Method for producing cathode active material for lithium ion secondary battery

Abstract

To solve a problem of increase in a volume handled in transportation and a producing process of a cathode material because nickel sulfate is a hexahydrate, so that mass% of Ni is about 20 to 25% and bulk specific gravity is small.SOLUTION: A method for producing a cathode active material for a lithium ion secondary battery containing lithium, a metal element M other than the lithium and nickel, and nickel, includes: a mixed powder generation step of mixing a compound containing the lithium and metal powder containing the metal element M and a metal of the nickel to generate mixed powder; and a firing step of firing the mixed powder in an atmosphere containing oxygen to obtain a cathode active material for the lithium ion secondary battery.SELECTED DRAWING: Figure 1

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] Lithium ion batteries are widely used in various fields such as electronics, automobiles, and infrastructure. Among these, in automobiles, lithium ion batteries are used as the power source for electric vehicles (EVs) and are important core components. From the perspective of extending the cruising range, the energy density of lithium ion batteries has been improving year by year, and high-capacity ternary layered materials are used as the positive electrode active materials for batteries. The ternary layered material is a composite oxide of a metal element such as Ni, Co, Mn, or Al and Li (hereinafter, a lithium metal composite oxide), and the higher the atomic ratio of Ni in the metal element, the higher the capacity. Therefore, high-Ni materials are expected as positive electrode active materials for EV batteries. Furthermore, from the perspectives of sustainable maintenance of the global environment, cost, and resource conservation measures, materials with reduced Co, which has a small reserve and is expensive, are expected. From the movement to improve the Ni ratio as an alternative, high-Ni conversion will be further promoted in the future.

[0003] Patent Document 1 describes a method for producing a positive electrode active material using a metal hydroxide as a precursor. A process for producing a positive electrode active material by reacting a Li source with a metal hydroxide is widely adopted. Also, Patent Document 2 describes a production method in which a nickel source is melted, nickel particles obtained by an atomization method are dissolved in an aqueous sulfuric acid solution to obtain nickel sulfate, and then a hydroxide containing Ni is obtained by a crystallization method, and a positive electrode active material for a secondary battery is obtained by a coprecipitation method using this hydroxide.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] As disclosed in Patent Document 1 and Patent Document 2, the positive electrode active material is produced by reacting a Li source with synthesized transition metal hydroxide particles through a coprecipitation reaction. In the coprecipitation reaction, an aqueous solution such as nickel sulfate is used as a raw material, which is generated by acid-dissolving high-purity refined nickel ingots to avoid impurities. Nickel sulfate is produced by purifying nickel from nickel ore mined from a mine and undergoing processes such as acid dissolution, so there is a problem of high processing costs. In addition, since nickel sulfate is a hexahydrate, the mass percentage of Ni is about 20 - 25% and the bulk specific gravity is small. To compensate for this, the volume handled in the manufacturing process of the positive electrode material becomes large. At the same time, the transportation cost also increases. For these reasons, the energy required for transportation and manufacturing increases, and the manufacturing process becomes complicated and lengthy. As a result, there is a problem that the amount of greenhouse gas (GHG) emitted also increases.

[0006] Therefore, the present invention aims to provide a method for manufacturing a positive electrode active material for a lithium-ion secondary battery that can reduce the volume handled in the transportation and the manufacturing process of the positive electrode material while using high-purity refined nickel ingots to avoid impurities, reduce the energy used for transportation and manufacturing, and eliminate the complexity of the manufacturing process. As a result, the amount of GHG emissions is suppressed.

Means for Solving the Problems

[0007] The manufacturing method of the positive electrode active material for a lithium-ion secondary battery of the present invention is a manufacturing method of a positive electrode active material for a lithium-ion secondary battery including lithium, a metal element M other than the lithium and nickel, and nickel, in which a compound containing the lithium and a metal powder containing the metal of the metal element M and the nickel are mixed to generate a mixed powder, and a firing step of firing the mixed powder in an atmosphere containing oxygen to obtain a positive electrode active material for a lithium-ion secondary battery. It is characterized by having these steps.

[0008] The firing step is preferably performed in a firing atmosphere containing 20% or more of oxygen by volume ratio. Further, the firing step preferably includes a main firing stage maintained at 750°C or higher and 900°C or lower.

[0009] The metal element M is preferably at least one element among Co, Mn, Al, Ti, Mg, Zr, Nb, and Mo.

[0010] Furthermore, between the mixed powder generation step and the firing step, an oxidation step of oxidizing the mixed powder is included, and the melting point of the compound containing the lithium is preferably higher than the oxidation temperature in the oxidation step.

Advantages of the Invention

[0011] According to the present invention, the energy used for transportation and manufacturing can be reduced, and the complexity of the manufacturing process can be eliminated. As a result, a manufacturing method of a positive electrode active material for a lithium-ion secondary battery with reduced GHG emissions can be provided.

Brief Description of the Drawings

[0012]

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Mode for Carrying Out the Invention

[0013] <Method for Manufacturing Positive Electrode Active Material for Lithium-Ion Secondary Battery> Hereinafter, a method for manufacturing a positive electrode active material for a lithium-ion secondary battery according to the present embodiment (hereinafter referred to as a method for manufacturing a positive electrode active material) will be described.

