Method for producing positive electrode active material

Abstract

This method for producing a positive electrode active material includes mixing a nickel metal powder, a first compound powder containing elemental lithium, and a second compound powder containing a metal element M other than elemental nickel and elemental lithium so as to prepare precursor particles in which the surface of the nickel metal powder is covered with the first compound powder and the second compound powder, and firing the precursor particles so as to prepare particles of a positive electrode active material.

Description

Method for manufacturing a positive electrode active material

[0001] The present disclosure relates to a method for manufacturing a positive electrode active material.

[0002] Patent Document 1 describes a method for manufacturing a positive electrode active material, which includes mixing metal nickel powder, a compound containing lithium, and a compound containing a metal element other than nickel and lithium, subjecting the obtained mixture to calcination, pulverization and mixing, and granulation, and subjecting the obtained granulated product to final firing.

[0003] International Publication No. 2022 / 209988

[0004] In the conventional method for manufacturing a positive electrode active material described above, the mixture is pulverized and granulated after mixing the respective raw materials. As a result of intensive studies on such a conventional manufacturing method, the inventors have found a novel manufacturing method capable of improving the performance of an electric storage device in which the positive electrode active material is used.

[0005] The present disclosure has been made in view of such a situation, and one of its objects is to provide a technique for improving the performance of an electric storage device.

[0006] One aspect of the present disclosure is a method for manufacturing a positive electrode active material. This manufacturing method includes mixing metal nickel powder, a first compound powder containing a lithium element, and a second compound powder containing a metal element M other than nickel element and lithium element to produce precursor particles in which the surface of the metal nickel powder is coated with the first compound powder and the second compound powder, and firing the precursor particles to produce particles of the positive electrode active material.

[0007] Any combination of the above components, and those obtained by converting the expressions of the present disclosure among methods, apparatuses, systems, etc. are also effective as aspects of the present disclosure.

[0008] According to the present disclosure, the performance of the electric storage device can be improved.

[0009] This is a process diagram of the method for manufacturing a positive electrode active material according to an embodiment. Figure 2(A) is an SEM image of a cross-section of the positive electrode active material according to Example 1. Figure 2(B) is a diagram showing the results of line analysis by SEM-EDX on the positive electrode active material according to Example 1. Figure 2(C) is an SEM image of a cross-section of the positive electrode active material according to Comparative Example 1. Figure 2(D) is a diagram showing the results of line analysis by SEM-EDX on the positive electrode active material according to Comparative Example 1. This figure shows the average detected value a, average detected value b, and ratio a / b of each element in the positive electrode active materials of Example 1 and Comparative Example 1.

[0010] The present disclosure will be described below with reference to the drawings, based on preferred embodiments. The embodiments are illustrative and not limiting, and not all features or combinations thereof described in the embodiments are necessarily essential to the present disclosure. The same or equivalent components, members, and processes shown in each drawing are denoted by the same reference numerals, and redundant descriptions are omitted where appropriate. The scale and shape of each part shown in each drawing are set for convenience to facilitate explanation and are not to be interpreted restrictively unless otherwise specified. Furthermore, where terms such as "first," "second," etc. are used in this specification or claims, unless otherwise specified, these terms do not indicate any order or importance, but are used to distinguish one configuration from another. In addition, some components that are not important for explaining the embodiments are omitted in each drawing.

[0011] First, a power storage device using a positive electrode active material manufactured by the method for manufacturing positive electrode active material according to this embodiment will be described. The power storage device is a rechargeable non-aqueous electrolyte secondary battery, such as a lithium-ion battery. In this embodiment, a positive electrode active material for a lithium-ion battery is used as an example, but the positive electrode active material can be used in other known devices as appropriate. The power storage device comprises an electrode group and an outer casing. The electrode group is, for example, cylindrical and has a wound structure in which a strip-shaped positive electrode plate and a strip-shaped negative electrode plate are stacked with a strip-shaped separator in between and wound in a spiral shape. The separator is, for example, made of polypropylene, polyethylene, or the like, a microporous film that has ion permeability and insulating properties. Leads are attached to the positive electrode plate and the negative electrode plate.

