Method for producing positive electrode active material for lithium secondary battery

Abstract

To provide a method for manufacturing a cathode active material for lithium secondary batteries, which is high in quality, high in productivity and low in production cost.SOLUTION: A method for manufacturing a cathode active material for lithium secondary batteries comprises: a mixing step of mixing a lithium compound and a compound including a metal element other than Li in the formula (1) below; and a firing step of firing a precursor obtained by the mixing step to gain a lithium composite compound represented by the following formula (1): Li1+aM1O2+α (1). (In the formula (1), M1 is a metal element other than Li, including at least Ni; the percentage of Ni in M1's is 70 atom% or more, and a and α are numbers that satisfy: -0.1≤a≤0.2 and -0.2≤α≤0.2.) The firing step includes at least a thermal treatment step of performing a thermal treatment while rolling the precursor in a furnace tube 10 of firing furnace 1. In the furnace tube, aluminum oxide is formed on an outermost layer of a powder-contact part, and as a layer underlying it, an alloy layer of aluminum and the furnace tube base is formed.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a method for producing a positive electrode active material used for a positive electrode of a lithium secondary battery. relates to.

Background Art

[0002] In lithium secondary batteries, which are widely popular as secondary batteries having a high energy density and being small and lightweight, an increase in the capacity of the active materials used for the positive electrode and the negative electrode is required.

[0003] As a positive electrode active material having a high charge-discharge capacity, a lithium composite compound represented by LiMO2 (M represents a metal element such as Ni, Co, Mn, etc.) having an α-NaFeO2 type layered structure is known. Since this positive electrode active material tends to have a higher capacity as the ratio of nickel increases, it is expected as a positive electrode active material for realizing a high energy density of the battery. And a lithium composite compound having high productivity and low production cost is required.

[0004] In the production process of the positive electrode active material, a technique using a rotary kiln as a firing furnace for performing the formation reaction of the lithium composite compound has been proposed. The rotary kiln has features such as not requiring a special firing container and being able to easily maintain an oxidizing atmosphere in the furnace.

[0005] For example, Patent Document 1 discloses that the hearth tube of the firing furnace used in the rolling heat treatment step is made of ceramics, or made of nickel, tungsten, molybdenum, titanium, or an alloy mainly composed of these metals.

[0006] Also, Patent Document 2 discloses that in a double-structured hearth tube having an inner cylinder tube, at least one of the inner layer of the hearth tube or the outer layer of the inner cylinder tube is made of a metal nickel material or a nickel alloy material.

Prior Art Documents

Patent Document

[0007]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0008] However, in the case of a core tube made of ceramics, it is difficult to increase the size of the device and obtain high productivity. In the case of an alloy mainly composed of metal, there is a risk that metal components will be mixed into the raw materials during long - term use of the core tube.

[0009] Therefore, an object of the present invention is to provide a positive electrode active material for a lithium secondary battery having high quality, high productivity, and low production cost.

Means for Solving the Problems

[0010] The manufacturing method of the positive electrode active material for a lithium secondary battery of the present invention One aspect comprises a mixing step of mixing a lithium compound and a compound containing a metal element other than Li in the following formula (1), and a firing step of firing the precursor obtained through the mixing step to obtain a lithium composite compound represented by the following (1). The firing step has at least a heat treatment step of performing heat treatment while rolling the precursor in a core tube of a firing furnace. In the core tube, Obtained by surface oxidation of pure aluminum aluminum oxide is formed on the outermost layer of the powder - contacting part, and Said pure an alloy layer of aluminum and the base material of the core tube is formed in the underlying layer, which is characterized. Li 1+a M1O 2+α ···(1) (However, in the above formula (1), M1 is a metal element other than Li and at least Ni And any one of Mn, Co, Al, Ti, Zr, Mo, Nb, W, V, Cr, Mg, Ca, Cu, Zn, Snincluding, the proportion of Ni in M1 being 70 atomic % or more, a and α being numbers satisfying -0.1 ≤ a ≤ 0.2 and -0.2 ≤ α ≤ 0.2.)

[0011] Further, in the method for producing a positive electrode active material for a lithium secondary battery of the present invention, the firing furnace includes a first air supply pipe that injects an oxidizing gas toward the inner peripheral surface side of the furnace core tube, and a second air supply pipe that allows an oxidizing gas to flow in the axial direction of the furnace core tube, and it is preferable that the first air supply pipe has a plurality of injection ports having the circumferential direction of the first air supply pipe as the opening direction.)

[0012] Further, in the method for producing a positive electrode active material for a lithium secondary battery of the present invention, it is preferable that legs for holding the first air supply pipe are not joined to the furnace core tube in the furnace core tube.)

[0013] Further, the method for producing a positive electrode active material for a lithium secondary battery of the present invention preferably includes a step of holding at 700°C or higher for 2 hours or more in the heat treatment step.) Moreover, the method for producing a positive electrode active material for a lithium secondary battery according to another aspect of the present invention includes a mixing step of mixing a lithium compound and a compound containing a metal element other than Li in the following formula (1), A firing step of firing the precursor obtained through the mixing step to obtain a lithium composite compound represented by the following (1), The firing step includes at least a heat treatment step of performing heat treatment while rolling the precursor in the hearth tube of the firing furnace, The firing furnace includes a first air supply pipe that injects an oxidizing gas toward the inner peripheral surface side of the hearth tube, and a second air supply pipe that flows an oxidizing gas in the axial direction of the hearth tube, The first air supply pipe has a plurality of injection ports whose circumferential direction of the first air supply pipe is the opening direction, In the hearth tube, an aluminum oxide is formed on the outermost layer of the powder contact portion, and an alloy layer of aluminum and the base material of the hearth tube is formed on the underlying layer, and the leg holding the first air supply pipe is not joined to the hearth tube Characterized by this. Li 1+a M1O 2+α ···(1) (However, in the above formula (1), M1 is a metal element other than Li and contains at least Ni, the proportion of Ni in M1 is 70 atomic% or more, and a and α are numbers satisfying -0.1 ≦ a ≦ 0.2, -0.2 ≦ α ≦ 0.2.)

