Positive electrode active material

Abstract

To improve the durability of a positive electrode active material.SOLUTION: A positive electrode active material includes crystallites. The crystallites exist independently or form a secondary particle. The secondary particle includes the crystallites of 2 to 20. Each of the crystallites has a maximum Feret's diameter of 1 μm or more. An open pore is formed in at least one of the crystallite and the secondary particle. The open pore has an opening diameter of 10 to 500 nm. A relationship of 0.006≤L / D<0.500 is satisfied. The L represents a path length of the opening pore. When the crystallites exist independently, the D represents a maximum Feret's diameter of the crystallites. When the crystallites form the secondary particle, the D represents a maximum Feret's diameter of the secondary particle.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] WO 2022 / 137360 discloses a lithium composite oxide sintered plate having a specific range of porosity, average pore diameter, and interface length. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2022 / 137360 Summary of the Invention [Problem to be solved by the invention]

[0004] The positive electrode active material contains crystallites. The crystallites may form secondary particles. Repeated charge and discharge cycles cause the crystallites to repeatedly expand and contract. The volume change of the crystallites can cause strain to accumulate within the crystallites. The accumulation of strain can cause cracks to occur within the crystallites. The occurrence of cracks exposes new surfaces. It is believed that the reaction of the new surfaces with the electrolyte accelerates the decrease in capacity. In other words, it is believed that durability is reduced.

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

[0006] 1. The positive electrode active material contains crystallites. The crystallites exist independently or form secondary particles. The secondary particles contain 2 to 20 crystallites. Each of the crystallites has a maximum Feret diameter of 1 μm or more. Open pores are formed in at least one of the crystallites and the secondary particles. The open pores have an opening diameter of 10 to 500 nm. The relationship "0.006≦L / D<0.500" is satisfied. "L" indicates the path length of the open pores. When the crystallites exist independently, "D" indicates the maximum Feret diameter of the crystallites. When the crystallites form secondary particles, "D" indicates the maximum Feret diameter of the secondary particles.

[0007] In the present disclosure, shallow micropores are formed in at least one of the crystallites and secondary particles. That is, the opening diameter of the open pores is 10 to 500 nm. Furthermore, the ratio (L / D) of the path length of the open pores to the maximum Feret diameter of the secondary particles (or crystallites) is 0.06 or more and less than 0.500. The shallow micropores can mitigate volumetric changes of the crystallites. By reducing the accumulation of strain, improved durability is expected. Note that if the ratio (L / D) is less than 0.006, the volumetric changes of the crystallites may not be sufficiently mitigated. If the ratio (L / D) is 0.500 or more, durability may actually decrease. This is thought to be because the deep micropores separate the particles, narrowing the buffer zone that can absorb volumetric changes.

[0008] 2. The positive electrode active material described in the above item "1" may include, for example, the following configuration: The relationship "0.010≦L / D≦0.490" is satisfied.

[0009] 3. The positive electrode active material described in the above item "1" or "2" may include, for example, the following configuration: The open pores are open to the surface of the crystallite.

[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 open pores are open between the crystallites.

[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 open pores extend linearly.

[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 showing a positive electrode active material according to the present embodiment. [Figure 2] FIG. 2 is a conceptual diagram showing a central region and a peripheral region in the present embodiment. [Figure 3] 1 is a schematic flowchart of a method for producing a positive electrode active material according to the present embodiment. [Figure 4] FIG. 10 is a schematic diagram showing an example of a jig. [Figure 5] 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. All numerical values ​​may be approximate values ​​that may vary depending on the application of the subject 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.

[0017] For example, the phrase "at least one of A and B" includes "A or B" as well as "A and B." "At least one of A and B" can also be written as "A and / or B."

[0018] "Crystallite" refers to the smallest unit of a particle, a solid particle with boundaries between particles that cannot be further subdivided. "Secondary particle" refers to an aggregate of two or more crystallites. The crystallites that form a secondary particle may also be referred to as primary particles.

[0019] FIG. 1 is a conceptual diagram illustrating a positive electrode active material according to this embodiment. The "maximum Feret diameter (D1) of crystallite 1," "maximum Feret diameter (D2) of secondary particle 2," "opening diameter (d) of open pore 3," and "path length (L) of open pore 3" are all measured in a cross-sectional SEM (Scanning Electron Microscope) image of the secondary particle 2. The observation magnification can be adjusted depending on the particle size. The observation magnification may be, for example, approximately 1000x. A cross-sectional sample of the particle can be prepared by a conventionally known method. For example, a cross-sectional sample may be prepared using a cross-section polisher (CP) or a focused ion beam (FIB). Various dimensions within the image are measured using image analysis software. For example, "ImageJ Fiji" or the like may be used. Note that "ImageJ Fiji" is merely an 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.

