Composite positive electrode active material

Abstract

To provide a composite positive electrode active material capable of reducing the resistance of a battery.SOLUTION: A composite positive electrode active material has a positive electrode active material and a lithium ion conductive oxide containing at least one of B element and P element on at least a part of the surface of the positive electrode active material. The composite positive electrode active material has a solid electrolyte on at least a part of the surface of the lithium ion conductive oxide. The interface length value A (μm-1) obtained by dividing the length (μm) of the interface between the positive electrode active material and the solid electrolyte confirmed from a SEM image of a cross-section of the composite positive electrode active material by the area (μm2) of the positive electrode active material in the SEM image is 1.326 or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This disclosure relates to composite cathode active materials. [Background technology]

[0002] Various technologies have been proposed regarding composite cathode active materials, such as those disclosed in Patent Document 1. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2021-163580 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] In conventional technology, the area ratio of the solid electrolyte at a predetermined distance from the surface of the composite positive electrode active material particles in a cross-sectional image of the positive electrode active material particles is set to 40% or more. However, even with a high area ratio, if the contact interface length between the positive electrode active material and the solid electrolyte in the electrode is short, the battery resistance will be high.

[0005] This disclosure has been made in view of the above circumstances, and its main purpose is to provide a composite positive electrode active material that can reduce the resistance of a battery. [Means for solving the problem]

[0006] In other words, this disclosure includes the following aspects: <1> A composite positive electrode active material, The composite positive electrode active material comprises a positive electrode active material and a lithium ion conductive oxide containing at least one element from among element B and element P on at least a portion of the surface of the positive electrode active material. The composite positive electrode active material has a solid electrolyte on at least a portion of the surface of the lithium ion conductive oxide, The length (μm) of the interface between the positive electrode active material and the solid electrolyte, confirmed from the SEM image of the cross-section of the composite positive electrode active material, is divided by the area (μm , , [Figure 1] ,

[0012] , ) of the positive electrode active material in the SEM image to obtain an interface length value A (μm -1 ), and the composite positive electrode active material has an interface length value A of 1.326 or more.

[0007] <2> The positive electrode active material is positive electrode active material particles, and the average particle diameter of the positive electrode active material particles is 3 μm or more and 4.5 μm or less. The composite positive electrode active material according to <1>.

[0008] <3> The solid electrolyte is a sulfide-based solid electrolyte. The composite positive electrode active material according to <1> or <2>.

[0009] <4> A positive electrode having a positive electrode layer containing the composite positive electrode active material according to any one of <1> to <3> and a positive electrode current collector.

[0010] <5> A method for manufacturing a composite positive electrode active material, comprising: a first step of coating at least a part of the surface of the positive electrode active material with a lithium ion conductive oxide containing at least one element selected from B element and P element; a second step of coating at least a part of the surface of the lithium ion conductive oxide with a solid electrolyte, and the length (μm) of the interface between the positive electrode active material and the solid electrolyte, confirmed from the SEM image of the cross-section of the composite positive electrode active material, is divided by the area (μm 2 ) of the positive electrode active material in the SEM image to obtain an interface length value A (μm -1 ), and the method for manufacturing a composite positive electrode active material has an interface length value A of 1.326 or more.

Effect of the Invention

[0011] The composite positive electrode active material of the present disclosure can reduce the resistance of the battery.

Brief Description of the Drawings

[0012] [Figure 1]Figure 1 is a graph showing the relationship between the interface length A and the battery resistance. [Modes for carrying out the invention]

[0013] Embodiments of this disclosure are described below. Matters other than those specifically mentioned herein but necessary for the implementation of this disclosure (for example, the general composition and manufacturing process of composite cathode active materials that do not characterize this disclosure) can be understood as design matters for those skilled in the art based on the prior art. This disclosure can be implemented based on the content disclosed herein and common technical knowledge in the art. In this disclosure, "fully charged battery" means when the battery's State of Charge (SOC) is 100%. SOC represents the ratio of the battery's charge capacity to its full charge capacity, with a full charge capacity of 100%. The State of Charge (SOC) can be estimated, for example, from the battery's open-circuit voltage (OCV).

