Positive electrode active material for lithium ion secondary battery

Abstract

The present disclosure relates to a positive electrode active material for a lithium ion secondary battery. The purpose of the present disclosure is to reduce initial resistance. A positive electrode active material for a lithium ion secondary battery includes single crystal particles. Crack groups are formed in cross sections of the single crystal particles. The crack group includes two or more cracks. The crack group includes a portion in which two or more cracks extend in parallel. The positive electrode active material for a lithium ion secondary battery satisfies the relationship of 3% < = L / D. 'L' represents the length of the crack. 'D' represents the diameter of the minimum circumcircle of the single crystal particle in the cross section.

Description

Technical Field

[0001] This disclosure relates to positive electrode active materials for lithium-ion secondary batteries. Background Technology

[0002] Japanese Patent Application Publication No. 2023-036570 discloses a ternary cathode material with a large-grained polymer exhibiting a single-crystal morphology. Summary of the Invention

[0003] The single-crystalization of positive electrode active materials has been proposed. Single-crystalization is expected to improve properties such as storage characteristics. This is attributed to the smaller specific surface area of ​​single-crystal particles compared to polycrystalline particles. However, there is still room for improvement in the initial resistance of single-crystal particles.

[0004] The purpose of this disclosure is to reduce the initial resistance.

[0005] 1. A positive electrode active material for lithium-ion secondary batteries, comprising single-crystal particles. Crack clusters are formed in the cross-section of the single-crystal particles. Each crack cluster contains two or more cracks. The crack cluster includes portions where two or more cracks extend in parallel. 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 particles in the cross-section.

[0006] By forming crack clusters in single-crystal particles, a reduction in initial resistance can be expected. This is believed to be due to the electrolyte penetrating through the crack clusters into the single-crystal particles, thereby creating ion diffusion pathways within the particles. Hereinafter, the number of cracks is referred to as "crack number," and the length of the crack is referred to as "crack length." The positive electrode active material for lithium-ion secondary batteries can be simply referred to as "positive electrode active material." Lithium-ion secondary batteries can be simply referred to as "batteries."

[0007] 2. The positive electrode active material for lithium-ion secondary batteries described in "1" above may also include, for example, the following: The crack cluster contains 4 or more but no more than 8 cracks.

[0008] Single-crystal particles undergo repeated expansion and contraction during charging and discharging. This volume change can lead to stress concentration. This stress concentration can cause cracking in the single-crystal particles, potentially promoting capacity degradation. By pre-forming four or more cracks in the single-crystal particles before charging and discharging within the battery, it is expected that, in addition to reducing initial resistance, cycle characteristics can be improved. This is believed to be because the cracks mitigate stress concentration.

[0009] 3. The positive electrode active material for lithium-ion secondary batteries described in "1" or "2" above may also include the following: The positive electrode active material for lithium-ion secondary batteries satisfies the relationship of "6% ≤ L / D ≤ ​​52%".

[0010] When the relationship “6%≤L / D≤52%” is satisfied, in addition to reducing the initial resistance, the cycling characteristics can also be improved.

[0011] 4. The positive electrode active material for lithium-ion secondary batteries described in any of "1" to "3" above may also include the following: The positive electrode active material consists of single crystal particles comprising 50% or more by number fraction and the remainder being polycrystalline particles.

[0012] In addition to single-crystal particles, the positive electrode active material can also contain polycrystalline particles. With a single-crystal particle fraction of 50% or more, improved cycle performance can be expected, for example.

[0013] 5. The positive electrode active material for lithium-ion secondary batteries described in any of "1" to "4" above may also include, for example, the following: The positive electrode active material has the general formula "Li x Ni a Co b Mn c O y The general formula represents the composition of x, a, b, c, y. In the formula, x, a, b, c, 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.

[0014] With a Ni composition ratio "a" of 0.5 or higher, an increase in initial discharge capacity can be expected, for example.

[0015] Hereinafter, an embodiment of the present disclosure (hereinafter referred to as "this embodiment") and an example of the present disclosure (hereinafter referred to 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-limiting. The technical scope of the present disclosure includes all modifications within the meaning and scope equivalent to the claims. For example, from the very beginning, it is intended to include schemes that extract arbitrary structures from this embodiment and combine them arbitrarily. Attached Figure Description

[0016] The features, advantages, and technical and industrial significance of exemplary embodiments of the present invention are described below with reference to the accompanying drawings, wherein the same reference numerals denote the same elements.

