Positive electrode material, all-solid-state battery and preparation method of positive electrode material

Abstract

The invention discloses a positive electrode material capable of improving battery characteristics of an all-solid-state battery, the all-solid-state battery and a preparation method of the positive electrode material. A positive electrode active material for an all-solid-state battery according to one embodiment of the present invention contains a lithium transition metal oxide, a solid electrolyte, and carbon nanotubes, and is characterized in that at least one of the carbon nanotubes has a length of 100 [mu] m or more.

Description

Technical Field

[0001] This application is based on and claims priority to Japanese Patent Application No. 2023-221543, filed on December 27, 2023, the disclosure of which is incorporated herein by reference in its entirety. The present invention relates to cathode materials, all-solid-state batteries, and methods for preparing cathode materials. Background Technology

[0002] Currently, lithium-ion secondary batteries using liquid electrolytes are widely used. However, when using liquid electrolytes, short circuits can occur, followed by overheating or explosion, if deformation or external impact occurs. To improve the safety of lithium-ion secondary batteries, all-solid-state batteries that do not use organic electrolytes have recently attracted attention as the next generation of batteries, and research and development of all-solid-state batteries have been diversified.

[0003] Carbon nanotubes are known to be conductive materials added to the electrode materials of all-solid-state batteries. The length of carbon nanotubes used as conductive materials in prior art all-solid-state batteries is typically less than 10 μm and at most tens of micrometers (see, for example, Patent Documents 1 to 3).

[0004] Exceptionally, Patent Document 4 discloses a positive electrode for an all-solid-state battery, comprising a molded body containing a positive electrode active material, a sulfide-based solid electrolyte, and a conductive additive, wherein the conductive additive includes fibrous carbon and particulate carbon, and the thickness of the molded body of the positive electrode mixture is 250 μm or more, and it is also stated that the fiber length of the fibrous carbon is preferably 3-600 μm. However, Patent Document 4 does not disclose why the above-defined numerical range is preferred, and only shows an embodiment using fibrous carbon VGCF (vapor-grown carbon fiber) available from Showa Denko as an embodiment to actually demonstrate its effect. Here, the fiber length of VGCF is approximately 10 μm, and Patent Document 4 does not disclose specific test results regarding carbon nanotubes with a length exceeding 10 μm.

[0005] [References]

[0006] [Patent Literature]

[0007] Patent Document 1: Japanese Patent Publication No. 2020-507893

[0008] Patent Document 2: Japanese Patent Publication No. 2022-529987

[0009] Patent Document 3: Japanese Patent Publication No. 2023-132317

[0010] Patent Document 4: Japanese Patent Publication No. 2021-144906 Summary of the Invention

[0011] Technical issues

[0012] Even with reference to the aforementioned patent documents, the reason for avoiding carbon nanotubes with lengths exceeding tens of micrometers remains unclear. However, the inventors of this invention have discovered that, when implementing this invention, carbon nanotubes with a length of approximately 50 μm strongly tend to aggregate together and are difficult to disperse sufficiently in the positive electrode. The inventors of this invention speculate that, due to the above characteristics, it has been recognized that using carbon nanotubes with lengths exceeding tens of micrometers as conductive materials cannot significantly improve battery performance.

[0013] The battery characteristics of all-solid-state batteries (such as charge / discharge capacity or rate capability) have been improved to some extent by adding conventionally used carbon nanotubes with a length equal to or less than tens of micrometers. However, the demand for higher-performance batteries is increasing, and further improvements in battery characteristics are needed.

[0014] The present invention aims to solve the problems of the prior art. Therefore, the present invention aims to provide a cathode material that can improve the battery characteristics of all-solid-state batteries, an all-solid-state battery, and a method for preparing the cathode material.

[0015] Technical solution

[0016] The inventors of this invention believe that using carbon nanotubes with a length of 100 μm or more as a conductive material can effectively form electronic conduction paths between electrode active materials, thereby providing improved battery characteristics. Therefore, the inventors have examined the effect of using carbon nanotubes of different lengths exceeding 50 μm on battery characteristics and have found that using carbon nanotubes with a length of 100 μm or more suppresses the tendency for aggregation that occurs in carbon nanotubes with a length of approximately 50 μm. As a result, the inventors have found that long carbon nanotubes are suitable for practical applications. This invention is based on this discovery.

[0017] The present invention may cover the following embodiments.

[0018] [1] A cathode material for all-solid-state batteries, comprising: lithium transition metal oxide, Solid electrolytes, and Carbon nanotubes Wherein, at least one of the carbon nanotubes has a length of 100 μm or more.

[0019] [2] As defined in [1], the cathode material, wherein at least one of the carbon nanotubes has a length of 100 μm-500 μm.

[0020] [3] As defined in [1], the cathode material, wherein at least one of the carbon nanotubes has a length of 100 μm-200 μm.

[0021] [4] A positive electrode material for all-solid-state batteries, comprising: positive electrode active material particles containing lithium transition metal oxide and having an average particle size of 1 μm-20 μm; Solid electrolytes, and Carbon nanotubes that contact multiple positive electrode active material particles to form electron conduction paths between the positive electrode active material particles.

[0022] [5] A cathode material as defined in any of [1] to [4], wherein the content of the carbon nanotubes in the cathode material is 0.01% to 10% by weight.

[0023] [6] A cathode material as defined in any of [1] to [5], wherein the carbon nanotube is a multi-walled carbon nanotube.

[0024] [7] A cathode material as defined in any of [1] to [6], wherein the carbon nanotubes form an electron conduction path between a plurality of particles that do not contact each other.

[0025] [8] A cathode material as defined in any of [1] to [7], wherein the carbon nanotubes are unentangled and substantially do not form aggregates or bundles.

[0026] [9] A cathode material as defined in any of [1] to [8], wherein the lithium transition metal oxide contains more than 50 mol% nickel based on the total weight of the transition metal.

[0027]

[10] The positive electrode material as defined in any of [1] to [9], wherein the solid electrolyte is a sulfide solid electrolyte.

[0028]

[11] A cathode material as defined in any of [1] to

[10] , wherein the average diameter of the carbon nanotubes is 2 nm to 20 nm.

[0029]

[12] A cathode material as defined in any of [1] to

[11] , wherein the aspect ratio of the carbon nanotubes is 10000-100000.

