Positive electrode active material for lithium-ion secondary batteries

Abstract

Improved cycle characteristics. [Solution] A positive electrode active material for a lithium-ion secondary battery, comprising a lithium transition metal composite oxide, wherein the lithium transition metal composite oxide mainly comprises a crystal structure belonging to space group R-3m, satisfies the relationship "a ≤ 2.876 Å", and a represents the lattice constant in the a-axis direction of the lithium transition metal composite oxide.

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. 2023-516229 (Patent Document 1) discloses a high-nickel positive electrode active material in which the particle size distribution of the positive electrode active material satisfies the relationship "1.5 ≤ (D95 - D5) / D50 ≤ 2.5". [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Special Publication No. 2023-516229 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] There is a demand for lithium-ion secondary batteries with high energy density. Conventionally, it has been proposed to improve energy density by adjusting the particle size distribution of the positive electrode active material, thereby increasing the packing efficiency of the positive electrode active material.

[0005] On the other hand, in high-nickel positive electrode active materials such as those described in Patent Document 1, shrinkage in the c-axis direction occurs when nickel is extracted from between layers. This may reduce the capacity retention rate of lithium-ion secondary batteries containing such positive electrode active materials.

[0006] The purpose of this disclosure is to improve cycle characteristics. [Means for solving the problem]

[0007] [1] Contains lithium transition metal composite oxide, The aforementioned lithium transition metal composite oxide mainly contains a crystal structure belonging to space group R-3m, a ≤ 2.876 Å satisfies the relationship of where a represents the lattice constant in the a-axis direction of the lithium transition metal composite oxide, and it is a cathode active material for a lithium-ion secondary battery.

[0008] Although the detailed mechanism is unclear, it has been found that when the lattice constant "a" in the a-axis direction of the lithium transition metal composite oxide is 2.876 Å or less, an effect of improving the cycle characteristics can be obtained. Hereinafter, "cathode active material for lithium-ion secondary battery" may be abbreviated as "cathode active material". "Lithium-ion secondary battery" may be abbreviated as "battery".

[0009] [2] 4.937 < c / a < 4.952 satisfies the relationship of where c represents the lattice constant in the c-axis direction of the lithium transition metal composite oxide, and it is the cathode active material for a lithium-ion secondary battery according to [1].

[0010] By satisfying the relationship of "4.937 < c / a < 4.952", further improvement in cycle characteristics is expected.

[0011] [3] 2.872 Å ≤ a The cathode active material for a lithium-ion secondary battery according to [1] or [2], which satisfies the relationship of

[0012] [4] The lithium transition metal composite oxide has a general formula: Li x Ni d Co e Mn f O y and has a composition represented by In the above general formula, x, d, e, f, and y satisfy the relationships of 0.1 ≤ x ≤ 1.5, 0.5 ≤ d ≤ 1.0, 0 ≤ e ≤ 0.3, 0 ≤ f ≤ 0.3, d + e + f = 1.0, and 1.5 ≤ y ≤ 2.1, and it is the cathode active material for a lithium-ion secondary battery according to any one of [1] to [3].

[0013] [5] The lithium transition metal composite oxide forms a powder, In the powder, the number ratio of the single crystal particles to the total of the single crystal particles and the polycrystalline particles is 70% or more, the single crystal particles contain 1 to 10 primary particles, and the polycrystalline particles contain more than 10 primary particles. The positive electrode active material for a lithium ion secondary battery according to any one of [1] to [4].

[0014] 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-limiting. 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

[0015] [Figure 1] FIG. 1 is a conceptual diagram of a layered structure. [Figure 2] FIG. 2 is a table showing experimental results.

Modes for Carrying Out the Invention

[0016] "Comprising", "including", "having", and their modifications 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 of "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 of "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 allowed.

[0017] 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.

[0018] 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.

[0019] The crystal structure is determined by Rietveld analysis of the XRD (X-Ray Diffraction) pattern. First, the powder XRD (X-Ray Diffraction) pattern of the cathode active material is obtained. For example, the XRD measurement may be performed 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.

[0020] 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 0.05The procedure for the O2 composition is described. At site 3a (Li layer site), the Li occupancy is expressed as "1-x" and the Ni occupancy is expressed as "x". At site 3b (transition metal (TM) layer site), 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 most likely structure is estimated. The values ​​of the lattice constants in the a-axis direction and the c-axis direction of the obtained phase are denoted as the a-axis length "a" and the c-axis length "c", respectively.

[0021] 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 intensity in the range of 44° to 45°. There is a split peak in the range of 63.5° to 65.5°.

[0022] 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.

[0023] 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.

