Method for producing positive electrode active material

Abstract

To provide a method of manufacturing a positive electrode active material capable of improving resistance of a nonaqueous electrolyte secondary battery while having a small Li seat occupancy.SOLUTION: The method for producing a positive electrode active material includes a first step and a second step. In the first step, a mixture obtained by mixing a lithium compound and a transition metal-containing compound containing a transition metal is fired at 750 to 1000 °C in an oxygen atmosphere using a heater to obtain a lithium transition metal composite oxide. In the second step, the lithium transition metal composite oxide is irradiated with microwaves while the temperature of the lithium transition metal composite oxide after the first step is 400 °C or higher and lower than the firing temperature in the first step.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a method for producing a positive electrode active material. [Background technology]

[0002] Patent Document 1 discloses that in producing a positive electrode active material for a lithium ion battery, a lithium-containing carbonate is calcined by using a heater and microwave heating in combination. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-210463 Summary of the Invention [Problem to be solved by the invention]

[0004] The present disclosure provides a method for producing a positive electrode active material that can improve the resistance of a non-aqueous electrolyte secondary battery while having a small Li site occupancy. [Means for solving the problem]

[0005] [1] A first step of calcining a mixture of a lithium compound and a transition metal-containing compound using a heater in an oxygen atmosphere at 750 to 1000°C to obtain a lithium transition metal composite oxide; a second step of irradiating the lithium transition metal composite oxide with microwaves while the temperature of the lithium transition metal composite oxide after the first step is 400°C or higher and lower than the baking temperature in the first step. [2] The method for producing a positive electrode active material according to [1], wherein the transition metal-containing compound is at least one of a nickel-containing hydroxide and a nickel-containing oxide. [3] The method for producing a positive electrode active material according to [1] or [2], wherein the transition metal-containing compound contains at least one of Mn and Co. 〔4〕 The lithium compound is at least one of lithium hydroxide and lithium carbonate, and the method for producing a positive electrode active material according to any one of 〔1〕 to 〔3〕. 〔5〕 The positive electrode active material is Li, Ni, Mn, Co, and M [M is one or more metal elements selected from the group consisting of Mg, Ca, Al, Ti, V, Cr, Fe, Cu, Zn, Zr, Nb, Mo, Ta, W, and Y.], and The molar ratio of Li, Ni, Mn, Co, and M is Li:Ni:Mn:Co:M = a:x:y:z:t [a, x, y, z, and t satisfy 1.0 ≦ a ≦ 1.3, x + y + z + t = 1, 0.25 ≦ x ≦ 0.9, 0 < y ≦ 0.6, 0 < z ≦ 0.6, and 0 ≦ t ≦ 0.1.], and the method for producing a positive electrode active material according to any one of 〔1〕 to 〔4〕. 〔6〕 The positive electrode active material contains secondary particles, and the method for producing a positive electrode active material according to any one of 〔1〕 to 〔5〕. 〔7〕 The first step is carried out using a continuous firing furnace that fires while transporting the mixture, and the method for producing a positive electrode active material according to any one of 〔1〕 to 〔6〕. 〔8〕 The second step is carried out on the lithium transition metal composite oxide taken out from the continuous firing furnace, and the method for producing a positive electrode active material according to 〔7〕. 〔9〕 The continuous firing furnace is a heater-heated kiln, In the first step, in the kiln, while transporting the crucible containing the mixture, the mixture is fired, and the method for producing a positive electrode active material according to 〔7〕 or 〔8〕. 〔10〕 The irradiation amount of the microwave in the second step is 200 to 1500 Wh per 1 kg of the lithium transition metal composite oxide, and the method for producing a positive electrode active material according to any one of 〔1〕 to 〔9〕. 〔11〕 In the second step, the output of the microwave is 4 kW or less, and the irradiation time of the microwave is 15 to 90 minutes, and the method for producing a positive electrode active material according to any one of 〔1〕 to 〔10〕.

