Precursor of positive electrode active material for lithium-ion secondary batteries

Abstract

To provide a precursor that obtains a positive electrode active material for a lithium ion secondary battery made of a Li-Ni-Co-Al or Li-Ni-Co-Mn composite oxide that has higher cycle characteristics than conventional positive electrode materials while minimizing the usage mount of Co, which is a rare metal.SOLUTION: In a positive electrode active material precursor for a lithium ion secondary battery obtained by connecting a plurality of thin polygonal crystals in a state of being partially integrated in the plane direction, the thin plate polygonal crystal is a metal hydroxide represented by the following general formula (1): A(1-z)Bz(OH)2 (1). In the formula (1), A is at least two or more selected from Ni, Co, Mn, and Al, B is Mg or Zr, and z represents 0.00≤z≤0.10.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a precursor of a cathode active material for a lithium-ion secondary battery composed of a Li-Ni-Co-Al-based or Li-Ni-Co-Mn-based composite oxide. In particular, as the precursor of the cathode active material for the Li-Ni-Co-Al-based or Li-Ni-Co-Mn-based cathode active material, it is a substance having a specific structure, and relates to the precursor of the cathode active material that can be used to form a secondary battery having high capacity, high charge-discharge cycle characteristics, and high rate characteristics.

Background Art

[0002] Lithium-ion secondary batteries are used in various applications such as power sources for mobile devices such as notebook computers and mobile phones, and power tools. From the perspective of building a low-carbon society and energy security, it is expected that their applications will further expand in the future, and there is an urgent need for their performance improvement.

[0003] In recent years, the demand for lithium-ion secondary batteries as power sources for hybrid vehicles and electric vehicles (hereinafter collectively referred to as EVs) or as energy storage materials for renewable power generation has been rapidly expanding. In these applications, in particular, it is desired to have high capacity, high charge-discharge cycles, and long life. For this reason, improvement in the materials of lithium-ion secondary batteries has become an urgent task. Among the materials constituting a lithium-ion secondary battery, as the cathode active material, LiCoO2 mainly composed of cobalt (Co) is widely used. In the current situation where the demand for power sources for EVs is rapidly expanding, Co, which is a rare metal, has a risk of resource depletion, and there is also concern that the cost will increase.

[0004] As cathode active materials to replace LiCoO2, there are cathode active materials composed of LiMn2O4 mainly composed of Mn or Ni-Co-Mn ternary composite oxides. However, these cathode active materials have advantages and disadvantages in battery characteristics, and at present, they do not fully meet the requirements for power tools and power sources for EVs.

[0005] Under such circumstances, a lithium-ion secondary battery using Li mainly composed of Ni, which has a high charge-discharge voltage and a large charge-discharge capacity, x Ni (1-y-α) Co y Al α O2 as a positive electrode active material has been studied. Conventionally, for improving high capacity and charge-discharge maintenance rate, etc., for example, a technique of improving conductivity and achieving long life by depositing an oxide containing Zn and Al on the surface of the LiNiO2 positive electrode active material (Patent Document 1), a technique of improving the charge-discharge capacity, packing property and storage property of the Li-Ni-Co-Al composite oxide by reducing the change rate of specific surface area before and after compression and the content of sulfate ion (Patent Document 2), a LiCoO2 particle material containing wire-shaped LiCoO2 particles and having fast electron conduction and improving the output characteristics of the battery (Non-Patent Document 1), etc. Various improvement techniques for Li x Ni (1-y-α) Co y Al α O2 powder have been proposed.

[0006] In Patent Documents 1 and 2 mentioned above, the Li x Ni (1-y-α) Co y Al α O2 positive electrode active material attempts to improve electron conductivity and achieve high capacity and long life by creating a coating layer composed of a surface modifier. In Patent Document 3, composite hydroxide secondary particles having different aggregation states in the particle center and outer periphery are synthesized and used as a precursor to form a positive electrode active material having a hollow structure in an attempt to improve cycle characteristics, but have not yet achieved sufficient effects. In the market, there is always a demand for the development of a positive electrode material for a lithium-ion secondary battery with better battery characteristics than conventional ones.

[0007] Furthermore, in response to the above demand, as a positive electrode material having excellent conductivity and capable of improving high capacity and charge-discharge maintenance rate, etc., the applicant of the present application has developed a spherical Li x Ni (1-y-α) Co y Al αWe have proposed a positive electrode active material for lithium-ion secondary batteries consisting of O2 (Patent Document 4, International Publication No. 2016 / 143844).

