Method of producing positive electrode active material, method of producing positive electrode plate, and method of producing non-aqueous electrolyte secondary battery

Abstract

To provide a method of producing a positive electrode active material capable of suppressing introduction of Cr, which is an impurity.SOLUTION: A method of producing a positive electrode active material includes calcining a mixture of a nickel-containing compound and a lithium compound introduced into a furnace of a rotary kiln at 750 to 1000°C under an oxygen atmosphere. The nickel-containing compound is at least one of a nickel-containing hydroxide and a nickel-containing oxide. A layer of yttrium-chromium composite oxide is formed on an outermost surface of an inner wall of the furnace.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing a positive electrode active material, a method for producing a positive electrode plate, and a method for producing a non-aqueous electrolyte secondary battery. [Background technology]

[0002] Positive electrode active materials used in lithium secondary batteries are manufactured by firing in a firing furnace. For example, Patent Document 1 discloses that, since metal components may be mixed into the raw materials when an alloy furnace tube is used for a long period of time, aluminum oxide is formed on the outermost surface layer of the inner wall of the furnace tube. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2022-146357 Summary of the Invention [Problem to be solved by the invention]

[0004] The present disclosure provides a method for producing a positive electrode active material that can suppress the incorporation of Cr as an impurity, even when a rotary kiln is used in which the material forming the furnace contains an alloy containing Cr. [Means for solving the problem]

[0005] [1] A method for producing a positive electrode active material containing a lithium transition metal composite oxide, comprising: The method includes a step of firing a mixture of a nickel-containing compound and a lithium compound charged into a rotary kiln at 750 to 1000°C in an oxygen atmosphere, the nickel-containing compound is at least one of a nickel-containing hydroxide and a nickel-containing oxide, In the method for producing a positive electrode active material, a layer of yttrium-chromium composite oxide is formed on the outermost surface of the inner wall of the furnace. 〔2〕 The layer covers the base material layer of the furnace. The method for producing a positive electrode active material according to [1], wherein the base material layer is formed of an alloy containing Cr. 〔3〕 The method for producing a positive electrode active material according to [2], wherein the layer is formed by spraying yttria on the base material layer. 〔4〕 The method for producing a positive electrode active material according to [2] or [3], wherein the alloy containing Cr further contains Fe and Ni. 〔5〕 Further includes a step of molding the mixture to obtain a molded body. The method for producing a positive electrode active material according to any one of [1] to [4], wherein the firing step fires the molded body. 〔6〕 The method for producing a positive electrode active material according to any one of [1] to [5], wherein the lithium compound is at least one of lithium hydroxide and lithium carbonate. 〔7〕 The method for producing a positive electrode active material according to any one of [1] to [6], wherein the content of Cr in the lithium transition metal composite oxide is 1 ppm or less. 〔8〕 The method for producing a positive electrode active material according to any one of [1] to [7], wherein the lithium transition metal composite oxide contains Li, Ni, and Mn. 〔9〕 The lithium transition metal composite oxide is Li, Ni, Mn, Co, and M [M is one or more metal elements selected from the group consisting of Mg, Ca, Al, Ti, V, Cr, Fe, Cu, Zn, Zr, Nb, Mo, Ta, W, and Y]. And The molar ratio of Li, Ni, Mn, Co, and M is Li:Ni:Mn:Co:M = a:x:y:z:t [a, x, y, z, and t satisfy 1.0 ≦ a ≦ 1.3, x + y + z + t = 1, 0.25 ≦ x ≦ 0.9, 0 < y ≦ 0.6, 0 < z ≦ 0.6, and 0 < t ≦ 0.1]. The method for producing a positive electrode active material according to any one of [1] to [8]. 〔10〕 A method for producing a positive electrode plate using a positive electrode active material, The method for producing a positive electrode plate, wherein the positive electrode active material is produced by the method for producing a positive electrode active material according to any one of [1] to [9].

