Method for producing positive electrode active material for lithium secondary battery, and lithium secondary battery

Abstract

To provide a method for producing a positive electrode active material for a lithium secondary battery, the positive electrode active material being capable of imparting excellent high-temperature storage characteristics and reducing impedance, when used as a positive electrode active material of a lithium secondary battery.SOLUTION: A method for producing a positive electrode active material for a lithium secondary battery includes: a first mixing step of mixing a lithium compound, a cobalt compound and an aluminum compound and obtaining a first mixture; a first calcination step of calcining the first mixture and obtaining, as a first calcined product, aluminum-containing lithium cobalt composite oxide particles which are such that aluminum is present in a solid solution at least inside the particles; and a second mixing step of dry mixing the first calcined product obtained in the first calcination step and MgF2 and AlF3 which are inorganic fluoride particles and obtaining, as a second mixture, a positive electrode active material.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing a positive electrode active material for a lithium secondary battery and a lithium secondary battery. [Background technology]

[0002] In recent years, with the rapid advancement of portable and cordless home appliances, lithium-ion secondary batteries have been put to practical use as power sources for small electronic devices such as laptop computers, mobile phones, video cameras, etc. Since Mizushima et al. reported in 1980 that lithium cobalt oxide is useful as a positive electrode active material for lithium-ion secondary batteries, research and development into lithium-based composite oxides has been actively carried out, and many proposals have been made to date.

[0003] However, as electronic devices become larger and more sophisticated, further improvements in various battery properties are required.

[0004] The present applicant has previously proposed various positive electrode active materials for lithium secondary batteries that can provide lithium secondary batteries with excellent cycle characteristics even under high voltage. For example, in Patent Document 1, a mixture of lithium-cobalt composite oxide particles and inorganic fluoride particles is used as the positive electrode active material for lithium secondary batteries, thereby providing lithium secondary batteries with excellent cycle characteristics, a high energy capacity retention rate, and a small decrease in average operating voltage. Furthermore, in Patent Document 2, a mixture of titanium-containing lithium-cobalt composite oxide particles and inorganic fluoride particles is used as the positive electrode active material for lithium secondary batteries, thereby providing lithium secondary batteries with excellent cycle characteristics, a small decrease in average operating voltage, a high average operating voltage retention rate, and a high energy density retention rate. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2020-064712 [Patent Document 2] Japanese Patent Application Publication No. 2020-064711 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in response to the recent demand for improved battery characteristics, the above-mentioned conventional techniques have not yet achieved a high level of performance in various battery performance areas.

[0007] In particular, batteries used in electric bicycles, electric vehicles, robots, drones, backup power supplies, etc. are expected to be used in high-temperature environments and at high voltages and high power outputs.

[0008] Therefore, it has been desired that the deterioration of battery characteristics be suppressed even in a high-temperature environment, that the battery have excellent high-temperature storage characteristics, and that the battery have low impedance.

[0009] Therefore, an object of the present invention is to provide an industrially advantageous method for producing a positive electrode active material for a lithium secondary battery that can impart excellent high-temperature storage characteristics and reduce impedance when used as a positive electrode active material for a lithium secondary battery, and to provide a lithium secondary battery that has excellent high-temperature storage characteristics and low impedance. [Means for solving the problem]

[0010] In view of the above circumstances, the present inventors have conducted extensive research and have found that by mixing aluminum-containing lithium-cobalt composite oxide particles, in which aluminum is contained in the form of a solid solution inside the lithium-cobalt composite oxide particles, with inorganic fluoride particles of MgF2 and AlF3, and using the resulting mixture as a positive electrode active material for a lithium secondary battery, a lithium secondary battery having excellent high-temperature storage characteristics and low impedance can be obtained, and have completed the present invention.

[0011] That is, the present invention (1) comprises a mixture of aluminum-containing lithium-cobalt composite oxide particles and inorganic fluoride particles, the aluminum-containing lithium-cobalt composite oxide particles have aluminum present in the form of a solid solution at least inside the aluminum-containing lithium-cobalt composite oxide particles, at least a part of the inorganic fluoride particles is present on the particle surface of the aluminum-containing lithium-cobalt composite oxide particles, The present invention provides a positive electrode active material for lithium secondary batteries, characterized in that, when observed under a scanning electron microscope (SEM) at 50,000 times magnification, the proportion of inorganic fluoride particles present on the particle surfaces of the aluminum-containing lithium cobalt composite oxide particles, which have a particle diameter of 200 nm or more as determined by SEM, is 4.0% or less.

[0012] The present invention (2) also provides a positive electrode active material for a lithium secondary battery according to (1), characterized in that, when observed under a scanning electron microscope (SEM) at a magnification of 50,000 times, the proportion of inorganic fluoride particles present on the particle surfaces of the aluminum-containing lithium cobalt composite oxide particles, which have a particle diameter of less than 120 nm as determined by SEM, is 92.0% or more.

[0013] The present invention (3) also provides a positive electrode active material for a lithium secondary battery according to (1) or (2), characterized in that, when observed under a scanning electron microscope (SEM) at a magnification of 50,000 times, the ratio of inorganic fluoride particles present on the particle surfaces of the aluminum-containing lithium-cobalt composite oxide particles having a ratio of their major axis to their minor axis (major axis / minor axis) of less than 1.5 accounts for 65.0% or more.

[0014] The present invention (4) also provides a positive electrode active material for a lithium secondary battery according to any one of (1) to (3), characterized in that, when observed under a scanning electron microscope (SEM) at a magnification of 50,000 times, the ratio of inorganic fluoride particles present on the particle surfaces of the aluminum-containing lithium-cobalt composite oxide particles having a ratio of their major axis to their minor axis (major axis / minor axis) of 3.0 or more is 2.0% or less.

[0015] The present invention (5) also provides the positive electrode active material for a lithium secondary battery according to any one of (1) to (4), characterized in that the inorganic fluoride particles are MgF2 and AlF3.

[0016] The present invention (6) also provides a positive electrode active material for a lithium secondary battery according to any one of (1) to (5), characterized in that the content of the inorganic fluoride particles is 0.05 to 5.0 mol % in terms of atomic molar percentage of F relative to Co in the aluminum-containing lithium-cobalt composite oxide particles ((F / Co) × 100).

[0017] The present invention (7) also provides a positive electrode active material for a lithium secondary battery according to any one of (1) to (6), characterized in that the aluminum-containing lithium-cobalt composite oxide particles contain one or more M elements selected from Ca, Mg, Sr, Zr, Nb, B, and W.

[0018] The present invention (8) also provides a method for producing a positive electrode active material for a lithium secondary battery, the method comprising: a first mixing step of mixing a lithium compound, a cobalt compound, and an aluminum compound to obtain a first mixture; a first firing step of firing the first mixture to obtain aluminum-containing lithium-cobalt composite oxide particles, in which aluminum is present in a solid solution at least inside the particles, as a first fired product; and a second mixing step of dry-mixing the first fired product obtained in the first firing step with inorganic fluoride particles of MgF and AlF to obtain a positive electrode active material as a second mixture, wherein the inorganic fluoride particles have an average particle size of 0.01 to 30 μm, as expressed as a 50% particle size (D50) converted into volume in a particle size distribution measured by a laser diffraction / scattering method.

[0019] The present invention (9) also provides the method for producing a positive electrode active material for a lithium secondary battery according to (8), characterized in that in the first mixing step, the aluminum compound is mixed so that the molar ratio of Al to Co in the first mixture, calculated as an atomic ratio ((Al / Co) × 100), is 0.05 to 5.00 molar %.