[0020] The "maximum Feret diameter" refers to the distance between the two most distant points on the particle contour in an SEM image. When crystallite 1 exists alone, the "maximum Feret diameter (D)" refers to the "maximum Feret diameter (D1) of crystallite 1." When crystallite 1 forms secondary particle 2, the "maximum Feret diameter (D)" refers to the "maximum Feret diameter (D2) of secondary particle 2."

[0021] In the cross-sectional SEM image of the secondary particle 2, the surface of the secondary particle 2 is observed. The pores that are in communication with the outside air are "open pores 3." The diameter of the opening of the open pore 3 is the "opening diameter (d)." "Communicating pores" refer to open pores 3 that have multiple openings. In the case of communicating pores, the arithmetic mean value of the multiple opening diameters is considered to be the "opening diameter (d)."

[0022] When multiple open pores 3 are formed within a secondary particle 2, the longest path length is considered to be the "path length (L)." When the open pores branch or merge, the sum of all path lengths is considered to be the "path length (L)."

[0023] The "linearity" of the open pores 3 is calculated by the following formula: The closer the linearity is to 1, the more linear the open pores are considered to be. S=L0 / L1 S:Straightness L0: Path length of open pore 3 in cross-sectional SEM image L1: Linear distance between the openings of the open pores 3 in the cross-sectional SEM image (linear distance between the inlet and outlet)

[0024] 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.

[0025] -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.

[0026] 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, 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. The D50 can be measured, for example, by laser diffraction or the like.

[0027] -Crystallite- As shown in FIG. 1, the positive electrode active material includes a crystallite 1. The crystallite 1 may exist singly. The crystallite 1 has a maximum Feret diameter of 1 μm or more. The maximum Feret diameter (D1) of the crystallite 1 may be, for example, 1.5 μm or more, 2 μm or more, or 2.5 μm or more. The maximum Feret diameter (D1) of the crystallite 1 may be, for example, 3 μm or less, 2.5 μm or less, 2 μm or less, or 1.5 μm or less.

[0028] The crystallite 1 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. The "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.

[0029] -Secondary particles- The crystallites 1 may form secondary particles 2. The secondary particles 2 include 2 to 20 crystallites 1. If the number of crystallites 1 in the secondary particles 2 exceeds 20, for example, durability may be reduced. The number of crystallites 1 included in the secondary particles 2 may be, for example, 15 or less, 10 or less, or 5 or less. The number of crystallites 1 included in the secondary particles 2 may be, for example, 5 or more, 10 or more, or 15 or more.

[0030] The maximum Feret diameter (D2) of the secondary particles 2 may be, for example, 2 to 30 μm. The maximum Feret diameter (D2) of the secondary particles 2 may be, for example, 3 μm or more, 6 μm or more, 9 μm or more, 12 μm or more, 15 μm or more, 18 μm or more, 21 μm or more, 24 μm or more, or 27 μm or more. The maximum Feret diameter (D2) of the secondary particles 2 may be, for example, 27 μm or less, 24 μm or less, 21 μm or less, 18 μm or less, 15 μm or less, 12 μm or less, 9 μm or less, or 6 μm or less.

[0031] -Open Pores- Open pores 3 are formed in at least one of the crystallites 1 and the secondary particles 2. There may be a single open pore 3 or multiple open pores 3. One open pore 3 may branch. Multiple open pores 3 may merge. The number of open pores 3 contained in one secondary particle 2 may be, for example, 1 or more, 2 or more, 3 or more, 4 or more, or 5 or more. The number of open pores 3 may be, for example, 10 or less, 9 or less, 8 or less, 7 or less, 6 or less, 5 or less, or 4 or less.

[0032] The open pores 3 may be open to the surface of the crystallite 1, for example. The open pores 3 may penetrate the crystallite 1, for example. The open pores 3 may terminate within the crystallite 1, for example. The open pores 3 may open between the crystallites 1, for example. The open pores 3 may penetrate the secondary particle 2, for example. The open pores 3 may terminate within the secondary particle 2, for example. The open pores 3 may be interconnected holes, for example. The interconnected holes have multiple openings within the secondary particle 2 (or the crystallite 1). Within the secondary particle 2, the open pores 3 do not have to pass through the crystallite 1. For example, the open pores 3 may be gaps between the crystallites 1. When multiple open pores 3 are formed within one secondary particle 2, both open pores 3 that pass through the crystallite 1 and open pores 3 that are gaps between the crystallites 1 may be formed. One open pore 3 may include both a path that passes through the crystallites 1 and a path that passes through the gaps between the crystallites 1 .

