Method for manufacturing positive electrode active material for lithium secondary batteries

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] This 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] Lithium-ion batteries, which are widely used as small, lightweight rechargeable batteries with high energy density, require increased capacity in the active materials used in their positive and negative electrodes.

[0003] Lithium composite compounds represented by LiMO2 (where M represents a metallic element such as Ni, Co, or Mn) with an α-NaFeO2 type layered structure are known as positive electrode active materials with high charge and discharge capacities. This positive electrode active material is expected to be a positive electrode active material that can achieve high energy density in batteries, as its capacity tends to increase as the proportion of nickel increases. Therefore, there is a need for lithium composite compounds that offer high productivity and low production costs.

[0004] In the manufacturing process of positive electrode active materials, a technology has been proposed that utilizes a rotary kiln as a firing furnace for the formation reaction of lithium composite compounds. Rotary kilns have the advantage of not requiring a special firing vessel and being able to easily maintain an oxidizing atmosphere inside the furnace.

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

[0006] Furthermore, Patent Document 2 discloses a double-walled reactor core tube having an inner tube, wherein at least one of the inner layer of the reactor core tube or the outer layer of the inner tube is made of metallic nickel or a nickel alloy. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] International Publication No. 2017 / 213002 [Patent Document 2] Japanese Patent Publication No. 2019-75253 [Overview of the project] [Problems that the invention aims to solve]

[0008] However, ceramic reactor tubes make it difficult to scale up the equipment and achieve high productivity, so alloys with metal as the main component are used. teeth There is a risk that metal components may become mixed into the raw materials with prolonged use.

[0009] Therefore, the present invention aims to provide a positive electrode active material for lithium secondary batteries that is of high quality, has high productivity and 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 aspect of the present invention comprises 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), wherein the calcination step involves the precursor against firing furnace inside The process includes at least a heat treatment step, and the In the firing furnace, in contact with the precursor On the outermost surface of the powder-contacting area teeth, An aluminum oxide obtained by surface oxidation of pure aluminum is formed, and the underlying layer is the pure aluminum and powder contact area It is characterized by the formation of an alloy layer with the base material. Li 1+a M1O 2+α ...(1) (However, in the above formula (1), M1 is a metal element other than Li that 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 said 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.)

[0011] Furthermore, in the In another aspect method for producing a positive electrode active material for a lithium secondary battery of the present invention, the firing furnace preferably It has a reactor core tube. comprises a first air supply pipe that injects oxidizing gas toward the inner peripheral surface side of the core tube, and a second air supply pipe that flows oxidizing gas in the axial direction of the core tube, and the first air supply pipe preferably has a plurality of injection ports whose opening direction is the circumferential direction of the first air supply pipe.

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

[0013] Furthermore, the In one embodiment of the method for producing a positive electrode active material for a lithium secondary battery, it is preferable that the alloy layer is an alloy layer of pure aluminum and nickel. 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. Effects of the Invention

[0014] According to the present invention, a positive electrode active material for a lithium secondary battery with high quality, high productivity and low production cost can be provided. Brief Description 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] Fig. 1 is a diagram showing the 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 the schematic structure of a rotary kiln 1B used in Examples. [Modes for carrying out the invention]

[0016] The following describes in detail a positive electrode active material for a lithium secondary battery (hereinafter sometimes simply referred to as "positive electrode active material") and a method for manufacturing the same, according to one embodiment of the present invention. The following description is merely a specific example of the content of the present invention, and the present invention is not limited thereto. The present invention can be modified in various ways by those skilled in the art within the scope of the technical ideas disclosed herein.

[0017] <Cathode active material> The positive electrode active material according to this embodiment is a lithium composite compound composed of lithium and a transition metal, having a layered rock salt-type crystalline structure (hereinafter sometimes referred to as a layered structure) belonging to space group R-3m. This positive electrode active material is capable of reversibly intercalating and releasing lithium ions by applying a voltage, and is suitably used 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 given by the following formula (1): Li 1+a M1O 2+α ...(1) (However, in formula (1) above, M1 is a metallic element other than Li and contains at least Ni, the proportion of Ni per M1 exceeds 70 atomic percent, and a and α are numbers satisfying -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 proportion of nickel (Ni) per metal element other than lithium (Li) (M1) exceeds 70 atomic percent, thereby enabling high energy density and high charge / discharge capacity. The proportion of nickel (Ni) per metal element other than lithium (Li) (M1) can be set to an appropriate value within the range of more than 70 atomic percent and less than 100 atomic percent. Because this positive electrode active material contains a high proportion of nickel, Ni2+ to Ni 3+ it is important that the oxidation reaction for oxidizing is efficiently performed.

