Method for producing positive electrode active material for lithium ion secondary battery

Abstract

[Problem] To provide a method which is for manufacturing a positive electrode active material for a lithium ion secondary battery, and by which the solid-phase reaction of a precursor is uniformly promoted to suppress the elution amount of lithium carbonate. [Solution] This method for manufacturing a positive electrode active material for a lithium ion secondary battery involves reacting at least 95 mass% of a lithium compound through a heat treatment step using a rotary firing furnace and having a batch firing process for heating a precursor while rolling the same in a heating region in a furnace tube, wherein the batch firing process has: a tilted input stage for tilting the furnace tube and inputting the precursor from an inlet of the firing furnace; a horizontal firing stage for performing firing while making the furnace tube horizontal; and a tilted discharge stage for tilting the furnace tube and discharging a fired body from an outlet of the firing furnace.

Description

Technical Field

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

Background Art

[0002] As a secondary battery having a high energy density, being small and lightweight, and contributing to energy saving, lithium ion secondary batteries have been widely spread. Compared with other secondary batteries such as nickel-hydrogen storage batteries and nickel-cadmium storage batteries, lithium ion secondary batteries have characteristics such as high energy density and small memory effect. Therefore, its applications are expanding from small power sources such as portable electronic devices and household electrical appliances to stationary power sources such as power storage devices, uninterruptible power supply devices, and power leveling devices, and medium to large power sources such as drive power sources for ships, railway vehicles, hybrid railway vehicles, hybrid automobiles, and electric automobiles.

[0003] With the expansion of applications and the like, lithium ion secondary batteries are required to have further increased capacity. As a kind of positive electrode active material for lithium ion secondary batteries, there is a lithium composite compound having a crystal structure of the α-NaFeO 2 type. Among this type of lithium composite compound, LiNiO 2 based oxides having a high nickel content are expected to be applied to various uses because they exhibit high charge and discharge capacities.

[0004] LiNiO 2 based oxides are obtained by reacting a transition metal compound with a lithium compound. At this time, lithium carbonate, lithium hydroxide, etc. are used as the lithium compound.

[0005] From the viewpoint of manufacturing cost, it is more advantageous to use inexpensive lithium carbonate. However, carbon dioxide gas (CO 2) occurs. As the reaction proceeds and carbon dioxide gas is generated, the concentration of carbon dioxide gas in the furnace increases, and there is a risk that the reverse reaction will occur and the forward reaction will stop. That is, the reverse reaction in which the generated lithium composite oxide decomposes into a transition metal compound and lithium carbonate proceeds, and the forward reaction to generate the lithium composite oxide is inhibited. Therefore, when lithium carbonate is used as the lithium compound, it is important to discharge the generated carbon dioxide gas outside the furnace.

[0006] A rotary kiln (hereinafter referred to as a rotary kiln in the present invention) is considered effective for quickly discharging the gas generated in the furnace outside the furnace. This is a furnace that can perform heat treatment while rolling the powder, and since the powder is moved, the generated carbon dioxide gas is quickly discharged.

[0007] For example, Patent Document 1 discloses a rotary kiln provided with a cylindrical furnace installed so as to be tiltable or horizontally. Patent Document 2 discloses a method for manufacturing a positive electrode active material for a lithium ion secondary battery in which a precursor (powder) of a lithium composite oxide is fired while being rolled in a rotary kiln, and the temperature of the precursor is made uniform by alternately switching the forward and reverse rotation directions of the cylindrical furnace and then performing firing. Patent Document 3 discloses a method for manufacturing a positive electrode active material for a lithium ion secondary battery in which a precursor of a lithium composite oxide is fired while being rolled in a rotary kiln, an oxidizing gas is blown onto the precursor advancing in the cylindrical furnace, and the carbon dioxide gas generated from the precursor is exhausted by the airflow of the oxidizing gas while performing firing. Further, Patent Document 4 discloses a method for manufacturing a positive electrode active material for a lithium ion secondary battery in which blades are provided in a furnace core tube, and the furnace core tube and the blades rotate to perform firing while suppressing the adhesion phenomenon by the rolling of the powder and the impact of the blades.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0009] Patent Documents 1 to 4 all perform firing of powder using a rotary kiln. In firing using a rotary kiln, a cylindrical furnace in an inclined state or a horizontal state is rotated, and the precursor charged into the hearth tube is advanced while being rolled to perform continuous firing. Here, since the precursor is constantly advancing, there is a problem that it is difficult to obtain a sufficient reaction by holding the precursor for a long time. Although it is conceivable to increase the reactivity by making the precursor reach a uniform temperature by switching the rotation of the cylindrical furnace forward and backward as in Patent Document 2, switching the rotation of the furnace alternately during firing places a large burden on the process. Also, when operating by alternately switching the rotation to move the powder forward and backward and then advancing it overall, ultimately, the forward speed is extremely slow and long-time firing is being carried out, making it difficult to increase productivity.

[0010] On the other hand, when increasing productivity as a continuous process, since the precursor is constantly advancing, it is difficult to hold it in an isothermal heating region (hereinafter referred to as the heating region) that is evenly heated in the furnace for a long time. Although there are countermeasures such as reducing the inclination angle or reducing the rotation speed to ensure the holding time in the heating region, the movement of the precursor deteriorates, and the lumps of the precursor adhere to the furnace wall to form an uneven adhesion layer in thickness. Once adhesion occurs, it becomes difficult for the precursor to exceed the lumps, and the adhered matter further increases. Eventually, not only does the forward speed slow down, but the problem of adhered matter also easily occurs, deteriorating productivity.

[0011] Even when the solid-phase reaction is allowed to proceed under a flowing oxygen gas stream, the reaction proceeds on the upstream side where the oxygen concentration is high, while on the downstream side, the generated carbon dioxide gas accumulates and the reaction ceases. As a result, the variation in the physical properties of the produced fired body increases.

[0012] From the above, the present invention aims to provide a method for producing a positive electrode active material for a lithium-ion secondary battery, in which, in a production method for obtaining a fired body using a rotary kiln, the solid-phase reaction of the precursor is uniformly promoted and the elution amount of a lithium compound (mainly lithium carbonate) is suppressed.

Means for Solving the Problems

[0013] The method for producing a positive electrode active material for a lithium-ion secondary battery according to the present invention includes a mixing step of mixing a lithium compound and a compound containing a metal element other than Li in the following formula (1) to obtain a mixed powder, a first heat treatment step of heat treating the mixed powder to obtain a precursor, a second heat treatment step of heat treating the precursor to obtain a fired body, and a third heat treatment step of heat treating the fired body to obtain a lithium composite compound represented by the following formula (1). The second heat treatment step includes Using a firing furnace having a rotatable hearth tube, rolling the precursor said while heating in a heating region inside the hearth tube, and has a batch firing treatment process. In the batch firing treatment process, an inclined charging step of tilting the hearth tube and charging the precursor from an inlet of the firing furnace, a horizontal firing step of making the hearth tube horizontal and firing, and an inclined discharging step of tilting the hearth tube and discharging the fired body from an outlet of the firing furnace are included, so that 95% by mass or more of the lithium compound reacts , the hearth tube includes a supply-side spiral blade provided between the inlet and the heating region, and a discharge-side spiral blade provided between the heating region and the outlet, and the supply-side spiral blade and the discharge-side spiral blade are cut with reverse spiral grooves, and in the batch firing process, the precursor is concentrated in the heating region which is characterized by. Li 1+a M1O 2+α ···(1) (However, in the above formula (1), M1 is a metal element other than Li and contains at least Ni, the proportion of Ni in M1 is 70 atomic% or more, and a and α are numbers satisfying -0.1 ≤ a ≤ 0.2 and -0.2 ≤ α ≤ 0.2.)

