Cathode active material, and, method for manufacturing cathode active material

Abstract

To reduce a battery resistance.SOLUTION: A cathode active material includes a plurality of secondary particles. Each of the plurality of secondary particles includes 3 to 20 primary particles. A primary particle includes a lithium metal composite oxide. The lithium metal composite oxide has layered rock-salt structure. The primary particle has a particle size distribution of "Dmin of 0.3 μm or more" and "DFWHM of 0.10 μm or more". The particle size distribution is on a number basis. In the particle diameter distribution, the minimum diameter is denoted by Dmin. The DFWHM is the full width at half maximum of the maximum peak of the particle size distribution.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a positive electrode active material and a method for producing the positive electrode active material. [Background technology]

[0002] Japanese Patent Application Laid-Open No. 2019-145204 discloses secondary particles formed by aggregation of primary particles having an average particle size of 1 μm or less. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-145204 Summary of the Invention [Problem to be solved by the invention]

[0004] Secondary particles are aggregates of primary particles. For example, adjusting the size of the primary particles is expected to increase the discharge capacity. However, there is still room for improvement in the battery resistance.

[0005] An object of the present disclosure is to reduce battery resistance. [Means for solving the problem]

[0006] 1. The positive electrode active material includes a plurality of secondary particles. Each of the plurality of secondary particles includes 3 to 20 primary particles. The primary particles include a lithium metal composite oxide. The lithium metal composite oxide has a layered rock salt structure. The particle size distribution of the primary particles is "D 0.3 μm or more." min " and "D of 0.10 μm or more FWHM ". The particle size distribution is based on the number of particles. D min indicates the minimum diameter in the particle size distribution. FWHM indicates the full width at half maximum of the maximum peak of the particle size distribution.

[0007] Conventionally, primary particles have a sharp particle size distribution. In the positive electrode active material of the present disclosure, the primary particles have a broad particle size distribution. That is, D FWHM The particle size is 0.1 μm or larger. Although the details of the mechanism are unknown, it is expected that the broad particle size distribution of the primary particles will reduce battery resistance. For example, the coexistence of primary particles of various sizes may promote the diffusion of lithium (Li) inside the secondary particles.

[0008] 2. The positive electrode active material described in the above "1" may include, for example, the following components: D FWHM is 0.21 μm or more.

[0009] D FWHM When the particle size is 0.21 μm or more, not only is the battery resistance reduced but durability is also expected to improve. A broader particle size distribution can further promote ion diffusion within the secondary particles. On the other hand, the contact area between the surface of the secondary particles and the electrolyte can be reduced. This is thought to result in improved durability.

[0010] 3. The positive electrode active material described in "1" or "2" above may include, for example, the following configuration: FWHM is 0.87 μm or less.

[0011] 4. The positive electrode active material according to any one of the above items 1 to 3 may include, for example, the following structure: The lithium metal composite oxide has a composition represented by the following general formula. Li 1-a MO2 In the formula, the relationship "-0.5≦a≦0.5" is satisfied. M includes at least one selected from the group consisting of Ni, Co, Mn, and Al.

[0012] 5. A method for producing a positive electrode active material includes the following steps (a) to (e): (a) Prepare a metal hydroxide. (b) A mixture is formed by mixing a metal hydroxide and a lithium compound. (c) The mixture is heat-treated in an oxygen atmosphere to synthesize a lithium metal composite oxide. (d) The lithium metal composite oxide is crushed to form secondary particles. (e) The secondary particles are washed with water. The lithium compound has a D50 of 20 μm or more, which indicates the particle size at which the cumulative value reaches 50% in the particle size distribution based on mass.

[0013] Lithium compounds are the Li source for lithium metal composite oxides. Large particle size lithium compounds are used. The secondary particles are then washed with water. While the details of the mechanism are unclear, the combination of these processes tends to broaden the particle size distribution of the primary particles.

[0014] Hereinafter, an embodiment of the present disclosure (hereinafter may be abbreviated as "the present embodiment") and an example of the present disclosure (hereinafter may be abbreviated as "the present example") will be described. However, the present embodiment and the example do not limit the technical scope of the present disclosure. The present embodiment and the example are illustrative in all respects. The present embodiment and the example are non-restrictive. The technical scope of the present disclosure encompasses all modifications within the meaning and scope equivalent to the description of the claims. For example, it is also intended from the beginning that any configuration may be extracted from the present embodiment and arbitrarily combined. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 2 is an explanatory diagram of particle size distribution. [Figure 2] 1 is a schematic flowchart of a method for producing a positive electrode active material according to the present embodiment. [Figure 3] 10 is a table showing experimental results. DETAILED DESCRIPTION OF THE INVENTION

[0016] <Key terms> Numerical ranges such as "m to n%" include the upper and lower limits unless otherwise specified. That is, "m to n%" indicates a numerical range of "m% or more and n% or less." Furthermore, "m% or more and n% or less" includes "more than m% but less than n%." "Equal to or more" and "equal to or less" are expressed by the inequality sign "≦" with an equal sign. "More than" and "less than" are expressed by the inequality sign "<" without an equal sign.

