Positive electrode for lithium-ion secondary batteries

Abstract

Reduction of initial resistance. [Solution] The positive electrode for a lithium-ion secondary battery includes a positive electrode active material layer. The positive electrode active material layer includes a positive electrode active material. The positive electrode active material includes single crystal particles. In a scanning electron microscope image of a cross-section of the positive electrode active material layer, 100 randomly selected single crystal particles include at least one first particle having a concave portion and at least one second particle having a convex portion. The relationships "0.01≦d / D1≦0.56" and "0.01≦h / D2≦0.58" are satisfied. "d" indicates the depth of the concave portion in the first particle. "D1" indicates the diameter of the smallest circumscribed circle of the first particle. "h" indicates the height of the convex portion in the second particle. "D2" indicates the diameter of the smallest circumscribed circle of the second particle.

Description

[Technical Field]

[0001] This disclosure relates to a positive electrode for lithium-ion secondary batteries. [Background technology]

[0002] Japanese Patent Publication No. 2023-036570 discloses a large-grain aggregate ternary cathode material having a single-crystal-like morphology. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2023-036570 [Overview of the project] [Problems that the invention aims to solve]

[0004] Single crystallization of the positive electrode active material has been proposed. Single-crystal particles, compared to polycrystalline particles, have a smaller specific surface area, which is expected to improve cycle characteristics, for example. However, there is room for improvement in initial resistance.

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

[0006] 1. The positive electrode for a lithium-ion secondary battery includes a positive electrode active material layer. The positive electrode active material layer includes positive electrode active material. The positive electrode active material includes single-crystal particles. In a scanning electron microscope image of a cross-section of the positive electrode active material layer, 100 randomly selected single-crystal particles each contain at least one first particle with a concave portion and at least one second particle with a convex portion. The relationships "0.01≦d / D1≦0.56" and "0.01≦h / D2≦0.58" are satisfied. "d" indicates the depth of the concave portion in the first particle. "D1" indicates the diameter of the smallest circumscribed circle of the first particle. "h" indicates the height of the convex portion in the second particle. "D2" indicates the diameter of the smallest circumscribed circle of the second particle.

[0007] Typically, single-crystal particles have smooth surfaces. In the positive electrode active material layer, single-crystal particles can be densely packed. It is thought that gaps for electrolyte diffusion are unlikely to form between the single-crystal particles. As a result, the initial resistance may increase.

[0008] The positive electrode active material layer in this disclosure includes single crystal particles having a specific shape. That is, the positive electrode active material layer includes first particles and second particles. The recesses of the first particles and the protrusions of the second particles can form gaps into which the electrolyte can diffuse within the positive electrode active material layer. Furthermore, if the recesses and protrusions are of an appropriate size relative to the size of the single crystal particles, the diffusion of the electrolyte can be promoted. As a result, a reduction in initial resistance can be expected. Hereinafter, "positive electrode for lithium-ion secondary battery" may be abbreviated as "positive electrode".

[0009] 2. The positive electrode for the lithium-ion secondary battery described in "1" above may include, for example, the following configuration: 100 randomly selected single crystal particles each contain one or more third particles including concave and convex portions.

[0010] A single crystal grain may have both concave and convex portions.

[0011] 3. The positive electrode for lithium-ion secondary batteries described in "1" or "2" above may include, for example, the following configuration: The positive electrode active material consists of 50% or more single-crystal particles and the remainder being polycrystalline particles.

[0012] A higher percentage of single-crystal grains is expected to lead to improvements in properties such as cycle characteristics.

[0013] 4. The positive electrode for a lithium-ion secondary battery described in any one of items "1" to "3" above may include, for example, the following configuration: The positive electrode active material includes a lithium transition metal composite oxide.

[0014] 5. The positive electrode for a lithium-ion secondary battery according to any one of the above "1" to "4" may include, for example, the following configuration. The positive electrode active material has a composition represented by the general formula "Li [Figure 5] , [Figure 4] , [Figure 3] , , [Figure 6] , , , , , Ni a Co b Mn c O y ". In the general formula, "x, a, b, c, y" satisfy the relationships of "0.1 ≦ x ≦ 1.5", "0.5 ≦ a ≦ 1.0", "0 ≦ b ≦ 0.3", "0 ≦ c ≦ 0.3", "a + b + c = 1.0" and "1.5 ≦ y ≦ 2.1".

