Positive electrode active material and manufacturing method of the positive electrode active material

Abstract

To improved durability.SOLUTION: A positive electrode active material includes secondary particles. The secondary particles include a plurality of crystallites. Each of the plurality of crystallites includes a lithium metal composite oxide. The lithium metal composite oxide has a layered rock-salt structure. In a cross section of the secondary particles, the relationships of "2.5≤dL / dS≤28.2", "0.125≤dL / D" and "θ≤45°" are satisfied. The "dL" indicates a major axis diameter of the crystallite. In the formula, the "dS" represents the minor axis diameter of the crystallite. The "D" indicates the maximum Feret diameter of the secondary particles. The "θ" indicates an angle formed by a first straight line and a second straight line. The first straight line is an extended line of the major axis diameter of the crystallite. The second straight line passes through an intersection of the circumscribed circle of the secondary particle and an extension line and a center of a circumscribed circle.SELECTED DRAWING: Figure 2

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 a positive electrode active material that contains voids at a rate of 20% or more and that contains long voids that connect to the interior of the particles. [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] The secondary particle structure of the positive electrode active material can affect battery performance. For example, the formation of long voids that connect to the interior of the secondary particles can improve output characteristics. However, there is still room for improvement in terms of battery durability.

[0005] The objective of the present disclosure is to improve durability. [Means for solving the problem]

[0006] 1. The positive electrode active material includes secondary particles. The secondary particles include a plurality of crystallites. Each of the plurality of crystallites includes a lithium metal composite oxide. The lithium metal composite oxide has a layered rock salt structure. In the cross section of the secondary particle, "2.5≦d L / d S ≦28.2", 0.125≦d L / D" and "θ≦45°" are satisfied. L " indicates the major axis diameter of the crystallite. S" indicates the minor axis diameter of the crystallite. "D" indicates the maximum Feret diameter of the secondary particle. "θ" indicates the angle between the first line and the second line. The first line is an extension of the major axis diameter of the crystallite. The second line passes through the intersection of the circumscribing circle of the secondary particle and the extension line, and the center of the circumscribing circle.

[0007] FIG. 1 is a conceptual diagram showing a first example of a secondary particle structure. Secondary particle 2 is an aggregate of crystallites 1. Crystallite 1 is also referred to as a "primary particle." Crystallite 1 includes a reactive surface 1a and a non-reactive surface 1b. Reactive surface 1a is more active than non-reactive surface 1b. Conventionally, crystallite 1 has a small aspect ratio. In crystallites 1 with a small aspect ratio, reactive surface 1a tends to be larger than non-reactive surface 1b. When the electrolyte reacts with reactive surface 1a, a coating is formed on crystallite 1. The formation of the coating can consume lithium (Li). The formation of the coating can reduce the amount of Li used for charging and discharging. In other words, durability is thought to be reduced.

[0008] Figure 2 is a conceptual diagram showing a second example of a secondary particle structure. In Figure 2, crystallite 1 has a large aspect ratio. In long crystallites 1, the non-reactive surface 1b tends to be larger than the reactive surface 1a. By reducing the contact area between the reactive surface 1a and the electrolyte, improved durability is expected. Furthermore, multiple crystallites 1 are arranged radially. That is, each of the multiple crystallites 1 extends from the center of the secondary particle 2 toward the surface. The radial arrangement of crystallites 1 can promote ion conduction in the direction from the surface of the secondary particle 2 toward the center of the secondary particle 2. Promotion of ion conduction toward the center is expected to improve durability.

[0009] In the above "1", "d L / d S " indicates the aspect ratio of the crystallite. "d L / D" indicates the size ratio of the crystallites to the secondary particles. "θ" is an index of arrangement. The smaller the angle (θ), the more radially the crystallites are considered to be arranged. "2.5≦d L / d S ≦28.2", 0.125≦d L / D" and "θ≦45°" relationships are satisfied, and this is expected to improve durability.

[0010] 2. The positive electrode active material described in the above item "1" may include, for example, the following structure: In the cross section of the secondary particle, "0.125≦d L / D<0.500", "0°≦θ≦10°" and "2.5≦d L / d S ≦7.9” relationship is further satisfied.

[0011] By satisfying the above relationship "2," further improvement in durability is expected.

[0012] 3. The positive electrode active material described in the above "1" or "2" may include, for example, the following configuration: In the cross section of the secondary particle, the secondary particle has a porosity of 10% or less.

[0013] 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.