[0014] The method for producing a positive electrode active material for a lithium-ion secondary battery according to the present invention isinside the hearth tube be provided with a lifter within the heating region 、 By having the height of the lifter lower than the height of the supply-side spiral blade and / or the discharge-side spiral blade, it is characterized by firing while circulating the precursor in the heating region.

[0015] In the method for producing a positive electrode active material for a lithium-ion secondary battery of the present invention, it is preferable that the lifter in the heating region is a spiral blade.

[0016] Also, in the method for producing a positive electrode active material for a lithium-ion secondary battery of the present invention, it is preferable that the rotation speed of the furnace core tube in the inclined charging stage and / or the inclined discharging stage is higher than the rotation speed of the furnace core tube in the horizontal firing stage.

[0017] In the method for producing a positive electrode active material for a lithium-ion secondary battery of the present invention, the composition formula (1) is represented by the following composition formula (2), the average particle size of the slurry in the mixing step is 0.1 μm or more and 0.5 μm or less, the average particle size of the secondary particles of the granulated powder obtained by granulating the slurry is 5 μm or more and 20 μm or less, the firing step optionally includes a first heat treatment step of heat-treating the granulated powder at a heat treatment temperature of 200°C or more and 500°C or less for 0.5 hours or more and 5 hours or less to obtain a precursor, a second heat treatment step of heat-treating the precursor at a heat treatment temperature of 600°C or more and less than 750°C for 10 hours or more and 100 hours or less in an oxidizing atmosphere to obtain a fired body, and a third heat treatment step of heat-treating the fired body at a heat treatment temperature of 750°C or more and 900°C or less for 0.5 hours or more and 50 hours or less in an oxidizing atmosphere to obtain a lithium composite compound, and the second heat treatment step can perform the batch firing treatment process. Li 1+a Ni b Co c M d X e O 2+α ···(2) [However, in the compositional formula (2), M represents at least one selected from Al and Mn, X represents one or more metal elements other than Li, Ni, Co, Al, and Mn, and a, b, c, d, e, and α are numbers satisfying -0.1 ≦ a ≦ 0.2, 0.7 ≦ b ≦ 1.0, 0 ≦ c ≦ 0.20, 0 ≦ d ≦ 0.20, 0 ≦ e ≦ 0.20, b + c + d + e = 1, and -0.2 < α < 0.2, respectively.]

Advantages of the Invention

[0018] According to the present invention, there is provided a method for manufacturing a positive electrode active material for a lithium ion secondary battery, in which a solid-phase reaction of a precursor is uniformly promoted and the elution amount of a lithium compound (mainly lithium carbonate) is suppressed, in a manufacturing method of obtaining a fired body using a rotary kiln.

Brief Description of the Drawings

[0019]

Figure 1

Figure 2

Figure 3

Figure 4

Modes for Carrying Out the Invention

[0020] Hereinafter, the method for manufacturing a positive electrode active material for a lithium ion secondary battery of the present invention will be described in detail. First, the positive electrode active material according to one embodiment and the method for manufacturing a positive electrode active material for a lithium ion secondary battery (hereinafter simply referred to as the method for manufacturing a positive electrode active material) will be described, and then the batch firing process will be described. In the present invention, the mixed powder after moisture removal and before firing is referred to as a precursor, the precursor fired in the furnace is referred to as a fired body, and the active material obtained after all the firing steps is referred to as a lithium composite compound. In addition, the same reference numerals are given to the common configurations in the following respective figures, and redundant explanations are omitted.

[0021] Conventionally, in firing using a rotary kiln, continuous firing is performed by advancing the precursor charged inside while rolling it. At this time, in order to enhance the firing reaction, oxygen is flowed from the outlet side toward the inlet side. However, the concentration of carbon dioxide gas becomes high on the inlet side, resulting in a concentration difference inside the furnace. Due to the concentration difference of carbon dioxide gas, the amount of unreacted lithium carbonate increases, and as a result, the amount of lithium carbonate remaining in the fired body cannot be reduced below the reference value. If it is attempted to reduce it below the reference, it is necessary to extend the firing time, which is not preferable in terms of the process steps. Therefore, in the present invention, a rotary kiln is used in the second heat treatment step in which the reaction of the lithium compound is most promoted, that is, the step of obtaining a fired body from the precursor. At this time, by performing a batch firing process involving charging, firing, and discharging, in which the precursor is charged into the furnace, the precursor is concentrated and fired in the heating region, and the fired body is discharged, the reaction rate of the lithium compound is increased. Further, since it is provided with supply-side spiral blades from the inlet to the heating region and discharge-side spiral blades from the heating region to the outlet, it is easy to concentrate the precursor in the central heating region by the rotation of these spiral blades. Furthermore, since the height of the lifter (agitating blade) in the heating region is lower than the heights of the supply-side spiral blades and the discharge-side spiral blades, the precursor in the heating region can be circulated in the axial direction within the heating region, improving the reaction efficiency. Therefore, the fired body in the region with a high carbon dioxide gas concentration can be moved to the region with a low carbon dioxide gas concentration, and it has become possible to fire all the mixed powders in the region with a low carbon dioxide gas concentration. As a result, unreacted lithium carbonate can be reduced, and the solid-phase reaction is promoted uniformly. Thereby, the amount of unreacted lithium carbonate can be effectively reduced, and productivity can be maintained.

[0022] [Positive electrode active material] The positive electrode active material according to the present embodiment has an α-NaFeO crystal structure 2It is composed of a lithium composite oxide (hereinafter referred to as a lithium composite compound) having a layered structure and composed of lithium and a transition metal. The main phase of the positive electrode active material is a lithium composite compound having a layered structure. However, as will be described later, depending on the manufacturing conditions, the positive electrode active material may contain lithium carbonate as an inevitable impurity. Lithium carbonate may exist in a state where it forms a heterogeneous phase or adheres to the particles of the positive electrode active material.

[0023] The positive electrode active material according to the present embodiment has the following formula (1) Li 1+a M1O 2+α ···(1) (However, in the formula (1), M1 is a metal element other than Li and contains at least Ni, the proportion of Ni in M1 is 70 atomic% or more, and a and α are numbers satisfying -0.1 ≤ a ≤ 0.2 and -0.2 ≤ α ≤ 0.2.) It is represented by

[0024] The positive electrode active material according to the present embodiment is a positive electrode active material capable of realizing a high energy density and a high charge-discharge capacity by having a composition in which the proportion of nickel (Ni) per metal element (M1) other than lithium (Li) is 70 atomic% or more. The proportion of nickel (Ni) per metal element (M1) other than lithium (Li) can be appropriately set within the range of 70 atomic% or more and 100 atomic% or less. Since it is a positive electrode active material containing nickel at a high ratio in this way, Ni 2+ to Ni 3+ It is important that the oxidation reaction that oxidizes is efficiently performed.

[0025] A more preferable specific composition of the positive electrode active material according to the present embodiment is the following formula (2): Li 1+a Ni b Co c M d X e O 2+α ···(2) [However, in the compositional formula (2), M represents at least one selected from Al and Mn, X represents one or more metal elements other than Li, Ni, Co, Al, and Mn, and a, b, c, d, e, and α are, respectively, -0.1 ≦ a ≦ 0.2, 0.7 ≦ b ≦ 1.0, 0 ≦ c ≦ 0.20, 0 ≦ d ≦ 0.20, 0 ≦ e ≦ 0.2, b + c + d + e = 1, and -0.2 < α < 0.2. It is represented by.]

