Positive electrode active material for lithium ion secondary battery

Abstract

The present disclosure is a positive electrode active material for a lithium ion secondary battery. The positive electrode active material for a lithium ion secondary battery contains single crystal particles. The single crystal particles include a first pyramidal portion and a second pyramidal portion. The first pyramid portion protrudes in a first direction. The first pyramid portion has a first outer diameter. The first outer diameter represents a diameter of a minimum circumscribed circle with respect to the first pyramid portion. The second pyramid portion protrudes in the second direction. The second pyramid portion has a second outer diameter. The second outer diameter represents a diameter of a minimum circumscribed circle with respect to the second pyramid portion. The first outer diameter is larger than the second outer diameter. An angle formed by the first direction and the second direction is greater than 0 DEG and 45 DEG or less.

Description

Technical Field

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

[0002] Japanese Patent Application Publication No. 2023-036570 discloses a ternary cathode material. Summary of the Invention

[0003] Generally, lithium-ion rechargeable batteries use positive electrode active materials (hereinafter referred to as "positive electrode active materials") to form secondary particles. Secondary particles are aggregates of multiple primary particles. Through repeated charge-discharge reactions, the primary particles repeatedly expand and contract. These volume changes can create voids between the primary particles. The creation of these voids can disrupt the conductive paths between primary and secondary particles. Therefore, the desired cycle characteristics may not be achieved.

[0004] The purpose of this disclosure is to improve the cyclic characteristics.

[0005] The positive electrode active material for lithium-ion secondary batteries contains single-crystal particles.

[0006] The single crystal particle contains a first pyramidal part and a second pyramidal part.

[0007] The first pyramidal part protrudes in the first direction.

[0008] The first pyramidal part has a first outer diameter.

[0009] The first outer diameter represents the diameter of the smallest circumcircle relative to the first pyramid.

[0010] The second pyramidal part protrudes in the second direction.

[0011] The second pyramidal part has a second outer diameter.

[0012] The second outer diameter refers to the diameter of the smallest circumcircle relative to the second pyramid.

[0013] The first outer diameter is larger than the second outer diameter. The angle formed by the first direction and the second direction is greater than 0° and less than 45°.

[0014] Previously, octahedral single-crystal particles were known. In an octahedron (double tetrahedron), the two pyramidal parts (quadratic pyramids) share a common base and protrude in opposite directions. That is, the angle formed by the protruding directions of the two pyramidal parts is considered to be approximately 180°.

[0015] The single-crystal particles in this disclosure have a novel structure. Specifically, the two pyramidal portions have different sizes, and the angle formed by the two pyramidal portions is less than 45°. According to the novel insights of this disclosure, by having the same structure, the volume change of the single-crystal particles can be reduced. Since the conductive paths between primary and secondary particles become more difficult to interrupt, improved cycle performance can be expected.

[0016] The positive electrode active material for lithium-ion secondary batteries described above may include, for example, the following composition.

[0017] The positive electrode active material for lithium-ion secondary batteries meets the requirement of "0.010". <D b / D a The relationship is ≤0.750.

[0018] “D a " indicates the first outer diameter.

[0019] “D b " indicates the second outer diameter.

[0020] Regarding the dimensions of the first pyramidal portion and the second pyramidal portion, by satisfying "0.010 <D b / D a A relationship of ≤0.750" suggests that improved cyclic characteristics can be expected.

[0021] The positive electrode active material for lithium-ion secondary batteries described above may include, for example, the following composition.

[0022] The bottom surface of the second pyramid is fused with the side surface of the first pyramid.

[0023] The positive electrode active material for lithium-ion secondary batteries described above may include, for example, the following composition.

[0024] The single-crystal particle also contains a third pyramidal portion. The third pyramidal portion protrudes in the third direction.

[0025] The first, second, and third directions are different from each other.

[0026] The bottom surface of the third pyramid merges with the bottom surface of the first pyramid.

[0027] The positive electrode active material for lithium-ion secondary batteries described above may include, for example, the following composition.

[0028] The positive electrode active material for lithium-ion secondary batteries has a crystal structure belonging to space group R-3m.

[0029] The following describes one embodiment of this disclosure (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 was initially intended that any configuration could be extracted from this embodiment and combined arbitrarily. Attached Figure Description

[0030] The features, advantages, and technical and industrial significance of exemplary embodiments of the present invention will now be described with reference to the accompanying drawings, wherein like reference numerals denote like elements.