[0020] As the metal element (M1) other than lithium (Li), in addition to nickel, transition metal elements may be contained, non-transition metal elements may be contained, or a combination of these may be contained. 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 contained.

[0021] A more preferable specific composition of the positive electrode active material according to the present embodiment is represented by the following formula (2): Li 1+a Ni b Mn c Co d M2 e O 2+α ···(2) (provided that in the 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 the present embodiment relates to a method for synthesizing a lithium composite compound which is a positive electrode active material used for a positive electrode of a lithium secondary battery, is represented by the above formula (1), and has a layered rock salt-type crystal structure.

[0023] (Production Method Flow) Figure 1 is a flow chart of a method for manufacturing a positive electrode active material according to one embodiment of the present invention. As shown in Figure 1, the method for producing a positive electrode active material according to this embodiment includes a mixing step S1 and a calcination step S2. A precursor is prepared from the raw material compounds through the mixing step S1, and the precursor is calcined in the calcination 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 as one of the steps constituting the calcination step S2, in which the precursor of the lithium composite compound before calcination is subjected to heat treatment while being tumbled in a rotary kiln used as a calcination furnace.

[0024] In mixing step S1, a lithium-containing compound (lithium compound) is mixed with a compound containing metal elements other than Li that make up 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 has a stable supply, is readily available, and is inexpensive. In addition, because it has a high melting point, it causes less damage to manufacturing equipment and has excellent industrial applicability and practicality.

[0025] Compounds containing metal elements other than Li that make up 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] Examples of nickel-containing compounds that can be used include pure nickel (included as a compound for convenience), oxides, hydroxides, carbonates, acetates, etc. Among these, oxides or hydroxides are particularly preferred. Unlike when using carbonates or acetates, oxides and hydroxides do not generate large amounts of carbon dioxide during the calcination process, so lithium composite compounds with a high nickel content and high purity can be stably produced.

[0027] As manganese-containing compounds and cobalt-containing compounds, for example, oxides, hydroxides, carbonates, acetates, etc., can be used. Among these, oxides, hydroxides, or carbonates are particularly preferred. Furthermore, as compounds containing other metal elements such as M2, for example, carbonates, oxides, hydroxides, acetates, nitrates, etc., can be used. Among these, carbonates, oxides, or hydroxides are particularly preferred.

[0028] In mixing step S1, specifically, each raw material compound is weighed in a predetermined elemental composition ratio corresponding to the above formula, and each compound is crushed and mixed to prepare a powdered mixture in which each compound is mixed. From the viewpoint of uniform mixing and uniform particle size, it is preferable to crush each compound until the average particle size is less than 1 μm. As a crushing machine for crushing the compounds, a general precision crushing machine such as a ball mill, jet mill, or sand mill can be used. The grinding of the raw material compounds is preferably done by wet grinding, and from an industrial standpoint, wet grinding using water as the dispersion medium is particularly preferred. The solid-liquid mixture obtained by wet grinding may be dried, for example, using a dryer. Examples of dryers that can be used include spray dryers, fluidized bed dryers, evaporators, etc.

[0029] In the calcination process S2, the precursor obtained through the mixing process S1 is calcined to obtain a lithium composite compound having a layered structure. The calcination process S2 includes at least a heat treatment step in which the precursor of the lithium composite compound before calcination is subjected to heat treatment while being tumbled in a rotary kiln used as the calcination furnace. Here, the rotary kiln used in the calcination process S2 will be described.

[0030] (Rotary kiln / firing furnace) Figure 2 shows a schematic diagram of a rotary kiln used for manufacturing positive electrode active materials for lithium secondary batteries. As shown in Figure 2, the rotary kiln 1 comprises a core 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, substantially cylindrical shape, with an input section for the material to be processed Ma at one end in the longitudinal direction 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 such that the input section for the material to be processed Ma is located above the recovery section. The precursor of the lithium composite compound is introduced into the furnace tube 10 from a powder input device (not shown) installed in the input section, and flows longitudinally through the inside of the furnace tube 10 to undergo heat treatment. 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 of the furnace tube 10 in the longitudinal direction is referred to as "upstream," and the recovery section side is referred to as "downstream."

