Cathode active material

Abstract

To provide a cathode active material capable of reducing initial resistance.SOLUTION: The cathode active material includes first particles and second particles. The first particle has a maximum Feret diameter of 1 μm or more. The second particles have a maximum Feret diameter of 50 nm or less. The second particles are attached to the surface of the first particle. The amount of second particles attached is 0.24 or more per 1 μm2.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a positive electrode active material. [Background technology]

[0002] JP 2017-228438 A discloses a metal oxide particle having a pore diameter in the range of 10 to 40 nm with a differential pore volume of 0.01 cm 3 3 / g or more. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2017-228438 Summary of the Invention [Problem to be solved by the invention]

[0004] For example, it has been proposed to improve rate performance by adjusting the pore structure inside the particles, but the desired initial resistance may not be achieved due to insufficient reactive area on the particle surface.

[0005] The objective of the present disclosure is to reduce the initial resistance. [Means for solving the problem]

[0006] 1. The positive electrode active material includes first particles and second particles. The first particles have a maximum Feret diameter of 1 μm or more. The second particles have a maximum Feret diameter of 50 nm or less. The second particles are attached to the surfaces of the first particles. The amount of attached second particles is 1 μm or less. 2 More than 0.24 pieces per unit.

[0007] The second particles are fine particles that adhere to the surface of the first particles. When the amount of the second particles attached is equal to or greater than a specific value, a reduction in initial resistance is expected. This is thought to be because the fine particles contribute to an increase in the reaction area and promote the movement of lithium (Li) on the surface of the first particles.

[0008] 2. The positive electrode active material described in the above item "1" may include, for example, the following configuration: The second particles have a maximum Feret's diameter of 14 to 21 nm.

[0009] 3. The positive electrode active material according to the above item "1" or "2" may include, for example, the following configuration: The amount of the second particles attached is 1 μm 2 There are 3.2 or fewer per unit.

[0010] 4. The positive electrode active material according to any one of the above items "1" to "3" may have, for example, the following configuration: The first particles have a maximum Feret's diameter of 20 μm or less.

[0011] 5. The positive electrode active material according to any one of the above items "1" to "4" may have the following configuration, for example: The second particles have the same chemical composition as the first particles.

[0012] 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]

[0013] [Figure 1] FIG. 2 is a conceptual diagram illustrating an example of a positive electrode active material according to the present embodiment. [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

[0014] -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.

[0015] 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 "<, >."

[0016] All numerical values ​​are modified by the term "about." The term "about" may mean, for example, ±5%, ±3%, ±1%, etc. Numerical values ​​may be approximate values ​​that may vary depending on the application of the disclosed technology unless otherwise specified. Numerical values ​​may be expressed in significant figures unless otherwise specified. Measured values ​​may be average values ​​of multiple measurements unless otherwise specified. The number of measurements may be three or more, five or more, or ten or more. In general, the more measurements are performed, the more reliable the average value is expected to be. Measured values ​​may be rounded off based on the number of significant figures. Measured values ​​may include errors, such as those associated with the detection limit of the measuring device.

[0017] 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 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.

[0018] FIG. 1 is a conceptual diagram showing an example of a positive electrode active material in this embodiment. The "maximum Feret diameter" refers to the distance between the two most distant points on the contour line of a particle. The maximum Feret diameter (D1) of the first particle 1, the maximum Feret diameter (D2) of the second particle 2, and the amount of attached second particle 2 are determined by image analysis. For example, a JEOL SEM (Scanning Electron Microscope) "JSM-IT710HR" may be used. This device has a function for calculating length and area within an image. 30 first particles 1 are randomly selected within an SEM image. The first particles 1 are particles with a maximum Feret diameter of 1 μm or more. The number of second particles 2 attached to the surface of each first particle 1 is counted. The second particles 2 are fine particles with a maximum Feret diameter of 50 nm or less. The area of ​​each first particle 1 is measured. The total number of second particles 2 is divided by the total area of ​​the first particles to determine the amount of attachment per μm (unit: particles / μm 2 ) is obtained.

[0019] In the present embodiment, unless otherwise specified, specific examples of measuring devices and the like are merely examples, and equivalent devices and the like to those specific examples may also be used.

[0020] -Cathode active material- Hereinafter, the positive electrode active material in this embodiment will also be referred to 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.

[0021] The positive electrode active material is a powder. The D50 of the positive electrode active material may be, for example, 0.1 μm or more, 1 μm or more, or 3 μm or more. The D50 may be, for example, 20 μm or less, 10 μm or less, 5 μm or less, or 3 μm or less.

