Positive electrode active material for lithium-ion secondary batteries and method for manufacturing the same

Abstract

Improved cycle characteristics. [Solution] The positive electrode active material for lithium-ion secondary batteries contains single crystal particles. The single crystal particles contain a lithium transition metal composite oxide. The lithium transition metal composite oxide mainly contains a crystal structure belonging to space group R-3m. In the crystal structure, the 3a site contains lithium ions, and the 3b site contains at least nickel ions. Cation mixing occurs between the lithium ions at the 3a site and the nickel ions at the 3b site. The cation mixing rate, which is the occupancy rate of nickel ions at the 3a site, is 2.5% to 7.3%.

Description

[Technical Field]

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

[0002] Japanese Patent Publication No. 2023-104990 discloses nickel-based lithium metal composite oxides. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2023-104990 [Overview of the project] [Problems that the invention aims to solve]

[0004] A lithium transition metal composite oxide with a layered structure has been proposed. The layered structure is formed by the alternating stacking of 3a sites and 3b sites in the c-axis direction. 6c sites exist between the 3a and 3b sites. The 3a sites contain lithium (Li) ions. The 3a sites are also referred to as Li layers. The 3b sites contain nickel (Ni) ions. The 3b sites are also referred to as transition metal (TM) layers. The 6c sites contain oxygen (O) ions. During charging, Li ions are extracted from the 3a sites. This extraction of Li ions from the 3a sites can cause the crystal structure to contract in the c-axis direction. For example, in the high-voltage region of 4.1V or higher, significant contraction of the crystal structure can cause cracks to form on the surface of the single-crystal grains. It is thought that the reaction between the newly formed surfaces created by the cracks and the electrolyte leads to capacity degradation. As a result, the desired cycle characteristics may not be obtained.

[0005] The purpose of this disclosure is to improve cycle characteristics. [Means for solving the problem]

[0006] 1. One aspect of this disclosure is a positive electrode active material for lithium-ion secondary batteries. The positive electrode active material includes single crystal particles. The single crystal particles include a lithium transition metal composite oxide. The lithium transition metal composite oxide mainly contains a crystal structure belonging to space group R-3m. In the crystal structure, the 3a sites contain lithium ions, and the 3b sites contain at least nickel ions. Cation mixing occurs between the lithium ions at the 3a sites and the nickel ions at the 3b sites. The cation mixing rate, which is the occupancy rate of nickel ions at the 3a sites, is between 2.5% and 7.3%.

[0007] Some Li ions at the 3a site may be exchanged for some Ni ions at the 3b site. This phenomenon is also called "cation mixing (CM)". The "CM rate" is the occupancy rate of Ni ions at the 3a site. By introducing a specific amount of Ni ions to the 3a site in advance, even before the first charge in the battery, it is expected that the contraction of the crystal structure in the high-voltage region of 4.1V or higher will be mitigated. By stabilizing the crystal structure in the high-voltage region, an improvement in cycle characteristics is expected. However, if there are too many Ni ions, the cycle characteristics may actually deteriorate. Therefore, the cation mixing rate is between 2.5% and 7.3%. Hereinafter, "positive electrode active material for lithium-ion secondary batteries" may be abbreviated as "positive electrode active material". "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 components: The cation mixing rate is 6.0% or less.

[0009] An improvement in cycle characteristics can be expected if the cycle rate is between 2.5% and 6.0%.

[0010] 3. The positive electrode active material for lithium-ion secondary batteries described in "1" or "2" above may include, for example, the following components: Lithium transition metal composite oxide having the general formula "Li x Nia Co b Mn c O y It has a composition represented by the formula. 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".

[0011] The 3b site may further contain other elements in addition to Ni. For example, the 3b site may further contain Co and Mn. When the Ni composition ratio "a" is 0.5 or more, an increase in the initial discharge capacity is expected, for example.

