Positive electrode active material, method for producing positive electrode active material, and lithium ion secondary battery

Abstract

To provide a cathode active material that restrains deterioration after charging and discharging of a lithium ion secondary battery is repeated, a method for producing the cathode active material, and a lithium ion secondary battery equipped with a cathode containing the cathode active material.SOLUTION: A cathode active material includes a first phase made of a composite oxide belonging to R-3m, and a second phase made of a composite oxide belonging to tetragonal or hexagonal crystal. The composite oxide belonging to R-3m contains at least one selected from Ni, Co, and Mn, and Li, and the composite oxide belonging to tetragonal or hexagonal crystal contains Sr and at least one selected from Li, Ni, Co, and Mn.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to a positive electrode active material, a method for producing a positive electrode active material, and a lithium ion secondary battery. [Background technology]

[0002] Composite oxides of lithium and transition metals (lithium transition metal composite oxides) are known as positive electrode active materials for lithium ion secondary batteries. To improve the properties of lithium transition metal composite oxides as positive electrode active materials, combinations of various different elements have been proposed. For example, Patent Document 1 describes a positive electrode active material in which the surface of a composite oxide containing nickel as a transition metal is modified with strontium. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2021 / 152996 Summary of the Invention [Problem to be solved by the invention]

[0004] It is desirable to suppress deterioration of lithium ion secondary batteries that use lithium transition metal composite oxides as the positive electrode active material after repeated charge and discharge. An object of the present disclosure is to provide a positive electrode active material that suppresses deterioration after repeated charging and discharging of a lithium ion secondary battery, a method for producing this positive electrode active material, and a lithium ion secondary battery including a positive electrode containing this positive electrode active material. [Means for solving the problem]

[0005] The means for solving the above problems include the following embodiments. <1> The first phase is made of a composite oxide belonging to R-3m, and the second phase is made of a composite oxide belonging to a tetragonal or hexagonal crystal, The composite oxide belonging to R-3m contains at least one selected from Ni, Co, and Mn, and Li, The composite oxide belonging to a tetragonal or hexagonal crystal system contains Sr and at least one selected from Li, Ni, Co, and Mn. <2> The X-ray diffraction pattern includes a peak corresponding to a complex oxide attributable to R-3m and a peak corresponding to a complex oxide attributable to a tetragonal or hexagonal crystal; The composite oxide belonging to R-3m contains at least one selected from Ni, Co, and Mn, and Li, The composite oxide belonging to a tetragonal or hexagonal crystal system contains Sr and at least one selected from Li, Ni, Co, and Mn. <3> a step of heating a mixture containing a compound containing at least one selected from Ni, Co, and Mn, a compound containing Li, and a compound containing Sr, <1> or <2> 10. A method for producing the positive electrode active material according to claim 9. <4> The step of heating the mixture includes, in this order, a step of heating the mixture at a temperature A and a step of heating the mixture at a temperature B higher than the temperature A; <3> 10. A method for producing the positive electrode active material according to claim 9. <5> <1> or <2> A lithium ion secondary battery comprising a positive electrode containing the positive electrode active material according to claim 1. [Effects of the Invention]

[0006] According to the present disclosure, there are provided a positive electrode active material that suppresses deterioration after repeated charging and discharging of a lithium ion secondary battery, a method for producing this positive electrode active material, and a lithium ion secondary battery including a positive electrode that includes this positive electrode active material. DETAILED DESCRIPTION OF THE INVENTION

[0007] In the present disclosure, a numerical range indicated using "to" means a range that includes the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In the numerical ranges described in stages in this disclosure, the upper or lower limit value described in a certain numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. In the numerical ranges described in this disclosure, the upper or lower limit value described in a certain numerical range may be replaced with the value shown in the examples. In the present disclosure, the term "process" includes not only an independent process but also a process that cannot be clearly distinguished from other processes, as long as the intended purpose of the process is achieved. In the present disclosure, a combination of two or more preferred embodiments is a more preferred embodiment. In the present disclosure, when there are multiple substances corresponding to each component, the amount of each component means the total amount of the multiple substances unless otherwise specified.

[0008] First Embodiment The first embodiment of the present disclosure is The first phase is made of a composite oxide belonging to R-3m, and the second phase is made of a composite oxide belonging to a tetragonal or hexagonal crystal, The composite oxide belonging to R-3m contains at least one selected from Ni, Co, and Mn, and Li, The composite oxide belonging to the tetragonal or hexagonal crystal system is a positive electrode active material containing Sr and at least one selected from Li, Ni, Co, and Mn.