[0015] When the Ni composition ratio "a" is 0.5 or more, for example, an increase in the initial discharge capacity is expected.

[0016] Hereinafter, an embodiment of the present disclosure (hereinafter may be abbreviated as "this embodiment"), and an example of the present disclosure (hereinafter may be abbreviated as "this example") will be described. However, this embodiment and this example do not limit the technical scope of the present disclosure. This embodiment and this example are illustrative in all respects. This embodiment and this example are non-restrictive. The technical scope of the present disclosure includes all modifications within the meaning and scope equivalent to the description of the claims. For example, any configuration is extracted from this embodiment, and their arbitrary combinations are also initially planned.

Brief Description of Drawings

[0017] [Figure 1] It is a conceptual diagram of the cross-section of the first particle. [Figure 2] It is a conceptual diagram of the cross-section of the second particle. [Figure 3] It is a schematic cross-sectional view of the positive electrode for a lithium-ion secondary battery in this embodiment. [Figure 4] It is a table showing experimental results. [Figure 5] It is the first temperature profile. [Figure 6] It is the second temperature profile.

Modes for Carrying Out the Invention

[0018] Terms, phrases "Equipped with," "includes," "possesses," and variations thereof are open-ended expressions. Configurations expressed in an open-ended manner may or may not include additional elements in addition to the essential elements. The statement "consists of" is a closed expression. However, even configurations expressed in a closed manner may include additional elements that are usually incidental impurities or irrelevant to the subject technology. The statement "substantially consists of..." is a semi-closed expression. In configurations expressed in a semi-closed manner, the addition of elements that do not substantially affect the basic and novel characteristics of the subject technology is permitted.

[0019] For example, the expression "at least one of A and B" includes both "A or B" and "A and B". "At least one of A and B" can also be written as "A and / or B".

[0020] Geometric terms should not be interpreted strictly. Examples of geometric terms include "parallel," "perpendicular," and "orthogonal." For example, direction, angle, distance, etc., may be relatively distorted within a range where substantially the same or similar function is obtained. Geometric terms may include tolerances, errors, etc., in design, operation, and manufacturing. Dimensional relationships in each figure may not match actual dimensional relationships. Dimensional relationships in each figure may be modified to aid the reader's understanding. For example, length, width, thickness, etc., may be changed. Some components may be omitted.

[0021] Numerical values ​​may be displayed with significant figures. Unless otherwise specified, measured values ​​may be the average of multiple measurements. The number of measurements may be three or more, five or more, or ten or more. Generally, the reliability of the average is expected to improve with a larger number of measurements. Measured values ​​may be rounded to the nearest significant figure. Measured values ​​may include errors, such as those associated with the detection limits of the measuring device.

[0022] The devices, software, etc., used for measuring various values ​​are merely examples. Equivalent devices may be used. If equivalent devices are used, the measurement conditions may be adjusted to suit the device.

[0023] The first and second particles can be identified in a scanning electron microscope (SEM) image of the cross-section of the positive electrode active material layer. Hereinafter, the SEM image of the cross-section of the positive electrode active material layer may be abbreviated as "cross-sectional SEM image". Five samples are taken from the positive electrode. The sampling locations may be dispersed throughout the positive electrode. The sampling locations may be equally spaced. For example, a smooth surface is formed by cross-sectional processing of the sample using a cross-section polisher or the like. A cross-sectional SEM image is obtained by observing the smooth surface with an SEM. The imaging magnification may be adjusted according to the particle size. The imaging magnification may be, for example, 5000x to 50000x. 100 single crystal particles are randomly selected from the five cross-sectional SEM images. The first and second particles are selected from the 100 single crystal particles.

[0024] Figure 1 is a conceptual diagram of the cross-section of the first particle. The first particle 10 includes a recess 11. The recess 11 indicates a portion where the particle's contour line is recessed inward. The depth "d" of the recess 11 is determined by the following procedure. Various dimensional measurements and shape analyses in the cross-sectional SEM image may be performed using, for example, image processing software such as "ImageJ". (1) The minimum circumscribed circle (MCC) is identified for the particle contour line (P). The diameter of the MCC is diameter (D1). (2) A line segment "L" is drawn that extends from the perimeter of the MCC toward the center of the MCC and reaches point P. (3) Of the intersections of L and P, the intersection where L is the longest is considered to be the bottom point "X" of the concave section. (4) A double tangent line "T" is identified that has tangency points "A" and "B" on both sides of the base point. (5) The distance between T and X is considered to be the depth "d" of the recess. The length of the line segment "AB" is considered to be the width "w1" of the recess.