Advantages of the Invention

[0006] The positive electrode active material of the present disclosure can improve the resistance of a non-aqueous electrolyte secondary battery while having a small Li site occupancy. [Brief explanation of the drawings]

[0007] [Figure 1] 2 is a flowchart illustrating an example of a method for manufacturing a positive electrode active material according to the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0008] In this specification, unless otherwise specified, a numerical range such as "m to n" includes both the upper and lower limits. That is, "m to n" represents a numerical range of "m or more and n or less." A numerical value arbitrarily selected from within the numerical range may be set as a new upper or lower limit. For example, a new numerical range may be set by arbitrarily combining a numerical value within the numerical range with a numerical value described elsewhere in this specification, in a table, or in a figure.

[0009] (Method of manufacturing positive electrode active material) 1 is a flowchart showing an example of a method for producing a positive electrode active material according to an embodiment. The positive electrode active material produced by the method for producing a positive electrode active material according to this embodiment (hereinafter also referred to as "this method") is used for a positive electrode plate of a non-aqueous electrolyte secondary battery (hereinafter also referred to as "secondary battery") such as a lithium ion battery.

[0010] The positive electrode active material produced by this method is a lithium transition metal composite oxide (hereinafter also referred to as "composite oxide") containing lithium and a transition metal. The positive electrode active material may be a layered rock salt type, a spinel type, or an olivine type. The positive electrode active material is preferably a layered rock salt type. The crystal structure of the positive electrode active material can be confirmed by measurement using an X-ray diffraction method (hereinafter also referred to as an "XRD method").

[0011] The composition of the positive electrode active material is not particularly limited, but preferably contains Ni, and more preferably contains Ni, Mn, and Co. The positive electrode active material is more preferably Li, Ni, Mn, Co, and M [M is one or more metal elements selected from the group consisting of Mg, Ca, Al, Ti, V, Cr, Fe, Cu, Zn, Zr, Nb, Mo, Ta, W, and Y.] are included, and The molar ratio of Li, Ni, Mn, Co, and M is Li:Ni:Mn:Co:M = a:x:y:z:t [a, x, y, z, and t satisfy 1.0 ≤ a ≤ 1.3, x + y + z + t = 1, 0.25 ≤ x ≤ 0.9, 0 < y ≤ 0.6, 0 < z ≤ 0.6, and 0 ≤ t ≤ 0.1.].

[0012] The molar ratio of Li is 1.0 ≤ a ≤ 1.3, and it may be 1.0 ≤ a ≤ 1.25, it may be 1.01 ≤ a ≤ 1.2, it may be 1.03 ≤ a ≤ 1.15, or it may be 1.04 ≤ a ≤ 1.1. The molar ratio of Ni is 0.25 ≤ x ≤ 0.9, and it may be 0.3 ≤ x ≤ 0.9, it may be 0.4 ≤ x ≤ 0.88, or it may be 0.5 ≤ x ≤ 0.85. The molar ratio of Mn is 0 < y ≤ 0.6, and it may be 0.05 ≤ y ≤ 0.5, it may be 0.08 ≤ y ≤ 0.3, or it may be 0.10 ≤ y ≤ 0.2. The molar ratio of Co is 0 < z ≤ 0.6, and it may be 0 < z ≤ 0.5, it may be 0.01 ≤ z ≤ 0.3, or it may be 0.02 ≤ z ≤ 0.1. The molar ratio of M is 0 ≤ t ≤ 0.1, and it may be 0 < t ≤ 0.08, it may be 0.001 ≤ t ≤ 0.05, or it may be 0.002 ≤ t ≤ 0.01. When the positive electrode active material contains two or more kinds of metal elements M, the molar ratio of M refers to the total amount of two or more kinds of metal elements.

[0013] The composition of the positive electrode active material can be adjusted by the types of raw materials used and the blending amounts of the raw materials when manufacturing the positive electrode active material. The composition of the positive electrode active material can be determined by ICP (Inductively Coupled Plasma) optical emission spectrometry (ICP-AES).