Advantages of the Invention

[0014] According to the present invention, it is possible to provide a positive electrode active material for a lithium secondary battery having high quality, high productivity, and low production cost.)

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0016] Hereinafter, a positive electrode active material for a lithium secondary battery (hereinafter, may be simply referred to as a positive electrode active material) according to an embodiment of the present invention and a method for manufacturing the same will be described in detail. Note that the following description shows specific examples of the content of the present invention, and the present invention is not limited thereto. The present invention can be variously modified by those skilled in the art within the scope of the technical idea disclosed in this specification.

[0017] <Positive electrode active material> The positive electrode active material according to this embodiment is a lithium composite compound composed of lithium and a transition metal and having a layered rock salt-type crystal structure (hereinafter, may be referred to as a layered structure) belonging to the space group R-3m. This positive electrode active material enables reversible occlusion and release of lithium ions by applying a voltage, and is suitably used as a positive electrode active material for a lithium secondary battery (for a lithium ion secondary battery). Note that the lithium composite compound is also referred to as a lithium composite oxide.

[0018] The positive electrode active material according to this embodiment has the following formula (1): Li 1+a M1O 2+α ···(1) (However, in the formula (1), M1 is a metal element other than Li and contains at least Ni, the ratio of Ni per M1 exceeds 70 atomic%, and a and α are numbers satisfying -0.1 ≤ a ≤ 0.2 and -0.2 ≤ α ≤ 0.2.)

[0019] The cathode active material according to this embodiment is a cathode active material capable of realizing a high energy density and a high charge-discharge capacity by having a composition in which the ratio of nickel (Ni) per metal element (M1) other than lithium (Li) exceeds 70 atomic %. Note that the ratio of nickel (Ni) per metal element (M1) other than lithium (Li) can take an appropriate value within the range exceeding 70 atomic % and less than 100 atomic %. Since it is a cathode active material containing nickel at a high ratio in this way, Ni 2+ to Ni 3+ It is important that the oxidation reaction that oxidizes it to proceeds efficiently.

[0020] As the metal element (M1) other than lithium (Li), in addition to nickel, a transition metal element may be included, a non-transition metal element may be included, or a combination thereof may be included. Specific examples of such metal elements (M1) include manganese (Mn), cobalt (Co), aluminum (Al), titanium (Ti), zirconium (Zr), molybdenum (Mo), niobium (Nb), tungsten (W), vanadium (V), chromium (Cr), magnesium (Mg), calcium (Ca), copper (Cu), zinc (Zn), tin (Sn), and the like. Among these, from the viewpoint of stabilizing the layered structure, it is preferable that aluminum (Al) and / or titanium (Ti) is included.

[0021] A more preferable specific composition of the cathode active material according to this embodiment is the following formula (2): Li 1+a Ni b Mn c Co d M2 e O 2+α ···(2) (However, in the above formula (2), M2 is at least one element selected from the group consisting of Mg, Al, Ti, Zr, Mo, and Nb, and a, b, c, d, e, and α are numbers satisfying -0.1 ≦ a ≦ 0.2, 0.7 < b ≦ 0.9, 0 ≦ c < 0.3, 0 ≦ d < 0.3, 0 ≦ e ≦ 0.25, b + c + d + e = 1, and -0.2 ≦ α ≦ 0.2.)

[0022] <Method for producing positive electrode active material> The method for producing a positive electrode active material according to this embodiment relates to a method for synthesizing a lithium composite compound which is a positive electrode active material used for the positive electrode of a lithium secondary battery, is represented by the above formula (1), and has a layered rock salt type crystal structure.

[0023] (Manufacturing method flow) FIG. 1 is a flowchart of a method for producing a positive electrode active material according to an embodiment of the present invention. As shown in FIG. 1, the method for producing a positive electrode active material according to this embodiment includes a mixing step S1 and a firing step S2. Through the mixing step S1, a precursor is prepared from the raw material compounds, and the precursor is fired in the firing step S2 to synthesize a lithium composite compound that can be used as a material for the positive electrode of a lithium secondary battery (lithium ion secondary battery). The manufacturing method according to this embodiment includes at least a heat treatment step of performing heat treatment while rolling a precursor of the lithium composite compound before firing using a rotary kiln as a firing furnace as one step constituting the firing step S2.

[0024] In the mixing step S1, a compound containing lithium (lithium compound) and a compound containing a metal element other than Li that composes the positive electrode active material are mixed. Examples of the compound containing lithium include lithium carbonate and lithium hydroxide. Lithium carbonate is stable in supply, has good availability, and is inexpensive compared to lithium acetate, lithium nitrate, lithium hydroxide, lithium chloride, lithium sulfate, etc. Also, since it has a high melting point, it causes less damage to the manufacturing equipment and is excellent in industrial applicability and practicality.

[0025] As the compound containing a metal element other than Li that composes the positive electrode active material, a compound containing nickel, a compound containing manganese, a compound containing cobalt, or a compound containing other metal elements such as M2 is mixed.

[0026] As the compound containing nickel, for example, pure nickel (included as a compound for convenience), oxide, hydroxide, carbonate, acetate, etc. can be used. Among these, in particular, it is preferable to use an oxide or a hydroxide. In the case of an oxide or a hydroxide, unlike the case of using a carbonate or an acetate, etc., a large amount of carbon dioxide gas is not generated during the firing process, so a lithium composite compound with a high proportion of nickel and high purity can be stably produced.

[0027] As the compound containing manganese and the compound containing cobalt, for example, oxide, hydroxide, carbonate, acetate, etc. can be used. Among these, in particular, it is preferable to use an oxide, a hydroxide, or a carbonate. Also, as the compound containing other metal elements such as M2, for example, carbonate, oxide, hydroxide, acetate, nitrate, etc. can be used. Among these, in particular, it is preferable to use a carbonate, an oxide, or a hydroxide.