[0014] 5. A method for producing a positive electrode active material includes the following steps (a) to (d): (a) Prepare a metal hydroxide. (b) forming a first mixture by combining a metal hydroxide and a lithium compound; (c) subjecting the first mixture to a first heat treatment to form a second mixture; (d) The second mixture is subjected to a second heat treatment to synthesize a positive electrode active material. The first and second heat treatments are carried out in an oxygen atmosphere. The first heat treatment is carried out at a temperature of 500 to 650°C for 48 to 60 hours. The second heat treatment is carried out at a temperature of 900 to 1100°C for 0.5 to 2 hours.

[0015] The first and second heat treatments are also referred to as "calcination." The first heat treatment is performed at a low temperature for a long time. The second heat treatment is performed at a high temperature for a short time. The combination of the first and second heat treatments is expected to form the secondary particles described in "1" above.

[0016] Hereinafter, one embodiment of the present disclosure (hereinafter may be abbreviated as "this embodiment") and one 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 encompasses all modifications within the meaning and scope equivalent to the claims. For example, it is originally intended that any configurations may be extracted from this embodiment and arbitrarily combined. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 2 is a conceptual diagram showing a first example of a secondary particle structure. [Figure 2] FIG. 4 is a conceptual diagram showing a second example of a secondary particle structure. [Figure 3] FIG. 1 is a conceptual diagram showing a method for measuring the angle (θ). [Figure 4] 1 is a schematic flowchart of a method for producing a positive electrode active material according to the present embodiment. [Figure 5] 1 is a table showing manufacturing conditions and experimental results. DETAILED DESCRIPTION OF THE INVENTION

[0018] <Terms and phrases> Geometric terms should not be interpreted in their strict sense. Examples of geometric terms include "parallel," "perpendicular," and the like. For example, "parallel" may deviate slightly from the strict meaning of "parallel." For example, directions, angles, distances, etc. may be relatively displaced as long as substantially the same function is obtained. Geometric terms may include, for example, tolerances, errors, etc. in design, work, manufacturing, etc. The dimensional relationships in each figure may not match the actual dimensional relationships. The dimensional relationships in each figure may be changed to aid the reader's understanding. For example, length, width, thickness, etc. may be changed. Some components may be omitted.

[0019] 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 inequality symbols with equality signs "≦, ≧." "More than" and "less than" are expressed by inequality symbols without equality signs "<, >."

[0020] 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.

[0021] A "crystallite" is the smallest unit of a particle and refers to a solid particle with boundaries between particles that cannot be further divided. A "secondary particle" refers to an aggregate of two or more crystallites.

[0022] Crystallite major axis diameter (d L) and "minor axis diameter of crystallite (d S ), "maximum Feret diameter (D) of the secondary particles," "angle (θ)," and "porosity" are measured in cross-sectional SEM (Scanning Electron Microscope) images of the secondary particles. The observation magnification can be adjusted according to the particle size. The observation magnification may be, for example, about 1000x. Cross-sectional samples of particles can be prepared by conventional methods. For example, cross-sectional samples may be prepared using a CP (Cross Section Polisher), FIB (Focused Ion Beam), etc. Various dimensions and angles in the image are measured using image analysis software. For example, "ImageJ Fiji" or the like may be used. Note that "ImageJ Fiji" is merely one example. Any image analysis software having equivalent functions to "ImageJ Fiji" may be used. For example, image analysis software included with various SEM devices may be used.

[0023] In a cross-sectional SEM image of a secondary particle, the smallest rectangle circumscribing the crystallite (hereinafter also referred to as the "circumscribing rectangle") is identified. The length of the long side of the circumscribing rectangle is called the "major axis diameter (d L )" The length of the short side of the circumscribed rectangle is the "minor axis diameter (d S )".

[0024] In a cross-sectional SEM image of a secondary particle, the distance between the two most distant points on the outline of the secondary particle is the "maximum Feret diameter (D)."

[0025] FIG. 3 is a conceptual diagram showing a method for measuring the angle (θ). In a cross-sectional SEM image of a secondary particle, the circumscribing circle 4 of the secondary particle is identified. A crystallite 1 exposed on the surface of the secondary particle is selected. The major axis diameter (d L ) is extended to identify the first straight line L1. That is, the first straight line L1 is the major axis diameter (d L ) is an extension of the circumscribing circle 4. The intersection 4i between the first line L1 and the circumscribing circle 4 is identified. The second line L2 passing through the intersection 4i and the center 4c of the circumscribing circle 4 is identified. The angle (θ) is the angle (acute angle) between the first line L1 and the second line L2.