[0014] The positive electrode active material may be a single particle or a secondary particle. The secondary particle is an aggregate particle formed by aggregation of primary particles. The positive electrode active material preferably contains secondary particles. The aggregation number of primary particles in the secondary particles is preferably 50 or more, may be 100 or more, or may be 1000 or more, and is usually 5 × 10 6 5 x 10 or less 5 The positive electrode active material may contain, in addition to the secondary particles having the above-mentioned agglomeration number, at least one of secondary particles formed by agglomeration of 2 to 10 primary particles and single particles. The content of secondary particles formed by agglomeration of 50 or more primary particles in the positive electrode active material is, for example, 70 to 100 mass%, or alternatively 85 to 98 mass%, or 90 to 95 mass%, when the total amount of the positive electrode active material is taken as 100 mass%. The agglomeration number of the secondary particles can be adjusted by the manufacturing conditions, such as the firing conditions (firing temperature, number of firings, firing time, etc.), when manufacturing the positive electrode active material. The agglomeration number of the primary particles contained in the secondary particles can be confirmed, for example, by SEM images obtained by a scanning electron microscope (hereinafter also referred to as "SEM").

[0015] The Li site occupancy at the 3b site of the positive electrode active material is, for example, 2 to 4%, but may be 2.15 to 3.95%, 2.18 to 3.9%, or 2.2 to 3.8%. As will be described in the Examples below, the Li site occupancy [%] is calculated as the amount of Li present at the site represented by 3b when expressed in Wyckoff notation based on measurement data obtained by XRD of the positive electrode active material. The Li site occupancy of the positive electrode active material can be adjusted by adjusting the firing conditions in the first step and the microwave irradiation conditions in the second step.

[0016] As shown in Fig. 1, this method includes the following first and second steps: In this method, a positive electrode active material can be produced through the first and second steps. First step: A step of calcining a mixture of a lithium compound and a transition metal-containing compound using a heater in an oxygen atmosphere at 750 to 1000°C to obtain a composite oxide. Second step: A step of irradiating the composite oxide with microwaves while the temperature of the composite oxide after the first step is 400°C or higher and lower than the firing temperature in the first step.

[0017] It is believed that microwave irradiation of a composite oxide obtained by calcining a mixture of a lithium compound and a transition metal-containing compound can regulate at least one of the electronic arrangement and crystal distortion of the composite oxide. This allows for the production of a cathode active material that has a small Li-site occupancy but can improve the resistance of secondary batteries. On the other hand, without microwave irradiation, it is difficult to improve the resistance of secondary batteries even if the Li-site occupancy is reduced. Furthermore, when heater heating and microwave heating are used in combination in the first step, the raw materials can be heated rapidly, thereby shortening the heating time. However, this does not lead to the regulation of the electronic arrangement and crystal distortion of the composite oxide, and it is thought that it is difficult to improve the Li-site occupancy and the resistance of secondary batteries.

[0018] Each step of this method will now be described in detail. (1st step) The first step is a step of calcining a mixture of a lithium compound and a transition metal-containing compound to obtain a composite oxide, and the calcination is performed using a heater. The first step may include a step of obtaining a mixture. The lithium compound and the transition metal-containing compound are compounds that serve as raw materials for the composite oxide. The mixture is usually in a powder or particulate form.

[0019] The lithium compound may be one or more selected from the group consisting of lithium hydroxide, lithium carbonate, lithium nitrate, and lithium acetate. The lithium compound is preferably at least one of lithium hydroxide and lithium carbonate, more preferably lithium hydroxide. The lithium compound may be anhydrous or hydrated. When the lithium compound is lithium hydroxide, the lithium hydroxide may be anhydrous lithium hydroxide or lithium hydroxide hydrate. An example of the lithium hydroxide hydrate is lithium hydroxide monohydrate.

[0020] The transition metal-containing compound preferably contains one or more elements selected from the group consisting of Ni, Mn, and Co, and may contain Ni and at least one of Mn and Co, or may contain Ni, Mn, and Co. The transition metal-containing compound is preferably at least one of a nickel-containing hydroxide and a nickel-containing oxide, and more preferably contains a nickel-containing oxide or is a nickel-containing oxide. The nickel-containing compound may contain a metal element other than Ni in addition to Ni, preferably contains a transition metal element other than Ni, more preferably contains at least one of Mn and Co, or may contain Mn and Co.

