Precursor of positive electrode active material

Abstract

The present invention relates to a precursor of a positive electrode active material. The precursor is a nickel composite hydroxide, the precursor comprising more than 0 mass% of precursor particles having a true circularity of more than 0.10 calculated from a scanning electron microscope (SEM) image.

Description

Technical Field

[0001] This invention relates to precursors of positive electrode active materials. Background Technology

[0002] Various technologies have been proposed for the positive electrode active materials disclosed in Japanese Patent Application Publication No. 2022-146720 and Japanese Patent Application Publication No. 2021-24764. Summary of the Invention

[0003] The problem that the invention aims to solve

[0004] In order to obtain a positive electrode with high battery characteristics such as high cycle performance and high output performance, a variety of positive electrode active materials have been proposed.

[0005] For example, Japanese Patent Application Publication No. 2022-146720 discloses a positive electrode active material for all-solid-state lithium-ion batteries, which has the following composition: Li a Ni x Co y Mn 1-x-y O2 (in this formula, 1.00≤a≤1.03, 0.8≤x≤0.9, 0≤y≤0.16) indicates that the 50% cumulative volumetric particle size D50 is 3.0μm~7.0μm, the tap density is 1.9g / cc~2.5g / cc, and the roundness is 0.90~0.93.

[0006] In Japanese Patent Application Publication No. 2022-146720, roundness is an index that indicates how close a particle's shape is to a sphere. For example, the roundness of a perfectly spherical particle is its upper limit of 1.00, and the following formula is recorded as a formula for calculating roundness.

[0007] Circularity = 4πS / L 2

[0008] (In the formula, S is the projected area of ​​the particle, L is the perimeter of the particle's projected image, and π is the mathematical constant pi.)

[0009] Positive electrodes using the positive electrode active material described in Japanese Patent Application Publication No. 2022-146720 suffer from low output characteristics. This is because the positive electrode layer constituting the positive electrode is prone to separation from the positive electrode current collector.

[0010] Methods for solving problems

[0011] The present invention was made based on the above-mentioned actual situation, and provides a precursor of positive electrode active material that can improve the peel strength between the positive electrode layer and the positive electrode current collector.

[0012] That is, the present invention includes the following methods.

[0013] 1. A precursor for a positive electrode active material, wherein the precursor is a nickel complex hydroxide, and the precursor contains more than 0% by mass of precursor particles with a sphericity greater than 0.10 as calculated from scanning electron microscopy (SEM) images.

[0014] 2. The precursor according to claim 1, wherein the sphericity is 0.20 or higher, and the long side diameter of the precursor particles is 2.1 μm or higher and 10.2 μm or less.

[0015] 3. The precursor according to 1 or 2, wherein the roundness is 0.20 or more, and the precursor contains 10% by mass or more of the precursor particles.

[0016] 4. The precursor according to any one of 1 to 3, wherein the roundness is 3.60 or less.

[0017] 5. The precursor according to any one of 1 to 4, wherein the nickel complex hydroxide is a nickel-cobalt-manganese complex hydroxide.

[0018] Invention Effects

[0019] According to the present invention, a precursor of a positive electrode active material can be provided that can improve the peel strength between the positive electrode layer and the positive electrode current collector. Attached Figure Description

[0020] The features, advantages, and technical and industrial significance of exemplary embodiments of the present invention are described with reference to the accompanying drawings, in which the same symbols denote the same elements.

[0021] Figure 1 This is an SEM image of the precursor particles with high sphericity contained in the precursor of the present invention.

[0022] Figure 2 This is an SEM image of precursor particles with low roundness contained in the precursor of the present invention. Detailed Implementation

[0023] The embodiments of the present invention will be described below. It should be noted that events other than those specifically mentioned in this specification, i.e., events necessary for implementing the present invention (e.g., the usual composition and manufacturing process of positive electrode active materials and their precursors that are not characteristic of the present invention), can be considered as design matters based on prior art by those skilled in the art. The present invention can be implemented based on the disclosure in this specification and common technical knowledge in the field.

