Precursor of positive electrode active material

Abstract

This invention provides a precursor for a positive electrode active material that can improve the delamination strength between the positive electrode layer and the positive electrode current collector. [Solution] A precursor for a positive electrode active material, wherein the precursor is a nickel composite hydroxide, and the precursor contains more than 0% by mass of precursor particles whose roundness calculated from scanning electron microscope (SEM) images exceeds 0.10.

Description

Technical Field

[0001] The present disclosure relates to a precursor of a positive electrode active material.

Background Art

[0002] Various techniques have been proposed regarding positive electrode active materials as disclosed in Patent Documents 1 to 2.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] Conventionally, various positive electrode active materials have been proposed in order to obtain a positive electrode having high battery characteristics such as high cycle characteristics and high output characteristics. For example, Patent Document 1 discloses a positive electrode active material for an all-solid-state lithium-ion battery represented by the composition formula: Li a Ni x Co y Mn 1-x-y O2 (in the above formula, 1.00 ≤ a ≤ 1.03, 0.8 ≤ x ≤ 0.9, 0 ≤ y ≤ 0.16). The 50% cumulative volume particle size D50 is 3.0 to 7.0 μm, the tap density is 1.9 to 2.5 g / cc, and the roundness is 0.90 to 0.93. In Patent Document 1, the roundness is an index indicating how close the shape of the particles is to a sphere. For example, it is stated that the roundness of a true spherical particle is 1.00, which is the upper limit. Also, the following formula is described as the formula for calculating the roundness. Roundness = 4πS / L 2 (In the formula, S is the projected area of the particle, L is the perimeter of the particle projection image, and π is the ratio of the circumference of a circle to its diameter)

[0005] Conventional positive electrodes using positive electrode active materials, such as those described in Patent Document 1, suffer from the problem of low output characteristics. This is because the positive electrode layer and the positive electrode current collector that constitute the positive electrode are prone to delamination.

[0006] This disclosure has been made in view of the above circumstances, and its main purpose is to provide a precursor of a positive electrode active material that can improve the peeling strength between the positive electrode layer and the positive electrode current collector. [Means for solving the problem]

[0007] In other words, this disclosure includes the following aspects: <1> A precursor of the positive electrode active material, The precursor is a nickel complex hydroxide, The precursor is a precursor containing more than 0% by mass of precursor particles whose roundness, calculated from scanning electron microscope (SEM) images, exceeds 0.10.

[0008] <2> The roundness is 0.20 or greater, and the particle size Dmax on the long side of the precursor particle is 2.1 μm or more and 10.2 μm or less. <1> The precursor described above.

[0009] <3> The aforementioned roundness is 0.20 or higher, and the precursor particles contain 10% by mass or more. <1> or <2> The precursor described above.

[0010] <4> The roundness is 3.60 or less. <1> ~ <3> A precursor described in any one of the following. <5> The aforementioned nickel composite hydroxide is a nickel-cobalt-manganese composite hydroxide. <1> ~ <4> A precursor described in any one of the following. [Effects of the Invention]

[0011] According to this disclosure, it is possible to provide a precursor of a positive electrode active material that can improve the delamination strength between the positive electrode layer and the positive electrode current collector.

Brief Description of the Drawings

[0012] [Figure 1] FIG. 1 is a SEM image of precursor particles with a high degree of circularity contained in the precursor of the present disclosure. [Figure 2] FIG. 2 is a SEM image of precursor particles with a low degree of circularity contained in the precursor of the present disclosure.

Modes for Carrying Out the Invention

[0013] Hereinafter, embodiments according to the present disclosure will be described. In addition, matters other than those specifically mentioned in this specification and necessary for the implementation of the present disclosure (for example, general configurations and manufacturing processes of cathode active materials and their precursors that do not characterize the present disclosure) can be grasped as design matters of those skilled in the art based on the prior art in the relevant field. The present disclosure can be implemented based on the content disclosed in this specification and common technical knowledge in the relevant field. [[ID=I9]]

[0014] In the present disclosure, there is provided a precursor of a cathode active material, wherein the precursor is a nickel composite hydroxide, and the precursor contains more than 0% by mass of precursor particles having a circularity exceeding 0.10 calculated from a scanning electron microscope (SEM) image.