[0024] A "primary particle" is the smallest unit of a particle, representing a solid particle that constitutes a boundary between particles that cannot be further divided. The number of primary particles in single-crystal or polycrystalline particles is counted in a Scanning Electron Microscope (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.

[0025] "D50" represents the particle size at which the cumulative frequency in the volume-based particle size distribution (cumulative distribution) reaches 50%. D50 can be measured, for example, by laser diffraction.

[0026] <Cathode 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.

[0027] The positive electrode active material contains 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".

[0028] FIG. 1 is a conceptual diagram of a layered structure. The layered structure 10 includes 3a sites 11 and 3b sites 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 stacked. The 3a site 11 is also referred to as a Li layer. Li ions (Li + ) are stored in the 3a site 11. 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 + ).

[0029] In the layered structure, the lattice constant "a" in the a-axis direction is 2.876 Å or less. By the lattice constant "a" in the a-axis direction being 2.876 Å or less, improvement in cycle characteristics is expected. The lattice constant "a" in the a-axis direction may be, for example, 2.875 Å or less, or 2.874 Å or less. The lattice constant "a" in the a-axis direction may be, for example, 2.872 Å or more, or 2.873 Å or more.

[0030] In the layered structure, the lattice constant "c" in the c-axis direction may be 14.195 Å or more, 14.198 Å or more, 14.200 or more, 14.205 Å or more, or 14.209 Å or more. The lattice constant "c" in the c-axis direction may be 14.230 Å or less, 14.225 Å or less, 14.220 Å or less, or 14.215 Å or less.

[0031] In a layered structure, the ratio of the lattice constant "c" in the c-axis direction to the lattice constant "a" in the a-axis direction (hereinafter referred to as the lattice constant ratio "c / a") may be greater than 4.937 and less than 4.952. Further improvement in cycle characteristics can be expected when the lattice constant ratio "c / a" is greater than 4.937 and less than 4.952. The lattice constant ratio "c / a" may be, for example, 4.940 or greater, 4.941 or greater, 4.942 or greater, 4.943 or greater, or 4.944 or greater. The lattice constant ratio "c / a" may be, for example, 4.949 or less, 4.948 or less, or 4.947 or less.

[0032] Lithium transition metal composite oxides are, for example, those with the general formula "Li x Ni d Co e Mn f O y It 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.

[0033] 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.

[0034] In the above general formula, the Ni composition ratio "d", Co composition ratio "e", and Mn composition ratio "f" may satisfy the relationship "d + e + f = 1.0". The Ni composition ratio "d" may, for example, satisfy the relationship "0.5 ≤ d ≤ 1.0". The Ni composition ratio "d" 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 "d" may, for example, be 0.9 or less, 0.8 or less, 0.7 or less, or 0.6 or less.

[0035] In the above general formula, the Co composition ratio "e" may satisfy, for example, the relationship "0 ≤ e ≤ 0.3". The Co composition ratio "e" 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 "e" may be, for example, 0.025 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.

[0036] In the general formula above, the Mn composition ratio "f" may satisfy, for example, the relationship "0 ≤ f ≤ 0.3". The Mn composition ratio "f" 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 "f" may be, for example, 0.025 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.

[0037] 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 d Co e Al f O y It may have a composition represented by ". The range of the Al composition ratio "f" is the same as that of the Mn composition ratio "f" above.

[0038] 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.

[0039] 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.

[0040] Lithium transition metal composite oxides may form single-crystal particles. Lithium transition metal composite oxides may form polycrystalline particles. That is, the positive electrode active material may contain both single-crystal particles and polycrystalline particles. Polycrystalline particles may have substantially the same crystal structure and composition as 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. 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 70% or more. This is thought to be because single-crystal particles have fewer grain boundaries compared to polycrystalline particles, making them less prone to cracking during charging and discharging (volume change).

[0041] The fewer the number of primary particles constituting the single crystal particles, the less likely cracks are to occur. The number of primary particles constituting the single crystal particles 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 the polycrystalline particles may be, for example, 15 or more, 20 or more, 25 or more, or 50 or more. The number of primary particles constituting the polycrystalline particles may be, for example, 100 or less, 50 or less, 25 or less, or 20 or less. For example, when the single crystal particles are redefined to contain 5 or less primary particles, the above number fraction shall indicate the number fraction of the redefined single crystal particles.

[0042] The positive electrode active material (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 a span of, for example, 1 or less. "Span" indicates a value obtained by the calculation formula "(D90 - D10) / D50". The smaller the span, the more likely it is to have a sharp particle size distribution. 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.

Examples

[0043] <Preparation of Samples> Figure 2 is a table showing the experimental results. Samples No. 1 to No. 7 were prepared according to the following procedure.