[0024] In this invention, a precursor for a positive electrode active material is provided, wherein the precursor is a nickel complex hydroxide, and the precursor contains more than 0% by mass of precursor particles with a sphericity greater than 0.10 calculated from scanning electron microscope (SEM) images.

[0025] In this invention, the sphericity of the precursor particles is calculated as follows.

[0026] First, in the SEM (Scanning Electron Microscope) image, the lengths D of the four diagonals formed when the precursor particles are divided into eight equal parts are measured. Using the longest length (long side particle diameter Dmax) and the shortest length (short side particle diameter Dmin), the sphericity is calculated according to the following formula.

[0027] Roundness = (Dmax - Dmin) / 2

[0028] Furthermore, in this invention, the roundness of a precursor particle refers to the average value calculated from the roundness of multiple precursor particles. That is, a precursor particle containing a roundness greater than 0.10 (greater than 0% by mass) means a precursor particle containing a roundness greater than 0.10 (greater than 0% by mass) calculated from multiple precursor particles. The number of precursor particles used to calculate the roundness can be two or more, for example, 60 or more, or 100 or more. For 100 precursor particles from which the roundness is calculated, the first 20 particles sorted from largest to smallest roundness and the first 20 particles sorted from smallest to largest roundness can be removed, and the average value of the remaining 60 particles can be calculated.

[0029] The closer the sphericity is to 0, the higher the sphericity of the precursor particles, meaning the closer their shape is to a perfect circle. On the other hand, the higher the sphericity, the lower the sphericity of the precursor particles, meaning they have a shape that is deformed relative to a perfect circle.

[0030] Figure 1 This is a SEM image of the highly spherical precursor particles contained in the precursor of this invention. Figure 2 This is a SEM image of the precursor particles with low roundness contained in the precursor of this invention. For example... Figure 1 As shown, the precursor particles contained in the precursor of the present invention, which have a low sphericity of 0.10 or less, have high sphericity, such as... Figure 2 As shown, the precursor particles contained in the precursor of the present invention, which have a high sphericity of greater than 0.10, have low sphericity.

[0031] Precursor particles with a sphericity greater than 0.10 can retain their deformed shape to a certain extent when mixed with metal compounds such as lithium compounds as metal sources and calcined to synthesize positive electrode active materials, thereby forming positive electrode active materials with deformed shapes. It should be noted that precursor particles with a sphericity greater than 0.10 are sometimes referred to as high-sphericity precursor particles. Positive electrode layers containing positive electrode active materials with deformed shapes tend to exhibit an anchoring effect, resulting in higher adhesion to adjacent positive electrode current collectors (such as metal foils) compared to positive electrode layers without such materials. Therefore, by using the precursor of the present invention, the peel strength between the positive electrode layer and the positive electrode current collector can be improved. Although the binder dosage in the positive electrode layer is sometimes increased to improve the peel strength between the positive electrode layer and the positive electrode current collector, increasing the binder dosage reduces the electronic conductivity of the positive electrode layer, leading to an increase in the battery's IV resistance (short-term IV resistance). According to the present invention, even without increasing the binder dosage, a positive electrode layer with excellent peel strength to the current collector can be obtained, thereby improving the output characteristics of the positive electrode.

[0032] In this invention, the sphericity of the high-sphericity precursor particles only needs to be greater than 0.10, but can be greater than 0.15, greater than 0.20, or greater than 0.30. Alternatively, the sphericity of the high-sphericity precursor particles can be less than 3.60 or less than 0.40.

[0033] Furthermore, in this invention, the precursor particles are typically secondary particles formed by the aggregation of multiple primary particles, but in addition to secondary particles, they may also include individual, unaggregated primary particles. The particle shape is not particularly limited; for example, approximately spherical or approximately elliptical shapes are possible.

[0034] There are no particular restrictions on the size of the precursor particles. The size of the precursor particles can be determined by measuring the cross-sectional dimensions of multiple particles in SEM images and TEM (Transmission Electron Microscope) images and calculating their average value.