[0020] The present invention (10) also provides the method for producing a positive electrode active material for a lithium secondary battery according to (8) or (9), characterized in that the first fired product contains, as the M element, one or more selected from the group consisting of Ca, Mg, Sr, Zr, Nb, B, and W.

[0021] The present invention (11) also provides the method for producing a positive electrode active material for a lithium secondary battery according to any one of (8) to (10), characterized in that in the first firing step, the firing temperature is 800 to 1150°C.

[0022] The present invention (12) also provides the method for producing a positive electrode active material for a lithium secondary battery according to any one of (8) to (11), characterized in that in the second mixing step, the inorganic fluoride particles are mixed so that the molar percentage of F relative to Co in the second mixture, calculated as atoms ((F / Co) × 100), is 0.05 to 5.00 molar percent.

[0023] The present invention (13) also provides a method for producing a positive electrode active material for a lithium secondary battery according to any one of (8) to (12), characterized in that the mixing ratio of the MgF2 to the AlF3 in the second mixing step is 1.00 or less in terms of molar ratio (MgF2 / AlF3).

[0024] The present invention (14) also provides a lithium secondary battery characterized in that any one of the positive electrode active materials for lithium secondary batteries (1) to (7) is used as the positive electrode active material. [Effects of the Invention]

[0025] According to the present invention, it is possible to provide a positive electrode active material for lithium secondary batteries that can impart excellent high-temperature storage characteristics and reduce impedance when used as a positive electrode active material for lithium secondary batteries, an industrially advantageous method for producing the same, and a lithium secondary battery that has excellent high-temperature storage characteristics and low impedance. [Brief explanation of the drawings]

[0026] [Figure 1] 1 is an SEM image of the positive electrode active material for a lithium secondary battery obtained in Example 1. [Figure 2] 1 is an SEM image of the positive electrode active material for a lithium secondary battery obtained in Example 1. [Figure 3] 1 is an SEM image of the positive electrode active material for a lithium secondary battery obtained in Example 1. [Figure 4] 10 is an SEM image of the positive electrode active material for a lithium secondary battery obtained in Comparative Example 2. [Figure 5] 10 is an SEM image of the positive electrode active material for a lithium secondary battery obtained in Comparative Example 2. [Figure 6] 10 is an SEM image of the positive electrode active material for a lithium secondary battery obtained in Comparative Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0027] The present invention will be described below based on preferred embodiments.

[0028] The positive electrode active material for a lithium secondary battery of the present invention comprises a mixture of aluminum-containing lithium-cobalt composite oxide particles and inorganic fluoride particles, wherein the aluminum-containing lithium-cobalt composite oxide particles have aluminum present in a solid solution at least inside the aluminum-containing lithium-cobalt composite oxide particles, and at least a portion of the inorganic fluoride particles, i.e., all or a portion of the inorganic fluoride particles, are present on the particle surfaces of the aluminum-containing lithium-cobalt composite oxide particles, and when observed at 50,000 times magnification with a scanning electron microscope (SEM), the proportion of inorganic fluoride particles with a particle diameter of 200 nm or more among the inorganic fluoride particles present on the particle surfaces of the aluminum-containing lithium-cobalt composite oxide particles is 4.0% or less.

[0029] The aluminum-containing lithium-cobalt composite oxide forming the aluminum-containing lithium-cobalt composite oxide particles according to the present invention is a composite oxide containing at least lithium, cobalt, and aluminum, and is a composite oxide obtained by adding aluminum as an additive element to a lithium-cobalt composite oxide. In the aluminum-containing lithium-cobalt composite oxide particles according to the present invention, Al is present as a solid solution at least inside the aluminum-containing lithium-cobalt composite oxide particles. In the aluminum-containing lithium-cobalt composite oxide particles according to the present invention, Al is present as a solid solution at least inside the aluminum-containing lithium-cobalt composite oxide particles, thereby stabilizing the crystal structure of the lithium-cobalt composite oxide particles themselves.

[0030] In the aluminum-containing lithium-cobalt composite oxide particles according to the positive electrode active material for a lithium secondary battery of the present invention, the presence of Al in the form of a solid solution inside the aluminum-containing lithium-cobalt composite oxide particles means that when the aluminum-containing lithium-cobalt composite oxide particles are subjected to X-ray diffraction analysis using CuKα radiation as a radiation source, the aluminum-containing lithium-cobalt composite oxide particles are single-phase aluminum-containing lithium-cobalt composite oxide particles in which substantially no diffraction peaks attributable to the raw material aluminum compound and / or Al2O3 are detected. The phrase "diffraction peaks attributable to the aluminum compound and Al2O3 in the raw material are not substantially detected" means that the diffraction peaks attributable to the aluminum compound and Al2O3 are below the detection limit of the analytical device.

[0031] In the aluminum-containing lithium-cobalt composite oxide according to the positive electrode active material for lithium secondary batteries of the present invention, Al may be present only inside the composite oxide particles, or may be present both inside and on the surface of the composite oxide particles. In the present invention, Al may be present inside the composite oxide particles or on the surface of the composite oxide particles. In the positive electrode active material for lithium secondary batteries of the present invention, it is preferable that the aluminum-containing lithium-cobalt composite oxide particles have Al present as a solid solution at least inside the composite oxide particles, in order to stabilize the structure of the composite oxide. Note that "Al present as a solid solution at least inside the composite oxide particles" refers to cases where Al is present only inside the particles or both inside and on the particle surface.

[0032] In the aluminum-containing lithium-cobalt composite oxide particles according to the present invention, the molar ratio of Li to Co in terms of atoms (Li / Co) is 0.90 to 1.20, particularly preferably 0.95 to 1.15, more preferably 0.97 to 1.12, and particularly preferably 0.98 to 1.10. When the molar ratio of Li to Co in terms of atoms (Li / Co) in the aluminum-containing lithium-cobalt composite oxide particles is within the above range, the capacity per volume of the positive electrode active material for lithium secondary batteries can be improved.

[0033] In the aluminum-containing lithium-cobalt composite oxide particles according to the positive electrode active material for a lithium secondary battery of the present invention, the molar percentage of Al relative to Co, calculated as an atom ((Al / Co)×100), is preferably 0.05 to 5.00 molar percent, and particularly preferably 0.50 to 2.00 molar percent. When the molar percentage of Al relative to Co, calculated as an atom ((Al / Co)×100), of the aluminum-containing lithium-cobalt composite oxide particles is within the above range, the cycle characteristics under high voltage and high temperature can be improved.

[0034] The aluminum-containing lithium-cobalt composite oxide particles according to the positive electrode active material for lithium secondary batteries of the present invention contain Al as an essential additive element for the lithium-cobalt composite oxide, but may also contain an element M, if necessary, for the purpose of improving performance or physical properties. The element M is one or more metal elements selected from Ca, Mg, Sr, Zr, Nb, B, and W.

[0035] The aluminum-containing lithium-cobalt composite oxide particles according to the positive electrode active material for lithium secondary batteries of the present invention preferably contain at least one of Ca and Sr as the M element, in order to further improve battery characteristics, and particularly preferably contain at least one of Ca and Sr and one or more selected from Mg, Zr, Nb, B, and W. The aluminum-containing lithium-cobalt composite oxide particles according to the positive electrode active material for lithium secondary batteries of the present invention preferably contain at least one or two or more selected from Ca, Mg, Sr, and Zr as the M element. The aluminum-containing lithium-cobalt composite oxide particles according to the positive electrode active material for lithium secondary batteries of the present invention preferably contain Mg, Sr, and Zr as the M element.