[11] A method for manufacturing a nonaqueous electrolyte secondary battery including a positive electrode plate, comprising:

[11] A method for producing a non-aqueous electrolyte secondary battery, wherein the positive electrode plate is produced by the method for producing a positive electrode plate according to

[10] . [Effects of the Invention]

[0006] According to the method for producing a positive electrode active material of the present disclosure, a positive electrode active material can be produced in which the inclusion of Cr as an impurity is suppressed. [Brief explanation of the drawings]

[0007] [Figure 1] 2 is a flowchart illustrating an example of a method for manufacturing a positive electrode active material according to the embodiment. [Figure 2] 4 is a flowchart showing another example of the method for manufacturing a positive electrode active material according to the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0008] In this specification, unless otherwise specified, a numerical range such as "m to n" includes both the upper and lower limits. That is, "m to n" represents a numerical range of "m or more and n or less." A numerical value arbitrarily selected from within the numerical range may be set as a new upper or lower limit. For example, a new numerical range may be set by arbitrarily combining a numerical value within the numerical range with a numerical value described elsewhere in this specification, in a table, or in a figure.

[0009] (Method for producing positive electrode active material (1)) 1 and 2 are flowcharts showing an example of a method for producing a positive electrode active material according to an embodiment. The positive electrode active material produced by the method for producing a positive electrode active material according to this embodiment (hereinafter also referred to as "this method") is used for a positive electrode plate of a non-aqueous electrolyte secondary battery (hereinafter also referred to as "secondary battery") such as a lithium ion battery.

[0010] The positive electrode active material produced by this method contains a first lithium transition metal composite oxide (lithium transition metal composite oxide) (hereinafter also referred to as "first composite oxide"). The Cr content of the first composite oxide is preferably 50 ppm or less, may be 25 ppm or less, may be 10 ppm or less, may be 5 ppm, more preferably 1 ppm or less, and may be 0.9 ppm or less. The Cr content of the first composite oxide refers to the mass ratio to the total mass of the first composite oxide. The Cr content of the first composite oxide can be adjusted, for example, by producing the positive electrode active material by the method described later.

[0011] The composition of the first composite oxide is not particularly limited, but the first composite oxide preferably contains Ni and Mn as transition metals in addition to Li. The first composite oxide is more preferably Li, Ni, Mn, Co, and M [M is one or more metal elements selected from the group consisting of Mg, Ca, Al, Ti, V, Cr, Fe, Cu, Zn, Zr, Nb, Mo, Ta, W, and Y.], and the molar ratio of Li, Ni, Mn, Co, and M is Li:Ni:Mn:Co:M = a:x:y:z:t [a, x, y, z, and t are 1.0 ≦ a ≦ 1.3, x + y + z + t = 1, 0.25 ≦ x ≦ 0.9, 0 < y ≦ 0.6, 0 < z ≦ 0.6, and 0 < t ≦ 0.1.].

[0012] The molar ratio of Li is 1.0 ≤ a ≤ 1.3, and it may also be 1.0 ≤ a ≤ 1.25, 1.01 ≤ a ≤ 1.2, 1.03 ≤ a ≤ 1.15, or 1.04 ≤ a ≤ 1.1. The molar ratio of Ni is 0.25 ≤ x ≤ 0.9, and it may also be 0.3 ≤ x ≤ 0.9, 0.4 ≤ x ≤ 0.88, or 0.5 ≤ x ≤ 0.85. The molar ratio of Mn is 0 < y ≤ 0.6, and it may also be 0.05 ≤ y ≤ 0.5, 0.08 ≤ y ≤ 0.3, or 0.10 ≤ y ≤ 0.2. The molar ratio of Co is 0 < z ≤ 0.6, and it may also be 0 < z ≤ 0.5, 0.01 ≤ z ≤ 0.3, or 0.02 ≤ z ≤ 0.1. The molar ratio of M is 0 < t ≤ 0.1, and it may also be 0 < t ≤ 0.08, 0.001 ≤ t ≤ 0.05, or 0.002 ≤ t ≤ 0.01. When the first composite oxide contains two or more metal elements M, the molar ratio of M refers to the total amount of the two or more metal elements.

[0013] The composition of the first composite oxide can be adjusted by the types of raw materials and the blending amounts of the raw materials used in producing the first composite oxide. The composition of the first composite oxide can be determined by ICP (Inductively Coupled Plasma) optical emission spectrometry (ICP-AES).