[0008] On the other hand, as a conventional LiCoO2 crystalline cathode material, hexagonal barrel-shaped or hexagonal plate-shaped crystals have been proposed and are said to have high rate characteristics (Non-Patent Document 2, Patent Document 5). However, as mentioned above, instead of Co, which is a rare metal and whose resources are feared to be depleted, Li, which is mainly composed of Ni and has a high charge / discharge voltage and large charge / discharge capacity, has been proposed. x Ni (1-y-α) Co y Al α O2 and Li x Ni (1-y-β) Co y Mn β In the case of O2, there is doubt as to whether the hexagonal barrel-shaped or plate-shaped crystals described above can be formed, given the properties of Ni, Mn, and Al. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Japanese Patent Publication No. 2011-129258 [Patent Document 2] Japanese Patent Publication No. 2008-166269 [Patent Document 3] Japanese Patent Publication No. 2020-71899 [Patent Document 4] International Publication No. 2016 / 143844 [Patent Document 5] International Publication No. 2019 / 189801 [Non-patent literature]

[0010] [Non-Patent Document 1] “Intrinsic Electrochemical Characteristics in the Individual Needle-like LiCoO2 Crystals Synthesized by Flux Growth”, The Electrochemical Society of Japan, February 5, 2010, Vol. 25, No. 2, pp. 72-75. [Non-Patent Document 2] Shinshu University, Faculty of Engineering, Oishi, Teshima, and Gata Laboratory, "Research on Cathode Materials for Lithium-ion Secondary Batteries," "Environmentally friendly growth of well-developed LiCoO2 crystals for lithium-ion rechargeable batteries using a NaCl flux"; Crystal Growth & Design, Publication Date: October 6, 2010, Vol. 10, No. 10, pp. 4471-4475. [Overview of the project] [Problems that the invention aims to solve]

[0011] Based on the above-mentioned prior art, the present invention aims to provide a precursor for obtaining a lithium-ion secondary battery cathode active material consisting of Li-Ni-Co-Al or Li-Ni-Co-Mn composite oxides that has higher cycle characteristics than conventional cathode materials, while minimizing the use of the rare metal Co. [Means for solving the problem]

[0012] The inventors conducted extensive research to solve the above problem and obtained the following findings. That is, Li x -N (1-y-β) -Co y -Mn β O2 system and Li x -N (1-y-α) -Co y -Al α Ni, a precursor of O2-based cathode active material particles (1-y-β) -Coy -Mn β Complex hydroxides and Ni (1-y-α) -Co y -Al α Complex hydroxides and Ni (1-y) -Co y Complex hydroxide oxides and their oxides are composed of multiple plate-like crystals that aggregate and grow in random directions. It is known that positive electrode active materials synthesized using such precursors develop cracks inside the particles when repeatedly charged and discharged as secondary batteries, and this is considered one of the causes of reduced cycle performance. This phenomenon is thought to be because the secondary particles of the positive electrode active material are composed of an aggregate of primary particle crystals oriented in random directions, and the direction of expansion and contraction of the crystals during charging and discharging is also irregular, causing cracks to form inside the particles due to expansion and contraction. Therefore, in order to obtain a positive electrode active material with high capacity and excellent cycle performance, it is expected that using a material system with a high Ni content and aligning the crystal orientation will suppress the formation of cracks inside the particles and result in a positive electrode active material with excellent cycle performance.

[0013] However, the inventors have confirmed that it is extremely difficult or impossible to create the special shapes described above using Li-Ni-Co-Al or Li-Ni-Co-Mn composite oxides, which are positive electrode active materials for lithium-ion secondary batteries. Therefore, after conducting extensive research on Co-Ni composite hydroxides, which are precursors to these positive electrode active materials for lithium-ion secondary batteries, they discovered that a positive electrode active material for lithium-ion secondary batteries that uses a series of plate-like, polygonal thin-plate crystalline bodies as its precursor exhibits excellent battery properties. Furthermore, although the above-mentioned Co-Ni composite hydroxide is hexagonal, the crystals may crack during manufacturing, handling, storage, or transportation, resulting in a series of polygonal shapes such as triangles, pentagons, or octagons. The inventors have also found that even when using these as precursors, a positive electrode active material for lithium-ion secondary batteries with excellent battery properties can be obtained.