[0026] The cross-sectional SEM image of the secondary particles is binarized to distinguish between voids and crystallites. The "void ratio" is calculated by dividing the number of void pixels by the total number of void and crystallite pixels. The void ratio is expressed as a percentage.

[0027] "D50" indicates the particle size at which the cumulative distribution reaches 50% in the volume-based particle size distribution (cumulative distribution). The particle size distribution can be measured by a laser diffraction method.

[0028] 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.

[0029] <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.

[0030] The present positive electrode active material includes secondary particles. The present positive electrode active material may be an aggregate (powder) of secondary particles. The D50 of the present positive electrode active material may be, for example, 0.1 μm or more, 1 μm or more, 5 μm or more, or 10 μm or more. The 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.

[0031] ·Secondary particles As shown in FIG. 2, the secondary particle 2 is an aggregate of crystallites 1. The secondary particle 2 has any shape. The secondary particle 2 may be, for example, spherical, oval-spherical, or lumpy. In a cross-sectional SEM image of the secondary particle 2, the outline of the secondary particle 2 may have a circularity of, for example, 0.8 or more. The circularity may be, for example, 0.85 or more, 0.90 or more, or 0.95 or more. The "circularity" is calculated by the following formula. Cr=4πS / L 2 Cr: Circularity π: Pi S: Cross-sectional area of ​​secondary particle 2 (area of ​​the region surrounded by the outline of secondary particle 2) L: Perimeter of secondary particle 2 (length of the outline of secondary particle 2)

[0032] The maximum Feret diameter (D) of the secondary particles 2 may be, for example, 1 μm or more, 5 μm or more, 10 μm or more, 11.5 μm or more, 12.4 μm or more, 16.5 μm or more, 18.2 μm or more, or 20 μm or more. The maximum Feret diameter (D) may be, for example, 30 μm or less, 25 μm or less, 20 μm or less, 18.2 μm or less, 16.5 μm or less, 12.4 μm or less, 11.5 μm or less, or 10 μm or less.

[0033] Crystallites The secondary particle 2 includes a plurality of crystallites 1. In a cross-sectional SEM image of the secondary particle 2, the number of crystallites 1 included in one secondary particle 2 may be, for example, 10 or more, 50 or more, 100 or more, 150 or more, or 200 or more. The number of crystallites 1 included in one secondary particle 2 may be, for example, 500 or less, 250 or less, 200 or less, 150 or less, 100 or less, or 50 or less.

[0034] In this embodiment, 0.125 (= 1 / 8)≦d L The size ratio (d L The size ratio (d / D) may be, for example, 0.127 or more, 0.254 or more, 0.375 or more, or 0.496 or more. L / D) may be, for example, less than 0.500 (= 1 / 2), 0.496 or less, 0.375 or less, 0.254 or less, or 0.127 or less. That is, 0.125≦d L The relationship of / D<0.500 may be satisfied.

[0035] In this embodiment, 2.5≦d L / d S The relationship ≦28.2 is satisfied. L / d S ) may be, for example, 2.6 or more, 5 or more, 7.9 or more, 10 or more, 15 or more, 20.6 or more, or 25 or more. L / d S ) may be, for example, 25 or less, 20.6 or less, 15 or less, 10 or less, 7.9 or less, or 5 or less. That is, 2.5≦d L / d S The relationship ≦7.9 may be satisfied.

[0036] The major axis diameter of crystallite 1 (d L The major axis diameter (d L ) may be, for example, 15 μm or less, 10 μm or less, 9.0 μm or less, 6.2 μm or less, 3.1 μm or less, or 1.5 μm or less.

[0037] The minor axis diameter of crystallite 1 (d S The minor axis diameter (d S ) may be, for example, 1.2 μm or less, 1.0 μm or less, 0.80 μm or less, 0.56 μm or less, 0.40 μm or less, 0.32 μm or less, or 0.30 μm or less.

[0038] Within the secondary particle 2, the multiple crystallites 1 are arranged radially. That is, in this embodiment, the relationship θ≦45° is satisfied. The angle (θ) may be, for example, 43.5° or less, 30° or less, 25.7° or less, 20.6° or less, 15° or less, 10° or less, 9.8° or less, 5° or less, 2.6° or less, or 1° or less. The angle (θ) may be, for example, 0° or more, 1° or more, 2.6° or more, 5° or more, 9.8° or more, 10° or more, 15° or more, 20.6° or more, 25.7° or more, 30° or more, or 43.5° or more. That is, the relationship 0°≦θ≦10° may be satisfied.