[0032] The reactor core tube 10 is connected to a power source such as a motor (not shown) via a drive gear or roller. Driven by such a motor, the reactor core tube 10 rotates around its cylindrical central axis. As a result, the material to be processed, Ma, introduced into the reactor core tube 10 from the input section flows down the inside of the reactor core tube 10 as it rotates, and is recovered in the recovery section by a powder recovery device (not shown). The rotation speed of the reactor 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 reactor core tube 10 is preferably made of a metal such as Ni, W, Mo, or Ti, or an alloy mainly composed of these metals, that does not emit harmful components such as chromium. In the reactor core tube, aluminum oxide is formed on the outermost surface of the powder contact area, which is the part that comes into contact with the precursor (processed material Ma), and an alloy layer of aluminum and the base material of the reactor core tube is formed in the underlying layer. If aluminum oxide is formed on the outermost surface of the base material of the furnace tube 10, the reaction between the lithium component contained in the precursor of the lithium composite compound and the furnace tube 10 becomes less likely, thus reducing the risk of deterioration or damage to the furnace tube 10. The aluminum oxide on the outermost surface of the base material of the furnace tube 10 can be easily formed by dry-heating the base material coated with aluminum and oxidizing the coated aluminum. The method for coating the base material with aluminum may be, for example, thermal spraying or, for example, hot-dip galvanizing. If the base material is coated with aluminum before the furnace tube 10 is manufactured, the base material will be exposed at the joints, so it is preferable to coat it after manufacturing.

[0034] The heater 20 is installed around the shell of the reactor core tube 10. The heater 20 covers a portion of the longitudinal section of the reactor core tube 10, which is shown as a dashed line in Figure 2, and can raise the temperature of the heating zone 120 to the target temperature. The heater 20 also preheats a preheating zone 110, located upstream of the heating zone 120 and extending for a predetermined distance, as shown as a double dashed line in Figure 2, to a temperature lower than the target temperature. Therefore, when a lithium composite compound precursor is introduced into the reactor 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 undergoes heat treatment while rolling. However, the placement and number of heaters 20 are not particularly limited, as long as uniform heat treatment can be performed on the heating zone 120. The heaters 20 may be concentrated in one location or divided into multiple locations, provided that a preheating zone 110 is secured to prevent rapid heat treatment.

[0035] The first air supply pipe 30 constitutes a first air supply system that supplies oxidizing gas from a gas source (not shown) into the core tube 10, and injects the oxidizing gas toward the inner circumferential surface of the core tube 10 when the material to be processed, Ma, is heat-treated. The first air supply pipe 30 is arranged along the longitudinal direction inside the core tube 10 and extends approximately the entire length of the core tube 10 from the downstream side to the upstream side. The first air supply pipe 30 is arranged along the longitudinal direction of the core tube 10 and has multiple injection ports 32 that branch off from the first air supply pipe 30 and open at the branched ends. Each of the injection ports 32 can inject oxidizing gas, which is pressurized from a gas source (not shown), in a shower-like manner toward the inside of the core tube 10. In other words, the first air supply system blows 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 generated from the precursor is stirred up by the oxidizing gas and quickly removed from the vicinity of the precursor. In other words, carbon dioxide generated from the precursor and lingering inside the reactor core tube 10 does not re-react with the precursor, preventing the re-generation of lithium carbonate and inhibiting the formation of lithium complex compounds. The opening direction of the injection port 32 may be vertically downward (radial direction of the reactor core tube) or in the circumferential direction of the first air supply pipe 30, but it is preferable to set the opening direction of the injection port 32 to the circumferential direction of the first air supply pipe 30, as injecting in the circumferential direction creates a swirling flow in the reactor, allowing for efficient oxygen supply to the precursor and discharge of lingering carbon dioxide.