[0014] In this disclosure, a composite positive electrode active material is provided, The composite positive electrode active material comprises a positive electrode active material and a lithium ion conductive oxide containing at least one element from among element B and element P on at least a portion of the surface of the positive electrode active material. The composite positive electrode active material has a solid electrolyte on at least a portion of the surface of the lithium ion conductive oxide, The length (μm) of the interface between the positive electrode active material and the solid electrolyte, as confirmed from the SEM image of the cross-section of the composite positive electrode active material, is defined as the area (μm) of the positive electrode active material in the SEM image. 2 The interface length A (μm) divided by ) -1 The present invention provides a composite positive electrode active material in which the ratio is 1.326 or higher.

[0015] The length (μm) of the interface between the positive electrode active material and the solid electrolyte, as confirmed from the SEM (scanning electron microscope) image of the cross-section of the composite positive electrode active material of this disclosure, is defined as the area (μm) of the positive electrode active material in the SEM image.2 ) divided by the interfacial length value A (μm -1 ) should be 1.326 or more, and the interfacial length value A is 1.326 μm -1 or more and 1.632 μm -1 or less may be acceptable. The interfacial length value A may be controlled by at least one method selected from the group consisting of changing the volume ratio of the solid electrolyte in the composite positive electrode active material, changing the coating rate of the solid electrolyte coated with a lithium ion conductive oxide, and changing the coating method of the solid electrolyte. In the present disclosure, by having the interfacial length value A be 1.326 or more, the resistance of the battery can be reduced.

[0016] Examples of the positive electrode active material include oxide active materials. Examples of the oxide active material include, for example, LiNi 0.8 Co 0.15 Al 0.05 O2, LiCoO2, LiMnO2, LiNiO2, LiVO2, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiMn2O4, Li(Ni 0.5 Mn 1.5 )O4, LiFePO4, LiMnPO4, LiNiPO4, LiCuPO4, and the like. The positive electrode active material may be positive electrode active material particles. The average particle diameter of the positive electrode active material particles may be 3 μm or more and 4.5 μm or less.

[0017] In the present disclosure, unless otherwise specified, the average particle diameter of the particles is the value of the median diameter (D50), which is the particle diameter at the integrated value of 50% in the volume-based particle size distribution measured by laser diffraction / scattering particle size distribution measurement.

[0018] The composite positive electrode active material of the present disclosure only needs to have a lithium ion conductive oxide containing B element and P element on at least a part of the surface of the positive electrode active material, and may have a lithium ion conductive oxide containing B element and P element on the entire surface of the positive electrode active material.

[0019] The lithium-ion conductive oxide may contain at least one element from among elements B and P. Examples of lithium-ion conductive oxides include B2O3, Li2B4O7, LiBPO4, Li3PO4, and LiPO3. The thickness of the lithium-ion conductive oxide may be, for example, 0.1 nm or more, and may be 1 nm or more. On the other hand, the thickness of the lithium-ion conductive oxide may be, for example, 100 nm or less, and may be 20 nm or less. The coverage rate of the lithium-ion conductive oxide coating the positive electrode active material is not particularly limited as long as it satisfies the interface length value A as defined in this disclosure. The coverage rate of the lithium-ion conductive oxide coating the positive electrode active material may be, for example, 70% or more, and may be 90% or more.

[0020] The composite cathode active material of this disclosure may have a solid electrolyte on at least a portion of the surface of the lithium-ion conductive oxide, or it may have a solid electrolyte on the entire surface of the lithium-ion conductive oxide. The coverage rate of the solid electrolyte coating the lithium-ion conductive oxide is not particularly limited as long as it satisfies the interface length value A as defined in this disclosure. The coverage rate of the solid electrolyte coating the lithium-ion conductive oxide may be, for example, 70% or more, and may also be 90% or more.

[0021] Examples of solid electrolytes include sulfide-based solid electrolytes and oxide-based solid electrolytes. Examples of sulfide-based solid electrolytes include solid electrolytes containing Li, M (where M is at least one of P, As, Sb, Si, Ge, Sn, B, Al, Ga, and In), and S. Furthermore, sulfide-based solid electrolytes may further contain at least one of O and halogen elements. Examples of sulfide-based solid electrolytes include Li2S-P2S5, Li2S-SiS2, LiX-Li2S-SiS2, LiX-Li2S-P2S5, LiX-Li2O-Li2S-P2S5, LiX-Li2S-P2O5, LiX-Li3PO4-P2S5, and Li3PS4. The term "Li2S-P2S5" above refers to a material made using a raw material composition containing Li2S and P2S5, and the same applies to other descriptions. Furthermore, the "X" in LiX above represents a halogen element. Examples of halogen elements include F, Cl, Br, and I. The raw material composition containing LiX may contain one or more types of LiX. When two or more types of LiX are included, the mixing ratio of the two or more types is not particularly limited. The molar ratio of each element in a sulfide-based solid electrolyte can be controlled by adjusting the content of each element in the raw materials. Furthermore, the molar ratio and composition of each element in a sulfide-based solid electrolyte can be measured, for example, by ICP emission spectrometry.

