Positive electrode for lithium ion secondary battery

Abstract

The present disclosure relates to a positive electrode for a lithium ion secondary battery. The purpose of the present disclosure is to reduce initial resistance. A positive electrode for a lithium ion secondary battery includes a positive electrode active material layer. The positive electrode active material layer contains a positive electrode active material. The positive electrode active material contains single crystal particles. In a scanning electron microscope image of a cross section of the positive electrode active material layer, 100 single crystal particles randomly extracted include at least one first particle having a concave portion and at least one second particle having a convex portion. The relationships of "0.01 < = d / D1 < = 0.56" and "0.01 < = h / D2 < = 0.58" are satisfied. "D" represents the depth of the recess of the first particle. 'D1' represents the diameter of the minimum circumcircle of the first particle. And h represents the height of the convex part of the second particle. And D2 represents the diameter of the minimum circumcircle of the second particle.

Description

Technical Field

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

[0002] Japanese Patent Application Publication No. 2023-036570 discloses a ternary cathode material with a large-grained aggregate having a single-crystal morphology. Summary of the Invention

[0003] The single-crystalization of the positive electrode active material has been proposed. Compared with polycrystalline particles, single-crystal particles have a smaller specific surface area, thus improving cycle performance is expected. However, there is room for improvement in the initial resistance.

[0004] The purpose of this disclosure is to reduce the initial resistance.

[0005] 1. A positive electrode for a lithium-ion secondary battery, comprising a positive electrode active material layer. The positive electrode active material layer comprises a positive electrode active material. The positive electrode active material comprises 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 comprise at least one first particle with a concave portion and at least one second particle with a convex portion. The relationships satisfy "0.01 ≤ d / D1 ≤ 0.56" and "0.01 ≤ h / D2 ≤ 0.58". "d" represents the depth of the concave portion of the first particle. "D1" represents the diameter of the smallest circumscribed circle of the first particle. "h" represents the height of the convex portion of the second particle. "D2" represents the diameter of the smallest circumscribed circle of the second particle.

[0006] Typically, single-crystal particles have smooth surfaces. In the positive electrode active material layer, single-crystal particles can be densely packed. It is considered difficult for gaps to form between single-crystal particles that allow electrolyte diffusion. As a result, the initial resistance may increase.

[0007] The positive electrode active material layer disclosed herein comprises single-crystal particles with a specific shape. Specifically, the positive electrode active material layer comprises a first particle and a second particle. The concave portion of the first particle and the convex portion of the second particle can form gaps within the positive electrode active material layer that allow electrolyte diffusion. Furthermore, the concave and convex portions have appropriate dimensions relative to the size of the single-crystal particles, thereby promoting electrolyte diffusion. As a result, a reduction in initial resistance is expected. Hereinafter, "positive electrode for lithium-ion secondary batteries" may be simply referred to as "positive electrode".

[0008] 2. The positive electrode for a lithium-ion secondary battery as described in "1" above may also include the following: 100 randomly selected single crystal particles include one or more third particles containing both concave and convex portions.

[0009] A single crystal particle can also have both concave and convex parts.

[0010] 3. The positive electrode for lithium-ion secondary batteries described in "1" or "2" above may also include the following: The positive electrode active material consists of monocrystalline particles comprising 50% or more by number fraction and the remainder being polycrystalline particles.

[0011] A higher fraction of single-crystal particles is expected to result in improved cycle performance, for example.

[0012] 4. The positive electrode for lithium-ion secondary batteries described in any of "1" to "3" above may also include the following: The positive electrode active material includes a lithium transition metal composite oxide.

[0013] 5. The positive electrode for a lithium-ion secondary battery described in any of "1" to "4" above may also include the following: The positive electrode active material has the general formula "Li x Ni a Co b Mn c O y The general formula represents the composition of x, a, b, c, y. In the formula, x, a, b, c, 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.

[0014] With a Ni composition ratio "a" of 0.5 or higher, an increase in initial discharge capacity can be expected, for example.

[0015] The following describes one embodiment (hereinafter referred to as "this embodiment") and one example of this disclosure (hereinafter referred to as "this example"). However, this embodiment and this example do not limit the technical scope of this disclosure. This embodiment and this example are illustrative in all respects. This embodiment and this example are non-limiting. The technical scope of this disclosure includes all modifications within the meaning and scope equivalent to the claims. For example, it is intended from the outset to include any solutions extracted from this embodiment and to combine them arbitrarily. Attached Figure Description

[0016] Hereinafter, the features, advantages, and technical and industrial significance of exemplary embodiments of the present invention will be described with reference to the accompanying drawings, wherein the same reference numerals denote the same elements.