[0030]

[13] An all-solid-state battery comprising: a positive electrode comprising a positive electrode material as defined in any one of [1] to

[12] , Negative electrode; and The solid electrolyte layer between the positive electrode and the negative electrode.

[0031]

[14] A method for preparing 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 with a length of more than 100 μm.

[0032] Beneficial effects

[0033] According to the present invention, a cathode material capable of improving the battery characteristics of an all-solid-state battery, an all-solid-state battery, and a method for preparing the cathode material can be provided. Attached Figure Description

[0034] Figure 1 This is a schematic diagram illustrating the microstructure of a cathode material according to one embodiment of the present invention.

[0035] Figure 2 This is a schematic diagram illustrating the structure of an all-solid-state battery according to one embodiment of the present invention.

[0036] Figure 3 This is a graph showing the change in discharge capacity relative to the discharge rate in Examples 1 to 3 and Comparative Examples 1 and 2.

[0037] Figure 4 This is a graph showing the change in discharge capacity relative to the discharge rate in Examples 1 to 3 and Comparative Examples 1 and 2.

[0038] Figure 5 This is a graph showing the cyclic characteristics of Examples 1 to 3 and Comparative Example 1. Detailed Implementation

[0039] The following describes the cathode material, all-solid-state battery, and method for preparing the cathode material according to embodiments of the present invention. Furthermore, the following embodiments illustrating one embodiment of the present invention do not limit the invention and can be modified without departing from its scope. Moreover, the configurations and features of the embodiments can be combined arbitrarily.

[0040] In the following text, the singular forms “a,” “one,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise.

[0041] As used herein, when describing a part of a layer, membrane, region, plate, etc., as being "above" another part, this includes not only the case where it is "directly above" the other part, but also the case where the other part exists in between. Conversely, when describing a part of a layer, membrane, region, plate, etc., as being "below" another part, this includes not only the case where it is "directly below" the other part, but also the case where the other part exists in between. Furthermore, the expression "positioned above" in the specification can include both the case where it is positioned at the bottom and the case where it is positioned at the top.

[0042] As used in this article, “average particle size (D)” 50 "50%" can be defined as the particle size corresponding to the 50% of the volumetric cumulative particle size distribution curve. For example, the average particle size (D) 50Particle sizes can be determined using laser diffraction. Laser diffraction can typically determine particle sizes ranging from submicron to several millimeters and provides highly reproducible and high-resolution results.

[0043] As used herein, “single-walled carbon nanotube (SWCNT)” refers to a type of carbon nanotube in which the tubular walls formed by carbon atoms comprise a single atomic layer (i.e., a single-layer graphene sheet). Conversely, “multi-walled carbon nanotube (MWCNT)” refers to a type of carbon nanotube in which the tubular walls formed by carbon atoms comprise multiple atomic layers (i.e., multi-layer graphene sheets).

[0044] <Cathode Materials>

[0045] The cathode material of one embodiment of the present invention 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.

[0046] Figure 1 This is a schematic diagram illustrating the microstructure of a cathode material according to one embodiment of the present invention. The cathode material comprises lithium transition metal oxide particles 10, solid electrolyte particles 12, and carbon nanotubes 14. Figure 1 In this configuration, the solid electrolyte particles 12 are smaller than the lithium transition metal oxide particles 10 and are located around the lithium transition metal oxide particles 10. During charging and discharging, lithium ions migrate between the lithium transition metal oxide particles 10 and the solid electrolyte particles 12, which are in contact with each other. Carbon nanotubes 14 are entangled with the lithium transition metal oxide particles 10. The carbon nanotubes 14 are in partial contact with multiple lithium transition metal oxide particles 10. During charging and discharging, the carbon nanotubes 14 can form electronic conduction pathways from the lithium transition metal oxide particles 10 to other particles 10 or current collectors. Electronic conduction occurs simultaneously with lithium ion conduction.

[0047] [Lithium transition metal oxides]

[0048] Lithium transition metal oxides act as positive electrode active materials, which cooperate with the negative electrode active materials described below, and allow lithium ions to be inserted / deintercalated at the positive electrode during charging and discharging.

[0049] The lithium transition metal oxide may be a metal oxide commonly used as a positive electrode active material in the art. The lithium transition metal oxide may be any compound as long as it can reversibly intercalate / deintercalate lithium, and is not limited to any specific type. Specific examples of the lithium transition metal oxide may include lithium metal composite oxides containing lithium and at least one metal (such as cobalt, manganese, nickel, copper, vanadium, aluminum, etc.). More specifically, such lithium metal composite oxides may include: lithium manganese oxides (such as LiMnO2, LiMnO3, LiMn2O3, LiMn2O4, etc.); lithium cobalt oxides (such as LiCoO2, etc.); lithium nickel oxides (such as LiNiO2, etc.); lithium copper oxides (such as Li2CuO2, etc.); lithium vanadium oxides (such as LiV3O8, etc.); lithium nickel manganese oxides (such as LiNi 1-z Mn z O2 (0 < z < 1), LiMn 2-z Ni z O4 (0 < z < 2), etc.); lithium nickel cobalt oxides (such as LiNi 1-y Co y O2 (0 < y < 1), etc.); lithium manganese cobalt oxides (such as LiCo 1-z Mn z O2 (0 < z < 1), LiMn 2-y Co y O4 (0 < y < 2), etc.); lithium nickel manganese cobalt oxides (such as 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 < 1, 0 < z < 1, x + y + z = 2), etc.); lithium nickel cobalt metal (M) oxides (such as Li(Ni x Co y Mn z M w )O2 (where 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.); lithium-rich solid solution positive electrodes (such as pLi2MnO3-(1-p)Li(Ni​​​​​)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 substituted by one or more different metal elements, etc. The positive electrode active material layer may contain one of the compounds listed above, or two or more of them. However, the scope of the present invention is not limited thereto.

