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] The present disclosure relates to a composite positive electrode active material.

Background Art

[0002] Various techniques have been proposed regarding composite positive electrode active materials as disclosed in Patent Document 1.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

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

[0005] The present disclosure has been made in view of the above circumstances, and the main object thereof is to provide a composite positive electrode active material capable of reducing the resistance of a battery.

Means for Solving the Problems

[0006] That is, the present disclosure includes the following aspects. <1> A composite positive electrode active material, wherein the composite positive electrode active material has a positive electrode active material and a lithium ion conductive oxide containing at least one element of B element and P element on at least a part of the surface of the positive electrode active material, and 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 length (μm) of the interface between the cathode active material and the solid electrolyte as confirmed from the SEM image of the cross-section of the composite cathode active material is divided by the area (μm 2 ) of the cathode active material in the SEM image to obtain an interface length value A (μm -1 ), and the composite cathode active material has an interface length value A of 1.326 or more.

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

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

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

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

Advantages of the Invention

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

Brief Description of the Drawings

[0012]

Figure 1

DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments according to the present disclosure will be described. Matters other than those specifically mentioned in this specification and necessary for the implementation of the present disclosure (for example, the general configuration and manufacturing process of a composite cathode active material that does not characterize the present disclosure) can be grasped as design matters of those skilled in the art based on the prior art in the relevant field. The present disclosure can be implemented based on the content disclosed in this specification and common general knowledge in the relevant field. In the present disclosure, the fully charged state of the battery means the state when the state of charge (SOC) value of the battery is 100%. SOC indicates the ratio of the charged capacity to the fully charged capacity of the battery, and the fully charged capacity is SOC 100%. SOC may be estimated, for example, from the open circuit voltage (OCV) of the battery.

[0014] In the present disclosure, a composite cathode active material The composite cathode active material has a cathode 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 cathode active material. The composite cathode active material has a solid electrolyte on at least a part of the surface of the lithium ion conductive oxide. The interface length (μm) between the cathode active material and the solid electrolyte confirmed from the SEM image of the cross section of the composite cathode active material of the present disclosure is divided by the area (μm 2 ) of the cathode active material in the SEM image to obtain an interface length value A (μm -1 ), and a composite cathode active material with an interface length value A of 1.326 or more is provided.

[0015] The interface length (μm) between the cathode active material and the solid electrolyte confirmed from the SEM (scanning electron microscope) image of the cross section of the composite cathode active material of the present disclosure is the area (μm of the cathode 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. 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 the lithium ion conductive oxide, and changing the coating method of the solid electrolyte. In the present disclosure, by the interfacial length value A being 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, etc. 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 50% of the integrated value 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 of the elements B and P. Examples of the lithium ion conductive oxide include B2O3, Li2B4O7, LiBPO4, Li3PO4, LiPO3, and the like. The thickness of the lithium ion conductive oxide is, 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 is, 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 interfacial length value A defined in the present disclosure. The coverage rate of the lithium ion conductive oxide coating the positive electrode active material is, for example, 70% or more, and may be 90% or more.

[0020] The composite positive electrode active material of the present disclosure may have a solid electrolyte on at least a part of the surface of the lithium ion conductive oxide, or 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 interfacial length value A defined in the present disclosure. The coverage rate of the solid electrolyte coating the lithium ion conductive oxide is, for example, 70% or more, and may be 90% or more.