[0012] 4 The positive electrode active material for a lithium ion secondary battery according to any one of the above "1" to "3" may include, for example, the following configuration. The positive electrode active material for a lithium ion secondary battery has a powder form. The powder consists of single crystal particles with a volume fraction of 70% or more and the balance of polycrystalline particles. The single crystal particles contain 1 to 10 primary particles. The polycrystalline particles contain more than 10 primary particles.

[0013] The single crystal particles have fewer grain boundaries between primary particles compared to the polycrystalline particles. It is expected that the fewer the grain boundaries, the less likely cracks will occur during charge and discharge. When the volume fraction of the single crystal particles is 70% or more, an improvement in cycle characteristics is expected.

[0014] ​​​​​​​​​​​During heat treatment (firing), the flux material不含Li is expected to extract Li ions from the 3a site. CM may occur due to the transfer of Ni ions at the 3b site to the vacancies generated at the 3a site. Therefore, for example, it is expected that the CM rate can be adjusted to a desired range depending on the blending amount of the flux material.

[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 initially planned.

Brief Description of Drawings

[0017] [Figure 1] It is a conceptual diagram of a layered structure. [Figure 2] It is a schematic flowchart of the method for manufacturing a positive electrode active material in this embodiment. [Figure 3] It is a table showing experimental results.

Modes for Carrying Out the Invention

[0018] Terms, Phrases "Equipped with," "includes," "possesses," and variations thereof are open-ended expressions. Configurations expressed in an open-ended manner may or may not include additional elements in addition to the essential elements. The statement "consists of" is a closed expression. However, even configurations expressed in a closed manner may include additional elements that are usually incidental impurities or irrelevant to the subject technology. The statement "substantially consists of..." is a semi-closed expression. In configurations expressed in a semi-closed manner, the addition of elements that do not substantially affect the basic and novel characteristics of the subject technology is permitted.

[0019] Numerical values ​​may be displayed with significant figures. Unless otherwise specified, measured values ​​may be the average of multiple measurements. The number of measurements may be three or more, five or more, or ten or more. Generally, the reliability of the average is expected to improve with a larger number of measurements. Measured values ​​may be rounded to the nearest significant figure. Measured values ​​may include errors, such as those associated with the detection limits of the measuring device.

[0020] 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.

[0021] The "CM ratio" is determined by Rietveld analysis of the XRD (X-Ray Diffraction) pattern. First, the XRD pattern is obtained by powder XRD measurement of the cathode active material. The XRD measurement may be performed, for example, at beamline "BL5S2" of the "Aichi Synchrotron Radiation Center". For the measurement, the sample (cathode active material) is sealed in a glass capillary (inner diameter: 0.5 mm). The irradiation energy is 17 keV. The measurement time is 10 minutes.

[0022] Next, the XRD pattern is analyzed using the analysis software "GSAS-II". Refinement is performed using the R-3m space group phase. As an example, if the positive electrode active material is LiNi 0.90 Co 0.05 Mn 0.05 The procedure for the O2 composition is described. At site 3a, the Li occupancy is expressed as "1-x", and the Ni occupancy is expressed as "x". At site 3b, the Ni occupancy is expressed as "0.90-x", the Co occupancy is expressed as "0.05", the Mn occupancy is expressed as "0.05", and the Li occupancy is expressed as "x". By refining "x", the Ni occupancy "x" at site 3a can be determined. The percentage expression of the occupancy "x" is the CM ratio.

[0023] The crystal structure is determined by the XRD pattern. If the XRD pattern obtained above contains the following characteristics, the sample (cathode active material) is considered to contain a crystal structure belonging to space group R-3m as its main component. The horizontal axis (2θ) of the XRD pattern is converted to the energy value of CuKα (8042.55 eV). In the converted XRD pattern, there is a peak showing maximum intensity in the range of 18° to 20°. There is a peak showing second-highest intensity in the range of 44° to 45°. There is a split peak in the range of 63.5° to 65.5°.