[0036] When the aluminum-containing lithium-cobalt composite oxide particles according to the present invention contain an M element, the atomic molar percentage of the M element relative to the Co atoms in the aluminum-containing lithium-cobalt composite oxide particles ((M / Co) × 100) is preferably 0.01 to 2.00 mol %, and particularly preferably 0.05 to 1.00 mol %. When the aluminum-containing lithium-cobalt composite oxide particles contain an M element, the atomic molar percentage of the M element relative to the Co atoms in the aluminum-containing lithium-cobalt composite oxide particles ((M / Co) × 100) is within the above range, thereby improving the battery characteristics without impairing the charge / discharge capacity of the lithium secondary battery positive electrode active material. When the aluminum-containing lithium-cobalt composite oxide contains two or more M elements, the number of moles of the M element in atomic terms, which is the basis for calculating the above molar percentage, refers to the sum of the number of moles of each M element.

[0037] The M element may be present inside the aluminum-containing lithium-cobalt-based composite oxide particles, or may be present on the particle surface of the aluminum-containing lithium-cobalt-based composite oxide particles, or may be present both inside and on the particle surface of the aluminum-containing lithium-cobalt-based composite oxide particles.

[0038] When the M element is present on the particle surface of the aluminum-containing lithium-cobalt composite oxide particles, the M element may be present in the form of an oxide, composite oxide, sulfate, phosphate, or the like.

[0039] The aluminum-containing lithium-cobalt composite oxide particles according to the positive electrode active material for a lithium secondary battery of the present invention preferably have an average particle size of 0.50 to 30 μm, particularly preferably 3 to 25 μm, as a particle size at 50% volume (D50) in a particle size distribution measured by a laser diffraction / scattering method. The aluminum-containing lithium-cobalt composite oxide particles also preferably have a BET specific surface area of ​​0.05 to 5.00 m. 2 / g, particularly preferably 0.15 to 1.00 m 2When the average particle size or BET specific surface area of ​​the aluminum-containing lithium-cobalt composite oxide particles is within the above range, problems are less likely to occur in the kneading step for preparing the positive electrode mixture and in the coating step for coating the obtained positive electrode mixture onto the positive electrode current collector, and an electrode with high packing properties can be obtained.

[0040] One of the features of the inorganic fluoride particles according to the present invention, which are related to the positive electrode active material for a lithium secondary battery, is that they contain MgF2 and AlF3 in combination.

[0041] In the positive electrode active material for lithium secondary batteries of the present invention, the content of the inorganic fluoride particles is, in atomic molar percentage ((F / Co) × 100) of F relative to Co in the aluminum-containing lithium-cobalt composite oxide particles, preferably 0.05 to 5.00 molar percentage, more preferably 0.10 to 5.00 molar percentage, more preferably 1.00 to 5.00 molar percentage, and particularly preferably 1.25 to 4.50 molar percentage. When the atomic molar percentage ((F / Co) × 100) of F relative to Co in the aluminum-containing lithium-cobalt composite oxide particles is within the above range, a lithium secondary battery having excellent high-temperature storage characteristics and low impedance is obtained.

[0042] The proportion of MgF2 relative to AlF3, expressed as a molar ratio (MgF2 / AlF3), is 1.00 or less, preferably 0.01 to 0.95, more preferably 0.05 to 0.90, more preferably 0.05 to 0.80, more preferably 0.06 to 0.70, and particularly preferably 0.07 to 0.65. When the proportion of MgF2 relative to AlF3 (MgF2 / AlF3) is within the above range, a lithium secondary battery having excellent high-temperature storage characteristics and low impedance is obtained.

[0043] At least a portion of the inorganic fluoride particles, i.e., all or a portion of the inorganic fluoride particles, are present on the particle surface of the aluminum-containing lithium-cobalt composite oxide particles. That is, the positive electrode active material for lithium secondary batteries of the present invention is composed of aluminum-containing lithium-cobalt composite oxide particles and inorganic fluoride particles present on the surface of the aluminum-containing lithium-cobalt composite oxide particles, or aluminum-containing lithium-cobalt composite oxide particles, inorganic fluoride particles present on the surface of the aluminum-containing lithium-cobalt composite oxide particles, and inorganic fluoride particles not present on the surface of the aluminum-containing lithium-cobalt composite oxide particles. Note that when the inorganic fluoride particles are present on the particle surface of the aluminum-containing lithium-cobalt composite oxide particles, it is preferable that the inorganic fluoride particles are present only partially on the surface of the aluminum-containing lithium-cobalt composite oxide particles, since this does not inhibit lithium intercalation and deintercalation on the surface of the aluminum-containing lithium-cobalt composite oxide.

[0044] The particle diameter of the inorganic fluoride particles present on the particle surface of the aluminum-containing lithium-cobalt composite oxide particles is determined by a scanning electron microscope (SEM). Specifically, a scanning electron microscope (SEM) is used to observe an arbitrary field of view of the positive electrode active material for a lithium secondary battery of the present invention at an arbitrary magnification. Next, the minor axis and the major axis perpendicular to the minor axis of the inorganic fluoride particles present on the surface of the aluminum-containing lithium-cobalt composite oxide are measured, and half of the sum of the two is calculated. This calculated value is the particle diameter of the inorganic fluoride particles determined by the scanning electron microscope (SEM). In the present invention, the particle diameter of the inorganic fluoride particles determined by a scanning electron microscope (SEM) is preferably determined by randomly selecting a field of view with the SEM, observing the surface of a positive electrode active material sample at a magnification of 50,000 times, measuring the minor axis of the inorganic fluoride particles partially present on the surface of the aluminum-containing lithium-cobalt composite oxide particles in the observed field of view, and then measuring the major axis in a direction perpendicular to the minor axis, and calculating half of the sum of both.

[0045] In the present invention, the proportion of inorganic fluoride particles having a particle diameter of 200 nm or more, as determined by SEM, among the inorganic fluoride particles present on the particle surfaces of the aluminum-containing lithium-cobalt composite oxide particles is 4.0% or less, preferably 3.0% or less, preferably 2.5% or less, preferably 1.5% or less, and more preferably 0%. When the proportion of inorganic fluoride particles having a particle diameter of 200 nm or more, as determined by SEM, among the inorganic fluoride particles present on the particle surfaces of the aluminum-containing lithium-cobalt composite oxide particles is within the above range, a lithium secondary battery having excellent high-temperature storage characteristics and low impedance is obtained. The proportion of inorganic fluoride particles having a particle diameter of 200 nm or more as determined by SEM among inorganic fluoride particles present on the particle surface of the aluminum-containing lithium cobalt composite oxide particles is calculated by the following formula. Number ratio (%) = (total number (pieces) of inorganic fluoride particles with a particle diameter of 200 nm or more present on the particle surface of aluminum-containing lithium cobalt composite oxide particles when three visual fields are randomly selected and observed at a magnification of 50,000 times with an SEM) / (total number (pieces) of inorganic fluoride particles present on the surface of aluminum-containing lithium cobalt composite oxide observed within the three visual fields) × 100

[0046] In the present invention, the proportion of inorganic fluoride particles having a particle diameter of 180 nm or more as determined by SEM among inorganic fluoride particles present on the particle surfaces of aluminum-containing lithium-cobalt composite oxide particles is 5.0% or less, preferably 4.0% or less, preferably 2.5% or less, preferably 2.0% or less, and more preferably 1.5% or less. When the proportion of inorganic fluoride particles having a particle diameter of 180 nm or more as determined by SEM among inorganic fluoride particles present on the particle surfaces of aluminum-containing lithium-cobalt composite oxide particles is within the above range, a lithium secondary battery having excellent high-temperature storage characteristics and low impedance is obtained. The proportion of inorganic fluoride particles having a particle diameter of 180 nm or more as determined by SEM among inorganic fluoride particles present on the particle surfaces of aluminum-containing lithium cobalt composite oxide particles is calculated by the following formula. Number ratio (%) = (total number (pieces) of inorganic fluoride particles having a particle diameter of 180 nm or more present on the particle surface of aluminum-containing lithium cobalt composite oxide particles when three visual fields are randomly selected and observed at a magnification of 50,000 times with an SEM) / (total number (pieces) of inorganic fluoride particles present on the surface of aluminum-containing lithium cobalt composite oxide observed within the three visual fields) × 100