[0012] The electrode group is housed in an outer container along with the electrolyte. The outer container is a bottomed cylindrical metal container. A sealing body is fitted into the opening of the outer container. This seals the electrode group and electrolyte inside the outer container. The leads of the positive electrode plate are electrically connected to the sealing body. Therefore, the sealing body constitutes the positive electrode terminal. The leads of the negative electrode plate are electrically connected to the outer container. Therefore, the outer container constitutes the negative electrode terminal. The structure of the energy storage device can be modified as appropriate. The positive electrode plate and the negative electrode plate each comprise a current collector and an electrode mixture layer. The current collector is made of a strip of metal foil. In the case of a typical lithium-ion secondary battery, the current collector is made of aluminum foil or the like for the positive electrode, and copper foil or the like for the negative electrode. The electrode mixture layer is provided on top of the current collector.

[0013] The electrode mixture layer contains an electrode mixture. An example electrode mixture contains an electrode active material, a conductive material, and a binder. In the case of a typical lithium-ion secondary battery, examples of positive electrode active materials include lithium nickel cobalt manganese composite oxide (NCM), lithium nickel cobalt aluminum composite oxide (NCA), and lithium nickel cobalt manganese aluminum composite oxide (NCMA). Examples of negative electrode active materials include graphite. Examples of conductive materials include carbon black (CB), acetylene black (AB), Ketjenblack, carbon nanotubes (CNT), and graphite. Examples of binders include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and fluororubber.

[0014] Next, a method for manufacturing the positive electrode active material will be described. Figure 1 is a process diagram of the method for manufacturing the positive electrode active material according to this embodiment. In the method for manufacturing the positive electrode active material according to this embodiment, first, in preparation step S101, metallic nickel powder, a first compound powder containing lithium element, and a second compound powder containing nickel element and a metallic element M other than lithium element are prepared.

[0015] As examples of metallic nickel powder, those produced by the carbonyl method and the water atomization method can be used. The metallic nickel powder may have a volume-based median diameter D50 of 0.2 μm or more and 20 μm or less. This allows the particle size of the final positive electrode active material to be adjusted to a size suitable for use in energy storage devices. In this disclosure, the median diameter D50 refers to the particle size at which the cumulative frequency of the smallest particle size accounts for 50% in the volume-based particle size distribution. The particle size distribution can be measured using a laser diffraction particle size distribution analyzer (for example, the MT3000II manufactured by Microtrac-Bell Corporation) with water as the dispersion medium.

[0016] Examples of the first compound powder containing lithium element include lithium hydroxide, lithium hydroxide monohydrate, and lithium carbonate powders. The first compound powder has a volume-based median diameter D50 of 0.01 μm to 5 μm. This allows the particle size of the final positive electrode active material to be adjusted to a size suitable for use in energy storage devices.

[0017] The metal element M may be a transition metal, and may also be cobalt (Co), manganese (Mn), or aluminum (Al). When the positive electrode active material is lithium nickel cobalt manganese composite oxide (NCM), the second compound powder contains cobalt and manganese as metal element M. When the positive electrode active material is lithium nickel cobalt aluminum composite oxide (NCA), the second compound powder contains cobalt and aluminum as metal element M. When the positive electrode active material is lithium nickel cobalt manganese aluminum composite oxide (NCMA), the second compound powder contains cobalt, manganese, and aluminum as metal element M. Examples of the second compound powder include carbonate, hydroxide, and oxide powders of metal element M. The second compound may also be a pure metal metal element M. The second compound powder has a volume-based median diameter D50 of 0.01 μm or more and 5 μm or less. This allows the particle size of the resulting positive electrode active material to be adjusted to a size suitable for use in energy storage devices.

[0018] If there are two or more types of metal element M, the second compound powder may be a separate powder for each type of metal element M. For example, if metal element M is cobalt and manganese, the second compound powder may be a powder of the cobalt compound and a powder of the manganese compound.

[0019] Next, in the precursor preparation step S102, the metallic nickel powder, the first compound powder, and the second compound powder are mixed together to produce precursor particles. The precursor particles have a structure in which the surface of the metallic nickel powder is coated with the first compound powder and the second compound powder. In other words, the metallic nickel powder becomes the core material, and the first compound powder and the second compound powder become the coating or shell. As an example, the metallic nickel powder used in the precursor preparation step S102 is not oxidized. In order to maintain the unoxidized state of the metallic nickel powder, no oxidation treatment such as calcination or exposure to air is performed on the metallic nickel powder before the precursor preparation step S102. However, the metallic nickel powder may be oxidized.