[0031] Figure 1 This is a first conceptual diagram representing the single-crystal particles in this embodiment;

[0032] Figure 2 This is a second conceptual diagram representing the single-crystal particles in this embodiment;

[0033] Figure 3 This is a table representing the experimental results. Detailed Implementation

[0034] Terms and statements

[0035] Geometric terms should not be interpreted in a strict sense. As geometric terms, they may exemplify concepts such as "parallel," "perpendicular," and "orthogonal." For example, directions, angles, and distances can also be relatively displaced within a range that achieves substantially the same or similar function. Geometric terms may also encompass tolerances and errors in design, operation, and manufacturing. The dimensional relationships in various drawings sometimes differ from the actual dimensional relationships. To aid understanding, the dimensional relationships in the drawings are sometimes altered. For example, length, width, and thickness may be changed. Sometimes, a portion of a component may be omitted.

[0036] "Single crystal particles" refer to particles that cannot be broken down into smaller units using a disperser or pulverizer. Single crystal particles can also be referred to as primary particles, elementary particles, etc. Aggregates of single crystal particles can be referred to as secondary particles, aggregates, etc.

[0037] The space group to which a crystal structure belongs can be identified by powder XRD (X-ray diffraction).

[0038] The shape of the single crystal particle and the shape of each pyramid can be determined using at least one of three-dimensional SEM (Scanning Electron Microscope) observation and cross-sectional SEM observation. The direction of the protruding pyramid indicates the direction from the center of the base of the pyramid towards the vertex. In the case of a cross-sectional image, it indicates the direction from the midpoint of the base of the triangle towards the vertex. The angle formed by two directions (two straight lines) is called the inferior angle.

[0039] The determination (fitting) of the minimum circumcircle of each pyramid can be performed using image analysis software. For example, ImageJ can be used. The diameter of the minimum circumcircle is considered as the outer diameter of the pyramid.

[0040] Positive electrode active material

[0041] The positive electrode active material is the positive electrode active material used in lithium-ion secondary batteries. Lithium-ion secondary batteries (hereinafter referred to as "batteries") can be liquid-based batteries or all-solid-state batteries. Batteries can have any structure. For example, batteries can have wound or stacked power generation units. Batteries can have, for example, unipolar or bipolar structures.

[0042] Figure 1 This is a first conceptual diagram representing the single-crystal particles in this embodiment. Figure 2 This is a second conceptual diagram representing the single-crystal particles in this embodiment. The second conceptual diagram represents... Figure 1 A cross-section of the single crystal particle 10 is shown.

[0043] The single-crystal particle 10 includes a first pyramidal portion 11 and a second pyramidal portion 12. The outer diameter of the first pyramidal portion 11 (the first outer diameter "D") a ") is greater than the outer diameter of the second cone 12 (the second outer diameter "D") b ) large. That is, satisfying "D b / D a The relationship is <1. The outer diameter ratio is "D". b / D a "For example, it can be below 0.900, 0.800, 0.750, 0.500, 0.250, 0.100, 0.075, 0.050, or 0.025. The outer diameter ratio "D" b / D a "For example, it can be 0.010 or higher, greater than 0.010, 0.013 or higher, 0.015 or higher, 0.020 or higher, 0.025 or higher, 0.050 or higher, 0.075 or higher, 0.100 or higher, 0.150 or higher, 0.300 or higher, 0.450 or higher, or 0.600 or higher. For example, it can also satisfy '0.010'." <D b / D aThe relationship is ≤0.750. By satisfying this relationship, improvements in cyclic properties can be expected.

[0044] The first pyramidal part 11 protrudes in the first direction A1. The second pyramidal part 12 protrudes in the second direction A2. The angle θ formed by the first direction A1 and the second direction A2 is... 12 Greater than 0° and less than 45°. This is achieved by satisfying "0° < θ". 12 A relationship of ≤45° can be expected to improve the cycling characteristics. The angle θ formed... 12 For example, it can be below 40°, below 39°, below 38°, below 36°, below 35°, below 34°, below 33°, below 32°, below 31°, or below 30°. The angle θ formed... 12 For example, it can be 5° or higher, 10° or higher, 15° or higher, 20° or higher, or 25° or higher.

[0045] The second pyramid 12 can also be directly connected to the first pyramid 11. For example, the bottom surface of the second pyramid 12 can also be fused with the side surface of the first pyramid 11. In the fused part, it can be a state in which the boundary between the two pyramids can be identified, or it can be a state in which the boundary cannot be identified.