[0009] In the present disclosure, a "positive electrode active material comprising a first phase made of a composite oxide belonging to R-3m and a second phase made of a composite oxide belonging to a tetragonal or hexagonal crystal system" refers to a positive electrode active material in which the first phase and the second phase exist within the bulk (e.g., within the same particle). Therefore, for example, a mixture of particles made of a composite oxide belonging to R-3m and particles made of a composite oxide belonging to a tetragonal or hexagonal crystal system does not fall under the category of a "positive electrode active material comprising a first phase and a second phase."

[0010] As will be shown in the examples described later, by using a positive electrode active material including a first phase made of a specific complex oxide belonging to R-3m and a second phase made of a specific complex oxide belonging to a tetragonal or hexagonal crystal system, deterioration of a lithium ion secondary battery after repeated charge and discharge can be suppressed.

[0011] (Phase 1) The first phase contained in the positive electrode active material has a crystal structure attributed to R-3m and is composed of a composite oxide containing at least one selected from Ni, Co, and Mn and Li. This composite oxide has a layered crystal structure in which transition metal layers composed of at least one transition metal selected from Ni, Co, and Mn and oxygen and lithium layers are arranged alternately. Specific examples of the composite oxide that constitutes the first phase include a compound represented by the composition LiMO2 (where M is at least one selected from Ni, Co, and Mn), and compounds obtained by adding a doping element to the above compound. Specific examples of doping elements include Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, La, W, B, Ru, Cd, Ag, Y, Sc, As, Sb, Pt, Au, and Si. The first phase may consist of only one type of composite oxide, or may consist of two or more types of composite oxides. The composite oxide constituting the first phase may or may not contain Sr.

[0012] The composite oxide constituting the first phase may contain any one transition metal selected from Ni, Co and Mn, or may contain two or three transition metals. The type of transition metal contained in the positive electrode active material may be selected depending on the application of the lithium ion secondary battery, etc. For example, when used as a positive electrode active material for a large-capacity lithium ion secondary battery such as an automobile battery, the first phase preferably contains at least Ni as a transition metal, more preferably contains Ni and at least one selected from Co and Mn, and even more preferably contains Ni, Co, and Mn (NCM, nickel cobalt manganese oxide).

[0013] The NCM may contain a high proportion of Ni (for example, 50 mol % or more, 60 mol % or more, or 70 mol % or more of the total transition metals).

[0014] The molar ratios of Ni, Co, and Mn contained in the NCM may be selected, for example, from the range of 1:0.1 to 1:1 for the molar ratio of Ni to Co (Ni:Co), and from the range of 1:0.1 to 1:1 for the molar ratio of Ni to Mn (Ni:Mn).

[0015] The molar ratio of Ni to Co (Ni:Co) may be selected from the range of 1:0.1 to 1:0.5, 1:0.1 to 1:0.3, or 1:0.1 to 1:0.2. The molar ratio of Ni to Mn (Ni:Mn) may be selected from the range of 1:0.1 to 1:0.5, 1:0.1 to 1:0.3, or 1:0.1 to 1:0.2.

[0016] (Phase 2) The second phase contained in the positive electrode active material has a crystal structure belonging to a tetragonal or hexagonal system, and is made of a composite oxide containing Sr and at least one element selected from Li, Ni, Co, and Mn. Specific examples of the composite oxide that constitutes the second phase include SrMnO3, Sr2MnO4, and SrNiMnO4. The second phase may consist of only one type of composite oxide, or may consist of two or more types of composite oxides.

[0017] The composition ratio of the composite oxide constituting the first phase and the composite oxide constituting the second phase in the positive electrode active material is not particularly limited. From the viewpoint of exhibiting the effect of suppressing battery degradation while maintaining the function as a positive electrode active material, the proportion of the complex oxide constituting the second phase in the total of the complex oxide constituting the first phase and the complex oxide constituting the second phase is preferably 0.01 mol % or more and 10 mol % or less, and more preferably 1 mol % or more and 5 mol % or less.

[0018] The composition ratio of the elements constituting the positive electrode active material is not particularly limited. From the viewpoint of exhibiting the effect of suppressing battery degradation while maintaining the function as a positive electrode active material, the proportion of Sr in the total of Ni, Co, Mn, Li, and Sr among the elements constituting the positive electrode active material is preferably 0.01 mol % or more and 10 mol % or less, and more preferably 1 mol % or more and 5 mol % or less.