[0017] Figure 1 This is a conceptual diagram of the cross-section of the first particle.

[0018] Figure 2 This is a conceptual diagram of the cross-section of the second particle.

[0019] Figure 3This is a schematic cross-sectional view of the positive electrode for a lithium-ion secondary battery according to this embodiment.

[0020] Figure 4 This is a table representing the experimental results.

[0021] Figure 5 This is the first temperature curve.

[0022] Figure 6 This is the second temperature curve. Detailed Implementation

[0023] Terms and phrases

[0024] "Possessing," "Including," "Having," and their variations are open-ended expressions. Structures expressed in an open-ended manner may include additional elements besides the necessary ones, or they may not include additional elements. The statement "Composed of..." is a closed-ended expression. However, even structures expressed in a closed-ended manner may include usually accompanying impurities and additional elements unrelated to the target technology. The statement "Substantially composed of..." is a semi-closed-ended expression. In structures expressed in a semi-closed manner, it is permissible to add elements that do not substantially affect the basic and new characteristics of the target technology.

[0025] For example, expressions for "at least one of A and B" include "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".

[0026] Geometric terms should not be interpreted strictly. Examples of geometric terms include "parallel," "perpendicular," and "orthogonal." For instance, directions, angles, and distances can also be relative displacements within a range where substantially the same or similar functions can be achieved. Geometric terms can also include tolerances and errors in design, operation, and manufacturing. Dimensional relationships in drawings sometimes differ from actual dimensional relationships. To aid understanding, dimensional relationships in drawings are sometimes altered. For example, length, width, and thickness may be changed. Sometimes, parts of the structure may be omitted.

[0027] Numerical values ​​can be expressed using significant figures. Unless otherwise specified, a measured value can be the average of multiple measurements. The number of measurements can be 3 or more, 5 or more, or 10 or more. Generally, the more measurements are expected, the higher the reliability of the average value. Measured values ​​can be rounded to the nearest significant figure. Measured values ​​may include, for example, errors such as the detection limits of the accompanying measuring device.

[0028] The apparatus and software used in the determination of various values ​​are just one example. Equivalent products to the illustrated apparatus can also be used. When using equivalent products, the measurement conditions can be adjusted in conjunction with the apparatus.

[0029] The first and second particles can be identified in scanning electron microscopy (SEM) images 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 will be referred to simply as a "cross-sectional SEM image." Five samples are prepared from the positive electrode. The sample preparation sites can be distributed throughout the entire positive electrode. The spacing between the preparation sites can also be equal. For example, the sample is cross-sectionally processed using a cross-section polishing machine to form a smooth surface. Cross-sectional SEM images are obtained by observing the smooth surface using SEM. The magnification can be adjusted according to the particle size. The magnification can be, for example, 5000x to 50000x. 100 single-crystal particles are randomly extracted from the five cross-sectional SEM images. The first and second particles are extracted from the 100 single-crystal particles.

[0030] Figure 1 This is a conceptual diagram of the cross-section of the first particle. The first particle 10 includes a recess 11. The recess 11 represents the portion of the particle's outline that is recessed inward. The depth "d" of the recess 11 is determined through the following steps. Various dimensional measurements and shape analyses in the cross-sectional SEM image can be performed, for example, using image processing software such as "Image J".

[0031] (1) Determine the smallest circumcircle “MCC” opposite to the particle’s outline “P”. The diameter of MCC is the diameter “D1”.

[0032] (2) Draw the line segment “L” that reaches P by extending from the circumference of the MCC toward the center of the MCC.

[0033] (3) Among the intersections of L and P, the intersection with the longest L is regarded as the bottom point "X" of the concave part.

[0034] (4) Determine the double tangent line "T" that sandwiches the bottom point and has tangent points "A" and "B" on both sides of the bottom point.

[0035] (5) The distance between T and X is considered as the depth “d” of the concave part. The length of line segment “AB” is considered as the width “w1” of the concave part.

[0036] Figure 2 This is a conceptual diagram of the cross-section of the second particle. The second particle 20 includes a protrusion 21. The protrusion 21 represents the portion of the particle's outline that protrudes outwards. The height "h" of the protrusion 21 is determined through the following steps.

[0037] (1) Determine the largest inscribed circle “MIC” relative to the particle’s outline “P”.

[0038] (2) Draw the line segment “L” that reaches P by extending radially from the circumference of MIC.