[0026] Since the cathode active material represented by the formula (2) has a high nickel content, it can exhibit a high charge-discharge capacity in the range up to around 4.3 V compared with LiCoO 2 etc. Further, since the nickel content is high, it is a cathode active material with a low raw material cost and easy availability of raw materials compared with LiCoO 2 etc.

[0027] Here, the significance of the numerical ranges of a, b, c, d, e, and α in the formulas (1) and (2) will be described.

[0028] In the formula, a is -0.1 or more and 0.2 or less. a represents the stoichiometric ratio of the lithium composite compound represented by the general formula: LiM1O 2 That is, it represents the excess or deficiency amount of lithium from Li:M1:O = 1:1:2. If lithium is excessively low, the charge-discharge capacity of the cathode active material will be low. On the other hand, if lithium is excessively high, the charge-discharge cycle characteristics will deteriorate. If a is within the above numerical range, high charge-discharge capacity and good charge-discharge cycle characteristics can be achieved simultaneously.

[0029] a may be -0.02 or more and 0.05 or less. If a is -0.02 or more, a sufficient amount of lithium for contributing to charge-discharge can be ensured, so that the charge-discharge capacity of the cathode active material can be increased. Also, if a is 0.05 or less, charge compensation due to the valence change of the transition metal can be sufficiently achieved, so that high charge-discharge capacity and good charge-discharge cycle characteristics can be achieved simultaneously.

[0030] The coefficient b of nickel shall be 0.7 or more and 1.0 or less. When b is 0.7 or more, a sufficiently high charge-discharge capacity can be obtained as compared with the case of using other transition metals. Therefore, if b is within the above numerical range, a positive electrode active material exhibiting a high charge-discharge capacity can be manufactured at a lower cost compared with 2 LiCoO and the like.

[0031] Preferably, b shall be 0.8 or more and 0.95 or less, and more preferably 0.85 or more and 0.95 or less. The larger b is when it is 0.8 or more, the higher the charge-discharge capacity obtained. Also, the smaller b is when it is 0.95 or less, the smaller the lattice distortion or crystal structure change accompanying the insertion and desorption of lithium ions becomes, and it becomes difficult for cation mixing in which nickel is mixed into the lithium site or a decrease in crystallinity to occur during firing. Therefore, deterioration of the charge-discharge capacity and charge-discharge cycle characteristics is suppressed.

[0032] The coefficient c of cobalt shall be 0 or more and 0.20 or less. When cobalt is added, effects such as stabilization of the crystal structure and suppression of cation mixing in which nickel is mixed into the lithium site can be obtained. Therefore, the charge-discharge capacity can be improved without significantly impairing it, and the charge-discharge cycle characteristics can be improved. On the other hand, if cobalt is excessive, the raw material cost increases, so the manufacturing cost of the positive electrode active material increases. If c is within the above numerical range, it is possible to achieve both a high charge-discharge capacity and good charge-discharge cycle characteristics with good productivity.

[0033] c may be 0.01 or more and 0.20 or less, or may be 0.03 or more and 0.20 or less. The larger c is when it is 0.01 or more, the more sufficient the effect of element substitution of cobalt is obtained, and the more improved the charge-discharge cycle characteristics are. Also, if c is 0.20 or less, the raw material cost becomes lower, and the productivity of the positive electrode active material becomes better.

[0034] The coefficient d of M shall be 0 or more and 0.20 or less. When at least one element (M) selected from the group consisting of manganese and aluminum is elementally substituted, the layered structure is more stably maintained even when lithium is desorbed by charging. On the other hand, when these elements (M) are excessive, the ratio of other transition metals such as nickel decreases, and the charge-discharge capacity of the positive electrode active material decreases. If d is within the above numerical range, the crystal structure of the positive electrode active material can be stably maintained, and good charge-discharge cycle characteristics, thermal stability, etc. can be obtained together with a high charge-discharge capacity.

[0035] As the element represented by M, manganese and aluminum are preferable. Such elements contribute to the stabilization of the crystal structure of the positive electrode material having a high nickel content. Among them, manganese is particularly preferable. When manganese is elementally substituted, a higher charge-discharge capacity can be obtained as compared with the case where aluminum is elementally substituted. Further, during the firing of the lithium composite compound, manganese also reacts with lithium carbonate as shown in the following formula (3). By such a reaction, the coarsening of crystal grains is suppressed, and the oxidation reaction of nickel can proceed at a high temperature, so that a positive electrode active material showing a high charge-discharge capacity can be efficiently obtained.

[0036] Li 2 CO 3 +2M´O+0.5O 2 →2LiM´O 2 +CO 2 ···(3) (However, in the above formula (3), M´ represents a metal element such as Ni, Co, Mn, etc.)

[0037] The coefficient d of M is preferably 0.02 or more, and more preferably 0.04 or more. The larger the coefficient d of M, the more sufficient the effect of elemental substitution of manganese can be obtained. That is, the oxidation reaction of nickel can proceed at a higher temperature, and a positive electrode active material showing a high charge-discharge capacity can be obtained more efficiently. Further, the coefficient d of M is preferably 0.18 or less. If the coefficient d of M is 0.18 or less, the charge-discharge capacity can be kept high even when elementally substituted.

[0038] The coefficient e of X shall be 0 or more and 0.20 or less. X represents one or more metal elements other than Li, Ni, Co, Al, and Mn. However, when at least one element selected from the group consisting of magnesium, titanium, zirconium, molybdenum, and niobium is elementally substituted, various performances such as charge-discharge cycle characteristics can be improved while maintaining the activity of the positive electrode active material. On the other hand, when these elements (X) are excessive, the proportion of other transition metals such as nickel becomes low, and the charge-discharge capacity of the positive electrode active material decreases. If e is within the above numerical range, high charge-discharge capacity and good charge-discharge cycle characteristics and the like can be achieved simultaneously.

[0039] α in the above formulas (1) and (2) shall be -0.2 or more and 0.2 or less. α represents the stoichiometric ratio of the lithium composite compound represented by the general formula: LiM´O 2 That is, it represents the excess or deficiency amount of oxygen from Li:M´:O = 1:1:2. If α is within the above numerical range, the crystal structure has few defects, and high charge-discharge capacity and good charge-discharge cycle characteristics can be obtained.

[0040] [Manufacturing method of positive electrode active material] Next, the manufacturing method of the positive electrode active material will be described. The above-described positive electrode active material can be manufactured, for example, according to general manufacturing methods of positive electrode active materials such as the solid-phase method, coprecipitation method, sol-gel method, and hydrothermal method. The present invention is based on a method of synthesizing using a lithium compound as a raw material by the solid-phase method among these methods. Examples of the compound containing lithium include lithium carbonate, lithium acetate, lithium nitrate, lithium hydroxide, lithium chloride, lithium sulfate, etc. Among them, it is preferable to use lithium carbonate, and it is more preferable to use lithium carbonate at a ratio of 80% by mass or more in the raw material containing lithium. Lithium carbonate is excellent in supply stability and inexpensive compared to other compounds containing lithium, so it can be easily obtained. In addition, since lithium carbonate is weakly alkaline, it causes little damage to the manufacturing equipment and is excellent in industrial applicability and practicality. Hereinafter, the manufacturing method using lithium carbonate will be described.