[0035] The long-side diameter (Dmax, the longest diagonal length) of the highly spherical precursor particles can be greater than 2.1 μm and less than 10.2 μm. Furthermore, the short-side diameter (Dmin, the shortest diagonal length) of the highly spherical precursor particles can be greater than 1.3 μm and less than 9.4 μm. Here, Dmax and Dmin are average values.

[0036] In the precursor of this invention, there is no particular limitation on the proportion of highly spherical precursor particles as long as it is greater than 0% by mass. The proportion of highly spherical precursor particles can be 10% by mass or more, 20% by mass or more, 50% by mass or more, or 100% by mass.

[0037] The precursor of this invention is a nickel complex hydroxide. A nickel complex hydroxide is a hydroxide containing nickel (Ni) and other metals besides nickel. The other metals besides nickel can be one or more. Examples of other metals besides nickel include manganese (Mn), cobalt (Co), and aluminum (Al). Specific examples of nickel complex hydroxides include nickel-cobalt complex hydroxides containing nickel and cobalt, nickel-cobalt-manganese complex hydroxides containing nickel, cobalt, and manganese, and nickel-cobalt-aluminum complex hydroxides containing nickel, cobalt, and aluminum. The nickel complex hydroxide can be a nickel-cobalt-manganese complex hydroxide.

[0038] In these nickel-cobalt hydroxide complexes, there are no particular restrictions on the molar ratio of nickel to other metals relative to the total amount of nickel and other metals. For nickel-cobalt hydroxide complexes, the molar ratios can be as follows: Ni / NiCo can be 0.5 or more and less than 1.0, and Co / NiCo can be greater than 0 and less than 0.5. For nickel-cobalt-manganese hydroxide complexes, the molar ratios can be as follows: Ni / NiCoMn can be 0.5 or more and less than 1.0, Co / NiCoMn can be greater than 0 and less than 0.3, and Mn / NiCoMn can be greater than 0 and less than 0.3. For nickel-cobalt-aluminum hydroxide complexes, the molar ratios can be as follows: Ni / NiCoAl can be 0.5 or more and less than 1.0, Co / NiCoAl can be greater than 0 and less than 0.3, and Al / NiCoAl can be greater than 0 and less than 0.3.

[0039] It should be noted that, in this invention, the nickel composite hydroxide may also contain other metals besides nickel, cobalt, aluminum, and manganese. These other metals may be, for example, at least one selected from the group consisting of Zr, Mo, Mg, Ca, Na, Fe, Cr, Zn, Si, Sn, and Ag.

[0040] The method for manufacturing the precursor of the positive electrode active material of the present invention is not particularly limited, and the following methods can be listed as examples.

[0041] First, a water-soluble nickel source (nickel compound) and other water-soluble metal sources (cobalt compound, manganese compound, aluminum compound, etc.) are dissolved in ion-exchanged water to prepare an aqueous metal source solution. Typically, the molar percentage (mol%) of nickel and other metals in this aqueous metal source solution is set to be equal to the molar percentage (mol%) of nickel and other metals constituting the nickel-containing hydroxide. There are no particular limitations on the water-soluble metal compounds; sulfates are an example. The concentration of the aqueous metal source solution is not particularly limited; for example, it can be set to a total metal concentration (the ratio of all raw materials to the aqueous metal source solution) of 1.5 mol%.

[0042] Next, a certain amount of NH3 aqueous solution (ammonium ion donor) is loaded into the reaction vessel, and nitrogen is purged while stirring with a stirrer to form a non-oxidizing atmosphere.

[0043] Subsequently, an aqueous solution of sodium hydroxide was added to the reaction vessel, and while maintaining the pH at an alkaline level (e.g., pH 12), the above-mentioned aqueous solutions of metal raw materials and NH3 were added dropwise to the reaction vessel.

[0044] At this point, by setting conditions for simultaneous nucleation and nucleus growth of the nickel complex hydroxide, precursor particles with a sphericity greater than 0.10 can be formed. It can be considered that under conditions of simultaneous nucleation and nucleus growth, primary particles of varying sizes are first formed, and when these form secondary particles, secondary particle formation occurs, resulting in particles with a high sphericity value, i.e., deformed relative to a perfect circle. For example, conditions for simultaneous nucleation and nucleus growth can be exemplified by setting the stirring speed during the dropwise addition of the metal raw material aqueous solution and the NH3 aqueous solution to a speed lower than the normal speed. Specifically, a stirring speed of 100–700 rpm can be set, for example.