[0025] Figure 2 is a conceptual diagram of the cross-section of the second particle. The second particle 20 includes a convex portion 21. The convex portion 21 indicates the part of the particle's contour line that protrudes outward. The height "h" of the convex portion 21 is determined by the following procedure. (1) The maximum inscribed circle "MIC" is identified for the particle contour line "P". (2) A line segment "L" is drawn that extends radially from the circumference of MIC and reaches P. (3) Of the intersections of L and P, the intersection where L is the longest is considered to be the vertex "Y" of the convex part. (4) Of the contact points between MIC and P, the contact points "A" and "B" that are closest to vertex "Y" are identified. (5) The distance between line segment "AB" and vertex "Y" is considered to be the height "h" of the convexity. The length of line segment "AB" is considered to be the width "w2" of the convexity. (6) The MCC for P is identified. The diameter of the MCC is diameter "D2".

[0026] A "single-crystal particle" refers to a solid particle that is the smallest unit of particle and is recognized as being unable to be divided any further. In a cross-sectional SEM image, a single-crystal particle appears to have no grain boundaries. Single-crystal particles are also referred to as primary particles. An aggregate of two or more primary particles is considered a "polycrystalline particle." In the above cross-sectional SEM image, 100 particles are randomly extracted. By counting the number of single-crystal particles contained in these 100 particles, the proportion of single-crystal particles is determined.

[0027] "Maximum Ferret diameter" refers to the length of the longest side of the minimum bounding rectangle (MBR) relative to the particle's contour in the SEM image. If the MBR is square, the length of one side of the MBR is considered the length of the longest side. The maximum Ferret diameter is the average value of 100 particles.

[0028] The chemical composition of the positive electrode active material can be measured by ICP-AES (Inductively Coupled Plasma Atomic Emission Spectroscopy). A sample solution is prepared by dissolving 0.1 g of the sample (positive electrode active material) in a mixed acid (10 ml) of hydrochloric acid and sulfuric acid. The sample solution is diluted to an appropriate concentration in a volumetric flask. After dilution, compositional analysis is performed using an ICP-AES instrument. For example, a product such as "PS3520 UVDD II (manufactured by Hitachi High-Tech Science Corporation)" may be used.

[0029] Positive electrode for lithium-ion secondary batteries Figure 3 is a schematic cross-sectional view of the positive electrode for a lithium-ion secondary battery in this embodiment. The positive electrode 100 is for a lithium-ion secondary battery. The lithium-ion secondary battery may be an electrolyte battery or an all-solid-state battery. The lithium-ion secondary battery may have any structure. The lithium-ion secondary battery may have, for example, a wound type or a stacked type power generation element. The lithium-ion secondary battery may have, for example, a unipolar structure or a bipolar structure.

[0030] The positive electrode 100 may be, for example, in the form of a sheet. The positive electrode 100 includes a positive electrode active material layer 102. The positive electrode 100 may further include a positive electrode substrate 101. The positive electrode substrate 101 can support the positive electrode active material layer 102. The positive electrode active material layer 102 may be formed on only one side of the positive electrode substrate 101. The positive electrode active material layer 102 may be formed on both sides of the positive electrode substrate 101. The positive electrode substrate 101 may function as a current collector. The positive electrode substrate 101 may include, for example, Al foil. The positive electrode substrate 101 may have a thickness of, for example, 5 μm to 50 μm.

[0031] The positive electrode active material layer 102 may have a thickness of, for example, 10 μm to 1000 μm. The positive electrode active material layer 102 contains positive electrode active material. In addition to positive electrode active material, the positive electrode active material layer 102 may further contain, for example, a conductive material, a binder, etc. The positive electrode active material layer may consist, for example, 0.1% to 10% by mass fraction of optional components (additives, etc.), 0.1% to 10% of a binder, 0.1% to 10% of a conductive material, and the remainder being positive electrode active material.