[0020] In one example of the precursor preparation process S102, metallic nickel powder, first compound powder, and second compound powder are introduced into a mixing container of a powder processing device, and a rotor with blades is rotated inside the mixing container. As a result, various forces such as compression, shear, and impact are uniformly applied to each powder located between the inner wall of the container and the tip of the rotor. Consequently, the metallic nickel powder rolls around, and the first compound powder and second compound powder are coated onto the surface of the metallic nickel powder. The coating containing the first compound powder and second compound powder is then shaped into a spherical form. This yields spherical precursor particles. In this disclosure, "spherical" does not mean a perfect sphere, but rather a shape that can be considered generally spherical overall.

[0021] Next, in the firing process S103, the precursor particles are fired. This produces positive electrode active material particles. In other words, the positive electrode active material is the fired product of the precursor particles. Known electric furnaces or gas furnaces can be used to fire the precursor particles. In the firing process, the precursor particles may be heated to a temperature of 700°C to 900°C. This makes it possible to obtain a positive electrode active material suitable for use in energy storage devices. The above temperature is, as an example, the maximum temperature in the firing process. In the manufacturing method according to this embodiment, the firing process is carried out only in the firing process S103.

[0022] The positive electrode active material of this embodiment may contain metal elements other than nickel, lithium, and metal element M. Furthermore, the proportion of nickel element to the total number of moles of all metal elements except lithium may be 50 mol% or more. For example, the positive electrode active material may contain 50 mol% or more of nickel element to the total number of moles of nickel and metal element M.

[0023] The positive electrode active material obtained through the preparation process S101 to the firing process S103 has an elemental concentration gradient, with a high concentration of nickel and a low concentration of metal element M in the center, and a low concentration of nickel and a high concentration of metal element M on the surface. In conventional methods for manufacturing positive electrode active materials, in which a mixture of raw material powders is crushed and mixed before granulation and firing, the concentrations of nickel and metal element M become uniform throughout the particles of the positive electrode active material. In contrast, in the manufacturing method according to this embodiment, since the mixture of raw material powders is fired without crushing and mixing, it is possible to form the elemental concentration distribution described above within the particles of the positive electrode active material.

[0024] By giving the positive electrode active material a concentration gradient of nickel, which is high in the center and decreases towards the surface, it is possible to improve the thermal stability of the positive electrode active material while suppressing a decrease in positive electrode capacity. Furthermore, by giving the positive electrode active material a concentration gradient of metal element M, which is low in the center and increases towards the surface, if metal element M is manganese, it is possible to improve the thermal stability of the positive electrode active material while suppressing a decrease in positive electrode capacity. Also, if metal element M is cobalt, since cobalt contributes to improving the electronic conductivity of the positive electrode, increasing the cobalt concentration on the particle surface can reduce the electronic resistance of the positive electrode surface while reducing the amount of cobalt used. Finally, if metal element M is aluminum, it is possible to improve the thermal stability and durability of the positive electrode active material while suppressing a decrease in positive electrode capacity.

[0025] As an example, in a cross-section of the positive electrode active material, if the average elemental concentration in the 10% range from the outer edge of the positive electrode active material is A (at%), and the average elemental concentration in the 10% range from the center of the positive electrode active material is B (at%), then the ratio A / B for nickel element is 0.95 or less, and the ratio A / B for metallic element M is 1.05 or more. This makes it easier to exhibit the effects described above. In this disclosure, the "cross-section of the positive electrode active material" refers to, for example, a cross-section passing through a core composed of metallic nickel powder and a shell composed of a first compound powder and a second compound powder, or a cross-section at the point where the particle cross-sectional area is maximum, or a cross-section passing through the centroid or geometric center of the particle. In this disclosure, the "center of the positive electrode active material" refers to, for example, the centroid or geometric center of the cross-sectional shape of the positive electrode active material. Furthermore, in this disclosure, the "outer edge" refers to, for example, the outer peripheral portion of the positive electrode active material in an SEM image of the positive electrode active material.