[0015] In the present disclosure, the circularity of the precursor particles is calculated as follows. First, in a SEM (Scanning Electron Microscope) image, the lengths D of the four diagonals formed when the precursor particles are divided into eight parts are measured respectively. Among them, using the longest length (particle size Dmax on the long side) and the shortest length (particle size Dmin on the short side), the circularity is calculated from the following formula. Circularity = (Dmax - Dmin) / 2

[0016] Furthermore, in this disclosure, the roundness of a precursor particle refers to the average value calculated from the roundness of multiple precursor particles. In other words, "containing more than 0% by mass of precursor particles with a roundness exceeding 0.10" means that the product contains more than 0% by mass of precursor particles whose average roundness calculated from multiple precursor particles exceeds 0.10. The number of precursor particles used to calculate roundness can be two or more, but it may be, for example, 60 or more, or 100 or more. Of the 100 precursor particles for which roundness has been calculated, the 20 with the highest roundness and the 20 with the lowest roundness may be excluded, and the average value may be calculated for the remaining 60 particles.

[0017] The closer the circularity is to 0, the higher the circularity of the precursor particle, meaning it is close to a perfect circle. Conversely, the greater the circularity, the lower the circularity of the precursor particle, meaning it has a distorted shape from a perfect circle. Figure 1 is an SEM image of a precursor particle with high circularity contained in the precursor of this disclosure, and Figure 2 is an SEM image of a precursor particle with low circularity contained in the precursor of this disclosure. Precursor particles with low circularity of 0.10 or less contained in the precursor of this disclosure have high circularity as shown in Figure 1, and precursor particles with high circularity exceeding 0.10 have low circularity as shown in Figure 2.

[0018] Precursor particles having a roundness exceeding 0.10 (hereinafter sometimes referred to as high-roundness precursor particles) can retain some of their irregular shape when synthesized as positive electrode active material by mixing them with a metal compound that serves as a metal source, such as a lithium compound, and firing them, thereby forming a positive electrode active material with an irregular shape. A positive electrode layer containing a positive electrode active material with an irregular shape is more likely to exhibit an anchoring effect and has higher bonding properties with adjacent positive electrode current collectors (metal foil, etc.) compared to a positive electrode layer that does not contain a positive electrode active material with an irregular shape. Therefore, by using the precursor of this disclosure, the peel strength between the positive electrode layer and the positive electrode current collector can be improved. To increase the peel strength between the positive electrode layer and the positive electrode current collector, the amount of binder in the positive electrode layer may be increased, but increasing the amount of binder reduces the electronic conductivity of the positive electrode layer and leads to an increase in the IV resistance (short-time IV resistance) of the battery. According to this disclosure, a positive electrode layer with excellent peel strength with the current collector can be obtained without increasing the amount of binder, thus improving the output characteristics of the positive electrode.

[0019] In this disclosure, the roundness of the highly round precursor particles may be greater than 0.10, 0.15 or greater, 0.20 or greater, or 0.30 or greater. Furthermore, the roundness of the highly round precursor particles may be 3.60 or less, or 0.40 or less.

[0020] Furthermore, in this disclosure, the precursor particles are usually secondary particles formed by the aggregation of multiple primary particles, but they may also include single primary particles that do not aggregate. The particle shape is not particularly limited and examples include a roughly spherical shape, a roughly elliptical shape, etc. The size of the precursor particles is not particularly limited and can be determined by measuring the cross-sectional sizes of multiple particles in SEM or TEM (Transmission Electron Microscope) images and calculating the average value. The highly round precursor particles may have a particle size Dmax (longest diagonal length) on the longer side of 2.1 μm or more, or 10.2 μm or less. Furthermore, the highly round precursor particles may have a particle size Dmin (shortest diagonal length) on the shorter side of 1.3 μm or more, or 9.4 μm or less. Here, Dmax and Dmin are average values.

[0021] In the precursor of this disclosure, the proportion of highly round precursor particles is not particularly limited as long as it is greater than 0% by mass, but may be 10% by mass or more, 20% by mass or more, 50% by mass or more, or 100% by mass.