[0021] Examples of the solid electrolyte include sulfide-based solid electrolytes and oxide-based solid electrolytes. Examples of the sulfide-based solid electrolyte include solid electrolytes containing Li element, M element (M is at least one of P, As, Sb, Si, Ge, Sn, B, Al, Ga, In), and S element. Further, the sulfide-based solid electrolyte may further contain at least one of O element and halogen element. Examples of sulfide 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. Note that the description of "Li2S-P2S5" means a material obtained using a raw material composition containing Li2S and P2S5, and the same applies to other descriptions. In addition, "X" in the above LiX represents a halogen element. Examples of halogen elements include the F element, Cl element, Br element, and I element. LiX may be contained in the raw material composition containing LiX in one or more than two kinds. When two or more kinds of LiX are contained, the mixing ratio of the two or more kinds is not particularly limited. The molar ratio of each element in the sulfide solid electrolyte can be controlled by adjusting the content of each element in the raw material. Also, the molar ratio and composition of each element in the sulfide solid electrolyte can be measured, for example, by ICP emission spectrometry.

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

[0023] The sulfide glass can be obtained by subjecting a raw material composition (for example, a mixture of Li2S and P2S5) to an amorphous treatment. Examples of the amorphous treatment include mechanical milling.

[0024] The glass ceramics can be obtained, for example, by heat-treating the sulfide glass. The heat treatment temperature may be higher than the crystallization temperature (Tc) observed by thermal analysis measurement of the sulfide glass, and is usually 195°C or higher. On the other hand, the upper limit of the heat treatment temperature is not particularly limited. The crystallization temperature (Tc) of the sulfide glass can be measured by differential thermal analysis (DTA). The heat treatment time is not particularly limited as long as it can obtain the desired degree of crystallization of the glass ceramics. For example, it is in the range of 1 minute to 24 hours, and among them, the range of 1 minute to 10 hours can be mentioned. The method of heat treatment is not particularly limited. For example, a method using a firing furnace can be mentioned.

[0025] Examples of the oxide-based solid electrolyte include substances having a garnet-type crystal structure containing, for example, an Li element, a La element, an A element (A is at least one of Zr, Nb, Ta, and Al), and an O element. Examples of the oxide-based solid electrolyte include, for example, 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) etc. may also be possible.

[0026] From the viewpoint of good handleability, the shape of the solid electrolyte may be particulate. Also, the average particle size (D50) of the particles of the solid electrolyte 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] In the present disclosure, a method for manufacturing a composite cathode active material, a first step of coating at least a part of the surface of the cathode active material with a lithium ion conductive oxide containing at least one of the B element and the 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 having, 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 the area (μm of the positive electrode active material in the SEM image 2 ) divided by the interface length value A (μm -1 ) is 1.326 or more, and a method for producing a composite positive electrode active material is provided.

[0028] In the present disclosure, as a first step, at least a part of the surface of the positive electrode active material is coated with a lithium ion conductive oxide, and as a second step, at least a part of the surface is further coated with a solid electrolyte. The coating methods in the first step and the second step are not particularly limited, and conventionally known methods can be appropriately adopted.

[0029] The composite positive electrode active material of the present disclosure is usually used for manufacturing the positive electrode of a battery. The method for manufacturing a positive electrode of the present disclosure includes a step of coating a positive electrode slurry on 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, and the like. The coating method of the positive electrode slurry is not particularly limited, and a conventionally known method can be adopted.

[0030] Examples of the material of the positive electrode current collector include metals such as aluminum, copper, SUS, 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 a sheet shape or the like.

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

[0032] Examples of the conductive material include carbon materials, metal particles, conductive polymers, and the like. Examples of the carbon materials include particulate carbon materials such as acetylene black (AB) and ketjen black (KB); fibrous carbon materials such as carbon fibers, carbon nanotubes (CNT), and carbon nanofibers (CNF).

[0033] Examples of the thickener include polysaccharides such as carboxymethyl cellulose (CMC) and methyl cellulose.

[0034] Examples of the solvent include aqueous solvents, organic solvents, and the like. The aqueous solvent means water or a mixed solvent containing water and a polar organic solvent. For example, an appropriate solvent can be selected according to the types of the positive electrode active material, binder, and the like. From the viewpoint of ease of handling, water can be preferably used as the aqueous solvent. Examples of the polar organic solvent that can be used in the mixed solvent include alcohols such as methanol, ethanol, and isopropyl alcohol; ketones such as acetone; ethers such as tetrahydrofuran. Examples of the organic solvent include 1,2,3,4-tetrahydronaphthalene, n-heptane, butyl butyrate, diisobutyl ketone, N-methyl-2-pyrrolidone (NMP), and the like.