[0017] Figure 1 This is a schematic cross-sectional view of the single crystal particle in this embodiment.

[0018] Figure 2 This is a table representing the experimental results.

[0019] Figure 3 This is the first temperature curve.

[0020] Figure 4 This is the second temperature curve. Detailed Implementation

[0021] Terms and phrases

[0022] The terms "possess," "comprise," and "have," and their variations, are open-ended expressions. Solutions expressed in an open-ended manner may include additional elements beyond the necessary elements, or they may not include any additional elements. The statement "composes of..." is a closed-ended expression. However, even solutions expressed in a closed-ended manner may include commonly accompanying impurities or additional elements unrelated to the target technology. The statement "substantially composed of..." is a semi-closed-ended expression. Solutions expressed in a semi-closed-ended manner allow for the addition of elements that do not substantially affect the fundamental and novel characteristics of the target technology.

[0023] Geometric terms should not be interpreted in a strict sense. As geometric terms, they may exemplify concepts such as "parallel," "perpendicular," and "orthogonal." For example, directions, angles, and distances can also be relatively displaced within a range that achieves substantially the same or similar function. Geometric terms may include, for example, tolerances and errors in design, operation, and manufacturing. Dimensional relationships in the drawings may sometimes differ from actual dimensional relationships. To aid understanding, dimensional relationships in the drawings may sometimes be altered. For example, length, width, and thickness may be changed. Sometimes, parts of the structure may be omitted.

[0024] Numerical values ​​can be expressed using significant figures. Unless otherwise specified, a measured value can be the average of multiple measurements. The number of measurements can be 3 or more, 5 or more, or 10 or more. Generally, the more measurements are expected, the higher the reliability of the average value. Measured values ​​can be rounded based on the number of significant figures. Measured values ​​may include, for example, errors such as the detection limits of the accompanying measuring device.

[0025] The apparatus and software used for measuring various values ​​are just one example. Equivalent products to the illustrated apparatus can also be used. When using equivalent products, the measurement conditions can be adjusted in conjunction with the apparatus.

[0026] Cross-sectional SEM (Scanning Electron Microscope) images of the positive electrode active material were obtained through the following steps. For example, a dispersion was formed by dispersing 1 g of the positive electrode active material (powder) in a mixture (10 g) of the main agent and curing agent of epoxy resin (product name "EPOTEX JP", manufactured by Nisshin EM Co., Ltd.). The dispersion was stirred for 1 minute using a mixer (product name "Awatori Rentaro", manufactured by Thinky Co., Ltd.). The stirring speed could be, for example, around 2000 rpm. The dispersion was then degassed under vacuum. After degassed, the dispersion was filled into a cylindrical container made of resin. The epoxy resin was cured by allowing the dispersion to stand for 1 day. After curing, the cured material was wet-milled to prepare a cross-sectional sample with a smooth cross-section. Cross-sectional SEM images of the positive electrode active material were obtained by performing SEM observation of the smooth cross-section.

[0027] A "single-crystal particle" is the smallest unit of particle, representing a solid particle that cannot be further subdivided. Single-crystal particles do not appear to have grain boundaries in cross-sectional SEM images. Single-crystal particles are also referred to as primary particles. Aggregates of two or more primary particles are considered "polycrystalline particles." One hundred particles are randomly extracted from a cross-sectional SEM image. The fractional number of single-crystal particles is determined by counting the number of single-crystal particles contained within each of the 100 particles.

[0028] One hundred single-crystal particles were randomly extracted from the cross-sectional SEM image. From these 100 single-crystal particles, single-crystal particles with crack clusters in the cross-section were further extracted. Figure 1 This is a schematic cross-sectional view of the single crystal particle in this embodiment. A crack group 5 is formed in the cross-section of the single crystal particle 10. The crack group 5 contains two or more cracks 2. The crack group 5 includes a portion in which two or more cracks 2 extend parallel to each other. Hereinafter, this portion is also referred to as "parallel portion 4". The diameter of the smallest circumcircle (MCC) of each extracted single crystal particle is measured. The diameter of the MCC is considered the particle 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 crack. Measurements of various dimensions and shape analysis in the cross-sectional SEM image can be performed, for example, using image analysis software such as "ImageJ". For example, dimensions can be measured at image magnifications from 10,000 to 30,000 times. The average particle diameter of all single crystal particles of the measured object is considered the particle diameter "D". The average crack length of all single crystal particles of the measured object is considered the crack length "L". The average number of cracks in all single-crystal particles of the measured object is considered as the crack number "n". The crack length "L" is calculated as the ratio of crack length to particle diameter "L / D" by dividing the crack length "L" by the diameter "D". The ratio "L / D" is expressed as a percentage.