[0022] The sulfide-based solid electrolyte may be sulfide glass, crystalline sulfide glass (glass ceramics), or a crystalline material obtained by solid-phase reaction treatment of the raw material composition. The crystalline state of sulfide-based solid electrolytes can be confirmed, for example, by performing powder X-ray diffraction measurements using CuKα radiation on the sulfide-based solid electrolyte.

[0023] Sulfide glass can be obtained by amorphous treatment of a raw material composition (e.g., a mixture of Li2S and P2S5). Examples of amorphous treatment include mechanical milling.

[0024] Glass ceramics can be obtained, for example, by heat-treating sulfide glass. The heat treatment temperature should be higher than the crystallization temperature (Tc) observed by thermal analysis of the sulfide glass, and is typically 195°C or higher. However, there is no particular upper limit to the heat treatment temperature. The crystallization temperature (Tc) of sulfide glass can be measured by differential thermal analysis (DTA). The heat treatment time is not particularly limited as long as it is sufficient to achieve the desired degree of crystallinity of the glass ceramic, but it is typically within the range of 1 minute to 24 hours, with a more typical range being 1 minute to 10 hours. The heat treatment method is not particularly limited, but one example is the use of a firing furnace.

[0025] Examples of oxide-based solid electrolytes include materials having a garnet-type crystal structure containing elements Li, La, A (where A is at least one of Zr, Nb, Ta, and Al), and O. Examples of oxide-based solid electrolytes include Li2O-B2O3-P2O5, Li2O-SiO2, Li2O-B2O3, Li 1.3 Al 0.3 Ti 0.7 (PO4)3, Li5La3Ta2O 12 Li7La3Zr2O 12 Li6BaLa2Ta2O 12 Li 3.6 Si 0.6 P 0.4 O4, Li4SiO4, Li3PO4, and Li 3+x PO 4-x N x (1 ≤ x ≤ 3) or similar conditions may also be acceptable.

[0026] The solid electrolyte may be in particulate form from the standpoint of ease of handling. Furthermore, the average particle size (D50) of the solid electrolyte particles is not particularly limited, but the lower limit may be 0.5 μm or more, and the upper limit may be 2 μm or less.

[0027] This disclosure describes a method for producing a composite cathode active material, A first step involves coating at least a portion of the surface of the positive electrode active material with a lithium-ion conductive oxide containing at least one element from among elements B and P, The process includes a second step of coating at least a portion of the surface of the lithium-ion conductive oxide with a solid electrolyte, The length (μm) of the interface between the positive electrode active material and the solid electrolyte, as confirmed from the SEM image of the cross-section of the composite positive electrode active material, is defined as the area (μm) of the positive electrode active material in the SEM image. 2 The interface length A (μm) divided by ) -1 The present invention provides a method for producing a composite positive electrode active material in which the ratio is 1.326 or higher.

[0028] In this disclosure, the first step involves coating at least a portion of the surface of the positive electrode active material with a lithium-ion conductive oxide, and the second step involves further coating at least a portion of that surface with a solid electrolyte. The coating methods in the first and second steps are not particularly limited, and conventionally known methods can be used as appropriate.

[0029] The composite positive electrode active material of this disclosure is typically used in the manufacture of positive electrodes for batteries. The method for manufacturing a positive electrode according to the present disclosure comprises the step of coating a positive electrode slurry onto at least one surface of a positive electrode current collector and drying it. The positive electrode slurry may contain a composite positive electrode active material, a conductive material, a binder, a thickener, a solvent, etc. The coating method for the positive electrode slurry is not particularly limited, and conventionally known methods can be employed.

[0030] Examples of materials for the positive electrode current collector include metals such as aluminum, copper, stainless steel, and nickel. The thickness of the positive electrode current collector is, for example, 0.1 μm or more and 100 μm or less. The shape of the positive electrode current collector may be sheet-like or the like.

[0031] Examples of binders include acrylonitrile butadiene rubber (ABR), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and styrene-butadiene rubber (SBR).