[0021] The nickel-containing hydroxide is preferably a nickel composite hydroxide containing Ni and a metal element other than Ni. The nickel-containing oxide is preferably a nickel composite oxide containing Ni and a metal element other than Ni. The metal element other than Ni contained in the nickel composite hydroxide and nickel composite oxide is preferably a transition metal element other than Ni, more preferably at least one of Mn and Co, and may be Mn and Co. The nickel-containing compound is preferably a nickel composite oxide.

[0022] The mixture may further contain a compound containing M (wherein M has the same meaning as above). Examples of the compound containing M include one or more compounds selected from the group consisting of oxides containing M, hydroxides containing M, sulfides containing M, oxyhydroxides containing M, and halides containing M.

[0023] The contents of the lithium compound, the transition metal-containing compound, and the compound containing M in the mixture may be set so as to obtain a composite oxide having a desired composition.

[0024] When the first step includes a step of obtaining a mixture, the step may involve mixing the lithium compound, the transition metal-containing compound, and, if necessary, the compound containing M, using, for example, a mixer. The mixer may be a general mixer, such as a jet mill, a ball mill, a rocking mixer, a shaker mixer, a V-blender, a ribbon mixer, a Julia mixer, or a Loedige mixer.

[0025] The mixture in the first step is fired in an oxygen atmosphere at 750 to 1000°C using a heater. The mixture is usually fired in a firing furnace. The firing furnace is preferably a continuous firing furnace in which the mixture is fired while being transported. Examples of continuous firing furnaces include heater-heated kilns, such as roller hearth kilns, shuttle kilns, pusher kilns, tunnel kilns, and externally heated rotary kilns.

[0026] The mixture may be fired by being directly charged into the firing furnace, or by introducing a sagger containing the mixture into the firing furnace. For example, when a heater-heated kiln is used as the continuous firing furnace, in the first step, the mixture may be fired while the sagger containing the mixture is being transported within the kiln.

[0027] The sagger is made of ceramics, and examples of the material for forming the sagger include one or more selected from the group consisting of alumina, magnesia, zirconium, mullite, silica, carbon, and cordierite.

[0028] The oxygen atmosphere in which the mixture is fired can be formed, for example, by supplying oxygen into the firing furnace. The first step is preferably carried out while continuously supplying oxygen into the firing furnace.

[0029] The firing temperature of the mixture is 750 to 1000°C, and may be 760 to 950°C, 770 to 900°C, 780 to 880°C, or 790 to 850°C. Within the firing temperature range, the time for maintaining firing at the target firing temperature (hereinafter also referred to as "firing time") is, for example, 1 to 20 hours, 3 to 15 hours, 4 to 12 hours, or 5 to 10 hours. This firing time does not include the time required to raise the temperature to the target firing temperature. By adjusting the firing temperature and firing time within the above ranges, a positive electrode active material having a small Li-site occupancy rate and capable of improving the resistance of a secondary battery can be easily obtained.

[0030] In the firing in the first step, the mixture may be heated and held at a temperature below 750°C, and then fired at 750 to 1000°C. For example, the mixture may be held at a temperature of 300 to 700°C for 1 to 5 hours, and then fired at 750 to 1000°C. The holding temperature may be 400 to 600°C, or 450 to 550°C. The holding time may be 2 to 4 hours, or 2.5 to 3.5 hours.

[0031] (2nd process) This is a step of irradiating the composite oxide with microwaves, and the microwave irradiation is carried out while the temperature of the composite oxide obtained in the first step is 400°C or higher and lower than the firing temperature in the first step. This microwave irradiation can adjust at least one of the electron arrangement and crystal distortion of the composite oxide obtained in the first step, making it easier to obtain a positive electrode active material that has a small Li-site occupancy and can improve the resistance of secondary batteries.