[0046] The single crystal particle 10 may also contain three or more pyramidal portions. The number of pyramidal portions contained in the single crystal particle 10 may be, for example, four or more, six or more, eight or more, or ten or more. Alternatively, the number of pyramidal portions contained in the single crystal particle 10 may be, for example, nine or fewer, seven or fewer, or five or fewer. Each pyramidal portion may also be independently shaped, for example, a triangular pyramid, a square pyramid, a pentagonal pyramid, a hexagonal pyramid, or an octagonal pyramid.

[0047] The single-crystal particle 10 may further include, for example, a third pyramidal portion 13. The third pyramidal portion 13 protrudes in a third direction A3. The first direction A1, the second direction A2, and the third direction A3 may be different from each other. The angle θ formed by the first direction A1 and the third direction A3... 13 For example, it can be greater than 45°, 60°, 75°, 90°, 105°, 120°, 135°, 150°, or 165°. The angle θ formed is... 13 For example, it can be below 180°, below 165°, below 150°, below 135°, below 120°, or below 105°.

[0048] When a single crystal particle 10 contains more than three pyramidal portions, for example, the first pyramidal portion 11 can be the largest pyramidal portion (the main pyramidal portion). For example, it can also satisfy "D b <D c ≤D a “D” c ≤Db <D a "etc." c "" indicates the outer diameter of the third pyramidal part 13 (the third outer diameter).

[0049] The third pyramid 13 can also be directly connected to the first pyramid 11. The connecting surface between the third pyramid 13 and the first pyramid 11 and the connecting surface between the second pyramid 12 and the first pyramid 11 can be the same or different. For example, the bottom surface of the third pyramid 13 can also be fused with the bottom surface of the first pyramid 11.

[0050] Single crystal particles 10 can exist alone. Single crystal particles 10 can also form secondary particles. The number of single crystal particles 10 contained in a secondary particle can be, for example, 2 or more, 4 or more, 6 or more, or 8 or more. The number of single crystal particles 10 contained in a secondary particle can be, for example, 9 or less, 7 or less, 5 or less, or 3 or less.

[0051] The positive electrode active material (powder) can be composed of single crystal particles 10. The positive electrode active material, as long as it contains single crystal particles 10, can also further contain single crystal particles, secondary particles, etc., of other shapes. The proportion of single crystal particles 10 in the positive electrode active material (powder) can, for example, be 5% or more, 10% or more, 25% or more, 50% or more, or 75% or more.

[0052] The positive electrode active material can have any chemical composition. For example, it can have a crystal structure belonging to space group R-3m. For example, it can contain at least one material selected from lithium nickel composite oxides, lithium nickel cobalt manganese composite oxides, and lithium nickel cobalt aluminum composite oxides. For example, it can have a crystal structure using the general formula "Li". x Ni a Co b Mn c O y The composition is represented by "". In the general formula, relationships such as "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" can also be satisfied. Similarly, relationships such as "0.9≤x≤1.1", "0.7≤a≤0.9", "0.05≤b≤0.15", and "0.05≤c≤0.15" can also be satisfied. Any dopant can be added to the positive electrode active material.

[0053] Sample preparation

[0054] No.1

[0055] Figure 3This is a table showing the experimental results. A raw material solution was formed by dissolving NiSO4, CoSO4, and MnSO4 in ion-exchange water. In the raw material solution, the molar ratio of Ni, Co, and Mn is "Ni / Co / Mn = 8 / 1 / 1". The solute concentration in the raw material solution is 30% (mass fraction).

[0056] Ammonia solution is added to the reaction vessel. While stirring the ammonia solution with a stirrer, the reaction vessel is purged with nitrogen gas. NaOH is then added to the reaction vessel, thus forming an alkaline reaction solution.

[0057] The raw material solution and ammonia were added dropwise to the reaction solution while maintaining a specific pH range, resulting in a precipitate (metal hydroxide). The metal hydroxide was recovered by filtering the reaction solution. A dispersion was formed by dispersing the metal hydroxide in ion-exchange water. The dispersion was then thoroughly stirred using a spatula. The metal hydroxide was washed with water. After washing, the dispersion was filtered, thereby recovering the metal hydroxide. The metal hydroxide was dried at 120°C for 16 hours to obtain a dried product.

[0058] A mixture was formed by mixing the dried material (metal hydroxide) and lithium compounds (LiOH, Li₂CO₃) in a mortar using a mortar and pestle. The molar ratio of Li to metal hydroxide was 1.1.

[0059] The positive electrode active material was synthesized by heat-treating the mixture in a muffle furnace. The heat treatment (calcination) conditions are described below. After heat treatment, the particle size of the positive electrode active material was adjusted using a jet mill.