[0041] Figure 1 is a flowchart showing an example of a method for manufacturing a positive electrode active material according to an embodiment of the present invention. As shown in Figure 1, the method for manufacturing a positive electrode active material according to this embodiment includes a mixing step S1 and a firing step S2. By passing through these steps, the lithium composite compound represented by the above formulas (1) and (2) is synthesized. Hereinafter, the case of the composition formula (2) will be described as an example.

[0042] In the mixing step S1, lithium carbonate and a compound containing a metal element other than Li in the formula (2) are mixed. That is, as raw materials for the positive electrode active material, lithium carbonate and a nickel compound containing nickel are mixed. Further, when the lithium composite compound represented by the formula (2) contains cobalt, an element represented by M, or an element represented by X, compounds containing these elements are added and mixed.

[0043] In the mixing step S1, at least lithium carbonate is used as a raw material containing lithium. As described above, it is inexpensive and easily available, and is preferable because it causes less damage to the manufacturing equipment. Further, since lithium carbonate has a relatively high melting point, it is possible to avoid the formation of a liquid phase and the coarsening of crystal grains during synthesis by the solid-phase method.

[0044] In the mixing step S1, nickel compounds such as nickel hydroxide, nickel carbonate, nickel oxide, nickel sulfate, and nickel acetate can be used as raw materials containing nickel. Among these nickel compounds, it is particularly preferable to use nickel hydroxide, nickel carbonate, or nickel oxide.

[0045] In the mixing step S1, nitrates, carbonates, sulfates, acetates, oxides, hydroxides, etc. can be used as cobalt compounds containing cobalt, metal compounds containing the element represented by M, and metal compounds containing the element represented by X. Among these, it is particularly preferable to use carbonates, oxides, or hydroxides.

[0046] In the mixing step S1, raw materials such as lithium carbonate are weighed, pulverized, and mixed to obtain a powdery mixed powder. As the pulverizer for pulverizing the raw materials, for example, general precision pulverizers such as ball mills, jet mills, and sand mills can be used. The pulverization of the raw materials may be either dry pulverization or wet pulverization. The raw material slurry obtained by wet pulverization can be dried using various dryers such as spray dryers, fluidized bed dryers, and evaporators.

[0047] In the mixing step S1, it is preferable to pulverize raw materials such as lithium carbonate until the average particle size becomes 0.5 μm or less, the average particle size of the slurry is 0.1 μm or more and 0.3 μm or less, and it is more preferable to pulverize until the average particle size becomes 0.2 μm or less. When the raw materials are pulverized to such a fine particle size, the reactivity between lithium carbonate and nickel compounds, etc. is improved, and components such as carbon dioxide are easily desorbed from the raw materials such as lithium carbonate. In addition, the mixing degree of the pulverized material is increased, and the firing proceeds uniformly, and the average particle size of the primary particles of the lithium composite compound can be easily controlled within an appropriate range.

[0048] In the mixing step S1, it is preferable to prepare a slurry by mixing with wet pulverization and granulate the obtained slurry by spray drying. When raw materials such as lithium carbonate are wet pulverized and mixed in a medium such as water and the obtained slurry is granulated by spray drying, after firing, the average particle size of the secondary particles is controlled, and secondary particles with an aspect ratio approximated to 1 can be stably obtained. As the spray dryer, various methods such as two-fluid nozzle type, four-fluid nozzle type, and disk type can be used.

[0049] The average particle size of the secondary particles granulated by spray drying is preferably 5 μm or more and 25 μm or less, and more preferably 5 μm or more and 20 μm or less. When granulated to such a particle size, the average particle size of the secondary particles of the lithium composite compound obtained after firing can be easily made within the target range. The average particle size of the secondary particles can be controlled, for example, by adjusting the concentration of the slurry, the viscosity of the slurry, the spraying amount of the slurry, the dispersion degree of the suspended matter, the spraying temperature, the spraying pressure, the air blowing speed, etc.

[0050] In the firing step S2, the unfired mixed powder obtained through the mixing step S1 is fired to obtain the lithium composite compound represented by the formula (2). In the firing step S2, the mixed powder that has been subjected to an appropriate treatment such as spray granulation through the mixing step S1 is fired under predetermined conditions to form a lithium composite compound having a layered structure. Note that the firing step S2 may be performed by a single heat treatment in which the heat treatment temperature is controlled within a certain range, or may be performed by a plurality of heat treatments in which the heat treatment temperatures are controlled within different ranges.

[0051] As shown in FIG. 1, the firing step S2 preferably includes a first heat treatment step S21, a second heat treatment step S22, and a third heat treatment step S23. By performing such a plurality of heat treatments, by adjusting the heat treatment temperature, heat treatment time, atmosphere, etc., while growing the particle size of the lithium composite compound within an appropriate range, the amount of lithium carbonate remaining in the lithium composite compound at the end of firing can be greatly reduced. Therefore, it is possible to achieve both a high charge-discharge capacity and good charge-discharge cycle characteristics, etc.

[0052] In the first heat treatment step S21, the mixed powder obtained in the mixing step S1 is heat-treated at a heat treatment temperature of 200°C or higher and 500°C or lower for 0.5 hours or longer and 5 hours or shorter to obtain a precursor. The first heat treatment step S21 is mainly performed to remove moisture and the like that hinder the synthesis reaction of the lithium composite compound from the mixed powder obtained in the mixing step S1. The first heat treatment step can be replaced with other conditions as long as the purpose can be achieved. The first heat treatment step can be performed arbitrarily.

[0053] In the first heat treatment step S21, if the heat treatment temperature is 200°C or higher, the combustion reaction of impurities and the thermal decomposition of raw materials, etc., proceed sufficiently, so it is possible to suppress the formation of inactive heterogeneous phases in subsequent heat treatments. Also, if the heat treatment temperature is 500°C or lower, since the crystals of the lithium composite compound are hardly formed in this step, it is possible to prevent the formation of a low-purity crystal phase in the presence of a gas containing moisture, impurities, etc.

[0054] The heat treatment temperature in the first heat treatment step S21 is preferably 250°C or higher and 400°C or lower, and more preferably 250°C or higher and 380°C or lower. If the heat treatment temperature is within this range, moisture, impurities, etc. can be efficiently removed, while it is possible to surely prevent the formation of crystals of the lithium composite compound in this step. Note that the heat treatment time in the first heat treatment step S21 can be appropriately set according to, for example, the heat treatment temperature, the amount of moisture and impurities contained in the mixed powder, the removal target of moisture and impurities, the degree of crystallization, etc.

[0055] The first heat treatment step S21 is preferably carried out under the airflow of the atmosphere gas or under evacuation by a pump. By performing the heat treatment in such an atmosphere, the gas containing moisture, impurities, etc. can be efficiently removed. The flow rate of the airflow of the atmosphere gas and the evacuation amount per unit time by the pump are preferably larger than the volume of the gas generated from the mixed powder. The volume of the gas generated from the mixed powder can be obtained based on, for example, the usage amount of the raw material and the molar ratio per raw material of the components gasified by combustion or thermal decomposition.

[0056] The first heat treatment step S21 may be carried out in an oxidizing gas atmosphere, a non-oxidizing gas atmosphere, or a reduced pressure atmosphere. As the oxidizing gas atmosphere, either an oxygen gas atmosphere or an air atmosphere may be used. Further, as the reduced pressure atmosphere, for example, a reduced pressure condition with an appropriate degree of vacuum such as below atmospheric pressure may be used.

[0057] In the second heat treatment step S22, the precursor obtained in the first heat treatment step S21 is heat treated at a heat treatment temperature of 600°C or higher and lower than 750°C for 10 hours or longer and 100 hours or shorter to obtain a fired body. The second heat treatment step S22 is mainly carried out for the purpose of removing the carbonate component by the reaction of lithium carbonate and nickel compound, etc., oxidizing nickel from divalent to trivalent, and generating crystals of the lithium composite compound.