[0022] The precursor of this disclosure is a nickel composite hydroxide. The nickel composite hydroxide is a hydroxide containing nickel (Ni) and other metal species other than nickel. The other metal species other than nickel may be one or two or more. Examples of other metal species other than nickel include manganese (Mn), cobalt (Co), and aluminum (Al). Specific examples of nickel composite hydroxides include nickel-cobalt composite hydroxide containing nickel and cobalt, nickel-cobalt-manganese composite hydroxide containing nickel, cobalt, and manganese, and nickel-cobalt-aluminum composite hydroxide containing nickel, cobalt, and aluminum. The nickel composite hydroxide may also be a nickel-cobalt-manganese composite hydroxide.

[0023] In these nickel composite hydroxides, the ratio (molar ratio) of nickel to each of the other metal species to the total amount of nickel and the other metal species is not particularly limited. In the case of nickel-cobalt composite hydroxide, the molar ratio may be as follows: Ni / NiCo may be 0.5 or more and less than 1.0, and Co / NiCo may be greater than 0 and less than or equal to 0.5. In the case of nickel-cobalt-manganese composite hydroxide, the molar ratio may be as follows: Ni / NiCoMn may be 0.5 or more and less than 1.0, Co / NiCoMn may be greater than 0 and less than or equal to 0.3, and Mn / NiCoMn may be greater than 0 and less than or equal to 0.3. In the case of nickel-cobalt-aluminum composite hydroxide, the molar ratio may be as follows: Ni / NiCoAl may be 0.5 or more and less than 1.0, Co / NiCoAl may be greater than 0 and less than or equal to 0.3, and Al / NiCoAl may be greater than 0 and less than or equal to 0.3.

[0024] In this disclosure, the nickel composite hydroxide may contain other metal species besides nickel, cobalt, aluminum, and manganese. The other metal species 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.

[0025] The method for producing the precursor of the positive electrode active material of this disclosure is not particularly limited, and examples include the following methods. First, a water-soluble nickel source (nickel compound) and other water-soluble metal sources (cobalt compound, manganese compound, aluminum compound, etc.), which will be used as raw materials for nickel composite hydroxides, are dissolved in deionized water to prepare an aqueous solution of metal raw materials. At this time, the ratio (mol%) of nickel and each of the other metal species to the total amount of nickel and other metal species in the aqueous solution of metal raw materials is typically equal to the ratio (mol%) of nickel and each of the other metal species that constitute the nickel composite hydroxide. The water-soluble metal compounds are not particularly limited, but examples include sulfates. The concentration of the aqueous solution of metal raw materials is not particularly limited, and for example, it can be a concentration such that the total concentration of the metal species (the ratio of all raw materials to the aqueous solution of metal raw materials) is 1.5 mol%. Next, a fixed amount of NH3 aqueous solution (ammonium ion supply) is added to the reaction vessel, and while stirring with a stirrer or the like, nitrogen is purged to create a non-oxidizing atmosphere.

[0026] Next, sodium hydroxide aqueous solution is added to the reaction vessel, and while maintaining the pH at an alkaline level (for example, pH 12), the above metal raw material aqueous solution and NH3 aqueous solution are added dropwise to the reaction vessel. In this case, by setting conditions such that nucleation and growth of nickel complex hydroxide proceed simultaneously, precursor particles with a roundness exceeding 0.10 can be formed. Under conditions where nucleation and growth proceed simultaneously, primary particles of different sizes are formed first, and when these form secondary particles, it is thought that secondary particles with a large roundness value, i.e., distorted secondary particles from perfectly circular ones, occur. Conditions for simultaneous nucleation and growth include, for example, reducing the stirring speed during the dropwise addition of the metal raw material aqueous solution and the NH3 aqueous solution compared to normal. Specifically, the stirring speed can be set to, for example, 100 to 700 rpm. Under the same stirring speed conditions, precursor particles with different degrees of roundness can be formed, for example, by changing the reaction time. For example, the degree of roundness can be increased by shortening the reaction time. The reaction time may be, for example, 1 to 20 hours. The reaction temperature is not particularly limited and can be, for example, 60°C.

[0027] After the reaction is complete, a drying treatment is performed. The drying treatment can be carried out, for example, at 120°C for 1 hour under an inert gas atmosphere.

