Manufacturing method for positive electrode active material for lithium secondary battery

Abstract

To provide a positive electrode active material for a lithium secondary battery having high quality, high productivity, and low production cost.SOLUTION: A manufacturing method of a positive electrode active material for a lithium secondary battery includes: 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 calcination step of calcinating a precursor obtained through the mixing step to obtain a lithium composite compound represented by the following formula (1). The calcination step includes at least a heat treatment step of performing a heat treatment while rolling the precursor in a furnace core tube of a calcination furnace. The furnace core, in which an aluminum oxide is formed on the outermost layer of a powder contact part, and an alloy layer of aluminum and the furnace core base material is formed on the underlayer: Li1+a M1O2+α ...(1)SELECTED DRAWING: Figure 2

Description

[Technical Field]

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

[0002] BACKGROUND ART Lithium secondary batteries have become widely used as small, lightweight secondary batteries with high energy density, and there is a demand for higher capacity active materials used in the positive and negative electrodes.

[0003] A lithium composite compound represented by LiMO2 (where M represents a metal element such as Ni, Co, or Mn) with an α-NaFeO2-type layered structure is known as a positive electrode active material with high charge / discharge capacity. This positive electrode active material shows a tendency for its capacity to increase particularly as the nickel ratio increases, and is therefore expected to be a positive electrode active material that can achieve high energy density in batteries. There is a demand for lithium composite compounds that have high productivity and low production costs.

[0004] In the manufacturing process of positive electrode active materials, a technology has been proposed in which a rotary kiln is used as a firing furnace for the formation reaction of a lithium composite compound. A rotary kiln has the advantage that it does not require a special firing vessel and can easily maintain an oxidizing atmosphere inside the furnace.

[0005] For example, Patent Document 1 discloses that the furnace tube of the firing furnace used in the rolling heat treatment process is made of ceramics, or nickel, tungsten, molybdenum, titanium, or an alloy containing these metals as its main component.

[0006] Patent Document 2 also discloses that in a furnace core tube having a double structure with an inner cylindrical tube, at least one of the inner layer of the furnace core tube or the outer layer of the inner cylindrical tube is made of metallic nickel material or nickel alloy material. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] International Publication No. 2017 / 213002 [Patent Document 2] Japanese Patent Application Publication No. 2019-75253 Summary of the Invention [Problem to be solved by the invention]

[0008] However, ceramic furnace tubes make it difficult to increase the size of the equipment and achieve high productivity, and when using a furnace tube made of an alloy whose main component is metal, there is a risk that metal components will become mixed into the raw material over long-term use.

[0009] Therefore, an object of the present invention is to provide a positive electrode active material for a lithium secondary battery that is high quality, highly productive, and has low production costs. [Means for solving the problem]

[0010] A method for producing a positive electrode active material for a lithium secondary battery according to one embodiment of the present invention includes a mixing step of mixing a lithium compound with a compound containing a metal element other than Li in formula (1) below, and a firing step of firing the precursor obtained through the mixing step to obtain a lithium composite compound represented by formula (1) below, wherein the firing step includes at least a heat treatment step of performing heat treatment on the precursor while rolling it in a muffle tube of a firing furnace, and the muffle tube is characterized in that an aluminum oxide obtained by surface oxidizing pure aluminum is formed on the outermost layer of the powder-contacting part, and an alloy layer of the pure aluminum and a base material of the muffle tube is formed on an underlying layer. Li 1+a M1O 2+α ···(1) (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, and Sn, the proportion of Ni in M1 being 70 atomic % or more, and a and α are numbers that satisfy the relationships -0.1≦a≦0.2 and -0.2≦α≦0.2.)

[0011] Furthermore, in the method for producing a positive electrode active material for a lithium secondary battery of the present invention, it is preferable that the firing furnace includes a first air intake pipe that injects an oxidizing gas toward the inner surface side of the furnace tube, and a second air intake pipe that flows the oxidizing gas in the axial direction of the furnace tube, and that the first air intake pipe has a plurality of injection ports whose opening direction is the circumferential direction of the first air intake pipe.

