacturing the cathode material. Figure 1 is a schematic diagram showing the microscopic structure of a cathode material according to an embodiment. Figure 2 is a schematic diagram showing the structure of an all-solid-state battery according to an embodiment. Figure 3 is a graph showing the change in discharge capacity with respect to the discharge rate of Examples 1 to 3 and Comparative Examples 1 and 2. Figure 4 is a graph showing the change in discharge capacity with respect to the discharge rate of Examples 1 to 3 and Comparative Examples 1 and 2. Figure 5 is a graph showing the cycle characteristics of Examples 1 to 3 and Comparative Example 1. Hereinafter, the cathode material, the all-solid-state battery, and the method for manufacturing the cathode material according to the embodiment will be described. In addition, the following embodiment shows one form of the present invention, and does not limit the present invention, and may be arbitrarily changed within the scope of the technical idea of ​​the present invention. In addition, each configuration and each feature of the embodiment may be arbitrarily combined. Hereinafter, singular expressions are used to mean 'one or plural' unless the context clearly indicates that only the singular is indicated. 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 underneath" the other part, but also the case where there is another part in between. Furthermore, in this specification, being placed "on" may include the case where it is placed below as well as above. In this specification, the average particle diameter (D 50 ) can be defined as the particle size corresponding to 50% of the volume accumulation in the particle size distribution curve. The average particle size (D 50) can be measured, for example, using the laser diffraction method. The laser diffraction method can generally measure particle diameters from the submicron range to several mm, and can obtain results with high reproducibility and high resolution. In this specification, a 'single-walled carbon nanotube' (SWCNT) means a carbon nanotube in which a tube-shaped wall made of carbon atoms is composed of a single atomic layer (i.e., one layer of graphene sheets). A 'multi-walled carbon nanotube' (MWCNT) means a carbon nanotube in which a tube-shaped wall made of carbon atoms is composed of multiple atomic layers (i.e., multiple layers of graphene sheets). <Bipolar material> A cathode material according to the present embodiment comprises a lithium transition metal oxide, a solid electrolyte, and carbon nanotubes, wherein at least one of the carbon nanotubes has a length of 100 μm or more. Fig. 1 is a schematic diagram showing the microscopic structure of a cathode material. The cathode material includes lithium transition metal oxide particles (10), solid electrolyte particles (12), and carbon nanotubes (14). In Fig. 1, the solid electrolyte particles (12) are smaller than the lithium transition metal oxide particles (10) and are scattered around the lithium transition metal oxide particles (10). During a charge and discharge process, lithium ions move between the lithium transition metal oxide particles (10) and the solid electrolyte particles (12) that are in contact with each other. The carbon nanotubes (14) are entangled with the lithium transition metal oxide particles (10). Some of the carbon nanotubes (14) are in contact with a plurality of lithium transition metal oxide particles (10). During a charge and discharge process, the carbon nanotubes (14) can form an electron conduction path from the lithium transition metal oxide particles (10) to other particles (10) or a current collector. The electron conduction occurs in conjunction with lithium ion conduction. [lithium transition metal oxide] During charging and discharging, the lithium transition metal oxide functions as a positive electrode active material that absorbs and releases lithium ions from the positive electrode in cooperation with the negative electrode active material described below. The lithium transition metal oxide may be one generally used as a positive electrode active material in the relevant technical field. The lithium transition metal oxide may be a compound capable of reversible insertion (intercalation) and extraction (deintercalation) of lithium, and its type is not particularly limited. Specific examples thereof include lithium metal composite oxides containing lithium and one or more metals such as cobalt, manganese, nickel, copper, vanadium, and aluminum. More specifically, examples of such lithium metal composite oxides include lithium-manganese oxides (e.g., LiMnO2, LiMnO3, LiMn2O3, LiMn2O4, etc.); lithium-cobalt oxides (e.g., LiCoO2, etc.); lithium-nickel oxides (e.g., LiNiO2, etc.); lithium-copper oxides (e.g., Li2CuO2, etc.); lithium-vanadium oxides (e.g., LiV3O8, etc.); lithium-nickel-manganese oxides (e.g., LiNiO2, etc.); 1-z Mn z O2(0<z<1), LiMn 2-z Ni z O4(0<z<2) etc.); lithium-nickel-cobalt oxides (e.g., LiNi 1-y Co y O2(0<y<1) etc.); lithium-manganese-cobalt oxides (e.g., LiCo 1-z Mn z O2(0<z<1), LiMn 2-y Co y O4(0<y<2) etc.); lithium-nickel-manganese-cobalt oxides (e.g., 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) oxide (e.g., 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, and 0<x<1, 0<y<1, 0<z<1, 0<w<1, x+y+z+w=1) etc.); Li-excessive solid solution anode (e.g., 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); Compounds in which the transition metal element in these compounds is partially replaced with one or more other metal elements may be mentioned. The positive electrode active material layer may contain one or more of these compounds. However, it is not limited to these. In particular, in lithium transition metal oxides with a high nickel content that are effective in increasing the capacity of batteries, it is preferable that the lithium transition metal oxide contains nickel of 50 mol% or more based on the total amount of transition metal. As an example of such lithium transition metal oxides, Li a NiO2(0.5≤a≤1.5); Li a (Ni x Co y Mn z )O2(0.5≤a≤1.50.5≤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 yO2(0.5≤a≤1.5, 0<y≤0.5); Li a Ni 1-z Mn z 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, and 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, and 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 a transition metal atom in these compounds is at least partially substituted with another one or more metal elements (for example, one or more of Al, Fe, V, Cr, Ti, Ta, Mg, Mo, Zr, Zn, Ga and In); compounds in which an oxygen atom in these compounds is partially substituted with another one or more non-metallic elements (for example, one or more of P, F, S and N). Preferably, the lithium transition metal oxide is Li a Ni x M yO2 (M is one or more metal elements other than Ni, and 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, and 0<a≤1.05, x+y=1), and the value of x can be, for example, 0.5 or more, 0.6 or more, 0.7 or more, 0.8 or more, or 0.9 or more, and can 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 may include one or more of the above, but is not limited thereto. In addition, even within the same particle, there may be a distribution in the substituted concentration in the interior and the surface layer. In addition, it may be coated on the surface of the particle. Examples include, but are not limited to, surfaces coated with metal oxides, lithium transition metal oxides, polymers, etc. 