Positive electrode active material for lithium-ion secondary batteries

Abstract

Reduction of initial resistance. [Solution] The positive electrode active material for lithium-ion secondary batteries contains single crystal particles. Crack groups are formed in the cross-section of the single crystal particles. Each crack group contains two or more cracks. Each crack group contains portions where two or more cracks extend parallel to each other. The positive electrode active material for lithium-ion secondary batteries satisfies the relationship "3% ≤ L / D". "L" represents the length of the crack. "D" represents the diameter of the smallest circumscribed circle of the single crystal particle in the cross-section.

Description

[Technical Field]

[0001] This disclosure relates to a positive electrode active material for lithium-ion secondary batteries. [Background technology]

[0002] Japanese Patent Publication No. 2023-036570 discloses a large-grain aggregate ternary cathode material exhibiting single-crystal-like morphology. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2023-036570 [Overview of the project] [Problems that the invention aims to solve]

[0004] Single crystallization of the positive electrode active material has been proposed. Single crystallization is expected to improve, for example, the storage properties. This is thought to be because single-crystal particles have a smaller specific surface area compared to polycrystalline particles. However, single-crystal particles still have room for improvement in initial resistance.

[0005] The purpose of this disclosure is to reduce initial resistance. [Means for solving the problem]

[0006] 1. The positive electrode active material for lithium-ion secondary batteries contains single crystal particles. Crack groups are formed in the cross-section of the single crystal particles. Each crack group contains two or more cracks. Each crack group contains portions where two or more cracks extend parallel to each other. The positive electrode active material for lithium-ion secondary batteries satisfies the relationship "3% ≤ L / D". "L" represents the length of the crack. "D" represents the diameter of the smallest circumscribed circle of the single crystal particle in the cross-section.

[0007] The formation of crack clusters in single-crystal particles is expected to reduce initial resistance. This is thought to be because the electrolyte penetrates into the single-crystal particles through the crack clusters, forming ion diffusion pathways within the particles. Hereafter, the number of cracks will also be referred to as "number of cracks," and the length of the cracks will also be referred to as "crack length." The positive electrode active material for lithium-ion secondary batteries may be abbreviated as "positive electrode active material." The lithium-ion secondary battery may be abbreviated as "battery."

[0008] 2. The positive electrode active material for lithium-ion secondary batteries described in "1" above may include, for example, the following configuration: The crack group includes 4 to 8 cracks.

[0009] Single-crystal particles repeatedly expand and contract due to repeated charging and discharging. This volume change in single-crystal particles can cause stress concentration. This stress concentration can lead to cracking of the single-crystal particles, potentially accelerating capacity degradation. However, if four or more cracks are pre-formed in the single-crystal particles before charging and discharging within the battery, in addition to reducing initial resistance, improvements in cycle characteristics can also be expected. This is thought to be because the cracks alleviate stress concentration.

[0010] 3. The positive electrode active material for lithium-ion secondary batteries described in "1" or "2" above may include, for example, the following configuration: The positive electrode active material for lithium-ion secondary batteries satisfies the relationship "6% ≤ L / D ≤ ​​52%".

[0011] When the relationship "6% ≤ L / D ≤ ​​52%" is satisfied, in addition to a reduction in initial resistance, an improvement in cycle characteristics can also be expected.

[0012] 4. The positive electrode active material for lithium-ion secondary batteries described in any one of items "1" to "3" above may include, for example, the following configuration: The positive electrode active material consists of 50% or more single crystal particles and the remainder being polycrystalline particles.

[0013] The positive electrode active material may contain polycrystalline particles in addition to single crystal particles. By the number fraction of the single crystal particles being 50% or more, for example, an improvement in cycle characteristics is expected.

[0014] 5. The positive electrode active material for a lithium ion secondary battery according to any one of the above "1" to "4" may contain, for example, the following configuration. The positive electrode active material has a composition represented by the general formula "Li ,

[0017] , [Figure 4] , , , , [Figure 3] ,

[0015] , [Figure 2] , [Figure 1] , , , y , , , ,

[0016] , , , , Ni a Co b Mn c O y ". In the general formula, "x, a, b, c, y" satisfy the relationships of "0.1 ≦ x ≦ 1.5", "0.5 ≦ a ≦ 1.0", "0 ≦ b ≦ 0.3", "0 ≦ c ≦ 0.3", "a + b + c = 1.0" and "1.5 ≦ y ≦ 2.1".