[0032] Examples of conductive materials include carbon materials, metal particles, and conductive polymers. Examples of carbon materials include particulate carbon materials such as acetylene black (AB) and Ketjenblack (KB); and fibrous carbon materials such as carbon fibers, carbon nanotubes (CNTs), and carbon nanofibers (CNFs).

[0033] Examples of thickening agents include polysaccharides such as carboxymethylcellulose (CMC) and methylcellulose.

[0034] Examples of solvents include aqueous solvents and organic solvents. An aqueous solvent refers to water or a mixed solvent containing water and a polar organic solvent. For example, an appropriate solvent may be selected depending on the type of positive electrode active material, binder, etc. As an aqueous solvent, water is preferably used due to its ease of handling. Examples of polar organic solvents that can be used as a mixed solvent include alcohols such as methanol, ethanol, and isopropyl alcohol, ketones such as acetone, and ethers such as tetrahydrofuran. Examples of organic solvents include 1,2,3,4-tetrahydronaphthalene, n-heptane, butyl butyrate, diisobutyl ketone, and N-methyl-2-pyrrolidone (NMP).

[0035] The positive electrode of this disclosure comprises a positive electrode current collector and a positive electrode layer formed by drying a positive electrode slurry coated on at least one surface of the positive electrode current collector. The positive electrode layer contains the above-mentioned composite positive electrode active material and may optionally contain the above-mentioned solid electrolyte, the above-mentioned binder, the above-mentioned conductive material, etc. The content ratio of the composite positive electrode active material in the positive electrode layer is not particularly limited and may be 50.0 to 81.2% by mass. The content ratio of the solid electrolyte in the positive electrode layer is not particularly limited and may be 0 to 16.5% by mass. The interface length A in this disclosure is the length (μm) of the interface between the positive electrode active material and the solid electrolyte as confirmed from the SEM image of the cross-section of the positive electrode layer, and the area (μm) of the positive electrode active material in the SEM image. 2The value obtained by dividing by ) is also acceptable.

[0036] The positive electrode of this disclosure is typically used in the manufacture of batteries. A battery comprises a positive electrode, an electrolyte layer, and a negative electrode.

[0037] The electrolyte layer may be a liquid electrolyte layer using an electrolyte solution, or it may be a solid electrolyte layer using a solid electrolyte. The electrolyte can be a conventionally known electrolyte used in lithium-ion secondary batteries. The solid electrolyte layer contains at least a solid electrolyte. As the solid electrolyte to be contained in the solid electrolyte layer, any known solid electrolyte usable in solid-state batteries can be used as appropriate, including the oxide-based solid electrolyte and sulfide-based solid electrolyte mentioned above. In order to suppress the peeling of the positive electrode layer and the negative electrode layer from the solid electrolyte layer, a relatively soft sulfide-based solid electrolyte may be used as the solid electrolyte.

[0038] Solid electrolytes can be used individually or in combination of two or more types. When using two or more types of solid electrolytes, they may be mixed together, or two or more layers of solid electrolytes may be formed to create a multilayer structure. The proportion of solid electrolyte in the solid electrolyte layer is not particularly limited, but may be, for example, 50% by mass or more, and may be in the range of 60% by mass or more and 100% by mass or less, or in the range of 70% by mass or more and 100% by mass or less, or 100% by mass.

[0039] The solid electrolyte layer may contain a binder to provide plasticity and other properties. Examples of such binders include the materials exemplified above as binders used in the positive electrode layer. However, in order to facilitate higher power output, the amount of binder contained in the solid electrolyte layer may be 5% by mass or less, from the viewpoint of preventing excessive aggregation of the solid electrolyte and enabling the formation of a solid electrolyte layer with uniformly dispersed solid electrolyte.

[0040] The thickness of the solid electrolyte layer is not particularly limited, but is usually between 0.1 μm and 1 mm.

[0041] The negative electrode includes a negative electrode layer and a negative electrode current collector. The negative electrode layer contains a negative electrode active material and, if necessary, a conductive material, binder, etc. Examples of negative electrode active materials include carbon active materials, oxide active materials, and metal active materials. Examples of carbon active materials include mesocarbon microbeads (MCMB), highly oriented graphite (HOPG), hard carbon, and soft carbon. Examples of oxide active materials include Nb2O5 and Li4Ti5O 12 Examples of metal active materials include SiO. Examples of metal active materials include In, Al, Si, and Sn. Examples of conductive materials and binders include the conductive materials and binders used in the positive electrode layer described above.