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), Lia (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 preferable. Here, the values ​​of a are all, for example, 0.5≤a≤1.5, and preferably 1.0≤a≤1.5. More specifically, LiNiO2, Li(Ni 0.5 Mn 0.3 Co 02 )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 Co0.05 Mn 0.05 )O 2, Li(Ni) 0.9 Co 0.05 Al 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. 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. When the particle size of the lithium transition metal oxide is 10 nm or more, surface deterioration of the particles can be suppressed. When 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 also, an electron conduction path by the carbon nanotube described later can be efficiently formed. 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. By means of such a coating, the internal resistance of the positive electrode can be reduced. [Solid electrolyte] The solid electrolyte assists in the transfer of lithium ions between the positive active material and the negative active material. The particles of the solid electrolyte can contact the positive active material particles, the negative active material particles, or other solid electrolyte particles to mediate the transfer of lithium ions. The solid electrolyte is a material with high ionic conductivity (e.g., ionic conductivity of 10) to mainly transfer lithium ions within the electrode. -5 s / m or more, preferably 10 -4All materials (s / m or higher) can be used, and are not limited to specific components. The solid electrolyte may be at least one selected from the group consisting of, for example, a sulfide-based solid electrolyte, an oxide-based solid electrolyte, and a polymer-based solid electrolyte. 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 containing an organic solvent and a lithium salt, an ionic liquid, a monomer, or an oligomer, etc., is incorporated into a polymer resin. Meanwhile, a sulfide-based solid electrolyte has high ionic conductivity, and an oxide-based solid electrolyte has excellent electrochemical stability. Therefore, an appropriate solid electrolyte component may be selected and used depending on the characteristics of the solid electrolyte and the intended use of the battery. Preferably, the solid electrolyte is a sulfide-based solid electrolyte. The sulfide-based solid electrolyte contains sulfur (S) and has the 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-PS-based glass or Li-PS-based glass ceramic. The sulfide-based solid electrolyte may include a glassy solid electrolyte, a crystalline solid electrolyte, and a glass ceramic solid electrolyte. 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, Argylodite-based solid electrolytes (e.g., Li6PS5X (X: Cl, Br, I)), LGPS-based solid electrolytes (Li 10 GeP2S 12 , Li 3.25 Ge 0.25 P 0.75 S4, etc.), Li7P3S 11Examples thereof include, but are not limited to, the solid electrolyte. For example, the solid electrolyte may contain one or more selected from the group consisting of the above compounds. Oxide-based solid electrolytes contain oxygen (O) and have the ionic conductivity of a metal belonging to Group 1 or Group 2 of the periodic table. Examples of oxide-based solid electrolytes include LLTO compounds, Li6La2CaTa2O 12 , Li6La2ANb2O 12 (A: Ca or Sr), Li2Nd3TeSbO 12 , Li3BO 2.5 N 0.5 , Li9SiAlO8, LAGP compounds, LATP compounds, Li 1+x Ti 2-x Al x Si y (PO4) 3-y (Here, 0≤x≤1, 0≤y≤1), LiAl x Zr 2-x (PO4)3(where, 0≤x≤1, 0≤y≤1), LiTi x Zr 2-x (PO4)3(0≤x≤1, 0≤y≤1), LISICON compounds, LIPON compounds, perovskite compounds, NASICON compounds, LLZO compounds, etc., but are not limited thereto. For example, the solid electrolyte may contain at least one selected from the group consisting of the above compounds. The polymer solid electrolyte may contain, for example, a polyester polymer, a polycarbonate polymer, an acrylate polymer, a polysiloxane polymer, a phosphazene polymer, a polyethylene derivative, an alkylene oxide derivative such as PEO (polyethylene oxide) or PPO (polypropylene oxide), a phosphoric acid ester polymer, polyagitation lysine, polyester sulfide, polyvinyl alcohol, polyvinylidene fluoride, a polymer containing an ionic dissociation group, and the like. For example, the solid electrolyte may contain at least one selected from the group consisting of the above compounds. The content of the solid electrolyte may be, for example, 1 mass% to 70 mass%, 5 mass% to 50 mass%, 10 mass% to 45 mass%, or 20 mass% to 40 mass% based on the total weight of the positive electrode active material. When the content of the solid electrolyte particles is 1 mass% or more, the solid electrolyte particles can sufficiently form a conduction path for lithium ions by surrounding the particle surfaces of the lithium transition metal oxide. When the content of the solid electrolyte particles is 50 mass% or less, the solid electrolyte particles can be suppressed from agglomerating and separating from the lithium transition metal oxide. 