[0032] Microwave irradiation is performed after the temperature of the composite oxide after the first step has been lowered below the calcination temperature, but under conditions where the temperature of the composite oxide is not too low (400°C or higher). Microwave irradiation is performed after cooling the composite oxide calcined in the first step, but before the composite oxide is cooled to below 400°C. The microwave irradiation temperature may be 400°C or higher but lower than the calcination temperature, but may also be 400°C or higher (calcination temperature -50°C), 420°C or higher (calcination temperature -100°C), or 450°C or higher (calcination temperature -150°C). If microwave irradiation is performed at a temperature equal to or higher than the calcination temperature, the composite oxide may overheat, releasing oxygen from the composite oxide and causing a flame. If the temperature of the composite oxide drops too low, microwave irradiation may be difficult to adjust the electronic arrangement and crystal distortion of the composite oxide, making it difficult to improve the Li-site occupancy and resistance of the secondary battery.

[0033] The method for cooling the composite oxide is not particularly limited. The composite oxide may be naturally cooled, or cold air may be blown onto the composite oxide. When the first step is performed using a continuous calcination furnace, the second step preferably involves irradiating the composite oxide removed from the continuous calcination furnace with microwaves. Since the temperature outside the continuous calcination furnace is usually lower than the temperature inside the continuous calcination furnace (e.g., 30 to 100°C), the composite oxide can be cooled by the temperature difference between the inside and outside of the continuous calcination furnace. This prevents the composite oxide from overheating, regulates the electron arrangement and crystal distortion of the composite oxide, reduces the Li site occupancy rate, and improves the resistance of the secondary battery.

[0034] The method of irradiating microwaves to the composite oxide removed from the continuous calcination furnace is not particularly limited. Preferably, as described above, in the first step, the mixture is calcined while the sagger containing the mixture is transported through a heater-heated kiln, and microwaves are irradiated to the composite oxide in the sagger discharged from the kiln. This allows the composite oxide in the sagger discharged from the kiln to be easily cooled. Therefore, microwaves can be irradiated to the cooled composite oxide, preventing overheating of the composite oxide due to microwave irradiation and preventing the generation of flames due to oxygen release from the composite oxide. The temperature of the composite oxide when irradiated with microwaves may be adjusted by holding the sagger discharged from the kiln for a certain period of time, or by adjusting the distance between the kiln and the microwave irradiation position, i.e., the transport distance of the sagger discharged from the kiln.

[0035] The method of irradiating microwaves is not limited to the above. For example, the composite oxide discharged from the continuous calcination furnace may be collected in a container and the composite oxide in the container may be irradiated with microwaves, or the composite oxide may be irradiated with microwaves while being discharged from the continuous calcination furnace.

[0036] Microwave irradiation can be carried out using a microwave irradiation device by placing the complex oxide in the microwave irradiation device. Alternatively, the complex oxide may be irradiated with microwaves by introducing a container in which the complex oxide has been collected or a sagger containing the complex oxide into the microwave irradiation device.

[0037] The microwave irradiation dose in the second step is preferably 200 to 1500 Wh, and may be 300 to 1200 Wh, 400 to 1100 Wh, or 500 to 1000 Wh per kg of composite oxide. The microwave output is preferably 4 kW or less, and may be 0.01 to 4 kW, 0.05 to 3.5 kW, 0.1 to 3 kW, 1 to 2.5 kW, or 1.2 to 2 kW. The microwave irradiation time is preferably 15 to 90 minutes, and may be 20 to 80 minutes, 25 to 70 minutes, or 30 to 60 minutes.

[0038] The microwave irradiation can be carried out in an oxygen atmosphere. The oxygen atmosphere can be formed by supplying oxygen into the space where the microwave irradiation is carried out, for example, by irradiating the material with microwaves while supplying oxygen.

[0039] The positive electrode active material obtained by this method can be used for a positive electrode plate. In the method for producing a positive electrode plate, the positive electrode active material produced by this method can be used to produce a positive electrode plate.

[0040] The positive electrode plate can have a positive electrode current collector foil and a positive electrode active material formed on one or both sides of the positive electrode current collector foil. The positive electrode active material is contained in a positive electrode active material layer, and the positive electrode active material layer can further include at least one of a binder and a conductive additive. The positive electrode active material layer can be formed by adding a solvent such as N-methyl-2-pyrrolidone (NMP) to materials that form the positive electrode active material layer, such as the positive electrode active material, binder, and conductive additive, to form a positive electrode mixture slurry, applying the slurry to the positive electrode current collector foil, drying, and compressing the slurry.