[0032] The positive electrode active material contains single-crystal particles. The positive electrode active material may further contain polycrystalline particles. The polycrystalline particles may have substantially the same composition and crystal structure as the single-crystal particles. The positive electrode active material may consist, for example, of 50% or more single-crystal particles and the remainder being polycrystalline particles. By having a single-crystal particle ratio of 50% or more, improvements in cycle characteristics can be expected. The single-crystal particle ratio may be, for example, 60% or more, 70% or more, 80% or more, 90% or more, or 95% or more. The single-crystal particle ratio may be, for example, 95% or less, 90% or less, 80% or less, 70% or less, or 60% or less.

[0033] In the cross-sectional SEM image of the positive electrode active material layer 102, the maximum Ferret diameter of the single crystal grains may be, for example, 0.1 μm or more, 0.5 μm or more, 1 μm or more, 2 μm or more, 3 μm or more, 4 μm or more, 5 μm or more, 6 μm or more, 7 μm or more, 8 μm or more, or 9 μm or more. The maximum Ferret diameter of the single crystal grains may also be, for example, 10 μm or less, 9 μm or less, 8 μm or less, 7 μm or less, 6 μm or less, 5 μm or less, 4 μm or less, 3 μm or less, 2 μm or less, or 1 μm or less.

[0034] In a cross-sectional SEM image of the positive electrode active material layer 102, 100 randomly selected single crystal particles each contain one or more first particles 10 and one or more second particles 20. The first particles 10 include a recess 11. The second particles 20 include a protrusion 21. The presence of both first particles 10 and second particles 20 within the positive electrode active material layer 12 can promote the diffusion of the electrolyte.

[0035] The recess 11 contained in the first particle 10 has a specific depth. That is, the relationship "0.01 ≤ d / D1 ≤ 0.56" is satisfied. The ratio of the depth of the recess 11 to the particle diameter "d / D1" may be, for example, 0.02 or more, 0.03 or more, 0.04 or more, 0.05 or more, 0.10 or more, 0.20 or more, 0.30 or more, 0.40 or more, 0.45 or more, 0.50 or more, or 0.56 or more. The ratio of the depth of the recess 11 to the diameter "d / D1" may be, for example, 0.70 or less, 0.60 or less, 0.56 or less, 0.50 or less, 0.45 or less, 0.40 or less, 0.30 or less, 0.20 or less, 0.10 or less, 0.05 or less, 0.04 or less, 0.03 or less, or 0.02 or less.

[0036] The width of the recess 11 may have a specific size. For example, the relationship "w1 / D1 ≤ 0.50" may be satisfied. The ratio of the width of the recess 11 to the particle diameter "w1 / D1" may be, for example, 0.40 or less, 0.30 or less, 0.20 or less, 0.10 or less, or 0.05 or less. The ratio of the width of the recess 11 to the particle diameter "w1 / D1" may be, for example, 0.01 or more, 0.03 or more, 0.05 or more, 0.10 or more, 0.20 or more, 0.30 or more, or 0.40 or more.

[0037] The first particle 10 may have a single recess 11. The first particle 10 may have multiple recesses 11. The number of recesses 11 contained in the first particle 10 may be, for example, 1 or more, 2 or more, 3 or more, 4 or more, or 5 or more. The number of recesses 11 contained in the first particle 10 may be, for example, 10 or less, 9 or less, 8 or less, 7 or less, 6 or less, 5 or less, 4 or less, 3 or less, or 2 or less. If the first particle 10 has multiple recesses 11, the depth "d" and width "w" are taken from the values ​​of the deepest recess 11.

[0038] The protrusions 21 contained in the second particle 20 have a specific height. That is, the relationship "0.01 ≤ h / D2 ≤ 0.58" is satisfied. The ratio of the height of the protrusions 21 to the particle diameter "h / D2" may be, for example, 0.02 or more, 0.03 or more, 0.04 or more, 0.05 or more, 0.10 or more, 0.20 or more, 0.30 or more, 0.40 or more, 0.50 or more, 0.52 or more, 0.58 or more, or 0.60 or more. The ratio of the height of the protrusions 21 to the particle diameter "h / D2" may also be, for example, 0.70 or less, 0.60 or less, 0.58 or less, 0.52 or less, 0.50 or less, 0.40 or less, 0.30 or less, 0.20 or less, 0.10 or less, 0.05 or less, 0.04 or less, 0.03 or less, or 0.02 or less.