[0017] All numerical values ​​are modified by the term "about." The term "about" may mean, for example, ±5%, ±3%, ±1%, etc. All numerical values ​​may be approximate values ​​that may vary depending on the application of the disclosed technology. All numerical values ​​may be expressed with significant figures. Unless otherwise specified, measured values ​​may be average values ​​of multiple measurements. The number of measurements may be three or more, five or more, or ten or more. In general, the more measurements are made, the more reliable the average value is expected to be. 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] "Primary particle" refers to the smallest unit of a particle, a solid particle with recognizable interparticle boundaries that cannot be further subdivided. Primary particles may also be referred to as "crystallites." "Secondary particle" refers to an aggregate of two or more primary particles.

[0019] The "particle size distribution of primary particles" is based on the number of particles. The particle size distribution can be created by the following procedure: The powder is observed using a SEM (Scanning Electron Microscope). For example, a JEOL product named "JSM-IT710HR" may be used. This SEM device is an example, and any SEM device with equivalent functionality may be used. 30 secondary particles are randomly selected. The particle size (maximum Feret diameter) of each primary particle of the 30 secondary particles is measured. The maximum Feret diameter is the distance between the two most distant points on the outline of the primary particle. A particle size number distribution is created.

[0020] FIG. 1 is an explanatory diagram of particle size distribution. The horizontal axis is particle size, and the vertical axis is frequency. min " is the minimum diameter of the particle size distribution. min For example, "D1" may be D1. "D1" indicates the particle diameter at which the cumulative value is 1%. max " is the maximum diameter of the particle size distribution. max may be, for example, D99. "D99" indicates the particle size at which the cumulative value is 99%. The particle size distribution may be multimodal. The particle size distribution may be unimodal. The particle size distribution has a maximum peak. The "maximum peak" indicates the peak with the greatest height among multiple peaks. When the distribution is unimodal, the single peak is considered to be the maximum peak. At the maximum peak, "D FWHM " is measured. D FWHM indicates the full width at half maximum (FWHM) of the maximum peak.

[0021] The "D50" of a lithium compound is a value in the particle size distribution based on mass. D50 indicates the particle size at which the cumulative value is 50%. Conventionally, mass-based is also sometimes expressed as "weight-based" or "volume-based." Mass-based particle size distribution can be measured, for example, by laser diffraction.

[0022] The stoichiometric composition formula indicates a representative example of a compound. The compound may have a non-stoichiometric composition. For example, "Al2O3" is not limited to a compound having a substance ratio (molar ratio) of "Al / O=2 / 3." Unless otherwise specified, "Al2O3" indicates a compound containing Al and O in any molar ratio. For example, the compound may be doped with a trace element. A portion of the Al and O may be substituted with another element.

[0023] <Cathode active material> Hereinafter, the positive electrode active material in this embodiment may be abbreviated as "the positive electrode active material." The positive electrode active material is for use in a secondary battery. That is, the present disclosure also provides a "positive electrode including the positive electrode active material" and a "secondary battery including the positive electrode active material." The secondary battery may be, for example, a liquid battery, a polymer battery, or an all-solid-state battery. The secondary battery may be, for example, a monopolar battery or a bipolar battery.

[0024] ·Secondary particles The present positive electrode active material is an aggregate (powder) of secondary particles. The present positive electrode active material includes a plurality of secondary particles. In the particle size distribution (by mass) of the present positive electrode active material, D50 may be, for example, 0.1 μm or more, 1 μm or more, 5 μm or more, or 10 μm or more. D50 may be, for example, 30 μm or less, 25 μm or less, 20 μm or less, 15 μm or less, or 10 μm or less.

[0025] The secondary particles include 3 to 20 primary particles. The number of primary particles refers to the number that can be confirmed in SEM observation (surface observation) of the secondary particles. The number of primary particles may be, for example, 15 or less, 10 or less, or 5 or less. The number of primary particles may be, for example, 5 or more, 10 or more, or 15 or more.