[0039] The porosity of the secondary particles 2 may be, for example, 10% or less. The porosity may be, for example, 9.5% or less, 7.4% or less, 6.3% or less, or 3.6% or less. The porosity may be, for example, 1% or more, 2% or more, 3.6% or more, 6.3% or more, 7.4% or more, or 9.5% or more.

[0040] Crystal structure Crystallite 1 contains a lithium metal composite oxide. Crystallite 1 may consist of, for example, a single crystal. Crystallite 1 may consist 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) analysis.

[0041] The layered rock salt structure has (100) planes and (003) planes. The (100) plane can be perpendicular to each layer of the layered rock salt structure. The gap between the layers of the layered rock salt structure can serve as an entrance and exit for Li. On the other hand, the (003) plane can be parallel to each layer of the layered rock salt structure. For example, on the outer surface of the crystallite 1, the plane where the (100) plane is detected may be regarded as the reaction surface 1a. For example, on the outer surface of the crystallite 1, the plane where the (003) plane is detected may be regarded as the non-reaction surface 1b. The (100) plane and the (003) plane can be detected, for example, by TEM (Transmission Electron Microscopy) analysis.

[0042] The crystallite 1 may have end faces at both ends in the long axis direction. For example, the (100) plane may be detected on the end face. The crystallite 1 may include a peripheral surface (side surface) in the short axis direction. The peripheral surface may connect two end faces. For example, the (003) plane may be detected on the peripheral surface.

[0043] ·Chemical composition The lithium metal composite oxide can have an arbitrary chemical composition. The lithium metal composite oxide may have, 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. 2 9>

[0044] The composition of the lithium metal composite oxide may be represented, for example, by 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.

[0045] 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.

[0046] A dopant may be added to the lithium metal composite oxide. The dopant may be diffused throughout the particles or may be locally distributed. 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 dopant may be an atom derived from the crystal control material described later. The dopant may contain, for example, at least one selected from the group consisting of W and B.

[0047] The ratio of the amount of the dopant to the amount of the lithium metal composite oxide may be, for example, 0.01 or more, 0.05 or more, or 0.1 or more. The same ratio may be, for example, 0.5 or less, 0.1 or less, or 0.05 or less.

[0048] <Method for manufacturing a positive electrode active material> Figure 4 is a schematic flowchart of the method for manufacturing a positive electrode active material in the present embodiment. Hereinafter, the method for manufacturing a positive electrode active material in the present embodiment may be abbreviated as "this manufacturing method". This manufacturing method includes "(a) preparation of metal hydroxide", "(b) mixing", "(c) first heat treatment", and "(d) second heat treatment". This manufacturing method may further include, for example, "(e) crushing", etc.

[0049] ·(a) Preparation of metal hydroxide 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.

[0050] ·(b) Mixture The method includes mixing a metal hydroxide and a lithium compound to form a first mixture. For example, the materials may be mixed and ground in a mortar or the like.

[0051] "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.

[0052] For example, a crystallinity control material may be added. That is, a first mixture may be formed by mixing a metal hydroxide, a lithium compound, and a crystallinity control material. The crystallinity control material is expected to increase the aspect ratio of the crystallites and to radially arrange the crystallites. The crystallinity control material may include, for example, at least one selected from the group consisting of H2WO4 and B2O3. The ratio of the amount of substance of the crystallinity control material to the amount of substance of the metal hydroxide (precursor) may be, for example, 0.1 to 1. The ratio may be, for example, 0.5 or more or 0.5 or less.

[0053] (c) First heat treatment, (d) Second heat treatment The method includes subjecting the first mixture to a first heat treatment to form a second mixture. The method further includes subjecting the second mixture to a second heat treatment to synthesize the cathode active material. The first and second heat treatments are performed in an oxygen atmosphere.

[0054] The first heat treatment is performed at a low temperature for a long time. The temperature of the first heat treatment is 500 to 650°C. The temperature of the first heat treatment may be, for example, 550°C or higher, or 600°C or higher. The temperature of the first heat treatment may be, for example, 600°C or lower, or 550°C or lower. The time of the first heat treatment is 48 to 60 hours. The time of the first heat treatment may be, for example, 54 hours or lower, or 52 hours or lower.

[0055] The second heat treatment is performed at a high temperature for a short time. The temperature of the second heat treatment is 900 to 1100°C. The temperature of the second heat treatment may be, for example, 950°C or higher. The temperature of the second heat treatment may be, for example, 1050°C or lower. The time of the second heat treatment is 0.5 to 2 hours. The time of the second heat treatment may be, for example, 1.5 hours or shorter.