[0041] The positive electrode current collector foil is, for example, a metal foil made of an Al material such as Al or an Al alloy. Examples of binders include fluororesins such as polyvinylidene fluoride (PVdF) and polytetrafluoroethylene; cellulose-based resins such as carboxymethyl cellulose (CMC), methyl cellulose, and hydroxypropyl cellulose; and styrene butadiene rubber, and one or more of these can be used. Examples of conductive additives include carbon materials. Examples of carbon materials include fibrous carbon such as carbon nanotubes and carbon black, and one or more of these can be used.

[0042] The positive electrode plate obtained as described above can be used in a secondary battery. In the method for manufacturing a secondary battery, a secondary battery can be manufactured using the positive electrode plate manufactured by the above-described method for manufacturing a positive electrode plate.

[0043] The secondary battery may include an electrode assembly including a positive electrode plate and a non-aqueous electrolyte, and may have a battery case that accommodates the electrode assembly and the non-aqueous electrolyte. The battery case and the non-aqueous electrolyte may be any known battery case and non-aqueous electrolyte used in secondary batteries.

[0044] The electrode body may include the above-described positive electrode plate, negative electrode plate, and separator. In the electrode body, the positive electrode active material layer of the positive electrode plate and the negative electrode active material layer of the negative electrode plate face each other via the separator. The electrode body may be a laminate type in which a positive electrode plate, a negative electrode plate, and a separator are laminated, or a wound type in which a strip-shaped laminate in which a strip-shaped positive electrode plate, a strip-shaped negative electrode plate, and a strip-shaped separator are laminated is wound. The wound type electrode body may have a flat shape that is pressed after winding the laminate.

[0045] A negative electrode plate typically includes a negative electrode current collector foil and a negative electrode active material layer. The negative electrode current collector foil is a metal foil made of, for example, a copper material such as copper or a copper alloy. The negative electrode active material layer contains a negative electrode active material and may further contain a conductive additive, a binder, etc. The negative electrode active material layer can be formed by applying a negative electrode mixture slurry to the negative electrode current collector foil, drying, and compressing the slurry. The negative electrode active material layer can be formed by adding a solvent such as water to materials that form the negative electrode active material layer, such as the negative electrode active material, the binder, and the conductive additive.

[0046] Examples of the negative electrode active material include carbon-based active materials such as graphite, and metal-based active materials such as Si, SiOx (x=0.5-1.5), a composite of Si and C, and Sn, and one or more of these can be used. Examples of the binder include the above-mentioned cellulose-based resin, polyacrylic acid, styrene-butadiene rubber, and one or more of these can be used. Examples of the conductive additive include those mentioned above.

[0047] The separator may have a substrate and a functional layer on at least one side of the substrate. The substrate may be a film or a porous sheet such as a nonwoven fabric made of a resin such as a polyolefin (e.g., polyethylene or polypropylene), polyester, cellulose, or polyamide. The substrate may have a single-layer structure or a multilayer structure, and in the case of a multilayer structure, the materials of the layers may be the same or different. Examples of the functional layer include an adhesive layer formed with an adhesive and a heat-resistant layer containing an inorganic filler, a binder, etc.

[0048] The non-aqueous electrolyte solution is preferably a non-aqueous solvent such as an organic solvent containing an electrolyte. Examples of the electrolyte include LiPF6, LiBF4, LiClO4, LiFSO3, and LiBOB, and one or more of these can be used. Examples of the non-aqueous solvent include ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, propylene carbonate, butylene carbonate, and diethyl carbonate, and one or more of these can be used. [Example]

[0049] Hereinafter, the present disclosure will be described more specifically with reference to examples and comparative examples.