[0047] In the present invention, the proportion of inorganic fluoride particles having a particle diameter of 160 nm or more, as determined by SEM, among the inorganic fluoride particles present on the particle surfaces of the aluminum-containing lithium-cobalt composite oxide particles is 6.0% or less, preferably 5.0% or less, preferably 3.5% or less, preferably 3.0% or less, and more preferably 2.5% or less. When the proportion of inorganic fluoride particles having a particle diameter of 160 nm or more, as determined by SEM, among the inorganic fluoride particles present on the particle surfaces of the aluminum-containing lithium-cobalt composite oxide particles is within the above range, a lithium secondary battery having excellent high-temperature storage characteristics and low impedance is obtained. The proportion of inorganic fluoride particles having a particle diameter of 160 nm or more as determined by SEM among inorganic fluoride particles present on the particle surface of the aluminum-containing lithium cobalt composite oxide particles is calculated by the following formula. Number ratio (%) = (total number (pieces) of inorganic fluoride particles with a particle diameter of 160 nm or more present on the particle surface of aluminum-containing lithium cobalt composite oxide particles when three visual fields are randomly selected and observed at a magnification of 50,000 times with an SEM) / (total number (pieces) of inorganic fluoride particles present on the surface of aluminum-containing lithium cobalt composite oxide observed within the three visual fields) × 100

[0048] In the present invention, the proportion of inorganic fluoride particles having a particle diameter of 140 nm or more, as determined by SEM, among the inorganic fluoride particles present on the particle surfaces of the aluminum-containing lithium-cobalt composite oxide particles is 6.0% or less, preferably 5.0% or less, preferably 3.5% or less, preferably 3.0% or less, and more preferably 2.5% or less. When the proportion of inorganic fluoride particles having a particle diameter of 140 nm or more, as determined by SEM, among the inorganic fluoride particles present on the particle surfaces of the aluminum-containing lithium-cobalt composite oxide particles is within the above range, a lithium secondary battery having excellent high-temperature storage characteristics and low impedance is obtained. The proportion of inorganic fluoride particles having a particle diameter of 140 nm or more as determined by SEM among inorganic fluoride particles present on the particle surface of the aluminum-containing lithium cobalt composite oxide particles is calculated by the following formula. Number ratio (%) = (total number (pieces) of inorganic fluoride particles having a particle diameter of 140 nm or more present on the particle surface of aluminum-containing lithium cobalt composite oxide particles when three visual fields are randomly selected and observed at a magnification of 50,000 times with an SEM) / (total number (pieces) of inorganic fluoride particles present on the surface of aluminum-containing lithium cobalt composite oxide observed within the three visual fields) × 100

[0049] In the present invention, the proportion of inorganic fluoride particles having a particle diameter of less than 120 nm as determined by SEM among inorganic fluoride particles present on the particle surfaces of aluminum-containing lithium-cobalt composite oxide particles is 92.0% or more, preferably 93.0% or more, preferably 94.5% or more, preferably 95.0% or more, and more preferably 96.0% or more. When the proportion of inorganic fluoride particles having a particle diameter of less than 120 nm as determined by SEM among inorganic fluoride particles present on the particle surfaces of aluminum-containing lithium-cobalt composite oxide particles is within the above range, a lithium secondary battery having excellent high-temperature storage characteristics and low impedance is obtained. The proportion of inorganic fluoride particles having a particle diameter of less than 120 nm as determined by SEM among inorganic fluoride particles present on the particle surface of the aluminum-containing lithium cobalt composite oxide particles is calculated by the following formula. Number ratio (%) = (total number (pieces) of inorganic fluoride particles having a particle diameter of less than 120 nm present on the particle surface of aluminum-containing lithium cobalt composite oxide particles when three visual fields are randomly selected and observed at a magnification of 50,000 times with an SEM) / (total number (pieces) of inorganic fluoride particles present on the surface of aluminum-containing lithium cobalt composite oxide observed within the three visual fields) × 100

[0050] In the present invention, the proportion of inorganic fluoride particles present on the particle surfaces of aluminum-containing lithium-cobalt composite oxide particles having a ratio of their major axis to their minor axis (major axis / minor axis) of less than 1.5 is 65.0% or more, preferably 70.0% or more, preferably 75.0% or more, and more preferably 78.0% or more. When the proportion of inorganic fluoride particles present on the particle surfaces of aluminum-containing lithium-cobalt composite oxide particles having a ratio of their major axis to their minor axis (major axis / minor axis) of less than 1.5 is within the above range, a lithium secondary battery having excellent high-temperature storage characteristics and low impedance is obtained. The proportion of inorganic fluoride particles present on the particle surface of the aluminum-containing lithium-cobalt composite oxide particles having a ratio of the major axis to the minor axis (major axis / minor axis) of less than 1.5 is calculated by the following formula. Number ratio (%) = (total number (pieces) of inorganic fluoride particles present on the particle surface of aluminum-containing lithium cobalt composite oxide particles, which have a ratio of the major axis to the minor axis (major axis / minor axis) of less than 1.5 when three visual fields are randomly selected and observed under a magnification of 50,000 times with an SEM) / (total number (pieces) of inorganic fluoride particles present on the surface of aluminum-containing lithium cobalt composite oxide observed within the three visual fields) × 100

[0051] In the present invention, the proportion of inorganic fluoride particles present on the particle surfaces of aluminum-containing lithium-cobalt composite oxide particles that have a ratio of their major axis to their minor axis (major axis / minor axis) of less than 1.3 is 60.0% or more, preferably 65.0% or more, preferably 70.0% or more, and more preferably 72.5% or more. When the proportion of inorganic fluoride particles present on the particle surfaces of aluminum-containing lithium-cobalt composite oxide particles that have a ratio of their major axis to their minor axis (major axis / minor axis) of less than 1.3 is within the above range, a lithium secondary battery with excellent high-temperature storage characteristics and low impedance is obtained. The proportion of inorganic fluoride particles present on the particle surface of the aluminum-containing lithium-cobalt composite oxide particles having a ratio of the major axis to the minor axis (major axis / minor axis) of less than 1.3 is calculated by the following formula. Number ratio (%) = (total number (pieces) of inorganic fluoride particles present on the particle surface of aluminum-containing lithium cobalt composite oxide particles, which have a ratio of the major axis to the minor axis (major axis / minor axis) of less than 1.3 when three visual fields are randomly selected and observed under a magnification of 50,000 times with an SEM) / (total number (pieces) of inorganic fluoride particles present on the surface of aluminum-containing lithium cobalt composite oxide observed within the three visual fields) × 100