[0022] As shown in FIG. 1 , the positive electrode active material 5 includes a first particle 1 and a second particle 2. The first particle 1 may be, for example, a crystallite. A "crystallite" refers to the smallest unit of a particle, a solid particle with boundaries between particles that are recognized as not being further subdivided. The crystallite may have any shape. For example, the crystallite may be spherical, plate-like, flaky, columnar, rod-like, needle-like, blocky, or angular. For example, when the crystallite is a plate-like particle, the crystallite may have a main surface and an end surface. For example, a 100-plane may be detected on the main surface (plate surface). The end surface intersects with the main surface at the periphery of the main surface. For example, a 003-plane may be detected on the end surface. Each crystal plane (003-plane, 100-plane) can be identified, for example, by transmission electron microscopy (TEM) analysis.

[0023] The maximum Feret diameter (D1) of the first particles 1 may be, for example, 1 μm or more, 2 μm or more, 3 μm or more, 5 μm or more, 7.5 μm or more, 10 μm or more, 12.5 μm or more, 15 μm or more, 17.5 μm or more, or 20 μm or more. The maximum Feret diameter (D1) of the first particles 1 may be, for example, 30 μm or less, 25 μm or less, 20 μm or less, 17.5 μm or less, 15 μm or less, 12.5 μm or less, 10 μm or less, 7.5 μm or less, 5 μm or less, 3 μm or less, or 2 μm or less.

[0024] The first particle 1 may be, for example, a secondary particle. "Secondary particle" refers to an aggregate (aggregate) of two or more crystallites. The number of crystallites contained in a secondary particle may be, for example, 3 or more, 5 or more, 10 or more, or 20 or more. The number of crystallites contained in a secondary particle may be, for example, 50 or less, 40 or less, 30 or less, 20 or less, 10 or less, or 5 or less.

[0025] The second particles 2 are attached to the first particles 1. The second particles 2 have any shape. The second particles 2 may be, for example, spherical, plate-like, flake-like, columnar, rod-like, needle-like, blocky, or angular. The second particles 2 may be crystallites or secondary particles.

[0026] The amount of the second particles 2 attached is 0.24 particles / μm 2 The adhesion amount is 0.24 particles / μm. 2 By setting the amount to above 0.78 particles / μm, a reduction in the initial resistance is expected. 2 More than 1.5 pieces / μm 2 More than 2.1 pieces / μm 2 or more than 3.2 particles / μm 2 The amount of adhesion may be, for example, 20 particles / μm 2 Below, 10 pieces / μm 2 Below, 5 pieces / μm 2 Below, 3.2 pieces / μm 2 Below, 2.1 pieces / μm 2 Below, 1.5 pieces / μm 2 or less, or 0.78 pieces / μm 2 It may be the following:

[0027] -Chemical composition- The first particle 1 and the second particle 2 can each independently have any chemical composition. For example, the second particle 2 can have a different chemical composition from the first particle 1. For example, the second particle 2 can have substantially the same chemical composition as the first particle 1. "Substantially the same" can allow for a difference of, for example, 0.1 to 3% for each component. For example, the second particle 2 can have the same chemical composition as the first particle 1.

[0028] The first particle 1 and the second particle 2 may each contain, for example, a lithium metal composite oxide. The first particle 1 and the second particle 2 may each be composed of, for example, a single crystal. The first particle 1 and the second particle 2 may each be composed of, for example, 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." The crystal structure can be identified by powder XRD (X-ray diffraction).

[0029] The first particle 1 and the second particle 2 may each independently have a composition represented by the following general formula, for example. 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.

[0030] The first particle 1 and the second particle 2 may each independently have a composition represented by the following general formula, for example: 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, relationships such as “0.5≦x<1”, “0<y≦0.25”, and “0<z≦0.25” may also be satisfied.

[0031] The first particle 1 and the second particle 2 may each independently have a composition 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, relationships such as “0.5≦x<1”, “0<y≦0.25”, and “0<z≦0.25” may also be satisfied.

[0032] A dopant may be added to the first particle 1 and the second particle 2. The dopant may be diffused throughout the particle 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 contain, for example, at least one selected from the group consisting of S, Na, Ca, Cl, N, W, and B.

[0033] 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.

[0034] -Method for manufacturing a positive electrode active material- Figure 2 is a schematic flowchart of the method for manufacturing the positive electrode active material of the present embodiment. Hereinafter, the method for manufacturing the positive electrode active material in the present embodiment may be abbreviated as “this manufacturing method”. This manufacturing method includes, for example, “(a) preparation of metal hydroxide”, “(b) mixing”, “(c) heat treatment”, and “(d) pulverization”.