[0060] Atmosphere: Oxygen atmosphere

[0061] Temperature: 700~1100℃

[0062] Time: 10 hours

[0063] No.2

[0064] Similar to No. 1, the dried product (metal hydroxide) was prepared using a co-precipitation method. In a mortar, the dried product (metal hydroxide), lithium compounds (LiOH, Li₂CO₃), and fluxes (Li₂SO₄, LiNO₃, LiCH₃COOH) were mixed using a mortar and pestle to form Mixture 1. The molar ratio of Li to the molar ratio of the metal hydroxide was 1.1.

[0065] In a muffle furnace, under an oxygen atmosphere and at the melting point of the flux, the first mixture is heat-treated for 5 hours, thereby forming a pre-fired product.

[0066] The first material was formed by crushing the pre-fired material in a mortar and pestle for 10 minutes.

[0067] The second material was formed by pulverizing the pre-burnt material in a jet mill.

[0068] A second mixture was formed by mixing the first and second materials. The positive electrode active material was synthesized by heat-treating the second mixture in a muffle furnace. The conditions for heat treatment (formal calcination) are described below. After heat treatment, the particle size of the positive electrode active material was adjusted using a jet mill.

[0069] Atmosphere: Oxygen atmosphere

[0070] Temperature: 700~1100℃

[0071] Time: 10 hours

[0072] No.3 to No.5

[0073] The second material was formed by changing the supply speed of the pre-burned material and the pressure of the compressed fluid in the jet mill. Otherwise, the positive electrode active material was manufactured in the same manner as No.2.

[0074] Evaluation of Cyclic Characteristics

[0075] A cylindrical lithium-ion secondary battery (evaluation unit) was manufactured. The composition of the evaluation unit is described below.

[0076] Power generation unit: wound type

[0077] Positive electrode: Positive electrode active material / AB / PVDF = 88 / 10 / 2 (mass ratio)

[0078] Negative electrode: Negative electrode active material (natural graphite), CMC, SBR

[0079] Electrolytes: LiPF6 (1 mol / L), EC / DMC / EMC = 3 / 4 / 3 (volume ratio)

[0080] The positive and negative electrodes are manufactured by coating the surface of a substrate (metal foil) with a slurry. An Allgood film coater (with film thickness adjustment function) was used as the coating device. After coating, the film was dried at 80°C for 5 minutes.

[0081] At room temperature, the evaluation cell underwent 200 charge-discharge cycles using a constant current of 2C within a voltage range of 3.0–4.1V. The capacity retention rate (percentage) was calculated by dividing the discharge capacity of the 200th cycle by the initial discharge capacity. A higher capacity retention rate is considered to indicate better cycle performance.

[0082] result

[0083] When the single crystal particle contains a first pyramidal portion and a second pyramidal portion, and the angle formed by the protruding directions of the first pyramidal portion and the second pyramidal portion is less than 45°, a tendency to improve the cycling characteristics is observed.

[0084] Regarding the dimensions of the first pyramidal portion and the second pyramidal portion, the requirement of "0.010" must be met. <D b / D a When the relationship is ≤0.750", a tendency to improve the cycle characteristics is observed.

Claims

1. A positive electrode active material for lithium-ion secondary batteries, comprising single-crystal particles, The single-crystal particle comprises a first pyramidal portion and a second pyramidal portion. The first pyramidal portion protrudes in the first direction. The first pyramidal portion has a first outer diameter. The first outer diameter refers to the diameter relative to the smallest circumcircle of the first pyramidal portion. The second pyramidal portion protrudes in the second direction. The second pyramidal portion has a second outer diameter. The second outer diameter refers to the diameter relative to the smallest circumcircle of the second pyramidal portion. The first outer diameter is larger than the second outer diameter, and, The angle formed by the first direction and the second direction is greater than 0° and less than 45°.

2. The positive electrode active material for lithium-ion secondary batteries according to claim 1, Satisfies 0.010 <D b / D a A relationship ≤0.750 The D a This represents the first outer diameter, and, The D b This refers to the second outer diameter.

3. The positive electrode active material for lithium-ion secondary batteries according to claim 1 or 2, The bottom surface of the second pyramidal portion merges with the side surface of the first pyramidal portion.

4. The positive electrode active material for lithium-ion secondary batteries according to claim 1 or 2, The single-crystal particle also includes a third pyramidal portion. The third pyramidal portion protrudes in the third direction. The first direction, the second direction, and the third direction are different from each other, and, The bottom surface of the third pyramidal portion merges with the bottom surface of the first pyramidal portion.

5. The positive electrode active material for lithium-ion secondary batteries according to claim 1 or 2, It has a crystal structure belonging to space group R-3m.

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