[0014] The positive electrode active material may contain only the first composite oxide, or may contain an active material other than the first composite oxide. The content of the first composite oxide in the positive electrode active material may be 85 to 100% by mass, 90 to 100% by mass, 92 to 99% by mass, or 95 to 98% by mass based on the total amount of the positive electrode active material.

[0015] This method includes a step of calcining a mixture of a nickel-containing compound and a lithium compound placed in a rotary kiln at 750 to 1000°C in an oxygen atmosphere (hereinafter also referred to as the "calcining step"), as shown in Figure 1. In this method, the nickel-containing compound is at least one of a nickel-containing hydroxide and a nickel-containing oxide, and a layer of yttrium-chromium composite oxide (hereinafter also referred to as "YCrO3") (hereinafter also referred to as the "YCrO3 layer") is formed on the outermost surface of the inner wall of the kiln.

[0016] In this method, a mixture of a nickel-containing compound and a lithium compound is fired using a rotary kiln in which a YCrO3 layer is formed on the outermost surface of the furnace's inner wall. Because YCrO3 has low reactivity with lithium compounds, this prevents the lithium compound from reacting with the YCrO3 on the outermost surface of the furnace's inner wall, thereby preventing corrosion of the furnace's inner wall. This prevents Cr contained in the furnace's inner wall from being mixed into the first composite oxide as an impurity, making it easier to obtain a first composite oxide with a Cr content within the above-mentioned range. Furthermore, because the YCrO3 layer is less likely to crack even at the firing temperature range of the mixture, the rotary kiln used in this method also has excellent furnace durability.

[0017] The nickel-containing compound and lithium compound contained in the mixture to be calcined in the calcination step of this method are compounds that serve as raw materials for the composite oxide. The mixture is usually in the form of powder or particles.

[0018] The nickel-containing compound is at least one of a nickel-containing hydroxide and a nickel-containing oxide, and more preferably contains a nickel-containing oxide or is a nickel-containing oxide. The nickel-containing compound may contain a metal element other than Ni in addition to Ni, preferably a transition metal element other than Ni, more preferably at least one of Mn and Co, and may contain Mn and Co. The Cr content in the nickel-containing compound is preferably 0.001% by mass or less, and may be 0.0001% by mass or less.

[0019] The nickel-containing hydroxide is preferably a nickel composite hydroxide containing Ni and a metal element other than Ni. The nickel-containing oxide is preferably a nickel composite oxide containing Ni and a metal element other than Ni. The metal element other than Ni contained in the nickel composite hydroxide and nickel composite oxide is preferably a transition metal element other than Ni, more preferably at least one of Mn and Co, and may be Mn and Co. The nickel-containing compound is preferably a nickel composite oxide.

[0020] The lithium compound may be one or more selected from the group consisting of lithium hydroxide, lithium carbonate, lithium nitrate, and lithium acetate. The Cr content in the lithium compound is preferably 0.001% by mass or less, and may be 0.0001% by mass or less. The lithium compound is preferably at least one of lithium hydroxide and lithium carbonate, and more preferably lithium hydroxide. The lithium compound may be anhydrous or hydrated. When the lithium compound is lithium hydroxide, the lithium hydroxide may be anhydrous lithium hydroxide or lithium hydroxide hydrate. An example of the lithium hydroxide hydrate is lithium hydroxide monohydrate.

[0021] The average particle size (D50) of the lithium compound is, for example, 3 to 20 μm, or may be 5 to 18 μm, or may be 8 to 15 μm. In this specification, the average particle size is the particle size (D50) at which the cumulative frequency of particles with smaller sizes in a volume-based particle size distribution reaches 50%. The volume-based particle size distribution can be measured using a laser diffraction particle size distribution analyzer.

[0022] The contents of the nickel-containing compound and the lithium compound in the mixture may be set so as to obtain a first composite oxide having the desired composition.