[0036] The first air supply pipe 30 is preferably provided in a way that allows for adjustment of the amount of oxidizing gas sprayed, the spraying angle, and the oxygen concentration, from the viewpoint of efficiently supplying oxygen and exhausting carbon dioxide, as well as preventing the scattering of powder from the material being processed. For example, the amount of sprayed can be adjusted by adjusting the gas flow rate of the first air supply system, or by providing an openable and closable nozzle 32 and adjusting the number of nozzle openings. The spraying angle can be adjusted by providing the first air supply pipe 30 so as to be rotatable around its central axis. For example, the spraying angle may be set to be greater than 0° and less than or equal to 45° in the forward or reverse direction of rotation of the core tube 10. The spraying angle can also be adjusted by moving the first air supply pipe 30 horizontally or in the opposite direction inside the core tube 10. For example, the first air supply pipe 30 may be stationary at an eccentric position from the central axis of the core tube 10 and spraying may be performed. Furthermore, the oxygen concentration can be adjusted by installing an oxygen concentration detection means near the inlet or outlet of the reactor 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. These injection amounts, injection angles, and oxygen concentrations can also be adjusted in appropriate combinations. Alternatively, a carbon dioxide concentration detection means can be installed instead of, or in combination with, the oxygen concentration detection means, and the amount of oxygen can be adjusted by monitoring and controlling it so that the detected carbon dioxide concentration reaches a specified value.

[0037] The first air supply pipe 30 is held inside the reactor core tube 10 by legs 34. By interposing a round bar or the like near the circumferential side of the contact area between the legs 34 and the reactor core tube 10, the legs 34 and the reactor core tube 10 can be restrained in the circumferential direction without being fixed in place. This allows the driving force of the reactor core tube 10 to be transmitted to the first air supply pipe 30, and the first air supply pipe and the reactor core tube 10 to rotate synchronously. This round bar or the like does not restrain the legs 34 in the axial direction of the reactor core tube, but only in the circumferential direction, and furthermore, by not joining the legs 34 and the reactor core tube 10, the first air supply pipe 30 can be easily removed from the reactor core tube 10, improving workability during cleaning and maintenance of the inside of the reactor core tube 10, and ultimately improving productivity.

[0038] The second air supply pipe 40 constitutes a second air supply system that supplies oxidizing gas from a gas source (not shown) into the core tube 10. When the material Ma to be processed is heat-treated, it generates a flow of oxidizing gas inside the core tube 10 in the axial direction of the core tube 10. The oxidizing gas may flow from the upstream side to the downstream side of the core tube 10, but it is preferable to flow from the downstream side to the upstream side of the core tube 10. The second air supply pipe 40 is located downstream of the heating zone 120 inside the core tube 10 and opens toward the upstream side of the core tube 10. Furthermore, when viewed from the axial direction, the second air supply pipe 40 is located radially outward from the first air supply pipe and opens into the space radially outward from the first air supply pipe 30. The second air supply pipe 40 flows an oxidizing gas in a substantially horizontal direction radially outward from 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 core tube 10. In other words, the second air supply system creates an oxidizing gas flow inside the core tube 10, so that carbon dioxide generated from the precursor by the heat treatment is exhausted along with the oxidizing gas in the airflow. If the oxidizing gas flow from the second air supply system flows in the opposite direction to the flow of the precursor, the carbon dioxide concentration decreases towards the downstream side of the core tube 10, thus reliably reducing the amount of carbon contamination in the processed material Ma, which completes its heat treatment downstream. The amount and direction of oxidizing gas supplied by the second air supply pipe 40 can also be adjusted as appropriate.

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

[0040] The lifter 50 is installed on the inner circumferential surface of the reactor tube 10. The lifter 50 protrudes inward from a part of the circumferential direction on the inner circumferential surface of the reactor tube 10, and as the reactor tube 10 rotates, it lifts and agitates the material to be processed, Ma. That is, agitation by the lifter 50 causes the surface powder and bottom powder in the precursor powder to flow while being exchanged, which increases the probability of contact with oxygen and its uniformity, and also efficiently removes carbon dioxide generated from the precursor from the interparticle gaps in the powder. Therefore, by agitating the precursor with the lifter 50 under the oxidizing gas supplied by the first and second supply air systems, the supply of oxygen and exhaust of carbon dioxide proceed effectively, and the solid-phase reaction that produces the lithium composite compound is greatly promoted.