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

[0013] In addition, the method for producing a positive electrode active material for a lithium secondary battery of the present invention preferably includes a step of maintaining the material at 700° C. or higher for 2 hours or longer in the heat treatment step. In another aspect of the present invention, a method for producing a positive electrode active material for a lithium secondary battery includes a mixing step of mixing a lithium compound with a compound containing a metal element other than Li in the following formula (1): and a calcination step of calcining 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 on the precursor while rolling it in a furnace tube of a firing furnace, the firing furnace includes a first air supply pipe that injects an oxidizing gas toward an inner peripheral surface of the furnace tube, and a second air supply pipe that flows the oxidizing gas in an axial direction of the furnace tube; The first air intake pipe has a plurality of injection ports whose opening direction is in the circumferential direction of the first air intake pipe, In the furnace 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 furnace core tube is formed on the underlying layer, and the legs that hold the first air supply pipe are not joined to the furnace core tube. It is characterized by: Li 1+a M1O 2+α ···(1) (In the 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 that satisfy −0.1≦a≦0.2 and −0.2≦α≦0.2.) [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a positive electrode active material for a lithium secondary battery that is high quality, highly productive, and has low production costs. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a flow diagram of a method for producing a positive electrode active material according to an embodiment of the present invention. [Figure 2] 1 is a diagram showing a schematic structure of a rotary kiln 1 used for producing a positive electrode active material according to an embodiment of the present invention. [Figure 3] FIG. 1 is a diagram showing a schematic structure of a rotary kiln 1B used in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, a cathode active material for a lithium secondary battery according to one embodiment of the present invention (hereinafter, sometimes simply referred to as a cathode active material) and a method for producing the same will be described in detail. Note that the following description shows specific examples of the contents of the present invention, and the present invention is not limited thereto. Various modifications of the present invention can be made by those skilled in the art within the scope of the technical idea disclosed in this specification.

[0017] <Cathode active material> The positive electrode active material according to this embodiment is a lithium composite compound containing lithium and a transition metal and having a layered rock-salt crystal structure (hereinafter sometimes referred to as a layered structure) belonging to the space group R-3m. This positive electrode active material is capable of reversibly absorbing and releasing lithium ions upon application of a voltage, and is suitable for use as a positive electrode active material for lithium secondary batteries (lithium-ion secondary batteries). The lithium composite compound is also called a lithium composite oxide.

[0018] The positive electrode active material according to this embodiment is a compound represented by the following formula (1): Li 1+a M1O 2+α ···(1) (In the formula (1), M1 is a metal element other than Li and contains at least Ni, the proportion of Ni per M1 exceeds 70 atomic %, and a and α are numbers that satisfy −0.1≦a≦0.2 and −0.2≦α≦0.2.)

[0019] The positive electrode active material according to this embodiment has a composition in which the ratio of nickel (Ni) to the metal element (M1) other than lithium (Li) exceeds 70 atomic %, and is therefore a positive electrode active material that can achieve high energy density and high charge / discharge capacity. The ratio of nickel (Ni) to the metal element (M1) other than lithium (Li) can be any value within the range of more than 70 atomic % and less than 100 atomic %. Because it is a positive electrode active material containing such a high ratio of nickel, Ni 2+ Ni 3+ It is important that the oxidation reaction to oxidize to HCl be carried out 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] The cathode active material according to this embodiment has a more preferable specific composition represented by 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] <Manufacturing method of cathode active material> The manufacturing method of the cathode active material according to this embodiment relates to a method for synthesizing a lithium composite compound represented by the above formula (1) and having a layered rock salt-type crystal structure, which is a cathode active material used for the cathode of a lithium secondary battery.

[0023] (Manufacturing method flow) FIG. 1 is a flowchart of a manufacturing method of a cathode active material according to an embodiment of the present invention. 1, the method for producing a positive electrode active material according to this embodiment includes a mixing step S1 and a calcining step S2. A precursor is prepared from raw material compounds through the mixing step S1, and the precursor is calcined in the calcining step S2, thereby synthesizing 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 production method according to this embodiment includes, as one of the steps constituting the calcining step S2, at least a heat treatment step in which the precursor of the lithium composite compound before calcination is heat-treated while being tumbled in a rotary kiln used as a calcination furnace.

[0024] In the mixing step S1, a lithium-containing compound (lithium compound) is mixed with a compound containing a metal element other than Li that constitutes the positive electrode active material. Examples of lithium-containing compounds include lithium carbonate and lithium hydroxide. Compared to lithium acetate, lithium nitrate, lithium hydroxide, lithium chloride, lithium sulfate, etc., lithium carbonate is in stable supply, easy to procure, and inexpensive. In addition, because it has a high melting point, it causes less damage to manufacturing equipment and is highly suitable for industrial use and practical application.

[0025] Compounds containing metal elements other than Li that constitute the positive electrode active material include compounds containing nickel, compounds containing manganese, compounds containing cobalt, and compounds containing other metal elements such as M2.

[0026] As the compound containing nickel, for example, pure nickel (conveniently included as a compound), oxide, hydroxide, carbonate, acetate, etc. can be used. Among these, it is particularly preferable to use an oxide or hydroxide. Unlike the case where a carbonate, acetate, etc. is used, an oxide or hydroxide does not generate a large amount of carbon dioxide gas during the firing process, so that a lithium composite compound having a high proportion of nickel and high purity can be stably produced.

[0027] Examples of compounds that can be used as manganese-containing compounds and cobalt-containing compounds include oxides, hydroxides, carbonates, and acetates. Of these, it is particularly preferable to use oxides, hydroxides, or carbonates. Furthermore, examples of compounds that can be used as compounds containing other metal elements such as M2 include carbonates, oxides, hydroxides, acetates, and nitrates. Of these, it is particularly preferable to use carbonates, oxides, or hydroxides.