[0052] In the present invention, the proportion of inorganic fluoride particles present on the particle surfaces of aluminum-containing lithium-cobalt composite oxide particles that have a ratio of their major axis to their minor axis (major axis / minor axis) of 2.0 or more is 13.5% or less, preferably 12.5% ​​or less, preferably 11.5% or less, and more preferably 10.5% or less. When the proportion of inorganic fluoride particles present on the particle surfaces of aluminum-containing lithium-cobalt composite oxide particles that have a ratio of their major axis to their minor axis (major axis / minor axis) of 2.0 or more is within the above range, a lithium secondary battery with excellent high-temperature storage characteristics and low impedance is obtained. The proportion of inorganic fluoride particles present on the particle surface of the aluminum-containing lithium-cobalt composite oxide particles having a ratio of the major axis to the minor axis (major axis / minor axis) of 2.0 or more is calculated by the following formula. Number ratio (%) = (total number (pieces) of inorganic fluoride particles present on the particle surface of aluminum-containing lithium cobalt composite oxide particles, which have a ratio of the major axis to the minor axis (major axis / minor axis) of 2.0 or more when three visual fields are randomly selected and observed under a magnification of 50,000 times with an SEM) / (total number (pieces) of inorganic fluoride particles present on the surface of aluminum-containing lithium cobalt composite oxide observed within the three visual fields) × 100

[0053] In the present invention, the proportion of inorganic fluoride particles present on the particle surfaces of aluminum-containing lithium-cobalt composite oxide particles that have a ratio of their major axis to their minor axis (major axis / minor axis) of 3.0 or more is 2.0% or less, preferably 1.8% or less, preferably 1.7% or less, and more preferably 1.6% or less. When the proportion of inorganic fluoride particles present on the particle surfaces of aluminum-containing lithium-cobalt composite oxide particles that have a ratio of their major axis to their minor axis (major axis / minor axis) of 3.0 or more is within the above range, a lithium secondary battery with excellent high-temperature storage characteristics and low impedance is obtained. The proportion of inorganic fluoride particles present on the particle surface of the aluminum-containing lithium-cobalt composite oxide particles having a ratio of the major axis to the minor axis (major axis / minor axis) of 3.0 or more is calculated by the following formula. Number ratio (%) = (total number (pieces) of inorganic fluoride particles present on the particle surface of aluminum-containing lithium cobalt composite oxide particles, which have a ratio of the major axis to the minor axis (major axis / minor axis) of 3.0 or more when three visual fields are randomly selected and observed at a magnification of 50,000 times with an SEM) / (total number (pieces) of inorganic fluoride particles present on the surface of aluminum-containing lithium cobalt composite oxide observed within the three visual fields) × 100

[0054] The average particle size of the positive electrode active material for a lithium secondary battery of the present invention is preferably 0.50 to 30 μm, particularly preferably 3 to 25 μm, as the particle size at 50% volume (D50) in the particle size distribution measured by a laser diffraction / scattering method. The BET specific surface area of ​​the positive electrode active material for a lithium secondary battery of the present invention is preferably 0.05 to 5.0 m 2 / g, particularly preferably 0.15 to 1.0 m 2 When the average particle size or BET specific surface area of ​​the positive electrode active material for a lithium secondary battery of the present invention is within the above range, problems are less likely to occur in the kneading step for preparing the positive electrode mixture and in the coating step for coating the obtained positive electrode mixture onto a positive electrode current collector.

[0055] The positive electrode active material for a lithium secondary battery of the present invention can impart excellent high-temperature storage characteristics and reduce impedance when used as a positive electrode active material for a lithium secondary battery.

[0056] The positive electrode active material for a lithium secondary battery of the present invention can also be used as a positive electrode active material for an all-solid-state battery, an electricity storage device, and the like.

[0057] The method for producing the positive electrode active material for a lithium secondary battery of the present invention is not particularly limited, but it is preferably produced by the method for producing the positive electrode active material for a lithium secondary battery of the present invention described below.

[0058] The method for producing a positive electrode active material for a lithium secondary battery of the present invention is characterized by mixing a lithium compound, a cobalt compound, and an aluminum compound to obtain a first mixture, firing the first mixture to obtain aluminum-containing lithium-cobalt composite oxide particles in which aluminum is present in a solid solution at least inside the particles as a first fired product, and dry-mixing the first fired product with inorganic fluoride particles of MgF and AlF to obtain a second mixture to obtain a positive electrode active material for a lithium secondary battery.

[0059] The positive electrode active material for lithium secondary batteries obtained by the method for producing a positive electrode active material for lithium secondary batteries of the present invention basically consists of a mixture of aluminum-containing lithium-cobalt composite oxide particles and inorganic fluoride particles of MgF2 and AlF3.

[0060] The positive electrode active material for lithium secondary batteries obtained by the method for producing a positive electrode active material for lithium secondary batteries of the present invention can impart excellent high-temperature storage characteristics and reduce impedance when used as a positive electrode active material for lithium secondary batteries.

[0061] The method for producing a positive electrode active material for a lithium secondary battery of the present invention is characterized by comprising: a first mixing step of mixing a lithium compound, a cobalt compound, and an aluminum compound to obtain a first mixture; a first firing step of firing the first mixture to obtain aluminum-containing lithium-cobalt composite oxide particles in which aluminum is present in a solid solution at least inside the particles, as a first fired product; and a second mixing step of dry-mixing the first fired product obtained in the first firing step with inorganic fluoride particles, MgF and AlF, to obtain a positive electrode active material as a second mixture.

[0062] The first mixing step is, for example, a step of mixing a lithium compound, a cobalt compound, and an aluminum compound to obtain a first mixture containing the lithium compound, the cobalt compound, and the aluminum compound.

[0063] The lithium compound to be used in the first mixing step is not particularly limited as long as it is a lithium compound that is usually used as a raw material for producing a lithium-cobalt composite oxide, and examples thereof include oxides, hydroxides, carbonates, nitrates, sulfates, and organic acid salts of lithium.

[0064] The cobalt compound used in the first mixing step is not particularly limited as long as it is a cobalt compound that is normally used as a raw material for producing a lithium-cobalt-based composite oxide, and examples thereof include oxides, oxyhydroxides, hydroxides, carbonates, nitrates, sulfates, and organic acid salts of cobalt.

[0065] The aluminum compound used in the first mixing step is not particularly limited as long as it is an aluminum compound that can be used as a raw material for producing an aluminum-added lithium-cobalt composite oxide, and examples thereof include aluminum oxide, hydroxide, carbonate, nitrate, sulfate, and organic acid salt, etc. Among these, aluminum hydroxide is preferred as the aluminum compound.

[0066] In the first mixing step, the lithium compound and the cobalt compound are mixed so that the molar ratio of Li to Co in the first mixture (Li / Co) in atomic terms is 0.90 to 1.20, preferably 0.95 to 1.15, more preferably 0.97 to 1.12, and particularly preferably 0.98 to 1.10. When the mixing ratio of the lithium compound and the cobalt compound is within the above range, a single phase lithium-cobalt composite oxide containing aluminum is easily obtained in X-ray diffraction analysis, and the capacity per volume of the positive electrode active material for a lithium secondary battery can be improved.

[0067] In the first mixing step, the aluminum compound is mixed so that the atomic molar percentage of Al relative to Co in the first mixture ((Al / Co)×100) is preferably 0.05 to 5.00 molar percent, and particularly preferably 0.50 to 2.00 molar percent. By having the aluminum compound mixed in the above range, the cycle characteristics under high voltage and high temperature can be improved.

[0068] In the first mixing step, a compound containing an M element can be mixed into the first mixture for the purpose of improving performance or physical properties.

[0069] The M element is one or more metal elements selected from Ca, Mg, Sr, Zr, Nb, B, and W. Examples of compounds containing the M element include oxides, hydroxides, carbonates, nitrates, and organic acid salts containing the M element. Compounds containing two or more M elements may also be used as the compound containing the M element.