[0014] [Method for Manufacturing Metal Nickel Powder] Before explaining the method for manufacturing the positive electrode active material, the method for manufacturing the metal nickel powder will be exemplified. In the present embodiment, for example, metal nickel powder manufactured by an atomization method or a carbonyl method can be used. According to the atomization method or the carbonyl method, it is preferable because metal nickel powder with a small amount of impurity elements can be obtained. For the purpose of avoiding short circuits in the battery, high-purity raw materials are used for the battery members. In particular, iron (Fe) is an impurity element that easily causes short circuits, so the Fe content of the metal nickel powder is preferably 100 ppm or less. More preferably, it is 30 ppm or less, and even more preferably 10 ppm or less. Also, as a high-purity nickel source, high-purity briquettes or cathodes of grade ClassI are suitable. In the present embodiment, high-purity metal nickel powder with few impurities is obtained without acid-dissolving these briquettes or cathodes.

[0015] Figure 2 is a schematic diagram of the production of metal nickel powder by the water atomization method. Although not limited to the water atomization method in this embodiment, it is shown as an example of the production method. Here, it has a first step of melting nickel briquettes or cathodes in a melting furnace 1 to obtain molten nickel 2, and a second step of obtaining metal nickel powder 4 by the atomization method of injecting high-pressure water 3 into the molten nickel 2. As the medium injected into the molten nickel, a gas can also be used instead of high-pressure water. In that case, it is called the gas atomization method. By using these atomization methods, high-purity metal nickel powder can be obtained.

[0016] In this embodiment, it is characterized in that the metal nickel powder is used as a raw material for the positive electrode active material without acid dissolution. At the stage of being produced by the atomization method, a part of the metal nickel powder is already oxidized. In order to promote oxidation, in FIG. 2, the high-pressure water 3 can use an acidic solution in addition to water. Also, when injecting a gas, in addition to inert gases such as argon and nitrogen, a gas containing oxygen may also be used. That the metal nickel powder is partially oxidized by the atomization method means that, for example, an oxygen content of 300 ppm or more is preferable. In addition, when the oxidation process is carried out separately, a mixed powder with a higher oxidation rate can be obtained. In this case, the oxygen concentration in the metal nickel powder is preferably 3,000 ppm or more, more preferably 5,000 ppm or more, and still more preferably 10,000 ppm or more.

[0017] Also, as another method of obtaining metal nickel powder, there is the carbonyl method. The carbonyl method reacts nickel briquettes or the like with carbon monoxide gas to obtain gaseous nickel carbonyl, and then thermally decomposes this nickel carbonyl under reduced pressure and low temperature to obtain metal nickel powder. By using the carbonyl method, high-purity metal nickel powder can also be obtained.

[0018] The particle size of the metallic nickel powder is preferably in the range of several μm to several tens of μm. When the pulverization process is omitted during the production of the positive electrode active material, the average particle size D50 of the metallic nickel powder is preferably in the range of 5 to 30 μm, more preferably 5 to 20 μm, and even more preferably 5 to 15 μm. The particle size of the metallic nickel powder can be controlled by the injection pressure of the water or gas injected in the atomization method, and can be controlled by the thermal decomposition conditions in the carbonyl method. Incidentally, powders exceeding 100 μm can be removed by sieving and recycled to dissolution or the like.

[0019] [Method I for manufacturing positive electrode active material] Hereinafter, Method I for manufacturing a positive electrode active material using the above metallic nickel powder will be described. As shown in the flowchart of FIG. 1, this manufacturing method I includes a step of firing a mixed powder obtained by mixing a metallic nickel powder, a compound containing Li, and a compound containing a metal element M other than Li and Ni to obtain a positive electrode active material having a layered structure. In the present embodiment, as described above, since metallic nickel powder is used as the nickel raw material, the acid dissolution step and the coprecipitation step are unnecessary. Further, since metallic nickel powder is used as the nickel raw material, the volume handled in the transportation and the manufacturing process of the positive electrode active material can be made smaller than that of compounds such as nickel sulfate and nickel hydroxide. Specifically, when the Ni content per unit volume of each compound is expressed in mass%, nickel sulfate (Ni(SO)4·6H2O) is 5% and nickel hydroxide (Ni(OH)2) is 29%, whereas the Ni content of metallic nickel is 100%, and the Ni content per unit volume becomes higher. As a result, the volume handled in the transportation and the manufacturing process of the positive electrode active material is about 1 / 20 compared with nickel sulfate and about 1 / 3 compared with nickel hydroxide in the present embodiment, so that the fuel consumption during transportation can be suppressed, and the production efficiency can be improved by space saving and driving force reduction during manufacturing. These lead to a reduction in the GHG emissions, and as a result, the positive electrode active material can be manufactured while suppressing GHG. It is preferable that the Ni content per unit volume is closer to 100%. For example, as the metallic nickel powder, the specific gravity is 8 g / cm 3As described above, it is preferable that the metal is nickel and the remainder is in an oxidized state and contains no elements other than inevitable impurities. At this time, the oxygen content of the metallic nickel powder is 7.3 mass% or less.

[0020] The composition of the positive electrode active material is not particularly limited, but the specific composition of this embodiment will be described later. Basically, if the ratio of Ni per total metal element other than Li contained in the positive electrode active material is 60% or more in atomic ratio, since the redox potential of Ni is relatively low, it is easy to exhibit a high capacity at a predetermined potential and can meet the required battery characteristics. More preferably, it is 80% or more, which is preferable for expecting a further high capacity. Also, since the ratio of nickel per total metal element other than Li is 60% or more in atomic ratio, in the firing process, the firing reaction proceeds while lithium and metal element M diffuse into the particles with the individual particles of the metallic nickel powder as nuclei, so it is considered that it becomes easy to control the powder properties of the positive electrode active material after firing according to the particle size of the metallic nickel powder. In addition, a metallic nickel powder in which at least a part is oxidized is used. This is because when at least a part of the metallic nickel powder is oxidized at the stage before firing, the firing reaction to the layered structure proceeds rapidly when firing in an oxygen-containing atmosphere.