[0014] The positive electrode active material precursor for lithium-ion secondary batteries of the present invention is based on the above findings and is characterized by being composed of a large number of thin polygonal crystals linked together in a state in which they are partially integrated in the planar direction. These thin polygonal crystals may be metal hydroxides represented by formula (1). A (1-z) B z (OH)2··· (1) A is at least two selected from Ni, Co, Mn, and Al. B is Mg or Zr, and z is 0.00 ≤ z ≤ 0.10. Furthermore, this positive electrode active material precursor for lithium-ion secondary batteries may consist of a composite oxide obtained by heat-treating the above-mentioned metal hydroxide. [Effects of the Invention]

[0015] The lithium-ion secondary battery positive electrode active material precursor (hereinafter sometimes abbreviated as "precursor") of the present invention is a composite oxide obtained by heat treatment of a large number of thin polygonal crystalline plates that are partially integrated in the planar direction, and can be used as a precursor for the positive electrode active material of a lithium-ion secondary battery. The thin polygonal crystals described above may also be metal hydroxides represented by formula (1) above. Batteries using the above precursor not only have a high initial discharge capacity, but also high capacity, high charge-discharge cycle characteristics, high rate characteristics, and a long lifespan, making them extremely effective as positive electrode active materials for future secondary batteries. [Brief explanation of the drawing]

[0016] [Figure 1] These are electron microscope images of the positive electrode active material precursor for lithium-ion secondary batteries obtained in Example 1 of the present invention, with (A) being a 5000x magnification, (B) a 10000x magnification, and (C) a 20000x magnification. [Figure 2] Figure 1 shows a schematic diagram illustrating a precursor of the positive electrode active material for lithium-ion secondary batteries. [Figure 3]This graph shows the initial discharge capacity (cycle characteristics) of batteries using cathode materials obtained from the precursors in Example 1, Comparative Example 1, and Comparative Example 2, as determined by the number of cycles. [Figure 4] This graph shows the discharge capacity retention rate (cycle characteristics) of batteries using cathode materials obtained from the precursors in Example 1, Comparative Example 1, and Comparative Example 2, based on the number of cycles. [Figure 5] The rate characteristics obtained in Example 1 and Comparative Example 1 are shown in Figure 5. In Figure 5, the solid line (A) represents the initial discharge capacity obtained in Example 1, the dashed line (B) represents the initial discharge capacity obtained in Comparative Example 1, and the dotted line (C) represents the initial discharge capacity obtained in Comparative Example 2. [Figure 6] This figure schematically shows a positive electrode active material obtained using the positive electrode active material precursor for lithium-ion secondary batteries shown in Figure 1. [Figure 7] These are electron microscope images showing the measurement method for the size of positive electrode material particles obtained using the caterpillar-shaped positive electrode active material precursor for lithium-ion secondary batteries manufactured in Example 1. Figure 7(A) shows the precursor particles, and Figure 7(B) shows the positive electrode active material particles. [Figure 8] These are electron microscope images used to measure the dimensions of cathode material particles obtained using the precursor produced in Comparative Example 1, using the same measurement method as in Example 1. Figure 8(A) shows the precursor particles, and Figure 8(B) shows the cathode active material particles. [Modes for carrying out the invention]

[0017] The positive electrode active material precursor for lithium-ion secondary batteries of the present invention has a structure in which many thin, plate-like crystalline bodies of polygonal shapes such as triangle 1, square 2, pentagon 3, and hexagon 4 are linked together, with adjacent faces in the planar direction (not the thickness direction) of the thin plates being partially integrated (bonded) with each other, as schematically shown in Figure 2, and exhibits a caterpillar-like appearance as shown in the micrograph in Figure 1.

[0018] The size of the precursor presenting the above caterpillar-like appearance (hereinafter sometimes referred to as "caterpillar-like precursor") is, for example, in FIG. 2, the diameter (hereinafter sometimes referred to as the short axis) T is approximately 0.5 to 4.0 μm, and the length (hereinafter sometimes referred to as the long axis) G is approximately 1.0 to 20.0 μm, and the ratio (G / T) is preferably 0.3 to 30.0, more preferably 1.0 to 20.0.

[0019] The caterpillar-like precursor of the present invention is used as a precursor of a composite oxide of the Li-Ni-Co-Al system or the Li-Ni-Co-Mn system. Specifically, the composition formula is Li x Ni (1-y-α) Co y Al α O2 (wherein, in the composition formula, 0.9 ≦ x ≦ 1.1, 0.03 ≦ y ≦ 0.3, 0.00 < α ≦ 0.05), or Li x Ni (1-y-β) Co y Mn β O2 (wherein, in the composition formula, 0.9 < x < 1.1, 0 ≦ y ≦ 0.33, 0.00 < β ≦ 0.33), etc., and is used as a precursor of a composite oxide, and is composed of a composite oxide of the Li-Ni-Co-Al system or the Li-Ni-Co-Mn system.

[0020] In the lithium ion secondary battery obtained by using the above-described caterpillar-like precursor of the present invention, in the lithium ion secondary battery using a positive electrode active material having a composition formula such as the above Li x Ni (1-y-α) Co y Al α O2, as shown in FIGS. 3 to 5 for example, compared with the lithium ion secondary battery using a conventional spherical positive electrode active material for a lithium ion secondary battery, both the initial charge-discharge capacity, cycle characteristics and rate characteristics are excellent.