[0050] In particular, in the case of a lithium transition metal oxide having a high nickel content that effectively provides a high-capacity battery, based on the total weight of the transition metal, the lithium transition metal oxide preferably contains 50 mol% or more of nickel. Specific examples of such lithium transition metal oxides may 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 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 Mn z)O4 (where M is at least one element 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 (where M is at least one element 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 the transition metal elements are partially substituted by more than one different metal element (e.g., one or more elements selected from Al, Fe, V, Cr, Ti, Ta, Mg, Mo, Zr, Zn, Ga, and In); compounds in which the oxygen atoms are partially substituted by at least one non-metallic metal element (e.g., one or more elements selected from P, F, S, and N), etc. Preferably, the lithium transition metal oxide is represented by Li a Ni x M y O2 (where M is at least one metal element other than Ni, e.g., at least one element 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), where 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 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 at least one of the compounds listed above, but is not limited thereto. In addition, in the same particle, there may be a distribution of substitution concentrations between the interior and the surface layer. In addition, the positive electrode material may be particles with a surface coating. For example, the particles may be surface-coated with a metal oxide, a lithium transition metal oxide, a polymer, etc., but are not limited thereto.

[0051] In particular, for improving the capacity characteristics and stability of the battery, the lithium transition metal oxide may preferably be 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), Lia (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. Here, all values ​​of a can satisfy 0.5≤a≤1.5, preferably 1.0≤a≤1.5.

[0052] More specifically, the lithium transition metal oxide can preferably be LiNiO2 or Li(Ni)O2. 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 Mn0.1 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 0.05 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.

[0053] For example, the particle size of lithium transition metal oxides can be 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 to 10 μm. When the particle size of lithium transition metal oxides is greater than 10 nm, surface degradation of the particles can be suppressed. When the particle size of lithium transition metal oxides is less than 20 μm, the lithium diffusion path is not excessively elongated, and the electronic conduction path can be effectively formed by carbon nanotubes as described below.

[0054] Preferably, a coating layer containing a metal oxide is formed on the 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 layer containing LiNbO2 is formed on the surface of the lithium transition metal oxide particles. Such a coating layer can reduce the internal resistance of the positive electrode.

[0055] [Solid electrolyte]

[0056] Solid electrolytes facilitate the transfer of lithium ions between the positive and negative electrode active materials. Solid electrolyte particles can come into contact with particles of the positive electrode active material, particles of the negative electrode active material, or particles of another solid electrolyte to mediate lithium ion transfer.

[0057] Any solid electrolyte can be used, as long as it is a material with high ionic conductivity (e.g., ionic conductivity of 10). -5 s / m or higher, preferably 10 -4 The solid electrolyte (with a ratio of s / m or higher) can primarily function as a transfer electrode for lithium ions, and its composition is not limited to any specific component.

[0058] For example, the solid electrolyte can be at least one selected from the group consisting of sulfide-based solid electrolytes, oxide-based solid electrolytes, and polymer-based solid electrolytes. Polymer-based solid electrolytes can be solid electrolytes formed by adding polymer resin to a solvated lithium salt, or polymer gel electrolytes formed by impregnating polymer resin with an organic electrolyte containing an organic solvent and lithium salt, an ionic liquid, a monomer, or an oligomer. Meanwhile, sulfide-based solid electrolytes exhibit high ionic conductivity, and oxide-based solid electrolytes exhibit excellent electrochemical stability. Therefore, suitable solid electrolyte components can be selected and used according to the characteristics of the solid electrolyte and the intended use of the battery. Preferably, the solid electrolyte is a sulfide-based solid electrolyte.

[0059] Sulfide solid electrolytes contain sulfur (S) and have the conductivity of metal ions belonging to Group 1 or Group 2 of the periodic table. For example, sulfide solid electrolytes can include Li-PS glass or Li-PS glass-ceramics. Sulfide solid electrolytes can include glassy solid electrolytes, crystalline solid electrolytes, and glassy ceramic solid electrolytes. Specific examples of sulfide solid electrolytes may include, but are not limited to: 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, sulfide-silver-germanium ore solid electrolytes (e.g., Li₆PS₅X (X: Cl, Br, I)), LGPS solid electrolytes (Li 10 GeP2S 12 Li 3.25 Ge 0.25 P 0.75 S4, etc.), Li7P3S 11 For example, solid electrolytes may include at least one selected from the compounds listed above.

[0060] Oxide solid electrolytes contain oxygen (O) and have the conductivity of metal ions belonging to Group 1 or Group 2 of the periodic table. Specific examples of oxide solid electrolytes may include, but are not limited to: LLTO compounds, Li6La2CaTa2O 12 、Li6La2ANb 212 (where A is 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 (where 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 (where 0 ≤ x ≤ 1, 0 ≤ y ≤ 1), LISICON compounds, LIPON compounds, perovskite compounds, NASICON compounds, and LLZO compounds. For example, a solid electrolyte may include at least one compound selected from the compounds listed above.

[0061] Polymer-based solid electrolytes may include, for example, polyester polymers, polycarbonate polymers, acrylate polymers, polysiloxane polymers, phosphazene polymers, polyethylene derivatives, alkylene oxide derivatives (e.g., polyethylene oxide (PEO) or polypropylene oxide (PPO)), phosphate polymers, polyaziridinium, polyester sulfides, polyvinyl alcohol, polyvinylidene fluoride, and polymers containing ionically dissociable groups. For example, a solid electrolyte may include at least one compound selected from the compounds listed above.

[0062] Based on the total weight of the positive electrode active material, the content of solid electrolyte can be 1%-70% by weight, 5%-50% by weight, 10%-45% by weight, or 20%-40% by weight. When the content of solid electrolyte particles is above 1% by weight, lithium-ion conduction pathways can be fully formed by having the solid electrolyte particles surround the surface of the lithium transition metal oxide particles. When the content of solid electrolyte is below 50% by weight, the separation of solid electrolyte particles from the lithium transition metal oxide and their agglomeration can be suppressed.

[0063] For example, the particle size of the solid electrolyte is 10 nm to 10 μm, 50 nm to 5 μm, 100 nm to 1 μm, or 200 nm to 500 μm. When the particle size of the solid electrolyte falls within the above range, lithium-ion transfer between active materials can proceed smoothly at an appropriate level of electrode density or tap density.

[0064] Carbon nanotubes

[0065] Carbon nanotubes primarily contact the surfaces of the positive electrode active material particles or the positive electrode current collector to facilitate the formation of a conductive network between or between the positive electrode active material particles and the positive electrode current collector. Carbon nanotubes are not in bundles, but rather exist mostly as single chains, and are primarily located on the surface of the positive electrode active material.