[0058] In the second heat treatment step S22, 95% by mass or more of the lithium carbonate introduced as a raw material is reacted. It is preferable to react 97% by mass or more, and more preferably 98% by mass or more. If the reaction of lithium carbonate is insufficient and the oxidation of nickel is insufficient, divalent nickel is likely to substitute for the lithium site, and there is a risk that the charge-discharge capacity of the positive electrode active material will decrease. Further, if a large amount of lithium carbonate remains at the end of the second heat treatment step S22, carbon dioxide is released into the atmosphere of the third heat treatment step S23, inhibiting the reaction to form crystals or causing the carbonate component to be incorporated into the crystals of the lithium composite compound, which may reduce the particle breakage strength and charge-discharge capacity. On the other hand, if most of the lithium carbonate is reacted in the second heat treatment step S22, a lithium composite compound exhibiting a high charge-discharge capacity can be fired in the third heat treatment step S23.

[0059] Also, if the reaction of lithium carbonate is insufficient and a large amount of lithium carbonate remains at the end of the second heat treatment step S22, there is a possibility that lithium carbonate melts and forms a liquid phase in the third heat treatment step S23. Firing the lithium composite compound in the liquid phase may cause the crystal grains to coarsen and deteriorate the output characteristics of the lithium-ion secondary battery. On the other hand, if most of the lithium carbonate is reacted in the second heat treatment step S22, it becomes difficult for a liquid phase to form in the third heat treatment step S23. Therefore, even if the heat treatment temperature is increased, the crystal grains of the lithium composite compound are less likely to coarsen. Therefore, while ensuring the output characteristics of the lithium-ion secondary battery, the oxidation of nickel can be advanced at a high temperature to fire a lithium composite compound exhibiting a high charge-discharge capacity.

[0060] In the second heat treatment step S22, if the heat treatment temperature is 600°C or higher, the formation of a layered structure proceeds due to the reaction between lithium carbonate and a nickel compound, etc., so that it is possible to prevent unreacted lithium carbonate from remaining. Therefore, it becomes difficult for lithium carbonate to form a liquid phase in subsequent heat treatments, the coarsening of crystal grains is suppressed, and a positive electrode active material exhibiting high particle breakage strength and charge-discharge capacity can be obtained. Further, if the heat treatment temperature is less than 750°C, grain growth does not proceed excessively, so that the charge-discharge capacity of the positive electrode active material increases.

[0061] The heat treatment temperature in the second heat treatment step S22 is preferably 650 °C or higher. The higher the heat treatment temperature, the more the reaction of lithium carbonate is promoted, and the more reliably the residual of lithium carbonate is prevented. In particular, when substituting manganese as the element represented by M in the formula (2), when the coefficient d of manganese exceeds 0 and is less than 0.075, it is preferably 600 °C or higher. On the other hand, when the coefficient d of manganese is 0.075 or more, since the reaction temperature decreases, it may be 550 °C or higher.

[0062] The heat treatment temperature in the second heat treatment step S22 is preferably 700 °C or lower, and more preferably 680 °C or lower. The lower the heat treatment temperature, the more grain growth is suppressed, so the charge and discharge capacity of the positive electrode active material increases. In addition, since lithium carbonate is less likely to melt and a liquid phase is less likely to be formed, coarsening of crystal grains can be more reliably suppressed.

[0063] The heat treatment time in the second heat treatment step S22 is preferably 10 hours or more and 50 hours or less, and more preferably 20 hours or more and 30 hours or less. When the heat treatment time is within this range, the reaction of lithium carbonate proceeds sufficiently, so that the carbonate component can be surely removed. In addition, the required time for heat treatment is shortened, and the productivity of the positive electrode active material is improved.

[0064] The second heat treatment step S22 is preferably carried out in an oxidizing atmosphere. The oxygen concentration of the atmosphere is preferably 80% or more, more preferably 90% or more, still more preferably 95% or more, and particularly preferably 100%. In addition, the carbon dioxide concentration of the atmosphere is preferably 5% or less, more preferably 1% or less, and still more preferably 0.1% or less. In addition, the second heat treatment step S22 is preferably carried out under a stream of oxidizing gas. When heat treatment is carried out under a stream of oxidizing gas, nickel can be surely oxidized, and carbon dioxide released into the atmosphere can be surely removed. The second heat treatment step S22 may be carried out as a calcination step in a plurality of times.

[0065] In the third heat treatment step S23, the fired body obtained in the second heat treatment step S22 is heat treated at a heat treatment temperature of 750°C or higher and 900°C or lower for 0.5 hours or more and 50 hours or less to obtain a lithium composite compound. The main purpose of the third heat treatment step S23 is to sufficiently oxidize nickel in the fired body from divalent to trivalent and to grow the crystal grains of the lithium composite compound having a layered structure to an appropriate size.

[0066] In the third heat treatment step S23, if the heat treatment temperature is 750°C or higher, nickel can be sufficiently oxidized to grow the primary particles to an appropriate particle size. Therefore, a lithium composite compound with high crystallinity and low open pore volume ratio is fired. As a result, the residual amount of lithium carbonate during firing and the amount of contamination after firing are reduced, and a positive electrode active material with a high charge-discharge capacity can be obtained. Also, if the heat treatment temperature is 900°C or lower, lithium is less likely to volatilize and the decomposition of the lithium composite compound having a layered structure is suppressed, so it is possible to avoid a decrease in the purity of the crystal obtained after firing and a decrease in the charge-discharge capacity.

[0067] The heat treatment temperature in the third heat treatment step S23 is preferably 800°C or higher, more preferably 840°C or higher, and even more preferably 850°C or higher. The higher the heat treatment temperature is in this way, the more surely nickel is oxidized and the grain growth of the lithium composite compound can be promoted.

[0068] The heat treatment temperature in the third heat treatment step S23 is preferably 890°C or lower. The lower the heat treatment temperature is in this way, the less likely lithium is to volatilize, so the decomposition of the lithium composite compound having a layered structure can be more surely prevented and a lithium composite compound showing a high charge-discharge capacity can be obtained.

[0069] In the third heat treatment step S23, the heat treatment time is preferably 0.5 hours or more and 15 hours or less. When the heat treatment time is within this range, nickel can be sufficiently oxidized to obtain a lithium composite compound with high crystal purity and a high charge-discharge capacity. In addition, the required time for heat treatment is shortened, and the productivity of the positive electrode active material is improved.

[0070] The third heat treatment step S23 is preferably performed in an oxidizing atmosphere. The oxygen concentration of the atmosphere is preferably 80% or more, more preferably 90% or more, still more preferably 95% or more, and particularly preferably 100%. In addition, the carbon dioxide concentration of the atmosphere is preferably 5% or less, more preferably 1% or less, and still more preferably 0.1% or less. Further, the third heat treatment step S23 is preferably performed under a gas flow of an oxidizing gas. When the heat treatment is performed under a gas flow of an oxidizing gas, nickel can be surely oxidized, and carbon dioxide released into the atmosphere can be surely removed. The third heat treatment step S23 is preferably performed once as the main firing step, but may be performed a plurality of times.

[0071] By passing through the above mixing step S1 and firing step S2, the lithium composite compound represented by the formula (2) can be obtained. The lithium composite compound may be classified as necessary so that the average particle diameter of the secondary particles becomes appropriate.