[0021] In order to obtain a positive electrode active material for a lithium-ion secondary battery having a layered structure by firing the positive electrode active material in an oxygen-containing atmosphere, a step of oxidizing the metallic nickel powder may be positively introduced for the purpose of performing the firing reaction rapidly. For example, an oxidation step such as exposing the metallic nickel powder to an air atmosphere or thermally oxidizing it in air or an oxidizing atmosphere may be provided separately. This will be described with reference to Manufacturing Method II. Also, when the metallic nickel powder is produced by the water atomization method, heating and drying are required, but this drying step can also serve as an oxidation step. In this case, it is preferable to determine the upper limit of the oxygen concentration in the metallic nickel powder in consideration of the balance with the amount of Ni per unit volume in transportation.

[0022] In FIG. 1, in the step of mixing a metal nickel powder, a compound containing lithium, and a compound containing a metal element M other than lithium and nickel to obtain a mixed powder, the metal nickel powder can be used as atomized powder as it is. The atomized powder may also be pulverized and adjusted to an appropriate particle size before use. Further, as the compound containing lithium, lithium hydroxide or lithium carbonate can be used. Considering the reactivity with metals, lithium hydroxide with a low melting point is conceivable, but considering environmental and safety aspects, lithium carbonate is preferable because lithium hydroxide is a toxic substance. Next, examples of the compound containing the metal element M other than lithium and nickel include oxides, carbonates, hydroxides, phosphates, and the like. For the sake of convenience, "compound" is assumed to include pure metals. Since the positive electrode active material is a composite oxide of lithium and a metal, pure metals, oxides, carbonates, and hydroxides are preferable as raw materials. The metal element M is preferably an element containing at least one selected from Co, Mn, Al, Ti, Mg, Zr, Nb, and Mo. Further, considering the reactivity with the metal nickel powder in the firing process, the average particle size of the compound containing the metal element M is preferably equal to or less than that of the metal nickel powder.

[0023] For the mixing of the raw material powders, a V-type mixer, a stirring mixer, an attritor, a media mill, or the like is used. In order to mix uniformly, it is preferable that the aggregation of each raw material powder can be broken up. The mixing method may be either a dry method of mixing only the raw material powders or a wet method of mixing with a liquid as a dispersion medium.

[0024] Next, a firing step is carried out in which the above mixed powder is fired to obtain a positive electrode active material having a layered structure. For the firing of the mixed powder, an electric furnace or a gas furnace is used. The firing atmosphere preferably contains oxygen at a volume ratio of 20% or more, and when the Ni content is 80% or more of all metal elements, the oxygen concentration is preferably 90% or more. The firing process preferably includes a pre-firing stage maintained at 450°C or higher and 730°C or lower, and a main firing stage maintained at 750°C or higher and 900°C or lower. The preferred firing temperature and holding time are adjusted according to the composition formulated during raw material mixing, and the firing is performed so that various physical properties (such as specific surface area) of the target cathode active material are within a suitable range after firing.

[0025] In this embodiment, until the step of firing the above-described mixed powder to obtain a cathode active material for a lithium-ion secondary battery having a layered structure, at least a part of the metal nickel powder is oxidized, and the oxidized metal nickel powder is used. This may also be expressed as containing a part of the oxide. This may use an acidic solution or a gas containing oxygen as described in the step of obtaining the metal nickel powder, or an oxidizing medium may be used in the step of obtaining the mixed powder, and in the firing step, it may be exposed to an oxidizing atmosphere or an oxidation step may be provided. Further, when moving between each step, a step of exposing the metal nickel powder to at least the air atmosphere may be added.

[0026] [Method for Manufacturing Cathode Active Material II] Next, the manufacturing method II of the positive electrode active material using the above-mentioned metallic nickel powder will be described. As shown in the flowcharts of FIGS. 3 and 4, this manufacturing method II is characterized in that a metallic nickel powder and a compound containing at least lithium are mixed, and an oxidation process is performed on this mixed powder. That is, the positive electrode active material is obtained by firing in an oxygen-containing atmosphere to obtain a positive electrode active material having a layered structure. However, it is different from the manufacturing method I in that an oxidation process for actively oxidizing the metallic nickel powder is introduced for the purpose of performing the firing reaction rapidly. The oxidation process is preferably a thermal oxidation in an oxidizing atmosphere because the time required for the oxidation treatment can be shortened. Further, in the case of thermal oxidation, the temperature is preferably about 100 to 700 ° C, more preferably 400 to 680 ° C, and still more preferably 600 to 680 ° C. This is because the oxidation rate becomes high when the temperature is 600 to 680 ° C. Also, the oxidation rate by the oxidation process is preferably 50% or more, more preferably 65% or more. When the oxidation rate is high and the residual amount of the metallic nickel component after oxidation is small, the pulverization in the subsequent pulverization and mixing process becomes easy, and in the pulverization and mixing process described later, a pulverized and mixed powder having a predetermined pulverized particle size can be obtained.

[0027] In addition, when the oxidation process is performed after mixing the metallic nickel powder and the compound containing lithium, the compound containing lithium becomes an inclusion, and sintering between the metallic nickel powders can be prevented, and the powder state can be maintained even after the oxidation process, which is preferable. Also, the amount of the compound containing lithium mixed with the metallic nickel powder is preferably 25% by mass or more of the compound containing lithium used in the production. This is because sintering between the metallic nickel powders can be prevented by performing thermal oxidation after mixing 25% by mass or more.