[0026] The particles constituting the positive electrode active material have a volume-based median diameter D50 of, for example, 1.0 μm or more and 20.0 μm or less. The particles constituting the positive electrode active material may have a single-particle shape or a multi-particle shape. In this disclosure, the above-mentioned "single particle" means a particle formed from a single primary particle, not a secondary particle formed by the aggregation of many, for example, 1000 or more primary particles. In other words, there are substantially no particle interfaces of primary particles inside the particle. Note that a particle formed by the aggregation of 10 or fewer primary particles approximates a single-particle shape and can be considered substantially as a single particle. The single particle may be a single-crystal particle in which there are substantially no grain boundaries inside the particle, or a polycrystalline particle in which there are several grain boundaries inside the particle. In this disclosure, the above-mentioned "multi-particle" means the secondary particle described above.

[0027] The embodiments of this disclosure have been described in detail above. The embodiments described above are merely examples of how to implement this disclosure. The content of the embodiments does not limit the technical scope of this disclosure, and many design changes, such as changes, additions, and deletions of components, are possible as long as they do not depart from the spirit of the invention as defined in the claims. A new embodiment with design changes will have the combined effects of both the embodiment and the variation. In the embodiments described above, the content in which such design changes are possible is emphasized with notations such as "of this embodiment" or "in this embodiment," but design changes are also permitted even if there are no such notations. Furthermore, any combination of components included in each embodiment is also valid as an embodiment of this disclosure. The hatching applied to the cross-section in the drawings does not limit the material of the object to which the hatching is applied.

[0028] The embodiments may be specified by the following items: [Item 1] A method for producing a positive electrode active material, comprising mixing metallic nickel powder, a first compound powder containing lithium element, and a second compound powder containing nickel element and a metal element M other than lithium element to produce precursor particles in which the surface of the metallic nickel powder is coated with the first compound powder and the second compound powder, and firing the precursor particles to produce particles of positive electrode active material. [Item 2] A method for producing a positive electrode active material according to Item 1, wherein, in the cross-section of the positive electrode active material, when the average elemental concentration in the range of 10% from the outer edge of the positive electrode active material is A (at%) and the average elemental concentration in the range of 10% from the center of the positive electrode active material is B (at%), the ratio A / B for nickel element is 0.95 or less and the ratio A / B for metal element M is 1.05 or more. [Item 3] A method for producing a positive electrode active material according to Item 1 or Item 2, wherein the positive electrode active material has a molar amount of nickel element of 50 mol% or more relative to the total number of moles of all metal elements excluding lithium element. [Item 4] A method for producing a positive electrode active material according to any of Items 1 to 3, comprising heating the precursor particles at a temperature of 700°C to 900°C during firing. [Item 5] A method for producing a positive electrode active material according to any of Items 1 to 4, wherein the metallic nickel powder has a volume-based median diameter D50 of 0.2 μm to 10 μm. [Item 6] A method for producing a positive electrode active material according to any of Items 1 to 5, wherein the first compound powder and the second compound powder have a volume-based median diameter D50 of 0.01 μm to 5 μm.

[0029] The following describes embodiments of the present invention, but these embodiments are merely illustrative examples for suitably illustrating the present invention and do not limit the present invention in any way.

[0030] (Example 1) As raw materials for the positive electrode active material, metallic nickel powder (manufactured by Vale), lithium hydroxide (manufactured by Fujifilm Wako Pure Chemical Industries), manganese carbonate (manufactured by Kojunsei Chemicals Co., Ltd.), and cobalt carbonate (manufactured by Kojunsei Chemicals Co., Ltd.) were prepared. The lithium hydroxide, manganese carbonate, and cobalt carbonate were finely pulverized in advance using a ball mill. Each raw material was put into a powder processing device (NOB-MINI®: manufactured by Hosokawa Micron Corporation) and mixed. This yielded precursor particles in which the surface of the metallic nickel powder was covered with powders of lithium hydroxide, manganese carbonate, and cobalt carbonate.