[0039] (3) Among the intersections of L and P, the longest intersection of L is considered as the vertex "Y" of the convex part.

[0040] (4) Determine the tangent points "A" and "B" that are closest to the vertex "Y" among the tangent points of MIC and P.

[0041] (5) The distance between line segment “AB” and vertex “Y” is considered as the height “h” of the convex part. The length of line segment “AB” is considered as the width “w2” of the convex part.

[0042] (6) Determine the MCC opposite to P. The diameter of the MCC is the diameter "D2".

[0043] A "single-crystal particle" is the smallest unit of particle, representing a solid particle that cannot be further subdivided. Single-crystal particles do not appear to have grain boundaries in cross-sectional SEM images. Single-crystal particles are also referred to as primary particles. Aggregates of two or more primary particles are considered "polycrystalline particles." In the aforementioned cross-sectional SEM image, 100 particles were randomly extracted. The fraction of single-crystal particles was determined by counting the number of single-crystal particles contained within these 100 particles.

[0044] The "Maximum Ferrette Diameter" refers to the length of the longer side of the minimum bounding rectangle (MBR) relative to the particle's outline in the SEM image. When the MBR is square, the length of the MBR's side is considered the length of the longer side. The maximum Ferrette diameter is calculated using the average of 100 particles.

[0045] The chemical composition of the positive electrode active material can be determined using 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 solution (10 ml) of hydrochloric acid and sulfuric acid. The sample solution is then diluted to an appropriate concentration using a volumetric flask. After dilution, compositional analysis is performed using an ICP-AES apparatus. For example, product names such as "PS3520 UVDD II (manufactured by Hitachi High Technology Co., Ltd.)" can also be used.

[0046] Positive electrode for lithium-ion secondary batteries

[0047] Figure 3 This is a schematic cross-sectional view of the positive electrode for a lithium-ion secondary battery according to this embodiment. The positive electrode 100 is used in the lithium-ion secondary battery. The lithium-ion secondary battery can be a liquid-system battery or an all-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 power generation element. For example, the lithium-ion secondary battery can have a unipolar structure or a bipolar structure.

[0048] The positive electrode 100 can be, for example, in sheet form. The positive electrode 100 includes a positive electrode active material layer 102. The positive electrode 100 may also 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 only on one side of the positive electrode substrate 101, or it may be formed on both sides of the positive electrode substrate 101. The positive electrode substrate 101 can also function as a current collector. The positive electrode substrate 101 may, for example, include an Al foil. The thickness of the positive electrode substrate 101 can, for example, be 5 μm to 50 μm.

[0049] The thickness of the positive electrode active material layer 102 can be, for example, 10 μm to 1000 μm. The positive electrode active material layer 102 contains a positive electrode active material. In addition to the positive electrode active material, the positive electrode active material layer 102 may also contain, for example, conductive materials, binders, etc. The positive electrode active material layer may, for example, be composed of 0.1% to 10% optional components (additives, etc.), 0.1% to 10% binder, 0.1% to 10% conductive material, and the balance being the positive electrode active material, by mass fraction.

[0050] The positive electrode active material comprises single-crystal particles. The positive electrode active material may also comprise polycrystalline particles. Polycrystalline particles may have substantially the same composition and crystal structure as single-crystal particles. For example, the positive electrode active material may consist of single-crystal particles comprising 50% or more in number fraction and the remainder being polycrystalline particles. With a single-crystal particle fraction of 50% or more, improved cycle characteristics can be expected, for example. The single-crystal particle fraction may, for example, be 60% or more, 70% or more, 80% or more, 90% or more, or 95% or more. The single-crystal particle fraction may, for example, be less than 95%, less than 90%, less than 80%, less than 70%, or less than 60%.

[0051] In the cross-sectional SEM image of the positive electrode active material layer 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, less than 10 μm, less than 9 μm, less than 8 μm, less than 7 μm, less than 6 μm, less than 5 μm, less than 4 μm, less than 3 μm, less than 2 μm, or less than 1 μm.

[0052] In the cross-sectional SEM image of the positive electrode active material layer 102, 100 randomly extracted single crystal particles include more than one first particle 10 and more than one second particle 20. The first particle 10 includes a recess 11. The second particle 20 includes a protrusion 21. By simultaneously including the first particle 10 and the second particle 20 within the positive electrode active material layer 12, the diffusion of the electrolyte can be promoted.