[0028] In the mixing step S1, specifically, each compound of the raw materials is weighed at a predetermined element composition ratio corresponding to the above formula, and each compound is pulverized and mixed to prepare a powdery mixture in which each compound is mixed. From the viewpoint of uniformly mixing each compound and also making the particle sizes uniform, it is preferable to pulverize until the average particle size becomes less than 1 μm. As the pulverizer for pulverizing the compound, for example, general precision pulverizers such as a ball mill, a jet mill, and a sand mill can be used. The pulverization of the compound of the raw material is preferably wet pulverization, and from an industrial viewpoint, wet pulverization using water as a dispersion medium is particularly preferable. The solid-liquid mixture obtained by wet pulverization may be dried, for example, using a dryer. As the dryer, for example, a spray dryer, a fluidized bed dryer, an evaporator, etc. can be used.

[0029] In the firing step S2, the precursor obtained through the mixing step S1 is fired to obtain a lithium composite compound having a layered structure. The firing step S2 includes at least a heat treatment step of performing heat treatment while rolling the precursor of the lithium composite compound before firing in a rotary kiln used as a firing furnace. Here, the rotary kiln used in the firing step S2 will be described.

[0030] (Rotary Kiln / Firing Furnace) FIG. 2 is a diagram showing a schematic structure of a rotary kiln used for manufacturing a positive electrode active material for a lithium secondary battery. As shown in FIG. 2, the rotary kiln 1 includes a hearth tube 10, a heater 20, a first air supply pipe 30, a second air supply pipe 40, and a lifter 50.

[0031] The hearth tube 10 has a hollow substantially cylindrical shape, and has an input section for the object to be processed Ma at one end side in the longitudinal direction and a recovery section for the heat-treated object at the other end side. The hearth tube 10 is installed inclined with respect to the horizontal plane such that the input section of the object to be processed Ma is located above the recovery section. The precursor of the lithium composite compound is introduced into the hearth tube 10 from a powder introduction device (not shown) installed at the input section, and flows through the inside of the hearth tube 10 in the longitudinal direction and is heat-treated. The inclination angle of the hearth tube 10 is not particularly limited, but is, for example, in the range of 0.5 to 3°. In this specification, the input section side in the longitudinal direction of the hearth tube 10 is referred to as "upstream", and the recovery section side is referred to as "downstream".

[0032] The hearth tube 10 is connected via a drive gear or roller by power from a motor or the like (not shown). The hearth tube 10 is configured to rotate about the central axis of the cylindrical shape as the rotation axis by driving such a motor or the like. Therefore, the object to be processed Ma introduced into the hearth tube 10 from the input section flows down while rolling inside the hearth tube 10 as the hearth tube 10 rotates, and is recovered by a powder recovery device (not shown) at the recovery section. The rotation speed of the hearth tube 10 is not particularly limited, but is, for example, in the range of 0.5 to 3 rpm.

[0033] The base material of the core tube 10 is preferably a metal such as Ni, W, Mo, Ti, etc. from which harmful components such as chromium are not discharged, or an alloy mainly composed of these metals. And in the core tube, aluminum oxide is formed on the outermost layer of the powder contact part which is the part in contact with the precursor (object to be processed Ma), and an alloy layer of aluminum and the base material of the core tube is formed on its underlying layer. When aluminum oxide is formed on the outermost layer of the base material of the core tube 10, it becomes difficult for the lithium component contained in the precursor of the lithium composite compound to react with the core tube 10, so the possibility of deterioration and damage of the core tube 10 is reduced. The aluminum oxide on the outermost layer of the base material of the core tube 10 can be easily formed by subjecting the base material coated with aluminum to air burning and oxidizing the coated aluminum. The method of coating the base material with aluminum may be, for example, thermal spraying or, for example, electroplating. If the base material is coated with aluminum before the core tube 10 is made into a can, the base material will be exposed at the joint part, so it is preferable to coat it after the can is made.

[0034] The heater 20 is installed around the circumference of the core tube 10. The heater 20 covers a heating zone 120 shown by a one-dot chain line in FIG. 2, which is a partial section in the longitudinal direction of the core tube 10, and can raise the temperature of the heating zone 120 to the target temperature. Also, the heater 20 preheats a preheating zone 110 at a predetermined distance shown by a two-dot chain line in FIG. 2, which is a section upstream of the heating zone 120, to a temperature lower than the target temperature. Therefore, when the precursor of the lithium composite compound is introduced into the core tube 10 while the heater 20 is operating, the precursor is preheated in the preheating zone 110 and then heated to the target temperature in the heating zone 120 and heat-treated while rolling. However, the arrangement position and the number of units of the heater 20 are not particularly limited as long as uniform heat treatment can be performed on the heating zone 120. The heater 20 may be arranged in a concentrated manner at one location or may be arranged separately at a plurality of locations as long as the preheating zone 110 is ensured so that rapid heat treatment does not proceed.