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. When the particle size of the solid electrolyte is within the above range, the transfer of lithium ions between the active material and the solid electrolyte can be smoothly performed at an appropriate electrode density or tap density. [Carbon Nanotubes] Carbon nanotubes mainly come into contact with the surface of the positive electrode active material particles or the positive electrode current collector, and contribute to forming a conductive network between the positive electrode active material particles or between the positive electrode active material particles and the positive electrode current collector. Carbon nanotubes are not in a bundle shape, but mostly exist in single strands, and are mainly arranged on the surface of the positive electrode active material. Among the carbon nanotubes included in the positive electrode material, at least one carbon nanotube has a length of 100 ㎛ or more. The length of the carbon nanotube may be, for example, 100 ㎛ to 500 ㎛, 100 ㎛ to 450 ㎛, 100 ㎛ to 400 ㎛, 100 ㎛ to 350 ㎛, 100 ㎛ to 300 ㎛, 100 ㎛ to 250 ㎛, or 100 ㎛ to 200 ㎛. The length of the carbon nanotube may be 110 ㎛ or more, 120 ㎛ or more, 130 ㎛ or more, 140 ㎛ or more, 150 ㎛ or more, 180 ㎛ or more, 200 ㎛ or more, 210 ㎛ or more, 230 ㎛ or more, or 250 ㎛ or more. From the results of the examples and comparative examples described below, it is presumed that carbon nanotubes having a length of 100 μm or more can efficiently form an electronic conduction path between active material particles or between active material particles and a current collector without excessive agglomeration of the carbon nanotubes. On the other hand, carbon nanotubes having a length of 500 μm or less can suppress extremely long carbon nanotubes from sticking closely to each other and agglomerating into bundles. 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, the carbon nanotubes having a length of 100 µm or more can efficiently form an electron conduction path between active material particles or between active material particles and a current collector without excessively agglomerating with each other during electrode manufacture. In addition, a conductive network can be formed efficiently even with a small amount of conductive material. Here, the 'average length' means the average of the lengths of the top 100 carbon nanotubes and the bottom 100 carbon nanotubes observed by SEM. Preferably, the carbon nanotubes are substantially free of aggregates or bundles. For example, when observed with a SEM, it is preferable that 50% (by number) 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 do not come into contact with two or more other carbon nanotubes. Alternatively, when observed with a SEM, it is preferable that 50% (by number) 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 do not come into contact with two or more other carbon nanotubes. Alternatively, when observed with a SEM, it is preferable that 50% (by number) 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 do not come into contact with other carbon nanotubes. The average diameter of the carbon nanotubes is not particularly limited, but may be, for example, 1 nm to 3 μm, 5 nm to 1 μm, 10 nm to 500 nm, or 20 nm to 100 nm. In a specific embodiment, the average diameter of the carbon nanotubes may be 2 nm to 5 nm, 7 nm to 10 nm, or 10 nm to 100 nm. In another specific embodiment, the average diameter of the carbon nanotubes may be 20 nm to 15 nm, or 10 nm to 10 nm. When the above range is satisfied, the carbon nanotubes are easily dispersed in the anode without being excessively aggregated. Here, the 'average diameter' means the average value of the diameters of the top 100 carbon nanotubes and the bottom 100 carbon nanotubes in terms of diameter size observed by SEM. Meanwhile, in a specific embodiment of the present invention, the carbon nanotube according to the present invention may have an aspect ratio in a range of 10,000 to 100,000. The 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 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 aspect ratio may be appropriately adjusted so that the diameter and length of the carbon nanotube described above satisfy the aspect ratio. In one embodiment, the term 'aspect ratio' may mean the ratio of length to diameter (length / diameter). In one embodiment, the aspect ratio may mean the ratio of average length to average diameter (average length / average diameter). Carbon nanotubes can be single-layer carbon nanotubes or multi-layer carbon nanotubes. Multi-layer carbon nanotubes are easier to manufacture than single-layer carbon nanotubes and are superior in cost. The content of carbon nanotubes is, for example, 0.01 mass% or more and 10 mass% or less, 0.05 mass% or more and 8 mass% or less, 0.1 mass% or more and 6 mass% or less, 0.2 mass% or more and 5 mass% or less, 0.5 mass% or more and 4 mass% or less, or 1 mass% or more and 3 mass% or less, based on the total mass of the positive electrode material. When the above range is satisfied, a conductive network within the positive electrode can be efficiently formed while maintaining the dispersibility and stability of the carbon nanotubes, thereby improving the charge / discharge characteristics of the battery. The content of carbon nanotubes is, for example, 0.01 mass% or more and 20 mass% or less, 0.05 mass% or more and 15 mass% or less, 0.1 mass% or more and 12 mass% or less, 0.2 mass% or more and 10 mass% or less, 0.5 mass% or more and 8 mass% or less, or 1 mass% or more and 5 mass% or less, based on the total mass of the lithium transition metal oxide. When the above range is satisfied, since a sufficient amount of carbon nanotubes is present with respect to the lithium transition metal oxide, a conductive network within the positive electrode can be efficiently formed. According to another embodiment, a cathode material for an all-solid-state battery includes cathode active material particles containing a lithium transition metal oxide and having an average particle diameter of 1 µm to 20 µm, a solid electrolyte, and carbon nanotubes in contact with a plurality of cathode active material particles to form an electron conduction path between the cathode active material particles. Since carbon nanotubes have high electron conductivity, by coming into contact with a plurality of positive electrode active material particles, an electron conduction path is formed between the active material particles. The carbon nanotubes have a length sufficient to be in contact with a plurality of positive electrode active material particles having an average particle diameter of 1 µm to 20 µm. For example, when observed with an SEM, it is preferable that 50% (based on the number) 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 positive electrode active material particles. When viewed from 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 with a SEM, it is preferable that 50% (by number) 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 the carbon nanotubes. In addition, when observed with a SEM, it is preferable that 50% (by number) 