[0028] The precursor of the positive electrode active material of this disclosure can be used as a positive electrode active material for lithium-ion batteries and the like by, for example, converting it into a lithium nickel composite oxide. Lithium nickel composite oxide can be produced from the precursor of the present disclosure by, for example, the following method: mixing the nickel composite hydroxide, which is the precursor of the present disclosure, with a lithium compound that serves as a lithium source, and calcining the resulting mixture.

[0029] Examples of lithium compounds include at least one selected from lithium carbonate, lithium nitrate, lithium acetate, lithium hydroxide, lithium oxide, lithium chloride, and the like. The ratio of lithium compound to precursor in the mixture is such that the ratio (mol%) of lithium and each of the other metal species relative to the total amount of lithium in the target cathode active material and the metal species contained in the precursor is typically equal to the ratio (mol%) of lithium and each of the other metal species in the mixture. The mixing method is not particularly limited, and known methods can be employed.

[0030] The resulting mixture can be calcined, for example, at 700-950°C for 8-10 hours to obtain a lithium nickel composite oxide. A known calcination furnace, such as a muffle furnace, can be used for calcination. The positive electrode active material obtained by calcining the precursor of this disclosure is generally considered to consist of single crystal particles. Here, a single-crystal particle refers to a single particle that does not constitute secondary particles, essentially meaning a particle composed of a single crystal. The fact that it is a single-crystal particle can be confirmed by the absence of grain boundaries in the SEM image. Furthermore, the cathode active material obtained by calcining the precursor of this disclosure is generally considered to have a layered rock salt structure.

[0031] The positive electrode active material precursor provided in this disclosure can be used, for example, as a precursor for the positive electrode active material that constitutes the positive electrode of a battery (such as a lithium-ion battery). That is, this disclosure provides a battery in which a positive electrode, an electrolyte layer, and a negative electrode are stacked in this order, and the positive electrode contains a positive electrode active material obtained from the precursor of this disclosure. The following explains batteries.

[0032] The positive electrode has a positive electrode layer and, if necessary, further has a positive electrode current collector. 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 described above, or it may further contain other active materials. The content of the positive electrode active material in the positive electrode layer is not particularly limited and may be, for example, 20 to 80% by mass. The positive electrode layer may optionally contain at least one of an electrolyte, a conductive material, and a binder. Examples of electrolytes include solid electrolytes. The solid electrolyte may be an inorganic solid electrolyte such as a sulfide solid electrolyte, a halide solid electrolyte, an oxide solid electrolyte, or a complex hydride solid electrolyte, or it may be an organic solid electrolyte such as a gel electrolyte. The proportion of the solid electrolyte in the positive electrode layer may be, for example, 10 to 60% by mass. 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 Ketjenblack (KB), and fibrous carbon materials such as vapor-processed carbon fiber (VGCF), carbon nanotubes (CNT), and carbon nanofibers (CNF). The proportion of conductive material in the positive electrode layer may be, for example, 0.1 to 5% by mass. 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 binder in the positive electrode layer may be, for example, 0.5 to 5% by mass. Examples of materials for the positive electrode current collector include SUS, Cr, Au, Pt, Zn, aluminum, copper, nickel, iron, titanium, and carbon. The thickness of the positive electrode current collector is, for example, 0.1 μm or more and 100 μm or less. The shape of the positive electrode current collector may be foil-like or plate-like. The planar shape of the positive electrode current collector is not particularly limited, but examples include circular, elliptical, rectangular, and any polygonal shape. The positive electrode current collector may have a buffer layer, an elastic layer, or a PTC (Positive Temperature Coefficient) thermistor layer on its surface.

[0033] The negative electrode has a negative electrode layer and, if necessary, further has a negative electrode current collector. The negative electrode layer is a layer containing at least a negative electrode active material. The negative electrode layer may also optionally contain at least one of an electrolyte, a conductive material, and a binder. Examples of negative electrode active materials for lithium-ion batteries include carbon materials such as natural graphite, elemental Li, and Li alloys. The electrolyte, conductive material, and binder used in the negative electrode layer are the same as those described for the positive electrode layer above. Examples of materials for the negative electrode current collector include SUS, aluminum, copper, nickel, iron, titanium, and carbon. The thickness of the negative electrode current collector is, for example, 0.1 μm or more and 100 μm or less. The shape of the negative electrode current collector may be foil-like or plate-like. The planar shape of the negative electrode current collector is not particularly limited, but examples include circular, elliptical, rectangular, and any polygonal shape. The negative electrode current collector may have a buffer layer, an elastic layer, or a PTC thermistor layer arranged on its surface.