[0035] The first gas supply pipe 30 constitutes a first gas supply system that supplies an oxidizing gas from a gas source (not shown) to the inside of the core pipe 10, and when heat-treating the object to be processed Ma, it injects the oxidizing gas toward the inner peripheral surface side of the core pipe 10. The first gas supply pipe 30 is arranged inside the core pipe 10 along the longitudinal direction, covering substantially the entire length from the downstream side to the upstream side of the core pipe 10. The first gas supply pipe 30 has a plurality of injection ports 32 that are arranged along the longitudinal direction of the core pipe 10, branch from the first gas supply pipe 30, and open at the branched tips. Each of the injection ports 32 can inject the oxidizing gas pumped from a gas source (not shown) in a shower-like manner toward the inside of the core pipe 10. That is, by the first gas supply system, the oxidizing gas is sprayed onto the precursor being heat-treated while rolling, so that oxygen is directly supplied to the precursor, and the oxidation reaction is efficiently promoted. Also, the carbon dioxide gas generated from the precursor is lifted by the oxidizing gas and quickly discharged from the vicinity of the precursor. That is, it is prevented that the carbon dioxide gas generated from the precursor and staying in the core pipe 10 reacts again with the precursor to regenerate lithium carbonate and inhibit the formation of the lithium composite compound. The opening direction of the injection port 32 may be the vertically downward side (radial direction of the core pipe) or the circumferential direction of the first gas supply pipe 30. However, by injecting in the circumferential direction, the airflow in the furnace becomes a swirling flow, and the oxygen supply to the precursor and the discharge of the staying carbon dioxide gas can be efficiently performed. Therefore, it is preferable that the opening direction of the injection port 32 is the circumferential direction of the first gas supply pipe 30.

[0036] The first air supply pipe 30 is preferably provided such that the supply of oxygen and the exhaust of carbon dioxide can be efficiently performed, and the blowing amount, blowing angle, and oxygen concentration of the oxidizing gas can be adjusted from the viewpoint of preventing the scattering of the powder of the object to be processed. For example, the blowing amount can be adjusted by adjusting the gas flow rate of the first air supply system, or by providing the injection port 32 so as to be openable and closable and adjusting the number of openings of the injection port 32. Further, the blowing angle can be adjusted by rotatably providing the first air supply pipe 30 with the central axis as the rotation axis. For example, it may be injected at an angle exceeding 0° and equal to or less than 45° in the forward or reverse direction with respect to the rotation direction of the core pipe 10. Further, the blowing angle can be adjusted by moving the first air supply pipe 30 horizontally or the like inside the core pipe 10. For example, the first air supply pipe 30 may be stationary at a position eccentric from the central axis of the core pipe 10 and injected. Regarding the oxygen concentration, oxygen concentration detecting means is provided near the inlet or outlet of the core pipe 10, or at an arbitrary location, and the oxygen amount can be adjusted by monitoring and controlling so that the detected oxygen concentration becomes a specified value. And these blowing amounts, blowing angles, and oxygen concentrations can be adjusted by appropriately combining them. In addition, instead of or in combination with the oxygen concentration detecting means, carbon dioxide concentration detecting means can be provided, and the oxygen amount can be adjusted by monitoring and controlling so that the detected carbon dioxide concentration becomes a specified value.

[0037] The first air supply pipe 30 is held inside the core pipe 10 by the legs 34. A round bar or the like can be interposed in the vicinity of the circumferential side of the contact portion between the legs 34 and the core pipe 10, and the legs 34 and the core pipe 10 can be restrained in the circumferential direction without being fixed. Thereby, the driving force of the core pipe 10 can be transmitted to the first air supply pipe 30, and the first air supply pipe and the core pipe 10 can rotate synchronously. This round bar or the like does not restrain the axial direction of the core pipe of the legs 34, restrains only the circumferential direction, and does not join the legs 34 and the core pipe 10, so that the first air supply pipe 30 can be easily removed from the core pipe 10, improving the workability during cleaning and maintenance inside the core pipe 10, and thus improving the productivity.

[0038] The second gas supply pipe 40 constitutes a second gas supply system that supplies an oxidizing gas from a gas source (not shown) to the inside of the core pipe 10. When heat-treating the workpiece Ma, an air current of the oxidizing gas is generated inside the core pipe 10 in the axial direction of the core pipe 10. The oxidizing gas may flow from the upstream side to the downstream side of the core pipe 10, but preferably flows from the downstream side to the upstream side of the core pipe 10. The second gas supply pipe 40 is disposed on the downstream side of the heating zone 120 inside the core pipe 10 and opens toward the upstream side of the core pipe 10. Further, when viewed in the axial direction, the second gas supply pipe 40 is located radially outside the first gas supply pipe and opens into the space radially outside the first gas supply pipe 30. The second gas supply pipe 40 flows the oxidizing gas in a substantially horizontal direction radially outside the first gas supply pipe 30. The oxidizing gas is exhausted to the outside from an exhaust port (not shown) provided on the upstream side of the core pipe 10 after passing through the heating zone 120 and the preheating zone 110. That is, by the second gas supply system, an air current of the oxidizing gas is formed inside the core pipe 10, so that the carbon dioxide gas generated from the precursor by the heat treatment rides on the air current together with the oxidizing gas and is exhausted. When the air current of the oxidizing gas by the second gas supply system is a flow that opposes the direction in which the precursor flows down, the concentration of the carbon dioxide gas becomes lower toward the downstream side of the core pipe 10. Therefore, the amount of carbon mixed into the workpiece Ma that finishes the heat treatment on the downstream side can be surely reduced. Note that the supply amount, supply direction, etc. of the oxidizing gas by the second gas supply pipe 40 can also be adjusted as appropriate.

[0039] As the oxidizing gas supplied by the first gas supply system and the second gas supply system, a gas that promotes the reaction with the oxygen element, such as oxygen gas and oxygen-enriched air, is used. The oxidizing gas supplied by the first gas supply system and the second gas supply system preferably has an oxygen concentration of 90% or more, more preferably 95% or more, and preferably 100%.

[0040] The lifter 50 is provided on the inner peripheral surface of the core tube 10. The lifter 50 protrudes inward from a part in the circumferential direction of the inner peripheral surface of the core tube 10, and stirs the object to be processed Ma as the core tube 10 rotates. That is, by being stirred by the lifter 50, the surface powder and the bottom powder in the precursor powder flow while being interchanged, the contact probability with oxygen and its uniformity are increased, and the carbon dioxide gas generated from the precursor is efficiently excluded from the interparticle gaps in the powder. Therefore, when the lifter 50 stirs the precursor under the oxidizing gas supplied by the first air supply system or the second air supply system, the supply of oxygen and the exhaust of carbon dioxide gas effectively proceed, and the solid-phase reaction for generating the lithium composite compound is greatly promoted.

