Cathode for lithium-ion secondary battery

Abstract

A cathode for a lithium-ion secondary battery comprises a layer of active cathode material. The active cathode material contains single-crystal particles. One hundred single-crystal particles, randomly selected from a scanning electron microscope image of a cross-section of the active cathode material layer, comprise at least one first particle with a depression and at least one second particle with a protrusion. The following relationships are satisfied: "0.01 ≤ d / D1 ≤ 0.56" and "0.01 ≤ h / D2 ≤ 0.58", where "d" is the depth of the depression of the first particle, "D1" is the diameter of the smallest circumscribing circle of the first particle, "h" is the height of the protrusion of the second particle, and "D2" is the diameter of the smallest circumscribing circle of the second particle.

Description

BACKGROUND OF THE INVENTION 1. Field of the invention

[0001] The present disclosure relates to cathodes for lithium-ion secondary batteries. 2. Description of the state of the art

[0002] The unpublished Japanese patent application No. 2023-036570 (JP 2023-036570 A) discloses a ternary cathode material with large grains and a single-crystal morphology. SUMMARY OF THE INVENTION

[0003] Single-crystallization of active cathode material has been proposed. Since single-crystal particles have a smaller specific surface area than polycrystalline particles, they are expected to improve cycle properties. However, there is still room for improvement regarding the initial resistance.

[0004] The aim of the present invention is to reduce the initial resistance. 1. A cathode for a lithium-ion secondary battery comprises a layer of active cathode material. The active cathode material contains single-crystal particles. One hundred single-crystal particles, randomly selected from a scanning electron microscope image of a cross-section of the active cathode material layer, comprise at least one first particle with a depression and at least one second particle with a protrusion. The following relationships are satisfied: "0.01 ≤ d / D1 ≤ 0.56" and "0.01 ≤ h / D2 ≤ 0.58", where "d" is the depth of the depression of the first particle, "D1" is the diameter of the smallest circumscribing circle of the first particle, "h" is the height of the protrusion of the second particle, and "D2" is the diameter of the smallest circumscribing circle of the second particle.

[0005] Single-crystal particles typically have a smooth surface. These particles can be densely packed within the layer of active cathode material. It is assumed that gaps are less likely to form between the single-crystal particles, through which an electrolyte solution could diffuse. Consequently, the initial resistance may increase.

[0006] The active cathode material layer of the present invention contains single-crystal particles with specific shapes. Specifically, the active cathode material layer contains the first particle and the second particle. The depression of the first particle and the protrusion of the second particle can form gaps in the active cathode material layer through which the electrolyte solution can diffuse. Furthermore, the depression and protrusion each have a suitable size relative to the size of the single-crystal particles. This facilitates the diffusion of the electrolyte solution. Consequently, a reduction in the initial resistance can be expected. Hereinafter, the “cathode for a lithium-ion secondary battery” can simply be referred to as the “cathode.”

[0007] 2. The cathode according to point “1” above may, for example, have the following configuration. The randomly selected one hundred single-crystal particles include at least one third particle that has the depression and the protrusion.

[0008] A single crystal particle can exhibit both depressions and elevations.

[0009] 3. The cathode according to point “1” above or „ 2" can, for example, include the following configuration. The active cathode material contains 50% or more of the number of single-crystal particles, with the remainder consisting of polycrystalline particles.

[0010] The higher the numerical proportion of single crystal particles, the more likely it is that, for example, the cycle characteristics will improve.

[0011] 4. The cathode according to any of the preceding points “1” to “3” may, for example, comprise the following configuration. The active cathode material contains a lithium transition metal composite oxide.

[0012] 5. The cathode according to any of the preceding points “1” to “4” may, for example, comprise the following configuration. The active cathode material has a composition represented by the following general formula: “Li x Ni a Co b Mn c O y “. In the 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”.

[0013] Since the Ni composition fraction or composition ratio or ratio fraction “a” is 0.5 or more, an increase in the initial discharge capacity can be expected, for example.