[0026] Aspect ratio of primary particles The primary particles may have an aspect ratio of, for example, 1 to 2. The aspect ratio may be, for example, 1.8 or less, 1.6 or less, 1.4 or less, or 1.2 or less. The aspect ratio may be, for example, 1.2 or more, 1.4 or more, 1.6 or more, or 1.8 or more. "Aspect ratio" is the ratio of the major axis diameter to the minor axis diameter. The major axis diameter indicates the maximum Feret diameter. The minor axis diameter indicates the minimum Feret diameter.

[0027] Primary particle size distribution The particle size distribution is D 0.3 μm or more min D min may be, for example, 0.6 μm or more, or 0.9 μm or more. minmay be, for example, 1.2 μm or less, 0.9 μm or less, or 0.6 μm or less.

[0028] The particle size distribution is, for example, D of 3.0 μm or less. max D max may be, for example, 2.7 μm or less, 2.4 μm or less, 2.1 μm or less, 1.8 μm or less, 1.5 μm or less, 1.2 μm or less, or 0.9 μm or less. max may be, for example, 0.6 μm or more, 0.9 μm or more, 1.2 μm or more, 1.5 μm or more, 1.8 μm or more, 2.1 μm or more, 2.4 μm or more, or 2.7 μm or more.

[0029] The particle size distribution is D 0.10 μm or more FWHM D FWHM may be, for example, 0.21 μm or more, 0.37 μm or more, 0.51 μm or more, or 0.87 μm or more. FWHM may be, for example, 1.8 μm or less, 1.5 μm or less, 1.2 μm or less, 0.87 μm or less, 0.51 μm or less, 0.37 μm or less, or 0.21 μm or less.

[0030] Crystal structure The primary particles include a lithium metal composite oxide. The primary particles may be composed of, for example, a single crystal. The primary particles may be composed of a lithium metal composite oxide. The lithium metal composite oxide has a layered rock salt structure. The layered rock salt structure is also referred to as an "α-NaFeO2 structure." The space group of the layered rock salt structure is "R-3m." Note that the "- (bar)" is normally placed above the "3," but is placed before the "3" for convenience. The crystal structure can be identified by powder X-ray diffraction (XRD).

[0031] ·Chemical composition The lithium metal composite oxide may have any chemical composition, for example, a composition represented by the following general formula: Li 1-a MO2 In the formula, the relationship of -0.5 ≤ a ≤ 0.5 is satisfied. M contains at least one selected from the group consisting of Ni, Co, Mn, and Al.

[0032] The composition of the lithium metal composite oxide may be represented by, for example, the following general formula. The compound represented by the following general formula may also be referred to as "NCM". Li 1-a Ni x Co y Mn z O2 In the formula, the relationships of -0.5 ≤ a ≤ 0.5, 0 < x < 1, 0 < y < 1, 0 < z < 1, and x + y + z = 1 are satisfied. For example, the relationships of 0.5 ≤ x < 1, 0 < y ≤ 0.25, and 0 < z ≤ 0.25 may be satisfied.

[0033] The composition of the lithium metal composite oxide may be represented by, for example, the following general formula. The compound represented by the following general formula may also be referred to as "NCA". Li 1-a Ni x Co y Al z O2 In the formula, the relationships of -0.5 ≤ a ≤ 0.5, 0 < x < 1, 0 < y < 1, 0 < z < 1, and x + y + z = 1 are satisfied. For example, the relationships of 0.5 ≤ x < 1, 0 < y ≤ 0.25, and 0 < z ≤ 0.25 may be satisfied.

[0034] A dopant may be added to the lithium metal composite oxide. The dopant may be diffused throughout the particle or distributed locally. For example, the dopant may be unevenly distributed on the particle surface. The dopant may be a substitutional solid solution atom or an interstitial solid solution atom. The amount of dopant added (substance amount fraction relative to the entire positive electrode active material) may be, for example, 0.01 to 5%, 0.1 to 3%, or 0.1 to 1%. One or more dopants may be added. Two or more dopants may form a composite. The dopant may include at least one selected from the group consisting of, for example, B, C, N, halogen, Si, Na, Mg, Al, Mn, Co, Cr, Sc, Ti, V, Cu, Zn, Ga, Ge, Se, Sr, Y, Zr, Nb, Mo, In, Pb, Bi, Sb, Sn, W, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and actinides.

[0035] <Method of manufacturing positive electrode active material> 2 is a schematic flowchart of a method for producing a positive electrode active material according to this embodiment. Hereinafter, the method for producing a positive electrode active material according to this embodiment may be abbreviated as "this production method." This production method includes "(a) preparation of metal hydroxide," "(b) mixing," "(c) heat treatment," "(d) crushing," and "(e) water washing."