[0023] The rotary kiln used in this method may be an externally heated type. The entire rotary kiln furnace may be made of YCrO3, as long as a YCrO3 layer is formed on the outermost surface of the inner wall. The remaining portions of the furnace other than the YCrO3 layer may be made of a material other than YCrO3. For example, the furnace may have a base material layer made of an alloy containing Cr and a YCrO3 layer covering the base material layer and formed on the outermost surface of the furnace's inner wall. When the base material layer is covered with a YCrO3 layer, the thickness of the YCrO3 layer may be, for example, 10 to 1,000 nm, 50 to 900 nm, or 100 to 800 nm. When the base material layer is covered with a YCrO3 layer, the YCrO3 layer preferably covers the entire surface of the base material layer facing the inner wall of the furnace. However, the YCrO3 layer may cover 80% or more, 90% or more, or 95% or more of the entire surface of the base material layer facing the inner wall of the furnace. When the coverage ratio of the YCrO3 layer is within the above range, it is easy to prevent Cr from being mixed into the first composite oxide as an impurity.

[0024] The base material layer is preferably formed of an alloy containing Cr. The Cr-containing alloy may contain one or more metals selected from the group consisting of Fe, Ni, Mn, and Mo, in addition to Cr, and preferably contains Fe and Ni. The Cr-containing alloy may contain non-metallic elements such as Si, P, S, and C, as long as the alloy exhibits its properties as an alloy. Examples of the Cr-containing alloy include at least one of SUS310S and SUS316L.

[0025] The method for forming the YCrO3 layer is not particularly limited. When the inner wall of the furnace has a base material layer formed of an alloy containing Cr and a YCrO3 layer covering the base material layer, the YCrO3 layer can be formed by thermally spraying yttria (yttrium oxide, hereinafter also referred to as "Y2O3") onto the base material layer. By thermally spraying Y2O3 onto the base material layer, the Cr in the base material layer and the sprayed Y2O3 are combined, and a YCrO3 layer can be formed on the outermost layer of the inner wall of the furnace. The combination of the Cr in the base material layer and the sprayed Y2O3 makes the YCrO3 layer less likely to peel off from the base material layer.

[0026] The YCrO3 layer can be formed, for example, as follows. First, Y2O3 powder is sprayed onto a base material layer to form a Y2O3 layer, resulting in a laminate of the base material layer and the Y2O3 layer. The thickness of the Y2O3 layer is, for example, 50 to 200 μm, and may be 80 to 150 μm. Next, the laminate is fired and then allowed to cool, thereby diffusing Y2O3 into the chromium oxide layer deposited on the base material to form the YCrO3 layer. This firing and cooling process is repeated two or more times, and the Y2O3 film present on the YCrO3 layer is expanded and peeled off, forming the YCrO3 layer on the outermost surface. The laminate may be fired, for example, at a temperature of 800 to 1200°C under atmospheric pressure for 1 to 20 hours. The firing and cooling process may be repeated, for example, two to five times, or may be two to four times, or may be two to three times.

[0027] The mixture is calcined in an oxygen atmosphere. The oxygen atmosphere can be formed, for example, by supplying oxygen into the furnace of a rotary kiln. The mixture is preferably calcined while continuously supplying oxygen into the furnace.

[0028] The firing temperature of the mixture is 750 to 1000° C., optionally 760 to 950° C., optionally 770 to 900° C., optionally 780 to 880° C., or optionally 790 to 850° C. The firing time at the above firing temperature is, for example, 1 to 20 hours, optionally 5 to 15 hours, or optionally 8 to 12 hours.

[0029] (Method for producing positive electrode active material (2)) As shown in FIG. 2, the firing step of the present method may include a first step of obtaining a mixture of a nickel-containing compound and a lithium compound, a second step of molding the mixture to obtain a molded body, and a third step of firing the molded body using a rotary kiln.

[0030] The step of obtaining a mixture in the first step may be carried out by mixing a nickel-containing compound and a lithium compound. The nickel-containing compound and the lithium compound may be the same as those described above. The mixing ratio of the nickel-containing compound and the lithium compound may be set so as to obtain a first composite oxide having the desired composition.

[0031] The nickel-containing compound and the lithium compound can be mixed using, for example, a mixer, such as a jet mill, a ball mill, a rocking mixer, a shaker mixer, a V-blender, a ribbon mixer, a Julia mixer, or a Loedige mixer.

[0032] The second step is a step of forming a molded body by molding the mixture obtained in the first step. The method of forming the mixture is not limited as long as it can form a molded body having the density described below, but compression molding is preferred.