[0070] When a compound containing an M element is mixed in the first mixing step, the compound containing the M element is mixed so that the molar percentage of the M element relative to the Co atoms in the first mixture ((M / Co) × 100) is preferably 0.01 to 2.00 mol %, and particularly preferably 0.05 to 1.00 mol %. By having the mixing percentage of the compound containing the M element within the above range, the battery characteristics can be improved without impairing the charge / discharge capacity of the positive electrode active material for lithium secondary batteries. When two or more compounds containing the M element are used, the number of moles of the M element in atomic terms, which is the basis for calculating the above molar percentage, refers to the sum of the number of moles of each M element.

[0071] In the first mixing step, examples of a method for mixing the lithium compound, the cobalt compound, the aluminum compound, and the compound containing the M element, which is used as needed, include mixing methods using a coffee mill, a household mixer, a ribbon mixer, a Henschel mixer, a Super mixer, a Nauta mixer, or the like.

[0072] The first firing step is a step of firing the first mixture obtained by carrying out the first mixing step to obtain aluminum-containing lithium-cobalt composite oxide particles in which aluminum is present as a solid solution inside the particles as a first fired product.

[0073] In the first firing step, the firing temperature when firing the first mixture to react the raw materials is 800 to 1150° C., preferably 830 to 1130° C., and more preferably 850 to 1100° C. When the firing temperature is within the above range, it is possible to reduce the production of unreacted cobalt oxide or overheat decomposition products of lithium-cobalt composite oxide, which are a cause of a decrease in capacity of the aluminum-containing lithium-cobalt composite oxide.

[0074] In the first firing step, the firing time for firing the first mixture to react the raw materials is 1 to 30 hours, preferably 5 to 28 hours, and more preferably 6 to 25 hours. The firing atmosphere in the first firing step is preferably an oxidizing atmosphere such as air or oxygen gas.

[0075] In the first firing step, firing may be carried out multiple times as necessary, and after firing, the fired product may be crushed or classified as necessary.

[0076] The second mixing step is a step of dry-mixing the aluminum-containing lithium-cobalt composite oxide particles, which are the first fired product, with inorganic fluoride particles, MgF2 and AlF3, to obtain a second mixture, which is a positive electrode active material for a lithium secondary battery, containing the aluminum-containing lithium-cobalt composite oxide particles and the inorganic fluoride particles, MgF2 and AlF3.

[0077] Another feature is that in the second mixing step, the inorganic fluoride particles are a combination of MgF2 and AlF3. In the following, MgF2 and AlF3 may be collectively referred to as "inorganic fluoride" or "inorganic fluoride particles."

[0078] In the second mixing step, the inorganic fluoride particles are mixed in such an amount that the atomic molar percentage of F relative to Co in the aluminum-containing lithium-cobalt composite oxide particles ((F / Co) × 100) is 0.05 to 5.00, more preferably 0.10 to 5.00, and even more preferably 0.10 to 4.50. By mixing the inorganic fluoride particles in the above range, a lithium secondary battery having excellent high-temperature storage characteristics and low impedance is obtained.

[0079] The inorganic fluoride particles in the second mixing step are MgF2 and AlF3. The mixing ratio of MgF2 to AlF3, in terms of molar ratio (MgF2 / AlF3), is 1.00 or less, preferably 0.01 to 0.95, more preferably 0.05 to 0.90, more preferably 0.05 to 0.80, more preferably 0.06 to 0.70, and particularly preferably 0.07 to 0.65. When the mixing ratio of MgF2 to AlF3 (MgF2 / AlF3) is within the above range, a lithium secondary battery having excellent high-temperature storage characteristics and low impedance is obtained.

[0080] The inorganic fluoride particles in the second mixing step have an average particle size of preferably 0.01 to 30 μm, particularly preferably 0.10 to 20 μm, as the particle size at 50% cumulative volume (D50) in the particle size distribution measured by a laser diffraction / scattering method. When the average particle size of the inorganic fluoride particles is within the above range, a lithium secondary battery having excellent high-temperature storage characteristics and low impedance is obtained. The inorganic fluoride particles may be pulverized or crushed using a known device such as a jet mill, pin mill, roll mill, ball mill, or bead mill.

[0081] In the second mixing step, examples of methods for dry-mixing the aluminum-containing lithium-cobalt composite oxide particles and the inorganic fluoride particles include mixing methods using a coffee mill, a household mixer, a high-speed mixer, a ribbon mixer, a Henschel mixer, a super mixer, a turbosphere mixer, a Nauta mixer, a V-type mixer, a ball mill, a bead mill, etc. In the second mixing step, the inorganic fluoride particles are mixed with the aluminum-containing lithium-cobalt composite oxide particles by mixing using the dry mixing device, thereby obtaining a mixture of the aluminum-containing lithium-cobalt composite oxide particles and the inorganic fluoride particles. Alternatively, in the second mixing step, the inorganic fluoride particles are pulverized or crushed by mixing using the dry mixing device to obtain inorganic fluoride particles with small particle sizes, which are then mixed with the aluminum-containing lithium-cobalt composite oxide particles, thereby obtaining a mixture of the aluminum-containing lithium-cobalt composite oxide particles and the inorganic fluoride particles. Alternatively, in the second mixing step, inorganic fluoride particles and aluminum-containing lithium-cobalt composite oxide particles are mixed by mixing treatment in the dry mixing device, and at the same time, the inorganic fluoride particles are pulverized or crushed, and the inorganic fluoride particles with reduced particle size are mixed with the aluminum-containing lithium-cobalt composite oxide particles, thereby obtaining a mixture of aluminum-containing lithium-cobalt composite oxide particles and inorganic fluoride particles.

[0082] The method for producing a positive electrode active material for a lithium secondary battery of the present invention includes a first mixing step, a first firing step, and a second mixing step. Therefore, when used as a positive electrode active material for a lithium secondary battery, it is possible to impart excellent high-temperature storage characteristics and reduce impedance, and it is possible to industrially advantageously produce a positive electrode active material for a lithium secondary battery.

[0083] The average particle size of the positive electrode active material for lithium secondary batteries obtained by the method for producing a positive electrode active material for lithium secondary batteries of the present invention is preferably 0.50 to 30 μm, particularly preferably 3 to 25 μm, as the particle size at 50% volume (D50) in the particle size distribution measured by a laser diffraction / scattering method. The BET specific surface area of ​​the positive electrode active material for lithium secondary batteries of the present invention is preferably 0.05 to 5.0 m 2 / g, particularly preferably 0.15 to 1.0 m 2 When the average particle size or BET specific surface area of ​​the positive electrode active material for a lithium secondary battery of the present invention is within the above range, problems are less likely to occur in the kneading step for preparing the positive electrode mixture and in the coating step for coating the obtained positive electrode mixture onto a positive electrode current collector.

[0084] The positive electrode active material for lithium secondary batteries obtained by the method for producing a positive electrode active material for lithium secondary batteries of the present invention can impart excellent high-temperature storage characteristics and reduce impedance when used as a positive electrode active material for lithium secondary batteries.

[0085] The lithium secondary battery of the present invention uses the positive electrode active material for lithium secondary batteries of the present invention as a positive electrode active material. The lithium secondary battery of the present invention comprises a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte containing a lithium salt.

[0086] The lithium secondary battery of the present invention may be in any shape, such as a button, sheet, cylinder, square, or coin shape.

[0087] The lithium secondary battery of the present invention is a lithium secondary battery that has excellent high-temperature storage characteristics and low impedance.

[0088] The impedance of the lithium secondary battery of the present invention is 100Ω or less, preferably 60Ω or less, and more preferably 30Ω or less. The impedance of the lithium secondary battery is determined by measuring the AC impedance using an impedance measuring device in a frequency measurement range of 0.02 Hz to 20 kHz after the coin-type lithium secondary battery is brought to SOC 100%, i.e., fully charged, and then applying a voltage of 0 V to the open circuit circuit, i.e., no voltage, and then measuring the AC impedance from the Cole-Cole plot obtained by the AC impedance measurement.