[0021] Figure 3 shows the initial discharge capacity by cycle count, and Figure 4 shows the discharge capacity retention rate by cycle count. In Figures 3 and 4, reference numeral 5 denotes a lithium-ion secondary battery in which the positive electrode active material of the above compositional formula obtained using the caterpillar-shaped precursor of the present invention obtained in Example 1 is used as a component, and is measured under the measurement conditions described in Example 1. Reference numeral 6 denotes a lithium-ion secondary battery in which the conventional spherical positive electrode active material for lithium-ion secondary batteries obtained in Comparative Example 1, described later, is used, and is measured under the same conditions as above. For reference, in Comparative Example 2, an extremely small spherical positive electrode active material for lithium-ion secondary batteries with a diameter about the same as the diameter of the positive electrode active material using the caterpillar-shaped precursor of the present invention (average diameter of dimensions T and G in Figure 2) is obtained, and the discharge capacity and discharge capacity retention rate by cycle count when this extremely small spherical positive electrode active material for lithium-ion secondary batteries is used and measured under the same conditions as above is also shown as reference numeral 7.

[0022] Figure 5 shows the rate characteristics of a lithium-ion secondary battery. In Figure 5, the solid line (A) represents the case where the positive electrode active material using the caterpillar-shaped precursor of the present invention obtained in Example 1 was used and measured under the same conditions as above; the dashed line (B) represents the case where the conventional spherical positive electrode active material for lithium-ion secondary batteries obtained in Comparative Example 1 was used and measured under the same conditions as above; and the dotted line (C) represents the case where the conventional extremely small spherical positive electrode active material for lithium-ion secondary batteries obtained in Comparative Example 2 was used and measured under the same conditions as above.

[0023] As described above, the positive electrode active material using the caterpillar-shaped precursor of the present invention is superior to spherical positive electrode active materials for lithium-ion secondary batteries in various properties required for such batteries, and this is presumed to be due to the following reasons. During charging and discharging of a positive electrode active material for lithium-ion secondary batteries, lithium ions migrate from the surface to the center of the material and then back to the surface. In the case of the positive electrode active material for lithium-ion secondary batteries using the caterpillar-shaped (in other words, columnar) precursor of the present invention, the center is located in the direction that penetrates from one end to the other along the long axis, and the entire surface of both end faces and the entire side of the columnar shape is the surface. Since lithium ions migrate from the surface to the center, the migration distance of lithium ions is extremely short. On the other hand, in the case of conventional spherical positive electrode active materials for lithium-ion secondary batteries, the center is a single point at the center of the sphere, and the surface is the entire surface of the sphere. Since lithium ions migrate from this surface to the center at the single point, the migration distance is longer compared to the columnar positive electrode active material for lithium-ion secondary batteries. As a result, the rate characteristics of the material using the caterpillar-shaped precursor of the present invention are superior to those of the conventional spherical material.

[0024] Furthermore, conventional spherical positive electrode active materials for lithium-ion secondary batteries expand and contract during charging and discharging, and this expansion and contraction causes cracks, ultimately leading to fracture of the positive electrode active material. In the case of the caterpillar-shaped precursor of the present invention, the crystal orientation is quite aligned, and the occurrence of cracks inside the particles as described above is suppressed. Of course, it is conceivable that cracks may occur inside the particles in the longitudinal direction or at both ends, but these internal cracks are partial, and it goes without saying that they will not lead to fracture of the positive electrode active material for lithium-ion secondary batteries, or only very rarely. In contrast, with conventional spherical positive electrode active materials for lithium-ion secondary batteries, even if cracks occur only in a part of the particle, it is undeniable that the entire sphere will fracture. It is presumed that this also contributes to the difference in cycle characteristics.

[0025] A positive electrode active material for lithium-ion secondary batteries using the caterpillar-shaped precursor of the present invention having the above-described characteristics can be produced, for example, by obtaining a co-precipitated oxide (secondary particle) of Ni-Co-Al, Ni-Co-Mn, or Ni-Co (i.e., a precursor exhibiting a caterpillar-like appearance), mixing it with a Li compound, and then calcining it. The above co-precipitated hydroxide is prepared by preparing an aqueous solution of Ni, Co, Al, or Mn compounds (water-soluble compounds such as sulfates, nitrates, or chlorides are used for each of the Ni, Co, Al, and Mn compounds). The concentration of this aqueous solution is adjusted so that the total amount of Ni, Co, Al, and Mn per liter of water is approximately 10 to 500 g / L. This aqueous solution is supplied to a slurry containing a metal hydroxide or metal oxide composed of two or more elements such as Ni, Co, Al, and Mn at a concentration of 100 g / L or more, which is stirred at 30 to 90°C, with a pH of 9 to 13, under nitrogen gas supply, and at 100 to 1200 rpm. Once the newly formed particles in the slurry have reached a predetermined shape (caterpillar-like) and size, stirring is stopped. The resulting slurry is then washed with water until the conductivity of the washing water is 300 mS / cm or less. After that, the caterpillar-like particles are classified and separated from the other particles. The resulting caterpillar-like particles are then subjected to solid-liquid separation, and the solid component is dried.