[0028] The compound containing lithium preferably has a melting point higher than the temperature of thermal oxidation. When the melting point of the compound containing lithium is higher than the thermal oxidation temperature, sintering between the metal nickel powders in the oxidation step can be prevented. Therefore, the compound containing lithium used in this manufacturing method is preferably lithium carbonate. This is because the melting point of lithium carbonate is 724°C, enabling the temperature of thermal oxidation to be increased to 720°C, preventing sintering between the powders and shortening the oxidation step. The average particle size of the compound containing Li is preferably from several μm to several hundred μm, more preferably from several μm to several tens of μm.

[0029] The particle size of the metal nickel powder preferably ranges from several μm to several tens of μm as described in the production of the above metal nickel powder. However, in this manufacturing method II, from the viewpoints of being able to uniformly mix the metal nickel powder and the compound containing lithium in the mixing step and being able to increase the oxidation rate in the oxidation step, the average particle size of the metal nickel powder preferably ranges from 5 to 30 μm, more preferably from 5 to 20 μm, and still more preferably from 5 to 15 μm.

[0030] Next, a compound containing a metal element M other than lithium and nickel is mixed. When an oxidation step is introduced, as shown in FIG. 3, after the oxidation step, the oxidized powder containing the oxidized metal nickel powder and the compound containing the metal element M other than lithium and nickel are mixed to form a mixed powder. The oxidation step may also be performed on a mixed powder obtained by further mixing a compound containing a metal element M other than lithium and nickel with a mixture of a metal nickel powder and a compound containing lithium, as shown in FIG. 4. However, from the viewpoint of being able to reduce the processing amount of the oxidation step, the step of mixing the compound containing the metal element M other than lithium and nickel after the oxidation step shown in FIG. 3 is preferred. The compound containing the metal element M other than Li and Ni is the same as in Manufacturing Method I, so the description is omitted, but examples of the composition of the positive electrode active material are given below.

[0031] Next, a step of pulverizing and mixing the mixed powder containing the above oxidation powder for the purpose of promoting the firing reaction (which may also be referred to as the mixed pulverization step) is introduced. This step corresponds to the step of obtaining the mixed powder of Manufacturing Method I. The pulverization and mixing can be carried out using an attritor, a media mill, etc. Since the mixed powder can be pulverized to a submicron size, it is preferable to use a media mill, and more preferably to use a bead mill.

[0032] The D50 of the primary particles of the mixed powder (pulverized and mixed powder) after pulverization and mixing is preferably 0.17 μm or less. When the D50 of the primary particles is 0.17 μm or less, the firing reaction is promoted and the voids of the cathode active material are suppressed. As a result, the particle strength of the cathode active material becomes high strength and the cycle characteristics become good. Also, the D95 of the primary particles of the pulverized and mixed powder is preferably 0.26 μm or less. When the D95 of the primary particles is 0.26 μm or less, the firing reaction is promoted and the voids of the cathode active material are suppressed. As a result, the particle strength of the cathode active material becomes high strength and the cycle characteristics become good.

[0033] Also, the specific surface area of the pulverized and mixed powder is preferably 28 m 2 / g or more. When the specific surface area of the pulverized and mixed powder is 28 m 2 / g or more, the firing reaction is promoted and the voids of the cathode active material are suppressed. As a result, the particle strength of the cathode active material becomes high strength and the cycle characteristics become good.

[0034] Next, the firing step of firing the above mixed powder or pulverized and mixed powder to obtain a cathode active material for a layered-structured lithium-ion secondary battery will be described. For firing the raw material mixed powder or pulverized and mixed powder, an electric furnace or a gas furnace is used. The firing atmosphere preferably contains 20% or more of oxygen by volume, and when the Ni content is 80% or more of all metal elements, an oxygen concentration of 90% or more is preferable. The firing process includes a pre-firing stage maintained at 450°C or higher and 730°C or lower, and a main firing stage maintained at 750°C or higher and 900°C or lower. The preferred firing temperature and holding time are adjusted according to the composition blended during raw material mixing, and the firing is performed so that various physical properties (such as specific surface area) of the target positive electrode active material are within a suitable range after firing.

[0035] In addition, the synthesized lithium transition metal composite oxide may be subjected to a washing process of washing with deionized water or the like, a drying process of drying the washed lithium transition metal composite oxide, etc. after the firing process for the purpose of removing impurities or the like. Further, it may be subjected to a crushing process of crushing the synthesized lithium transition metal composite oxide, a classification process of classifying the lithium transition metal composite oxide into a predetermined particle size, etc.

[0036] Next, the composition of the positive electrode active material of the present embodiment will be described. As described above, the composition of the positive electrode active material of the present embodiment is not particularly limited, but the preferred composition will be described below. First, the positive electrode active material according to the present embodiment is represented by the following formula (1). Li 1+a NiMO 2+α ···(1) (However, in the above formula (1), M is a metal element other than Li and Ni, and the ratio of Ni among all metal elements is 60 atomic% or more, and a and α are numbers satisfying -0.1 ≤ a ≤ 0.2 and -0.2 ≤ α ≤ 0.2.)

[0037] The positive electrode active material according to the present embodiment can achieve a high energy density and a high charge-discharge capacity by having a composition in which the ratio of Ni per total metal element other than Li is 60 atomic% or more. Note that the ratio of Ni per total metal element other than Li can be appropriately set within the range of 60 atomic% or more and 100 atomic% or less. Since it is a positive electrode active material containing nickel at a high ratio, it is important that the oxidation reaction of oxidizing Ni to Ni is efficiently performed. 2+ to 3+ is efficiently performed.