[0024] The stoichiometric composition formula shows a representative example of a compound. The compound may have a non-stoichiometric composition. For example, "Al2O3" is not limited to compounds with a molar ratio of "Al / O = 2 / 3". Unless otherwise specified, "Al2O3" refers to a compound containing Al and O in any molar ratio. For example, the compound may be doped with trace elements. Some of the Al and O may be substituted with other elements.

[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] A "primary particle" refers to a solid particle that is the smallest unit of particle and has a boundary between particles that is recognized as being unable to be divided any further. Primary particles appear to have no grain boundaries in SEM (Scanning Electron Microscope) images. The number of primary particles contained in single-crystal or polycrystalline particles is counted in the SEM image of the powder. The image magnification is, for example, 10,000x. A single primary particle or an aggregate of 2 to 10 primary particles is considered a single-crystal particle. An aggregate of more than 10 primary particles is considered a polycrystalline particle. In an SEM image, the percentage of single-crystal particles can be determined from 100 randomly selected particles.

[0027] "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."

[0028] positive electrode active material The positive electrode active material is for a battery. The battery may be a liquid-based battery or an all-solid-state battery. The battery may have any structure. The battery may have, for example, a wound or stacked power generation element. The battery may have, for example, a unipolar or bipolar structure.

[0029] The positive electrode active material may be in powder form. The D50 of the positive electrode active material 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 be, for example, 30 μm or less, 20 μm or less, or 10 μm or less.

[0030] 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 (powder) 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. An improvement in cycle characteristics can be expected when the percentage of single-crystal particles is 70% or more. The percentage of single-crystal particles may be, for example, 100% or less, 90% or less, or 80% or less.

[0031] It is expected that the fewer the number of primary particles constituting a single crystal particle, the less likely cracks are to occur. The number of primary particles constituting a single crystal particle may be, for example, 9 or less, 8 or less, 7 or less, 6 or less, 5 or less, 4 or less, 3 or less, or 2 or less. The number of primary particles constituting a polycrystalline particle may be, for example, 15 or more, 20 or more, 25 or more, or 50 or more. The number of primary particles constituting a polycrystalline particle may be, for example, 100 or less, 50 or less, 25 or less, or 20 or less. For example, if a single crystal particle is redefined to contain 5 or fewer primary particles, the above particle count percentages shall represent the particle count percentage of the single crystal particle after the redefinition.

[0032] The powder may be a monodisperse system. Since the powder is mainly composed of single crystal particles and is a monodisperse system, an improvement in cycle characteristics is expected. The powder may have, for example, a span of 1 or less. "Span" indicates a value obtained by the calculation formula "(D90 - D10) / D50". The smaller the span, the sharper the particle size distribution is considered to be. The span of the powder 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 span of the powder may be, for example, 0.1 or more, 0.2 or more, 0.3 or more, 0.4 or more, or 0.5 or more.

[0033] The single crystal particles contain a lithium transition metal composite oxide. The lithium transition metal composite oxide represents a compound containing Li, TM, and O. The lithium transition metal composite oxide mainly contains a crystal structure belonging to the space group R-3m. The lithium transition metal composite oxide may substantially consist of a crystal structure belonging to the space group R-3m. The crystal structure belonging to the space group R-3m is also referred to as a "layered structure".

[0034] FIG. 1 is a conceptual diagram of a layered structure. The layered structure 10 includes a 3a site 11 and a 3b site 12. There is a 6c site (not shown) between the 3a site 11 and the 3b site 12. In the c-axis direction, the 3a site | 11 and the 3b site 12 are alternately laminated. The 3a site 11 is also referred to as a Li layer. Li ions (Li + ) are stored in it. The 3b site 12 is also referred to as a TM layer. Transition metal ions (TM n+ ) are stored in the 3b site 12. The 3b site 12 contains at least Ni ions (Ni 3+ ). While some Ni ions transfer to the 3a site, some Li ions transfer to the 3b site. That is, cation exchange (cation mixing) occurs.