[0029] The chemical composition of the positive electrode active material can be determined using 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 solution (10 ml) of hydrochloric acid and sulfuric acid. The sample solution is then diluted to an appropriate concentration using a volumetric flask. After dilution, compositional analysis is performed using an ICP-AES apparatus. For example, a product name such as "PS 3520 UVDDII (manufactured by Hitachi High Technology Co., Ltd.)" can be used.

[0030] "D50" indicates the particle size at which the cumulative frequency reaches 50% in a volumetric particle size distribution (cumulative distribution). D50 can be determined, for example, by laser diffraction. Similarly, the particle size at which the cumulative frequency reaches 10% is also denoted as "D10", and the particle size at which the cumulative frequency reaches 90% is also denoted as "D90".

[0031] Positive electrode active material

[0032] The positive electrode active material is used in batteries. Batteries can be liquid-based or all-solid-state batteries. For example, in all-solid-state batteries, improved cycle characteristics can be expected by mitigating stress concentration. Batteries can have any structure. Batteries can, for example, have wound or stacked power generation elements. Batteries can, for example, have a unipolar or bipolar structure.

[0033] The positive electrode active material is an aggregate of particles (powder). The D50 of the powder can 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 can be, for example, less than 30 μm, less than 20 μm, or less than 10 μm.

[0034] The positive electrode active material comprises single-crystal particles. In addition to single-crystal particles, the positive electrode active material may also contain polycrystalline particles. Polycrystalline particles may have substantially the same crystal structure and composition as single-crystal particles. For example, the positive electrode active material may consist of single-crystal particles comprising 50% or more of the material and the remainder being polycrystalline particles. The percentage of single-crystal particles may, for example, be 60% or more, 70% or more, 80% or more, or 90% or more. The percentage of single-crystal particles may, for example, be less than 100%, less than 90%, or less than 80%. For example, by having a single-crystal particle percentage of 50% or more, improved cycle performance can be expected.

[0035] The positive electrode active material can be a monodisperse system. Since the powder is mainly composed of single-crystal particles and is a monodisperse system, improved cycle characteristics are expected. The powder can, for example, have a span of less than 1. "Span" is the value calculated using the formula "(D90-D10) / D50". A smaller span is considered to indicate a sharper particle size distribution. The powder span can, for example, be less than 0.9, less than 0.8, less than 0.7, less than 0.6, or less than 0.5. The powder span can, for example, be greater than 0.1, greater than 0.2, greater than 0.3, greater than 0.4, or greater than 0.5.

[0036] like Figure 1 As shown, a crack group 5 is formed in the cross-section of the single crystal particle 10. By utilizing the crack group 5 to allow the electrolyte to diffuse into the single crystal particle 10, it is expected to reduce the initial resistance. Furthermore, by mitigating stress concentration through the crack group 5, it is also expected to improve the cycling characteristics.

[0037] A single crack group 5 can be formed in the single crystal particle 10, or multiple crack groups 5 can be formed. The number of crack groups 5 can be, for example, more than 2, more than 3, or more than 4. The number of crack groups 5 can be, for example, less than 10, less than 5, less than 4, less than 3, or less than 2.

[0038] Crack group 5 contains two or more cracks 2. The number of cracks can 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. Alternatively, the number of cracks can be, for example, less than 20, less than 15, less than 10, less than 9, less than 8, less than 7, less than 6, less than 5, less than 4, or less than 3. For example, by having four or more but less than eight cracks, in addition to reducing the initial resistance, it is expected that the cycling characteristics will also be improved.

[0039] Each crack 2 has a starting point on the surface of the single crystal particle 10. Each crack 2 extends from the surface of the single crystal particle 10 into the interior of the single crystal particle 10. The crack 2 can extend in a straight line or in a curved line. For example, it is also possible for crack 2 to extend in a curved line. Figure 1 The crack 2 appears as a dashed line because it extends in the depth direction.