[0045] Under the same stirring speed, precursor particles with different sphericities can be formed, for example, by changing the reaction time. For example, by shortening the reaction time, the sphericity can be increased. The reaction time can be, for example, 1 to 20 hours. There are no particular limitations on the reaction temperature; for example, it can be set to 60°C.

[0046] After the reaction is complete, a drying process is performed. This drying process can be carried out, for example, at 120°C for 1 hour under an inert gas atmosphere.

[0047] The precursor of the positive electrode active material of the present invention can be used as a positive electrode active material for lithium-ion batteries, for example, by being converted into lithium-nickel composite oxide.

[0048] Lithium-nickel composite oxides can be manufactured, for example, from the precursors of the present invention by the following method: mixing a nickel composite hydroxide, which is a precursor of the present invention, with a lithium compound, which is a lithium source, and then calcining the resulting mixture.

[0049] Examples of lithium compounds include at least one selected from lithium carbonate, lithium nitrate, lithium acetate, lithium hydroxide, lithium oxide, and lithium chloride.

[0050] The ratio of lithium compound to precursor in the mixture is typically set as follows: the molar percentage (mol%) of lithium and other metals in the target cathode active material relative to the total stoichiometry of lithium and metals in the precursor is equal to the molar percentage (mol%) of lithium and other metals in the mixture. There are no particular restrictions on the mixing method; known methods can be used.

[0051] The resulting mixture can be calcined at 700–950°C for 8–10 hours to obtain lithium-nickel composite oxide, for example. Calcination can be performed using a known calcining furnace such as a muffle furnace.

[0052] The positive electrode active material obtained by sintering the precursor of the present invention is generally considered to be composed of single crystal particles.

[0053] Here, a single-crystal particle refers to a single particle that does not constitute a secondary particle; it is a particle that is essentially composed of a single crystal. This can be confirmed by the fact that grain boundaries cannot be identified in SEM images.

[0054] In addition, the positive electrode active material obtained by calcining the precursor of the present invention is generally considered to have a layered rock salt structure.

[0055] The precursors for the positive electrode active material provided by the present invention can be used, for example, as precursors for the positive electrode active material constituting the positive electrode of a battery (such as a lithium-ion battery). That is, the present invention provides a battery in which a positive electrode, an electrolyte layer, and a negative electrode are stacked sequentially, wherein the positive electrode contains a positive electrode active material obtained from the precursors of the present invention.

[0056] The following is an explanation of the battery.

[0057] The positive electrode has a positive electrode layer and, if necessary, a positive electrode current collector.

[0058] The positive electrode layer is a layer containing at least a positive electrode active material. The positive electrode active material may contain only the positive electrode active material obtained from the precursor of the present invention described above, or it may also contain other active materials. The content of the positive electrode active material in the positive electrode layer is not particularly limited, and may, for example, be 20–80% by mass.

[0059] The positive electrode layer may contain at least one of an electrolyte, a conductive material, and a binder, as needed.

[0060] Examples of electrolytes include solid electrolytes. Solid electrolytes can be inorganic solid electrolytes such as sulfide solid electrolytes, halide solid electrolytes, oxide solid electrolytes, and complexed hydride solid electrolytes, or organic solid electrolytes such as gel electrolytes. The proportion of solid electrolyte in the positive electrode layer can be, for example, 10–60% by mass.

[0061] Examples of conductive materials include carbon materials, metal particles, and conductive polymers. Examples of carbon materials include particulate carbon materials such as acetylene black (AB) and Ketjen black (KB), and fibrous carbon materials such as vapor-grown carbon fibers (VGCF), carbon nanotubes (CNTs), and carbon nanofibers (CNFs). The proportion of conductive material in the positive electrode layer can be, for example, 0.1–5% by mass.