[0035] The CM ratio is between 2.5% and 7.3%. Improvement in cycle characteristics is expected within this range of CM ratios. The CM ratio may be, for example, 3.0% or more, 3.5% or more, 4.0% or more, 4.5% or more, 5.0% or more, 5.5% or more, 6.0% or more, 6.5% or more, or 7.0% or more. The CM ratio may be, for example, 7.0% or less, 6.5% or less, 6.0% or less, 5.5% or less, 5.0% or less, 4.5% or less, 4.0% or less, 3.5% or less, or 3.0% or less. The CM ratio may be, for example, between 2.5% and 6.0%. The CM ratio may be, for example, between 3.5% and 6.0%.

[0036] Single-crystal grains may consist of a single crystalline material. That is, the CM ratio may be between 2.5% and 7.3% throughout the entire single-crystal grain. For example, a single-crystal grain may have a gradient composition from the center to the outer edge, or it may have a core-shell structure, resulting in different CM ratios depending on the location within the single-crystal grain. In this configuration, localized cracks may occur in areas where the CM ratio is outside the range of 2.5% to 7.3%. Compared to this configuration, a single-crystal grain consisting of a single crystalline material is expected to have superior cycle characteristics.

[0037] The Ni ion that has moved to the 3a site is divalent (Ni 2+ ) may also be acceptable. + The ionic radius of is 0.76 Å. Trivalent Ni ion (Ni 3+ The ionic radius of the divalent Ni ion (Ni) is 0.56 Å to 0.60 Å. 2+ The ionic radius of ) is 0.69 Å. The divalent Ni ion is Li + Because its ionic radius is close to that of the ion, the divalent Ni ion is considered stable within the 3a site.

[0038] Lithium transition metal composite oxides are, for example, those with the general formula "Li x Ni a Co b Mn c O yIt may have a composition represented by the formula ". In the general formula, the Li composition ratio "x" may satisfy, for example, the relationship "0.1 ≤ x ≤ 1.5". The Li composition ratio "x" may be, for example, 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 be, for example, 1.4 or less, or 1.2 or less.

[0039] In the general formula above, the O composition ratio "y" may satisfy, for example, the relationship "1.5 ≤ y ≤ 2.1". The O composition ratio "y" may be, for example, 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 be, for example, 2.0 or less, 1.9 or less, 1.8 or less, 1.7 or less, or 1.6 or less.

[0040] In the above general formula, the Ni composition ratio "a", Co composition ratio "b", and Mn composition ratio "c" may satisfy the relationship "a + b + c = 1.0". The Ni composition ratio "a" may, for example, satisfy the relationship "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, or 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] Method for manufacturing positive electrode active material Figure 2 is a schematic flowchart of the method for producing the positive electrode active material in this embodiment. Hereinafter, "the method for producing the positive electrode active material in this embodiment" may be abbreviated as "this method". This method includes "(a) preparation of the precursor", "(b) mixing", and "(c) heat treatment". Heat treatment is also called calcination. This method may further include, for example, washing with water, crushing, etc.

[0046] (a) Preparation of precursors This method includes preparing a precursor. The precursor can be prepared by any method. For example, the precursor may be synthesized by coprecipitation, hydrothermal synthesis, etc. The precursor contains a TM compound. The TM compound contains at least Ni. The TM compound may also contain a TM hydroxide.

[0047] (b) Mixture This method involves preparing a mixture by mixing a precursor, a lithium compound, and a flux material. The mixing method is arbitrary. For example, an agate mortar and pestle, a lab mill, etc., may be used. The lithium compound is a Li source. The lithium compound may include, for example, LiOH, Li2CO3, etc.

[0048] The flux material has a melting point lower than the firing temperature. The flux material can promote the single crystallization of lithium transition metal composite oxides by forming molten salts during firing. Specifically, it is expected that the single-crystal particle ratio in the powder will exceed 70%.