[0014] An embodiment of the present invention (hereinafter also referred to as the "present embodiment") and an example of the present invention (hereinafter also referred to as the "present example") are described. However, the present embodiment and the present example are not intended to limit the technical scope of the present invention. The present embodiment and the present example serve in every respect for illustrative purposes. The present embodiment and the present example are not limiting. The technical scope of the present invention includes all modifications that fall within the meaning and scope that correspond to the claims. For example, it is originally intended that any configurations can be extracted from the embodiment and combined as desired. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Features, advantages and technical and industrial significance of exemplary embodiments of the invention are described below with reference to the accompanying drawings, in which the same reference numerals denote the same elements, wherein: Fig. 1 a conceptual diagram of a cross-section of a first particle; Fig. 2 is a conceptual diagram of a cross-section of a second particle; Fig. 3 a schematic cross-sectional view of a cathode for a lithium-ion secondary battery according to one embodiment; Fig. 4 is a table with test results; Fig. 5 is a first temperature profile; and Fig. 6 is a second temperature profile. DETAILED DESCRIPTION OF THE EXECUTION FORMS Terms and expressions

[0016] The terms "comprise," "contain," "exhibit," and their variations are open expressions. An openly formulated configuration may or may not include additional elements beyond the essential ones. The term "consist of" is a closed expression. Even a configuration expressed in closed form may include additional elements, such as accidental impurities or elements irrelevant to the target technology. The term "essentially consist of" is a semi-closed expression. In a configuration expressed in semi-closed form, it is permissible to add elements that do not substantially affect the fundamental and novel properties of the target technology.

[0017] For example, the expression "either A or B or both" includes "A or B" and "A and B". The expression "either A or B or both" can also be written as "A and / or B".

[0018] Geometric terms should not be interpreted strictly. Examples of geometric terms are "parallel," "perpendicular," and "orthogonal." For instance, direction, angle, distance, etc., may be relatively shifted as long as essentially the same or similar functions are achieved. Geometric terms can encompass tolerances, errors, etc., for example, in relation to design, operation, and manufacturing. The dimensional relationships in any diagram may be relatively shifted as long as essentially the same or similar functions are achieved. Geometric terms can encompass tolerances, errors, etc., for example, those relating to design, operation, and manufacturing.The dimensional relationships in the individual figures may not correspond to the actual dimensional relationships. These relationships may have been altered to facilitate understanding for the reader. For example, length, width, thickness, etc., may have been changed. Some components may also have been omitted.

[0019] Numerical values ​​can be expressed in significant figures. Unless otherwise specified, measured values ​​can be the mean of several measurements. The number of measurements can be three or more, five or more, or ten or more. As a rule, the larger the number of measurements, the higher the expected reliability of the mean. Measured values ​​can be rounded based on the number of significant figures. Measured values ​​may include errors, such as those related to the detection limits of measuring instruments.

[0020] The devices, software, etc. used to measure various values ​​are merely examples. Devices, etc., that correspond to those mentioned herein as examples may also be used. If an equivalent device is used, the measurement conditions can be adjusted accordingly.

[0021] The first and second particles can be observed in scanning electron microscope (SEM) images of a cross-section of a layer of active cathode material. Hereinafter, SEM images of the cross-section of the active cathode material layer can be abbreviated as "cross-sectional SEM images." Five samples are taken from a cathode. The sampling points can be distributed across the entire cathode. The sampling points can be evenly spaced. For example, a smooth surface is created on the sample by cutting it with a cross-sectional polisher, etc. Cross-sectional SEM images are obtained by observing the smooth surface with an SEM. The image magnification can be adjusted according to particle size. The image magnification can be, for example, 5,000× to 50,000×. From five cross-sectional SEM images, 100 single-crystal particles can be randomly selected.From the 100 single-crystal particles, a first and a second particle can be extracted.

[0022] Fig. Figure 1 is a conceptual diagram of a cross-section of a first particle. A first particle 10 includes a depression 11. The depression 11 shows a portion where the particle's contour line is indented. The depth "d" of the depression 11 is determined by the following procedure. Various dimensional measurements and shape analyses of the cross-sectional SEM images can be performed using image processing software such as "ImageJ". (1) The smallest circumscribing or surrounding circle “MCC” for the contour line “P” of the particle is determined. The diameter of the MCC is the diameter “D1”. (2) A line segment “L” is drawn extending from the perimeter of the MCC to the center of the MCC and reaching P. (3) Of the points of intersection of L and P, the point where L is longest is considered to be the lowest point “X” of the depression. (4) A double tangent “T” with tangent points “A” and “B” on both sides of the lowest point is determined. (5) The distance between T and X is considered the depth “d” of the depression. The length of line “AB” is considered the width “w1” of the depression.