[0042] The material of the negative electrode current collector may be a material that does not alloy with Li, and examples include SUS, copper, and nickel. Examples of negative electrode current collector shapes include foil and plate shapes. The planar shape of the negative electrode current collector is not particularly limited, but examples include circular, elliptical, rectangular, and any polygonal shape. The thickness of the negative electrode current collector varies depending on the shape, but may be in the range of 1 μm to 50 μm, or in the range of 5 μm to 20 μm.

[0043] The type of battery is not particularly limited, but lithium-ion secondary batteries are an example. The battery may be a liquid-type battery using an electrolyte solution, or a solid-state battery using a solid electrolyte solution. Applications of the battery include, for example, powering vehicles such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), battery electric vehicles (BEVs), gasoline cars, and diesel cars. In particular, it may be used as a power source for hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), or electric vehicles (BEVs). The battery may also be used as a power source for mobile devices other than vehicles (e.g., trains, ships, aircraft), or as a power source for electrical products such as information processing devices. [Examples]

[0044] (Examples 1-3, Comparative Examples 1-3) [Fabrication of composite cathode active materials] LiNi 0.8 Co 0.15 Al 0.05 O2 particles were used. As a lithium-ion conductive oxide, particles of a compound containing at least one element from among elements B and P were used. As a solid electrolyte, particles of sulfide glass solid electrolyte were used. The surface of the positive electrode active material particles was coated with lithium-ion conductive oxide particles. The positive electrode active material particles coated with lithium-ion conductive oxide particles and the solid electrolyte particles were stirred and mixed to obtain composite positive electrode active material particles in which the surface of the lithium-ion conductive oxide particles was coated with solid electrolyte particles. [Positive electrode fabrication] 1,2,3,4-tetrahydronaphthalene was used as the solvent. Particles of the composite cathode active material obtained above were used as the cathode active material. Particles of sulfide glass solid electrolyte were used as the solid electrolyte. SBR was used as the binder. Carbon nanotubes were used as the conductive material. The composite cathode active material particles, solid electrolyte particles, binder, and conductive material were mixed in the solvent in the following mass composition ratios to prepare a cathode slurry. Mass composition ratio: Composite cathode active material: Solid electrolyte: Binder: Conductive material = 81.2:16.5:0.3:1.9 The prepared positive electrode slurry was coated onto the positive electrode current collector. The coated positive electrode slurry was then dried. This resulted in a positive electrode having a positive electrode layer on the positive electrode current collector.

[0045] [Fabrication of solid electrolyte layer] n-heptane and butyl butyrate were used as solvents. A sulfide glass solid electrolyte was used as the solid electrolyte. ABR (acrylonitrile butadiene rubber) was used as the binder. The solid electrolyte and binder were mixed in the solvent to prepare a solid electrolyte slurry. The prepared solid electrolyte slurry was coated onto a release film. The coated solid electrolyte slurry was then dried. After drying, the release film was peeled off the solid electrolyte coated foil to obtain the solid electrolyte layer.

[0046] [Negative electrode fabrication] Diisobutyl ketone was used as the solvent. Li4Ti5O was used as the negative electrode active material. 12 The following materials were used. A sulfide glass solid electrolyte was used as the solid electrolyte. SBR was used as the binder. Carbon nanotubes were used as the conductive material. The negative electrode slurry was prepared by mixing the negative electrode active material, solid electrolyte, binder, and conductive material in a solvent in the following mass composition ratios. Mass composition ratio: Negative electrode active material: Solid electrolyte: Binder: Conductive material = 72.2:24.3:1.8:2.4 The prepared negative electrode slurry was coated onto the negative electrode current collector. The coated negative electrode slurry was then dried. This resulted in a negative electrode having a negative electrode layer on the negative electrode current collector.

[0047] [Cell creation] The fabricated positive electrode, the fabricated solid electrolyte layer, and the fabricated negative electrode were arranged in this order to obtain a laminate. A positive electrode tab was attached to the positive electrode, and a negative electrode tab was attached to the negative electrode. The laminate was then placed inside a laminate film, and the laminate was sealed by creating a vacuum inside the laminate film to produce a laminate cell (sometimes referred to as a cell). The cell confinement pressure was set to 5 MPa relative to the electrode area.