[0049] Flux materials may react with Li during firing. Compounds formed by the reaction between the flux material and Li may be water-soluble. Water-soluble compounds can be easily removed by washing with water. For example, water-soluble compounds may dissolve in 100 mL of water at 20°C in amounts of 1 g or more. The amount that dissolves in 100 mL of water at 20°C (saturation amount) is also called "solubility." For example, if a water-soluble compound dissolves in 100 mL of water at 20°C in amounts of 1 g or more, the solubility of the water-soluble compound in water is 1 g / 100 mL (20°C).

[0050] The flux material does not contain Li. + The extraction of the Li can be promoted. The flux material may contain, for example, B2O3. During firing, B2O3 can react with Li2O to produce Li3BO3. This reaction can lead to the extraction of Li from the 3a site. + It is thought that abstraction occurs. It is thought that cation mixing occurs when Ni ions move into the vacant space created at the 3a site. The CM ratio can be adjusted to a desired range by the amount of flux material used. The ratio of the amount of flux material to the total amount of TM contained in the TM compound, "Flux / TM", may be, for example, 0.04 or more, 0.06 or more, 0.08 or more, 0.10 or more, or 0.12 or more. The ratio "Flux / TM" may also be, for example, 0.18 or less, 0.16 or less, 0.14 or less, 0.12 or less, 0.10 or less, or 0.08 or less.

[0051] The solubility of Li3BO3 in water may be 2.57 g / 100 mL (20°C). Li3BO3 can be removed from the system by washing with water. Therefore, after washing with water, components derived from the flux material (B2O3) may not be detected in the compositional analysis of the positive electrode active material.

[0052] (c) Heat treatment This method involves producing a positive electrode active material by heat-treating a mixture. For example, the heat treatment may be carried out in a calcination furnace. For example, a muffle furnace or the like may be used. The calcination atmosphere may be, for example, an oxygen atmosphere. The calcination temperature may be, for example, 650°C to 1100°C. The calcination time may be, for example, 5 hours to 15 hours.

[0053] This method may include washing the positive electrode active material with water. As mentioned above, if the reaction product between the flux material and Li is water-soluble, the reaction product can be discharged from the system by washing the positive electrode active material with pure water.

[0054] This method may include crushing the positive electrode active material. For example, after washing with water, the positive electrode active material is dried. After drying, the positive electrode active material can be crushed to achieve a predetermined particle size distribution. The crushing method is arbitrary. For example, crushing may be carried out using an agate mortar, a lab mill, or the like. [Examples]

[0055] Sample preparation Figure 3 is a table showing the experimental results. Samples No. 1 to No. 7 were prepared according to the following procedure.

[0056] (a) Preparation of precursors The raw material solution is prepared by dissolving NiSO4, CoSO4, and MnSO4 in deionized water. The mixing ratio of NiSO4, CoSO4, and MnSO4 is adjusted to "Ni / Co / Mn = 90 / 5 / 5 (mole ratio)". The mole concentration of the raw material solution is 0.2%.

[0057] A predetermined amount of NH3 aqueous solution is placed in the reaction vessel. The reaction vessel is stirred with a stirrer while being purged with nitrogen. NaOH is added to the reaction vessel to adjust the pH of the aqueous solution to alkaline. The raw material solution is added dropwise while the temperature and pH of the aqueous solution are adjusted to maintain a constant range (effectively constant value), forming a precipitate of TM hydroxide. The precipitate is dehydrated and calcined. The calcination temperature is 120°C to 220°C. The calcination time is 4 to 10 hours. The calcination pressure is 0.2 MPa to 1.0 MPa.

[0058] After calcination, the precipitate is washed with water. The residue (TM hydroxide) is recovered by filtration. The TM hydroxide is dried at 110°C for 12 hours to remove the water. Thus, the precursor (TM hydroxide) is prepared.