[0038] The cathode active material according to this embodiment has a more preferable specific composition represented by the formula (2). Li 1+a Ni b Co c M1 d X e O 2+α ···(2) [However, in the formula (2), M1 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 ≦ 0.1, b + c + d + e = 1, and -0.2 < α < 0.2.].

[0039] Since the cathode active material represented by the formula (2) has a high Ni content, it can exhibit a high charge-discharge capacity in the range up to around 4.3 V as compared with LiCoO2 and the like. Further, since the Ni content is high, it is a cathode active material with a low raw material cost and easy availability of raw materials as compared with LiCoO2 and the like.

[0040] Here, the significance of the numerical ranges of a, b, c, d, e, and α in the formulas (1) and (2) will be described.

[0041] 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: LiM'O2, that is, the excess or deficiency amount of lithium from Li:M':O = 1:1:2. If lithium is excessively less, 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, it is possible to achieve both a high charge-discharge capacity and good charge-discharge cycle characteristics.

[0042] a may be -0.02 or more and 0.07 or less. If a is -0.02 or more, a sufficient amount of lithium to contribute to charge and discharge is ensured, so that the charge and discharge capacity of the positive electrode active material can be increased. Further, if a is 0.07 or less, charge compensation due to the valence change of the transition metal is sufficient, so that both a high charge and discharge capacity and good charge and discharge cycle characteristics can be achieved.

[0043] The coefficient b of nickel is 0.7 or more and 1.0 or less. When b is 0.7 or more, a sufficiently high charge and 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 showing a high charge and discharge capacity can be manufactured at a lower cost compared with LiCoO2 or the like.

[0044] b is preferably 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 and discharge capacity obtained. Further, when b is 0.95 or less, the smaller it is, the smaller the lattice strain 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 and deterioration of crystallinity to occur during firing. Therefore, deterioration of the charge and discharge capacity and charge and discharge cycle characteristics is suppressed.

[0045] The coefficient c of cobalt is 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 and discharge capacity can be improved without significantly impairing it, and the charge and discharge cycle characteristics can be improved. On the other hand, if cobalt is excessive, the raw material cost increases, so that 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 and discharge capacity and good charge and discharge cycle characteristics with good productivity.

[0046] 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 and the larger it is than 0.01, the more sufficient the effect of cobalt element substitution is obtained, and the better 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.

[0047] The coefficient d of M1 is 0 or more and 0.20 or less. When at least one element (M1) selected from the group consisting of manganese and aluminum is element-substituted, the layered structure is maintained more stably even when lithium is desorbed by charging. On the other hand, if these elements (M1) are excessive, the ratio of other transition metals such as nickel becomes low, 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.

[0048] As the element represented by M1, manganese is particularly preferable. When manganese is element-substituted, a higher charge-discharge capacity can be obtained as compared with the case where aluminum is element-substituted. Also, during the firing of the lithium composite compound, manganese also reacts with lithium carbonate as shown in the following formula (3). Such a reaction suppresses the coarsening of crystal grains and can promote the oxidation reaction of nickel at high temperature, so that a positive electrode active material showing a high charge-discharge capacity can be efficiently obtained.

[0049] Li2CO3 + 2M´O + 0.5O2 → 2LiM´O2 + CO2 ···(3) (However, in the above formula (3), M´ represents a metal element such as Ni, Co, Mn, etc.)

[0050] The coefficient d of M1 is preferably 0.02 or more, more preferably 0.04 or more. The larger the coefficient d of M1, the more sufficient the effect of element substitution by at least one element selected from the group consisting of manganese and aluminum can be obtained. When M1 is manganese, it becomes possible to proceed with the oxidation reaction of nickel at a higher temperature, and a positive electrode active material showing a high charge and discharge capacity can be obtained more efficiently. Further, the coefficient d of M1 is preferably 0.18 or less. If the coefficient d of M1 is 0.18 or less, the charge and discharge capacity can be kept high even when element substitution has occurred.

[0051] The coefficient e of X is set to be 0 or more and 0.10 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 element-substituted, various performances such as charge and discharge cycle characteristics can be improved while maintaining the activity of the positive electrode active material. On the other hand, if these elements (X) are excessive, the proportion of other transition metals such as nickel becomes low, and the charge and discharge capacity of the positive electrode active material decreases. If e is within the above numerical range, high charge and discharge capacity and good charge and discharge cycle characteristics can be achieved simultaneously.

[0052] α in the above formulas (1) and (2) is set to be -0.2 or more and 0.2 or less. α represents the oxygen excess or deficiency amount from the stoichiometric ratio of the lithium composite compound represented by the general formula: LiM'O2, that is, Li:M':O = 1:1:2. If α is within the above numerical range, the crystal structure has few defects, and high charge and discharge capacity and good charge and discharge cycle characteristics can be obtained.

Examples

[0053] Hereinafter, the present invention will be specifically described with reference to examples, but the technical scope of the present invention is not limited thereto. Hereinafter, the measurement means of characteristic values and a preliminary experiment of the oxidation process will be described, and then the examples will be described.

[0054] (Average particle size, specific surface area) The D50 and D95 of the primary particles of the pulverized mixed powder and the secondary particles of the calcined powder of the positive electrode active material were measured by a laser diffraction particle size distribution analyzer. The specific surface area was measured by the BET method using gas adsorption with an automatic specific surface area measuring device.