[0015] By the Ni composition ratio "a" being 0.5 or more, for example, an increase in the initial discharge capacity is expected.

[0016] Hereinafter, an embodiment of the present disclosure (hereinafter may be abbreviated as "this embodiment"), and an example of the present disclosure (hereinafter may be abbreviated as "this example") will be described. However, this embodiment and this example do not limit the technical scope of the present disclosure. This embodiment and this example are illustrative in all respects. This embodiment and this example are non-restrictive. The technical scope of the present disclosure includes all modifications within the meaning and scope equivalent to the description of the claims. For example, any configuration is extracted from this embodiment and their arbitrary combinations are also contemplated from the beginning.

Brief Description of Drawings

[0017] [Figure 1] It is a schematic cross-sectional view of single crystal particles in this embodiment. [Figure 2] It is a table showing experimental results. [Figure 3] It is the first temperature profile. [Figure 4] It is the second temperature profile.

Mode for Carrying Out the Invention

[0018] Terms and Phrases "Comprising", "including", "having", and their variants are open-ended expressions. A configuration expressed in an open-ended manner may further include additional elements in addition to the essential elements, or may not include them. The description "consisting of" is a closed expression. However, even a configuration expressed in a closed manner may include additional elements that are normally accompanying impurities or are irrelevant to the target technology. The description "substantially consisting of" is a semi-closed expression. In a configuration expressed in a semi-closed manner, the addition of elements that do not substantially affect the basic and novel characteristics of the target technology is permitted.

[0019] Geometric terms should not be construed in a strict sense. Examples of geometric terms include "parallel", "perpendicular", "orthogonal", etc. For example, within a range where substantially the same or similar functions can be obtained, directions, angles, distances, etc. may be relatively displaced. Geometric terms may include, for example, tolerances, errors, etc. in design, operation, manufacturing, etc. The dimensional relationships in each figure may not match the actual dimensional relationships. The dimensional relationships in each figure may be changed to assist the reader's understanding. For example, the length, width, thickness, etc. may be changed. Some configurations may be omitted.

[0020] Numerical values may be expressed with significant figures. A measured value may be, unless otherwise specified, the average value in multiple measurements. The number of measurements may be three or more, five or more, or ten or more. Generally, the greater the number of measurements, the more the reliability of the average value is expected to improve. A measured value may be rounded off based on the number of significant figures. A measured value may include errors associated with, for example, the detection limit of the measuring device.

[0021] The devices, software, etc., used for measuring various values ​​are merely examples. Equivalent devices may be used. If equivalent devices are used, the measurement conditions may be adjusted to suit the device.

[0022] Cross-sectional SEM (Scanning Electron Microscope) images of the positive electrode active material are obtained by the following procedure. For example, a dispersion is formed by dispersing 1 g of positive electrode active material (powder) in a mixture (10 g) of the main component and curing agent of epoxy resin (product name "EPOTEX JP", manufactured by Nisshin EM Co., Ltd.). The dispersion is stirred and mixed for 1 minute using a mixer (product name "Awatori Rentaro", manufactured by Shinky Co., Ltd.). The stirring speed may be, for example, around 2000 rpm. The dispersion is degassed under vacuum. After degassing under vacuum, the dispersion is filled into a cylindrical resin container. The epoxy resin hardens when the dispersion is left for 1 day. After hardening, a cross-sectional sample with a smooth cross-section is prepared by wet polishing of the hardened material. A cross-sectional SEM image of the positive electrode active material is obtained by performing SEM observation of the smooth cross-section.