[0041] The lifter 50 can be provided in an appropriate shape and number. The lifter 50 is provided, for example, in a blade shape, a protrusion shape, a pipe shape, a prism shape, etc. extending in the longitudinal direction of the core tube 10, and a plurality of them may be arranged at appropriate intervals with respect to the circumferential direction of the core tube 10. The lifter 50 may be continuous without a gap in the longitudinal direction of the core tube 10, or may be intermittent with a gap.

[0042] The lifter 50 may be provided over the entire length inside the core tube 10, but it is preferably provided only in the band (heating band 120) that is directly heated to the target heat treatment temperature by the heater 20 in the heat treatment among the inner peripheral surfaces of the core tube 10, and is not provided on the upstream side or the downstream side of the heating band 120. In the preheating band 110 and the like on the upstream side of the heating band 120, the generation of carbon dioxide gas is remarkable. If the precursor powder is stirred in such a region, the precursor and the carbon dioxide gas may react to form lithium carbonate, which may hinder the formation reaction of the lithium composite compound. On the other hand, even if the lifter 50 is provided only in the heating band 120, it is possible to sufficiently promote the solid-phase reaction. By not providing it on the upstream side or the downstream side of the heating band 120, it is possible to suppress the situation where the fine powder of the precursor that has been stirred more than necessary is discharged together with the airflow of the oxidizing gas and the recovery rate decreases.

[0043] The rotary kiln 1 preferably has an exhaust port for exhausting the atmospheric gas inside the hearth tube 10 on the side surface of the upstream side of the hearth tube 10. When the exhaust port is provided on the side surface instead of the upper surface side of the hearth tube 10, carbon dioxide gas with a high specific gravity can be surely discharged from the hearth tube 10 by riding on the air flow of the oxidizing gas. More specifically, the position where the exhaust port is provided is preferably the inner surface of the upstream side inside the hearth tube 10, and more preferably located in the lower half part where the height is lower than the rotation axis of the hearth tube 10 among the inner surfaces.

[0044] According to the above rotary kiln 1, since the supply of oxygen, the exhaust of carbon dioxide gas, and the feeding of the precursor are continuously carried out, the heat treatment of the precursor can be performed in a short time. In particular, since the supply of oxygen is performed on the hearth tube 10 that forms a closed space, it can be performed at a low cost compared with a transfer furnace or the like that performs heat treatment in an open space. Further, since the first air supply system directly blows the oxidizing gas onto the precursor, high-concentration oxygen can be supplied to the precursor, and the carbon dioxide gas generated from the precursor can be lifted up and surely separated and removed from the flowing precursor. Further, since the second air supply system quickly exhausts the carbon dioxide gas lifted upward inside the hearth tube 10 to the outside of the furnace, it is possible to prevent the heat-treated precursor from contacting the carbon dioxide gas. That is, only with the first air supply system, the carbon dioxide gas generated from the precursor stays without being discharged from the hearth tube 10, and only with the second air supply system, it is difficult to remove the carbon dioxide gas staying in the interparticle gaps in the powder of the precursor. When the first air supply system and the second air supply system are used in combination, the circulation of the supply of oxygen and the exhaust of carbon dioxide gas is efficiently continued, and a heat-treated product with few crystal defects and impurities can be obtained.

[0045] (Firing process) Next, the details of the firing process S2 will be described.

[0046] As shown in FIG. 1, the firing process S2 preferably includes a first heat treatment process S21 for forming a first precursor, a second heat treatment process S22 for forming a second precursor, and a third heat treatment process S23 which is a finishing heat treatment. The rotary kiln 1 having the configuration shown in FIG. 2 may be used in any of these heat treatment processes, but it is preferably used in at least one of the second heat treatment process S22 and the third heat treatment process S23, and more preferably used in the second heat treatment process S22.

[0047] [First Heat Treatment Process S21] In the first heat treatment process S21, the mixture obtained in the mixing process S1 is heat-treated at a heat treatment temperature of 200 °C or higher and 400 °C or lower for 0.5 hours or longer and 5 hours or shorter to obtain a first precursor. The first heat treatment process S21 is mainly performed to remove highly volatile components such as moisture that hinder the synthesis reaction of the positive electrode active material from the mixture obtained in the mixing process S1. In this process, carbon dioxide gas and the like generated along with the thermal decomposition of raw materials such as lithium carbonate and the combustion of impurities are removed from the mixture together with the moisture.

[0048] The first heat treatment process S21 can be carried out using an appropriate heat treatment apparatus. Specifically, for example, a roller hearth kiln, a tunnel furnace, a pusher furnace, a rotary kiln, a batch furnace, etc. can be used. When the rotary kiln 1 is not used in the second heat treatment process S22 and the third heat treatment process S23, the above-mentioned roller hearth kiln, tunnel furnace, etc. can be used.

[0049] [Second Heat Treatment Process] In the second heat treatment step S22, the second precursor is obtained by heat-treating the first precursor obtained in the first heat treatment step S21 at a heat treatment temperature of 450 °C or higher and 900 °C or lower for 0.1 hour or longer and 50 hours or shorter. The second heat treatment step S22 is mainly performed for the purpose of oxidizing nickel in the first precursor from divalent to trivalent and crystallizing a lithium composite compound having a layered structure. That is, this step is a heat treatment step that forms a layered structure with the oxidation reaction of nickel in the first precursor using lithium carbonate (Li2CO3) and the oxide of M´ (M´O) as reactants. In the second heat treatment step S22, if the heat treatment temperature is less than 450 °C, the reaction rate of the solid-phase reaction becomes slow and lithium carbonate remains in excess, and there is a risk that the amount of carbon dioxide gas generated in the third heat treatment step S23 increases. On the other hand, if the heat treatment temperature exceeds 900 °C, the grain growth of the lithium composite compound proceeds excessively in this step, and there is a high possibility that a high-capacity positive electrode active material cannot be obtained. In contrast, at the above heat treatment temperature, a second precursor with few coarse crystal grains can be obtained while the solid-phase reaction proceeds as a whole. The reaction of lithium carbonate that proceeds in the second heat treatment step S22 is represented by the following formula (3).