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 the carbon nanotubes, and further, the carbon nanotubes are in contact with other positive electrode active material particles. From another viewpoint, it is preferable that the length of at least one carbon nanotube is larger than the average particle diameter of the positive electrode active material particles. For example, the length of at least one carbon nanotube is at least 5 times, at least 10 times, at least 20 times, or at least 30 times the average particle diameter of the positive electrode active material particles. Preferably, the average length of the carbon nanotube is larger than the average particle diameter of the positive electrode active material particles, for example, at least 5 times, at least 10 times, at least 20 times, or at least 30 times the average particle diameter of the positive electrode active material particles. As described above, carbon nanotubes can form an electron conduction path between a plurality of particles that are not in contact with each other. Accordingly, electron conduction within the positive electrode can be facilitated, so that the active material particles within the positive electrode can be uniformly involved in the charge / discharge reaction. As a result, the battery capacity can be improved. In addition, since carbon nanotubes have excellent electron conductivity, energy loss such as heat generation under high current can be suppressed. As a result, the rate characteristics (especially, high-speed charge / discharge characteristics) can be improved. Meanwhile, 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 come into contact with the positive electrode active material particles through the coating, but 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 contact, but may be indirect contact through another configuration, as long as the carbon nanotubes form an electron conduction path that mediates electron conduction between the positive electrode active material particles. In addition, the carbon nanotubes may support the positive electrode active material particles by adsorption or the like, but they do not have to be in simple contact and do not have to support the positive electrode active material particles. For example, the carbon nanotubes and the positive electrode active material particles do not have to be adsorbed. <Method for manufacturing positive electrode material> A method for manufacturing a cathode 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. The lithium transition metal oxide, the solid electrolyte, and the carbon nanotubes are as described above. By mixing them, an electronic conduction path can be formed between the particles of the lithium transition metal oxide with the carbon nanotubes intervening therebetween. The above mixing step can be performed using, but is not limited to, 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. that are generally used for mixing powders. 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 any liquid medium. The order of adding the lithium transition metal oxide, the solid electrolyte, and the carbon nanotubes is not particularly limited. In addition, other additives may be further added. <All-solid-state battery> An all-solid-state battery according to the present embodiment comprises a cathode including the cathode material, a cathode, and a solid electrolyte layer between the cathode and the anode. FIG. 2 is a schematic diagram showing the structure of an all-solid-state battery (100) according to an 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 solely of metallic lithium, the negative electrode material layer (132) and the negative electrode current collector (134) are provided integrally. Overall, 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. [Polar (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 side or both sides of the positive electrode current collector (112). The positive electrode active material layer (114) may be formed on the entire surface of the positive electrode current collector (112) or may be formed on only a portion of the surface. (Positive current collector (112)) The cathode current collector (112) used in the cathode (110) is not particularly limited as long as it does not cause a chemical change in the battery and has conductivity. For example, stainless steel; aluminum; nickel; titanium; sintered carbon; aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. may be used as the cathode current collector (112). The positive electrode current collector (112) may have a thickness of, for example, 3 ㎛ or more and 500 ㎛ or less. Fine unevenness may be formed on the surface of the positive electrode current collector (112) to increase adhesion to the positive electrode active material. The positive electrode current collector (112) may have various forms, for example, a film, a sheet, a foil, a net, a porous body, a foam, a non-woven body, etc. (Positive active material layer (114)) The positive electrode active material layer (114) may include the positive electrode material described above. In addition to the positive electrode material, the positive electrode active material layer (114) may also include other conductive materials, binders, optional additives, etc. 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. (challenge) The positive electrode active material layer (114) may further include a conductive material other than the carbon nanotubes. The conductive material is not particularly limited as long as it does not cause a chemical change and has conductivity. For example, it may include one or a mixture of two or more selected from the following conductive materials: graphite such as natural graphite or artificial graphite; carbon black such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, etc.; conductive fibers such as carbon fibers or metal fibers such as VGCF (vapor-grown carbon fibers); metal powders such as fluorinated carbon, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives. The positive electrode active material layer (114) may include carbon nanotubes having a length of less than 100 μm as the conductive material. (bookbinder) The positive electrode active material layer (114) may further include a binder. The binder is for securing adhesion 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 relevant technical field can be used, and its type is not particularly limited. As the binder, for example, vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, starch, hydroxy propyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof may be mentioned. These materials may be used alone or as a mixture of two or more. The content of the binder may be, for example, 10 mass% or less, and preferably 0.1 mass% to 5 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. [Cathode (130)] The negative electrode (130) may be composed solely of lithium metal, or may include a negative electrode current collector (134) and a negative electrode active material layer (132) formed on one or both sides of the negative electrode current collector (134). The negative electrode active material layer (132) may be formed on the entire surface of the negative electrode current collector (134) or only on a portion of the surface. (Cathode current collector (134)) The negative electrode current collector (134) used in the negative electrode is not particularly limited as long as it is conductive and does not cause a chemical change in the battery. For example, as the negative electrode current collector (134), copper; stainless steel; aluminum; nickel; titanium; sintered carbon; copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc.; aluminum-cadmium alloy, etc. can be used. The negative electrode current collector (134) may have a thickness of 3 ㎛ or more and 500 ㎛ or less. Fine unevenness may be formed on the surface of the negative electrode current collector (134) to increase adhesion to the negative electrode active material. The negative electrode current collector (134) may have various forms, such as a film, a sheet, a foil, a net, a porous body, a foam, a non-woven fabric, etc. (Cathode active material layer (132)) The negative electrode active material layer (132) includes a negative electrode active material and a solid electrolyte. The negative electrode active material layer (132) may include a conductive material, a binder, and other optional additives as needed. The solid electrolyte, conductive material, and binder are the same as those described for the positive electrode active material layer (114), so they are omitted. Meanwhile, the negative electrode active material layer (132), like the positive electrode active material layer (114), may include carbon nanotubes having a length of 100 μm or more. (negative active material) As the negative electrode active material, for example, 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 x Examples thereof include metal oxides capable of lithium doping and dedoping, such as WO2 (0<x≤1); composites containing the above metal compounds and carbonaceous materials, such as Si-C composites and Sn-C composites. These may be used alone or as a mixture of two or more. The negative electrode active material may be included in an amount of 70 mass% to 100 mass%, preferably 80 mass% to 99 mass%, relative to 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. [Solid electrolyte layer (120)] The solid electrolyte layer (120) is arranged 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 physically separates the positive electrode (110) and the negative electrode (130) and prevents short circuits between the electrodes. The solid electrolyte layer (120) is a layer formed of a material such as a solid electrolyte included in the positive electrode material, for example. However, the solid electrolyte layer (120) may further include another solid electrolyte or additive, or may be formed of another solid electrolyte. <Method for manufacturing an all-solid-state battery> The method for manufacturing the all-solid-state battery (100) is not particularly limited. For example, the all-solid-state battery (100) may be manufactured by sequentially putting the battery's constituent materials into a cylindrical mold and applying pressure. Alternatively, the all-solid-state battery (100) may be manufactured by molding each layer of the all-solid-state battery (100) separately, and then stacking and applying pressure to them. In addition, any method may be used. <Effect> In order to explain the technical effect of the above embodiment, first, as a premise, the significance of carbon nanotubes in conventional all-solid-state batteries is explained. All-solid-state batteries have merits such as reduced risk of electrolyte leakage and ignition, but they also have demerits due to being solid. For example, in order to smoothly move lithium ions and electrons in the charge / discharge reaction of an all-solid-state battery, it is important to bring the active material particles and the solid electrolyte particles into contact and suppress the resistance at their interface. For this reason, a coating with high ion conductivity such as LiNbO2 is often provided on the surface of the positive electrode. However, since the battery characteristics deteriorate when the particles become separated and isolated due to volume expansion accompanying charge / discharge, it is impossible to maintain contact between the particles using conventional surface coating technologies. Meanwhile, in order to provide conductivity to the electrode material, it is also common to add a conductive material including carbon nanotubes such as VGCF having high conductivity. However, in order to secure conductivity of the entire electrode by the VGCF of needle-like fibers having a length of several micrometers, it is necessary to introduce a large amount of conductive material. In addition, if the conductive material is insufficient and the active material particles are isolated, adverse effects such as accelerated decomposition of the solid electrolyte may occur. In this regard, carbon nanotubes longer than 100 μm have not been used as conductive materials for solid electrolytes. It is presumed that this is because those skilled in the art have recognized that long carbon nanotubes can have a negative effect on battery characteristics. According to Comparative Example 2 described below, 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 that when VGCF was used. Thus, it can be said that those skilled in the art have not had any motivation to use carbon nanotubes longer than that because even 50 μm carbon nanotubes deteriorate battery characteristics in the past. In this regard, the inventors of the present invention found that, as in the examples described below, 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. It is thought that carbon nanotubes having a length of 100 μm or more are widely dispersed in the positive electrode material, since the aggregation tendency observed in carbon nanotubes having a length of about 