[0048] [Method for calculating the interface length between the positive electrode active material and the solid electrolyte in the positive electrode layer] First, a binary image of the positive electrode active material and the solid electrolyte was created from the cross-sectional SEM image of the positive electrode layer. The creation of the binary image was performed using image analysis software. In this case, it was performed using "ImageJ". Next, the positive electrode active material area in the image and the contact interface length between the positive electrode active material and the solid electrolyte were determined using image analysis software. In this case, the calculation was performed using "MATLAB (registered trademark)". Using these values, the normalized interface length value A was calculated by the following formula. The results are shown in Table 1. Interface length value A [μm -1 = (Contact interface length between positive electrode active material and solid electrolyte in the analysis image [μm]) / (Area of positive electrode active material in the analysis image [μm 2 ) Each of the cells of Examples 1 to 3 and Comparative Examples 1 to 3 has the same configuration except that the interface length value A between the positive electrode active material and the solid electrolyte in the positive electrode layer is the value shown in Table 1. The interface length value A was controlled by at least one method selected from the group consisting of changing the volume ratio of the solid electrolyte in the composite positive electrode active material contained in the positive electrode layer, changing the coating rate of the solid electrolyte coated with the lithium ion conductive oxide, and changing the coating method of the solid electrolyte. A positive electrode layer with a larger interface length value A than a positive electrode layer with a smaller interface length value A has a larger area where the positive electrode active material can undergo insertion / desorption reactions of lithium ions, leading to a reduction in battery resistance.

[0049] [Charge-discharge evaluation] The fabricated laminate cell was subjected to charge-discharge evaluation. The implementation test was as follows. Activation, capacity measurement, and battery resistance measurement were performed at 25°C. · Activation: CCCV charge 0.333C - 0.01C cut upper limit 2.80V → CCCV discharge 0.333C - 0.01C cut lower limit 1.5V · Capacity measurement: The program is the same as that for activation. · Battery resistance measurement: When a current value at a 2.5C rate was passed at SOC20% (discharge resistance), the voltage change ΔV value was read, and the resistance value was calculated from Ohm's law V = IR. Durability Test: A cycle test was conducted at a cycle voltage range of 1.45 to 2.80 V, 60°C, and a rate of 1C. After the durability test, the capacity was measured again, and then the battery resistance after the durability test was measured. The battery resistance after the durability test is shown in Table 1.

[0050] [Table 1]

[0051] Figure 1 is a graph showing the relationship between the interface length A and the battery resistance. As shown in Figure 1 and Table 1, in this disclosure, the resistance of the battery can be reduced by having the interface length value A be 1.326 or greater.

Claims

1. A composite positive electrode active material, The composite positive electrode active material comprises a positive electrode active material and a lithium ion conductive oxide containing at least one element from among element B and element P on at least a portion of the surface of the positive electrode active material. The composite positive electrode active material has a solid electrolyte on at least a portion of the surface of the lithium ion conductive oxide, The length (μm) of the interface between the positive electrode active material and the solid electrolyte, as confirmed from the SEM image of the cross-section of the composite positive electrode active material, is defined as the area (μm) of the positive electrode active material in the SEM image. ２ The interface length A (μm) is obtained by dividing by ). －１ ) is 1.326 or higher, The positive electrode active material is a positive electrode active material particle, The average particle size of the positive electrode active material particles is 3 μm or more and 4.5 μm or less. The solid electrolyte is a sulfide-based solid electrolyte, which is a composite cathode active material.

2. A positive electrode having a positive electrode layer containing the composite positive electrode active material described in claim 1, and a positive electrode current collector.

3. A method for producing a composite positive electrode active material, A first step involves coating at least a portion of the surface of the positive electrode active material with a lithium-ion conductive oxide containing at least one element from among element B and element P, The process includes a second step of coating at least a portion of the surface of the lithium-ion conductive oxide with a solid electrolyte, The length (μm) of the interface between the positive electrode active material and the solid electrolyte, as confirmed from the SEM image of the cross-section of the composite positive electrode active material, is defined as the area (μm) of the positive electrode active material in the SEM image. ２ The interface length A (μm) is obtained by dividing by ). －１ ) is 1.326 or higher, The positive electrode active material is a positive electrode active material particle, The average particle size of the positive electrode active material particles is 3 μm or more and 4.5 μm or less. The method for producing a composite cathode active material, wherein the solid electrolyte is a sulfide-based solid electrolyte.

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