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

[0029] In the calcination step S2, the precursor obtained through the mixing step S1 is calcined to obtain a lithium composite compound having a layered structure. The calcination step S2 includes at least a heat treatment step in which the precursor of the lithium composite compound before calcination is heat-treated while being rolled in a rotary kiln used as a calcination furnace. Here, the rotary kiln used in the calcination 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 producing a positive electrode active material for a lithium secondary battery. As shown in FIG. 2, the rotary kiln 1 includes a furnace tube 10, a heater 20, a first air supply pipe 30, a second air supply pipe 40, and a lifter 50.

[0031] The furnace tube 10 has a hollow, generally cylindrical shape and has an input section for the workpiece Ma at one longitudinal end and a recovery section for the heat-treated material at the other end. The furnace tube 10 is installed at an incline with respect to the horizontal plane so that the input section for the workpiece Ma is located above the recovery section. A precursor of a lithium composite compound is input into the furnace tube 10 from a powder input device (not shown) installed in the input section, and is heat-treated while flowing longitudinally inside the furnace tube 10. The inclination angle of the furnace 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 furnace tube 10 is referred to as the "upstream" and the recovery section side as the "downstream."

[0032] The furnace core tube 10 is connected to a power source such as a motor (not shown) via a drive gear or rollers. Driven by such a motor, the furnace core tube 10 rotates around the central axis of its cylindrical shape. Therefore, the workpiece Ma introduced into the furnace core tube 10 from the introduction section flows down while rolling inside the furnace core tube 10 as the furnace core tube 10 rotates, and is collected by a powder collection device (not shown) in the collection section. The rotation speed of the furnace core 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 furnace tube 10 is preferably made of a metal such as Ni, W, Mo, or Ti, which does not emit harmful components such as chromium, or an alloy containing these metals as its main component. In the furnace tube, aluminum oxide is formed on the outermost surface of the powder contact portion, which is the portion that comes into contact with the precursor (workpiece Ma), and an alloy layer of aluminum and the base material of the furnace tube is formed on the underlying layer. If aluminum oxide is formed on the outermost layer of the base material of the furnace tube 10, the lithium component contained in the precursor of the lithium composite compound will not easily react with the furnace tube 10, thereby reducing the risk of deterioration or damage to the furnace tube 10. The aluminum oxide on the outermost layer of the base material of the furnace tube 10 can be easily formed by baking an aluminum-coated base material and oxidizing the coated aluminum. The base material may be coated with aluminum by, for example, thermal spraying or hot-dip plating. If aluminum is coated on the base material before can manufacturing of the furnace tube 10, the base material will be exposed at the joints, so it is preferable to coat the aluminum after can manufacturing.

[0034] The heater 20 is installed around the core tube 10. The heater 20 covers a heating zone 120, which is a section in the longitudinal direction of the core tube 10 and is indicated by a dashed line in FIG. 2, and can heat the heating zone 120 to a target temperature. The heater 20 also preheats a preheating zone 110, which is a section upstream of the heating zone 120 and is a predetermined distance away and indicated by a dashed line in FIG. 2, to a temperature lower than the target temperature. Therefore, when a precursor of a 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, where it is heat-treated while being tumbled. However, the arrangement position and number of heaters 20 are not particularly limited as long as uniform heat treatment can be performed in the heating zone 120. The heaters 20 may be arranged in one place or in multiple places, as long as the preheating zone 110 is secured so that the heat treatment does not proceed too rapidly.

[0035] The first air supply pipe 30 constitutes a first air supply system that supplies an oxidizing gas from a gas source (not shown) into the muffle tube 10. The oxidizing gas is sprayed toward the inner circumferential surface of the muffle tube 10 when heat-treating the workpiece Ma. The first air supply pipe 30 is arranged longitudinally inside the muffle tube 10 and extends substantially the entire length of the muffle tube 10, from the downstream side to the upstream side. The first air supply pipe 30 is arranged longitudinally of the muffle tube 10 and has multiple injection ports 32 that branch off from the first air supply pipe 30 and open at their ends. Each of the injection ports 32 can spray the oxidizing gas, which is pressure-fed from a gas source (not shown), in a shower-like manner toward the inside of the muffle tube 10. In other words, the first air supply system sprays the oxidizing gas onto the precursor that is being heat-treated while rolling, thereby directly supplying oxygen to the precursor and efficiently promoting the oxidation reaction. Furthermore, carbon dioxide gas generated from the precursor is blown up by the oxidizing gas and quickly removed from the vicinity of the precursor. In other words, carbon dioxide gas generated from the precursor and remaining in the furnace core tube 10 is prevented from reacting again with the precursor to regenerate lithium carbonate, which would inhibit the generation of a lithium composite compound. The opening direction of the injection port 32 may be vertically downward (in the radial direction of the furnace core tube) or in the circumferential direction of the first air supply pipe 30. However, injection in the circumferential direction creates a swirling airflow in the furnace, which allows for efficient supply of oxygen to the precursor and efficient discharge of remaining carbon dioxide gas. Therefore, it is preferable to set the opening direction of the injection port 32 in the circumferential direction of the first air supply pipe 30.