[0039] The width of the protrusion 21 may have a specific size. For example, the relationship "w2 / D1 ≤ 0.50" may be satisfied. The ratio of the width of the protrusion 21 to the particle diameter "w2 / D2" may be, for example, 0.40 or less, 0.30 or less, 0.20 or less, 0.10 or less, or 0.05 or less. The ratio of the width of the protrusion 21 to the particle diameter "w2 / D2" may be, for example, 0.01 or more, 0.03 or more, 0.05 or more, 0.10 or more, 0.20 or more, 0.30 or more, or 0.40 or more.

[0040] The number of first particles 10 contained in 100 single crystal particles may be, for example, 2 or more, 5 or more, 10 or more, 20 or more, 30 or more, 40 or more, 50 or more, 60 or more, 70 or more, 80 or more, or 90 or more. The number of first particles 10 contained in 100 single crystal particles may be, for example, 90 or less, 80 or less, 70 or less, 60 or less, 50 or less, 40 or less, 30 or less, 20 or less, 10 or less, 5 or less, or 2 or less.

[0041] The number of the second particles 20 contained in 100 single crystal particles may be, for example, 2 or more, 5 or more, 10 or more, 20 or more, 30 or more, 40 or more, 50 or more, 60 or more, 70 or more, 80 or more, or 90 or more. The number of the second particles 20 contained in 100 single crystal particles may be, for example, 90 or less, 80 or less, 70 or less, 60 or less, 50 or less, 40 or less, 30 or less, 20 or less, 10 or less, 5 or less, or 2 or less.

[0042] The 100 single crystal particles randomly extracted may contain a third particle. The third particle includes both the concave portion 11 and the convex portion 21 described above. The number of the third particles contained in 100 single crystal particles may be, for example, 2 or more, 5 or more, 10 or more, 20 or more, 30 or more, 40 or more, 50 or more, 60 or more, 70 or more, 80 or more, or 90 or more. The number of the third particles contained in 100 single crystal particles may be, for example, 90 or less, 80 or less, 70 or less, 60 or less, 50 or less, 40 or less, 30 or less, 20 or less, 10 or less, 5 or less, or 2 or less. Incidentally, the second particle 20 in FIG. 2 is also a third particle that includes both the concave portion 11 and the convex portion 21.

[0043] The positive electrode active material may contain, for example, a lithium transition metal composite oxide. The positive electrode active material may have a crystal structure belonging to the space group R-3m. The crystal structure is also referred to as a layered structure. The crystal structure can be specified by an XRD (X-ray Diffraction) pattern. The lithium transition metal composite oxide contains Li, a transition metal, and oxygen.

[0044] The positive electrode active material is, for example, of the general formula "Li x Ni a Co b Mn c O yIt may have a composition represented by the formula ". In the general formula, the Li composition ratio "x" may satisfy, for example, the relationship "0.1 ≤ x ≤ 1.5". The Li composition ratio "x" may be, for example, 0.4 or more, 0.6 or more, 0.8 or more, 1.0 or more, 1.2 or more, or 1.4 or more. The Li composition ratio "x" may be, for example, 1.4 or less, or 1.2 or less.

[0045] In the general formula above, the O composition ratio "y" may satisfy, for example, the relationship "1.5 ≤ y ≤ 2.1". The O composition ratio "y" may be, for example, 1.6 or more, 1.7 or more, 1.8 or more, 1.9 or more, or 2.0 or more. The O composition ratio "y" may be, for example, 2.0 or less, 1.9 or less, 1.8 or less, 1.7 or less, or 1.6 or less.

[0046] In the above general formula, the Ni composition ratio "a", Co composition ratio "b", and Mn composition ratio "c" may satisfy the relationship "a + b + c = 1.0". The Ni composition ratio "a" may, for example, satisfy the relationship "0.5 ≤ a ≤ 1.0". The Ni composition ratio "a" may, for example, be 0.6 or more, 0.7 or more, 0.8 or more, or 0.9 or more. The Ni composition ratio "a" may, for example, be 0.9 or less, 0.8 or less, 0.7 or less, or 0.6 or less.