[0036] (a) Preparation of metal hydroxides The production method includes preparing a metal hydroxide. The metal hydroxide is a precursor of a lithium metal composite oxide. The metal hydroxide may be synthesized, for example, by a coprecipitation method. For example, a sulfate may be prepared. The sulfate may include, for example, at least one selected from the group consisting of NiSO4, CoSO4, MnSO4, and Al2(SO4)3. A raw material solution is prepared by dissolving the sulfate in water. The mass concentration of the raw material solution may be, for example, 10 to 50%. A precipitate of the metal hydroxide may be generated by dropping the raw material solution into an alkaline aqueous solution. For example, the precipitate (metal hydroxide) may be collected by filtration. After collection, the metal hydroxide may be washed with water. After washing with water, the metal hydroxide may be dried.

[0037] ·(b) Mixture The method includes mixing a metal hydroxide and a lithium compound to form a mixture. For example, the materials may be mixed and ground in a mortar or the like.

[0038] "Lithium compound" refers to a compound containing Li. The lithium compound may, for example, include at least one selected from the group consisting of LiOH and Li2CO3. The lithium compound is a Li source for a lithium metal composite oxide. The ratio of the amount of substance of Li to the amount of substance of the metal hydroxide (precursor) may, for example, be 0.5 or more, 0.75 or more, 1 or more, 1.1 or more, or 1.25 or more. The ratio may, for example, be 1.5 or less, 1.25 or less, 1.1 or less, 1 or less, or 0.75 or less.

[0039] The lithium compound is a powder. The lithium compound has a D50 of 20 μm or more. The D50 of the lithium compound may be, for example, 32 μm or more, 39 μm or more, 51 μm or more, or 72 μm or more. The D50 of the lithium compound may be, for example, 100 μm or less, 90 μm or less, 80 μm or less, 72 μm or less, 51 μm or less, 39 μm or less, or 32 μm or less.

[0040] (c) Heat treatment This production method includes synthesizing a lithium metal composite oxide by subjecting the mixture to heat treatment in an oxygen atmosphere. Any heat treatment device or calcination furnace may be used. For example, a muffle furnace, an electric furnace, or the like may be used.

[0041] The heat treatment temperature may be, for example, 800 to 1100°C. The heat treatment temperature may be, for example, 900°C or higher, or 1000°C or higher. The heat treatment temperature may be, for example, 1000°C or lower, or 900°C or lower. The heat treatment time may be, for example, 8 to 12 hours. The heat treatment time may be, for example, 9 hours or higher, 10 hours or higher, or 11 hours or higher. The heat treatment time may be, for example, 11 hours or lower, 10 hours or lower, or 9 hours or lower.

[0042] ·(d) Crushing This manufacturing method includes crushing a lithium metal composite oxide to form secondary particles. Crushing can be performed so that the secondary particles have a predetermined particle size. For example, crushing can be performed using a crusher. Any crusher (e.g., a jet mill, etc.) can be used.

[0043] ·(e) Washing with water This production method includes washing the secondary particles with water. Washing with water can reduce the calcination residue of the lithium compound. The reduction in the calcination residue is expected to result in, for example, a reduction in battery resistance and an improvement in durability. [Example]

[0044] <Production of positive electrode active material> No.1 A raw material solution was formed by dissolving NiSO4, CoSO4, and MnSO4 in ion-exchanged water. The molar ratio of Ni, Co, and Mn in the raw material solution was "Ni / Co / Mn = 1 / 1 / 1". The solute concentration in the raw material solution was 30% (mass fraction).

[0045] Ammonia water was placed in a reaction vessel. While stirring the ammonia water with a stirrer, the atmosphere in the reaction vessel was replaced with nitrogen. NaOH was then added to the reaction vessel to form a reaction liquid.

[0046] The raw material solution and ammonia water were added dropwise to the reaction solution so that the pH of the reaction solution remained within a certain range, thereby forming a precipitate (metal hydroxide). The reaction solution was filtered, thereby recovering the metal hydroxide. The metal hydroxide was dispersed in ion-exchanged water, thereby forming a dispersion. The dispersion was thoroughly stirred with a spatula. That is, the metal hydroxide was washed with water. After washing with water, the dispersion was filtered, thereby recovering the metal hydroxide. The metal hydroxide was dried at 120°C for 16 hours, thereby forming a dried product.

[0047] The dry material (metal hydroxide) and a lithium compound (Li2CO3) were mixed in a mortar to form a mixture. The ratio of the amount of Li to the amount of metal hydroxide was 1.1. The D50 of the lithium compound was 9 μm.