[0040] By satisfying a relationship of "3% ≤ L / D" between the crack length "L" and the particle diameter "D", a reduction in initial resistance can be expected. For example, the ratio "L / D" can be 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. Alternatively, the ratio "L / D" can be less than 60%, less than 54%, less than 52%, less than 48%, less than 42%, less than 36%, less than 31%, less than 30%, less than 24%, less than 18%, less than 12%, or less than 6%. For example, when the ratio "L / D" is 6% or more, a reduction in initial resistance and improved cycling characteristics can be expected. If the ratio "L / D" is too large, particle cracks initiating from the crack may occur. When the ratio "L / D" is less than 52%, improved cycling characteristics can be expected.

[0041] The particle diameter "D" can 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" can also be, for example, less than 15 μm, less than 10 μm, less than 9 μm, less than 8 μm, less than 7 μm, less than 6 μm, less than 5 μm, less than 4 μm, less than 3 μm, less than 2 μm, or less than 1 μm.

[0042] Crack group 5 includes parallel portion 4. That is, crack group 5 is composed of multiple cracks 2 forming parallel portion 4. In parallel portion 4, two or more cracks 2 do not intersect. Two or more cracks 2 extend in parallel. When the angle formed by the directions of travel of two or more cracks 2, starting from the particle surface side, is less than 10°, this portion is considered parallel portion 4. In parallel portion 4, the angle formed by two or more cracks 2 can be, for example, less than 5°, less than 3°, or less than 1°. Parallel portion 4 may also extend along the entire length of crack 2. Parallel portion 4 may also be a part of the entire length of crack 2. The ratio of the length of parallel portion 4 to the crack length "L" can be, for example, greater than 0%, more than 5%, more than 10%, more than 20%, more than 30%, more than 40%, more than 50%, more than 60%, more than 70%, more than 80%, or more than 90%. The length of the parallel portion 4 relative to the crack length "L" can be, for example, less than 100%, less than 90%, less than 80%, less than 70%, less than 60%, less than 50%, less than 40%, less than 30%, less than 20%, less than 10%, or less than 5%.

[0043] The width of crack 2 refers to the dimension in a direction orthogonal to the length direction. The width of crack 2 can be, for example, less than one-hundredth or one-tenth of the crack length "L". The width of crack 2 can be, for example, less than 10 nm. The width of crack 2 can be, for example, less than 9 nm, less than 8 nm, less than 7 nm, less than 6 nm, less than 5 nm, less than 4 nm, less than 3 nm, less than 2 nm, or less than 1 nm. The width of crack 2 can be, for example, greater than 0.1 nm, greater than 1 nm, greater than 2 nm, greater than 3 nm, greater than 4 nm, greater than 5 nm, greater than 6 nm, greater than 7 nm, greater than 8 nm, or greater than 9 nm.

[0044] The spacing between cracks 2 in the width direction can be constant or variable. For example, the spacing between cracks 2 can be less than one-half, one-third, one-quarter, or one-fifth of the crack length "L". For example, the spacing between cracks 2 can be more than one-hundredth or more than one-tenth of the crack length "L".

[0045] The positive electrode active material may, for example, comprise a lithium transition metal complex oxide. This active material may have a crystal structure belonging to space group R-3m. This crystal structure is also known as a layered structure. The crystal structure can be determined using XRD (X-ray diffraction) patterns. Lithium transition metal complex oxides comprise Li, a transition metal, and oxygen.

[0046] Positive electrode active materials, for example, can have the general formula "Li x Ni a Co b Mn c O y The composition is indicated by "". In the general formula, the Li composition ratio "x" can, for example, satisfy the relationship "0.1≤x≤1.5". The Li composition ratio "x" can, for example, be 0.4 or higher, 0.6 or higher, 0.8 or higher, 1.0 or higher, 1.2 or higher, or 1.4 or higher. The Li composition ratio "x" can, for example, be less than 1.4 or less than 1.2.

[0047] In the general formula, the ratio "y" of component O can, for example, satisfy the relationship "1.5≤y≤2.1". The ratio "y" of component O can, for example, be 1.6 or higher, 1.7 or higher, 1.8 or higher, 1.9 or higher, or 2.0 or higher. The ratio "y" of component O can, for example, be 2.0 or lower, 1.9 or lower, 1.8 or lower, 1.7 or lower, or 1.6 or lower.