[0023] A "single-crystal particle" refers to a solid particle that is the smallest unit of particle and is recognized as being unable to be divided any further. In a cross-sectional SEM image, a single-crystal particle appears to have no grain boundaries. Single-crystal particles are also referred to as primary particles. An aggregate of two or more primary particles is considered a "polycrystalline particle." In a cross-sectional SEM image, 100 particles are randomly extracted. By counting the number of single-crystal particles contained in these 100 particles, the proportion of single-crystal particles is determined.

[0024] In the cross-sectional SEM image, 100 single crystal grains are randomly selected. From these 100 single crystal grains, single crystal grains having a group of cracks in their cross-section are further selected. Figure 1 is a schematic cross-sectional view of a single crystal grain in this embodiment. A group of cracks 5 is formed in the cross-section of a single crystal grain 10. The group of cracks 5 includes two or more cracks 2. The group of cracks 5 includes a portion where two or more cracks 2 extend in parallel. Hereinafter, this portion will also be referred to as the "parallel portion 4". For each selected single crystal grain, the diameter of the minimum circumscribed circle (MCC) of the single crystal grain is measured. The diameter of the MCC is considered to be the grain diameter. Furthermore, the number of cracks and the crack length are measured. The number of cracks indicates the number of cracks that do not intersect with other cracks. The crack length indicates the path length (total length) of the cracks. Measurement of various dimensions and shape analysis in the cross-sectional SEM image can be performed using, for example, image analysis software such as "ImageJ". For example, dimensions and other parameters may be measured at image magnifications ranging from 10,000 to 30,000 times. The average particle diameter of all single crystal particles being measured is considered to be the particle diameter "D". The average crack length of all single crystal particles being measured is considered to be the crack length "L". The average number of cracks in all single crystal particles being measured is considered to be the number of cracks "n". The ratio of crack length to particle diameter "L / D" is calculated by dividing the crack length "L" by the diameter "D". The ratio "L / D" is expressed as a percentage.

[0025] The chemical composition of the positive electrode active material can be measured by ICP-AES (Inductively Coupled Plasma Atomic Emission Spectroscopy). A sample solution is prepared by dissolving 0.1 g of the sample (positive electrode active material) in a mixed acid (10 ml) of hydrochloric acid and sulfuric acid. The sample solution is diluted to an appropriate concentration in a volumetric flask. After dilution, compositional analysis is performed using an ICP-AES instrument. For example, a product such as "PS3520 UVDD II (manufactured by Hitachi High-Tech Science Corporation)" may be used.

[0026] "D50" indicates the particle size at which the cumulative frequency reaches 50% in the volume-based particle size distribution (cumulative distribution). D50 can be measured, for example, by laser diffraction. Similarly, the particle size at which the cumulative frequency reaches 10% is also written as "D10," and the particle size at which the cumulative frequency reaches 90% is also written as "D90."

[0027] positive electrode active material The positive electrode active material is for use in batteries. The battery may be a liquid-based battery or an all-solid-state battery. For example, even in an all-solid-state battery, improvement in cycle characteristics due to stress concentration relief can be expected. The battery may have any structure. For example, the battery may have a wound or stacked power generation element. For example, the battery may have a unipolar structure or a bipolar structure.

[0028] The positive electrode active material is an aggregate of particles (powder). The D50 of the powder may be, for example, 1 μm or more, 3 μm or more, 5 μm or more, 10 μm or more, or 15 μm or more. The D50 of the powder may also be, for example, 30 μm or less, 20 μm or less, or 10 μm or less.

[0029] The positive electrode active material contains single-crystal particles. In addition to single-crystal particles, the positive electrode active material may further contain polycrystalline particles. The polycrystalline particles may have substantially the same crystal structure and composition as the single-crystal particles. The positive electrode active material may consist, for example, of 50% or more single-crystal particles and the remainder being polycrystalline particles. The percentage of single-crystal particles may be, for example, 60% or more, 70% or more, 80% or more, or 90% or more. The percentage of single-crystal particles may be, for example, 100% or less, 90% or less, or 80% or less. For example, an improvement in cycle characteristics can be expected when the percentage of single-crystal particles is 50% or more.