[0033] The open pores 3 have an opening diameter (d). The opening diameter (d) is 10 to 500 nm. If the opening diameter (d) is less than 10 nm, the initial resistance may increase. If the opening diameter (d) is more than 500 nm, cracks may occur in the secondary particles 2, for example, starting from the open pores 3. The opening diameter (d) may be, for example, 25 nm or more, 50 nm or more, 75 nm or more, 100 nm or more, 150 nm or more, 200 nm or more, 250 nm or more, 300 nm or more, 350 nm or more, 400 nm or more, or 450 nm or more. The opening diameter (d) may be, for example, 450 nm or less, 400 nm or less, 350 nm or less, 300 nm or less, 250 nm or less, 200 nm or less, 150 nm or less, 100 nm or less, 75 nm or less, 50 nm or less, or 25 nm or less.

[0034] In this positive electrode active material, the relationship "0.006≦L / D<0.500" is satisfied. The ratio (L / D) is the ratio of the path length (L) of the open pores 3 to the maximum Feret diameter (D). The ratio (L / D) may be, for example, 0.010 or more, 0.025 or more, 0.050 or more, 0.075 or more, 0.086 or more, 0.100 or more, 0.200 or more, 0.300 or more, 0.400 or more, or 0.490 or more. The ratio (L / D) may be, for example, 0.490 or less, 0.400 or less, 0.300 or less, 0.200 or less, 0.100 or less, 0.086 or less, 0.075 or less, 0.050 or less, 0.025 or less, or 0.010 or less. In other words, the relationship "0.010≦L / D≦0.490" may be satisfied.

[0035] The open pores 3 may extend, for example, in a curved line. The open pores 3 may extend, for example, linearly. Having the open pores 3 linear may improve durability. The open pores 3 may have a linearity of, for example, 1 to 1.5. The linearity may be, for example, 1.4 or less, 1.3 or less, 1.2 or less, 1.15 or less, 1.10 or less, or 1.05 or less. The linearity may be, for example, 1.05 or more, 1.1 or more, 1.15 or more, or 1.2 or more. The linearly extending open pores 3 may be formed, for example, by the hole processing described below.

[0036] FIG. 2 is a conceptual diagram showing the central region and peripheral region in this embodiment. The cross section of the secondary particle 2 includes a central region 2c and a peripheral region 2p. The central region 2c has a shape similar to the outline of the secondary particle 2. The central region 2c shares a geometric center with the outline of the secondary particle 2. The maximum Feret diameter of the central region 2c is 0.5D. The remaining portion of the cross section of the secondary particle 2 excluding the central region 2c is the peripheral region 2p. The peripheral region 2p surrounds the central region 2c. Note that while FIG. 2 shows the cross section of the secondary particle 2, when a crystallite 1 is present alone, the cross section of the crystallite 1 is considered to include the central region 2c and the peripheral region 2p.

[0037] For example, the open pores 3 may extend toward the central region 2c. For example, the open pores 3 may extend toward the geometric center of the secondary particle 2 (or crystallite 1). For example, the open pores 3 may reach the central region 2c. For example, the open pores 3 may pass through the central region 2c. For example, the open pores 3 may not reach the central region 2c and may terminate within the peripheral region 2p. If the open pores 3 are appropriately shallow, durability may be improved. The maximum Feret diameter of the central region 2c may be, for example, 0.6D, 0.7D, 0.8D, or 0.9D. The larger the maximum Feret diameter of the central region 2c is set, the more likely the peripheral region 2p is limited to the particle surface.

[0038] -Chemical composition- Crystallite 1 may contain, for example, a lithium metal composite oxide. Crystallite 1 may be made of, for example, a lithium metal composite oxide. The lithium metal composite oxide may have, for example, 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).

[0039] The lithium metal composite oxide may have any chemical composition, for example, a composition represented by the following general formula: Li , z , x , y ,

[0042] , , 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.

[0040] ​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​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.

[0043] -Method of manufacturing positive electrode active material- 3 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 method." This method includes "(a) preparation of metal hydroxide," "(b) mixing," "(c) heat treatment," "(d) crushing," and "(e) hole processing."

[0044] -(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.

[0045] -(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.

[0046] "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.

[0047] -(c) Heat treatment- This production method involves synthesizing a lithium metal composite oxide by heat treating the mixture in an oxygen atmosphere. In this production method, any heat treatment device or calcination furnace can be used. For example, a muffle furnace, an electric furnace, or the like may be used.

[0048] 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.

[0049] -(d) Crushing- The present manufacturing method includes forming crystallites by crushing a lithium metal composite oxide. The level of agglomeration of the crystallites can be adjusted by crushing. Crushing may be performed so that the crystallites exist independently. Crushing may be performed so that the crystallites form secondary particles. For example, crushing may be performed using a crusher. Any crusher (e.g., a jet mill, etc.) can be used.