[0033] The maximum diameter of the compact obtained in the second step is, for example, 18 to 50 mm, and may be 20 to 48 mm, 21 to 45 mm, or 22 to 40 mm. When the maximum diameter of the compact is within the above range, the contact area of ​​the lithium compound with the inner wall of the rotary kiln furnace can be reduced, thereby reducing the amount of Cr mixed into the first composite oxide. In addition, the compact can be fired efficiently. On the other hand, when the maximum diameter of the compact is smaller, the contact area of ​​the lithium compound with the inner wall of the furnace tends to increase. When the maximum diameter of the compact is larger, it becomes difficult to sufficiently fire the interior of the compact, or the firing takes a long time, making it difficult to fire the compact efficiently. In this specification, the maximum diameter of the compact refers to the longest length among the lengths connecting any two points on the periphery of the compact (the length when connecting two points passing through the interior of the compact).

[0034] The density of the molded body obtained in the second step is, for example, 1.5 to 4 g / cm 3 and preferably 1.6 to 3.5 g / cm 3 and 1.8 to 3.0 g / cm 3and 2.0 to 2.8 g / cm 3 and 2.0 to 2.5 g / cm 3 When the density of the molded body is within the above range, cracking of the molded body during firing can be suppressed, and firing can be performed efficiently. On the other hand, when the density of the molded body is low, the molded body is more likely to crack during firing in the third step. When the density of the molded body is high, oxygen is less likely to enter the interior of the molded body, making it difficult to fire the molded body sufficiently to the inside, or firing takes a long time, making it difficult to fire the molded body efficiently.

[0035] The shape of the molded body obtained in the second step is not particularly limited, but is preferably spherical, elliptical, or cylindrical, and more preferably elliptical or cylindrical. The above-mentioned shape of the molded body reduces the contact area with the inner wall of the rotary kiln furnace, thereby reducing the amount of Cr mixed into the first composite oxide. Furthermore, since the above-mentioned molded body has reduced corners compared to a prismatic shape, etc., powdering of the molded body due to chipping of the corners can be suppressed when fired in the rotary kiln in the third step. Elliptical or cylindrical molded bodies are preferred because they are more likely to move at a speed suitable for firing in the rotary kiln used in the third step than spherical molded bodies.

[0036] The molded article may have a single-layer structure formed by molding the mixture, or may have a multi-layer structure of two or more layers, including a core layer formed by molding the mixture and a coating layer covering the core layer. The core layer may be formed by compression molding the mixture. In a molded article having a core layer and a coating layer (hereinafter also referred to as a "multi-layer molded article"), the coating layer may cover the entire surface of the core layer, or may cover only a portion of the core layer. The coating layer may cover preferably 70% or more, or may cover 80% or more, or may cover 90% or more of the entire surface of the core layer. The multi-layer molded article may have, for example, a three-layer structure in which a coating layer, a core layer, and a coating layer are laminated in this order.

[0037] The coating layer preferably contains a second lithium transition metal composite oxide (hereinafter also referred to as "second composite oxide"), and more preferably contains the second composite oxide and a binder. When the coating layer contains the second composite oxide, the core layer formed using a mixture containing a lithium compound is less likely to be exposed. Therefore, in the firing step (third step), direct contact between the inner wall of the rotary kiln furnace and the lithium compound is less likely to occur, and the incorporation of Cr as an impurity into the first composite oxide can be further suppressed.

[0038] The second composite oxide is not particularly limited as long as it is an oxide containing lithium and a transition metal. The second composite oxide preferably has the composition described for the first composite oxide, and is more preferably the first composite oxide produced by the present method. The second composite oxide may have the same composition as the first composite oxide contained in the molded body, or may have a different composition. The coating layer may contain, for example, a cathode active material produced by the present method, i.e., a cathode active material containing the first composite oxide. The composition of the second composite oxide can be determined by ICP-AES, as described for the first composite oxide.

[0039] The coating layer may contain a binder to improve the coverage of the core layer. Mixing the second composite oxide with a binder improves the granulation properties of the second composite oxide, thereby improving the coverage of the core layer with the coating layer.