[0023] Fig. Figure 2 is a conceptual diagram of a cross-section of a second particle. A second particle 20 includes a protrusion 21. The protrusion 21 marks a part where the contour line of the particle projects outwards. The height “h” of the protrusion 21 is determined by the following procedure. (1) The maximum circumscribing circle “MIC” for the contour line “P” of the particle is determined. (2) A line segment “L” is drawn extending radially from the circumference of the MIC and reaching P. (3) Of the points of intersection of L and P, the point where L is longest shall be considered the highest point “Y” of the elevation. (4) From the points of tangency between MIC and P, the points “A” and “B” that are closest to the uppermost point “Y” are determined. (5) The distance between line segment “AB” and the highest point “Y” is considered the height “h” of the elevation. The length of line segment “AB” is considered the width “w2” of the elevation. (6) The MCC for P is determined. The diameter of the MCC is the diameter “D2”.

[0024] The term "single-crystal particle" refers to the smallest unit of a particle that can be recognized as a solid particle and cannot be further subdivided. A single-crystal particle appears to have no particle boundaries in cross-sectional SEM images. A single-crystal particle is also called a "primary particle." An aggregate of two or more primary particles is considered a "polycrystalline particle." From the preceding cross-sectional SEM images, 100 particles are randomly selected. The proportion of single-crystal particles is determined by counting the number of single-crystal particles among the 100 particles.

[0025] The "maximum Feret diameter" refers to the length of the long side of the minimum bounding rectangle (MBR) for the contour line of a particle in a SEM image. If the MBR is a square, the length of one side of the MBR is considered the length of the long side. The mean of the 100 particles is used as the maximum Feret diameter.

[0026] The chemical composition of the active cathode material can be measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES). A sample solution is prepared by dissolving 0.1 g of the sample (active cathode material) in a mixed acid solution (10 ml) of hydrochloric and sulfuric acid. The sample solution is diluted to a suitable concentration using a volumetric flask. After dilution, the composition analysis is performed using an ICP-AES instrument. For example, a product such as "PS3520UVDDII (manufactured by Hitachi High-Tech Corporation)" can be used. Cathode for lithium-ion secondary battery

[0027] Fig. Figure 3 is a schematic cross-sectional view of a cathode for a lithium-ion secondary battery according to the embodiment. A cathode 100 is provided for use in a lithium-ion secondary battery. The lithium-ion battery can be a liquid-filled or solid-state battery. The lithium-ion secondary battery can have any structure. For example, the lithium-ion secondary battery can have a wound or stacked current-generating element. The lithium-ion secondary battery can, for example, have a unipolar or a bipolar structure.

[0028] The cathode 100 can, for example, be in the form of a foil. The cathode 100 comprises a layer of active cathode material 102. The cathode 100 can also comprise a cathode substrate 101. The cathode substrate 101 can support the layer of active cathode material 102. The layer of active cathode material 102 can be formed on one side of the cathode substrate 101. The layer of active cathode material 102 can be formed on both sides of the cathode substrate 101. The cathode substrate 101 can serve as a current collector. The cathode substrate 101 can, for example, comprise aluminum foil (Al), etc. The cathode substrate 101 can, for example, have a thickness of 5 µm to 50 µm.

[0029] The layer of active cathode material 102 can, for example, have a thickness of 10 µm to 1000 µm. The layer of active cathode material 102 contains active cathode material. In addition to the active cathode material, the layer of active cathode material 102 can, for example, contain an electrically conductive material, a binder, etc. The layer of active cathode material can, for example, contain 0.1 to 10% by mass of an optional component (such as an additive), 0.1 to 10% by mass of the binder, and 0.1 to 10% by mass of the electrically conductive material, with the remainder being the active cathode material.

[0030] The active cathode material contains single-crystal particles. The active cathode material may also contain polycrystalline particles. The polycrystalline particles may have essentially the same composition and crystal structure as the single-crystal particles. For example, the active cathode material may contain 50% or more of the number of single-crystal particles, with the remainder consisting of polycrystalline particles. Since the numerical fraction of single-crystal particles is 50% or more, an improvement in cycle characteristics can be expected. The numerical fraction of single-crystal particles may be, for example, 60% or more, 70% or more, 80% or more, 90% or more, or 95% or more. The numerical fraction of single-crystal particles may be, for example, 95% or less, 90% or less, 80% or less, 70% or less, or 60% or less.