[0030] The positive electrode active material may be a monodisperse system. Improved cycle characteristics are expected when the powder is mainly composed of single-crystal particles and is a monodisperse system. The powder may have a span of, for example, 1 or less. "Span" refers to the value obtained by the calculation formula "(D90-D10) / D50". A smaller span indicates a sharper particle size distribution. The powder span may be, for example, 0.9 or less, 0.8 or less, 0.7 or less, 0.6 or less, or 0.5 or less. The powder span may also be, for example, 0.1 or more, 0.2 or more, 0.3 or more, 0.4 or more, or 0.5 or more.

[0031] As shown in Figure 1, a group of cracks 5 is formed in the cross-section of the single crystal grain 10. The diffusion of the electrolyte into the single crystal grain 10 due to the crack group 5 is expected to reduce the initial resistance. Furthermore, the easing of stress concentration by the crack group 5 is also expected to improve the cycle characteristics.

[0032] A single crack group 5 may be formed on the single crystal grain 10, or multiple crack groups 5 may be formed. The number of crack groups 5 may be, for example, two or more, three or more, or four or more. The number of crack groups 5 may be, for example, 10 or less, 5 or less, 4 or less, 3 or less, or 2 or less.

[0033] Crack group 5 includes two or more cracks 2. The number of cracks may be, for example, three or more, four or more, five or more, six or more, seven or more, eight or more, nine or more, or ten or more. The number of cracks may also be, for example, 20 or less, 15 or less, 10 or less, nine or less, eight or less, seven or less, six or less, five or less, four or less, or three or less. For example, having four or more cracks and eight or fewer is expected to reduce initial resistance as well as improve cycle characteristics.

[0034] Each crack 2 originates on the surface of the single crystal grain 10. Each crack 2 extends from the surface of the single crystal grain 10 toward the interior of the single crystal grain 10. The cracks 2 may extend in a straight line or in a curved line. For example, if the cracks 2 extend in the depth direction of Figure 2, there may be parts where the cracks 2 appear as dotted lines.

[0035] A reduction in initial resistance is expected when the relationship "3% ≤ L / D" is satisfied between the crack length "L" and the particle diameter "D". The ratio "L / D" may be, for example, 6% or more, 12% or more, 18% or more, 24% or more, 30% or more, 31% or more, 36% or more, 42% or more, 48% or more, 52% or more, or 54% or more. The ratio "L / D" may also be, for example, 60% or less, 54% or less, 52% or less, 48% or less, 42% or less, 36% or less, 31% or less, 30% or less, 24% or less, 18% or less, 12% or less, or 6% or less. For example, when the ratio "L / D" is 6% or more, a reduction in initial resistance and improvement in cycle characteristics can be expected. If the ratio "L / D" is excessively large, there is a possibility that particle cracking may occur starting from the crack. When the ratio "L / D" is 52% or less, an improvement in cycle characteristics can be expected.

[0036] The particle diameter "D" may be, for example, 0.1 μm or more, 0.5 μm or more, 1 μm or more, 2 μm or more, 3 μm or more, 4 μm or more, 5 μm or more, 6 μm or more, 7 μm or more, 8 μm or more, 9 μm or more, or 10 μm or more. The particle diameter "D" may also be, for example, 15 μm or less, 10 μm or less, 9 μm or less, 8 μm or less, 7 μm or less, 6 μm or less, 5 μm or less, 4 μm or less, 3 μm or less, 2 μm or less, or 1 μm or less.

[0037] Crack group 5 includes a parallel section 4. That is, crack group 5 is composed of multiple cracks 2 that form a parallel section 4. In the parallel section 4, two or more cracks 2 do not intersect. Two or more cracks 2 extend in parallel. When the angle between the directions of propagation of two or more cracks 2, starting from the particle surface, is 10° or less, that section is considered to be a parallel section 4. In the parallel section 4, the angle between two or more cracks 2 may be, for example, 5° or less, 3° or less, or 1° or less. The parallel section 4 may extend over the entire length of the crack 2. The parallel section 4 may be a part of the entire length of the crack 2. The ratio of the length of the parallel section 4 to the crack length "L" may be, for example, greater than 0%, 5% or more, 10% or more, 20% or more, 30% or more, 40% or more, 50% or more, 60% or more, 70% or more, 80% or more, or 90% or more. The ratio of the length of the parallel portion 4 to the crack length "L" may be, for example, 100% or less, 90% or less, 80% or less, 70% or less, 60% or less, 50% or less, 40% or less, 30% or less, 20% or less, 10% or less, or 5% or less.