50 μm is eliminated. It is presumed that, due to such dispersion characteristics and the length of the carbon nanotubes themselves, carbon nanotubes having a length of 100 μm or more can form an electronic conduction network between active material particles in the positive electrode material and between the active material particles and the current collector. This characteristic of carbon nanotubes is particularly significant in all-solid-state batteries. Since liquids are fluid, it is thought that in the process of mixing the positive electrode active material and the conductive material to form a slurry, the conductive material can be dispersed to a certain extent in the 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 included in the conductive material have high cohesion, it may be difficult for the carbon nanotubes to disperse, making it difficult to form an electron conduction path between active material particles that are far apart. However, if carbon nanotubes of 100 μm or more in length, which are long in themselves and can form an electron conduction path far away and have been found to have low cohesion, are used, all of the above problems can be solved. As a result, in the examples described below, it is thought that a high-capacity all-solid-state battery that could not be realized with VGCF or carbon nanotubes of about 50 μm can be realized. In addition, in an all-solid-state battery, since a solid electrolyte is used instead of an electrolyte, it is more important to bring the active material particles into contact with the solid electrolyte particles than when a fluid electrolyte is used. For this reason, the cathode material of an all-solid-state battery is manufactured by mixing a cathode active material and a solid electrolyte. Accordingly, contact between the cathode active material particles and the solid electrolyte particles is secured, but the contact point between the cathode active material particles and the cathode current collector is reduced accordingly. Therefore, the significance of long carbon nanotubes that can form a relatively long-distance electron conduction path between the cathode active material particles and the current collector located far from the current collector can be said to be very great. In addition, since the carbon nanotubes, which are considerably longer than the active material particles, are entangled with the active material particles, it is thought that even if the volume expansion and contraction of the active material particles due to charge and discharge are repeated, the contact between the carbon nanotubes and the active material particles is easily maintained. In this way, since the long carbon nanotubes can form a matrix structure, so to speak, that absorbs the expansion and contraction of the active material particles, it is thought to be desirable from the viewpoint of the cycle characteristics and stability of the battery. The advantages of long carbon nanotubes such as the above were not previously recognized, and rather, as mentioned above, there was a perception that long carbon nanotubes should be avoided in all-solid-state batteries. Considering this, the technical significance of the present invention can be said to be very great. 《Example》 Hereinafter, examples and comparative examples will be described. However, the present invention is not limited thereto. In addition, the considerations described below are merely exemplary conjectures to assist in understanding the invention, and do not limit the present invention at all. <Example 1> [Manufacture of bipolar materials] LiNi, a cathode active material with an average particle size of approximately 5㎛ 0.8 Co 0.1 Mn 0.1 After mixing 61.86 parts by mass of O2 powder coated with LiNbO2 and 36.08 parts by mass of argyrodite-based solid electrolyte Li6PS5Cl powder, 2.06 parts by mass of multilayer carbon nanotubes (MWCNTs) having an average length of about 400 μm were added and mixed uniformly in a mortar. Thus, a cathode material was obtained. [Manufacturing of all-solid-state batteries] An all-solid-state battery (100) was manufactured by stacking and compressing 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 ㎛), and a negative electrode current collector (SUS plate) (134) in this order as shown in Fig. 2. <Example 2> A cathode material and an all-solid-state battery including the cathode material were manufactured in the same manner as in Example 1, except that the average length of the MWCNT was set to about 250 μm. <Example 3> A cathode material and an all-solid-state battery including the cathode material were manufactured in the same manner as in Example 1, except that the average length of the MWCNT was set to about 125 μm. <Comparative Example 1> A cathode material and an all-solid-state battery including the cathode material were manufactured in the same manner as in Example 1, except that carbon nanofibers having an average length of less than 10 μm (product name: VGCF-H, Resonac product) were used instead of MWCNTs. <Comparative Example 2> A cathode material and an all-solid-state battery including the cathode material were manufactured in the same manner as in Example 1, except that the average length of the MWCNT was set to about 50 μm. <Evaluation Example 1: Shape Observation> As a result of observing the positive electrode materials of Examples 1 to 3 and Comparative Example 2 with a scanning electron microscope (SEM), in Examples 1 to 3, it was confirmed that relatively large positive electrode active material particles (10), relatively small solid electrolyte particles (12), and carbon nanotubes (14) were randomly mixed, as shown in FIG. 1. The carbon nanotubes (14) were generally dispersed in loose strands. 