[0066] The carbon nanotubes in the cathode material have a length of 100 μm or more. For example, the length of the carbon nanotubes can be 100 μm-500 μm, 100 μm-450 μm, 100 μm-400 μm, 100 μm-350 μm, 100 μm-300 μm, 100 μm-250 μm, or 100 μm-200 μm. The length of the carbon nanotubes can also 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 can be seen from the results of the following examples and comparative examples, carbon nanotubes with a length of 100 μm or more can effectively form electron conduction pathways between active material particles or between active material particles and current collectors, without causing excessive aggregation. Meanwhile, carbon nanotubes with a length of less than 500 μm can suppress the close contact and clustering of extremely long carbon nanotubes.

[0067] Preferably, the average length of the carbon nanotubes in the cathode material is 100 μm or more, 100 μm-500 μm, 100 μm-450 μm, 100 μm-400 μm, 100 μm-350 μm, 100 μm-300 μm, 100 μm-250 μm, or 100 μm-200 μm. The length of the carbon nanotubes can 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, when manufacturing the electrode, carbon nanotubes with a length of 100 μm or more will not cause excessive aggregation between each other and can effectively form electron conduction paths between active material particles or between active material particles and current collectors. Furthermore, even with a low content of conductive material, these carbon nanotubes can effectively form a conductive network. Here, "average length" refers to the average length of the 100 largest and 100 smallest carbon nanotubes observed by scanning electron microscopy (SEM).

[0068] Preferably, the carbon nanotubes are unentangled and substantially do not form aggregates or bundles. For example, when observed by SEM, it is preferred that at least 50% (based on quantity), at least 60%, at least 70%, at least 80%, at least 90%, or 100% of the carbon nanotubes in the cathode material are not in contact with more than two other carbon nanotubes. In one variation, when observed by SEM, it is preferred that at least 50% (based on quantity), at least 60%, at least 70%, at least 80%, at least 90%, or 100% of the carbon nanotubes in the cathode material are not in contact with other carbon nanotubes at more than two points. In another variation, when observed by SEM, it is preferred that at least 50% (based on quantity), at least 60%, at least 70%, at least 80%, at least 90%, or 100% of the carbon nanotubes in the cathode material are not in contact with other carbon nanotubes.

[0069] Although there are no particular limitations, the average diameter of carbon nanotubes can be from 1 nm to 3 μm, 5 nm to 1 μm, 10 nm to 500 nm, or 20 nm to 100 nm. In specific embodiments, the average diameter of carbon nanotubes can be greater than 2 nm, greater than 5 nm, greater than 7 nm, or greater than 10 nm. In another specific embodiment, the average diameter of carbon nanotubes can be less than 20 nm, less than 15 nm, or less than 10 nm. When the above ranges are met, carbon nanotubes are easily dispersed in the positive electrode without causing excessive aggregation. Here, "average diameter" refers to the average of the diameters of the largest 100 and smallest 100 carbon nanotubes observed by scanning electron microscopy (SEM).

[0070] Meanwhile, according to one embodiment of the present invention, the aspect ratio of the carbon nanotubes can be 10,000-100,000. The aspect ratio can be above 10,000, above 12,000, above 13,000, above 15,000, above 20,000, above 30,000, above 50,000, or above 70,000. The aspect ratio can be below 100,000, below 90,000, below 85,000, below 80,000, below 75,000, below 70,000, below 50,000, below 40,000, or below 30,000. The diameter and length of the carbon nanotubes can be appropriately controlled within the ranges satisfying the above aspect ratios. In one embodiment, the term "aspect ratio" can refer to the ratio of length to diameter (length / diameter). In one embodiment, the aspect ratio can refer to the ratio of average length to average diameter (average length / average diameter).

[0071] Carbon nanotubes can be single-walled or multi-walled. Compared to single-walled carbon nanotubes, multi-walled carbon nanotubes are easier to manufacture and are more cost-effective.

[0072] Based on the total weight of the cathode material, the carbon nanotube content is 0.01 wt%-10 wt%, 0.05 wt%-8 wt%, 0.1 wt%-6 wt%, 0.2 wt%-5 wt%, 0.5 wt%-4 wt%, or 1 wt%-3 wt%. When the above ranges are met, a conductive network can be efficiently formed in the cathode while maintaining the dispersion and stability of the carbon nanotubes, thus improving the charge and discharge characteristics of the battery.

[0073] Based on the total weight of the lithium transition metal oxide, the carbon nanotube content is, for example, 0.01 wt%-20 wt%, 0.05 wt%-15 wt%, 0.05 wt%-15 wt%, 0.1 wt%-12 wt%, 0.2 wt%-10 wt%, 0.5 wt%-8 wt%, or 1 wt%-5 wt%. When the above ranges are met, sufficient carbon nanotubes are present based on the lithium transition metal oxide, thus a conductive network can be effectively formed in the positive electrode.

[0074] Another embodiment of the cathode material for all-solid-state batteries includes a lithium transition metal oxide and cathode active material particles with an average particle size of 1 μm-20 μm, a solid electrolyte, and carbon nanotubes that are in contact with multiple cathode active material particles to form electron conduction pathways between the cathode active material particles.

[0075] Because carbon nanotubes possess high electronic conductivity, they create electronic conduction pathways between multiple positive electrode active material particles when in contact with them. The length of the carbon nanotubes allows them to contact multiple positive electrode active material particles with an average particle size of 1 μm–20 μm. For example, when observed via SEM, it is preferable that at least 50% (based on quantity), at least 60%, at least 70%, at least 80%, at least 90%, or 100% of the carbon nanotubes in the positive electrode material are in contact with multiple positive electrode active material particles.

[0076] When observed from the positive electrode active material particles, it is preferable that an electronic conduction path is formed with other active material particles. For example, when observed by SEM, it is preferable that at least 50% (based on quantity), at least 60%, at least 70%, at least 80%, at least 90%, or 100% of the positive electrode active material particles are in contact with carbon nanotubes. Furthermore, when observed by SEM, it is preferable that at least 60% (based on quantity), at least 60%, at least 70%, at least 80%, at least 90%, or 100% of the positive electrode active material particles are in contact with carbon nanotubes, and that the carbon nanotubes are also in contact with other positive electrode active material particles.