[0026] A precursor exhibiting a caterpillar-like appearance obtained as described above, i.e., a compound containing Ni-Co-Al, Ni-Co, or Ni-Co-Mn coprecipitated oxide exhibiting a caterpillar-like appearance, and a Li compound (a Li compound that can become an oxide at high temperatures, such as hydroxides, carbonates, halides, etc., with an average particle size of about 5 to 50 μm) are dry-mixed in a proportion that satisfies the stoichiometric relationship of a predetermined compositional formula to prepare a raw material mixture. Dry mixing should preferably be carried out for about 0.5 to 1.5 hours under conditions of normal temperature, normal pressure, and closed (such as closing the raw material input section of the powder mixing device).

[0027] The raw material mixture prepared as described above is fired in an oxidizing atmosphere at 700-800°C for 5-20 hours. After firing, it is rapidly cooled outside the firing furnace or slowly cooled inside the furnace. There are no particular restrictions on the heating conditions during firing, but for example, the heating is increased over 5-15 hours, preferably 8-12 hours, from the start of heating the furnace. As described above, a positive electrode active material for lithium-ion secondary batteries using the caterpillar-shaped precursor of the present invention is obtained, which exhibits a corn-like shape as schematically shown in Figure 6 (in Figure 6, reference numerals 11, 11, 11, ...11 indicate a plurality of granular materials of various shapes and dimensions covering the surface).

[0028] The caterpillar-shaped precursor in the present invention can be used as a precursor even when mixed with other substances that have different particle shapes and compositions. That is, the (c) lithium-ion secondary battery positive electrode active material mixed precursor according to the present invention may be a (c) lithium-ion secondary battery positive electrode active material mixed precursor comprising the above-mentioned caterpillar-shaped precursor (a) lithium-ion secondary battery positive electrode active material precursor and (b) lithium-ion secondary battery positive electrode active material precursor for mixing. In other words, by mixing the product of the present invention with other precursors (additive precursors) and using it as a precursor for a lithium-ion secondary battery, it is expected that a positive electrode active material with improved cycle characteristics, rate characteristics, and binding properties with electrode foil can be obtained. Furthermore, it is preferable that the content of (a) lithium-ion secondary battery positive electrode active material precursor, which is the above-mentioned caterpillar-shaped precursor, in (c) lithium-ion secondary battery positive electrode active material mixed precursor is 10% by mass or more. The reason for this is that if the mixing ratio of (a) lithium-ion secondary battery positive electrode active material precursor, which is the caterpillar-shaped precursor of the present invention, is 10% or more, the cycle characteristics and rate characteristics can be improved by 10% or more. Note that (b) lithium-ion secondary battery positive electrode active material precursor for mixing, which can be used in combination with (a) lithium-ion secondary battery positive electrode active material precursor, which is the caterpillar-shaped precursor of the present invention, is a conventional precursor, for example, the lithium-ion secondary battery positive electrode active material precursor described in WO2016-143844A1, or Ni (1-y-γ) Co y A γThese include (OH)2 and metal oxides obtained by dehydrating these metal hydroxides. Among these, (b) metal hydroxides or metal oxides having the same composition ratio of Ni, Co, Al, and Mn as the positive electrode active material precursor for lithium-ion secondary batteries used, which is the caterpillar-shaped precursor used, are preferred. The composition of the precursors used in the mixture may consist of one or more types. The above general formula Ni shows the positive electrode active material precursor for lithium-ion secondary batteries used in the mixture. (1-y-γ) Co y A γ In (OH)2, A is Mn or Al, y is 0.00 ≤ y ≤ 1.00, and γ is 0.00 ≤ γ ≤ 1.00. Specifically, Ni (1-y-γ) Co y A γ (OH)2 is Ni when γ is 0. (1-y) Co y (OH)2, and when A is Al, Ni (1-y-γ) Co y Al γ (OH)2, and when A is Mn, Ni (1-y-γ) Co y Mn γ When (OH)2, y is 1, and γ is 0, it is Co(OH)2. When A is Mn, y is 0, and γ is 1, it is Mn(OH)2.