[0031] In the cross-sectional SEM images of the layer of more active cathode material 102, the maximum Feret diameter of the single crystal particles can 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 Feret diameter of the single crystal particles can 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.

[0032] One hundred single-crystal particles, randomly selected from cross-sectional SEM images of the active cathode material layer 102, comprise at least one first particle 10 and at least one second particle 20. The first particle 10 comprises a depression 11. The second particle 20 comprises a protrusion 21. Since the active cathode material layer 12 contains both the first particle 10 and the second particle 20, diffusion of the electrolyte solution can be facilitated.

[0033] The depression 11 contained in the first particle 10 has a specific depth. In other words, the following relationship is satisfied: “0.01 ≤ d / D1 ≤ 0.56”. The ratio “d / D1” of the depth of the depression 11 to the particle diameter can 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 “d / D1” of the depth of the depression 11 to the particle diameter can 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.

[0034] The depression 11 can have a specific width. For example, the following relationship may be satisfied: “w1 / D1 ≤ 0.50”. The ratio “w1 / D1” of the width of the depression 11 to the particle diameter 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 “w1 / D1” of the width of the depression 11 to the particle diameter 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.

[0035] The first particle 10 can contain a single depression 11. The first particle 10 can contain multiple depressions 11. The number of depressions 11 contained in the first particle 10 can be, for example, one or more, two or more, three or more, four or more, or five or more. The number of depressions 11 contained in the first particle 10 can be, for example, 10 or fewer, nine or fewer, eight or fewer, seven or fewer, six or fewer, five or fewer, four or fewer, three or fewer, or two or fewer. If the first particle 10 contains multiple depressions 11, the values ​​for the depth "d" and the width "w" are determined based on the deepest depression 11.

[0036] The protrusions 21 contained in the second particle 20 have a specific height. In other words, the following relationship is satisfied: “0.01 ≤ h / D2 ≤ 0.58”. The ratio “h / D2” of the height of the protrusion 21 to the particle diameter can 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 “h / D2” of the height of the elevation 21 to the particle diameter can 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.

[0037] The elevation 21 can have a specific width. For example, the following relationship may be satisfied: “w2 / D1 ≤ 0.50”. The ratio “w2 / D2” of the width of the elevation 21 to the particle 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 “w2 / D2” of the width of the elevation 21 to the particle diameter 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.

[0038] The number of the first 10 particles among the 100 single-crystal particles can be, for example, two or more, five 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 first 10 particles among the 100 single-crystal particles can also be, for example, 90 or fewer, 80 or fewer, 70 or fewer, 60 or fewer, 50 or fewer, 40 or fewer, 30 or fewer, 20 or fewer, 10 or fewer, five or fewer, or two or fewer.

[0039] The number of second particles 20 among the 100 single-crystal particles can be, for example, two or more, five 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 second particles 20 among the 100 single-crystal particles can also be, for example, 90 or fewer, 80 or fewer, 70 or fewer, 60 or fewer, 50 or fewer, 40 or fewer, 30 or fewer, 20 or fewer, 10 or fewer, five or fewer, or two or fewer.

[0040] The randomly selected 100 single-crystal particles may contain a third particle. The third particle comprises both a recess 11 and a protrusion 21, as described above. The number of third particles among the 100 single-crystal particles may be, for example, two or more, five 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 third particles among the 100 single-crystal particles may be, for example, 90 or fewer, 80 or fewer, 70 or fewer, 60 or fewer, 50 or fewer, 40 or fewer, 30 or fewer, 20 or fewer, 10 or fewer, five or fewer, or two or fewer. The second particle 20 in Fig. 2 is also a third particle that includes both a depression 11 and a raised area 21.

[0041] The active cathode material can, for example, comprise a lithium transition metal compound oxide. The active cathode material can have a crystal structure belonging to the space group R-3m. This crystal structure is also referred to as a "layered structure." The crystal structure can be determined by an X-ray diffraction pattern (XRD pattern). The lithium transition metal compound oxide contains lithium, transition metals, and oxygen.

[0042] The active cathode material can have a composition that is represented, for example, by the following general formula: “Li x Ni a Co b Mn c O yIn the general formula, the Li addition ratio "x" can, for example, satisfy the following relationship: "0.1 ≤ x ≤ 1.5". The Li composition fraction "x" can, for example, be 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 fraction "x" can, for example, be 1.4 or less, or 1.2 or less.