[0089] The uses of the lithium secondary battery of the present invention are not particularly limited, and it can be used to supply power to various products such as small electronic devices, large electronic devices, electric vehicles, and power storage devices. Examples of small electronic devices include notebook computers, laptop computers, pocket word processors, mobile phones, cordless handsets, portable CD players, radios, LCD televisions, electric shavers, memory cards, video cameras, game devices, and power tools. Examples of large electronic devices include robots, drones, and medical equipment. Examples of electric vehicles include electric vehicles (including hybrid vehicles), electric bicycles, and railroad cars. Examples of power storage devices include backup power sources for buildings or power generation facilities. [Example]

[0090] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0091] Example 1 <First mixing process> Tricobalt tetroxide (average particle size 2.6 μm), lithium carbonate (average particle size 5.7 μm), and aluminum hydroxide (average particle size 1.6 μm) were weighed and mixed in a coffee mill to obtain a first mixture having a molar ratio of Li to Co (Li / Co) of 1.040 and a molar percentage of Al to Co ((Al / Co) × 100) of 1.00 mol%. <First firing step> Next, the obtained first mixture was fired in an alumina pot at 900°C for 5 hours. X-ray diffraction analysis of the fired product revealed that it was single-phase LiCoO, confirming that Al was contained in the lithium-cobalt composite oxide particles as a solid solution. After the firing was completed, the fired product was pulverized and classified to obtain aluminum-containing lithium cobalt composite oxide particles containing 1.00 mol % of Al relative to Co. <Second mixing process> Next, the obtained aluminum-containing lithium-cobalt composite oxide particles, MgF2 (average particle diameter D50 = 0.9 μm), and AlF3 (average particle diameter D50 = 2.2 μm) were weighed and mixed in a coffee mill to obtain a positive electrode active material sample having a molar percentage of F relative to Co ((F / Co) × 100) of 0.90 molar%, and a molar ratio (MgF2 / AlF3) of MgF2 relative to AlF3 of 0.1.

[0092] Examples 2 to 5 By the same procedure as in Example 1, the first mixing step and the first baking step shown in Table 1 were carried out to obtain a baked product. The fired product was then crushed and classified to obtain an aluminum-containing lithium-cobalt composite oxide, which was then subjected to a second mixing step in the same manner as in Example 1 to obtain a positive electrode active material sample.

[0093] (Comparative Example 1) <First mixing process> Tricobalt tetroxide (average particle size 2.6 μm), lithium carbonate (average particle size 5.7 μm), and aluminum hydroxide (average particle size 1.6 μm) were weighed and mixed in a coffee mill to obtain a first mixture having a molar ratio of Li to Co (Li / Co) of 1.040 and a molar percentage of Al to Co ((Al / Co) × 100) of 1.00 mol%. <First firing step> The resulting first mixture was then calcined in an alumina pot at 900°C for 5 hours. X-ray diffraction analysis of the calcined product revealed that it was single-phase LiCoO2. After the firing was completed, the fired product was crushed and classified to obtain a positive electrode active material sample.

[0094] (Comparative Example 2) <First mixing process> Tricobalt tetroxide (average particle size 2.6 μm), lithium carbonate (average particle size 5.7 μm), and aluminum hydroxide (average particle size 1.6 μm) were weighed and mixed in a coffee mill to obtain a first mixture having a molar ratio of Li to Co (Li / Co) of 1.040 and a molar percentage of Al to Co ((Al / Co) × 100) of 1.00 mol%. <First firing step> The resulting first mixture was then fired in an alumina pot at 900° C. for 5 hours. After the firing was completed, the fired product was pulverized and classified to obtain aluminum-containing lithium cobalt composite oxide particles containing 1.00 mol % of Al relative to Co. <Second mixing process> Next, the obtained aluminum-containing lithium-cobalt composite oxide particles, MgF2 (average particle diameter D50 = 46.8 μm), and AlF3 (average particle diameter D50 = 107.5 μm) were weighed and mixed in a coffee mill to obtain a positive electrode active material sample having a molar percentage of F relative to Co ((F / Co) × 100) of 0.90 molar%, and a molar ratio (MgF2 / AlF3) of MgF2 relative to AlF3 of 0.1.

[0095] The following physical properties (average particle diameter) were determined for the positive electrode active material samples obtained in Examples 1 to 5 and Comparative Examples 1 and 2. The results are shown in Table 1.

[0096] (1) Average particle size It was calculated from the particle size at 50% of the volume (D50) in the particle size distribution measured by the laser diffraction / scattering method.

[0097] [Table 1]

[0098] Next, for the positive electrode active material samples obtained in Example 1 and Comparative Example 2, three visual fields were randomly selected and magnified 50,000 times using a scanning electron microscope (SEM) to observe the surfaces of the positive electrode active material samples. SEM images of the positive electrode active material sample obtained in Example 1 are shown in Figures 1 to 3, and SEM images of the positive electrode active material sample obtained in Comparative Example 2 are shown in Figures 4 to 6. In each visual field, the number of inorganic fluoride particles having the particle diameters shown in Table 2 was visually counted, and the total value for all three visual fields was calculated. In addition, using the same three visual fields, the number of inorganic fluoride particles having the ratios of the major axis to the minor axis (major axis / minor axis) shown in Table 3 was visually counted, and the total value for all three visual fields was calculated. A scanning electron microscope (SU8220, manufactured by Hitachi High-Technologies Corporation) was used for SEM observation. The particle diameter was determined by measuring the minor axis of the inorganic fluoride particles, then measuring the major axis in a direction perpendicular to the minor axis, and calculating half of the sum of the minor axis and the major axis.

[0099] [Table 2]

[0100] [Table 3]

[0101] From Tables 2 and 3, it was confirmed that in the positive electrode active material sample obtained in Example 1, of the inorganic fluoride particles present on the particle surfaces of the aluminum-containing lithium-cobalt composite oxide particles, the number ratio of inorganic fluoride particles having a particle diameter of 200 nm or more was 0%, the number ratio of inorganic fluoride particles having a particle diameter of less than 120 nm was 96.4%, the number ratio of inorganic fluoride particles showing a ratio of major axis to minor axis (major axis / minor axis) of less than 1.5 was 80.2%, and the number ratio of inorganic fluoride particles showing a ratio of major axis to minor axis (major axis / minor axis) of 3.0 or more was 1.5%. On the other hand, in the positive electrode active material sample obtained in Comparative Example 2, it was confirmed that, of the inorganic fluoride particles present on the particle surfaces of the aluminum-containing lithium-cobalt composite oxide particles, the number ratio of inorganic fluoride particles having a particle diameter of 200 nm or more was 4.5%, the number ratio of inorganic fluoride particles having a particle diameter of less than 120 nm was 91.6%, the number ratio of inorganic fluoride particles showing a ratio of major axis to minor axis (major axis / minor axis) of less than 1.5 was 61.5%, and the number ratio of inorganic fluoride particles showing a ratio of major axis to minor axis (major axis / minor axis) of 3.0 or more was 2.2%.

[0102] Next, a battery performance test was carried out as follows.