[0038] The width of crack 2 is the dimension in the direction perpendicular to the length direction. The width of crack 2 may be, for example, 1 / 100th or less of the crack length "L", or 1 / 10th or less. The width of crack 2 may be, for example, less than 10 nm. The width of crack 2 may be, for example, 9 nm or less, 8 nm or less, 7 nm or less, 6 nm or less, 5 nm or less, 4 nm or less, 3 nm or less, 2 nm or less, or 1 nm or less. The width of crack 2 may be, for example, 0.1 nm or more, 1 nm or more, 2 nm or more, 3 nm or more, 4 nm or more, 5 nm or more, 6 nm or more, 7 nm or more, 8 nm or more, or 9 nm or more.

[0039] The spacing between cracks 2 in the width direction may be constant or variable. The spacing between cracks 2 may be, for example, less than or equal to half, one-third, one-quarter, or one-fifth of the crack length "L". The spacing between cracks 2 may also be, for example, more than or equal to one-hundredth or one-tenth of the crack length "L".

[0040] The positive electrode active material may, for example, contain a lithium transition metal composite oxide. The positive electrode active material may have a crystal structure belonging to the space group R-3m. The same crystal structure is also referred to as a layered structure. The crystal structure can be specified by an XRD (X-ray Diffraction) pattern. The lithium transition metal composite oxide contains Li, a transition metal, and oxygen.

[0041] The positive electrode active material may, for example, have a composition represented by the general formula "Li x Ni a Co b Mn c O y ". In the general formula, the Li composition ratio "x" may, for example, satisfy the relationship of "0.1 ≦ x ≦ 1.5". The Li composition ratio "x" may, for example, be 0.4 or more, 0.6 or more, 0.8 or more, 1.0 or more, 1.2 or more, or 1.4 or more. The Li composition ratio "x" may, for example, be 1.4 or less, or 1.2 or less.

[0042] In the above general formula, the O composition ratio "y" may, for example, satisfy the relationship of "1.5 ≦ y ≦ 2.1". The O composition ratio "y" may, for example, be 1.6 or more, 1.7 or more, 1.8 or more, 1.9 or more, or 2.0 or more. The O composition ratio "y" may, for example, be 2.0 or less, 1.9 or less, 1.8 or less, 1.7 or less, or 1.6 or less.

[0043] In the above general formula, the Ni composition ratio "a", the Co composition ratio "b", and the Mn composition ratio "c" may satisfy the relationship of "a + b + c = 1.0". The Ni composition ratio "a" may, for example, satisfy the relationship of "0.5 ≦ a ≦ 1.0". The Ni composition ratio "a" may, for example, be 0.6 or more, 0.7 or more, 0.8 or more, 0.9 or more. The Ni composition ratio "a" may, for example, be 0.9 or less, 0.8 or less, 0.7 or less, or 0.6 or less.

[0044] In the general formula above, the Co composition ratio "b" may satisfy, for example, the relationship "0 ≤ b ≤ 0.3". The Co composition ratio "b" may be, for example, 0.01 or more, 0.02 or more, 0.03 or more, 0.04 or more, 0.05 or more, 0.06 or more, 0.07 or more, 0.08 or more, 0.09 or more, 0.10 or more, 0.15 or more, 0.20 or more, or 0.25 or more. The Co composition ratio "b" may be, for example, 0.25 or less, 0.20 or less, 0.15 or less, 0.10 or less, 0.09 or less, 0.08 or less, 0.07 or less, 0.06 or less, 0.05 or less, 0.04 or less, 0.03 or less, 0.02 or less, or 0.01 or less.