[0029] Preferably, the mixing ratio of the caterpillar-like precursor of the present invention with the other substances is such that the caterpillar-like precursor of the present invention accounts for at least 10% by mass of the total precursor (i.e., the mixture of the caterpillar-like precursor of the present invention and the other substances (precursors), hereinafter referred to as "mixed precursor").

[0030] Even when using this mixed precursor, a raw material mixture can be prepared by dry mixing it with a Li compound, similar to when using the caterpillar-like precursor of the present invention alone. This mixture is then calcined in an oxidizing atmosphere, and after calcination, it is rapidly cooled outside the furnace or slowly cooled inside the furnace to obtain a cathode material exhibiting a corn-like shape as schematically shown in Figure 6. [Examples]

[0031] [Example 1] At room temperature, the Ni:Co molar ratio of NiSO4 to CoSO4 was set to 89:11, and an aqueous Ni-Co solution was prepared by mixing NiSO4·6H2O and CoSO4·7H2O to a total volume of 265 g / L (L = liters, mL = milliliters). Meanwhile, pure water was placed in a 50 L capacity stainless steel reaction vessel (with a lid and overflow port), and the mixture was stirred at 60°C. While maintaining this condition, N2 gas was introduced to create positive pressure in the reaction vessel to prevent air from entering. The above aqueous Ni-Co solution, along with (NH4)2SO4 and NaOH aqueous solutions, were added dropwise, and stirring was continued at a pH of 12.0 and a stirring speed of 4.1 m / s to obtain a slurry containing Ni-Co coprecipitated oxide.

[0032] The obtained slurry was concentrated to a solid content concentration of 500 g / L, then placed in a lidded SUS reaction vessel (capacity 5 L), and while maintaining a stirring state of 1000 rpm at 55°C, N2 gas was introduced, and the above Ni-Co aqueous solution, (NH4)2SO4, and NaOH aqueous solution were added dropwise, and stirring was continued. During this stirring, the addition of (NH4)2SO4 was adjusted so that the ammonia concentration of the slurry in the vessel became 15.0 g / L, and the addition of NaOH was adjusted so that the pH of the slurry in the vessel became 12.1. Furthermore, a concentration operation was performed to periodically remove the liquid portion of the slurry in the tank, maintaining a solid content concentration of 500 g / L or higher. During the above procedure, a 10 mL sample of the caterpillar-shaped Ni-Co coprecipitate hydroxide slurry was collected, and solid-liquid separation was performed. The solid portion (precipitate) was washed with water, dehydrated, and dried. The particle shape was observed at 1500x magnification using a microscope (JEOL Ltd.; product name "JSM-6700F" was used), and the dimensions were measured every 7 hours from the start of stirring. Subsequently, stirring and liquid supply were stopped when the average dimensions measured by the electron microscope reached a diameter (short axis) T = 1.8 μm and a length (long axis) G = 4.1 μm. Subsequently, the slurry in the water tank was removed and washed with water until the conductivity of the washing water was 300 mS / cm or less to remove impurities. Then, it was dehydrated and dried to obtain the solid content. The reason for washing with water until the conductivity of the washing water is 300 mS / cm or less is to confirm the target of removing impurities adhering to the product by washing during the manufacturing process, and in this invention, 300 mS / cm or less is set as the target value. Next, the solid components were dispersed in pure water, allowed to stand for 5 minutes, and the supernatant slurry was separated. The supernatant slurry was then subjected to solid-liquid separation, and the solid components were dehydrated and dried to obtain a precursor consisting of a caterpillar-shaped metal hydroxide with a Ni:Co molar ratio of 89:11.

[0033] The caterpillar-like Ni-Co coprecipitate oxide obtained as described above, aluminum oxide (commercially available with an average particle size of 10 μm or less), and lithium hydroxide (commercially available with an average particle size of 0.5 to 50 μm) were dry-mixed in stoichiometric proportions to prepare a raw material mixture containing the caterpillar-like precursor. The above raw material mixture was calcined at 730°C for 15.3 hours under an oxidizing atmosphere to obtain a material with the composition formula Li x Ni (1-y-α) Co y Al α A positive electrode active material was obtained with a molar ratio (ratio of gram atomic numbers of each element) of O2, Ni:Co:Al of 86:11:3.

[0034] The positive electrode active material described above exhibited a corn-like shape, schematically shown in Figure 6, and the average particle dimensions were 1.6 μm for the short axis T and 4.2 μm for the long axis G. Figure 7 shows the measurement method for particle dimensions (caterpillar-like precursor and corn-like positive electrode material, in this case the corn-like positive electrode material) using an electron microscope. The dimensions of a predetermined amount of product sampled from appropriate locations are measured, and the average value is calculated.