[0062] Examples of binders include styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber (NBR), butadiene rubber (BR), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), styrene-isoprene-styrene block copolymer (SIS), and ethylene-propylene-diene copolymer (EPDM). The proportion of the binder in the positive electrode layer can be, for example, 0.5 to 5% by mass.

[0063] Materials used as positive current collectors include, for example, SUS, Cr, Au, Pt, Zn, aluminum, copper, nickel, iron, titanium, and carbon. The thickness of the positive current collector is, for example, 0.1 μm or more and 100 μm or less. The shape of the positive current collector can be foil-like or plate-like. There are no particular limitations on the top view shape of the positive current collector; for example, circular, elliptical, rectangular, and arbitrary polygonal shapes are possible. The positive current collector can also be composed of a buffer layer, an elastic layer, or a PTC (Positive Temperature Coefficient) thermistor layer disposed on its surface.

[0064] The negative electrode has a negative electrode layer and, if necessary, a negative electrode current collector.

[0065] The negative electrode layer is a layer containing at least a negative electrode active material. Additionally, the negative electrode layer may contain at least one of an electrolyte, a conductive material, and a binder, depending on requirements. Examples of negative electrode active materials for lithium-ion batteries include carbon materials such as natural graphite, elemental Li, and Li alloys.

[0066] Regarding the electrolyte, conductive material, and binder used in the negative electrode layer, the same substances as those described in the positive electrode layer can be listed.

[0067] Materials used as negative current collectors include, for example, SUS, aluminum, copper, nickel, iron, titanium, and carbon. The thickness of the negative current collector is, for example, 0.1 μm or more and 100 μm or less. The shape of the negative current collector can be foil-like or plate-like. There are no particular limitations on the top view shape of the negative current collector; for example, circular, elliptical, rectangular, and arbitrary polygonal shapes are possible. The negative current collector can also be composed of a buffer layer, an elastic layer, or a PTC thermistor layer disposed on its surface.

[0068] The electrolyte layer is a layer formed between the positive electrode layer and the negative electrode layer, and it contains at least an electrolyte. Examples of electrolytes, besides the solid electrolytes described in the positive electrode layer above, include liquid electrolytes.

[0069] As an electrolyte, aqueous electrolytes and non-aqueous electrolytes can be used. These can be used individually or in combination of two or more.

[0070] Aqueous electrolytes contain water as a main solvent component. That is, based on the total amount of solvent (liquid component) constituting the electrolyte (100 mol%), water can account for 50 mol% or more, particularly 70 mol% or more, and further 90 mol% or more. On the other hand, there is no particular upper limit on the proportion of water in the solvent.

[0071] Although the solvent contains water as the main component, it may also contain solvents other than water. Examples of solvents other than water include one or more selected from ethers, carbonates, nitriles, alcohols, ketones, amines, amides, sulfur compounds, and hydrocarbons. Based on the total amount of the solvent (liquid component) constituting the electrolyte (100 mol%), the solvents other than water may be 50 mol% or less, particularly 30 mol% or less, and further, 10 mol% or less.

[0072] Aqueous electrolytes contain electrolytes. Electrolytes known for use in aqueous electrolytes can be used. Examples of electrolytes include lithium salts, nitrates, acetates, and sulfates of imine compounds. Specific examples include lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(pentafluoroethanesulfonyl)imide (LiBETI), lithium bis(nonafluorobutyryl)imide, lithium nonafluoro-N-[(trifluoromethane)sulfonyl]butyrylamide, lithium N,N-hexafluoro-1,3-disulfonylimide, CH3COOLi, LiPF6, LiBF4, Li2SO4, and LiNO3.

[0073] Regarding the concentration of the electrolyte in an aqueous electrolyte solution, as long as it does not exceed the saturation concentration range of the electrolyte in the solvent, it can be appropriately set according to the desired battery characteristics. This is because if there are residual solid electrolytes in the aqueous electrolyte solution, these solids may hinder the battery reaction.