[0045] In the above general formula, the Mn composition ratio "c" may satisfy, for example, the relationship "0 ≤ c ≤ 0.3". The Mn composition ratio "c" may be, for example, 0.01 or more, 0.02 or more, 0.03 or more, 0.04 or more, 0.05 or more, 0.06 or more, 0.07 or more, 0.08 or more, 0.09 or more, 0.10 or more, 0.15 or more, 0.20 or more, or 0.25 or more. The Mn composition ratio "c" may be, for example, 0.25 or less, 0.20 or less, 0.15 or less, 0.10 or less, 0.09 or less, 0.08 or less, 0.07 or less, 0.06 or less, 0.05 or less, 0.04 or less, 0.03 or less, 0.02 or less, or 0.01 or less.

[0046] In addition, in the above general formula, all or part of Mn may be substituted with Al, etc. That is, lithium transition metal composite oxides are, for example, those with the general formula "Li x Ni a Co b Al c O y It may have a composition represented by ". The range of the Al composition ratio "c" is the same as that of the Mn composition ratio "c" above.

[0047] Lithium transition metal composite oxides may contain any dopant. The dopant represents an element other than Li, Ni, Co, Mn, and O. For example, the dopant may contain at least one selected from the group consisting of Zr, Mo, W, Mg, Ca, Na, Fe, Cr, Zn, Si, Sn, Al, and Ag. The composition ratio of the dopant may be, for example, 0.005 or more, 0.01 or more, 0.02 or more, 0.03 or more, or 0.04 or more. The composition ratio of the dopant may be, for example, 0.05 or less, 0.04 or less, 0.03 or less, 0.02 or less, or 0.01 or less. [Examples]

[0048] Sample preparation Figure 2 is a table showing the experimental results. The positive electrode active materials No. 1 to No. 5 were manufactured according to the following procedure.

[0049] Preparation of the precursor The raw material solution is formed by dissolving NiSO4, CoSO4, and MnSO4 in deionized water. In the raw material solution, the molar ratio of Ni, Co, and Mn is "Ni / Co / Mn = 90 / 5 / 5". The solute concentration in the raw material solution is 30% (mass fraction).

[0050] Ammonia water is added to the reaction vessel. The ammonia water is stirred by a stirrer, and the inside of the reaction vessel is replaced with nitrogen. Then, NaOH is added to the reaction vessel, forming an alkaline reaction solution.

[0051] The reaction solution is dropped with the raw material solution and ammonia water to maintain a pH within a certain range, forming a precipitate (metal hydroxide). The reaction solution is filtered to recover the metal hydroxide. The metal hydroxide is dispersed in deionized water to form a dispersion. The dispersion is thoroughly stirred with a spatula, that is, the metal hydroxide is washed with water. After washing, the dispersion is filtered to recover the metal hydroxide. The metal hydroxide is dried at 120°C for 16 hours to form a dried product.

[0052] Adding Li sources In a mortar, a mixture is formed by mixing a dried material (metal hydroxide) and a lithium compound (LiOH, Li2CO3) using a pestle. By adding an excess of Li relative to the total amount of transition metals, a molten salt is formed during firing, and a lithium transition metal composite oxide can be single-crystallized. That is, the percentage of single-crystal particles can exceed 50%. The ratio of the amount of Li to the total amount of transition metals is, for example, 1.5 or more.

[0053] firing In a firing furnace (e.g., a muffle furnace), the mixture is subjected to firing (heat treatment) to synthesize a lithium transition metal composite oxide. The firing atmosphere is an oxygen atmosphere. Figure 3 shows the first temperature profile. In No. 1, firing is carried out according to the first temperature profile. The furnace temperature is raised to a firing temperature in the range of 700°C to 1100°C. The firing temperature is substantially maintained for 10 hours. After 10 hours, the furnace temperature is cooled to room temperature by natural cooling.

[0054] Figure 4 shows the second temperature profile. For samples No. 2 to No. 5, firing is performed according to the second temperature profile. The furnace temperature is raised to a firing temperature within the range of 700°C to 1100°C. The firing temperature is substantially maintained for 10 hours. After 10 hours, the furnace is cooled by supplying nitrogen gas. The nitrogen gas supply rate for each sample is shown in the "Nitrogen supply rate during cooling" section of Figure 2.