[0035] The positive electrode of the present disclosure includes 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 composite positive electrode active material and may contain the solid electrolyte, the binder, the conductive material, and the like as necessary. 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 interfacial length value A of the present disclosure is 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 positive electrode layer, divided by the area (μm 2It may also be a value divided by

[0036] The positive electrode of the present disclosure is usually used in the manufacture of batteries. The battery includes a positive electrode, an electrolyte layer, and a negative electrode.

[0037] The electrolyte layer may be a liquid electrolyte layer using an electrolytic solution as the electrolyte, or may be a solid electrolyte layer using a solid electrolyte as the electrolyte. As the electrolytic solution, a conventionally known electrolytic solution used in a lithium-ion secondary battery can be used. The solid electrolyte layer contains at least a solid electrolyte. As the solid electrolyte to be contained in the solid electrolyte layer, a known solid electrolyte that can be used in a solid battery can be appropriately used, and examples include the above-described oxide-based solid electrolyte and sulfide-based solid electrolyte. In order to suppress 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] The solid electrolyte can be used alone or in combination of two or more. When using two or more solid electrolytes, two or more solid electrolytes may be mixed, or layers of two or more solid electrolytes may be formed to form a multilayer structure. The proportion of the solid electrolyte in the solid electrolyte layer is not particularly limited, but is, 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 may be in the range of 70% by mass or more and 100% by mass or less, or may be 100% by mass.

[0039] From the viewpoint of expressing plasticity, etc., the solid electrolyte layer can also contain a binder. Examples of such a binder include the materials exemplified as the binder used in the above-described positive electrode layer. However, from the viewpoint of facilitating high output and forming a solid electrolyte layer having a solid electrolyte that prevents excessive aggregation of the solid electrolyte and is uniformly dispersed, the binder contained in the solid electrolyte layer may be 5% by mass or less.

[0040] The thickness of the solid electrolyte layer is not particularly limited and is usually 0.1 μm or more and 1 mm or less.

[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, a binder, and the like. Examples of the negative electrode active material include carbon active materials, oxide active materials, and metal active materials. Examples of the carbon active material include mesocarbon microbeads (MCMB), highly oriented pyrolytic graphite (HOPG), hard carbon, and soft carbon. Examples of the oxide active material include Nb2O5, Li4Ti5O 12 and SiO. Examples of the metal active material include In, Al, Si, and Sn. Examples of the conductive material and the binder include the materials exemplified as the conductive material and the binder used for the above-described positive electrode layer.

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

[0043] The type of battery is not particularly limited, and examples include lithium-ion secondary batteries. The battery may be a liquid-based battery using an electrolytic solution as an electrolyte, or a solid battery using a solid electrolyte as an electrolyte. Examples of the uses of the battery include power sources for vehicles such as hybrid vehicles (HEVs), plug-in hybrid vehicles (PHEVs), battery electric vehicles (BEVs), gasoline vehicles, and diesel vehicles. Among them, it may be used as a driving power source for a hybrid vehicle (HEV), a plug-in hybrid vehicle (PHEV), or a battery electric vehicle (BEV). Further, the battery may be used as a power source for a moving body other than a vehicle (e.g., railway, ship, aircraft), or may be used as a power source for electrical products such as information processing devices.