[0050] Examples 1 and 2 (Preparation of positive electrode active material) A nickel-containing oxide containing Ni, Co, and Mn in a molar ratio of Ni:Co:Mn = 83:5:12 was prepared as the transition metal-containing compound, and lithium hydroxide monohydrate (average particle size (D50): 10 μm) was prepared as the lithium compound. The transition metal-containing compound and the lithium compound were mixed to a molar ratio of Li:Ni:Co:Mn = 1.06:0.83:0.05:0.12 to obtain a mixture. The mixture was filled into an alumina crucible as a sagger, and the crucible was placed in an electric furnace to fire the mixture, thereby obtaining a lithium transition metal composite oxide (first step). The mixture was fired by first heating it to 500°C at an oxygen flow rate of 4 L / min and a heating rate of 5°C / min while measuring the temperature with an alumina-coated K-type thermocouple inserted into the mixture in the crucible, holding it at 500°C for 3 hours, then heating it to 805°C at a heating rate of 5°C / min, and firing it at this temperature for the time shown in Table 1.

[0051] The crucible was removed from the electric furnace and held outside the furnace for a certain period of time. 50 g of the lithium transition metal composite oxide was then removed and placed in a microwave irradiation device. It was then irradiated with microwaves at an output of 100 W in an oxygen atmosphere for the period of time shown in Table 1, yielding a positive electrode active material. The temperature of the lithium transition metal composite oxide when irradiated with microwaves was in the range of 400°C or higher and lower than 805°C. The positive electrode active material had a layered rock salt structure and consisted of secondary particles formed by agglomerations of 50 or more primary particles.

[0052] (Preparation of positive electrode plate) A positive electrode plate was fabricated using the positive electrode active material obtained above. The positive electrode active material, acetylene black (AB) as a conductive additive, and polyvinylidene fluoride (PVdF) as a binder were prepared in a mass ratio of positive electrode active material:AB:PVdF = 100:1:1, and these were mixed with N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode mixture slurry. The positive electrode mixture slurry was applied to aluminum foil as a positive electrode current collector foil, dried, compressed, and then cut to the specified size to obtain a positive electrode plate.

[0053] (negative plate) A negative electrode active material was prepared as a mixture of graphite and SiO. Styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC) were used as binders. The negative electrode active material, SBR, and CMC were prepared in a mass ratio of 100:1:1 (negative electrode active material:SBR:CMC), and these were mixed with water to prepare a negative electrode mixture slurry. The negative electrode slurry was applied to copper foil as a negative electrode current collector foil, dried, compressed, and then cut to the specified size to obtain a negative electrode plate.

[0054] (Fabrication of non-aqueous electrolyte secondary battery) A separator with a three-layer structure of polypropylene / polyethylene / polypropylene was prepared. A positive electrode plate and a negative electrode plate were stacked with the separator interposed therebetween to obtain an electrode assembly. A positive electrode tab formed of aluminum foil in the region of the positive electrode current collector foil where the positive electrode active material layer was not formed, and a negative electrode tab formed of copper foil in the region of the negative electrode current collector foil where the negative electrode active material layer was not formed, were exposed at both ends of the electrode assembly. The positive electrode tab was welded to an aluminum plate serving as an external positive electrode current collector, and the negative electrode tab was welded to a copper plate serving as an external negative electrode current collector. The electrode assembly was then inserted into an exterior body made of aluminum laminate film, and the film was welded to form a liquid injection port. A nonaqueous electrolyte was injected through the liquid injection port, and the liquid injection port was sealed to obtain a battery.

[0055] The non-aqueous electrolyte was prepared by dissolving lithium hexafluorophosphate (LiPF) as an electrolyte at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a volume ratio of EC:EMC = 1:3.

[0056] Comparative Example 1 A positive electrode active material was obtained in the same manner as described in Examples 1 and 2, except that microwave irradiation was not performed. The positive electrode active material had a layered rock salt structure and consisted of secondary particles formed by aggregation of 50 or more primary particles. A positive electrode plate and a nonaqueous electrolyte secondary battery were obtained in the same manner as described in Examples 1 and 2, except that this positive electrode active material was used.

[0057] Comparative Example 2 A positive electrode active material was obtained in the same manner as described in Examples 1 and 2, except that the baking time for baking the mixture at 805°C was changed to the time shown in Table 1 and microwave irradiation was not performed. The positive electrode active material had a layered rock salt structure and was composed of secondary particles formed by aggregation of 50 or more primary particles. A positive electrode plate and a nonaqueous electrolyte secondary battery were obtained in the same manner as described in Examples 1 and 2, except that this positive electrode active material was used.