[0041] The lifters 50 can be provided in an appropriate shape and number. For example, the lifters 50 can be provided in the form of wing-like, rib-like, pipe-like, or prismatic shape extending in the longitudinal direction of the reactor core tube 10, and multiple lifters 50 can be arranged at appropriate intervals in the circumferential direction of the reactor core tube 10. The lifters 50 may be continuous without gaps in the longitudinal direction of the reactor core tube 10, or they may be intermittent with gaps between them.

[0042] The lifter 50 may be provided along the entire length inside the core tube 10, but it is preferable that it be provided only in the zone (heating zone 120) of the inner circumferential surface of the core tube 10 that is directly heated to the target heat treatment temperature by the heater 20 during heat treatment, and not upstream or downstream of the heating zone 120. In the preheating zone 110 and other areas upstream of the heating zone 120, carbon dioxide is generated significantly, and if the precursor powder is stirred in such a region, the precursor and carbon dioxide will react to produce 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 zone 120, it is possible to sufficiently promote the solid-phase reaction, while by not providing it upstream or downstream of the heating zone 120, it is possible to suppress the situation in which fine precursor powder that has been stirred more than necessary is discharged with the oxidizing gas stream, which reduces the recovery rate.

[0043] The rotary kiln 1 preferably has an exhaust port on the upstream side of the core tube 10 for exhausting the atmospheric gas inside the core tube 10. If the exhaust port is located on the side of the core tube 10 rather than on the top surface, the high-density carbon dioxide can be reliably discharged from the core tube 10 by being carried by the oxidizing gas flow. More specifically, the location of the exhaust port is preferably on the upstream inner surface inside the core tube 10, and more preferably in the lower half of the inner surface, which is lower in height than the rotation axis of the core tube 10.

[0044] With the rotary kiln 1 described above, oxygen supply, carbon dioxide exhaust, and precursor powder supply are carried out continuously, so the heat treatment of the precursor can be performed in a short time. In particular, since oxygen supply is performed to the core tube 10 which forms a closed space, it can be done at a lower cost compared to transport furnaces and the like which perform heat treatment in an open space. Furthermore, the first supply system blows an oxidizing gas directly onto the precursor, so a high concentration of oxygen can be supplied to the precursor, and carbon dioxide generated from the precursor can be stirred up and reliably separated and removed from the flowing precursor. In addition, the second supply system quickly exhausts the carbon dioxide that has been stirred up inside the core tube 10 to the outside of the furnace, so that the heat-treated precursor can not come into contact with carbon dioxide. That is, with only the first supply system, carbon dioxide generated from the precursor is not discharged from the core tube 10 and remains there, and with only the second supply system, it is difficult to remove the carbon dioxide that remains in the interparticle gaps in the precursor powder. By using both the first and second air supply systems in combination, the circulation of oxygen supply and carbon dioxide exhaust is efficiently maintained, resulting in heat-treated materials with fewer crystal defects and impurities.

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

[0046] 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, as shown in Figure 1. The rotary kiln 1 with the configuration shown in Figure 2 may be used in any of these heat treatment processes, but it is preferable to use it in at least one of the second heat treatment process S22 and the third heat treatment process S23, and more preferably in the second heat treatment process S22.

[0047] [First heat treatment process 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 a period of 0.5 hours or more and 5 hours or less to obtain the first precursor. The first heat treatment step S21 is performed primarily to remove highly volatile components such as water that would hinder the synthesis reaction of the positive electrode active material from the mixture obtained in the mixing step S1. In this step, carbon dioxide and other gases generated as a result of the thermal decomposition of raw materials such as lithium carbonate and the combustion of impurities are removed from the mixture along with water.