[0040] The binder contains one or more selected from the group consisting of polyvinylidene fluoride (PVdF), carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA), and polyacrylamide (PAM), and preferably contains PVdF. The binder is used to improve the granulation properties of the second composite oxide, and may be present in an amount of 0.1 to 5 mass %, 0.5 to 3 mass %, or 0.8 to 2 mass % relative to the total amount of the second composite oxide.

[0041] The compact can be obtained by molding the mixture using a molding machine, for example, by compression molding the mixture using a powder molding machine. As the powder molding machine, a general powder molding machine can be used, for example, a hydraulic press, a hydrostatic press, a briquetting machine, a single-punch tablet press, a rotary tablet press, or the like.

[0042] The method for forming the multilayer molded body is not particularly limited, and it can be obtained, for example, by simultaneously molding the material for forming the coating layer and the mixture for forming the core layer. For example, a layer of a material containing the second composite oxide and a layer of the mixture are laminated, and then the laminate is molded to obtain a multilayer molded body. When the coating layer contains a binder, for example, the multilayer molded body can be obtained as follows. First, the second composite oxide and the binder are mixed, dispersed in a solvent such as N-methyl-2-pyrrolidone (NMP), and then the NMP is volatilized and removed by heating to obtain a granulated material containing the second composite oxide and the binder. Next, the granulated material is placed in a mold, the mixture is placed on top of it, and the granulated material is then placed, and the resulting mixture is compression molded to obtain a multilayer molded body. The molding used to form the multilayer molded body may be compression molding.

[0043] The third step is to fire the molded body obtained in the second step using a rotary kiln. The rotary kiln may be the one described above. The firing conditions for the molded body may be the same as those described above for the mixture.

[0044] (Manufacturing method of positive electrode plate) The method for manufacturing a positive electrode plate according to this embodiment is a method for manufacturing a positive electrode plate using a positive electrode active material, and the positive electrode active material is manufactured by this method.

[0045] The positive electrode plate can have a positive electrode current collector foil and a positive electrode active material formed on one or both sides of the positive electrode current collector foil. The positive electrode active material is contained in a positive electrode active material layer, and the positive electrode active material layer can further include at least one of a binder and a conductive material. The positive electrode active material layer can be formed by adding a solvent such as N-methyl-2-pyrrolidone (NMP) to materials that form the positive electrode active material layer, such as the positive electrode active material, binder, and conductive material, to form a positive electrode slurry, applying the slurry to the positive electrode current collector foil, drying, and compressing the slurry.

[0046] The positive electrode current collector foil is, for example, a metal foil made of an Al material such as Al or an Al alloy. Examples of binders include fluororesins such as polyvinylidene fluoride (PVdF) and polytetrafluoroethylene; cellulose-based resins such as carboxymethyl cellulose (CMC), methyl cellulose, and hydroxypropyl cellulose; and styrene butadiene rubber, and one or more of these can be used. Examples of conductive materials include carbon materials. Examples of carbon materials include fibrous carbon such as carbon nanotubes and carbon black, and one or more of these can be used.

[0047] (Method of manufacturing non-aqueous electrolyte secondary battery) The method for manufacturing a non-aqueous electrolyte secondary battery (secondary battery) of this embodiment is a method for manufacturing a secondary battery using the above-described positive electrode plate, and the positive electrode plate is manufactured by the above-described method for manufacturing a positive electrode plate.

[0048] The secondary battery may include an electrode assembly including a positive electrode plate and a non-aqueous electrolyte, and may have a battery case that accommodates the electrode assembly and the non-aqueous electrolyte. The battery case and the non-aqueous electrolyte may be any known battery case and non-aqueous electrolyte used in secondary batteries.

[0049] The electrode assembly may include the above-described positive electrode plate, negative electrode plate, and separator. In the electrode assembly, the positive electrode active material layer of the positive electrode plate and the negative electrode active material layer of the negative electrode plate face each other via the separator. The electrode assembly may be a laminated type in which a positive electrode plate, a negative electrode plate, and a separator are stacked, or a wound type in which a strip-shaped laminate in which a strip-shaped positive electrode plate, a strip-shaped negative electrode plate, and a strip-shaped separator are stacked is wound. The wound type electrode assembly may have a flat shape that is pressed after rolling the laminate. The negative electrode plate usually has a negative electrode current collector foil and a negative electrode active material layer. The negative electrode current collector foil, negative electrode active material layer, and separator may be known negative electrode current collector foils, negative electrode active material layers, and separators used in secondary batteries.