[0036] The first air inlet pipe 30 is preferably configured to allow adjustment of the oxidizing gas spray amount, spray angle, and oxygen concentration, from the viewpoints of efficiently supplying oxygen and exhausting carbon dioxide while preventing the scattering of powder from the workpiece. For example, the spray amount can be adjusted by adjusting the gas flow rate of the first air inlet system, or by providing the nozzles 32 with open / close capabilities and adjusting the aperture number of the nozzles 32. The spray angle can be adjusted by providing the first air inlet pipe 30 with a rotatable central axis. For example, the spray may be performed at an angle of more than 0° and less than 45° in either the forward or reverse direction relative to the rotational direction of the furnace core tube 10. The spray angle can also be adjusted by moving the first air inlet pipe 30 horizontally or otherwise within the furnace core tube 10. For example, the first air inlet pipe 30 may be stationary and positioned eccentrically from the central axis of the furnace core tube 10 during spraying. The oxygen concentration can be adjusted by providing an oxygen concentration detector near the inlet or outlet of the furnace tube 10, or at any other location, and monitoring and controlling the amount of oxygen so that the detected oxygen concentration reaches a specified value. The blowing amount, blowing angle, and oxygen concentration can also be adjusted by combining them as appropriate. Incidentally, a carbon dioxide concentration detector can be provided instead of, or in addition to, the oxygen concentration detector, and the amount of oxygen can be monitored and controlled so that the detected carbon dioxide concentration reaches a specified value.

[0037] The first air inlet pipe 30 is held inside the muffle tube 10 by legs 34. A round bar or similar may be inserted near the circumferential side of the contact point between the legs 34 and the muffle tube 10 to constrain the legs 34 and the muffle tube 10 in the circumferential direction without fastening them together. This allows the driving force of the muffle tube 10 to be transmitted to the first air inlet pipe 30, allowing the first air inlet pipe and the muffle tube 10 to rotate synchronously. This round bar or similar does not constrain the legs 34 in the axial direction of the muffle tube, but only in the circumferential direction. Furthermore, by not joining the legs 34 to the muffle tube 10, the first air inlet pipe 30 can be easily removed from the muffle tube 10, improving the ease of cleaning and maintenance of the interior of the muffle tube 10 and ultimately improving productivity.

[0038] The second air supply pipe 40 constitutes a second air supply system that supplies an oxidizing gas from a gas source (not shown) into the muffle tube 10, and generates an oxidizing gas flow inside the muffle tube 10 in the axial direction of the muffle tube 10 when heat-treating the workpiece Ma. The oxidizing gas may flow from the upstream side to the downstream side of the muffle tube 10, but it is preferable to flow the oxidizing gas from the downstream side to the upstream side of the muffle tube 10. The second air supply pipe 40 is located downstream of the heating zone 120 inside the muffle tube 10 and opens toward the upstream side of the muffle tube 10. Furthermore, the second air supply pipe 40 is located radially outward of the first air supply pipe when viewed in the axial direction, and opens into a space radially outward of the first air supply pipe 30. The second air supply pipe 40 flows oxidizing gas in a substantially horizontal direction radially outward of the first air supply pipe 30. After passing through the heating zone 120 and the preheating zone 110, the oxidizing gas is exhausted to the outside through an exhaust port (not shown) located upstream of the furnace tube 10. That is, the second air supply system creates an oxidizing gas flow inside the furnace tube 10, so that carbon dioxide gas generated from the precursors during the heat treatment is carried along with the oxidizing gas and exhausted along the flow. If the oxidizing gas flow from the second air supply system flows in the opposite direction to the precursor flow, the carbon dioxide concentration decreases toward the downstream side of the furnace tube 10. This reliably reduces the amount of carbon contamination in the workpiece Ma that completes heat treatment downstream. The amount and direction of the oxidizing gas supplied by the second air supply pipe 40 can also be adjusted as needed.

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

[0040] The lifter 50 is provided on the inner circumferential surface of the furnace core tube 10. The lifter 50 protrudes inward from a portion of the inner circumferential surface of the furnace core tube 10 in the circumferential direction and stirs the workpiece Ma as the furnace core tube 10 rotates. That is, stirring by the lifter 50 causes the surface powder and the bottom powder in the precursor powder to alternate and flow, increasing the probability and uniformity of contact with oxygen and efficiently removing carbon dioxide gas generated from the precursor from the gaps between particles in the powder. Therefore, stirring the precursor by the lifter 50 under oxidizing gas supplied by the first and second gas supply systems effectively advances the supply of oxygen and the exhaust of carbon dioxide gas, greatly accelerating the solid-state reaction that produces a lithium composite compound.