[0074] For example, when using LiTFSI as the electrolyte, the aqueous electrolyte may contain more than 1 mole of LiTFSI relative to 1 kg of water, particularly more than 5 moles, and further more than 7.5 moles. There is no particular upper limit, for example, it may be less than 25 moles.

[0075] As a non-aqueous electrolyte, an electrolyte containing lithium salt and non-aqueous solvent is usually used.

[0076] Examples of lithium salts include inorganic lithium salts such as LiPF6, LiBF4, LiClO4, and LiAsF6; and organic lithium salts such as LiCF3SO3, LiN(SO2CF3)2(Li-TFSI), LiN(SO2C2F5)2, and LiC(SO2CF3)3.

[0077] Examples of non-aqueous solvents include ethylene carbonate (EC), propylene carbonate (PC), butyl carbonate (BC), dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), γ-butyrolactone, sulfolane, acetonitrile (AcN), dimethoxymethane, 1,2-dimethoxyethane (DME), 1,3-dimethoxypropane, diethyl ether, tetraethylene glycol dimethyl ether (TEGDME), tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide (DMSO), and mixtures thereof. From the viewpoint of ensuring high dielectric constant and low viscosity, it can be a mixture of cyclic carbonate compounds such as EC, PC, and BC with high dielectric constant and high viscosity and chain carbonate compounds such as DMC, DEC, and EMC with low dielectric constant and low viscosity, or it can be a mixture of EC and DEC.

[0078] The concentration of lithium salt in non-aqueous electrolytes can be, for example, 0.3–5 M.

[0079] Non-aqueous electrolytes can also contain ionic liquids. Ionic liquids can include, for example, sulfonium salts, ammonium salts, and pyridine. Salt, piperidine Salt, pyrrolidine Salt, Morpholine Salt, Salt, imidazole At least one of the group consisting of salts and their derivatives.

[0080] The electrolyte layer can be a membrane impregnated with the above-mentioned electrolyte and used to prevent the positive electrode layer from contacting the negative electrode layer.

[0081] There are no particular limitations on the material used for the membrane, as long as it is a porous membrane. Examples include resins such as polyethylene (PE), polypropylene (PP), polyester, polyvinyl alcohol, cellulose, and polyamide, as well as polyethylene and polypropylene. Furthermore, the membrane can be a single-layer or multi-layer structure. Examples of multi-layer membranes include two-layer membranes like PE / PP, or three-layer membranes like PP / PE / PP or PE / PP / PE.

[0082] The diaphragm can also be made of resin nonwoven fabric, glass fiber nonwoven fabric, or other nonwoven fabrics.

[0083] The battery may also include a constraint fixture that applies constraint pressure to the positive electrode layer, electrolyte layer, and negative electrode layer along the thickness direction. The constraint pressure can be, for example, 0.1 MPa to 100 MPa.

[0084] There are no particular restrictions on the type of battery; generally, it is a battery in which metal ions conduct between the positive and negative electrode layers. Lithium-ion batteries are an example of such batteries. Furthermore, the battery can be a primary battery or a secondary battery, but a secondary battery is preferred. This is because it can be repeatedly charged and discharged, making it useful, for example, as a battery for automotive applications.

[0085] There are no particular restrictions on the shape of the battery; for example, it can be coin-shaped, cylindrical, square, sheet-shaped, button-shaped, flat, or stacked.

[0086] Batteries are used in various applications, including as power sources for vehicles such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), battery electric vehicles (BEVs), gasoline vehicles, and diesel vehicles. In particular, they can be used as drive power sources for HEVs, PHEVs, and BEVs. Furthermore, batteries can also be used as power sources for mobile bodies other than vehicles (such as railway vehicles, ships, and aircraft), and for electrical products such as information processing devices.

[0087] Preparation of precursors for positive electrode active materials

[0088] Synthesis of precursor particles A to F

[0089] NiSO4, CoSO4, and MnSO4 were dissolved in ion-exchanged water to prepare an aqueous solution of the metal raw materials. The Ni / Co / Mn ratio in the aqueous solution was 80 / 10 / 10 (mol%). The concentration of the aqueous solution (the ratio of all raw materials to the aqueous solution) was set to 1.5 mol%.