[0034] The electrolyte layer is a layer formed between the positive electrode layer and the negative electrode layer, and contains at least an electrolyte. Examples of electrolytes include the solid electrolyte described for the positive electrode layer, as well as an electrolyte solution. Aqueous electrolytes and non-aqueous electrolytes can be used as the electrolyte. These may be used individually or in combination of two or more types.

[0035] The solvent in an aqueous electrolyte contains water as its main component. That is, water may account for 50 mol% or more, particularly 70 mol% or more, and even 90 mol% or more, of the total amount of solvent (liquid component) constituting the electrolyte, based on a standard (100 mol%). On the other hand, there is no particular upper limit to the proportion of water in the solvent. The solvent mainly consists of water, but 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. The amount of solvent other than water may be 50 mol% or less, particularly 30 mol% or less, and even 10 mol% or less, based on the total amount of solvent (liquid component) constituting the electrolyte.

[0036] Aqueous electrolytes contain an electrolyte. Conventionally known electrolytes can be used for aqueous electrolytes. Examples of electrolytes include lithium salts, nitrates, acetates, and sulfates of imidic acid compounds. Specific examples of electrolytes include lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(pentafluoroethanesulfonyl)imide (LiBETI), lithium bis(nonafluorobutanesulfonyl)imide, lithium nonafluoro-N-[(trifluoromethane)sulfonyl]butanesulfonylamide, lithium N,N-hexafluoro-1,3-disulfonylimide, CH3COOLi, LiPF6, LiBF4, Li2SO4, and LiNO3.

[0037] The electrolyte concentration in an aqueous electrolyte can be appropriately set according to the desired battery characteristics, as long as it does not exceed the saturation concentration of the electrolyte relative to the solvent. This is because if solid electrolyte remains in the aqueous electrolyte, that solid may inhibit the battery reaction. For example, when LiTFSI is used as the electrolyte, the aqueous electrolyte may contain 1 mole or more of LiTFSI per 1 kg of water, and may contain 5 moles or more, and may contain 7.5 moles or more. The upper limit is not particularly limited and may be, for example, 25 moles or less.

[0038] As a non-aqueous electrolyte, one containing a lithium salt and a non-aqueous solvent is typically used. 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. Examples of non-aqueous solvents include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl 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, a mixture of cyclic carbonate compounds such as EC, PC, and BC having high dielectric constant and high viscosity, and chain-like carbonate compounds such as DMC, DEC, and EMC having low dielectric constant and low viscosity may be used, or a mixture of EC and DEC may be used. The concentration of the lithium salt in the non-aqueous electrolyte may be, for example, 0.3 to 5 M.

[0039] The non-aqueous electrolyte may contain an ionic liquid. The ionic liquid may contain, for example, at least one selected from the group consisting of sulfonium salts, ammonium salts, pyridinium salts, piperidinium salts, pyrrolidinium salts, morpholinium salts, phosphonium salts, imidazolium salts, and derivatives thereof.

[0040] The electrolyte layer may be impregnated with the aforementioned electrolyte solution, and a separator may be used to prevent contact between the positive electrode layer and the negative electrode layer. The material for the separator is not particularly limited as long as it is a porous membrane. Examples include polyethylene (PE), polypropylene (PP), polyester, polyvinyl alcohol, cellulose, and polyamide resins, with polyethylene and polypropylene being particularly preferred. The separator may have a single-layer structure or a multi-layer structure. Examples of multi-layer separators include a PE / PP two-layer separator, or a PP / PE / PP or PE / PP / PE three-layer separator. The separator may be made of nonwoven fabrics such as resin nonwoven fabric and glass fiber nonwoven fabric.

[0041] The battery may further include a restraining jig that applies restraining pressure to the positive electrode layer, electrolyte layer, and negative electrode layer along the thickness direction. The restraining pressure may be, for example, 0.1 MPa to 100 MPa.

[0042] The type of battery is not particularly limited, but it is usually a battery in which metal ions conduct between the positive and negative electrode layers. Lithium-ion batteries are an example of such batteries. The battery may be a primary battery or a secondary battery, but a secondary battery is preferable because it can be repeatedly charged and discharged and is useful, for example, as a battery for vehicles. The shape of the battery is not particularly limited and may be, for example, coin-type, cylindrical, prismatic, sheet-type, button-type, flat-type, or stacked-type.