[0072] In the firing process S2, as the firing furnace for heat treatment, a rotary furnace such as a rotary kiln, a roller hearth kiln, a tunnel furnace, a continuous furnace such as a pusher furnace, or an appropriate heating device such as a batch furnace can be used. The first heat treatment step S21, the second heat treatment step S22, and the third heat treatment step S23 may be carried out using the same heating device or different heating devices. Further, each heat treatment step may be carried out intermittently by changing the atmosphere, or may be carried out continuously when performing the heat treatment while exhausting the gas in the atmosphere. However, in the present invention, the firing process of the above-described second heat treatment step S22 is carried out using a rotary furnace such as a rotary kiln. Hereinafter, the batch firing treatment process using a rotary kiln will be described.

[0073] [Batch Firing Treatment Process] The rotary kiln has a hearth tube that can rotate and tilt, an inlet for supplying the precursor, and an outlet for discharging the fired body after heat treatment. Usually, in a rotary kiln, the inclined hearth tube rotates to advance the powder while rolling it for firing. In the present invention, a batch type that ensures the holding time of the fired body and separately performs the charging and discharging operations is adopted in the rotary kiln, and this is called the batch firing treatment process in the present invention. As shown in FIG. 2, this firing process includes an inclined charging stage (a) in which the furnace is in an inclined state and the precursor is charged into the heating region, a horizontal firing stage (b) in which the furnace is in a horizontal state and the precursor is evenly fired while being held in the heating region of the furnace for a long time, and an inclined discharging stage (c) in which the furnace is in an inclined state and the fired body with the progress of the firing reaction is discharged.

[0074] In the inclined feeding stage, the precursor (mixed powder) is fed into the hearth tube of the rotary kiln. Inside the hearth tube, there is a heating region in the central part of the furnace, and a discharge-side spiral blade from the central part to the outlet of the furnace and a supply-side spiral blade to the inlet of the furnace are provided respectively. The supply-side spiral blade and the discharge-side spiral blade are cut with reverse spiral grooves. Thus, when the discharge-side spiral blade and the supply-side spiral blade are rotated, the spiral grooves rotate in opposite directions respectively, and the powder placed on the spiral grooves can be moved in opposite directions. Here, by rotating the supply-side spiral blade and the discharge-side spiral blade in the forward rotation direction, the fed precursor can be scraped and collected in the heating region in the central part of the furnace, that is, it can be moved to the heating region and concentrated.

[0075] In the horizontal firing stage, the hearth tube is set to a horizontal state with an inclination angle of 0 degrees, and the hearth tube is rotated in the forward rotation direction at a rotation speed of about 1 to 3 rpm while starting the heating and firing. When the rotation direction of the hearth tube is the forward rotation direction and the rotation speed is about 1 rpm, the firing time is about 15 to 50 hours, so that the firing can proceed while rolling the precursor and keeping the fired body in the heating region.

[0076] In the inclined discharge stage, after the batch firing process in the horizontal firing stage is carried out for a predetermined time, the inclination angle of the hearth tube is changed to, for example, 3 degrees, and further the rotation directions of the discharge-side spiral blade and the supply-side spiral blade are changed from the forward rotation direction to the reverse rotation direction. As a result, the spiral grooves rotate in opposite directions respectively, and the fired body in the furnace is moved to the outlet side by the discharge-side spiral blade and discharged. On the other hand, the inclination angle of the rotary kiln can be, for example, about 1 to 5 degrees, preferably 2 to 3 degrees. By making the angle steeper, the fired body that has reached the outlet can be taken out in a short time. At this time, the rotation speed can also be increased.

[0077] The supply-side helical blade and the discharge-side helical blade each have helical grooves cut in opposite directions as described above. Also, as an example of a lifter in the heating region, there is a configuration example as shown in FIG. 4. FIG. 4(a) is an example in which cylindrical convex pieces are arranged in a helical shape, (b) is an example in which cylindrical convex pieces are arranged in the axial direction and the radial direction, and (c) is an example in which it is in the shape of a helical blade. In all cases, the height HL of the lifter F is lower than the height H of the blades in the other outer regions. The reason for this is that the blades in the regions other than the heating region need to retain the fired powder in the heating region. Therefore, the supply-side and discharge-side helical blades outside the heating region need to be higher than the filling height of the fired powder to prevent the fired powder from leaking outside the heating region. On the other hand, in the heating region, it is important to circulate the fired powder. Therefore, a lifter (agitating blade) with an agitating action is used. The lifter does not need to be high and rather should be low and shaped so that it can easily scoop up in small increments, enabling the precursor in the heating region to be fired while circulating in the longitudinal direction within the heating region. During firing, the oxygen concentration in the heating region is higher on the discharge side than on the supply side. Therefore, if there is no circulation, the fired powder will be fired in a state where there is always a difference in oxygen concentration even within the heating region. As a result, it is easy to form fired bodies with different degrees of progress of the solid-phase reaction in the fired powder on the supply side and the discharge side. On the other hand, when there is circulation, the fired body in the region with a low oxygen concentration can be moved to the region with a high oxygen concentration, and it becomes possible to fire all the mixed powder in the region with a high oxygen concentration (low carbon dioxide gas concentration). As a result, unreacted lithium carbonate can be reduced.

[0078] The lifter in the heating region described above may be formed continuously and integrally with the supply-side helical blade and / or the discharge-side helical blade described above, or may be formed separately. However, a height of about 1 / 3 to 2 / 3 of the filling height of the fired powder is sufficient. Also, as the shape of the lifter, any shape that can easily scoop up and cause circulation is acceptable, but it is preferably the same helical shape as the above helical blade.

Example

[0079] Hereinafter, the present invention will be specifically described with reference to examples. Note that the technical scope of the present invention is not limited thereto.

[0080] [Example 1] As starting materials for the positive electrode active material, lithium carbonate, nickel hydroxide, cobalt carbonate, manganese carbonate, and titanium oxide were prepared. These starting materials were weighed so that the atomic ratio of Li:Ni:Co:Mn:Ti was 1.04:0.85:0.03:0.10:0.02, and the mixing step S1 was carried out. Specifically, ion-exchanged water was added and mixed so that the total weight of the starting materials was 20 mass%, and pulverization and mixing were carried out using a bead mill to obtain a pulverized powder with an average particle size of 0.16 μm. The obtained solid-liquid mixture was dried using a spray dryer to obtain a raw material mixed powder with an average particle size of 11.6 μm.

[0081] Next, the obtained raw material mixed powder was filled into a firing container made of alumina, and heat treatment (first heat treatment step S21) was carried out at 360 °C for 1 hour in an air atmosphere using a roller hearth kiln to obtain a precursor (mixture). By the first heat treatment step S21, not only the moisture absorbed by the raw material mixed powder was removed, but also the thermal decomposition of nickel hydroxide and the partial thermal decomposition of each carbonate were carried out, and a certain amount of carbon dioxide gas was removed.

[0082] Next, the rotary kiln furnace described in FIG. 2 was set at an inclination angle of 3 degrees, and the precursor C equivalent to a filling rate of 30% was charged (inclined charging stage). Next, with the inclination angle set to 0 degrees in the horizontal state, while flowing an oxygen stream from the outlet side toward the inlet side in the hearth tube rotating forward at 1 rpm, a batch heat treatment was carried out at 700 °C for 15 hours in an oxidizing atmosphere with an oxygen concentration of 95% or more. Here, in the rotary kiln of this configuration, the spiral blades are attached along the inner peripheral surface of the hearth tube, and the supply-side spiral blade B and the discharge-side spiral blade A are composed of spiral blades wound in opposite directions. During forward rotation, the spiral blades on the supply (inlet) side and the discharge (outlet) side move the precursor to the heating region D. Here, in order to keep the fired body in the regions other than the heating region in the heating region D, the height of the spiral blades is set to be higher than the filling height of the fired powder. On the other hand, the height of the lifter F installed in the heating region to circulate the fired body in the heating region is lower than that of the supply-side and discharge-side spiral blades, and is about half the height of the other parts. Since the blades have different heights in the heating region and the other regions in this way, the fired body in the heating region can circulate within the heating region (horizontal firing stage).