[0090] A certain amount of NH3 aqueous solution was added to the reaction vessel, and nitrogen was purged while stirring with a stirrer. NaOH aqueous solution was then added to the reaction vessel, maintaining the pH at alkaline (pH=12), while the above-mentioned metal raw material aqueous solution and NH3 aqueous solution were added dropwise. During the dropwise addition, the stirrer speed was set to 400 rpm and the reaction temperature to 60°C, synthesizing precursor particles A to E. Regarding the reaction time of precursor particles A to E, precursor particles B and C were longer than precursor particle A, while precursor particles D and E were shorter. Additionally, during the dropwise addition, the stirrer speed was set to 800 rpm, the reaction temperature to 60°C, and the reaction time to 10 hours, synthesizing precursor particle F.

[0091] After the reaction is complete, the mixture is dried at 120°C for 1 hour under an inert gas atmosphere.

[0092] The obtained precursor particles A to F were observed by SEM, and their roundness was calculated according to the aforementioned method. The roundness, Dmin, and Dmax are shown in Table 1.

[0093] (Table 1)

[0094] Table 1 compares precursor particle F (stirring speed 800 rpm) with precursor particles A to E (stirring speed 400 rpm), showing that by reducing the stirring speed, high-sphericity precursor particles with a sphericity of 0.20 or higher (greater than 0.10) can be synthesized. Furthermore, comparing precursor particles A to E at the same stirring speed, it is found that, based on precursor particle A, extending the reaction time decreases the sphericity of the precursor particles (precursor particles B and C), while shortening the reaction time increases the sphericity of the precursor particles (precursor particles D and E).

[0095] Preparation of the precursor in Example 1

[0096] The precursor particles A with a high sphericity of 0.30 were mixed at a ratio of 10% by mass, and the precursor particles F with a low sphericity of 0.10 were mixed at a ratio of 90% by mass, as the precursor of Example 1.

[0097] Preparation of the precursor in Example 2

[0098] The precursor particles A with a high sphericity of 0.30 were mixed at a ratio of 20% by mass, and the precursor particles F with a low sphericity of 0.10 were mixed at a ratio of 80% by mass, as the precursor of Example 2.

[0099] Preparation of the precursor in Example 3

[0100] High-roundness precursor particles A with a roundness of 0.30 were mixed at a ratio of 50% by mass, and low-roundness precursor particles F with a roundness of 0.10 were mixed at a ratio of 50% by mass, as the precursor of Example 3.

[0101] Preparation of the precursor in Example 4

[0102] 100% by mass of high-roundness precursor particles A with a roundness of 0.30 were used as the precursor in Example 4.

[0103] Preparation of the precursor in Example 5

[0104] The precursor particles B with a high sphericity of 0.25 and the precursor particles F with a low sphericity of 0.10 were mixed at a ratio of 50% by mass to form the precursor of Example 5.

[0105] Preparation of the precursor in Example 6

[0106] 100% by mass of high-roundness precursor particles C with a roundness of 0.20 were used as the precursor in Example 6.

[0107] Preparation of the precursor in Example 7

[0108] High-roundness precursor particles D with a roundness of 0.40 were mixed at a ratio of 50% by mass, and low-roundness precursor particles F with a roundness of 0.10 were mixed at a ratio of 50% by mass, as the precursor of Example 7.

[0109] Preparation of the precursor in Example 8

[0110] The precursor particles E with a high sphericity of 3.60 were mixed at a ratio of 50% by mass, and the precursor particles F with a low sphericity of 0.10 were mixed at a ratio of 50% by mass, as the precursor of Example 8.

[0111] Preparation of the precursor of Comparative Example 1

[0112] 100% by mass of low-sphericity precursor particles F with a sphericity of 0.10 were used as the precursor for Comparative Example 1.

[0113] Synthesis of positive electrode active material

[0114] The precursors of Examples 1-8 and Comparative Example 1 were respectively mixed with a lithium compound (LiOH) as a lithium source in a mortar. The resulting mixtures were calcined in a calcining furnace at 950°C for 10 hours, thereby synthesizing the positive electrode active materials (LiNi) of Examples 1-8 and Comparative Example 1. 0.8 Co 0.1 Mn 0.1 O2).