[0043] Applications of batteries include, for example, powering vehicles such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), battery electric vehicles (BEVs), gasoline cars, and diesel cars. In particular, they may be used as power sources for hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), or battery electric vehicles (BEVs). Batteries may also be used as power sources for mobile devices other than vehicles (e.g., trains, ships, aircraft), and as power sources for electrical products such as information processing devices. [Examples]

[0044] [Preparation of precursors for positive electrode active material] (Synthesis of precursor particles A-F) Metal raw material aqueous solutions were prepared by dissolving NiSO4, CoSO4, and MnSO4 in deionized water. The Ni / Co / Mn ratio in the metal raw material aqueous solutions was 80 / 10 / 10 in mol%. The concentration of the metal raw material aqueous solutions (the ratio of all raw materials to the metal raw material aqueous solution) was 1.5 mol%. A fixed amount of NH3 aqueous solution was added to the reaction vessel, and the vessel was purged with nitrogen while stirring with a stirrer. NaOH aqueous solution was added to the reaction vessel, and while maintaining the pH at an alkaline level (pH=12), the above metal raw material aqueous solution and NH3 aqueous solution were added dropwise. During the dropwise addition, the stirring speed of the stirrer was set to 400 rpm and the reaction temperature to 60°C to synthesize precursor particles A to E. The reaction times for precursor particles A to E were adjusted so that precursor particles B and C had longer reaction times and precursor particles D and E had shorter reaction times, with precursor particle A being the baseline. Furthermore, during the dropwise addition, the stirring speed of the stirrer was set to 800 rpm, the reaction time to 60°C was set to 10 hours, and precursor particle F was synthesized. After the reaction was complete, the mixture was dried at 120°C for 1 hour under an inert gas atmosphere. For each of the obtained precursor particles A to F, SEM observation was performed, and the roundness was calculated using the method described above. The roundness, Dmin, and Dmax are shown in Table 1.

[0045] [Table 1]

[0046] Table 1 shows that comparing precursor particle F (stirring speed 800 rpm) with precursor particles A-E (stirring speed 400 rpm), it can be seen that reducing the stirring speed allows for the synthesis of highly round precursor particles with a roundness of 0.20 or higher (greater than 0.10). Furthermore, comparing precursor particles A-E at the same stirring speed, it can be seen that, using precursor particle A as a baseline, increasing the reaction time reduces the roundness of the precursor particles (precursor particles B and C), while decreasing the reaction time increases the roundness of the precursor particles (precursor particles D and E).

[0047] (Preparation of the precursor in Example 1) The precursor for Example 1 was prepared by mixing 10% by mass of high-roundness precursor particles A, which have a roundness of 0.30, and 90% by mass of low-roundness precursor particles F, which have a roundness of 0.10. (Preparation of the precursor in Example 2) The precursor for Example 2 was prepared by mixing 20% ​​by mass of high-roundness precursor particles A, which have a roundness of 0.30, and 80% by mass of low-roundness precursor particles F, which have a roundness of 0.10. (Preparation of the precursor in Example 3) The precursor for Example 3 was prepared by mixing 50% by mass of high-roundness precursor particles A, which have a roundness of 0.30, and 50% by mass of low-roundness precursor particles F, which have a roundness of 0.10. (Preparation of the precursor in Example 4) The precursor for Example 4 was prepared using 100% by mass of high-roundness precursor particles A, which had a roundness of 0.30. (Preparation of the precursor in Example 5) The precursor for Example 5 was prepared by mixing 50% by mass of high-roundness precursor particles B, which have a roundness of 0.25, and 50% by mass of low-roundness precursor particles F, which have a roundness of 0.10. (Preparation of the precursor in Example 6) The precursor for Example 6 was prepared using 100% by mass of high-roundness precursor particles C, which had a roundness of 0.20. (Preparation of the precursor in Example 7) The precursor for Example 7 was prepared by mixing 50% by mass of high-roundness precursor particles D, which have a roundness of 0.40, and 50% by mass of low-roundness precursor particles F, which have a roundness of 0.10. (Preparation of the precursor in Example 8) The precursor for Example 8 was prepared by mixing 50% by mass of high-roundness precursor particles E, which have a roundness of 3.60, and 50% by mass of low-roundness precursor particles F, which have a roundness of 0.10. (Preparation of the precursor in Comparative Example 1) 100% by mass of low-roundness precursor particles F, with a roundness of 0.10, was used as the precursor for Comparative Example 1.