[0055] (Oil absorption amount) The oil absorption amount of the powder sample was measured in accordance with JIS K5101-13-1, and NMP (N-methylpyrrolidone) was used as the solvent. Weigh 5.0 g of the powder sample and place it in a mountain shape on a flat bat. Absorb NMP with a polysyringe (2 mL capacity) and measure its mass. Next, while dropping NMP onto the powder sample, knead it with a spatula, and continue dropping and kneading until the powder sample becomes clay-like throughout. When NMP becomes excessive, it can be visually confirmed that the droplets are not absorbed by the powder sample and remain on the surface. The amount of NMP dropped until this point was converted per 100 g of the powder sample and defined as the oil absorption amount.

[0056] (X-ray diffraction pattern) The X-ray diffraction (XRD) pattern in the X-ray powder diffraction measurement of the positive electrode active material was measured using an X-ray diffractometer "X'Pert PRO MPD" (manufactured by PANalyticalsei) under the conditions of a line source of CuKα, a tube voltage of 45 kV, a tube current of 40 mA, a sampling interval of 0.02° / step, a divergence slit of 0.5°, a scattering slit of 0.5°, a receiving slit of 0.15 mm, and a scanning range of 15° ≤ 2θ ≤ 80°.

[0057] [Oxidation process of metal nickel powder] (Preliminary experiment 1) The metal nickel powder (manufactured by Nippon Atomizing & Processing Co., Ltd.) produced by the water atomization method with D50 of 8 μm and lithium carbonate were weighed so that the molar ratio of Li:Ni as metal elements was 1.03:0.85. These raw material powders were put into a V-type mixer with a volume of 45 L in a total amount of 5 kg and mixed for 90 minutes to obtain a raw material mixed powder. Next, this raw material mixed powder was heat-treated in a firing furnace in an air atmosphere at 650 °C for 10 hours to obtain an oxidized powder. The obtained oxidized powder had a weight increase of 18% compared to the raw material mixed powder. From this weight increase rate, it was confirmed that 70% of the metal nickel powder had become nickel oxide. That is, the oxidation rate was 70%. Also, a part of the oxidized powder was caked. By crushing this with a mortar, an oxidized powder with D50 of 8 μm was obtained.

[0058] (Preliminary Experiment 2) An oxidation process similar to that in Preliminary Experiment 1 was carried out except that the metal nickel powder (manufactured by Vale) produced by the carbonyl method with D50 of 8 μm was used, and an oxidized powder with D50 of 8 μm was obtained. The oxidation rate of this metal nickel powder was 70%.

[0059] (Preliminary Experiment 3) An oxidation process similar to that in Preliminary Experiment 1 was carried out except that the metal nickel powder with D50 of 67 μm was used, and an oxidized powder with D50 of 32 μm was obtained. The oxidation rate of this metal nickel powder was 10%.

[0060] From Preliminary Experiment 1, Preliminary Experiment 2, and Preliminary Experiment 3, it was found that when the D50 of the metal nickel powder is small, the oxidation rate becomes high, and when it is at least 8 μm or less, an oxidized powder with a high oxidation rate of 70% or more can be obtained.

[0061] (Preliminary Experiment 4) Except that nickel metal powder and lithium carbonate were weighed so that the molar ratio of Li:Ni was 0.26:0.85 in terms of metal elements, the same oxidation process as in Preliminary Experiment 1 was carried out. Although part of the oxidized powder was caked, oxidized powder with a D50 of 8 μm was obtained by crushing it with a mortar. Note that the molar ratio of Li:Ni being 0.26:0.85 in terms of metal elements for nickel metal powder and lithium carbonate means that 25% by mass of the lithium-containing compound used in the production was mixed with the nickel metal powder.

[0062] (Preliminary Experiment 5) Nickel metal powder and lithium carbonate were weighed so that the molar ratio of Li:Ni was 0:0.85 in terms of metal elements. That is, oxidation was carried out only with nickel metal. Otherwise, the same oxidation process as in Preliminary Experiment 1 was carried out. The nickel metal powder was sintered and could not be crushed with a mortar.

[0063] From Preliminary Experiment 1, Preliminary Experiment 4, and Preliminary Experiment 5, it was confirmed that sintering of the nickel metal powder can be prevented by performing an oxidation treatment after mixing 25% by mass or more of the nickel metal powder and the lithium-containing compound used in the production.

[0064] [Example 1] Example 1 implemented Manufacturing Method I. That is, plate-shaped metallic nickel with an Fe content of 30 ppm was melted in a melting furnace, and high-pressure water was sprayed onto the molten nickel that flowed out and dropped to obtain metallic nickel powder with an average particle size of 8 μm by the water atomization method. When the oxygen content of this metallic nickel powder was measured with an oxygen-nitrogen analyzer, it was 3,000 ppm. In addition to the obtained metallic nickel powder, the prepared raw materials are as follows. As a lithium-containing compound, lithium hydroxide was prepared, and as compounds containing metal elements M other than lithium and nickel, cobalt oxide, manganese oxide, and titanium oxide were prepared. Each raw material was weighed so that the molar ratio of Li:Ni:M was 1.03:0.85:0.15 in terms of metal elements. These raw material powders were put into a V-type mixer with a volume of 45 L in a total amount of 5 kg and mixed for 90 minutes to obtain a raw material mixed powder. Next, this raw material mixed powder was calcined temporarily at 500°C for 10 hours in an oxygen gas atmosphere in a firing furnace replaced with an oxygen gas atmosphere, and then sintered at 820°C for 10 hours. Between each process, the metallic nickel powder was transported in an environment where it was exposed to the atmosphere without being enclosed in a vacuum or non-oxidizing atmosphere. Thus, a positive electrode active material made of a lithium metal composite oxide was obtained.