[0041] The lifters 50 may be provided in any suitable shape and number. The lifters 50 may be provided, for example, in the shape of blades, ridges, pipes, or square columns extending in the longitudinal direction of the furnace core tube 10, and a plurality of lifters may be arranged at suitable intervals in the circumferential direction of the furnace core tube 10. The lifters 50 may be continuous with no gaps in the longitudinal direction of the furnace core tube 10, or may be discontinuous with gaps.

[0042] Lifters 50 may be provided throughout the entire length of the furnace core tube 10. However, it is preferable that lifters 50 be provided only in the zone (heating zone 120) of the inner circumferential surface of the furnace core tube 10 that is directly heated to the target heat treatment temperature by the heater 20 during heat treatment, and not provided upstream or downstream of the heating zone 120. Carbon dioxide gas is generated significantly in the preheating zone 110, etc., upstream of the heating zone 120. If the precursor powder is stirred in such a zone, the precursor and carbon dioxide gas may react to produce lithium carbonate, potentially hindering the formation reaction of the lithium composite compound. While providing lifters 50 only in the heating zone 120 can sufficiently promote the solid-state reaction, not providing lifters 50 upstream or downstream of the heating zone 120 can prevent the precursor powder from being stirred more than necessary from being discharged with the oxidizing gas flow, thereby reducing the recovery rate.

[0043] The rotary kiln 1 preferably has an exhaust port on the upstream side surface of the furnace tube 10 for exhausting the atmospheric gas inside the furnace tube 10. If the exhaust port is provided on the side surface of the furnace tube 10 rather than on the top surface, carbon dioxide gas, which has a high specific gravity, can be carried along with the oxidizing gas flow and reliably exhausted from the furnace tube 10. More specifically, the exhaust port is preferably provided on the upstream inner surface inside the furnace tube 10, and more preferably in the lower half of the inner surface that is lower than the rotation axis of the furnace tube 10.

[0044] The rotary kiln 1 described above allows for the continuous supply of oxygen, exhaust of carbon dioxide, and powder supply of precursor, thereby enabling the heat treatment of the precursor in a short time. In particular, because oxygen is supplied to the furnace tube 10, which forms a closed space, the heat treatment can be performed at a lower cost than a transfer furnace or the like, which performs heat treatment in an open space. Furthermore, the first gas supply system sprays oxidizing gas directly onto the precursor, thereby supplying high-concentration oxygen to the precursor and reliably separating and removing carbon dioxide generated from the precursor from the flowing precursor. Furthermore, the second gas supply system quickly exhausts carbon dioxide gas blown upward inside the furnace tube 10 to the outside of the furnace, thereby preventing the heat-treated precursor from coming into contact with carbon dioxide. Specifically, with only the first gas supply system, carbon dioxide generated from the precursor remains without being discharged from the furnace tube 10, while with only the second gas supply system, it is difficult to remove carbon dioxide gas remaining in the gaps between particles of the precursor powder. By using the first and second gas supply systems in combination, the supply of oxygen and the exhaust of carbon dioxide gas are efficiently and continuously circulated, and a heat-treated product with fewer crystal defects and impurities can be obtained.

[0045] (Firing process) Next, the firing step S2 will be described in detail.

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

[0047] [First heat treatment step S21] In the first heat treatment step S21, the mixture obtained in the mixing step S1 is heat-treated at a heat treatment temperature of 200°C or higher and 40°C or lower for 0.5 hours or higher and 5 hours or lower to obtain a first precursor. The first heat treatment step S21 is performed mainly for the purpose of removing highly volatile components, such as moisture, that interfere with the synthesis reaction of the positive electrode active material from the mixture obtained in the mixing step S1. In this step, carbon dioxide gas and other gases generated by the thermal decomposition of raw materials such as lithium carbonate and the combustion of impurities are removed from the mixture together with moisture.

[0048] The first heat treatment step S21 can be performed using an appropriate heat treatment device. Specifically, for example, a roller hearth kiln, a tunnel furnace, a pusher furnace, a rotary kiln, a batch furnace, etc. can be used. Note that if the rotary kiln 1 is not used in the second heat treatment step S22 and the third heat treatment step S23, the roller hearth kiln, tunnel furnace, etc. can be used.

[0049] [Second heat treatment process] In the second heat treatment step S22, the first precursor obtained in the first heat treatment step S21 is heat-treated at a heat treatment temperature of 450°C to 900°C for 0.1 to 50 hours to obtain a second precursor. The second heat treatment step S22 is performed mainly 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 in which a layered structure is formed by oxidizing nickel in the first precursor using lithium carbonate (Li2CO3) and an oxide of M' (MO) as reactants. If the heat treatment temperature in the second heat treatment step S22 is less than 450°C, the reaction rate of the solid-state reaction will be slow, resulting in excess lithium carbonate remaining, and there is a risk of an increased amount of carbon dioxide gas being generated in the third heat treatment step S23. On the other hand, if the heat treatment temperature exceeds 900°C, the grain growth of the lithium composite compound will proceed excessively in this step, and there is a high risk that a high-capacity positive electrode active material will not be obtained. In contrast, if the heat treatment temperature is set to the above range, a second precursor with few coarse crystal grains can be obtained while the solid-state reaction proceeds overall. 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 above formula (3) is further improved. If it is 700°C or higher, the reaction efficiency is further improved, and this is even more preferable. Furthermore, 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, crystal grains are less likely to become coarse.