[0035] A lithium-ion secondary battery was fabricated in the same manner as conventionally, except for using the corn-like positive electrode active material obtained as described above. This battery was subjected to 80 charge-discharge cycles at a measurement temperature of 20°C, a voltage range of 4.25 to 2.5V, and a rate of 1C. The cycle characteristics (the ratio of the discharge capacity after 80 cycles to the discharge capacity at the initial discharge) and rate characteristics are shown in Table 1 and the solid line (A) in Figures 3 to 5. In Figures 3 and 4, reference numeral 5 indicates the data for Example 1.

[0036] [Comparative Example 1] (Same as Example 1 in WO2016-143844A1) An aqueous Ni-Co solution was prepared at room temperature with a Ni:Co molar ratio of 89:11 for NiSO4 and CoSO4. Meanwhile, pure water was placed in a 50L stainless steel reaction vessel (capacity) with a lid and overflow port, and a stirrer was operated at 60°C. While maintaining this condition, N2 gas was introduced, and the above aqueous Ni-Co solution, along with (NH4)2SO4 and NaOH aqueous solutions, were added dropwise. Stirring was continued for more than 24 hours at a blade tip speed of 4.1 m / s. During this stirring, it was confirmed that the N2 gas being circulated into the reaction vessel was continuously incorporated into the aqueous solution. Furthermore, the addition of (NH4)2SO4 was adjusted so that the ammonia concentration of the aqueous solution in the vessel became 12.0 g / L, and the addition of NaOH was adjusted so that the pH of the aqueous solution in the vessel became 12.0. The precipitate obtained from the overflow port of the SUS reaction vessel was removed in slurry form, washed with water, dehydrated, and dried at 110°C for 5 hours to obtain the precursor of Comparative Example 1, which is a Ni-Co coprecipitated oxide.

[0037] 950 g (molar ratio 0.97) of the above Ni-Co coprecipitated hydroxide, 160 g (molar ratio 0.03) of alumina (commercially available product with an average particle size of 10 μm or less), and 445 g (molar ratio 1.03) of lithium hydroxide (commercially available product with an average particle size of 0.5 to 50 μm) were dry-mixed for 1 hour. After mixing, the mixture was fired in an electric furnace at 750°C for 20 hours, including the heating time, under an oxidizing atmosphere. After firing, when the furnace temperature reached 200°C, the mixture was removed from the furnace and allowed to cool to room temperature to obtain a positive electrode active material with an aspect ratio greater than 0.9, consisting of nearly spherical particles with a smooth surface. In this example, as in Example 1, a method for measuring the dimensions of particles (generally spherical precursors and cathode materials) using an electron microscope was employed, and Figure 8 shows electron microscope images of predetermined amounts of the product taken from appropriate locations.

[0038] A lithium-ion secondary battery was fabricated in the same manner as in Example 1, except for using the positive electrode active material described above. The initial capacity (discharge capacity), cycle characteristics (ratio of discharge capacity after 80 cycles to discharge capacity at initial discharge), and rate characteristics of this battery were measured under the same conditions as in Example 1. The results are shown in Table 1 and along the dashed line (B) in Figures 3 to 5. In Figures 3 and 4, reference numeral 6 indicates the data for Comparative Example 1.

[0039] [Comparative Example 2] An aqueous Ni-Co solution was prepared at room temperature with a Ni:Co molar ratio of 89:11 for NiSO4 and CoSO4. Meanwhile, pure water was placed in a lidded, stainless steel reaction vessel (capacity 5L), and the stirrer was operated at 55°C. While maintaining this condition, N2 gas was introduced to create positive pressure in the reaction vessel to prevent air from entering, and the above-mentioned aqueous Ni-Co solution, along with aqueous (NH4)2SO4 and NaOH solutions, were added dropwise. Stirring was continued for 35 hours at a pH of 12.0 and a stirring speed of 4.1 m / s. During this stirring, the mother liquor was periodically removed to prevent the slurry level from exceeding the initial reaction level. The addition of (NH4)2SO4 was adjusted so that the ammonia concentration in the slurry in the vessel reached 15.0 g / L, and the addition of NaOH was adjusted so that the slurry in the vessel reached a pH of 12.1. The obtained precipitate was removed in slurry form, washed with water, dehydrated, and dried at 110°C for 5 hours to obtain Ni-Co co-precipitated oxide (precursor to Comparative Example 2). The cathode active material was produced in the same manner as in Example 1, except that this precursor was used.

[0040] A lithium-ion secondary battery was fabricated in the same manner as in Example 1, except for using the positive electrode active material described above. The initial capacity (discharge capacity), cycle characteristics (ratio of discharge capacity after 80 cycles to discharge capacity at initial discharge), and rate characteristics of this battery were measured under the same conditions as in Example 1, and the results are shown in Table 1 and Figures 3 to 5. In Figures 3 to 4, reference numeral 7 represents the data for Comparative Example 2. In Figure 5, the dotted line (c) represents the data for Comparative Example 2.