[0077] From another perspective, preferably, the length of at least one carbon nanotube is greater than the average particle size of the positive electrode active material particles. For example, the length of at least one carbon nanotube corresponds to at least 5 times, at least 10 times, at least 20 times, or at least 30 times the average particle size of the positive electrode active material particles. Preferably, the average length of the carbon nanotube is greater than the average particle size of the positive electrode active material particles, for example, corresponding to at least 5 times, at least 10 times, at least 20 times, or at least 30 times the average particle size of the positive electrode active material particles.

[0078] In this way, carbon nanotubes can form electronic conduction pathways between multiple particles that are not in contact with each other. This promotes electron conduction in the positive electrode, allowing the active material particles in the positive electrode to participate uniformly in the charge-discharge reaction. As a result, battery capacity can be improved. Furthermore, due to the excellent electronic conductivity of carbon nanotubes, energy loss, such as heat generation at high current levels, can be suppressed. Therefore, rate characteristics (especially high-rate charge-discharge characteristics) can be improved.

[0079] Meanwhile, direct contact between carbon nanotubes and the positive electrode active material is generally not required. 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 particles through this coating, thus forming an electron conduction path. Therefore, the contact between carbon nanotubes and the positive electrode active material particles does not have to be direct; indirect contact can be achieved through another configuration, as long as the carbon nanotubes form an electron conduction path between the particles that mediates electron conduction. Furthermore, carbon nanotubes can support the positive electrode active material particles through adsorption, but they can also contact the particles without supporting them. For example, carbon nanotubes and positive electrode active material particles may not adsorb onto each other.

[0080] <Preparation Methods of Cathode Materials>

[0081] One embodiment of the present invention provides a method for preparing a cathode material for an all-solid-state battery, which includes the step of mixing a lithium transition metal oxide, a solid electrolyte, and carbon nanotubes with a length of 100 μm or more.

[0082] Refer to the above description of lithium transition metal oxides, solid electrolytes, and carbon nanotubes. When they are mixed, electron conduction pathways can be formed between lithium transition metal oxide particles via carbon nanotubes.

[0083] The mixing step can be performed using, but is not limited to, dry mixers, agitators, oscillators (e.g., orbital oscillators), mortar mixers, and grinders (such as planetary ball mills), which are commonly used for powder mixing. The mixing process is not limited to dry mixing; it can also be a wet mixing process that mixes lithium transition metal oxides, solid electrolytes, and carbon nanotubes in any liquid medium. The addition of lithium transition metals, solid electrolytes, and carbon nanotubes is not limited to any particular order. Furthermore, other additives can be added.

[0084] All-solid-state batteries

[0085] An all-solid-state battery according to one embodiment of the present invention comprises: a positive electrode containing the above-described positive electrode material, a negative electrode, and a solid electrolyte layer located between the positive electrode and the negative electrode.

[0086] Figure 2 This is a schematic diagram illustrating the structure of an all-solid-state battery 100 according to one embodiment of the present invention. Figure 2As shown, the all-solid-state battery 100 sequentially comprises a positive electrode 110, a solid electrolyte layer 120, and a negative electrode 130. The positive electrode 110 comprises a positive current collector 112 and a positive electrode material layer 114. The negative electrode 130 comprises a negative electrode material layer 132 and a negative electrode current collector 134. However, for example, when the negative electrode is formed only of metallic lithium, the negative electrode material layer 132 and the negative electrode current collector 134 are integrally disposed. Overall, the all-solid-state battery 100 is formed by sequentially stacking the positive current collector 112, the positive electrode material layer 114, the solid electrolyte layer 120, the negative electrode material layer 132, and the negative electrode current collector 134.

[0087] [Positive electrode 110]

[0088] In one embodiment of the all-solid-state battery 100 of the present invention, 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 on all or part of the surface of the positive electrode current collector 112.

[0089] (Positive current collector 112)

[0090] There are no particular limitations on the positive current collector 112 used for the positive electrode 110, as long as it does not cause any chemical changes in the battery and is conductive. Specific examples of the positive current collector 112 may include: stainless steel; aluminum; nickel; titanium; calcined carbon; aluminum or stainless steel that has been surface-treated with carbon, nickel, titanium, silver, etc.

[0091] For example, the thickness of the positive electrode current collector 112 can be 3 μm to 500 μm. Fine surface irregularities can be formed on the surface of the positive electrode current collector 112 to enhance adhesion to the positive electrode active material. For example, the positive electrode current collector 112 can be in various shapes such as sheets, foils, meshes, porous materials, foams, nonwovens, etc.

[0092] (Positive electrode active material layer 114)

[0093] The positive electrode active material layer 114 may contain the aforementioned positive electrode material. In addition to the positive electrode material, the positive electrode active material layer 114 may also contain other conductive materials, binders, optional additives, etc.

[0094] For example, the thickness of the positive electrode active material layer 114 can be 1 μm-500 μm, 5 μm-250 μm, 10 μm-200 μm, 20 μm-150 μm or 50 μm-100 μm.

[0095] (Conductive materials)

[0096] The positive electrode active material layer 114 may also contain conductive materials other than the aforementioned carbon nanotubes. There are no particular limitations on the conductive material, as long as it does not cause any chemical change and is conductive. Specific examples of conductive materials may include any of the following: graphite, such as natural or artificial graphite; carbon black materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermally cracked carbon black; conductive fibers, such as carbon fibers (including vapor-grown carbon fibers (VGCF)) or metal fibers; fluorocarbons; metal powders, such as aluminum or nickel; conductive whiskers, such as zinc oxide or potassium titanate; conductive metal oxides, such as titanium oxide; and conductive materials, such as polyphenylene derivatives, or mixtures of two or more of these. The positive electrode active material layer 114 may contain carbon nanotubes with a length of less than 100 μm as conductive materials.

[0097] (Adhesive)

[0098] The positive electrode active material layer 114 may also include an adhesive. The adhesive is used to ensure adhesion between the positive electrode active material particles or between the positive electrode active material particles and the current collector. The adhesive can be any conventional adhesive used in the art and is not limited to any particular type. Specific examples of adhesives may include polyvinylidene fluoride-co-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. These adhesive materials can be used alone or in combination.

[0099] Based on the total weight of the positive electrode active material layer 114, the amount of adhesive can be less than 10% by weight, preferably 0.1% to 5% by weight. When the content of adhesive meets the above range, excellent electrode adhesion can be achieved while minimizing the increase in electrode resistance.

[0100] [Negative electrode (130)]

[0101] The negative electrode 130 may contain only lithium metal, or it may contain 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 all or part of the surface of the negative electrode current collector 134.