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

[0053] To achieve high capacity in a positive electrode active material containing more than 70 atomic % nickel, it is particularly important to sufficiently oxidize the nickel valence from divalent to trivalent. This is because divalent nickel easily substitutes for lithium sites in LiM'O2, which has a layered structure, causing 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 where sufficient oxygen is supplied, thereby reliably changing the nickel valence from divalent to trivalent. Furthermore, carbon dioxide gas generated in the reaction of formula (3) inhibits the progress of the reaction of 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 in an airflow that does not easily allow carbon dioxide gas to stagnate.

[0054] Specifically, the second heat treatment step S22 is preferably performed in 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%. The second heat treatment step S2 is preferably performed under an oxidizing gas flow. When the heat treatment is performed under an oxidizing gas flow with a high oxygen concentration, nickel can be reliably oxidized and the carbon dioxide gas generated by the above formula (3) can be reliably 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 probability of contact between the powdered first precursor and oxygen can be increased, allowing nickel and the like to be sufficiently oxidized. Furthermore, by rolling the powdered first precursor, the generated carbon dioxide gas is less likely to remain in the gaps between the particles, allowing the carbon dioxide gas to be efficiently removed and promoting the solid-state reaction.

[0056] When the second heat treatment step S22 is performed using a rotary kiln 1 configured as shown in FIG. 2, the first precursor is loaded into a furnace tube 10, which is adjusted to an oxidizing atmosphere, and the first and second air supply systems and heater 20 are activated to rotate the furnace tube 10 at a predetermined rotational speed. Specifically, the first air supply system sprays oxidizing gas onto the first precursor as it rolls and flows downward from upstream to downstream within the furnace tube 10 of the rotary kiln 1, which is adjusted to an oxygen atmosphere with an oxygen concentration of 90% or higher. At the same time, carbon dioxide gas generated from the first precursor is exhausted with a stream of oxidizing gas from the second air supply system. The carbon dioxide gas generated from the first precursor is preferably exhausted axially of the furnace tube 10 through an exhaust port provided on the upstream side of the furnace tube 10. Furthermore, it is preferable to perform heat treatment by adjusting at least one of the amount, blowing angle, and oxygen concentration of the oxidizing gas blown by the first gas supply system in accordance with the amount of the first precursor introduced, the heat treatment temperature, the oxygen concentration of the atmosphere, the rotation speed of the furnace tube 10, and the like. However, as described above, the main purpose of the second heat treatment step S22 is to prevent the large amount of carbon dioxide gas generated from the first precursor from inhibiting the reaction. In order to proceed with the series of steps, it is preferable to exhaust as much carbon dioxide gas as possible in this second heat treatment step S22 and efficiently exhaust it from the furnace tube 10. For this reason, the second heat treatment step S22 is a step in which the second gas supply system, which exhausts carbon dioxide gas, is highly important. Therefore, in the second heat treatment step S22, it is preferable to adjust at least the amount of oxidizing gas blown by the second gas supply pipe 40, the blowing pressure, and the like. It is more preferable to adjust both the second gas supply system and the first gas supply system.

[0057] [Third heat treatment process] In the third heat treatment step S23, the second precursor obtained in the second heat treatment step S22 is heat-treated at a heat treatment temperature of 700°C or higher and 900°C or lower to obtain a lithium composite compound having a layered structure. The third heat treatment step S23 is carried out mainly for the purpose of sufficiently oxidizing nickel in the second precursor from divalent to trivalent and growing crystal grains of the lithium composite compound having a layered structure. That is, this step is a heat treatment step in which an oxidation reaction of nickel in the second precursor and grain growth of crystal grains of the lithium composite compound are carried out.

[0058] In the third heat treatment step S23, the heat treatment may be performed while the second precursor is left stationary, or while the second precursor is being rolled. By performing the heat treatment while rolling the second precursor, the probability of contact between the powdered second precursor and oxygen can be increased, and nickel and the like can be sufficiently oxidized. In addition, rolling the powdered second precursor has the advantage of more uniformly firing the lithium composite compound.

[0059] The third heat treatment step S23 is preferably carried out after the second heat treatment step S22 is completed, by completely evacuating the atmospheric gas used in the second heat treatment step S22 and introducing a new atmospheric gas. Furthermore, when both the second heat treatment step S22 and the third heat treatment step S23 are carried out using a rotary kiln 1 having the configuration shown in Fig. 2, the second heat treatment step S22 may be carried out using a single rotary kiln 1, and then the third heat treatment step S23 may be carried out using the same rotary kiln 1. Alternatively, the second heat treatment step S22 and the third heat treatment step S23 may be carried out sequentially using multiple rotary kilns 1, or the second heat treatment step S22 and the third heat treatment step S23 may be carried out continuously at the same time in a single rotary kiln 1. [Example]

[0060] The present invention will be specifically explained below by showing examples, but the technical scope of the present invention is not limited to these examples.