[0056] (e) Crushing The present production method may include crushing the lithium metal composite oxide. Any crusher (e.g., a jet mill, etc.) can be used. By crushing, the particle size of the lithium metal composite oxide can be adjusted. [Example]

[0057] <Production of positive electrode active material> No.1 Figure 5 is a table showing the manufacturing conditions and experimental results. No. 1 positive electrode active material was manufactured by the first synthesis method. NiSO4, CoSO4, and MnSO4 were dissolved in ion-exchanged water to form a raw material solution. In the raw material solution, the molar ratio of Ni, Co, and Mn was "Ni / Co / Mn = 8 / 1 / 1." The solute concentration in the raw material solution was 30% (mass fraction).

[0058] 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.

[0059] 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.

[0060] The dried material 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.

[0061] The mixture was heat-treated in a muffle furnace to synthesize a lithium metal composite oxide. The heat treatment consisted of one stage. The heat treatment conditions were as follows. After the heat treatment, the particle size of the lithium metal composite oxide was adjusted using a jet mill.

[0062] Atmosphere: Oxygen atmosphere Temperature: 800 to 1100°C Duration: 10 hours

[0063] No.2 Positive electrode active material No. 2 was produced by the second synthesis method. The second synthesis method differs from the first synthesis method in the use of a crystal control material and heat treatment. As with No. 1, a dried material (metal hydroxide) was prepared by coprecipitation.

[0064] The dried material, lithium compound (Li2CO3), and crystallization control materials (H2WO4, B2O3) were mixed in a mortar to form a mixture. The ratio of the amount of crystallization control material to the amount of metal hydroxide was 0.1.

[0065] A lithium metal composite oxide was synthesized by carrying out a first heat treatment and a second heat treatment in this order in a muffle furnace. The heat treatment conditions were as follows. After the heat treatment, the particle size of the lithium metal composite oxide was adjusted using a jet mill.

[0066] First heat treatment Atmosphere: Oxygen atmosphere Temperature: 500℃ Duration: 50 hours

[0067] Second heat treatment Atmosphere: Oxygen atmosphere Temperature: 1000℃ Time: 1 hour

[0068] No.3, No.4, No.5 As shown in FIG. 5, a positive electrode active material was prepared in the same manner as in No. 2, except that the temperature of the first heat treatment was changed.

[0069] <Evaluation> A cylindrical lithium-ion secondary battery (evaluation cell) was manufactured. The evaluation cell had the following configuration:

[0070] 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)

[0071] 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.

[0072] 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. A higher capacity retention rate is considered to indicate better durability.

[0073] <Result> As shown in Figure 5, No. 2 to No. 5 have improved durability compared to No. 1. No. 2 to No. 5 have a durability of 2.5 ≤ d L / d S ≦28.2, 0.125≦d L / D and the relationship θ≦45° is satisfied. [Explanation of symbols]

[0074] 1 crystallite, 1a reactive surface, 1b non-reactive surface, 2 secondary particle, 4 circumscribed circle, 4c center, 4i intersection, L1 first straight line, L2 second straight line.

Claims

1. Contains secondary particles, the secondary particles include a plurality of crystallites, each of the plurality of crystallites includes a lithium metal composite oxide; The lithium metal composite oxide has a layered rock salt structure, In the cross section of the secondary particle, 2.5≦d L / d S ≦28.2, 0.125≦d L / D, and the relationship θ≦45° is satisfied, The above d L represents the major axis diameter of the crystallite, and the d S represents the minor axis diameter of the crystallite, The D represents the maximum Feret diameter of the secondary particles, The θ indicates the angle between the first line and the second line, The first straight line is an extension line of the major axis diameter of the crystallite, and the second straight line passes through an intersection point between the circumscribing circle of the secondary particle and the extension line and through the center of the circumscribing circle; Cathode active material.

2. In the cross section of the secondary particle, 0.125≦d L / D<0.500, 0°≦θ≦10°, and 2.5≦d L / d S The relationship ≦7.9 is further satisfied, The positive electrode active material according to claim 1 .

3. In the cross section of the secondary particle, the secondary particle has a porosity of 10% 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) combining the metal hydroxide and a lithium compound to form a first mixture; (c) subjecting the first mixture to a first heat treatment to form a second mixture; and (d) synthesizing a positive electrode active material by subjecting the second mixture to a second heat treatment; Including, the first heat treatment and the second heat treatment are performed in an oxygen atmosphere; The first heat treatment is carried out at a temperature of 500 to 650° C. for 48 to 60 hours; and The second heat treatment is carried out at a temperature of 900 to 1100°C for 0.5 to 2 hours. A method for producing a positive electrode active material.

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