[0102] (Negative electrode current collector 134)

[0103] The negative electrode current collector 134 for the negative electrode is not particularly limited as long as it does not cause any chemical changes in the battery and has conductivity. Specific examples of the negative electrode current collector 134 may include: copper; stainless steel; aluminum; nickel; titanium; calcined carbon; copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc.; aluminum-cadmium alloy, etc.

[0104] For example, the thickness of the negative electrode current collector 134 can be 3 μm - 500 μm. The negative electrode current collector 134 can form fine surface irregularities on its surface to enhance adhesion to the negative electrode active material. For example, the negative electrode current collector 134 can be various shapes such as sheets, foils, meshes, porous bodies, foams, non-woven fabrics, etc.

[0105] (Negative electrode active material layer 132)

[0106] The negative electrode active material layer 132 contains a negative electrode active material and a solid electrolyte. When necessary, the negative electrode active material layer 132 can contain a conductive material, a binder, and other optional additives. The descriptions of the solid electrolyte, conductive material, and binder are the same as those described for the positive electrode active material layer 114 and will be omitted here. At the same time, similar to the positive electrode active material layer 114, the negative electrode active material layer 132 can contain carbon nanotubes with a length of 100 μm or more.

[0107] (Negative electrode active material)

[0108] Specific examples of the negative electrode active material may 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, or amorphous carbon; metal compounds capable of alloying with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloy, Sn alloy, or Al alloy; metal oxides capable of doping / dedoping lithium, such as SiO v (0 < v < 2), SnO2, vanadium oxides, lithium vanadium oxides, Li x Fe2O3 (0 ≤ x ≤ 1) or Li x WO2 (0 ≤ x ≤ 1); composites containing metal compounds and carbonaceous materials, such as Si-C composites or Sn-C composites, etc. These materials can be used alone or in combination.

[0109] Based on the total weight of the negative electrode active material layer 132, the content of the negative electrode active material can be 70 wt% - 100 wt%, preferably 80 wt% - 99 wt%. When the content of the electrode active material satisfies the above range, excellent energy density, electrode adhesion, and electronic conductivity can be achieved.

[0110] [Solid electrolyte layer 120]

[0111] A 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 and negative active materials. The solid electrolyte layer 120 also functions as a separator layer that physically separates the positive electrode 110 and the negative electrode 130 to prevent short circuits between the two electrodes.

[0112] For example, the solid electrolyte layer 120 is a layer formed of the same solid electrolyte as that contained in the positive electrode material. However, the solid electrolyte layer 120 may also contain other solid electrolytes or additives, or may be formed of other solid electrolytes.

[0113] <Manufacturing Methods of All-Solid-State Batteries>

[0114] There are no particular limitations on the manufacturing method of the all-solid-state battery 100. For example, the all-solid-state battery 100 can be manufactured by sequentially introducing the battery-forming materials into a cylindrical mold and applying pressure. In one variation, the all-solid-state battery 100 can be manufactured by forming the layers of the all-solid-state battery 100 and then stacking and pressurizing these layers. Any method other than those described above can be used.

[0115] <Effect>

[0116] To clarify the technical effects of the above embodiments, the significance of carbon nanotubes in conventional all-solid-state batteries will first be described.

[0117] While all-solid-state batteries offer advantages such as reduced risk of electrolyte leakage or fire, their solid state also presents disadvantages. For example, in order to facilitate the smooth migration of lithium ions and electrons during the charge-discharge reaction of all-solid-state batteries, it is crucial to ensure contact between the active material particles and the solid electrolyte particles and to suppress their interfacial resistance. This is achieved by applying a coating layer with high ionic conductivity, such as LiNbO2, to the surface of the positive electrode. However, as charge and discharge proceed, the battery characteristics deteriorate when the particles are spaced apart and isolated due to volume expansion, etc. Therefore, conventional surface coating techniques cannot maintain contact between the particles.

[0118] Simultaneously, conductive materials containing carbon nanotubes with high conductivity, such as VGCF, are typically added to impart conductivity to the electrode material. However, to ensure the conductivity of the entire electrode through needle-like fibers of VGCF with lengths of several micrometers, a large amount of conductive material needs to be introduced. Furthermore, when the active material particles are isolated due to a lack of conductive material, adverse effects such as accelerated decomposition of the solid electrolyte may occur.

[0119] In light of the above, according to existing technology, long carbon nanotubes with a length of 100 μm or more have not been used as conductive materials for solid electrolytes. This is because those skilled in the art believe that long carbon nanotubes may adversely affect battery characteristics. Comparative Example 2 below shows that when using carbon nanotubes with a length of 50 μm, compared to using VGCF, the carbon nanotubes aggregate, leading to a decrease in battery capacity. Therefore, it can be said that, according to existing technology, since even carbon nanotubes with a length of 50 μm can cause degradation in battery characteristics, those skilled in the art have no incentive to use longer carbon nanotubes.

[0120] Against this backdrop, the inventors of the present invention have discovered that, compared to Comparative Example 1 using VGCF, as described in the following examples, using carbon nanotubes with a length of 100 μm or more as a conductive material can improve battery capacity and rate performance. These carbon nanotubes with a length of 100 μm or more are believed to overcome the agglomeration tendency of carbon nanotubes with a length of approximately 50 μm in the cathode material and are widely dispersed in the cathode material. Carbon nanotubes with a length of 100 μm or more are believed to be able to form an electron conduction network between active material particles and between active material particles and current collectors in the electrode material due to this dispersion characteristic and the length of the carbon nanotubes themselves.

[0121] This property of carbon nanotubes is particularly important in all-solid-state batteries. Due to the fluidity of liquids, the conductive material can be widely dispersed to a certain extent in the slurry prepared by mixing the cathode material with the conductive material. However, in all-solid-state batteries, the cathode material and conductive material are typically mixed in a powder state. Therefore, when the carbon nanotubes contained in the conductive material have high agglomeration properties, they may be difficult to disperse, making it difficult to form electronic conduction paths between the spaced-apart active material particles. However, when carbon nanotubes with a length of 100 μm or more are used, their greater length allows for the formation of longer electronic conduction paths, and they exhibit low agglomeration properties, thus solving all the aforementioned problems. As a result, high-capacity all-solid-state batteries are considered achievable in the following embodiments, which is not possible when using VGCF or carbon nanotubes with a length of approximately 50 μm.