[0061] Example 1 Lithium carbonate, nickel hydroxide, cobalt carbonate, and manganese carbonate were prepared as starting materials for the positive electrode active material. These starting materials were weighed out so that the atomic ratio of Li:Ni:Co:Mn was 1.04:0.80:0.15:0.05, and a 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 the mixture was pulverized and mixed using a bead mill. The resulting solid-liquid mixture was dried using a spray dryer to obtain a raw material mixture powder.

[0062] Next, the obtained raw material mixed powder was filled into an alumina firing container and subjected to heat treatment in a roller hearth kiln at 360°C for 1 hour in an air atmosphere (first heat treatment step S21), to obtain a first precursor. This heat treatment not only removed the moisture absorbed by the raw material mixed powder, but also thermally decomposed the nickel hydroxide and partially decomposed each carbonate, thereby removing a certain amount of carbon dioxide (CO2).

[0063] Next, the obtained first precursor was placed in the rotary kiln 1 shown in FIG. 2, and in the rotating furnace core tube 10, while air was being supplied through the first air supply pipe 30 and the second air supply pipe 40, heat treatment was carried out at 650°C for 0.9 hours, and then at 700°C for 3.5 hours. That is, the second heat treatment step S22 was carried out to obtain the second precursor. At this time, the furnace core tube 10 in the rotary kiln 1 had a total tube length L1 of 3500 mm, a tube inner diameter D1 of 214 mm, and a volume V1 of 0.126 m 3 The total length of the first air supply pipe 30, L2, is 3500 mm, the outer diameter of the pipe, D2, is 120 mm, and the volume, V2, is 0.04 m 3 , V2 / V1 = 0.32 (32%), and D2 / D1 = 0.56. The inner layer (inner shell) of the furnace core tube 10 was made of metallic nickel, the outer layer (outer shell) was made of stainless steel, and the inner layer of the first air supply pipe 30 was made of stainless steel and the outer layer was made of metallic nickel. Pure aluminum was sprayed onto the inner layer of the furnace core tube 10 and the outer layer of the first air supply pipe 30, and then the resulting mixture was baked in argon gas. Oxygen was then introduced to oxidize the surface, forming aluminum oxide on the outermost layer of the powder-contacting portion of the heat-treated portion Ma, and an alloy layer of aluminum and nickel (base material) was formed as an underlying layer.

[0064] Next, this second precursor is charged into the rotary kiln 1B shown in FIG. 3 and subjected to a heat treatment at 840° C. for 0.7 hours (third heat treatment step S23) to obtain Li 1.0 Ni 0.8 0Co 0.15 Mn 0.05 A lithium composite compound (positive electrode active material) having a composition of O was obtained. At this time, an alumina furnace tube 10B was used in the rotary kiln 1B. The amounts 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. The measurement results are shown in Table 1.

[0065] Next, a lithium secondary battery was fabricated using the obtained positive electrode active material as a positive electrode material in the following procedure. First, the positive electrode active material, a binder, and a conductive material were mixed to prepare a positive electrode mixture slurry. Then, the prepared positive electrode mixture slurry was applied to a 20 μm thick aluminum foil as a positive electrode current collector, and after drying at 120°C, the electrode density was adjusted to 2.0 g / cm. 3 The cathode was then compression-molded using a press to form a cathode, which was then punched into a disk with a diameter of 15 mm. A negative electrode was also fabricated using metallic lithium as the negative electrode material. A lithium secondary battery was then fabricated using the fabricated cathode and negative electrode and a non-aqueous electrolyte. The non-aqueous electrolyte used was a solution in which LiPF6 was dissolved to a concentration of 1.0 mol / L in a solvent containing a mixture of ethylene carbonate and dimethyl carbonate in a volume ratio of 3:7.

[0066] Next, the initial discharge capacity of the fabricated lithium secondary battery was measured using the following procedure. First, the battery was charged at a constant current and voltage of 0.2 CA up to a charge cut-off voltage of 4.3 V. Then, the battery was discharged at a constant current of 0.2 CA down to a discharge cut-off voltage of 2.5 V, and the discharge capacity was calculated from the discharge current. The results are shown in Table 1.

[0067] [Measurement of core tube weight loss rate] Using the first precursor obtained in the same manner, the second heat treatment step S22 shown in Example 1 was performed, and the weight of the furnace tube was then measured. A process including temperature increase and decrease was repeated as one cycle, and the weight of the furnace tube was measured after each cycle. That is, the weight of the furnace tube after the cycle was divided by the weight of the furnace tube before use, and the value was used as the weight loss rate (wall loss) to evaluate durability. The results after 300 and 500 cycles are shown in Table 1.