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, the carbon nanotubes were observed to be aggregated with each other. In Comparative Example 2, many carbon nanotubes were in contact with the surface of a single positive electrode active material particle (10) and / or a single solid electrolyte particle (12). <Evaluation Example 2: Battery Capacity and Rate Characteristics> For each all-solid-state battery of Examples and Comparative Examples, aging was performed by performing charge and discharge twice at a charge rate of 0.05 C and a discharge rate of 0.05 C (converted to 1 C = 200 mAh / g) in a constant temperature bath maintained at 60°C, with an upper limit voltage of 4.25 V and a lower limit voltage of 3 V. Here, the 'initial capacity' is defined as the value obtained by dividing the discharge capacity in the first charge and discharge process by the discharge capacity (reference value) of Comparative Example 1 using the following formula. Thereafter, the charge current was fixed to 0.1 C, and the discharge rate was changed in the order of 0.1 C → 0.2 C → 0.33 C → 0.5 C → 1.0 C to conduct a rate test. FIGS. 3 and 4 are graphs showing the change in discharge capacity with respect to the discharge rate of Examples 1 to 3 and Comparative Examples 1 and 2. The vertical axis of FIG. 3 shows the measured value of the discharge capacity, and the vertical axis of FIG. 4 shows the discharge capacity normalized by setting the discharge capacity at a rate of 0.1 C as 100% for each example to make it easy to compare the change in discharge capacity of each Example and Comparative Example (this normalized discharge capacity is called the 'rate characteristic'). The manufacturing conditions of Examples 1 to 3 and Comparative Examples 1 and 2, the values ​​of the initial capacities, and whether sufficient electron conduction paths were formed as confirmed by SEM observation are shown in Table 1 below. Here, if 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 insufficient electron conduction paths were formed. Initial capacity of conductive material (%) Type of electronic conduction path Quantity (mass%) Average length (μm) Average diameter (nm) Aspect ratio (average length / average diameter) Example 1 MWCNT 2.0 6 400 5 - 10 80,000 - 40,000 104.1 Sufficiently formed Example 2 MWCNT 2.0 6 250 5 - 10 50,000 - 25,000 107.6 Sufficiently formed Example 3 MWCNT 2.0 6 125 5 - 10 25,000 - 12,500 109.6 Sufficiently formed Comparative Example 1 VGCF 2.0 6 <10 150 33 - 67 100 Insufficient Comparative Example 2 MWCNT 2.0 6 50 5 - 10 10,000 - 5,000 97.5 Insufficient <Evaluation Example 3: Lifespan Characteristics of Battery> Subsequently, the cycle characteristics of Examples 1 to 3 and Comparative Example 1 were investigated. Specifically, for the all-solid-state batteries of Examples 1 to 3 and Comparative Example 1, after aging 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, in addition to the first and second charging and discharging processes, a total of 10 charging and discharging processes were repeated. 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 excellent cycle characteristics compared to Comparative Example 1. <Consideration of Evaluation Results of Examples and Comparative Examples> Hereinafter, the above evaluation results will be considered. However, the following consideration is a hypothesis at the current stage 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 Comparative Example 1, while the initial capacity of Comparative Example 2 with a carbon nanotube length of about 50 μm was inferior to Comparative Example 1. Considering the observation results by SEM, it is thought that the carbon nanotubes having a length of 100 μm or more are long enough to come into contact with multiple positive electrode active material particles and are easy to be unraveled and dispersed one by one, so that the electron conductivity of the entire positive electrode material (in particular, 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, first, the charge / discharge reaction is activated and stabilized because the carbon nanotubes mediate the electron conduction from the positive electrode active material near the current collector to the current collector. Second, the carbon nanotubes mediate the electron conduction from the positive electrode active material located away from the current collector to the positive electrode active material near the current collector, resulting in the formation of an electron conduction path from the positive electrode active material located away from the current collector to the current collector. As a result, since more (ideally, all) positive electrode active material particles can contribute to the charge / discharge reaction, each positive electrode active material particle in the positive electrode active material layer can equally participate in the charge / discharge reaction. As a result, it is thought that the battery capacity is improved, and a battery capacity close to the theoretical capacity is obtained. In addition, 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 much better than that of Comparative Example 1. That is, even when high-speed discharge was performed, the battery characteristics were not excessively degraded compared to low-speed discharge, and it was confirmed that the battery had excellent high-speed discharge characteristics. Since the carbon nanotubes having a length of 100 μm or more form 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, more positive electrode active materials can participate in the charge and discharge reaction more uniformly and efficiently. Accordingly, it is thought that capacity decrease in 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, heat loss under high current can be suppressed, so it is thought that efficient charge and discharge become possible. In addition, 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. Since the very long carbon nanotubes are entangled with the positive electrode active material particles, the electron conduction path can be maintained even if the positive electrode active material repeatedly shrinks and expands due to charge and discharge. In this way, it is presumed that the life characteristics of the battery are improved as a result of the carbon nanotubes functioning as a conductive matrix that absorbs the shrinkage and expansion of the positive electrode active material. In comparison, carbon nanotubes having a length of 50 μm tend to aggregate with each other. For this reason, the provision of conductivity by the carbon nanotubes is limited, and electronic conduction between active material particles and electronic conduction between the active material particles and the current collector cannot be efficiently promoted. As a result, it is thought that sufficient battery capacity and rate characteristics cannot be obtained. On the other hand, shorter carbon nanotubes (for example, 10 μm or less in length) generally tend to be more easily dispersed without aggregating than carbon nanotubes having a length of 50 μm, but it is thought that such short carbon nanotubes cannot sufficiently form an electronic conduction path between active material particles. [Explanation of symbols] 10: Positive active material particles 12: Solid electrolyte particles 14: Carbon nanotubes