[0122] Furthermore, since a solid electrolyte is used instead of a liquid electrolyte in all-solid-state batteries, it is crucial to ensure contact between the active material particles and the solid electrolyte particles compared to the case of using a fluid liquid electrolyte. To this end, the positive electrode material for all-solid-state batteries is prepared by mixing the positive electrode active material with the solid electrolyte. This ensures contact between the positive electrode material particles and the solid electrolyte particles, but correspondingly reduces the contact points between the positive electrode active material particles and the positive electrode current collector. Therefore, it can be said that long carbon nanotubes capable of forming relatively long electron conduction paths between the current collector and the positive electrode active material spaced apart from the current collector are of significant importance.

[0123] Furthermore, because the carbon nanotubes, which are significantly longer than the active material particles, are entangled with the active material particles, the contact between the carbon nanotubes and the active material particles can be easily maintained even when the active material particles repeatedly expand / contract due to charging and discharging. Therefore, long carbon nanotubes can form a so-called matrix structure that absorbs the expansion / contraction of the active material particles, which is also preferred in terms of battery cycle characteristics and stability.

[0124] The advantages of long carbon nanotubes have not been fully understood in the prior art, and as mentioned above, there is a perception that "long carbon nanotubes should be avoided in all-solid-state batteries." Considering this, it can be said that the present invention has significant technical implications.

[0125] Embodiments of the present invention

[0126] Examples and comparative examples will be described below. However, the invention is not limited thereto. Furthermore, the discussion below is merely an exemplary assumption to aid in understanding the invention and is not intended to limit the invention.

[0127] <Example 1>

[0128] [Preparation of cathode materials]

[0129] First, 61.86 parts by weight of LiNi, with an average particle size of approximately 5 μm, coated with LiNbO2, were used as the positive electrode active material. 0.8 Co 0.1 Mn 0.1 O2 powder and 36.08 parts by weight of Li6PS5Cl powder, which serves as a sulforaphite-germanium ore-based solid electrolyte, were mixed. Then, 2.06 parts by weight of multi-walled carbon nanotubes (MWCNTs) with an average length of approximately 400 μm were added, and the resulting mixture was stirred until homogeneous using a mortar and pestle. This yielded the cathode material.

[0130] [Manufacturing of all-solid-state batteries]

[0131] A solid-state battery 100 is obtained by sequentially stacking and compressing a positive current collector (SUS plate) 112, a positive electrode material layer 114, a sulfide solid electrolyte membrane layer 120, a Li metal 132 (thickness 0.1 μm) as a negative electrode, and a negative current collector (SUS plate) 134.

[0132] <Example 2>

[0133] The cathode material and the all-solid-state battery containing the cathode material were obtained in the same manner as in Example 1, except that MWCNTs with an average length of about 250 μm were used.

[0134] <Example 3>

[0135] The cathode material and the all-solid-state battery containing the cathode material were obtained in the same manner as in Example 1, except that MWCNTs with an average length of about 125 μm were used.

[0136] <Comparative Example 1>

[0137] The cathode material and the all-solid-state battery containing the cathode material were obtained in the same manner as in Example 1, except that carbon nanofibers (VGCF-H) with an average length of less than 10 μm were used instead of MWCNTs.

[0138] <Comparative Example 2>

[0139] The cathode material and the all-solid-state battery containing the cathode material were obtained in the same manner as in Example 1, except that MWCNTs with an average length of about 50 μm were used.

[0140] <Test Example 1: Shape Observation>

[0141] The cathode materials in Examples 1 to 3 and Comparative Example 2 were observed using scanning electron microscopy (SEM). It was observed that in Examples 1 to 3, as... Figure 1 As shown, relatively large positive electrode active material particles 10, relatively small solid electrolyte particles 12, and carbon nanotubes 14 are randomly distributed. The carbon nanotubes 14 are essentially single-unit unentangled and dispersed. Multiple carbon nanotubes 14 are in contact with the surfaces of multiple positive electrode active material particles 10 and multiple solid electrolyte particles 12. Meanwhile, in Comparative Example 2, agglomeration of carbon nanotubes was observed. In Comparative Example 2, multiple 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.

[0142] <Test Example 2: Battery Capacity and Rate Characteristics>

[0143] The all-solid-state batteries in each embodiment and comparative example were aged by performing two charge-discharge cycles at a charge rate of 0.05 C and a discharge rate of 0.05 C (1C = 200 mAh / g), with a charge cut-off voltage of 4.25 V and a discharge cut-off voltage of 3 V, respectively, in a thermostat maintained at 60°C. Here, the discharge capacity of the initial charge-discharge cycle divided by the discharge capacity (reference value) of Comparative Example 1 is defined as the "initial capacity" according to the following mathematical formula 1.

[0144] [Mathematical Expression 1]

[0145] Then, while keeping the charging rate at a fixed 0.1 C and changing the discharging rate sequentially to 0.1 C → 0.2 C → 0.33 C → 0.5 C → 1.0 C, a rate test was conducted. Figure 3 and Figure 4 This is a graph showing the change in discharge capacity relative to the discharge rate in Examples 1 to 3 and Comparative Examples 1 and 2. Figure 3 The vertical axis in the figure shows the measured value of the discharge capacity, and Figure 4 The vertical axis shows the discharge capacity obtained by normalizing each embodiment to 100% of the discharge capacity at 0.1 C for easy comparison of the discharge capacity variations between the embodiments and comparative examples (the normalized discharge capacity is called the "rate characteristic").

[0146] Table 1 below shows the manufacturing conditions, initial capacity values, and results of whether an electron conduction path was sufficiently formed, as determined by SEM observation, for Examples 1 to 3 and Comparative Examples 1 and 2. Here, when the carbon nanotubes in the scanning electron microscope (SEM) image are in contact with the surface of multiple positive electrode active material particles 10, it is determined that an electron conduction path has been sufficiently formed. Otherwise, it is determined that an electron conduction path has not been sufficiently formed.