[0048] [Synthesis of positive electrode active material] Each of the precursors from Examples 1-8 and Comparative Example 1 was mixed with a lithium compound (LiOH), which is a lithium source, in a mortar. The resulting mixture was calcined in a furnace at 950°C for 10 hours to obtain the positive electrode active material (LiNi) from Examples 1-8 and Comparative Example 1. 0.8 Co 0.1 Mn0.1 O2) was synthesized.

[0049] [Fabrication of the positive electrode] The positive electrodes of Examples 1-8 and Comparative Example 1 were prepared using the respective positive electrode active materials of Examples 1-8 and Comparative Example 1. Specifically, first, a positive electrode composite paste containing each positive electrode active material, 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) which served as the positive electrode current collector, using a film applicator with a film thickness adjustment function (manufactured by AllGrid Co., Ltd.). Then, it was dried in a dryer at 80°C for 5 minutes to produce positive electrodes for Examples 1 to 8 and Comparative Example 1, which had a positive electrode layer on the positive electrode current collector.

[0050] [Measurement of peel strength] First, a polyethylene tape approximately 2 cm wide and 20 cm long was attached to the positive electrode layer side of the positive electrode prepared as described above, and the air bubbles were removed and the tape was adhered. Next, using a dumbbell cutter, the positive electrode with the polyethylene tape attached was cut to a size of 1 cm wide and 15 cm long to prepare a sample for the peel test. In the peel test specimen, the polyethylene tape and positive electrode layer were peeled from the current collector surface, and the peeling force (load, N / m) was measured. The peeling conditions were a peeling angle of 90°±5°, a peeling speed of 20 mm / min, and a peeling length of 60 mm. The frequency distribution of the obtained peeling force data was taken at 0.05 N / m intervals, and the mode was defined as the peeling strength (peeling strength between the positive electrode current collector and the positive electrode layer). The peel strength of Examples 1 to 8 was normalized using the following formula, based on the peel strength of Comparative Example 1. Normalized peel strength = (Peel strength of each example) / (Peel strength of Comparative Example 1) The results are shown in Table 2. Table 2 also shows the percentage of highly round precursor particles (precursor particles A to E), roundness, Dmin, and Dmax in the precursor for each example. In addition, in Table 2, for Comparative Example 1 (containing 100% by mass of precursor particle F), which does not contain highly round precursor particles, the roundness, Dmin, and Dmax of precursor particle F are shown.

[0051] [Table 2]

[0052] As shown in Table 2, the positive electrodes of Examples 1 to 8 showed higher peel strength compared to Comparative Example 1. This is thought to be because the positive electrode active material obtained from a precursor containing highly rounded precursor particles (precursor particles A to E with roundness exceeding 0.10) improved the anchoring effect of the positive electrode layer on the positive electrode current collector. From a comparison of Examples 1 to 4, which contain precursor particles A with a roundness of 0.30 in different proportions, it can be concluded that the higher the proportion of highly round precursor particles, the higher the peel strength. Furthermore, a comparison of Examples 3, 5, 7, and 8, which contain 50% by mass of highly round precursor particles, and a comparison of Examples 4 and 6, which contain 100% by mass of highly round precursor particles, shows that in the roundness range of 0.25 to 0.40, the higher the roundness of the precursor particles, the higher the peel strength.

Claims

1. A precursor of the positive electrode active material, The precursor is a nickel complex hydroxide, The precursor comprises more than 0% by mass of precursor particles whose roundness, calculated from scanning electron microscope (SEM) images, exceeds 0.

10.

2. The precursor according to claim 1, wherein the roundness is 0.20 or greater, and the particle size Dmax on the long side of the precursor particle is 2.1 μm or greater and 10.2 μm or less.

3. The precursor according to claim 1, wherein the roundness is 0.20 or more and the precursor particles are present in an amount of 10% by mass or more.

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.

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