[0048] The mixture was subjected to heat treatment in a muffle furnace to synthesize a lithium metal composite oxide under the following heat treatment conditions:

[0049] Atmosphere: Oxygen atmosphere Temperature: 800 to 1100°C Duration: 10 hours

[0050] After the heat treatment, the lithium metal composite oxide was crushed in a jet mill, thereby producing a positive electrode active material.

[0051] No.2 A dry material (metal hydroxide) was prepared in the same manner as in No. 1. The dry material (metal hydroxide) and a lithium compound (Li2CO3) were mixed in a mortar to form a mixture. The ratio of the amount of Li to the amount of metal hydroxide was 1.1. The D50 of the lithium compound was 20 μm.

[0052] The mixture was subjected to heat treatment in a muffle furnace to synthesize a lithium metal composite oxide under the following heat treatment conditions:

[0053] Atmosphere: Oxygen atmosphere Temperature: 800 to 1100°C Duration: 10 hours

[0054] After the heat treatment, the lithium metal composite oxide was crushed by a jet mill to form secondary particles.

[0055] After crushing, 5 g of the powder (secondary particles) was added to 50 mL of ion-exchanged water to prepare a dispersion. The dispersion was stirred for 5 minutes using a stirrer. After stirring, the dispersion was filtered to recover the secondary particles. The secondary particles were dried at 120°C for 16 hours. This produced a positive electrode active material.

[0056] No.3 to No.6 A positive electrode active material was prepared in the same manner as in No. 2, except that the D50 of the lithium compound was changed.

[0057] <Evaluation> A cylindrical lithium-ion secondary battery (evaluation cell) was manufactured. The evaluation cell had the following configuration:

[0058] Power generating element: Wound type Positive electrode: Positive electrode active material / AB / PVDF=88 / 10 / 2 (mass ratio) Negative electrode: Negative electrode active material (natural graphite), CMC, SBR Electrolyte: LiPF6 (1 mol / L), EC / DMC / EMC = 3 / 4 / 3 (volume ratio)

[0059] The positive and negative electrodes were manufactured by coating the surface of the substrate (metal foil) with the slurry. An Allgood film applicator (with film thickness adjustment function) was used as the coating device. After coating the slurry, the coating was dried at 80°C for 5 minutes.

[0060] The initial resistance of the evaluation cells was measured. Figure 3 is a table showing the experimental results. In Figure 3, the relative values ​​are shown in the "Initial Resistance" section. The relative values ​​(percentages) were calculated by dividing the initial resistance of each evaluation cell by the initial resistance of No. 1. The smaller the initial resistance, the more the battery resistance is evaluated to be reduced.

[0061] A durability test was conducted on the evaluation cell. Specifically, the cell was charged and discharged 200 times at a constant current of 2C in a voltage range of 3.0 to 4.1V at room temperature. The capacity retention rate (percentage) was calculated by dividing the 200th discharge capacity by the initial discharge capacity. The higher the capacity retention rate, the better the durability was evaluated.

[0062] <Result> As shown in Figure 3, D FWHM When the thickness is 0.10 μm or more, the battery resistance tends to decrease. FWHM When the thickness was 0.21 μm or more, the durability tended to improve.

Claims

1. Contains a plurality of secondary particles, each of the plurality of secondary particles includes 3 to 20 primary particles; the primary particles include a lithium metal composite oxide; The lithium metal composite oxide has a layered rock salt structure, The particle size distribution of the primary particles is D of 0.3 μm or more min , and D of 0.10 μm or more FWHM and The particle size distribution is based on the number of particles, The above D min indicates the minimum diameter in the particle size distribution, and The above D FWHM indicates the full width at half maximum of the maximum peak of the particle size distribution, Cathode active material.

2. The above D FWHM is 0.21 μm or more, The positive electrode active material according to claim 1 .

3. The above D FWHM is 0.87 μm or less, The positive electrode active material according to claim 1 .

4. The lithium metal composite oxide has the general formula: Li 1-a MO 2 It has a composition represented by In the general formula, The relationship of -0.5≦a≦0.5 is satisfied, and M includes at least one selected from the group consisting of Ni, Co, Mn, and Al; The positive electrode active material according to claim 1 .

5. (a) providing a metal hydroxide; (b) mixing the metal hydroxide and the lithium compound to form a mixture; (c) synthesizing a lithium metal composite oxide by subjecting the mixture to a heat treatment in an oxygen atmosphere; (d) crushing the lithium metal composite oxide to form secondary particles; and (e) washing the secondary particles with water; Including, The lithium compound has a D50 of 20 μm or more, and The D50 indicates the particle size at which the cumulative value becomes 50% in the particle size distribution based on mass. A method for producing a positive electrode active material.

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