[0059] (b) Mixture The mixture is prepared by mixing the precursor (TM hydroxide), lithium compound (LiOH), and flux material (B2O3). The ratio of the amount of Li to the total amount of TM, "Li / TM," is 1. The ratio of the amount of B2O3 to the total amount of TM, "Flux / TM," is shown in "Flux / TM" in Figure 2. In No. 6, no flux material is used.

[0060] (c) Heat treatment In a firing furnace, the mixture is subjected to heat treatment (main firing) to produce the positive electrode active material. The firing temperature is between 650°C and 1100°C. The firing time is between 5 and 15 hours.

[0061] After firing, the positive electrode active material is crushed in an agate mortar until the particle size is 0.2 mm or less. The positive electrode active material is dispersed in 500 mL of pure water to form a slurry. The slurry is vigorously stirred for 1 minute. The slurry is filtered using filter paper and a Buchner funnel. The residue is rinsed with 500 mL of pure water to form a cake. The cake is vacuum-dried at 90°C. After drying, the cake is crushed in an agate mortar to adjust it to the desired particle size. Thus, the positive electrode active material is manufactured. The CM ratio is measured for each sample by Rietveld analysis.

[0062] evaluation A laminated cell is prepared. A laminated cell is a cell in which the power generation elements are housed in a pouch made of aluminum laminate film. The configuration of the power generation elements is as follows: Positive electrode: Positive electrode active material, conductive material (acetylene black) Negative electrode: Negative electrode active material (natural graphite) Electrolytes: LiPF6 (1 ml / L), EC / DMC / EMC = 3 / 4 / 3 (volume ratio)

[0063] 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.

[0064] 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

[0065] At a current rate of 1C, the rated capacity of the cell is drawn in one hour. 0.3C is 0.3 times 1C. The capacity retention rate can be calculated by dividing the discharge capacity at 100 cycles by the discharge capacity at 1 cycle. A higher capacity retention rate indicates better cycle characteristics. In Figure 3, "<50" indicates a capacity retention rate of less than 50%.

[0066] Experimental results In Figure 3, when the CM ratio is between 2.5% and 7.3%, there is a tendency for the cycle characteristics to improve.

[0067] When the cycle rate is between 3.5% and 6.0%, there is a tendency for cycle characteristics to improve. [Explanation of symbols]

[0068] 10 Layered structure, 11 Site 3a, 12 Site 3b.

Claims

1. Contains single crystal particles, The single crystal particles contain a lithium transition metal composite oxide. The aforementioned lithium transition metal composite oxide mainly contains a crystal structure belonging to space group R-3m, In the aforementioned crystal structure, Site 3a contains lithium ions, Site 3b contains at least nickel ions, Cationic mixing occurs between the lithium ion at site 3a and the nickel ion at site 3b, and The cation mixing rate, which is the occupancy rate of nickel ions at site 3a, is 2.5% or more and 7.3% or less. Positive electrode active material for lithium-ion secondary batteries.

2. The cation mixing rate is 6.0% or less. The positive electrode active material for a lithium-ion secondary battery according to claim 1.

3. The lithium transition metal composite oxide has the general formula: Li x Ni a Co b Mn c O y Having 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.

4. Having the form of a powder, The aforementioned powder consists of single-crystal particles making up 70% or more of the total particle size, and the remainder being polycrystalline particles. The single crystal grain comprises one to ten primary particles, and The aforementioned polycrystalline particles include more than 10 primary particles. The positive electrode active material for a lithium-ion secondary battery according to claim 1 or claim 2.

5. (a) Prepare the precursor, (b) Preparing a mixture by mixing the precursor, lithium compound, and flux material, (c) A positive electrode active material is produced by subjecting the mixture to heat treatment. Includes, The precursor comprises a transition metal compound, The transition metal compound contains at least nickel, The flux material does not contain lithium. A method for producing positive electrode active material for lithium-ion secondary batteries.

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