[0053] The recess 11 contained in the first particle 10 has a specific depth. That is, it satisfies the relationship "0.01 ≤ d / D1 ≤ 0.56". The ratio of the depth of the recess 11 to the particle diameter "d / D1" 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 of the depth of the recess 11 to the particle diameter "d / D1" 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.

[0054] The width of the recess 11 can also have a specific dimension. For example, it can satisfy the relationship "w1 / D1 < 0.50". The ratio of the width of the recess 11 to the particle diameter "w1 / D1" can, for example, be less than 0.40, less than 0.30, less than 0.20, less than 0.10, or less than 0.05. The ratio of the width of the recess 11 to the particle diameter "w1 / D1" can, for example, be greater than 0.01, greater than 0.03, greater than 0.05, greater than 0.10, greater than 0.20, greater than 0.30, or greater than 0.40.

[0055] The first particle 10 may have a single recess 11. The first particle 10 may also have multiple recesses 11. The number of recesses 11 contained in the first particle 10 may be, for example, one or more, two or more, three or more, four or more, or five or more. The number of recesses 11 contained in the first particle 10 may be, for example, less than 10, less than 9, less than 8, less than 7, less than 6, less than 5, less than 4, less than 3, or less than 2. When the first particle 10 has multiple recesses 11, the depth "d" and width "w" are taken from the value of the deepest recess 11.

[0056] The protrusion 21 contained in the second particle 20 has a specific height. That is, it satisfies the relationship "0.01 ≤ h / D2 ≤ 0.58". The ratio of the height of the protrusion 21 to the particle diameter "h / D2" 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 of the height of the protrusion 21 to the particle diameter "h / D2" can be, for example, less than 0.70, less than 0.60, less than 0.58, less than 0.52, less than 0.50, less than 0.40, less than 0.30, less than 0.20, less than 0.10, less than 0.05, less than 0.04, less than 0.03, or less than 0.02.

[0057] The width of the protrusion 21 can also have a specific dimension. For example, it can satisfy the relationship "w2 / D1≤0.50". The ratio of the width of the protrusion 21 to the particle diameter "w2 / D2" can, for example, be 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" can, for example, be 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.

[0058] The number of first particles 10 in 100 single crystal particles can 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 in 100 single crystal particles can be, for example, less than 90, less than 80, less than 70, less than 60, less than 50, less than 40, less than 30, less than 20, less than 10, less than 5, or less than 2.

[0059] The number of second particles 20 contained in 100 single crystal particles can be, for example, more than 2, more than 5, more than 10, more than 20, more than 30, more than 40, more than 50, more than 60, more than 70, more than 80, or more than 90. The number of second particles 20 contained in 100 single crystal particles can be, for example, less than 90, less than 80, less than 70, less than 60, less than 50, less than 40, less than 30, less than 20, less than 10, less than 5, or less than 2.

[0060] The 100 randomly selected single-crystal particles may also include a third particle. The third particle includes both the aforementioned concave portion 11 and convex portion 21. The number of third particles in the 100 single-crystal particles can 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. Alternatively, the number of third particles in the 100 single-crystal particles can be, for example, less than 90, less than 80, less than 70, less than 60, less than 50, less than 40, less than 30, less than 20, less than 10, less than 5, or less than 2. Furthermore, Figure 2 The second particle 20 is also the third particle, which contains both the concave part 11 and the convex part 21.

[0061] The positive electrode active material can also contain, for example, lithium transition metal composite oxides. The positive electrode active material can also have a crystal structure belonging to space group R-3m. This crystal structure is also known as a layered structure. The crystal structure can be determined by XRD (X-ray Diffraction) patterns. Lithium transition metal composite oxides contain Li, a transition metal, and oxygen.

[0062] Positive electrode active materials can also have, for example, properties derived from the general formula "Li". x Ni a Co b Mn c O y The composition is indicated by "". In the general formula, the Li composition ratio "x" can, for example, satisfy the relationship "0.1≤x≤1.5". The Li composition ratio "x" can, for example, be 0.4 or higher, 0.6 or higher, 0.8 or higher, 1.0 or higher, 1.2 or higher, or 1.4 or higher. The Li composition ratio "x" can, for example, be less than 1.4 or less than 1.2.

[0063] In the above general formula, the ratio "y" of component O can, for example, satisfy the relationship "1.5≤y≤2.1". The ratio "y" of component O can, for example, be 1.6 or higher, 1.7 or higher, 1.8 or higher, 1.9 or higher, or 2.0 or higher. The ratio "y" of component O can, for example, be less than 2.0, less than 1.9, less than 1.8, less than 1.7, or less than 1.6.