[0047] In the general formula above, the Co composition ratio "b" may satisfy, for example, the relationship "0 ≤ b ≤ 0.3". The Co composition ratio "b" may be, for example, 0.01 or more, 0.02 or more, 0.03 or more, 0.04 or more, 0.05 or more, 0.06 or more, 0.07 or more, 0.08 or more, 0.09 or more, 0.10 or more, 0.15 or more, 0.20 or more, or 0.25 or more. The Co composition ratio "b" may be, for example, 0.25 or less, 0.20 or less, 0.15 or less, 0.10 or less, 0.09 or less, 0.08 or less, 0.07 or less, 0.06 or less, 0.05 or less, 0.04 or less, 0.03 or less, 0.02 or less, or 0.01 or less.

[0048] In the above general formula, the Mn composition ratio "c" may satisfy, for example, the relationship "0 ≤ c ≤ 0.3". The Mn composition ratio "c" may be, for example, 0.01 or more, 0.02 or more, 0.03 or more, 0.04 or more, 0.05 or more, 0.06 or more, 0.07 or more, 0.08 or more, 0.09 or more, 0.10 or more, 0.15 or more, 0.20 or more, or 0.25 or more. The Mn composition ratio "c" may be, for example, 0.25 or less, 0.20 or less, 0.15 or less, 0.10 or less, 0.09 or less, 0.08 or less, 0.07 or less, 0.06 or less, 0.05 or less, 0.04 or less, 0.03 or less, 0.02 or less, or 0.01 or less.

[0049] In addition, in the above general formula, all or part of Mn may be substituted with Al, etc. That is, lithium transition metal composite oxides are, for example, those with the general formula "Li x Ni a Co b Al c O y It may have a composition represented by ". The range of the Al composition ratio "c" is the same as that of the Mn composition ratio "c" above.

[0050] Lithium transition metal composite oxides may contain any dopant. The dopant represents an element other than Li, Ni, Co, Mn, and O. For example, the dopant may contain at least one selected from the group consisting of Zr, Mo, W, Mg, Ca, Na, Fe, Cr, Zn, Si, Sn, Al, and Ag. The composition ratio of the dopant may be, for example, 0.005 or more, 0.01 or more, 0.02 or more, 0.03 or more, or 0.04 or more. The composition ratio of the dopant may be, for example, 0.05 or less, 0.04 or less, 0.03 or less, 0.02 or less, or 0.01 or less. [Examples]

[0051] Sample preparation Figure 4 is a table showing the experimental results. Positive electrodes No. 1 to No. 5 were manufactured using the following procedure.

[0052] Preparation of the precursor The raw material solution is formed by dissolving NiSO4, CoSO4, and MnSO4 in deionized water. In the raw material solution, the molar ratio of Ni, Co, and Mn is "Ni / Co / Mn = 90 / 5 / 5". The solute concentration in the raw material solution is 30% (mass fraction).

[0053] Ammonia water is added to the reaction vessel. The ammonia water is stirred by a stirrer, and the inside of the reaction vessel is replaced with nitrogen. Then, NaOH is added to the reaction vessel, forming an alkaline reaction solution.

[0054] The reaction solution is dropped with the raw material solution and ammonia water to maintain a pH within a certain range, forming a precipitate (metal hydroxide). The reaction solution is filtered to recover the metal hydroxide. The metal hydroxide is dispersed in deionized water to form a dispersion. The dispersion is thoroughly stirred with a spatula, that is, the metal hydroxide is washed with water. After washing, the dispersion is filtered to recover the metal hydroxide. The metal hydroxide is dried at 120°C for 16 hours to form a dried product.

[0055] Adding Li sources In a mortar, a mixture is formed by mixing a dried material (metal hydroxide) and a lithium compound (LiOH, Li2CO3) using a pestle. By adding an excess of Li relative to the total amount of transition metals, a molten salt is formed during firing, and a lithium transition metal composite oxide can be single-crystallized. That is, the percentage of single-crystal particles can exceed 50%. The ratio of the amount of Li to the total amount of transition metals is, for example, 1.5 or more.

[0056] firing In a firing furnace (e.g., a muffle furnace), the mixture is subjected to firing (heat treatment) to synthesize a lithium transition metal composite oxide. The firing atmosphere is an oxygen atmosphere. Figure 5 shows the first temperature profile. In No. 1, firing is carried out according to the first temperature profile. The furnace temperature is raised to X°C, which is in the range of 700°C to 1100°C. X°C is substantially maintained for 10 hours. After 10 hours, the furnace temperature is lowered to room temperature.