[0050] -(e) Hole processing- This manufacturing method involves forming open pores by pressing a needle-shaped jig into at least one of the crystallites and the secondary particles. FIG. 4 is a schematic diagram showing an example of the jig. The pin-holder jig 10 includes a plurality of needle-shaped jigs 11. The plurality of needle-shaped jigs 11 are arranged at a predetermined pitch (p). The pitch (p) may be, for example, 5 to 15 μm. The needle-shaped jig 11 includes a tip portion 11a and a body portion 11b. The tip portion 11a may be continuous with the body portion 11b. The tip portion 11a may be tapered, for example. The length of the tip portion 11a may be, for example, about 10 μm. The body diameter of the tip portion 11a may be, for example, 1 μm or less. The tip diameter (φ) may be, for example, 10 to 500 nm. The tip diameter (φ) may be, for example, 50 nm or more. The tip diameter (φ) may be, for example, 400 nm or less, 300 nm or less, or 200 nm or less.

[0051] For example, by pressing the powder using the pin holder jig 10 as the upper mold, open pores can be formed in at least one of the crystallites and secondary particles. For example, pressing may be performed using a small hydraulic press for tablet molding. The pressing pressure may be, for example, 0.1 to 10 MPa. The pressing pressure may be, for example, 0.5 MPa or more, 1.0 MPa or more, 2.5 MPa or more, 5.0 MPa or more, or 7.5 MPa or more. The pressing pressure may be, for example, 7.5 MPa or less, 5.0 MPa or less, 2.5 MPa or less, 1.0 MPa or less, or 0.5 MPa or less. [Example]

[0052] -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.

[0053] 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.

[0054] 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.

[0055] In a mortar, the dried material (metal hydroxide) and a lithium compound (Li2CO3) were mixed to form a mixture.

[0056] The mixture was subjected to heat treatment in a muffle furnace to synthesize a lithium metal composite oxide under the following heat treatment conditions:

[0057] Atmosphere: Oxygen atmosphere Temperature: 800 to 1100°C Duration: 10 hours

[0058] After the heat treatment, the lithium metal composite oxide was crushed in a jet mill, thereby producing a positive electrode active material.

[0059] No.2 Lithium metal composite oxide was synthesized in the same manner as in No. 1. The lithium metal composite oxide was crushed to prepare powder. A needle-like jig was prepared. The tip diameter of the needle-like jig was 100 nm. The length of the tip was 10 μm. The body diameter of the tip was less than 1 μm. A mini hydraulic press for tablet molding manufactured by ST JAPAN was prepared. The powder was filled into the lower die of the die set. A needle-like jig was attached to the upper die of the die set. Holes were drilled into the powder using a pressing pressure of P0.

[0060] No.3 to No.6 Positive electrode active materials were prepared in the same manner as No. 2, except that the indentation pressure was changed. The indentation pressure was changed in the order of No. 2 (minimum) to No. 6 (maximum).

[0061] -evaluation- A cylindrical lithium-ion secondary battery (evaluation cell) was manufactured. The evaluation cell had the following configuration:

[0062] 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)

[0063] 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.

[0064] 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.

[0065] -result- Figure 5 is a table showing the experimental results. When the relationship "0.006≦L / D<0.500" is satisfied, durability tends to improve. The smaller the ratio (L / D), the more durability tends to improve. [Explanation of symbols]

[0066] 1 crystallite, 2 secondary particle, 2c central region, 2p peripheral region, 3 open pore, 10 pinholder jig, 11 needle-shaped jig, 11a tip, 11b body.

Claims

1. Contains crystallites, The crystallites exist independently or form secondary particles, the secondary particles contain 2 to 20 of the crystallites, Each of the crystallites has a maximum Feret diameter of 1 μm or more, open pores are formed in at least one of the crystallites and the secondary particles, the open pores have an opening diameter of 10 to 500 nm, 0.006≦L / D<0.500 The relationship is satisfied, L represents the path length of the open pore, When the crystallite exists alone, D indicates the maximum Feret diameter of the crystallite, and When the crystallites form the secondary particles, D represents the maximum Feret diameter of the secondary particles; Cathode active material.

2. 0.010≦L / D≦0.490 The relationship is satisfied The positive electrode active material according to claim 1 .

3. The open pores are open to the surface of the crystallite. The positive electrode active material according to claim 1 .

4. The open pores are open between the crystallites. The positive electrode active material according to claim 1 .

5. The open pores extend linearly. The positive electrode active material according to claim 1 .

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