[0043] In the general formula above, the oxygen composition fraction "y" can, for example, satisfy the following relationship: "1.5 ≤ y ≤ 2.1". The oxygen composition fraction "y" can, for example, be 1.6 or more, 1.7 or more, 1.8 or more, 1.9 or more, or 2.0 or more. The oxygen composition fraction "y" can, for example, be 2.0 or less, 1.9 or less, 1.8 or less, 1.7 or less, or 1.6 or less.

[0044] In the general formula above, the Ni composition fraction "a", the Co composition fraction "b", and the Mn composition fraction "c" can satisfy the following relationship: "a + b + c = 1.0". For example, the Ni composition fraction "a" can satisfy the following relationship: "0.5 ≤ a ≤ 1.0". The Ni composition fraction "a" can be, for example, 0.6 or more, 0.7 or more, 0.8 or more, or 0.9 or more. The Ni composition fraction "a" can be, for example, 0.9 or less, 0.8 or less, 0.7 or less, or 0.6 or less.

[0045] In the general formula above, the co-composition fraction “b” can, for example, satisfy the following relationship: “0 ≤ b ≤ 0.3”. The co-composition fraction “b” can, for example, be 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 fraction “b” may, for example, be 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.

[0046] In the general formula above, the Mn composition fraction “c” can, for example, satisfy the following relationship: “0 ≤ c ≤ 0.3”. The Mn composition fraction “c” can, for example, be 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 fraction “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.

[0047] In the above general formula, Mn can be wholly or partially replaced by Al, etc. That is, the lithium transition metal compound oxide can have a composition represented, for example, by the following general formula: “Li xN i aCObAlcOy “. The area of ​​the Al composition fraction “c” is the same as that of the Mn composition fraction “c” described above.

[0048] Any dopant can be added to the lithium transition metal compound oxide. The dopant refers to an element other than Li, Ni, Co, Mn, and O. For example, the dopant can include at least one element from the group consisting of Zr, Mo, W, Mg, Ca, Na, Fe, Cr, Zn, Si, Sn, Al, and Ag. The proportion of the dopant can be, for example, 0.005 or more, 0.01 or more, 0.02 or more, 0.03 or more, or 0.04 or more. Alternatively, the proportion of the dopant can be, for example, 0.05 or less, 0.04 or less, 0.03 or less, 0.02 or less, or 0.01 or less. Preparation of the samples

[0049] Fig. Table 4 shows the experimental results. Cathodes No. 1 to No. 5 were manufactured according to the following procedure. Production of the precursor

[0050] A raw material solution is prepared by dissolving NiSO4, CoSO4, and MnSO4 in deionized water. The molar ratio of Ni, Co, and Mn in the raw material solution is Ni / Co / Mn = 90 / 5 / 5. The concentration of the dissolved substances in the raw material solution is 30% by mass.

[0051] Aqueous ammonia solution is added to a reaction vessel. While stirring with a stirrer, the reaction vessel is purged with nitrogen. NaOH is also added to the reaction vessel to create an alkaline solution.

[0052] The raw material solution and aqueous ammonia solution are added dropwise to the reaction solution while maintaining the pH of the reaction solution within a specific range, resulting in the formation of a precipitate (metal hydroxide). The reaction solution is filtered to recover the metal hydroxide. A dispersion is formed by dispersing the metal hydroxide in deionized water. The dispersion is thoroughly stirred with a spatula, effectively washing the metal hydroxide with water. After washing, the dispersion is filtered again to recover the metal hydroxide. The metal hydroxide is then dried for 16 hours at 120 °C to produce a dried material. Addition of Li source

[0053] The dry material (metal hydroxide) and the lithium compounds (LiOH, Li₂CO₃) are mixed in a mortar with a pestle to form a mixture. By adding an excess amount of Li in moles, relative to the total amount of transition metals, a molten salt is formed during firing. This allows a lithium-transition metal compound oxide to crystallize into single crystals. That is, the proportion of single-crystal particles can be 50% or more. The molar ratio of Li to the total amount of transition metals is, for example, 1.5 or more. Burn

[0054] The mixture is calcined (heat-treated) in a furnace (e.g., a muffle furnace) to synthesize a lithium transition metal composite oxide. The calcination atmosphere is an oxygen atmosphere. Fig. 5 is a first temperature profile. In No. 1, firing is carried out according to the first temperature profile. The kiln temperature is increased to X °C, which is in the range of 700 °C to 1100 °C. The firing temperature is held essentially at X °C for 10 hours. After the 10 hours, the kiln temperature is cooled to room temperature.