[0115] Positive electrode production

[0116] The positive electrodes of Examples 1-8 and Comparative Example 1 were prepared using the positive electrode active materials of Examples 1-8 and Comparative Example 1.

[0117] Specifically, firstly, using a coating applicator (manufactured by AllGlide Co., Ltd.) with film thickness adjustment function, a positive electrode composite slurry containing various positive electrode active materials, acetylene black as a conductive material, and PVDF (4% by mass) as a binder was coated onto the surface of a metal foil (Al foil) serving as the positive electrode current collector. Then, the positive electrode with a positive electrode layer on the positive electrode current collector, as described in Examples 1-8 and Comparative Example 1, was dried at 80°C for 5 minutes using a dryer.

[0118] Determination of peel strength

[0119] First, attach a piece of polyethylene tape approximately 2cm wide and 20cm long to the positive electrode layer side of the prepared positive electrode, removing any air bubbles to ensure adhesion. Next, using a dumbbell-shaped cutter, cut the positive electrode with the polyethylene tape attached into pieces 1cm wide and 15cm long to prepare a sample for the peel test.

[0120] For the specimens used in the peel test, the polyethylene tape and the positive electrode layer were peeled from the surface of the current collector, and the peel force (load, N / m) was measured. The peel conditions were set as follows: peel angle 90°±5°, peel speed 20 mm / min, and peel length 60 mm. From the obtained peel force data, a frequency distribution was plotted at intervals of 0.05 N / m, and the most frequent value was taken as the peel strength (peel strength between the positive current collector and the positive electrode layer).

[0121] Using the peel strength of Comparative Example 1 as a benchmark, the peel strengths of Examples 1 to 8 were normalized according to the following formula.

[0122] Normalized peel strength = (peel strength of each embodiment) / (peel strength of Comparative Example 1)

[0123] The results are shown in Table 2. Table 2 also shows the proportion, roundness, Dmin, and Dmax of the highly rounded precursor particles (precursor particles A to E) in the precursors of each embodiment. It should be noted that in Table 2, for Comparative Example 1 (containing 100% by mass of precursor particles F) which does not contain highly rounded precursor particles, the roundness, Dmin, and Dmax of precursor particle F are shown.

[0124] (Table 2)

[0125] As shown in Table 2, the cathodes of Examples 1-8 exhibited higher peel strength compared to Comparative Example 1. This is believed to be because the cathode active material obtained from precursors containing highly spherical precursor particles (precursor particles A-E with a sphericity greater than 0.10) improved the anchoring effect of the cathode layer on the cathode current collector.

[0126] A comparison of Examples 1 to 4, which contain precursor particles A with a sphericity of 0.30 in different proportions, suggests that the higher the proportion of high-sphericity precursor particles, the higher the peel strength.

[0127] Furthermore, by comparing Examples 3, 5, 7 and 8, which contain 50% by mass of high-sphericity precursor particles, and Examples 4 and 6, which contain 100% by mass of high-sphericity precursor particles, it can be seen that within the range of sphericity of 0.25 to 0.40, the higher the sphericity of the precursor particles, the higher the peel strength.

Claims

1. A precursor of a positive electrode active material, wherein, The precursor is a nickel complex hydroxide. The precursor contains more than 0% by mass of precursor particles with a roundness greater than 0.10 as calculated from scanning electron microscope (SEM) images.

2. The precursor according to claim 1, wherein, The sphericity is 0.20 or higher, and the long side diameter of the precursor particles is 2.1 μm or higher and 10.2 μm or lower.

3. The precursor according to claim 1, wherein, The roundness is 0.20 or higher, and the precursor contains 10% by mass or more of the precursor particles.

4. The precursor according to claim 1, wherein, The roundness is 3.60 or less.

5. The precursor according to claim 1, wherein, The nickel composite hydroxide is a nickel-cobalt-manganese composite hydroxide.

Images