[0050] Li2CO3 + 2M´O + 0.5O2 → 2LiM´O2 + CO2 ···(3)

[0051] The heat treatment temperature in the second heat treatment step S22 is more preferably 600 °C or higher. If it is 600 °C or higher, the reaction efficiency of the formula (3) is further improved. If it is 700 °C or higher, the reaction efficiency is further improved, which is more preferable. Also, the heat treatment temperature in the second heat treatment step S22 is more preferably 800 °C or lower. If it is 800 °C or lower, it becomes difficult for crystal grains to become coarser.

[0052] The heat treatment time in the second heat treatment step S22 is more preferably 0.1 hour or longer and 5 hours or shorter. When the heat treatment time is 5 hours or shorter, the time required for manufacturing the positive electrode active material can be shortened, and productivity can be improved.

[0053] In order to achieve high capacity in the positive electrode active material with a nickel ratio exceeding 70 atomic %, it is particularly important to sufficiently oxidize the valence of nickel from divalent to trivalent. This is because divalent nickel easily substitutes for lithium sites in LiM'O2 having a layered structure, which causes a decrease in the capacity of the positive electrode active material. Therefore, in the second heat treatment step S22, it is preferable to heat-treat the first precursor in an oxidizing atmosphere with sufficient oxygen supply to surely change the valence of nickel from divalent to trivalent. Further, the carbon dioxide gas generated in the formula (3) inhibits the progress of the reaction of the formula (3) and causes a decrease in the capacity of the positive electrode active material. Therefore, in the second heat treatment step S22, it is preferable to perform the heat treatment under an air flow where carbon dioxide gas hardly stays.

[0054] Specifically, in the second heat treatment step S22, it is preferable to set an oxidizing atmosphere with an oxygen concentration of 90% or more, more preferably an oxidizing atmosphere with an oxygen concentration of 95% or more, and even more preferably an oxidizing atmosphere with an oxygen concentration of 100%. Further, the second heat treatment step S2 is preferably performed under an air flow of an oxidizing gas. When the heat treatment is performed under an air flow of an oxidizing gas with a high oxygen concentration, nickel can be surely oxidized and the carbon dioxide gas generated in the formula (3) can be surely removed.

[0055] In the second heat treatment step S22, it is preferable to perform the heat treatment while rolling the first precursor. By performing the heat treatment while rolling the first precursor, the contact probability between the powdery first precursor and oxygen can be increased, and nickel etc. can be sufficiently oxidized. Further, due to the rolling of the powdery first precursor, the generated carbon dioxide gas hardly stays in the particle gaps, the carbon dioxide gas can be efficiently removed, and the solid-phase reaction can be promoted.

[0056] When the second heat treatment step S22 is carried out using the rotary kiln 1 configured as shown in FIG. 2, the first precursor is introduced into the hearth tube 10 adjusted to an oxidizing atmosphere, and the first air supply system, the second air supply system, and the heater 20 are operated to rotate the hearth tube 10 at a predetermined rotational speed. That is, an oxidizing gas is sprayed by the first air supply system onto the first precursor flowing down while rolling from the upstream side to the downstream side in the hearth tube 10 of the rotary kiln 1 adjusted to an oxygen atmosphere with an oxygen concentration of 90% or more, and while exhausting the carbon dioxide gas generated from the first precursor with the airflow of the oxidizing gas by the second air supply system, heat treatment is performed at a predetermined heat treatment temperature and heat treatment time. The carbon dioxide gas generated from the first precursor is preferably discharged from the axial direction of the hearth tube 10 through an exhaust port provided on the side surface of the upstream side in the hearth tube 10. Further, it is preferable to perform heat treatment by adjusting at least one of the spraying amount, spraying angle, and oxygen concentration of the oxidizing gas by the first air supply system according to the input amount of the first precursor, heat treatment temperature, oxygen concentration of the atmosphere, rotational speed of the hearth tube 10, etc. However, the main purpose of the second heat treatment step S22 is to suppress the fact that a large amount of carbon dioxide gas generated from the first precursor becomes an inhibition factor for the reaction. It is preferable for proceeding with the series of steps to discharge as much carbon dioxide gas as possible in this second heat treatment step S22 and efficiently discharge it from the inside of the hearth tube 10. For this reason, the second heat treatment step S22 is a step in which the importance of the second air supply system for discharging carbon dioxide gas is high. Therefore, in the second heat treatment step S22, it is preferable to adjust at least the air supply amount of the oxidizing gas by the second air supply pipe 40, the blowing pressure, etc., and it is more preferable to perform both the adjustment of the second air supply system and the adjustment of the first air supply system.

[0057] [Third Heat Treatment Step] In the third heat treatment step S23, a lithium composite compound having a layered structure is obtained by heat-treating the second precursor obtained in the second heat treatment step S22 at a heat treatment temperature of 700°C or higher and 900°C or lower. The third heat treatment step S23 is mainly performed to sufficiently oxidize nickel in the second precursor from divalent to trivalent and to grow the crystal grains of the lithium composite compound having a layered structure. That is, this step is a heat treatment step that performs the oxidation reaction of nickel in the second precursor and the grain growth of the crystal grains of the lithium composite compound.

[0058] In the third heat treatment step S23, the second precursor may be allowed to stand for heat treatment, or heat treatment may be performed while it is being rolled. By heat-treating while rolling the second precursor, the contact probability between the powdery second precursor and oxygen can be increased, and nickel and the like can be sufficiently oxidized. Further, when the powdery second precursor rolls, there is an advantage that the lithium composite compound is fired more uniformly.