Examples

[0044] (Examples 1 to 3, Comparative Examples 1 to 3) [Preparation of Composite Cathode Active Material] Particles of LiNi 0.8 Co 0.15 Al 0.05 O2 were used. Particles of a compound containing at least one of B element and P element were used as the lithium-ion conductive oxide. Particles of a sulfide glass solid electrolyte were used as the solid electrolyte. The surface of the particles of the cathode active material was coated with particles of the lithium-ion conductive oxide. The particles of the cathode active material coated with the particles of the lithium-ion conductive oxide and the particles of the solid electrolyte were stirred and mixed to obtain particles of a composite cathode active material in which the surface of the particles of the lithium-ion conductive oxide was coated with the particles of the solid electrolyte. [Preparation of Cathode] 1,2,3,4-tetrahydronaphthalene was used as the solvent. The particles of the composite cathode active material obtained above were used as the cathode active material. Particles of a 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 particles of the composite cathode active material, the particles of the solid electrolyte, the binder, and the conductive material were mixed in the following mass composition ratio in a solvent 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 on a positive electrode current collector. Thereafter, the coated positive electrode slurry was dried. As a result, a positive electrode having a positive electrode layer on the positive electrode current collector was obtained.

[0045] [Preparation 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 a binder. The solid electrolyte and the binder were mixed in a solvent to prepare a solid electrolyte slurry. The prepared solid electrolyte slurry was coated on a release film. Thereafter, the coated solid electrolyte slurry was dried. The release film was peeled off from the dried solid electrolyte-coated foil to obtain a solid electrolyte layer.

[0046] [Preparation of Negative Electrode] Diisobutyl ketone was used as a solvent. Li4Ti5O 12 was used as the negative electrode active material. A sulfide glass solid electrolyte was used as the solid electrolyte. SBR was used as a binder. Carbon nanotubes were used as a conductive material. The negative electrode active material, the solid electrolyte, the binder, and the conductive material were mixed in a solvent at the following mass composition ratio to prepare a negative electrode slurry. 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 on a negative electrode current collector. Thereafter, the coated negative electrode slurry was dried. As a result, a negative electrode having a negative electrode layer on the negative electrode current collector was obtained.

[0047] [Preparation of Cell] The prepared positive electrode, the prepared solid electrolyte layer, and the prepared 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. Thereafter, the laminate was housed in a laminate film, the inside of the laminate film was evacuated, and the laminate was sealed to prepare a laminate cell (sometimes referred to as a cell). The cell restraint pressure was set to 5 MPa with respect to the electrode area.

[0048] [Calculation Method for Interface Length between Positive Electrode Active Material and Solid Electrolyte in 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 and deinsertion reactions of lithium ions, leading to a reduction in battery resistance.

[0049] [Charge-discharge evaluation] Charge-discharge evaluation was performed on the fabricated laminate cell. 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. Endurance test: A cycle test was carried out at a cycle voltage range of 1.45 to 2.80 V, 60 °C, and a 1C rate. After the endurance test, the capacity was measured again, and then the battery resistance after the endurance test was measured. The battery resistance after the endurance test is shown in Table 1.

[0050]

Table 1

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

Claims

1. A composite positive electrode active material, wherein the composite positive electrode active material has a positive electrode active material and a lithium ion conductive oxide containing at least one element of B element and P element on at least a part of the surface of the positive electrode active material, and 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 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, divided by the area (μm ２ 2) of the positive electrode active material in the SEM image, gives an interface length value A (μm －１ 2) of 1.326 or more. A composite positive electrode active material.

2. The positive electrode active material is positive electrode active material particles, and an 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 Claim 1.

3. The solid electrolyte is a sulfide-based solid electrolyte. The composite positive electrode active material according to Claim 1.

4. A positive electrode having a positive electrode layer containing the composite positive electrode active material according to Claim 1 and a positive electrode current collector.

5. A method for manufacturing a composite positive electrode active material, including a first step of coating at least a part of the surface of a positive electrode active material with a lithium ion conductive oxide containing at least one element of B element and P element, and a second step of coating at least a part 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 divided by the area (μm ２ 2) of the positive electrode active material in the SEM image to obtain an interface length value A (μm －１ 2), and the method for producing a composite positive electrode active material, wherein the value is 1.326 or more.

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