[0064] In the above general formula, the Ni composition ratio "a", the Co composition ratio "b", and the Mn composition ratio "c" can satisfy the relationship "a + b + c = 1.0". The Ni composition ratio "a" can, for example, satisfy the relationship "0.5 ≤ a ≤ 1.0". The Ni composition ratio "a" can, for example, be 0.6 or higher, 0.7 or higher, 0.8 or higher, or 0.9 or higher. The Ni composition ratio "a" can, for example, be less than 0.9, less than 0.8, less than 0.7, or less than 0.6.

[0065] In the above general formula, the Co composition ratio "b" can, for example, satisfy the relationship "0 ≤ b ≤ 0.3". The Co composition ratio "b" can, for example, be 0.01 or higher, 0.02 or higher, 0.03 or higher, 0.04 or higher, 0.05 or higher, 0.06 or higher, 0.07 or higher, 0.08 or higher, 0.09 or higher, 0.10 or higher, 0.15 or higher, 0.20 or higher, or 0.25 or higher. The Co composition ratio "b" can, for example, be less than 0.25, less than 0.20, less than 0.15, less than 0.10, less than 0.09, less than 0.08, less than 0.07, less than 0.06, less than 0.05, less than 0.04, less than 0.03, less than 0.02, or less than 0.01.

[0066] In the above general formula, the composition ratio "c" of Mn can, for example, satisfy the relationship "0 ≤ c ≤ 0.3". The composition ratio "c" of Mn can, for example, be 0.01 or higher, 0.02 or higher, 0.03 or higher, 0.04 or higher, 0.05 or higher, 0.06 or higher, 0.07 or higher, 0.08 or higher, 0.09 or higher, 0.10 or higher, 0.15 or higher, 0.20 or higher, or 0.25 or higher. The composition ratio "c" of Mn can, for example, be less than 0.25, less than 0.20, less than 0.15, less than 0.10, less than 0.09, less than 0.08, less than 0.07, less than 0.06, less than 0.05, less than 0.04, less than 0.03, less than 0.02, or less than 0.01.

[0067] Furthermore, in the above general formula, all or part of Mn can be replaced by Al, etc. That is, lithium transition metal composite oxides can, for example, have the general formula "Li x Ni a Co b Al c O y The composition is indicated by "c". The range of the Al composition ratio "c" is the same as that of the Mn composition ratio "c" mentioned above.

[0068] Optional dopants may also be added to the lithium transition metal composite oxide. Dopants refer to elements other than Li, Ni, Co, Mn, and O. Dopants may, for example, contain at least one selected from Zr, Mo, W, Mg, Ca, Na, Fe, Cr, Zn, Si, Sn, Al, and Ag. The composition ratio of dopants may, for example, be 0.005 or more, 0.01 or more, 0.02 or more, 0.03 or more, or 0.04 or more. Alternatively, the composition ratio of dopants may, for example, be less than 0.05, less than 0.04, less than 0.03, less than 0.02, or less than 0.01.

[0069] Sample preparation

[0070] Figure 4This is a table showing the experimental results. The positive electrodes No. 1 through No. 5 were manufactured using the following steps.

[0071] Preparation of precursors

[0072] A feedstock solution is formed by dissolving NiSO4, CoSO4, and MnSO4 in ion-exchanged water. In the feedstock solution, the molar ratio of Ni, Co, and Mn is "Ni / Co / Mn = 90 / 5 / 5". The solute concentration in the feedstock solution is 30% (mass fraction).

[0073] Ammonia solution is added to the reaction vessel. While stirring the ammonia solution with a stirrer, nitrogen is used to displace the contents of the reaction vessel. Then, NaOH is added to the reaction vessel to form an alkaline reaction solution.

[0074] The raw material solution and ammonia water are added dropwise to the reaction solution while maintaining a specific pH range, thereby forming a precipitate (metal hydroxide). The metal hydroxide is recovered by filtering the reaction solution. A dispersion is formed by dispersing the metal hydroxide in ion-exchanged water. The dispersion is thoroughly stirred with a spatula. The metal hydroxide is then washed with water. After washing, the metal hydroxide is recovered by filtering the dispersion. A dried product is formed by drying the metal hydroxide at 120°C for 16 hours.