[0048] In the general formula, the Ni composition ratio "a", the Co composition ratio "b", and the Mn composition ratio "c" can satisfy the relationship "a + b + c = 1.0". The Ni composition ratio "a" can, for example, satisfy the relationship "0.5 ≤ a ≤ 1.0". The Ni composition ratio "a" can, for example, be 0.6 or higher, 0.7 or higher, 0.8 or higher, or 0.9 or higher. The Ni composition ratio "a" can, for example, be less than 0.9, less than 0.8, less than 0.7, or less than 0.6.

[0049] In the general formula, the Co composition ratio "b" can, for example, satisfy the relationship "0 ≤ b ≤ 0.3". The Co composition ratio "b" can, for example, be 0.01 or higher, 0.02 or higher, 0.03 or higher, 0.04 or higher, 0.05 or higher, 0.06 or higher, 0.07 or higher, 0.08 or higher, 0.09 or higher, 0.10 or higher, 0.15 or higher, 0.20 or higher, or 0.25 or higher. The Co composition ratio "b" can, for example, be less than 0.25, less than 0.20, less than 0.15, less than 0.10, less than 0.09, less than 0.08, less than 0.07, less than 0.06, less than 0.05, less than 0.04, less than 0.03, less than 0.02, or less than 0.01.

[0050] In the general formula, the composition ratio "c" of Mn can, for example, satisfy the relationship "0 ≤ c ≤ 0.3". The composition ratio "c" of Mn can, for example, be 0.01 or higher, 0.02 or higher, 0.03 or higher, 0.04 or higher, 0.05 or higher, 0.06 or higher, 0.07 or higher, 0.08 or higher, 0.09 or higher, 0.10 or higher, 0.15 or higher, 0.20 or higher, or 0.25 or higher. The composition ratio "c" of Mn can, for example, be less than 0.25, less than 0.20, less than 0.15, less than 0.10, less than 0.09, less than 0.08, less than 0.07, less than 0.06, less than 0.05, less than 0.04, less than 0.03, less than 0.02, or less than 0.01.

[0051] Furthermore, in the stated general formula, all or part of Mn can be replaced by Al, etc. That is, lithium transition metal composite oxides can, for example, have the general formula "Li x Ni a Co b Al c O y The composition is indicated by "c". The range of the Al composition ratio "c" is the same as that of the Mn composition ratio "c".

[0052] Any dopant may be added to the lithium transition metal composite oxide. The dopant refers to an element other than Li, Ni, Co, Mn, and O. For example, the dopant may contain at least one element selected from Zr, Mo, W, Mg, Ca, Na, Fe, Cr, Zn, Si, Sn, Al, and Ag. The dopant composition ratio may, for example, be 0.005 or more, 0.01 or more, 0.02 or more, 0.03 or more, or 0.04 or more. Alternatively, the dopant composition ratio may, for example, be less than 0.05, less than 0.04, less than 0.03, less than 0.02, or less than 0.01.

[0053] Sample preparation

[0054] Figure 2 This is a table showing the experimental results. The positive electrode active materials No. 1 to No. 5 were manufactured through the following steps.

[0055] Preparation of precursors

[0056] A feedstock solution is formed by dissolving NiSO4, CoSO4, and MnSO4 in ion-exchanged water. In the feedstock solution, the molar ratio of Ni, Co, and Mn is "Ni / Co / Mn = 90 / 5 / 5". The solute concentration in the feedstock solution is 30% (mass fraction).

[0057] Ammonia solution is added to the reaction vessel. While stirring the ammonia solution with a stirrer, nitrogen is used to displace the contents of the reaction vessel. Then, NaOH is added to the reaction vessel to form an alkaline reaction solution.

[0058] The raw material solution and ammonia water are added dropwise to the reaction solution while maintaining a specific pH range, forming a precipitate (metal hydroxide). The metal hydroxide is recovered by filtering the reaction solution. The metal hydroxide is then dispersed in ion-exchange water to form a dispersion. The dispersion is thoroughly stirred using a spatula. The metal hydroxide is then washed with water. After washing, the dispersion is filtered to recover the metal hydroxide. Finally, the metal hydroxide is dried at 120°C for 16 hours to form a dried product.

[0059] Li Yuan's addition

[0060] In a mortar, a mixture is formed by mixing a dried substance (metal hydroxide) and a lithium compound (LiOH, Li₂CO₃) using a mortar and pestle. By adding Li in excess relative to the total mass of the transition metals, a molten salt is formed during calcination, and the lithium transition metal composite oxide can be single-crystalled. That is, the number fraction of single crystal particles can be 50% or more. The ratio of the mass of Li to the total mass of the transition metals is, for example, 1.5 or more.