[0083] After the batch heat treatment, the hearth tube was tilted at an inclination angle of 3 degrees, the rotation speed was 3 rpm, and the rotation direction of the hearth tube was reversed to discharge the fired precursor (i.e., the fired body) E (inclined discharge stage). During reverse rotation, the supply-side spiral blade B moves the fired body to the inlet side, and the discharge-side spiral blade A moves the fired body to the outlet side. Then, when discharging the fired powder after batch operation, it was collected by weight corresponding to 10% of the total input amount, and the lithium carbonate elution amount of each collected powder was measured to confirm that it was uniformly fired from the discharge side to the supply side. Note that the powder discharged first was the fired body on the outlet side in the heating region, and the powder discharged last can be said to be the fired body on the inlet side in the heating region. Therefore, by collecting in batches, the firing variation in the furnace during batch operation can be observed. The results are shown together in FIG. 3.

[0084] Next, the obtained fired body was heat-treated at 840 °C for 10 hours in an oxidation atmosphere with an oxygen concentration of 95% or more using a roller hearth kiln (third heat treatment step S23), and Li 1.0 Ni 0.85 Co 0.03 Mn 0.10 Ti 0.02 O 2 A fired powder of a lithium composite compound having the composition of was obtained. The obtained fired powder of the lithium composite compound was classified to have an opening size of 45 μm or less and used as a positive electrode active material.

[0085] Regarding the fired body obtained by the batch firing treatment in Example 1, the elution amount of lithium carbonate from the fired body and the reaction rate of lithium carbonate are shown in Table 1, and the transition of the elution amount with respect to the cumulative recovery weight is shown in FIG. 3. In addition, the elution amount of lithium carbonate of the positive electrode active material (lithium composite compound) is shown in Table 1. The measurement method is described below.

[0086] [Example 2] A fired body was obtained in the same manner as in Example 1 except that a rotary kiln without a lifter in the heating region was used in the second heat treatment step and the holding time was 30 hours. In the batch operation, a precursor equivalent to a filling rate of 30% was charged and operated at 700 °C for 30 hours in an oxidizing atmosphere with an oxygen concentration of 95% or more. Then, all the fired bodies in the furnace were discharged. Also at this time, when the second heat treatment step was completed, the fired bodies were divided and recovered in the same manner as in Example 1, and the elution amount of lithium carbonate of each fired body was measured. The results are also shown in FIG. 3. In addition, the elution amount and reaction rate of lithium carbonate of the fired body were measured in the same manner as in Example 1. The results are shown in Table 1. Next, a positive electrode active material was obtained in the same manner as in Example 1 using the above fired body. The elution amount of lithium carbonate of this positive electrode active material was measured in the same manner as in Example 1. The results are shown in Table 1.

[0087] [Example 3] A fired body was obtained in the same manner as in Example 2, except that the operation time of the second heat treatment step was 18 hours. At this time as well, the fired body was divided and recovered in the same manner as in Example 1, and the elution amount of lithium carbonate in each fired body was measured. The results are shown together in FIG. 3. Also, the elution amount and reaction rate of lithium carbonate in the fired body were measured in the same manner as in Example 1. The results are shown in Table 1. Next, a positive electrode active material was obtained in the same manner as in Example 1 using the above fired body. The elution amount of lithium carbonate in this positive electrode active material was measured in the same manner as in Example 1. The results are shown in Table 1.

[0088] [Comparative Example 1] A raw material mixed powder was prepared in the same manner as in Example 1, except that the starting materials were weighed so that the atomic ratio of Li:Ni:Co:Mn:Ti was 1.04:0.90:0.03:0.05:0.02, and the first heat treatment was performed. The second heat treatment step was carried out using a continuous operation type rotary kiln in which the raw material powder was continuously supplied and passed through the heating zone while the core tube was rotated at 1 rpm and inclined at an inclination angle of 1 degree. Otherwise, a fired body was obtained in the same manner as in Example 1. The charging rate in the continuous operation was 120 g / hr (equivalent to a filling rate of 3%). The continuous operation type is a method in which the precursor is supplied with the core tube inclined, fired while being rolled forward as it is, and the fired body is continuously discharged. However, in the second heat treatment step, the fired body adhered to the inner wall and could not be discharged.

[0089] [Comparative Example 2] Heat treatment was carried out in the same manner as in Comparative Example 1, except that the heat treatment temperature in the second heat treatment step was set to 650°C. In Comparative Example 2, the fired body could be recovered. Here, in the case of continuous operation, the holding time was 1 hour per pass through the rotary kiln, and a total of 5 passes were made to perform a heat treatment equivalent to 5 hours. The fired body was recovered in the same manner as in Example 1, in portions equivalent to 10% of the total input weight, and the amount of lithium carbonate eluted from the obtained fired body was measured. Further, the reaction rate was calculated from the results. The results are shown in Table 1. Although adhesion could be avoided by lowering the temperature, the amount of lithium carbonate eluted was large, and the reaction rate was less than 95%. Also, when the third heat treatment step was carried out in a continuous rotary kiln, the amount of lithium carbonate eluted was as much as 0.40%, and lithium carbonate could not be reacted sufficiently.

[0090] (Amount of lithium carbonate eluted) The amount of lithium carbonate eluted from the fired body or the positive electrode active material was measured by neutralization titration using an automatic titrator "COM-1700A" (manufactured by Hiranuma Sangyo Co., Ltd.). First, 0.5 g of the positive electrode active material was dispersed in 30 mL of pure water bubbled with argon gas. Then, after stirring the dispersion for 60 minutes, the filtrate was recovered by suction filtration, and the filtrate was titrated with hydrochloric acid. The titration curve had two stages. Up to the first equivalence point corresponded to the total amount of hydroxide ions of lithium hydroxide and carbonate ions of lithium carbonate, and from the first equivalence point to the second equivalence point corresponded to the amount of hydrogen carbonate ions generated from the carbonate ions. Therefore, the amount (mass%) of lithium carbonate was calculated from the titration amount from the first equivalence point to the second equivalence point.

[0091] (Reaction rate of lithium carbonate) The reaction rate was calculated based on the following formula (4). Reaction rate (%) = [(L1 - L2) ÷ L1] × 100 ··· (4) L1: Amount of lithium carbonate eluted after the first heat treatment (precursor) L2: Amount of lithium carbonate eluted after the second heat treatment (fired body)

[0092] (Discharge capacity of the positive electrode active material) Next, a positive electrode and a lithium-ion secondary battery were fabricated using the positive electrode active material obtained in the examples, and the discharge capacity of the lithium-ion secondary battery was evaluated. Note that in Comparative Example 1, the positive electrode active material could not be recovered, and in Comparative Example 2, although it could be recovered, the reaction was insufficient, so no evaluation was performed. First, a 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 current collector made of an aluminum foil with a thickness of 20 μm and dried at 120 °C to form a positive electrode mixture layer. After that, it was compression-molded by pressing so that the electrode density became 2.6 g / cm 3 and punched into a disk shape with a diameter of 15 mm to fabricate a positive electrode.