[0075] Li Yuan's addition

[0076] In a mortar, a mixture is formed by mixing dried material (metal hydroxide) and lithium compounds (LiOH, Li₂CO₃) using a mortar and pestle. By adding Li in excess relative to the total mass of the transition metals, a molten salt is formed during calcination, allowing the lithium transition metal composite oxide to be single-crystalled. That is, the fraction of single-crystal particles can be 50% or more. The ratio of the mass of Li to the total mass of the transition metals is, for example, 1.5 or more.

[0077] Firing

[0078] Lithium transition metal composite oxides are synthesized by calcining (heat treatment) a mixture in a calcining furnace (e.g., a muffle furnace). The calcining atmosphere is an oxygen atmosphere. Figure 5 This is temperature curve number 1. In No. 1, firing is carried out according to temperature curve number 1. The furnace temperature is raised until X℃ is within the range of 700℃~1100℃. X℃ is essentially maintained for 10 hours. After 10 hours, the furnace temperature is cooled to room temperature.

[0079] Figure 6This is the second temperature curve. In No. 2 to No. 5, firing was carried out according to the second temperature curve. The furnace temperature was increased until X℃ was within the range of 700℃~1100℃. The temperature was then decreased to X-100℃ at a rate of Y. Next, the temperature was increased to X℃ at a rate of Y. The rates of decrease and increase, “Y”, are also denoted as amplitude velocity. The amplitude velocity “Y” in each sample represents… Figure 4 The amplitude and velocity of the furnace were measured. Afterwards, the furnace underwent alternating cooling and heating for 10 hours. After 10 hours, the furnace temperature was reduced to room temperature.

[0080] 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 the positive electrode active material.

[0081] Positive electrode production

[0082] A slurry is formed by mixing a positive electrode active material, a conductive material (acetylene black), a binder (polyvinylidene fluoride), and a dispersion medium. The positive electrode is fabricated by coating the slurry onto the surface of a substrate (Al foil). An Allgood film coater (with film thickness adjustment function) is used as the coating apparatus. After coating, the film is dried at 80°C for 5 minutes. A cross-sectional SEM image of the positive electrode is obtained. The presence or absence of the first particle (concave particle) and the second particle (convex particle), the depth of the concave portion "d", the height of the convex portion "h", and the particle diameters "D1, D2" are determined.

[0083] Determination of initial resistance

[0084] The initial resistance is determined by the following steps.

[0085] Fabricate laminating units. The structure of a laminating unit is described below.

[0086] Outer casing: A soft package made of Al laminated film

[0087] Power generation element: Stacked type (single layer)

[0088] Positive electrode: Positive electrode active material / conductive material / binder = 88 / 10 / 2 (mass ratio)

[0089] Negative electrode: Negative electrode active material (natural graphite), CMC, SBR

[0090] Electrolytes: LiPF6 (1 mol / L), EC / DMC / EMC = 3 / 4 / 3 (volume ratio)

[0091] The laminated unit is held between two stainless steel plates, thereby applying a predetermined pressure to the power generation element. The state of charge (SOC) of the laminated unit is adjusted to 50%. The IV resistance is measured at a temperature of -10°C. Figure 4The value shown for the "Initial Resistance" item is a relative value with the initial resistance of No. 1 set to 100%.

[0092] Experimental results

[0093] like Figure 4 As shown, when the relationships “0.01 ≤ d / D1 ≤ 0.56” and “0.01 ≤ h / D2 ≤ 0.58” are satisfied, a tendency for the initial resistance to decrease is observed.

Claims

1. A positive electrode for a lithium-ion secondary battery, comprising a positive electrode active material layer, The positive electrode active material layer contains positive electrode active material. The positive electrode active material contains 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 comprise at least one first particle with a concave portion and at least one second particle with a convex portion. The following relationship must be satisfied: 0.01 ≤ d / D1 ≤ 0.56, and 0.01 ≤ h / D2 ≤ 0.58, The d represents the depth of the recess of the first particle. D1 represents the diameter of the smallest circumcircle of the first particle. The h represents the height of the protrusion of the second particle, and D2 represents the diameter of the smallest circumcircle of the second particle.

2. The positive electrode for a lithium-ion secondary battery according to claim 1, The 100 randomly selected single crystal particles include: one or more third particles containing the concave portion and the convex portion.

3. The positive electrode for a lithium-ion secondary battery according to claim 1 or 2, The positive electrode active material is composed of single crystal particles comprising more than 50% by number and the remainder being polycrystalline particles.

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

5. The positive electrode for a lithium-ion secondary battery according to claim 4, The positive electrode active material has a composition represented by the following general formula: Li x Ni a Co b Mr c O y In the general formula, x, a, b, c, and y satisfy the 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.

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