[0061] Firing

[0062] Lithium transition metal composite oxides are synthesized by calcining (heat treatment) a mixture in a calcining furnace (e.g., a muffle furnace). The calcining atmosphere is an oxygen atmosphere. Figure 3 This is the first temperature profile. In No.1, firing is carried out using the first temperature profile. The furnace temperature is raised until it reaches a firing temperature in the range of 700°C to 1100°C. The firing temperature is maintained for approximately 10 hours. After 10 hours, the furnace temperature is cooled to room temperature by natural cooling.

[0063] Figure 4 This is the second temperature profile. In samples No. 2 to No. 5, firing was carried out using the second temperature profile. The furnace temperature was raised until it reached a firing temperature within the range of 700°C to 1100°C. The firing temperature was maintained for approximately 10 hours. After 10 hours, nitrogen was supplied to the furnace, thereby cooling the furnace. The nitrogen supply rate in each sample was... Figure 2 The "Nitrogen supply rate during cooling" item is shown.

[0064] After firing, the particle size of the lithium transition metal composite oxide is adjusted using a pulverizer such as a jet mill. This process produces the positive electrode active material.

[0065] Determination of initial resistance

[0066] The initial resistance is measured using the following steps. Laminated cells are then fabricated. The structure of the laminated cell is described below.

[0067] Outer casing: A soft package made of Al laminated film

[0068] Power generation element: Stacked type (single layer)

[0069] Positive electrode: Positive electrode active material / conductive material / binder = 88 / 10 / 2 (mass ratio)

[0070] Negative electrode: Negative electrode active material (natural graphite), CMC, SBR

[0071] Electrolytes: LiPF6 (1 mol / L), EC / DMC / EMC = 3 / 4 / 3 (volume ratio)

[0072] The positive and negative electrodes are manufactured by coating a slurry onto the surface of a substrate (metal foil). For example, a film applicator (with film thickness adjustment function) manufactured by All Good is used as the coating device. After coating, the film is dried at 80°C for 5 minutes.

[0073] The laminated unit is clamped between two stainless steel plates, thereby applying a specified pressure to the power generation element. The state of charge (SOC) of the laminated unit is adjusted to 50%. The IV resistance is measured at a temperature of -10°C. Figure 2The value shown in the "Initial Resistance" item is a relative value with the initial resistance of No.1 set to 100%.

[0074] Determination of Cyclic Characteristics

[0075] In the laminated unit, a cyclic test was performed under the following conditions.

[0076] Ambient temperature: 60℃

[0077] Number of cycles: 100

[0078] Current ratio: 0.3C

[0079] Voltage range: 4.25V to 2.5V

[0080] The rated capacity of the cell was measured over 1 hour at a current rate of 1C. 0.3C is 0.3 times 1C. Discharge capacity was measured at a current rate of 0.2C in the 1st, 25th, 50th, 75th, and 100th cycles. Capacity retention was calculated by dividing the discharge capacity of the 100th cycle by the discharge capacity of the 1st cycle. A higher capacity retention is considered to indicate better cycle characteristics.

[0081] Experimental results

[0082] like Figure 2 As shown, when the relationship “3%≤L / D” is satisfied, a tendency to decrease the initial resistance is observed.

[0083] When the number of cracks is 4 or more but less than 8, a tendency is observed to decrease the initial resistance and thus improve the cycling characteristics.

[0084] When the relationship “6%≤L / D≤52%” is satisfied, a tendency is observed to decrease the initial resistance, thereby improving the cycling characteristics.

Claims

1. A positive electrode active material for lithium-ion secondary batteries, Contains single crystal particles, Crack clusters are formed in the cross-section of the single crystal particle. The crack cluster contains two or more cracks. The crack cluster comprises two or more parallel extending portions of the cracks. And it satisfies the relationship 3% ≤ L / D. L represents the length of the crack, and D represents the diameter of the smallest circumcircle of the single crystal particle in the cross section.

2. The positive electrode active material for lithium-ion secondary batteries according to claim 1, The crack cluster comprises 4 or more but no more than 8 cracks.

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

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

5. The positive electrode active material for lithium-ion secondary batteries according to claim 1 or 2, It has a composition represented by the following general formula: Li x Ni a Co b Mr c O y In the general formula, x, a, b, c, and y satisfy the 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.

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