[0055] After firing, the particle size of the lithium transition metal composite oxide is adjusted using a pulverizer such as a jet mill. Thus, the positive electrode active material is manufactured.

[0056] Measurement of initial resistance The initial resistance was measured using the following procedure. A laminate cell was then fabricated. The configuration of the laminate cell is as follows.

[0057] Outer packaging: Pouch made of aluminum laminate film Power generation element: Stacked type (single layer) Positive electrode: positive electrode active material / conductive material / binder = 88 / 10 / 2 (mass ratio) Negative electrode: Negative electrode active material (natural graphite), CMC, SBR Electrolytes: LiPF6 (1 ml / L), EC / DMC / EMC = 3 / 4 / 3 (volume ratio)

[0058] The positive and negative electrodes are manufactured by coating the surface of a substrate (metal foil) with a slurry. For example, a film applicator (with film thickness adjustment function) manufactured by Allgood is used as the coating equipment. After coating with the slurry, the coating is dried at 80°C for 5 minutes.

[0059] The laminate cell is sandwiched between two stainless steel plates, thereby applying a predetermined pressure to the power generation element. The state of charge (SOC) of the laminate cell is adjusted to 50%. The IV resistance is measured in a -10°C temperature environment. The value shown in the "Initial Resistance" column of Figure 2 is a relative value, with the initial resistance value of No. 1 set to 100%.

[0060] Measurement of cycle characteristics A cycle test will be performed on the laminated cell under the following conditions. Ambient temperature: 60℃ Number of cycles: 100 Current rate: 0.3C Voltage range: 4.25V to 2.5V

[0061] At a current rate of 1C, the cell's rated capacity is supplied in one hour. 0.3C is 0.3 times 1C. At the 1st, 25th, 50th, 75th, and 100th cycles, the discharge capacity is measured at a current rate of 0.2C. The capacity retention rate is calculated by dividing the discharge capacity at the 100th cycle by the discharge capacity at the 1st cycle. A higher capacity retention rate indicates better cycle characteristics.

[0062] Experimental results As shown in Figure 2, when the relationship "3% ≤ L / D" is satisfied, there is a tendency for the initial resistance to decrease.

[0063] When the number of cracks is between 4 and 8, there is a tendency for the initial resistance to decrease and the cycle characteristics to improve.

[0064] When the relationship "6% ≤ L / D ≤ ​​52%" is satisfied, there is a tendency for the initial resistance to decrease and the cycle characteristics to improve. [Explanation of Symbols]

[0065] 2 cracks, 4 parallel sections, 5 crack groups, 10 single crystal grains.

Claims

1. Contains single crystal particles, A group of cracks is formed in the cross-section of the aforementioned single crystal grain. The aforementioned group of cracks includes two or more cracks, The aforementioned group of cracks includes a portion in which two or more of the aforementioned cracks extend parallel to each other. 3% ≤ L / D Satisfying the relationship, The L indicates the length of the crack, and, D indicates the diameter of the smallest circumscribed circle of the single crystal grain in the cross-section. Positive electrode active material for lithium-ion secondary batteries.

2. The aforementioned group of cracks includes four to eight of the aforementioned cracks. The positive electrode active material for a lithium-ion secondary battery according to claim 1.

3. 6% ≤ L / D ≤ ​​52% Satisfying the relationship, The positive electrode active material for a lithium-ion secondary battery according to claim 1 or claim 2.

4. It consists of single-crystal particles making up 50% or more of the total number of particles, and the remainder being polycrystalline particles. The positive electrode active material for a lithium-ion secondary battery according to claim 1 or claim 2.

5. General formula: Li x Ni a Co b Mn c O y It has a composition represented by, In the above general formula, x, a, b, c, and y satisfy the following relationships: 0.1 ≤ x ≤ 1.5, 0.5 ≤ a ≤ 1.0, 0 ≤ b ≤ 0.3, 0 ≤ c ≤ 0.3, a + b + c = 1.0, and 1.5 ≤ y ≤ 2.

1. The positive electrode active material for a lithium-ion secondary battery according to claim 1 or claim 2.

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