Claims

1. Lithium transition metal oxide, solid electrolyte, and Contains carbon nanotubes, An all-solid-state battery cathode material, wherein at least one of the above carbon nanotubes has a length of 100 μm or more.

2. In paragraph 1, An anode material, wherein at least one of the above carbon nanotubes has a length of 100 ㎛ to 500 ㎛.

3. In paragraph 1, An anode material, wherein at least one of the above carbon nanotubes has a length of 100 ㎛ to 200 ㎛.

4. Positive electrode active material particles containing lithium transition metal oxide and having an average particle size of 1 ㎛ to 20 ㎛. solid electrolyte, and An all-solid-state battery cathode material comprising carbon nanotubes that come into contact with a plurality of cathode active material particles and form an electronic conduction path between the cathode active material particles.

5. In any one of paragraphs 1 to 4, A cathode material, wherein the content of the carbon nanotubes in the cathode material is 0.01 mass% or more and 10 mass% or less.

6. In any one of paragraphs 1 to 4, A cathode material wherein the above carbon nanotubes are multilayer carbon nanotubes.

7. In any one of paragraphs 1 to 4, An anode material in which the carbon nanotubes form an electronic conducting path between a plurality of particles that are not in contact with each other.

8. In any one of paragraphs 1 to 4, A cathode material wherein the carbon nanotubes are substantially free of aggregates or bundles.

9. In any one of paragraphs 1 to 4, A cathode material, wherein the lithium transition metal oxide contains nickel in an amount of 50 mol% or more based on the total amount of transition metals.

10. In any one of paragraphs 1 to 4, A cathode material wherein the solid electrolyte is a sulfide-based solid electrolyte.

11. In paragraph 1, The above carbon nanotube is an anode material having an average diameter of 2 nm to 20 nm.

12. In paragraph 1, The above carbon nanotube is an anode material having an aspect ratio of 10,000 to 100,000.

13. An anode comprising a cathode material as described in any one of clauses 1 to 4; cathode, and An all-solid-state battery having a solid electrolyte layer between the positive electrode and the negative electrode.

14. A method for producing a cathode material for an all-solid-state battery, comprising a step of mixing a lithium transition metal oxide, a solid electrolyte, and carbon nanotubes having a length of 100 μm or more.

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