[0048] The first heat treatment step S21 can be carried out using an appropriate heat treatment apparatus. Specifically, for example, a roller hearth kiln, tunnel furnace, pusher furnace, rotary kiln, batch furnace, etc. can be used. 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 or higher and 900°C or lower for a period of 0.1 hours or more and 50 hours or less to obtain the second precursor. The main purpose of the second heat treatment step S22 is to oxidize the nickel in the first precursor from divalent to trivalent and crystallize a lithium composite compound having a layered structure. In other words, this step is a heat treatment step in which lithium carbonate (Li2CO3) and an oxide of M' (M'O) are reactants, and the nickel in the first precursor is oxidized to form a layered structure. If the heat treatment temperature in the second heat treatment step S22 is less than 450°C, the reaction rate of the solid-phase reaction will be slow, resulting in an excess of lithium carbonate remaining, which may increase the amount of carbon dioxide generated in the third heat treatment step S23. On the other hand, if the heat treatment temperature exceeds 900°C, there is a high risk that the grain growth of the lithium composite compound will proceed excessively in this process, making it impossible to obtain a high-capacity positive electrode active material. In contrast, at the aforementioned heat treatment temperature, the solid-phase reaction proceeds throughout, while a second precursor with fewer coarse crystal grains can be obtained. The reaction of lithium carbonate that proceeds in the second heat treatment step S22 is represented by the following equation (3).

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

[0051] It is more preferable that the heat treatment temperature in the second heat treatment step S22 be 600°C or higher. If it is 600°C or higher, the reaction efficiency of formula (3) is further improved. If it is 700°C or higher, the reaction efficiency is further improved and is even more preferable. Furthermore, it is more preferable that the heat treatment temperature in the second heat treatment step S22 be 800°C or lower. If it is 800°C or lower, the crystal grains are less likely to coarseen.

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

[0053] In order to achieve high capacity in a positive electrode active material with a nickel content exceeding 70 atomic percent, it is particularly important to sufficiently oxidize the nickel valency from divalent to trivalent. This is because divalent nickel readily substitutes for lithium sites in the layered structure of LiM'O2, 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 with sufficient oxygen supply to reliably change the nickel valency from divalent to trivalent. Furthermore, the carbon dioxide generated in equation (3) inhibits the progress of the reaction in equation (3), causing 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 airflow that does not easily accumulate carbon dioxide.

[0054] Specifically, the second heat treatment step S22 is preferably carried out in an oxidizing atmosphere with an oxygen concentration of 90% or higher, more preferably in an oxidizing atmosphere with an oxygen concentration of 95% or higher, and even more preferably in an oxidizing atmosphere with an oxygen concentration of 100%. Furthermore, the second heat treatment step S2 is preferably carried out under a stream of oxidizing gas. Performing the heat treatment under a stream of oxidizing gas with a high oxygen concentration ensures that nickel is reliably oxidized and that the carbon dioxide generated in formula (3) above is 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, and nickel and the like can be sufficiently oxidized. In addition, as the powdered first precursor rolls, the generated carbon dioxide gas is less likely to accumulate in the interparticle gaps, and the carbon dioxide gas can be efficiently removed, thereby promoting the solid-phase reaction.

[0056] The second heat treatment step S22, when performed using a rotary kiln 1 configured as shown in Figure 2, involves introducing the first precursor into a furnace tube 10 adjusted to an oxidizing atmosphere, and operating the first air supply system, the second air supply system, and the heater 20 to rotate the furnace tube 10 at a predetermined rotational speed. Specifically, the first precursor, which is rolling and flowing down from the upstream side to the downstream side within the furnace tube 10 of the rotary kiln 1 adjusted to an oxygen atmosphere with an oxygen concentration of 90% or higher, is subjected to oxidizing gas blown in by the first air supply system, while carbon dioxide generated from the first precursor is exhausted by the oxidizing gas flow from the second air supply system, and heat treatment is performed at a predetermined heat treatment temperature and for a predetermined heat treatment time. It is preferable that the carbon dioxide generated from the first precursor is discharged from the axial direction 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 following: the amount of oxidizing gas blown in by the first air supply system, the blowing angle, and the oxygen concentration, according to 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, etc. However, as mentioned above, the main purpose of the second heat treatment step S22 is to suppress the large amount of carbon dioxide generated from the first precursor from becoming an inhibitor of the reaction. It is preferable to remove as much carbon dioxide as possible in this second heat treatment step S22 and to efficiently discharge it from inside the furnace tube 10 in order to proceed with the series of processes. For this reason, the second heat treatment step S22 is a step in which the second air supply system that discharges carbon dioxide is of high importance. Therefore, in the second heat treatment step S22, it is preferable to adjust at least the amount of oxidizing gas supplied by the second air supply pipe 40, the blowing pressure, etc., and it is even more preferable to adjust both the second air supply system and the first air 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 main purpose of the third heat treatment step S23 is to sufficiently oxidize the nickel in the second precursor from divalent to trivalent and to grow the crystal grains of the lithium composite compound having a layered structure. In other words, this step is a heat treatment step that carries out 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 heat-treated while standing, or it may be heat-treated while being tumbled. Heat-treating the second precursor while tumbling increases the probability of contact between the powdered second precursor and oxygen, allowing for sufficient oxidation of nickel and other elements. Furthermore, the tumbling of the powdered second precursor has the advantage of resulting in more uniform firing of the lithium composite compound.