[0065] In Example 1, by manufacturing a positive electrode active material mainly composed of nickel using metallic nickel powder, impurities could be reduced. Also, in the conventional coprecipitation process, nickel ingots are processed into water-soluble compounds such as nickel sulfate, and then processed into an aqueous solution such as nickel sulfate (acid dissolution step). Further, nickel hydroxide powder is manufactured from this aqueous solution of nickel sulfate or the like by the coprecipitation method (coprecipitation step), and this nickel hydroxide powder is used as a precursor. In this regard, in the example, metallic nickel powder directly manufactured from nickel ingots is used as a precursor without going through the process of processing into compounds such as nickel sulfate or nickel hydroxide. Therefore, the acid dissolution step and the coprecipitation step are not required, and the positive electrode active material can be manufactured simply. Also, the volume handled in the transportation and the manufacturing process of the positive electrode material can be reduced.

[0066] [Example 2] Example 2 implemented Manufacturing Method II. That is, as raw materials, lithium carbonate, metallic nickel powder, cobalt carbonate, manganese carbonate, titanium oxide, and aluminum oxide were prepared, and each raw material was weighed so that the molar ratio of metal elements Li:Ni:Co:Mn:Ti:Al was 1.03:0.85:0.03:0.09:0.03:0.01. Note that for the metallic nickel powder, a metallic nickel powder with a D50 of 8 μm produced by the above-described water atomization method was used. First, the metallic nickel powder and lithium carbonate were put into a V-type mixer and mixed for 90 minutes to obtain a raw material mixed powder. Next, this raw material mixed powder was subjected to an oxidation treatment (oxidation step) at 650°C for 10 hours in a firing furnace in an air atmosphere to obtain an oxidized powder containing oxidized metallic nickel powder. The obtained oxidized powder was mixed with metal element M composed of cobalt carbonate, manganese carbonate, titanium oxide, and aluminum oxide, and pure water was added thereto so that the solid content ratio became 30% by mass. Then, wet pulverization (wet mixing) was performed with a pulverizer to prepare a raw material slurry so that the D50 of the primary particles became 0.30 μm (pulverization and mixing step). Subsequently, the obtained raw material slurry was spray-dried with a nozzle-type spray dryer (manufactured by Okawara Chemical Machinery Co., Ltd., model ODL-20) to obtain granulated bodies with a D50 of about 10 μm (granulation step). Then, the dried granulated bodies were fired to obtain a lithium transition metal composite oxide (firing step). Specifically, it was calcined at 700°C for 24 hours in an oxygen gas stream in a firing furnace replaced with an oxygen gas atmosphere. Thereafter, the lithium transition metal composite oxide was obtained by performing the main firing at 840°C for 10 hours in an oxygen gas stream in a firing furnace replaced with an oxygen gas atmosphere. The fired powder obtained in the firing step was classified using a sieve with an opening of 53 μm, and the powder passing through the sieve was used as the positive electrode active material of the sample.

[0067] [Example 3] A positive electrode active material was produced in the same manner as in Example 2, except that wet pulverization (wet mixing) was performed with a pulverizer to prepare a raw material slurry so that the D50 of the primary particles became 0.17 μm.

[0068] [Example 4] The positive electrode active material was produced in the same manner as in Example 2, except that the raw material slurry was prepared by wet grinding (wet mixing) with a pulverizer so that the D50 of the primary particles was 0.13 μm.

[0069] The specific surface areas of the granulated bodies of Examples 2 to 4 were measured. The results are shown in Table 1. Also, the positive electrode active materials of Examples 2 to 4 were observed by SEM and X-ray diffraction was measured. The photographs are shown in FIGS. 5 to 7. Also, the X-ray diffraction patterns are shown in FIGS. 8 to 10. Furthermore, the specific surface areas and oil absorption amounts of the positive electrode active materials of Examples 2 to 4 were measured. These are also shown in Table 1.

[0070] (Fabrication of Positive Electrode) Next, a lithium-ion secondary battery was fabricated using the synthesized positive electrode active material as the positive electrode material, and the discharge capacity and capacity retention rate of the lithium-ion secondary battery were measured. First, the fabricated positive electrode active material, a carbon-based conductive material, and a binder previously dissolved in N-methyl-2-pyrrolidone (NMP) were mixed so that the mass ratio was 94:4.5:1.5. Then, the uniformly mixed positive electrode mixture slurry was applied onto a positive electrode current collector of an aluminum foil with a thickness of 15 μm so that the coating amount was 13 mg / cm 2 Next, the positive electrode mixture slurry applied to the positive electrode current collector was heat-treated at 120 °C to distill off the solvent, thereby forming a positive electrode mixture layer. Then, the positive electrode mixture layer was pressure-molded by hot pressing and punched into a circular shape with a diameter of 15 mm to obtain a positive electrode.

[0071] (Initial Capacity, Charge / Discharge Cycle Characteristics (Capacity Retention Rate)) Subsequently, a lithium-ion secondary battery was fabricated using the fabricated positive electrode, negative electrode, and separator. As the negative electrode, metallic lithium punched into a circular shape with a diameter of 16 mm was used. As the separator, a porous separator made of polypropylene with a thickness of 30 μm was used. The positive electrode and the negative electrode were opposed to each other in a non-aqueous electrolyte through the separator to assemble a lithium-ion secondary battery. As the non-aqueous electrolyte, a solution in which LiPF6 was dissolved to a concentration of 1.0 mol / L in a solvent obtained by mixing ethylene carbonate and dimethyl carbonate so that the volume ratio was 3:7 was used.