[0044] <Synthesis of TM Precursor (TM Hydroxide)> A raw material solution is prepared by dissolving NiSO₄, CoSO₄, and MnSO₄ in ion-exchanged water. The mixing ratio (molar ratio) of NiSO₄, CoSO₄, and MnSO₄ is "Ni / Co / Mn = 90 / 5 / 5". The molar concentration of the raw material solution is 0.2%.

[0045] 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.

[0046] 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 moisture. After drying, the TM hydroxide is mixed with a Li compound (LiOH) to form a mixture. The Li compound may be, for example, Li2CO3. The mixing ratio is adjusted so that the ratio of the amount of Li to the total amount of TM, "Li / TM," is the value shown in the "Charging Ratio" of the "Synthesis Conditions" in Figure 2. When Li / TM is greater than 1, it is considered that there is an excess of LiOH relative to the TM hydroxide. An excess of LiOH tends to cause the lithium transition metal composite oxide to become single crystals. That is, it is expected that the number ratio of single crystal particles in the powder will be 70% or more.

[0047] In the firing furnace, the mixture is subjected to final firing to form the fired product. The firing furnace may be, for example, a muffle furnace. The heating rate, final firing temperature, and final firing time are shown in "Heating Rate," "Final Firing Temperature," and "Final Firing Time" in the "Synthesis Conditions" section of Figure 2.

[0048] In an agate mortar, the calcined material is crushed until the particle size is 0.2 mm or less, forming a crushed material. The crushed 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 a predetermined particle size. Instead of an agate mortar, the cake may be crushed using, for example, a lab mill. From the above, a lithium transition metal composite oxide is produced. In Nos. 1 to 7, the composition of the lithium transition metal composite oxide is LiNi 0.90 Co 0.05 Mn 0.05 The material is O2. Rietveld analysis is used to measure the lattice constant "a" in the a-axis direction and the lattice constant "c" in the c-axis direction for each sample. SEM images are used to confirm the proportion of single-crystal particles for each sample. In Figure 2, under "Particle Morphology" for "Positive Electrode Active Material," if the proportion of single-crystal particles is 70% or more, it is indicated as "Single-crystal particles," and if the proportion of polycrystalline particles is 70% or more, it is indicated as "Polycrystalline particles."

[0049] <Rating> 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 (lithium transition metal composite oxide), 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)

[0050] 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.

[0051] In a laminated cell, a cycle test under the following conditions is carried out. Ambient temperature: 60 °C Number of cycles: 50 Current rate: 0.3C Voltage range: from 4.25V to 2.5V

[0052] At a current rate of 1C, the rated capacity of the cell flows in 1 hour. 0.3C is 0.3 times that of 1C. The capacity retention rate is obtained by dividing the discharge capacity at the 50th cycle by the discharge capacity at the 1st cycle.

[0053] <Experimental results> In FIG. 2, when the relationship of "a ≦ 2.876 Å" is satisfied, the cycle characteristics tend to be improved. When the relationship of "4.937 < c / a < 4.952" is satisfied, the cycle characteristics tend to be further improved.

Explanation of symbols

[0054] 10 Layered structure, 11 3a site, 12 3b site.

Claims

1. Contains lithium transition metal composite oxide, The aforementioned lithium transition metal composite oxide mainly contains a crystal structure belonging to space group R-3m, a ≤ 2.876 Å Satisfying the relationship, The above a represents the lattice constant in the a-axis direction of the lithium transition metal composite oxide, and is a positive electrode active material for a lithium-ion secondary battery.

2. 4.937<c / a<4.952 Satisfying the relationship, The positive electrode active material for a lithium-ion secondary battery according to claim 1, wherein c represents the lattice constant in the c-axis direction of the lithium transition metal composite oxide.

3. 2.872 Å ≤ a A positive electrode active material for a lithium-ion secondary battery according to claim 1 or claim 2, satisfying the relationship.

4. The lithium transition metal composite oxide has the general formula: Li x Ni d Co e Mn f O y Having a composition represented by, The positive electrode active material for a lithium-ion secondary battery according to claim 1 or claim 2, wherein x, d, e, f, and y satisfy the following relationships in the general formula: 0.1 ≤ x ≤ 1.5, 0.5 ≤ d ≤ 1.0, 0 ≤ e ≤ 0.3, 0 ≤ f ≤ 0.3, d + e + f = 1.0, and 1.5 ≤ y ≤ 2.

1.

5. The lithium transition metal composite oxide is in the form of a powder. In the aforementioned powder, the ratio of single-crystal particles to the total number of single-crystal particles and polycrystalline particles is 70% or more. The single crystal grain comprises one to ten primary particles, and The positive electrode active material for a lithium-ion secondary battery according to claim 1 or claim 2, wherein the polycrystalline particles include more than 10 primary particles.

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