[0031] The obtained precursor particles were placed in an electric furnace (FT-1200G-300: manufactured by Furutech Co., Ltd.) and fired at 750°C for 10 hours. This yielded particulate positive electrode active material, in other words, lithium transition metal composite oxide. The obtained positive electrode active material was subjected to cross-sectional processing using an ion milling device (IM4000II: manufactured by Hitachi High-Tech Corporation). Line analysis of the obtained cross-section was then performed on the obtained cross-section using a SEM-EDX (SU8600: manufactured by Hitachi High-Tech Corporation) for nickel, manganese, cobalt, and oxygen. From the results of the line analysis, the average detection value a (cps) in the 10% range from the outer edge of the positive electrode active material, the average detection value b (cps) in the 10% range from the center of the positive electrode active material, and the ratio a / b were calculated for nickel, manganese, and cobalt. The average detected value a is equal to the average elemental concentration A, the average detected value b is equal to the average elemental concentration B, and the ratio a / b is equal to the ratio A / B.

[0032] (Comparative Example 1) A positive electrode active material was prepared in the same manner as in Example 1, except that metallic nickel powder, lithium hydroxide, manganese carbonate, and cobalt carbonate were uniformly mixed without preparing precursor particles. SEM-EDX analysis was performed, and the average detection values ​​a, b and the ratio a / b were calculated.

[0033] Figure 2(A) is an SEM image of a cross-section of the positive electrode active material according to Example 1. Figure 2(B) shows the results of line analysis by SEM-EDX on the positive electrode active material according to Example 1. Figure 2(C) is an SEM image of a cross-section of the positive electrode active material according to Comparative Example 1. Figure 2(D) shows the results of line analysis by SEM-EDX on the positive electrode active material according to Comparative Example 1. Figure 3 shows the average detected value a, average detected value b, and ratio a / b for each element in the positive electrode active materials of Example 1 and Comparative Example 1. The solid lines shown in Figures 2(A) and 2(C) indicate the locations where line analysis was performed. The range X shown in Figures 2(B) and 2(D) indicates the estimated range of presence of the positive electrode active material.

[0034] As shown in Figures 2(B), 2(D), and 3, in Comparative Example 1, the ratios a / b of nickel, manganese, and cobalt were all greater than 0.95 and less than 1.05. Therefore, in Comparative Example 1, the concentrations of each element were approximately uniform between the surface and the center of the positive electrode active material, and no concentration gradients for each element were observed. On the other hand, in Example 1, the ratio a / b of nickel was 0.95 or less, and the ratios a / b of manganese and cobalt were 1.05 or more. Therefore, in Example 1, a concentration gradient was observed for nickel, with higher concentrations at the center and lower concentrations at the surface, while a concentration gradient was observed for manganese and cobalt, with lower concentrations at the center and higher concentrations at the surface. Thus, it was confirmed that the method for producing positive electrode active material according to this embodiment can produce a positive electrode active material that can improve the performance of energy storage devices.

[0035] This disclosure can be used in a method for producing a positive electrode active material.

[0036] S101 Preparation process, S102 Precursor preparation process, S103 Sintering process.

Claims

1. A method for producing a positive electrode active material, comprising: mixing metallic nickel powder, a first compound powder containing lithium element, and a second compound powder containing nickel element and a metallic element M other than lithium element to produce precursor particles in which the surface of the metallic nickel powder is coated with the first compound powder and the second compound powder; and firing the precursor particles to produce particles of positive electrode active material.

2. The method for producing a positive electrode active material according to claim 1, wherein, in a cross-section of the positive electrode active material, when the average elemental concentration in the range of 10% from the outer edge of the positive electrode active material is A (at%) and the average elemental concentration in the range of 10% from the center of the positive electrode active material is B (at%), the ratio A / B for nickel element is 0.95 or less and the ratio A / B for metal element M is 1.05 or more.

3. The method for producing a positive electrode active material according to claim 1 or 2, wherein the positive electrode active material has a molar amount of nickel element relative to the total number of moles of all metal elements excluding lithium element of 50 mol% or more.

4. A method for producing a positive electrode active material according to claim 1 or 2, comprising heating the precursor particles at a temperature of 700°C or higher and 900°C or lower in the firing process.

5. The method for producing a positive electrode active material according to claim 1 or 2, wherein the metallic nickel powder has a volume-based median diameter D50 of 0.2 μm or more and 10 μm or less.

6. The method for producing a positive electrode active material according to claim 5, wherein the first compound powder and the second compound powder have a volume-based median diameter D50 of 0.01 μm or more and 5 μm or less.

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