[0093] Also, a negative electrode was fabricated using metallic lithium as the negative electrode active material. Then, a lithium-ion secondary battery was fabricated using the fabricated positive electrode and negative electrode. As the non-aqueous electrolyte, LiPF was dissolved in a solvent in which ethylene carbonate and dimethyl carbonate were mixed at a volume ratio of 3:7 so that the concentration became 1.0 mol / L. 6 A solution was used. For each of the fabricated lithium-ion secondary batteries, charging was performed at a constant current and constant voltage up to the charging cut-off voltage (4.3 V) with a charging current of 0.2 CA, and discharging was performed at a constant current up to the discharging cut-off voltage (3.3 V or 2.5 V) with a discharging current of 0.2 CA to measure the discharge capacity.

[0094] Table 1 shows the chemical composition of the positive electrode active material, the firing temperature and firing time (the heat treatment temperature and heat treatment time of the second and third heat treatment steps), etc., related to the above examples and comparative examples, as well as the elution amount of lithium carbonate, the reaction rate, and the measurement results of the discharge capacity. Also, FIG. 3 shows the transition of the elution amount of lithium carbonate in the fired bodies of Example 1, Example 2, and Example 3.

[0095]

Table 1-1

[0096]

Table 1-2

[0097] According to FIG. 3, in Example 1 and Example 2, the working of the helical blades results in little variation in the elution amount of lithium carbonate. In particular, in Example 1, by making the height of the lifter (helical blades) in the heating region lower than others, it has become possible to effectively circulate the fired body in the heating region. As a result, the fired body that was conventionally in a region with a high carbon dioxide concentration was only fired in the region with a high carbon dioxide concentration during firing. However, by being more circulated, it was able to react by being exposed to a gas with a high oxygen concentration despite the short firing time. Since all the powder characteristics obtained in Example 1 are excellent, it can be said that a fired body with excellent powder characteristics can be efficiently obtained. In Example 2, the reaction was able to proceed by setting the firing time longer. In Example 3, the holding time is shorter than that of Example 2 (30 hours) by 18 hours. However, there is also variation in the elution amount of lithium carbonate, suggesting that the firing time is insufficient. It can be said that by setting the holding time to 30 hours instead of 18 hours, the amount of lithium carbonate between the recovered lots becomes uniform.

[0098] Also, as shown in Table 1, it has been possible to increase the reaction rate of lithium carbonate in the second heat treatment step and suppress the variation in the elution amount. And the discharge capacity of the positive electrode active material obtained in the third heat treatment step has increased. This is because the elution amount of lithium carbonate could be reduced in the second heat treatment step, that is, the reaction rate of lithium carbonate could be improved. Therefore, in the third heat treatment step, it becomes difficult for lithium carbonate to melt and it becomes difficult to form a liquid phase. As a result, grain growth is more suppressed, and consequently, it is considered that the charge-discharge capacity of the positive electrode active material has increased.

[0099] On the other hand, when the rotary kiln of the comparative example was used in a continuous operation mode, the recovery rate decreased and the elution amount of lithium carbonate increased. Also, in the second heat treatment step, the fired body was not discharged. This is because the viscosity of the powder increases around the melting point of lithium carbonate, and since the powder with increased viscosity was operated at a low rotational speed and a low inclination angle, powder adhesion occurred. Once adhesion occurs, it is considered that the powder has difficulty crossing the step caused by the mass of the adhered material, resulting in an increase in adhesion and reaching a dead end. In addition, even when the rotational speed and inclination angle were increased, it was confirmed that it was difficult to secure the firing time in the heating region and the solid-phase reaction could not proceed.

Explanation of symbols

[0100] S1: Mixing step, S2: Firing step, S21: First heat treatment step, S22: Second heat treatment step, S23: Third heat treatment step A: Discharge side spiral blade, B: Supply side spiral blade, C: Precursor to be input, D: Fired body, E: Discharged fired body, F: Lifter

Claims

1. A mixing step of mixing a lithium compound and a compound containing a metal element other than Li in the following formula (1) to obtain a mixed powder, a first heat treatment step of heat-treating the mixed powder to obtain a precursor, a second heat treatment step of heat-treating the precursor to obtain a fired body, and a third heat treatment step of heat-treating the fired body to obtain a lithium composite compound represented by the following formula (1), in the second heat treatment step, a batch firing treatment process is carried out using a firing furnace having a rotatable hearth tube, in which the precursor is heated while being rolled in a heating region inside the hearth tube, in the batch firing treatment process, an inclined charging stage of tilting the hearth tube and charging the precursor from an inlet of the firing furnace, a horizontal firing stage of making the hearth tube horizontal and oxidizing and firing the precursor in an oxygen atmosphere, an inclined discharging stage of tilting the hearth tube and discharging the fired body from an outlet of the firing furnace, by having the above, more than 95% by mass of the lithium compound is reacted, the hearth tube, a supply-side spiral blade provided between the inlet and the heating region, a discharge-side spiral blade provided between the heating region and the outlet, and the supply-side spiral blade and the discharge-side spiral blade are provided with reverse spiral grooves, In the batch firing treatment process, the precursor is concentrated in the heating region. A method for producing a positive electrode active material for a lithium ion secondary battery, characterized by this. Li １＋ａ M1O ２＋α ...(1) (However, in the formula (1), M1 is a metal element other than Li and contains at least Ni, the proportion of Ni in M1 is 70 atomic% or more, and a and α are numbers satisfying -0.1 ≦ a ≦ 0.2, -0.2 ≦ α ≦ 0.2.)

2. A lifter is provided in the heating region inside the hearth tube, and the height of the lifter is lower than the height of the supply-side spiral blade and / or the discharge-side spiral blade, so that the precursor in the heating region is circulated while being fired. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 1, characterized by this.

3. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 2, characterized in that the lifter is a spiral blade.

4. The rotation speed of the hearth tube in the inclined charging stage and / or the inclined discharging stage is made higher than the rotation speed of the hearth tube in the horizontal firing stage. The method for producing a positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 3, characterized by this.

5. The composition formula (1) is represented by the following composition formula (2), the average particle size of the slurry in the mixing step is 0.1 μm or more and 0.5 μm or less, the average particle size of the secondary particles of the granulated powder obtained by granulating the slurry is 5 μm or more and 20 μm or less, the firing step is, optionally includes a first heat treatment step of obtaining the precursor by heat-treating the granulated powder at a heat treatment temperature of 200°C or more and 500°C or less for 0.5 hours or more and 5 hours or less, a second heat treatment step of obtaining the fired body by heat-treating the precursor in an oxidizing atmosphere at a heat treatment temperature of 600°C or more and less than 750°C for 10 hours or more and 100 hours or less, and a third heat treatment step of obtaining the lithium composite compound by heat-treating the fired body in an oxidizing atmosphere at a heat treatment temperature of 750°C or more and 900°C or less for 0.5 hours or more and 50 hours or less, The method for producing a positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 4, characterized by comprising the above steps. Li １＋ａ Ni ｂ Co ｃ M ｄ X e O ２＋α ··· (2) [However, in the composition formula (2), M represents at least one selected from Al and Mn, X represents one or more metal elements other than Li, Ni, Co, Al and Mn, and a, b, c, d, e and α are numbers satisfying -0.1 ≦ a ≦ 0.2, 0.7 ≦ b ≦ 1.0, 0 ≦ c ≦ 0.20, 0 ≦ d ≦ 0.20, 0 ≦ e ≦ 0.20, b + c + d + e = 1, and -0.2 < α < 0.2, respectively.]

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