[0050] (Rotary Kiln Disclosure) Although the rotary kiln used for producing a positive electrode active material has been described above, the rotary kiln can also be used for purposes other than the production of a positive electrode active material. As described below, the present disclosure also discloses a rotary kiln that can be used for the production of a positive electrode active material and other purposes. The rotary kiln of the present disclosure can be an externally heated type.

[0051] The rotary kiln of the present disclosure has a furnace, and a layer of yttrium-chromium composite oxide (YCrO3 layer) is formed on the outermost surface of the furnace's inner wall. As long as a YCrO3 layer is formed on the outermost surface of the inner wall, the entire furnace may be made of YCrO3, or portions of the furnace other than the YCrO3 layer may be made of a material other than YCrO3.

[0052] The rotary kiln of the present disclosure may have a furnace, and the furnace may have a base material layer formed of an alloy containing Cr, and a YCrO layer covering the base material layer and formed on the outermost surface of the furnace's inner wall. The coverage ratio of the YCrO layer to the base material layer and the material forming the base material layer may be as described in the method for producing a positive electrode active material.

[0053] The rotary kiln equipped with a furnace having a base material layer and a YCrO layer may include a step of forming the YCrO layer by thermally spraying yttria (YO) onto the base material layer formed of an alloy containing Cr. Examples of a method for forming the YCrO layer on the base material layer include the methods described in the method for producing a positive electrode active material. [Example]

[0054] Hereinafter, the present disclosure will be described more specifically with reference to examples and comparative examples.

[0055] Example 1 (Preparation of the mixture) A nickel composite oxide containing Ni, Co, and Mn in a molar ratio of Ni:Co:Mn = 83:5:12 was prepared as the nickel-containing compound, and lithium hydroxide monohydrate (average particle size (D50): 10 μm) was prepared as the lithium compound. The nickel-containing compound and the lithium compound were mixed to a molar ratio of Li:Ni:Co:Mn = 1.06:0.83:0.05:0.12 to obtain a mixture (first step).

[0056] (Production of molded body) 5.0 g of the mixture was placed in a powder molding die ("DT6025A-2025" manufactured by NPA Systems Co., Ltd.). The mixture in the die was compression molded at 20 MPa using a hydraulic press (manufactured by Riken Kikai) to produce a cylindrical powder with a diameter of 20 mm, a height of 10 mm, and a density of 2.2 g / cm. 3 A molded body having the following maximum diameter was obtained (second step).

[0057] (Preparation of metal plate with YCrO3 layer) A 100mm x 100mm SUS310S plate was prepared to simulate the base material layer of a rotary kiln furnace, a Cr-containing alloy. Yttrium (YO) powder was sprayed onto the SUS310S plate to form a 100µm-thick YO layer, resulting in a laminate of the SUS310S plate and the YO layer. The laminate was then fired at 1000°C for 10 hours under atmospheric pressure and allowed to cool. YO diffused into the chromium oxide layer deposited on the SUS310S plate, forming an yttrium-chromium composite oxide layer (YCrO layer). The firing and cooling process was repeated twice, causing the YO film on the YCrO layer to expand and peel off. This resulted in a SUS310S plate with a YCrO layer formed on its outermost surface (hereinafter referred to as a "metal plate with a YCrO layer").

[0058] (Sintering process) The compact was placed on the YCrO3 layer of the metal plate with the YCrO3 layer, placed in an electric furnace, and fired at 805°C for 10 hours in an oxygen atmosphere (Step 3), yielding a lithium-transition metal composite oxide as the positive electrode active material. The Cr content of the positive electrode active material was measured by inductively coupled plasma (ICP) atomic emission spectroscopy (ICP-AES). Specifically, measurements were performed in accordance with JIS K 0116:2014, General Rules for Optical Emission Spectroscopy. The lithium-transition metal composite oxide was dissolved by the alkali fusion method and then diluted to a specified volume with ultrapure water, tartaric acid, or hydrochloric acid using a high-resolution ICP atomic emission spectrometer (Hitachi High-Tech Science Corporation, PS3500DDII). The results are shown in Table 1.