[0059] It is preferable that the third heat treatment step S23 is performed after the completion of the second heat treatment step S22, 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 performed using a rotary kiln 1 configured as shown in Figure 2, the second heat treatment step S22 may be performed using a single rotary kiln 1, followed by the third heat treatment step S23 using the same rotary kiln 1, or the second heat treatment step S22 and the third heat treatment step S23 may be performed sequentially using multiple rotary kilns 1, or the second heat treatment step S22 and the third heat treatment step S23 may be performed simultaneously and continuously in a single rotary kiln 1. [Examples]

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

[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 so that the atomic ratio of Li:Ni:Co:Mn was 1.04:0.80:0.15:0.05, and 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 then the mixture was crushed 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 mixture was filled into an alumina firing container and subjected to a first heat treatment (first heat treatment step S21) at 360°C for 1 hour in an air atmosphere using a roller hearth kiln to obtain the first precursor. This heat treatment not only removed the moisture absorbed by the raw material mixture, but also caused thermal decomposition of nickel hydroxide and partial thermal decomposition of each carbonate, and removed a certain amount of carbon dioxide (CO2).

[0063] Next, the obtained first precursor was placed in the rotary kiln 1 shown in Figure 2, and heat treatment was performed at 650°C for 0.9 hours while supplying air through the first air supply pipe 30 and the second air supply pipe 40 inside the rotating furnace tube 10, followed by heat treatment at 700°C for 3.5 hours. That is, the second heat treatment step S22 was performed to obtain the second precursor. At this time, in the rotary kiln 1, the furnace tube 10 had a total length L1 = 3500 mm, an inner diameter D1 = 214 mm, and a volume V1 = 0.126 m³. 3 The total length of the first air supply pipe 30 is L2 = 3500 mm, the outer diameter of the pipe is D2 = 120 mm, and the volume is V2 = 0.04 m³. 3 The ratios V2 / V1 was set to 0.32 (32%) and D2 / D1 to 0.56. The inner layer (inner shell) of the core tube 10 was made of metallic nickel, and the outer layer (outer shell) 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. After spraying pure aluminum onto the inner layer of the core tube 10 and the outer layer of the first air supply pipe 30, the pipes were preheated in argon gas, and then oxygen was introduced to oxidize the surface, thus performing the heat treatment. thing Ma and An aluminum oxide layer was formed on the outermost surface of the powder-contacting area, and an alloy layer of aluminum and the base material, nickel, was formed in the underlying layer.

[0064] Next, this second precursor is placed in the rotary kiln 1B shown in Figure 3 and subjected to heat treatment at 840°C for 0.7 hours (third heat treatment step S23), and Li 1.0 Ni 0.80 Co 0.15 Mn 0.05 A lithium composite compound (positive electrode active material) having an O2 composition was obtained. In this process, an alumina furnace tube 10B was used in rotary kiln 1B. The amount of unreacted lithium carbonate and lithium hydroxide remaining in the obtained positive electrode active material, as well as 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 the positive electrode material by following the procedure below. First, the positive electrode active material, binder, and 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, which served as the positive electrode current collector, and dried at 120°C. After drying, the electrode density was set to 2.0 g / cm³. 3 The material was compressed and molded using a press, and then punched out into a 15mm diameter disc to create the positive electrode. A negative electrode was also made using metallic lithium as the negative electrode material. A lithium secondary battery was then constructed using the prepared positive and negative electrodes and a non-aqueous electrolyte. As the non-aqueous electrolyte, a solution was used in which LiPF6 was dissolved in a solvent prepared by mixing ethylene carbonate and dimethyl carbonate in a volume ratio of 3:7 to a concentration of 1.0 mol / L.