[0035] (a) Preparation of metal hydroxides This manufacturing 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 concentration of the raw material solution may be, for example, 10 to 50% by mass fraction. A metal hydroxide precipitate 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.

[0036] (b) Mixture The method may include mixing a metal hydroxide and a lithium compound to form a mixture. For example, mixing and grinding the materials may be performed in a mortar or the like.

[0037] "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.

[0038] Various additives may be added. The additives may impart anisotropy to the grain growth of crystallites during heat treatment. For example, LiSO4, Li2SO4, NaCl, CaCl, LiCl, LiNO3, etc. may be added. These additives may promote plate-like growth of crystallites. For example, H2WO4, B2O3, etc. may be added. These additives may promote rod-like growth of crystallites.

[0039] (c) Heat treatment The present production method may include 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.

[0040] 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.

[0041] (d) Crushing The present production method may include pulverizing the lithium metal composite oxide. The pulverization may be carried out so that second particles 2 (fine particles) are generated and the second particles 2 adhere to the first particles 1 (base particles). For example, the lithium metal composite oxide (base particles) may be subjected to a pulverization treatment using a ball mill. Fragments (second particles 2) may be generated by the pulverization. Furthermore, the mechanochemical effect of the ball mill may cause the second particles 2 to adhere to the surfaces of the base particles (first particles 1). The size and amount of adhesion of the second particles 2 (fine particles) may be adjusted, for example, by the rotation speed and treatment time of the ball mill. The rotation speed may be, for example, 100 rpm or more, 200 rpm or more, 300 rpm or more, or 400 rpm or more. The rotation speed may be, for example, 500 rpm or less, 400 rpm or less, 300 rpm or less, or 200 rpm or less. The treatment time may be, for example, 60 minutes or more, 120 minutes or more, 180 minutes or more, or 240 minutes or more. The treatment time may be, for example, 300 minutes or less, 240 minutes or less, 180 minutes or less, or 120 minutes or less. The balls (grinding media) may be made of, for example, ZrO2. The diameter of the balls may be, for example, 1 to 10 mm.

[0042] It should be noted that, for example, depending on the crusher such as a jet mill, although the secondary particles are crushed, the crystallites (primary particles) may not be pulverized, and the second particles 2 (fine particles) may not be generated. [Example]

[0043] -Production of positive electrode active material- No.1 A raw material solution was prepared 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=8 / 1 / 1. The concentration of the raw material solution was 30% by mass.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] The mixture was subjected to heat treatment in a muffle furnace to synthesize a lithium metal composite oxide. The heat treatment conditions were as follows. After the heat treatment, the lithium metal composite oxide was crushed using a jet mill to produce a positive electrode active material.

[0048] Atmosphere: Oxygen atmosphere Temperature: 800 to 1100°C Duration: 10 hours

[0049] No.2 to No.6 Lithium metal composite oxides were synthesized in the same manner as in No. 1. A planetary ball mill ("P-5", manufactured by Fritsch) and balls (made of ZrO2, diameter: 5 mm) were prepared. Figure 3 is a table showing the experimental results. The lithium metal composite oxides were milled under the milling conditions shown in Figure 2 to produce the respective positive electrode active materials.

[0050] -evaluation- A cylindrical lithium-ion secondary battery (evaluation cell) was manufactured. The evaluation cell had the following configuration:

[0051] 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)

[0052] 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.

[0053] -evaluation- The initial resistance of the evaluation cell was measured. The initial resistance in Figure 2 is a relative value when the initial resistance of No. 1 is considered to be 100%. In No. 1, no secondary particles (fine particles of 50 nm or less) were observed. In Nos. 2 and 3, secondary particles adhering to the surface of the primary particles were observed. The amount of adhering secondary particles was 0.24 particles / μm 2 When this is the case or more, the initial resistance tends to decrease. [Explanation of symbols]

[0054] 1 first particle, 2 second particle, 5 positive electrode active material.

Claims

1. comprising first particles and second particles; The first particles have a maximum Feret's diameter of 1 μm or more, the second particles have a maximum Feret's diameter of 50 nm or less; the second particles are attached to the surfaces of the first particles, and The amount of the second particles attached is 1 μm 2 0.24 or more per Cathode active material.

2. The second particles have a maximum Feret's diameter of 14 to 21 nm. The positive electrode active material according to claim 1 .

3. The amount of the second particles attached is 1 μm 2 3.2 or less per The positive electrode active material according to claim 1 or 2.

4. The first particles have a maximum Feret's diameter of 20 μm or less. The positive electrode active material according to claim 1 or 2.

5. the second particles have the same chemical composition as the first particles; The positive electrode active material according to claim 1 or 2.

Images