[0059] In the third heat treatment step S23, after the completion of the second heat treatment step S22, it is preferable to completely exhaust the atmosphere gas used in the second heat treatment step S22 and introduce a new atmosphere gas. Further, when both the second heat treatment step S22 and the third heat treatment step S23 are carried out using the rotary kiln 1 having the configuration shown in FIG. 2, after performing the second heat treatment step S22 using a single rotary kiln 1, the third heat treatment step S23 may be performed using the same rotary kiln 1, or the second heat treatment step S22 and the third heat treatment step S23 may be sequentially carried out using a plurality of rotary kilns 1, or the second heat treatment step S22 and the third heat treatment step S23 may be continuously carried out simultaneously in a single rotary kiln 1.

Example

[0060] Hereinafter, the present invention will be specifically described with reference to examples, but the technical scope of the present invention is not limited thereto.

[0061] (Example 1) As starting materials for the positive electrode active material, lithium carbonate, nickel hydroxide, cobalt carbonate, and manganese carbonate were prepared. These starting materials were weighed so that the atomic ratio of Li:Ni:Co:Mn was 1.04:0.80:0.15:0.05, and the mixing step S1 was carried out. Specifically, ion-exchanged water was added and mixed so that the total weight of the starting materials was 20 mass%, and pulverization and mixing were carried out using a bead mill. The obtained solid-liquid mixture was dried using a spray dryer to obtain a raw material mixed powder.

[0062] Next, the obtained raw material mixed powder was filled into a firing container made of alumina, and heat treatment (first heat treatment step S21) was carried out at 360 °C for 1 hour in an air atmosphere using a roller hearth kiln to obtain a first precursor. By this heat treatment, not only the moisture absorbed by the raw material mixed powder was removed, but also the thermal decomposition of nickel hydroxide and the partial thermal decomposition of each carbonate were carried out, and a certain amount of carbon dioxide gas (CO2) was removed.

[0063] Next, the obtained first precursor was put into the rotary kiln 1 shown in FIG. 2, and heat treatment was carried out at 650 °C for 0.9 hour while supplying air through the first air supply pipe 30 and the second air supply pipe 40 in the rotating hearth tube 10, and then heat treatment was carried out at 700 °C for 3.5 hours. That is, the second heat treatment step S22 was carried out to obtain a second precursor. At this time, in the rotary kiln 1, the hearth tube 10 had a total tube length L1 = 3500 mm, an inner diameter D1 = 214 mm, and a volume V1 = 0.126 m 3 , the first air supply pipe 30 had a total tube length L2 = 3500 mm, an outer diameter D2 = 120 mm, and a volume V2 = 0.04 m 3 , V2 / V1 = 0.32 (32%), D2 / D1 = 0.56. The inner layer (inner shell) of the hearth tube 10 was made of a metallic nickel material, the outer layer (outer shell) was made of a stainless steel material, the inner layer of the first air supply pipe 30 was made of a stainless steel material, and the outer layer was made of a metallic nickel material. Then, after spraying pure aluminum on the inner layer of the hearth tube 10 and the outer layer of the first air supply pipe 30, air burning was carried out in argon gas, and then oxygen was introduced to oxidize the surface, so that an aluminum oxide was formed on the outermost layer of the powder contact part of the heat-treated part Ma, and an alloy layer of aluminum and nickel as the base material was formed on the underlying layer.

[0064] Next, this second precursor was charged into a rotary kiln 1B shown in FIG. 3 and heat-treated at 840° C. for 0.7 hours (third heat treatment step S23) to obtain a lithium composite compound (cathode active material) having a composition of Li 1.0 Ni 0.8 0Co 0.15 Mn 0.05 O2. At this time, an alumina core tube 10B was used in the rotary kiln 1B. Then, the amounts of unreacted lithium carbonate and lithium hydroxide remaining in the obtained cathode active material and the specific surface area of the cathode active material were measured. The measurement results are shown in Table 1.

[0065] Subsequently, using the obtained cathode active material as a cathode material, a lithium secondary battery was fabricated by the following procedure. First, the cathode active material, a binder, and a conductive material were mixed to prepare a cathode mixture slurry. Then, the prepared cathode mixture slurry was applied to an aluminum foil with a thickness of 20 μm, which is a cathode current collector, dried at 120° C., and then compression-molded by pressing so that the electrode density became 2.0 g / cm 3 and punched into a disk shape with a diameter of 15 mm to fabricate a cathode. Also, a cathode was fabricated using metallic lithium as a cathode material. Then, a lithium secondary battery was fabricated using the fabricated cathode and anode and a non-aqueous electrolyte. As the non-aqueous electrolyte, a solution in which LiPF6 was dissolved at a concentration of 1.0 mol / L in a solvent obtained by mixing ethylene carbonate and dimethyl carbonate so that the volume ratio was 3:7 was used.

[0066] Next, for the fabricated lithium secondary battery, the initial discharge capacity was measured by the following procedure. First, with a charging current of 0.2 CA, it was charged at a constant current and constant voltage up to a charging cut-off voltage of 4.3 V. Then, with a discharging current of 0.2 CA, it was discharged at a constant current up to a discharging cut-off voltage of 2.5 V, and the discharge capacity was determined from the discharge current amount at that time. The results are shown in Table 1.

[0067] [Measurement of the reduction rate of the core tube weight] Using the first precursor obtained in the same manner, after performing the firing in the second heat treatment step S22 shown in Example 1, the weight of the furnace core tube was measured. The process including heating and cooling was repeated as one cycle, and the weight of the furnace core tube after each cycle was measured. That is, the durability was evaluated by using the value obtained by dividing the weight of the furnace core tube after the cycle by the weight of the furnace core tube before use as the weight reduction rate (weight loss). The results after 300 cycles and 500 cycles are shown in Table 1.