[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 0.2CA until the charging termination voltage reached 4.3V. Then, it was discharged at a constant current of 0.2CA until the discharge termination voltage reached 2.5V, and the discharge capacity was determined from the discharge current at that time. The results are shown in Table 1.

[0067] [Measurement of the rate of decrease in core tube weight] Using the first precursor obtained in a similar manner, the second heat treatment step S22 shown in Example 1 was performed, and the weight of the furnace tube was measured. The process, including heating and cooling, was repeated as one cycle, and the weight of the furnace tube was measured after the completion of each cycle. That is, the weight of the furnace tube after the completion of the cycle was divided by the weight of the furnace tube before use, and the durability was evaluated as the weight reduction rate (amount of wall thinning). The results after 300 and 500 cycles are shown in Table 1.

[0068] (Comparative Example 1) The first precursor, obtained in a similar manner, was placed in the rotary kiln 1 shown in Figure 2 and subjected to heat treatment at 650°C for 3.5 hours, i.e., the second heat treatment step S22, to obtain the second precursor. Next, this second precursor is placed in rotary kiln 1B and subjected to heat treatment at 840°C for 0.7 hours (third heat treatment step S23), Li 1.0 Ni 0.80 Co 0.15 Mn 0.05 A lithium composite compound (positive electrode active material) having an O2 composition was obtained. The amount of unreacted lithium carbonate and lithium hydroxide remaining in the obtained positive electrode active material, as well as the specific surface area of ​​the positive electrode active material, were measured. Furthermore, a lithium secondary battery was constructed, and its discharge capacity was determined. These results are shown in Table 1. The difference between Example 1 and Comparative Example 1 lies in the temperature and time of the second heat treatment step S22.

[0069] (Comparative Example 2) Similarly, the first precursor obtained was placed in the rotary kiln 1' shown in Figure 2 and subjected to heat treatment at 650°C for 0.9 hours, followed by heat treatment 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 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 contact area of ​​the heat-treated section Ma was occupied by nickel. Similarly, the process including heating and cooling was repeated as one cycle, and the weight reduction rate of the furnace 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 inclusion of a step (stage) in the second heat treatment step S22 of Example 1, where the temperature is maintained at 700°C or higher for 2 hours or more, indicates that the amount of residual unreacted lithium carbonate is small and the solid-phase reaction is progressing. Furthermore, in Example 1, by occupying the powder-contacting portion of the reactor core tube with aluminum oxide, the reaction between the lithium component contained in the precursor of the lithium composite compound and the reactor core tube becomes less likely, reducing the amount of wall thinning of the reactor core tube and improving durability. As a result, the extended lifespan of the reactor core tube can contribute 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 reactor core tubes 20, 20B heater 30, 30B 1st air supply pipe 32 Nozzle 34 legs 40, 40B Second Air Supply Pipe 50, 50B Lifter 110, 110B preheating zone 120, 120B heating zone Ma (Material to be processed)

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), The process includes a calcination step in which the precursor obtained through the mixing step is calcined to obtain a lithium composite compound represented by (1) below, The calcination step includes at least a heat treatment step of performing heat treatment on the precursor in a calcination furnace, In the firing furnace, the outermost layer of the powder contact area in contact with the precursor is formed of aluminum oxide obtained by surface oxidation of pure aluminum, and an alloy layer of the pure aluminum and the base material of the powder contact area is formed beneath it. A method for producing a positive electrode active material for lithium secondary batteries, characterized by the above. Li １＋ａ M1O ２＋α ・・・(1) (However, in formula (1) above, M1 is a metallic element other than Li that includes at least Ni and one of Mn, Co, Al, Ti, Zr, Mo, Nb, W, V, Cr, Mg, Ca, Cu, Zn, or Sn, and the proportion of Ni in M1 is 70 atomic percent or more, and a and α are numbers that satisfy -0.1 ≤ a ≤ 0.2 and -0.2 ≤ α ≤ 0.2.)

2. The method for producing a positive electrode active material for a lithium secondary battery according to Claim 1, characterized in that the alloy layer is an alloy layer of pure aluminum and nickel.

3. The heat treatment process includes a step of holding the temperature at 700°C or higher for 2 hours or more. A method for producing a positive electrode active material for a lithium secondary battery according to feature 1.

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