[0058] [Calculation of Li seat occupancy rate] The positive electrode active materials obtained in the examples and comparative examples were subjected to X-ray diffraction measurement using an X-ray diffractometer (Rigaku's "SmartLab"). The amount of Ni present at the sites represented by 3a and 3b in Wyckoff notation (hereinafter sometimes referred to as the "3a site" and "3b site") was calculated by Rietveld analysis of the measurement data obtained by X-ray diffraction measurement. The Li composition at the 3a site and the Ni composition at the 3b site were variable, while the Co and Mn compositions were constant. The amount of Li present at the 3b site was calculated as the Li site occupancy rate [%]. The results are shown in Table 1.

[0059] [Resistance measurement] The secondary batteries obtained in the examples and comparative examples were activated by charging them to 4.2 V at a current rate of 0.1 C, followed by discharging at 0.1 C to 3.0 V. Subsequently, the secondary batteries were charged at a temperature of 25°C until the state of charge (SOC) reached 50%. After a one-hour rest, the batteries were discharged at a current value of 1 C for 10 seconds. The OCV (open circuit voltage) immediately before discharge was defined as V0 [V], and the voltage after 10 seconds was defined as V1 [V]. Resistance R was calculated according to the following formula. The smaller the resistance R, the more improved the resistance of the secondary battery. The results are shown in Table 1. Resistance R [Ω] = (V0-V1) [V] / 1C current value [A]

[0060] [Table 1]

[0061] In Examples 1 and 2, the microwave irradiation dose per 1 kg of the lithium transition metal composite oxide was 667 Wh and 1000 Wh, respectively.

Claims

1. a first step of calcining a mixture of a lithium compound and a transition metal-containing compound using a heater in an oxygen atmosphere at 750 to 1000°C to obtain a lithium transition metal composite oxide; a second step of irradiating the lithium transition metal composite oxide with microwaves while the temperature of the lithium transition metal composite oxide after the first step is 400°C or higher and lower than the baking temperature in the first step.

2. The method for producing a positive electrode active material according to claim 1 , wherein the transition metal-containing compound is at least one of a nickel-containing hydroxide and a nickel-containing oxide.

3. The method for producing a positive electrode active material according to claim 2 , wherein the transition metal-containing compound contains at least one of Mn and Co.

4. The method for producing a positive electrode active material according to claim 1 , wherein the lithium compound is at least one of lithium hydroxide and lithium carbonate.

5. The positive electrode active material is Li, Ni, Mn, Co, and M (M is one or more metal elements selected from the group consisting of Mg, Ca, Al, Ti, V, Cr, Fe, Cu, Zn, Zr, Nb, Mo, Ta, W, and Y), and 2. The method for producing a positive electrode active material according to claim 1, wherein a molar ratio of Li, Ni, Mn, Co, and M is Li:Ni:Mn:Co:M=a:x:y:z:t, where a, x, y, z, and t are 1.0≦a≦1.3, x+y+z+t=1, 0.25≦x≦0.9, 0<y≦0.6, 0<z≦0.6, and 0≦t≦0.

1.

6. The method for producing a positive electrode active material according to claim 1 , wherein the positive electrode active material includes secondary particles.

7. The method for producing a positive electrode active material according to claim 1 , wherein the first step is carried out using a continuous firing furnace that fires the mixture while conveying it.

8. The method for producing a positive electrode active material according to claim 7 , wherein the second step is performed on the lithium transition metal composite oxide removed from the continuous firing furnace.

9. The continuous firing furnace is a heater-heated kiln, 9. The method for producing a positive electrode active material according to claim 7, wherein in the first step, the mixture is fired in the kiln while a sagger containing the mixture is transported.

10. 2. The method for producing a positive electrode active material according to claim 1, wherein the microwave irradiation amount in the second step is 200 to 1500 Wh per 1 kg of the lithium transition metal composite oxide.

11. 2. The method for producing a positive electrode active material according to claim 1, wherein in the second step, an output of the microwave is 4 kW or less, and a time of irradiation of the microwave is 15 to 90 minutes.

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