[0103] <Fabrication of lithium secondary batteries> A positive electrode agent was prepared by mixing 95% by mass of the positive electrode active materials obtained in Examples 1 to 5 and Comparative Examples 1 and 2, 2.5% by mass of graphite powder, and 2.5% by mass of polyvinylidene fluoride, and this was dispersed in N-methyl-2-pyrrolidinone to prepare a kneaded paste. The kneaded paste was applied to aluminum foil, dried, pressed, and punched into a disk with a diameter of 15 mm to obtain a positive electrode plate. Using this positive electrode plate, a coin-type lithium secondary battery was fabricated using components such as a separator, negative electrode, positive electrode, current collector, mounting hardware, external terminals, and electrolyte. Metallic lithium foil was used for the negative electrode, and the electrolyte was a 1-liter mixture of ethylene carbonate, dimethyl carbonate, and diethyl carbonate in a ratio of 2.5:6:1.5, in which 1 mol of LiPF6 and 1 wt% of vinylene carbonate were dissolved.

[0104] <Battery performance evaluation> The produced coin-type lithium secondary batteries were evaluated for the following battery performance (high-temperature storage characteristics and impedance). The results are shown in Table 4. (1) High-temperature storage characteristics (1-1) Test conditions First, the battery was charged at 25°C at 0.5C for 2 hours to 4.55V, followed by a constant current / constant voltage (CCCV) charge in which the voltage was maintained at 4.55V for 3 hours. Subsequently, the battery was discharged at a constant current (CC) discharge of 0.2C to 2.7V at 25°C to measure the initial discharge capacity per unit weight of active material. The coin-type lithium secondary battery was then charged a second time in the same manner, after which it was stored in a thermostatic chamber at 60°C for 10 days. The coin-type lithium secondary battery was then removed from the chamber and returned to 25°C. The second discharge was performed in the same manner, and the second discharge capacity per unit weight of active material was measured. The battery was then charged and discharged a third time in the same manner, and the third discharge capacity per unit weight of active material was measured. (1-2)Remaining capacity The remaining capacity was calculated from the initial discharge capacity and the second discharge capacity per weight of the active material in the above-mentioned high-temperature storage characteristic evaluation, using the following formula. Remaining capacity (%) = (2nd discharge capacity / initial discharge capacity) x 100 (1-3) Recovery capacity The recovered capacity was calculated from the initial discharge capacity and the third discharge capacity per weight of the active material in the above-mentioned high-temperature storage characteristic evaluation, using the following formula. Recovery capacity (%) = (3rd discharge capacity / initial discharge capacity) x 100 (2) Impedance First, the coin-type lithium secondary battery was charged at 0.5C for 2 hours to 4.6V, and then constant current / constant voltage charging (CCCV charging) was performed, maintaining the voltage at 4.6V for 3 hours, until the battery reached 100% SOC, i.e., a fully charged state. Then, AC impedance measurements were performed using an impedance measurement device with 0V applied to the open circuit circuit, i.e., no voltage applied, over a frequency measurement range of 0.02Hz to 20kHz. Resistance values ​​were then calculated from the Cole-Cole plot obtained from the AC impedance measurements.

[0105] [Table 4]

[0106] From Table 4, it was confirmed that the lithium secondary batteries fabricated using the positive electrode active materials obtained in Examples 1 to 5 had superior high-temperature storage characteristics and lower impedance than the lithium secondary batteries fabricated using the positive electrode active materials obtained in Comparative Examples 1 and 2.

Claims

1. The aluminum-containing lithium-cobalt composite oxide particles and the inorganic fluoride particles are mixed together. the aluminum-containing lithium-cobalt composite oxide particles have aluminum present in the form of a solid solution at least inside the aluminum-containing lithium-cobalt composite oxide particles, at least a part of the inorganic fluoride particles is present on the particle surface of the aluminum-containing lithium-cobalt composite oxide particles, a positive electrode active material for a lithium secondary battery, characterized in that, when observed with a scanning electron microscope (SEM) at a magnification of 50,000 times, the proportion of inorganic fluoride particles present on the particle surfaces of the aluminum-containing lithium cobalt composite oxide particles, which have a particle diameter of 200 nm or more as determined by SEM, is 4.0% or less.

2. 2. The positive electrode active material for a lithium secondary battery according to claim 1, wherein, when observed with a scanning electron microscope (SEM) at a magnification of 50,000 times, the proportion of inorganic fluoride particles present on the particle surfaces of the aluminum-containing lithium cobalt composite oxide particles having a particle diameter of less than 120 nm as determined by SEM is 92.0% or more.

3. 3. The positive electrode active material for a lithium secondary battery according to claim 1, wherein, when observed at a magnification of 50,000 times with a scanning electron microscope (SEM), the ratio of inorganic fluoride particles present on the particle surfaces of the aluminum-containing lithium-cobalt composite oxide particles having a ratio of their major axis to their minor axis (major axis / minor axis) of less than 1.5 accounts for 65.0% or more.

4. 3. The positive electrode active material for a lithium secondary battery according to claim 1, wherein, when observed at 50,000 times magnification with a scanning electron microscope (SEM), the ratio of inorganic fluoride particles present on the particle surfaces of the aluminum-containing lithium cobalt composite oxide particles having a ratio of their major axis to their minor axis (major axis / minor axis) of 3.0 or more is 2.0% or less.

5. The inorganic fluoride particles are MgF ２ and AlF ３ 3. The positive electrode active material for a lithium secondary battery according to claim 1, wherein

6. 3. The positive electrode active material for lithium secondary batteries according to claim 1, wherein the content of the inorganic fluoride particles is 0.05 to 5.0 mol % in terms of atomic mol % of F relative to Co in the aluminum-containing lithium-cobalt composite oxide particles ((F / Co) × 100).

7. 3. The positive electrode active material for a lithium secondary battery according to claim 1, wherein the aluminum-containing lithium-cobalt composite oxide particles contain, as an M element, one or more selected from Ca, Mg, Sr, Zr, Nb, B, and W.

8. a first mixing step of mixing a lithium compound, a cobalt compound, and an aluminum compound to obtain a first mixture; a first firing step of firing the first mixture to obtain, as a first fired product, aluminum-containing lithium-cobalt composite oxide particles in which aluminum is present as a solid solution at least inside the particles; The first fired product obtained in the first firing step and inorganic fluoride particles, MgF ２ and AlF ３ a second mixing step of dry-mixing the above to obtain a positive electrode active material as a second mixture; and the inorganic fluoride particles have an average particle size of 0.01 to 30 μm in terms of a 50% particle size (D50) converted into volume in a particle size distribution measured by a laser diffraction / scattering method; A method for producing a positive electrode active material for a lithium secondary battery, characterized by:

9. 9. The method for producing a positive electrode active material for a lithium secondary battery according to claim 8, wherein in the first mixing step, the aluminum compound is mixed so that the content of Al relative to Co in the first mixture is 0.05 to 5.00 mol % in atomic terms ((Al / Co)×100).

10. 10. The method for producing a positive electrode active material for a lithium secondary battery according to claim 8 or 9, wherein the first fired product contains, as an M element, one or more elements selected from the group consisting of Ca, Mg, Sr, Zr, Nb, B, and W.

11. 10. The method for producing a positive electrode active material for a lithium secondary battery according to claim 8, wherein the firing temperature in the first firing step is 800 to 1150°C.

12. 10. The method for producing a positive electrode active material for a lithium secondary battery according to claim 8, wherein in the second mixing step, the inorganic fluoride particles are mixed so that the molar percentage of F relative to Co in the second mixture ((F / Co)×100) is 0.05 to 5.00 mol % in terms of atoms.

13. The MgF in the second mixing step ２ The mixing ratio of AlF ３ Molar ratio (MgF ２ / AlF ３ 10. The method for producing a positive electrode active material for a lithium secondary battery according to claim 8, wherein the value of σ is 1.00 or less.

14. A lithium secondary battery, comprising the positive electrode active material for lithium secondary batteries according to claim 1 or 2 as a positive electrode active material.

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