[0057] Figure 6 shows the second temperature profile. For samples No. 2 to No. 5, firing is performed according to the second temperature profile. The furnace temperature is raised to X°C, which is within the range of 700°C to 1100°C. The temperature is then lowered to X-100°C at a rate of Y. Next, the temperature is raised to X°C at a rate of Y. The rate of cooling and heating "Y" is also referred to as the amplitude rate. The amplitude rate "Y" for each sample is shown in the amplitude rate section of Figure 4. Subsequently, cooling and heating are repeated alternately for 10 hours. After 10 hours, the furnace temperature is lowered to room temperature.

[0058] After firing, the particle size of the lithium transition metal composite oxide is adjusted using a pulverizer such as a jet mill. Thus, the positive electrode active material is manufactured.

[0059] Fabrication of the positive electrode A slurry is formed by mixing the positive electrode active material, conductive material (acetylene black), binder (polyvinylidene fluoride), and dispersion medium. The positive electrode is fabricated by coating the surface of the substrate (Al foil) with the slurry. An Allgood film applicator (with film thickness adjustment function) is used as the coating device. After coating with slurry, the coating is dried at 80°C for 5 minutes. A cross-sectional SEM image of the positive electrode is acquired. The presence or absence of the first particle (concave particle) and the second particle (convex particle), the depth of the concave part "d", the height of the convex part "h", and the particle diameters "D1, D2" are measured.

[0060] Measurement of initial resistance The initial resistance was measured using the following procedure. Laminate cells are fabricated. The composition of the laminate cells is as follows: Outer packaging: Pouch made of aluminum laminate film Power generation element: Stacked type (single layer) Positive electrode: positive electrode active material / conductive material / binder = 88 / 10 / 2 (mass ratio) Negative electrode: Negative electrode active material (natural graphite), CMC, SBR Electrolytes: LiPF6 (1 ml / L), EC / DMC / EMC = 3 / 4 / 3 (volume ratio)

[0061] The laminate cell is sandwiched between two stainless steel plates, thereby applying a predetermined pressure to the power generation element. The state of charge (SOC) of the laminate cell is adjusted to 50%. The IV resistance is measured in a -10°C temperature environment. The value shown in the "Initial Resistance" column of Figure 4 is a relative value, with the initial resistance value of No. 1 set to 100%.

[0062] Experimental results As shown in Figure 4, when the relationships "0.01≦d / D1≦0.56" and "0.01≦h / D2≦0.58" are satisfied, there is a tendency for the initial resistance to decrease. [Explanation of Symbols]

[0063] 10 First particle, 11 Recess, 20 Second particle, 21 Protrusion, 100 Positive electrode, 101 Positive electrode substrate, 102 Positive electrode active material layer.

Claims

1. It contains a positive electrode active material layer, The positive electrode active material layer includes a positive electrode active material, The positive electrode active material includes single crystal particles, In the scanning electron microscope image of the cross-section of the positive electrode active material layer, The 100 randomly selected single crystal particles each contain one or more first particles having a concave portion and one or more second particles having a convex portion. 0.01 ≤ d / D1 ≤ 0.56, and, 0.01 ≤ h / D² ≤ 0.58 The relationship is satisfied, The above d indicates the depth of the recess in the first particle, D1 indicates the diameter of the smallest circumscribed circle of the first particle. The aforementioned h indicates the height of the protrusion in the second particle, and, D2 indicates the diameter of the smallest circumscribed circle of the second particle. Positive electrode for lithium-ion secondary batteries.

2. One hundred randomly selected single crystal particles each contain one or more third particles including the concave and convex portions. The positive electrode for a lithium-ion secondary battery according to claim 1.

3. The positive electrode active material consists of 50% or more single crystal particles and the remainder being polycrystalline particles. A positive electrode for a lithium-ion secondary battery according to claim 1 or claim 2.

4. The positive electrode active material includes a lithium transition metal composite oxide. A positive electrode for a lithium-ion secondary battery according to claim 1 or claim 2.

5. The positive electrode active material has the general formula: Li x Ni a Co b Mn c O y It has a composition represented by, In the above general formula, x, a, b, c, and y satisfy the following relationships: 0.1 ≤ x ≤ 1.5, 0.5 ≤ a ≤ 1.0, 0 ≤ b ≤ 0.3, 0 ≤ c ≤ 0.3, a + b + c = 1.0, and 1.5 ≤ y ≤ 2.

1. The positive electrode for a lithium-ion secondary battery according to claim 4.

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