[0055] Fig. 6 is a second temperature profile. In Nos. 2 to 5, the firing is carried out according to the second temperature profile. The furnace temperature is increased to X °C, which is in the range of 700 °C to 1100 °C. The temperature is then decreased at a rate of Y to X - 100 °C. Subsequently, the temperature is increased at a rate of Y to X °C. The cooling and heating rate “Y” is also referred to as the “ramp rate”. The ramp rate “Y” for each sample is shown in the “ramp rate” column of the table in Fig. The specified temperature is then used. Cooling and heating are then repeated alternately for 10 hours. After 10 hours, the oven temperature is cooled to room temperature.

[0056] After firing, the particle size of the lithium transition metal composite oxide is adjusted using a pulverizer, such as a jet mill. This process produces an active cathode material. Cathode manufacturing

[0057] A slurry is formed by mixing the active cathode material, an electrically conductive material (acetylene carbon black), a binder (polyvinylidene fluoride), and a dispersion medium. A cathode is produced by coating the surface of a substrate (aluminum film) with the slurry. A film applicator (with a film thickness control function) from Allgood Co. Ltd. is used as the coating device. After application of the slurry, the film is dried for five minutes at 80 °C. Cross-sectional SEM images of the cathode are obtained. The presence or absence of first particles (particles with depressions) and second particles (particles with protrusions), the depths "d" of the depressions, the heights "h" of the protrusions, and the diameters "D1, D2" of the particles are measured. Measurement of the initial resistance

[0058] The initial resistance was measured using the following procedure.

[0059] A laminated cell is manufactured. The laminated cell has the following configuration. Outer casing: Pouch made of an aluminum laminate Power-generating element: stacked construction (single layer) Cathode: active cathode material / electrically conductive material / binder = 88 / 10 / 2 (mass ratio) Anode: active anode material (natural graphite), CMC, SBR Electrolyte: LiPF6 (1 mol / L), EC / DMC / EMC = 3 / 4 / 3 (volume ratio)

[0060] The laminated cell is clamped between two stainless steel plates, applying a predetermined pressure to the generating element. The state of charge (SOC) of the laminated cell is set to 50%. The IV resistance is measured at a temperature of -10 °C. The values ​​in the "Initial Resistance" column of the table in Fig. The values ​​given are relative values, with the initial resistance value of No. 1 set to 100%. Experimental results

[0061] As in Fig. As shown in Figure 4, the initial resistance tends to decrease when the following relationships are satisfied: “0.01 ≤ d / D1 ≤ 0.56” and “0.01 ≤ h / D2 ≤ 0.58”. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] JP 2023-036570

[0002] JP 2023-036570 A

[0002]

Claims

Cathode for a lithium-ion secondary battery, wherein the cathode comprises a layer of active cathode material, wherein: the layer of more active cathode material contains an active cathode material; the active cathode material contains single-crystal particles; one hundred single-crystal particles randomly selected from a scanning electron microscope image of a cross-section of the layer of active cathode material comprise at least one first particle with a depression and at least one second particle with a protrusion; and the following relationships are satisfied: 0.01 ≤ d / D1 ≤ 0.56 and 0.01 ≤ h / D2 ≤ 0.58, where d represents the depth of the depression of the first particle, D1 represents the diameter of a smallest circumscribing circle of the first particle, h represents the height of the elevation of the second particle and D2 represents the diameter of a smallest circumscribing circle of the second particle. Cathode according to claim 1, wherein the randomly selected hundred single crystal particles comprise at least one third particle having the depression and the elevation. Cathode according to claim 1 or 2, wherein the active cathode material contains 50% or more of the number of single crystal particles, the remainder consisting of polycrystalline particles. Cathode according to claim 1 or 2, wherein the active cathode material comprises a lithium transition metal composite oxide. Cathode according to claim 4, wherein the active cathode material has a composition represented by the following general formula: LixNiaCobMncOy where 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.

Images