[0147] [Table 1]

[0148] <Test Example 3: Battery Life Characteristics>

[0149] Subsequently, the cycling characteristics of Examples 1 to 3 and Comparative Example 1 were examined. Specifically, the all-solid-state batteries of Examples 1 to 3 and Comparative Example 1 were aged by charging and discharging twice in the same manner as in Test Example 2, and then the same charge-discharge cycle was repeated eight times at a constant current of 0.5 C. That is, a total of 10 charge-discharge cycles were repeated, including the first and second charge-discharge cycles. Figure 5 This is a graph illustrating the cyclic characteristics of Examples 1 to 3 and Comparative Example 1. (See figure) Figure 5 As shown, Examples 1 to 3 exhibited superior cycling characteristics compared to Comparative Example 1.

[0150] <Discussion of Test Results of Examples and Comparative Examples>

[0151] The test results will be discussed below. However, the following discussion is based on current assumptions, and the invention is not limited to theory.

[0152] As described above, when comparing the initial capacity of Comparative Example 1 based on carbon nanofibers (VGCF) commonly used as conductive materials, the initial capacity of Examples 1 to 3 using carbon nanotubes with a length of 100 μm or more was better than that of Comparative Example 1, and the initial capacity of Comparative Example 2 using carbon nanotubes with a length of about 50 μm was worse than that of Comparative Example 1.

[0153] Based on the results observed by SEM, it is believed that the overall electronic conductivity of the cathode material (especially the electronic conductivity within the cathode active material particles and between the cathode active material and the cathode current collector) is improved. This is because the carbon nanotubes, with lengths exceeding 100 μm, are longer, allowing them to contact multiple cathode active material particles and easily untangle and disperse individually. As a result, firstly, carbon nanotubes mediate electron conduction from the cathode active material near the current collector to the current collector, thereby activating and stabilizing the charge-discharge reaction. Secondly, carbon nanotubes mediate electron conduction from the cathode active material spaced apart from the current collector to the cathode active material near the current collector, thus forming an electron conduction path from the cathode active material spaced apart from the current collector to the cathode active material near the current collector. Therefore, more (ideally all) cathode active material particles can participate in the charge-discharge reaction, and thus each cathode active material particle in the cathode active material layer can participate in the charge-discharge reaction uniformly. Consequently, it is believed that the battery capacity is improved, providing a capacity close to the theoretical capacity.

[0154] In addition, such as Figure 3 and Figure 4 As shown, Examples 1 to 3, which use carbon nanotubes with a length of 100 μm or more, exhibit rate characteristics significantly superior to those of Comparative Example 1. That is, even during high-speed discharge, the battery characteristics do not deteriorate excessively compared to low-speed discharge, and the battery exhibits excellent high-speed discharge characteristics. Because the carbon nanotubes with a length of 100 μm or more form electron conduction paths between the positive electrode active material particles and between the positive electrode active material particles and the positive electrode current collector, more positive electrode active material participates in the charge-discharge reaction more uniformly and efficiently. Therefore, it is believed that capacity reduction can be suppressed during high-speed charge-discharge. It is also believed that because the widely dispersed carbon nanotubes reduce the internal resistance of the electrode, heat loss at high current levels can be suppressed, thereby allowing for efficient charge-discharge.

[0155] In addition, such as Figure 5As shown, Examples 1 to 3, which use carbon nanotubes with a length of 100 μm or more, exhibited cycling characteristics superior to those of Comparative Example 1. Because these long carbon nanotubes are entangled with the positive electrode active material particles, the aforementioned electron conduction pathway is maintained even as the positive electrode active material repeatedly contracts / expands during charge and discharge. Since the carbon nanotubes act as a conductive matrix absorbing this contraction / expansion of the positive electrode active material, the battery is considered to have improved lifespan characteristics.

[0156] Conversely, carbon nanotubes with a length of 50 μm tend to aggregate. This limits their ability to impart conductivity, hindering effective electron conduction between the active material particles and the current collector. Consequently, sufficient battery capacity and rate performance are not expected. Meanwhile, shorter carbon nanotubes (e.g., less than 10 μm in length) generally tend to disperse more easily and are less prone to aggregation compared to 50 μm long carbon nanotubes; however, such short carbon nanotubes are considered insufficient to form adequate electron conduction pathways between the active material particles.

[0157] [Figure Labels]

[0158] 10: Positive electrode active material particles

[0159] 12: Solid electrolyte particles

[0160] 14: Carbon nanotubes

Claims

1. A cathode material for all-solid-state batteries, comprising: Lithium transition metal oxides Solid electrolytes, and Carbon nanotubes in, At least one of the carbon nanotubes has a length of 100 μm or more.

2. The positive electrode material as described in claim 1, in, At least one of the carbon nanotubes has a length of 100 μm-500 μm.

3. The cathode material as described in claim 1, in, At least one of the carbon nanotubes has a length of 100 μm-200 μm.

4. A cathode material for all-solid-state batteries, comprising: Positive electrode active material particles, wherein the positive electrode active material particles contain lithium transition metal oxide and have an average particle size of 1 μm-20 μm; Solid electrolytes, and Carbon nanotubes that contact multiple positive electrode active material particles to form electron conduction paths between the positive electrode active material particles.

5. The cathode material as described in any one of claims 1 to 4, in, In the cathode material, the content of carbon nanotubes is 0.01%-10% by weight.

6. The cathode material as described in any one of claims 1 to 4, in, The carbon nanotubes are multi-walled carbon nanotubes.

7. The cathode material as described in any one of claims 1 to 4, in, The carbon nanotubes form electron conduction pathways between multiple particles that do not contact each other.

8. The cathode material as described in any one of claims 1 to 4, in, The carbon nanotubes are unentangled and do not form aggregates or bundles.

9. The cathode material as described in any one of claims 1 to 4, in, Based on the total weight of the transition metals, the lithium transition metal oxide contains more than 50 mol% nickel.

10. The cathode material as described in any one of claims 1 to 4, in, The solid electrolyte is a sulfide-based solid electrolyte.

11. The cathode material as described in claim 1, in, The average diameter of the carbon nanotubes is 2 nm-20 nm.

12. The cathode material as described in claim 1, in, The aspect ratio of the carbon nanotubes is 10000-100000.

13. An all-solid-state battery, comprising: The positive electrode comprises the positive electrode material according to any one of claims 1 to 4. Negative electrode; and The solid electrolyte layer between the positive electrode and the negative electrode.

14. A method for preparing a positive electrode material for an all-solid-state battery, comprising: The step involves mixing lithium transition metal oxide, solid electrolyte, and carbon nanotubes with a length of 100 μm or more.

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