[0068] (Comparative Example 1) The first precursor obtained in the same manner was placed in the rotary kiln 1 shown in FIG. 2 and subjected to a heat treatment at 650° C. for 3.5 hours, ie, a second heat treatment step S22, to obtain a second precursor. Next, this second precursor is charged into a rotary kiln 1B and subjected to a heat treatment at 840°C for 0.7 hours (third heat treatment step S23) to obtain Li 1.0 Ni 0.80 Co 0.15 Mn 0.05 A lithium composite compound (positive electrode active material) having a composition of O2 was obtained. The amounts 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. Furthermore, 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 temperature and time of the second heat treatment step S22.

[0069] (Comparative Example 2) The first precursor obtained in the same manner was placed in a rotary kiln 1' shown in Figure 2 and heat-treated at 650°C for 0.9 hours, followed by a heat treatment at 700°C for 3.5 hours. The rotary kiln 1' had the same dimensions as the example, and the inner layer of the furnace core tube 10 was made of metallic nickel and the outer layer was made of stainless steel. The inner layer of the first air supply pipe 30 was made of stainless steel and the outer layer was made of metallic nickel, and the powder-contacting portion of the heat-treated portion Ma was made of nickel. Similarly, a cycle consisting of a temperature increase and decrease was repeated, and the weight loss rate of the furnace core tube was measured. The results after 300 and 500 cycles are shown in Table 1.

[0070] [Table 1]

[0071] In Example 1 and Comparative Example 1, the second heat treatment step S22 of Example 1 includes a step (stage) of maintaining the temperature at 700°C or higher for 2 hours or longer, which indicates that the amount of residual unreacted lithium carbonate is small and the solid-phase reaction progresses. Furthermore, in Example 1, by making the powder-contacting portion of the furnace tube consist of aluminum oxide, it is found that the lithium component contained in the precursor of the lithium composite compound is less likely to react with the furnace tube, reducing the amount of thinning of the furnace tube and improving its durability. As a result, the life of the furnace tube is extended, which contributes to lower production costs. [Explanation of symbols]

[0072] S1 Mixing process S2 firing process S21 First heat treatment process S22 Second heat treatment process S23 Third heat treatment process 1, 1B Rotary kiln (firing furnace) 10, 10B furnace tube 20, 20B heater 30, 30B 1st air supply pipe 32 Nozzle 34 legs 40, 40B second air intake pipe 50, 50B lifter 110, 110B Preheating Zone 120, 120B heating zone Ma processed material

Claims

1. a mixing step of mixing a lithium compound with a compound containing a metal element other than Li in the following formula (1); and a calcination step of calcining 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 on the precursor while rolling it in a furnace tube of a firing furnace, In the furnace tube, an aluminum oxide obtained by surface oxidizing pure aluminum is formed on the outermost surface of the powder-contacting portion, and an alloy layer of the pure aluminum and the base material of the furnace tube is formed on the underlying layer.

1. A method for producing a positive electrode active material for a lithium secondary battery, comprising: Li １＋ａ M1O ２＋α ・・・(1) (In the formula (1), M1 is a metal element other than Li and contains at least Ni and any one of Mn, Co, Al, Ti, Zr, Mo, Nb, W, V, Cr, Mg, Ca, Cu, Zn, and Sn, the proportion of Ni in M1 is 70 atomic % or more, and a and α are numbers that satisfy −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 an inner peripheral surface of the furnace tube, and a second air supply pipe that flows the oxidizing gas in an axial direction of the furnace tube; 2. 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 opening direction is in the circumferential direction of the first air supply pipe.

3. 3. The method for producing a positive electrode active material for a lithium secondary battery according to claim 2, wherein the legs of the furnace tube that hold the first air supply pipe are not joined to the furnace tube.

4. The heat treatment step includes a step of maintaining the temperature at 700° C. or higher for 2 hours or more.

4. The method for producing a positive electrode active material for a lithium secondary battery according to claim 1.

5. a mixing step of mixing a lithium compound with a compound containing a metal element other than Li in the following formula (1); and a calcination step of calcining 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 on the precursor while rolling it in a furnace tube of a firing furnace, the firing furnace includes a first air supply pipe that injects an oxidizing gas toward an inner peripheral surface of the furnace tube, and a second air supply pipe that flows the oxidizing gas in an axial direction of the furnace tube; the first air intake pipe has a plurality of injection ports whose opening direction is in the circumferential direction of the first air intake pipe, In the furnace 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 furnace core tube is formed on the underlying layer, and the legs holding the first air supply pipe are not joined to the furnace core tube.

1. A method for producing a positive electrode active material for a lithium secondary battery, comprising: Li １＋ａ M1O ２＋α ・・・(1) (In the 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 that satisfy −0.1≦a≦0.2 and −0.2≦α≦0.2.)

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