[0059] Comparative Example 1 (Preparation of metal plate with chromium oxide film) A 100mm x 100mm SUS310S plate was prepared to simulate the base material layer of a rotary kiln furnace, which is an alloy containing Cr. This SUS310S plate was fired at 1000°C for 10 hours under atmospheric pressure to obtain a SUS310S plate with a chromium oxide film formed on its outermost surface (hereinafter also referred to as a "metal plate with a chromium oxide film").

[0060] (Sintering process) The compact was fired in the same manner as in Example 1, except that a metal plate with a chromium oxide film was used instead of the metal plate with a YCrO layer, to obtain a lithium transition metal composite oxide as a positive electrode active material. The Cr content in the positive electrode active material was measured by ICP-AES as described in Example 1. The results are shown in Table 1.

[0061] Comparative Example 2 The mixture prepared in the same manner as in Example 1 was placed on a metal plate with a chromium oxide film prepared in the same manner as in Comparative Example 1, and the mixture was placed in an electric furnace and fired at 805°C for 10 hours in an oxygen atmosphere to obtain a lithium transition metal composite oxide as a positive electrode active material. As described in Example 1, the Cr content in the positive electrode active material was measured by ICP-AES. The results are shown in Table 1.

[0062] [Table 1]

Claims

1. A method for producing a positive electrode active material containing a lithium transition metal composite oxide, comprising: The method includes a step of firing a mixture of a nickel-containing compound and a lithium compound charged into a furnace of a rotary kiln at 750 to 1000°C in an oxygen atmosphere, the nickel-containing compound is at least one of a nickel-containing hydroxide and a nickel-containing oxide, In the method for producing a positive electrode active material, a layer of yttrium-chromium composite oxide is formed on the outermost surface of the inner wall of the furnace.

2. the layer covers a base material layer of the furnace; The method for producing a positive electrode active material according to claim 1 , wherein the base layer is formed of an alloy containing Cr.

3. The method for producing a positive electrode active material according to claim 2 , wherein the layer is formed by thermally spraying yttria onto the base material layer.

4. The method for producing a positive electrode active material according to claim 2 , wherein the alloy containing Cr further contains Fe and Ni.

5. Further, the method includes a step of forming the mixture to obtain a molded body, The method for producing a positive electrode active material according to claim 1 , wherein the firing step includes firing the compact.

6. The method for producing a positive electrode active material according to claim 1 , wherein the lithium compound is at least one of lithium hydroxide and lithium carbonate.

7. The method for producing a positive electrode active material according to claim 1 , wherein the Cr content of the lithium transition metal composite oxide is 1 ppm or less.

8. The method for producing a positive electrode active material according to claim 1 , wherein the lithium transition metal composite oxide contains Li, Ni, and Mn.

9. The lithium transition metal composite oxide is Li, Ni, Mn, Co, and M (M is one or more metal elements selected from the group consisting of Mg, Ca, Al, Ti, V, Cr, Fe, Cu, Zn, Zr, Nb, Mo, Ta, W, and Y), and 2. The method for producing a positive electrode active material according to claim 1, wherein a molar ratio of Li, Ni, Mn, Co, and M is Li:Ni:Mn:Co:M=a:x:y:z:t, where a, x, y, z, and t are 1.0≦a≦1.3, x+y+z+t=1, 0.25≦x≦0.9, 0<y≦0.6, 0<z≦0.6, and 0<t≦0.

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10. A method for manufacturing a positive electrode plate using a positive electrode active material, A method for producing a positive electrode plate, wherein the positive electrode active material is produced by the method for producing a positive electrode active material according to any one of claims 1 to 9.

11. A method for manufacturing a nonaqueous electrolyte secondary battery including a positive electrode plate, comprising: A method for producing a non-aqueous electrolyte secondary battery, comprising producing the positive electrode plate by the method for producing a positive electrode plate according to claim 10.

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