[0072] The fabricated lithium-ion secondary battery was charged at a constant current / constant voltage of 38 A / kg based on the weight of the positive electrode mixture and an upper limit potential of 4.3 V under an environment of 25°C. Then, it was discharged at a constant current of 40 A / kg based on the weight of the positive electrode mixture to a lower limit potential of 2.5 V, and the charge capacity and discharge capacity were measured. Thereafter, it was charged at a constant current / constant voltage of 190 A / kg based on the weight of the positive electrode mixture and an upper limit potential of 4.3 V. Then, a cycle of discharging at a constant current of 190 A / kg based on the weight of the positive electrode mixture to a lower limit potential of 2.5 V was performed 30 cycles in total, and the discharge capacity after 30 cycles was measured. The fraction of the discharge capacity after 10 cycles with respect to the initial capacity was calculated as the capacity retention rate. The results are also shown in Table 1.

[0073]

Table 1-1

[0074]

Table 1-2

[0075] From the SEM observation images of FIGS. 5 to 7, it was found that the secondary particle diameter of the positive electrode active material in Examples 2 to 4 was about 10 μm and the primary particle diameter was about 400 nm. Further, compared with the positive electrode active material of Example 2 in which the D50 of the primary particles of the mixed powder (ground and mixed powder) after grinding and mixing was 0.30 μm, the positive electrode active materials of Examples 3 and 4 in which the D50 of the primary particles of the mixed powder (ground and mixed powder) after grinding and mixing was 0.17 μm or less had fewer voids, and it was confirmed that when the D50 of the primary particles of the mixed powder (ground and mixed powder) after grinding and mixing was 0.17 μm or less, the firing reaction was promoted and the voids of the positive electrode active material could be suppressed.

[0076] From the XRD patterns of FIGS. 8 to 10, all of the positive electrode active materials in Examples 2 to 4 had peaks attributed to the 003 plane near 2θ = 18°, the 101 plane near 2θ = 36°, the 006 plane and 012 plane near 2θ = 38°, the 104 plane near 2θ = 44°, the 015 plane near 2θ = 48°, and the 107 plane near 2θ = 58°, and thus it was confirmed that they belonged to the space group R3-m and were layered lithium metal composite oxides.

[0077] From Table 1, the positive electrode active materials of Examples 2 to 4 have a high charge capacity of 222 Ah / kg and a discharge capacity of 195 Ah / kg or more. Also, the capacity retention rate is 81% or more, indicating good cycle characteristics. That is, it was confirmed that by the method for producing a positive electrode active material of the present invention, a positive electrode active material with high capacity and good cycle characteristics can be obtained using metallic nickel powder as a raw material without processing it into compounds such as nickel sulfate or nickel hydroxide. Further, for the positive electrode active materials of Example 3 and Example 4 where the D50 of the primary particles after pulverization and mixing is 0.17 μm or less, the D95 is 0.26 μm or less, and the specific surface area is 28 m 2 / g or more, it was confirmed that the capacity retention rate is even better at 84% or more.

[0078] From the above, also in this example, compared with the conventional coprecipitation process, the nickel raw material does not require processing into compounds such as nickel sulfate or nickel hydroxide. That is, the acid dissolution step and the coprecipitation step are not required, and it can be easily manufactured. Also, since the positive electrode active material can be manufactured using metallic nickel powder as it is or by subjecting it to an oxidation treatment without passing through compounds such as nickel sulfate or nickel hydroxide, the manufacturing process is short and the transportation between manufacturing processes is small. Moreover, metallic nickel powder has a high nickel content and a large specific gravity compared to nickel sulfate, nickel hydroxide, etc., so the volume to be transported becomes small, and the energy required for transportation can be reduced. Due to these factors, the greenhouse gas (GHG) emissions can be reduced by about 30 to 40%, and as a result, the production of the positive electrode active material while suppressing GHG emissions can be achieved.

Explanation of Symbols

[0079] 1: Melting furnace 2: Molten nickel 3: High-pressure water injection 4: Metallic nickel powder

Claims

1. In a method for producing a positive electrode active material for a lithium-ion secondary battery, comprising lithium, a metal element M other than the lithium and nickel, and nickel, a mixed powder generation step of mixing a compound containing the lithium and a metal powder containing the metal of the metal element M and the nickel to generate a mixed powder; a firing step of firing the mixed powder in an atmosphere containing oxygen to obtain a positive electrode active material for a lithium-ion secondary battery, characterized in that the method for producing a positive electrode active material for a lithium-ion secondary battery has the above steps.

2. The method for producing a positive electrode active material for a lithium-ion secondary battery according to claim 1, characterized in that the firing step is carried out in a firing atmosphere containing 20% or more of oxygen by volume ratio.

3. The method for producing a positive electrode active material for a lithium-ion secondary battery according to claim 2, characterized in that the firing step includes a main firing stage maintained at 750 °C or higher and 900 °C or lower.

4. The method for producing a positive electrode active material for a lithium-ion secondary battery according to claim 3, characterized in that the metal element M is at least one element selected from Co, Mn, Al, Ti, Mg, Zr, Nb, and Mo.

5. The method for producing a positive electrode active material for a lithium-ion secondary battery according to claim 3, characterized in that an oxidation step of oxidizing the mixed powder is included between the mixed powder generation step and the firing step, and the melting point of the compound containing the lithium is higher than the oxidation temperature in the oxidation step.

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