[0041] [Example 2] In Example 2, a caterpillar-shaped precursor was produced in the same manner as in Example 1, except that stirring and liquid supply were stopped when the measurement results by electron microscope showed that the dimensions were, on average, 0.5 μm on the short axis and 1 μm on the long axis. The cathode material was then produced in the same manner as in Example 1, except that this precursor was used. In Example 2, a lithium-ion secondary battery was prepared in the same manner as in Example 1, except that this cathode material (average dimensions: 0.6 μm on the short axis and 0.8 μm on the long axis) was used. The initial capacity (discharge capacity), cycle characteristics (ratio of discharge capacity after 80 discharges to discharge capacity at initial discharge), and rate characteristics of this battery were measured under the same conditions as in Example 1, and the results are shown in Table 1.

[0042] [Example 3] When the measurement results by electron microscope showed that the dimensions were, on average, 4 μm in the short axis and 20 μm in the long axis, the stirring and liquid supply were stopped. In addition, a caterpillar-shaped precursor was produced in the same manner as in Example 1, except that this precursor was used. A cathode material was then produced in the same manner as in Example 1, except that this precursor was used. In Example 3, a lithium-ion secondary battery was fabricated in the same manner as in Example 1, except for the use of this positive electrode material. The initial capacity (discharge capacity), cycle characteristics (ratio of discharge capacity after 80 discharges to discharge capacity at initial discharge), and rate characteristics of this battery were measured under the same conditions as in Example 1, and the results are shown in Table 1.

[0043] [Table 1]

[0044] [Examples 4-6] Lithium-ion secondary batteries were prepared in the same manner as in Example 1, except that the caterpillar-shaped precursor obtained in Example 1 and the roughly spherical precursor obtained in Comparative Example 1 were mixed in the proportions shown in Table 2 to form the precursor. The initial capacity (discharge capacity), cycle characteristics (ratio of discharge capacity after 80 discharges to discharge capacity at initial discharge), and rate characteristics of these batteries were measured under the same conditions as in Example 1, and the results are shown in Table 2.

[0045] [Table 2]

[0046] [Examples 7-12] Lithium-ion secondary batteries were prepared in the same manner as in Example 1, except that the caterpillar-like precursor obtained in Example 1 was mixed with the precursors shown in Table 3 in predetermined ratios. The initial capacity (discharge capacity), cycle characteristics (ratio of discharge capacity after 80 discharges to discharge capacity at initial discharge), and rate characteristics of these prepared batteries were measured under the same conditions as in Example 1. The measurement results are shown in Table 3.

[0047] [Table 3] [Industrial applicability]

[0048] The present invention provides a positive electrode active material precursor for lithium-ion secondary batteries characterized by being composed of numerous thin polygonal crystals linked together in a partially integrated state in the planar direction. By using this precursor as a positive electrode material for a lithium-ion secondary battery, it is possible to obtain a lithium-ion secondary battery with higher charge / discharge capacity and longer lifespan compared to conventional lithium-ion secondary batteries. Therefore, the caterpillar-shaped positive electrode active material precursor for lithium-ion secondary batteries of the present invention can be used in various known applications, including EV power supplies, power supplies for personal computers and mobile phones, and backup power supplies, which always require high capacity.

Claims

1. A precursor for obtaining a positive electrode active material for lithium-ion secondary batteries, characterized by having a caterpillar-like shape in which a large number of thin polygonal crystals are linked together in a partially integrated state in the planar direction, comprising a Li-Ni-Co-Al composite oxide or a Li-Ni-Co-Mn composite oxide.

2. The positive electrode active material precursor for lithium-ion secondary batteries according to claim 1, characterized in that the thin polygonal crystal is a metal hydroxide represented by the following general formula (1). A(1-z)Bz(OH) 2 ・・・ (1) In the general formula (1) above, A is at least two selected from Ni, Co, Mn, and Al, B is Mg or Zr, and z represents 0.00 ≤ z ≤ 0.

10.

3. The positive electrode active material precursor for lithium-ion secondary batteries according to claim 2, characterized in that the thin polygonal crystal is an oxide of the metal hydroxide.

4. A lithium-ion secondary battery positive electrode active material mixed precursor comprising (a) a lithium-ion secondary battery positive electrode active material precursor and (b) a lithium-ion secondary battery positive electrode active material precursor for mixing, characterized in that the content of (a) a lithium-ion secondary battery positive electrode active material precursor for mixing contained in the (c) lithium-ion secondary battery positive electrode active material mixed precursor is 10% by mass or more.

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