[0068] (Comparative Example 1) The first precursor obtained in the same manner was put into the rotary kiln 1 shown in FIG. 2, and heat treatment at 650° C. for 3.5 hours, that is, the second heat treatment step S22 was performed to obtain a second precursor. Next, this second precursor was put into the rotary kiln 1B and heat treatment (third heat treatment step S23) was performed at 840° C. for 0.7 hours to obtain a lithium composite compound (positive electrode active material) having a composition of Li 1.0 Ni 0.80 Co 0.15 Mn 0.05 O2. Then, the amount of unreacted lithium carbonate and lithium hydroxide remaining in the obtained positive electrode active material and the specific surface area of the positive electrode active material were measured. Further, a lithium secondary battery was fabricated and the discharge capacity was determined. The results are shown in Table 1. The difference between Example 1 and Comparative Example 1 is the difference in the temperature and time of the second heat treatment step S22.

[0069] (Comparative Example 2) The first precursor obtained in the same manner was put into the rotary kiln 1'shown in FIG. 2, heat treatment was performed at 650° C. for 0.9 hours, and then heat treatment was performed at 700° C. for 3.5 hours. At this time, the rotary kiln 1'had the same dimensions as in the example, the inner layer of the furnace core tube 10 was made of a metal nickel material, the outer layer was made of a stainless steel material, the inner layer of the first air supply pipe 30 was made of a stainless steel material, the outer layer was made of a metal nickel material, and the powder contact part of the heat treatment part Ma was occupied by a nickel material. Similarly, the process including heating and cooling was repeated as one cycle, and the weight reduction rate of the furnace core tube was measured. The results after 300 cycles and 500 cycles are shown in Table 1.

[0070]

Table 1

[0071] In Example 1 and Comparative Example 1, in the second heat treatment step S22 of Example 1, since the step (stage) of holding at 700 °C or higher for 2 hours or longer is included, it shows that the amount of residual unreacted lithium carbonate is small and the solid-phase reaction is progressing. In addition, in Example 1, by adopting a structure in which the powder-loading part of the hearth tube is occupied by aluminum oxide, it becomes difficult for the lithium component contained in the precursor of the lithium composite compound to react with the hearth tube, and it can be seen that the wall thickness reduction amount of the hearth tube can be reduced and the durability can be improved. As a result, the long life of the hearth tube can contribute to low production costs.

Explanation of Reference Signs

[0072] S1 Mixing step S2 Firing step S21 First heat treatment step S22 Second heat treatment step S23 Third heat treatment step 1, 1B Rotary kiln (firing furnace) 10, 10B Hearth tube 20, 20B Heater 30, 30B First air supply pipe 32 Injection port 34 Leg 40, 40B Second air supply pipe 50, 50B Lifter 110, 110B Preheating zone 120, 120B Heating zone Ma Object to be processed

Claims

1. A mixing step of mixing a lithium compound and a compound containing a metal element other than Li in the following formula (1); A firing step of firing the precursor obtained through the mixing step to obtain a lithium composite compound represented by the following (1), The firing step includes at least a heat treatment step of performing heat treatment while rolling the precursor in a core tube of a firing furnace, In the core tube, an aluminum oxide obtained by surface oxidation of pure aluminum is formed on the outermost surface of the powder contact part, and an alloy layer of the pure aluminum and the base material of the core tube is formed in the underlying layer. A method for producing a positive electrode active material for a lithium secondary battery, characterized by the above. Li １＋ａ M1O ２＋α ...(1) (However, in the formula (1), M1 is a metal element other than Li and includes at least Ni and any one of Mn, Co, Al, Ti, Zr, Mo, Nb, W, V, Cr, Mg, Ca, Cu, Zn, Sn. The proportion of Ni in M1 is 70 atomic% or more, and a and α are numbers satisfying -0.1 ≤ a ≤ 0.2 and -0.2 ≤ α ≤ 0.2.)

2. The firing furnace includes a first air supply pipe that injects an oxidizing gas toward the inner peripheral surface side of the core tube, and a second air supply pipe that allows the oxidizing gas to flow in the axial direction of the core tube. The method for producing a positive electrode active material for a lithium secondary battery according to claim 1, wherein the first air supply pipe has a plurality of injection ports whose circumferential direction of the first air supply pipe is the opening direction.

3. The method for producing a positive electrode active material for a lithium secondary battery according to claim 2, wherein, in the core tube, the leg that holds the first air supply pipe is not joined to the core tube.

4. The heat treatment step includes a step of holding at 700 °C or higher for 2 hours or longer. The method for producing a positive electrode active material for a lithium secondary battery according to any one of claims 1 to 3, characterized by the above.

5. A mixing step of mixing a lithium compound and a compound containing a metal element other than Li in the following formula (1); A firing step of firing the precursor obtained through the mixing step to obtain a lithium composite compound represented by the following formula (1); The firing step includes at least a heat treatment step of performing heat treatment while rolling the precursor in a core tube of a firing furnace, The firing furnace includes a first air supply pipe that injects an oxidizing gas toward the inner peripheral surface side of the core tube, and a second air supply pipe that allows the oxidizing gas to flow in the axial direction of the core tube. The first air supply pipe has a plurality of injection ports whose circumferential direction of the first air supply pipe is the opening direction. In the core tube, aluminum oxide is formed on the outermost surface of the powder contact portion, and an alloy layer of aluminum and the base material of the core tube is formed on the underlying layer, and the leg holding the first air supply pipe is not joined to the core tube. A method for producing a positive electrode active material for a lithium secondary battery, characterized by the above. Li 1 + a M1O 2 + α... (1) (However, in the formula (1), M1 is a metal element other than Li and contains at least Ni, the ratio of the Ni in M1 is 70 atomic% or more, and a and α are numbers satisfying -0.1 ≤ a ≤ 0.2 and -0.2 ≤ α ≤ 0.2.)

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