[0019] The positive electrode active material may be in a particulate form. The volume average particle diameter of the particulate positive electrode active material is not particularly limited and can be selected, for example, from the range of 5 μm to 30 μm. When the positive electrode active material is in the form of secondary particles that are an aggregate of multiple primary particles, the volume average particle diameter is the volume average particle diameter of the secondary particles. The volume average particle size of the positive electrode active material particles is not particularly limited and can be selected, for example, from the range of 5 μm to 30 μm.

[0020] In the present disclosure, the volume average particle size of particles is the particle size (D50) at which the cumulative volume reaches 50% in the volume-based particle size distribution. The volume-based particle size distribution can be obtained, for example, by a laser diffraction / scattering method.

[0021] Whether or not the positive electrode active material contains a composite oxide that constitutes the first phase and a composite oxide that constitutes the second phase can be confirmed by a known analytical method such as XRD (X-ray diffraction).

[0022] Second Embodiment A second embodiment of the present disclosure is The X-ray diffraction pattern includes a peak corresponding to a complex oxide attributable to R-3m and a peak corresponding to a complex oxide attributable to a tetragonal or hexagonal crystal; The composite oxide belonging to R-3m contains at least one selected from Ni, Co, and Mn, and Li, The composite oxide belonging to the tetragonal or hexagonal crystal system is a positive electrode active material containing Sr and at least one selected from Li, Ni, Co, and Mn.

[0023] As will be shown in the examples described later, by using a positive electrode active material that exhibits an X-ray diffraction pattern including a peak corresponding to a specific composite oxide attributed to R-3m and a peak corresponding to a specific composite oxide attributed to a tetragonal or hexagonal crystal, deterioration of a lithium ion secondary battery after repeated charge and discharge can be suppressed.

[0024] The method for obtaining the X-ray diffraction pattern of the positive electrode active material is not particularly limited, and can be carried out by a known method.

[0025] The details and preferred aspects of the positive electrode active material of the second embodiment are the same as the details and preferred aspects of the positive electrode active material of the first embodiment.

[0026] <Third embodiment> A third embodiment of the present disclosure is a method for producing the positive electrode active material of the first or second embodiment described above, The method includes a step of heating a mixture containing a compound containing at least one selected from Ni, Co, and Mn, a compound containing Li, and a compound containing Sr.

[0027] In the method of the present disclosure, examples of the compound containing at least one selected from Ni, Co, and Mn, the compound containing Li, and the compound containing Sr include hydroxides, carbonates, and oxides. The compound containing at least one selected from Ni, Co, and Mn may be a composite compound containing two or more selected from Ni, Co, and Mn, or may be a compound containing Ni, a compound containing Co, a compound containing Mn, or a combination thereof.

[0028] In the method of the present disclosure, the step of heating a mixture containing a compound containing at least one selected from Ni, Co, and Mn, a compound containing Li, and a compound containing Sr more preferably includes a step of heating the mixture at temperature A and a step of heating the mixture at temperature B higher than temperature A, in this order. The mixture is heated at temperature A and then heated at temperature B, which is higher than temperature A, to efficiently produce the positive electrode active material of the first or second embodiment.

[0029] The temperature at which the heating step is carried out is not particularly limited as long as it is a temperature at which a desired composite oxide containing the elements in the mixture can be obtained. The heating temperature may be selected, for example, from the range of 600°C to 980°C. When the heating step includes a step of heating the mixture at temperature A and a step of heating the mixture at temperature B higher than temperature A, in that order, temperature A is preferably selected from the range of 600°C to 800°C, more preferably selected from the range of 600°C to 750°C, and even more preferably selected from the range of 600°C to 700°C. The time for the step of heating the mixture at temperature A is not particularly limited, and may be selected, for example, from between 1 hour and 10 hours. When the heating step includes a step of heating the mixture at temperature A and a step of heating the mixture at temperature B higher than temperature A, in that order, temperature B is preferably selected from the range of 680°C to 950°C, more preferably selected from the range of 750°C to 920°C, and even more preferably selected from the range of 800°C to 900°C. The time for the step of heating the mixture at temperature B is not particularly limited, and may be selected, for example, from between 1 hour and 10 hours.

[0030] The heating step can be carried out, for example, in an atmosphere with an oxygen content of 40% by volume to 100% by volume. The oxygen content during the heating step may be constant from the start to the end of the heating step, or may be varied.

[0031] <Fourth embodiment> A fourth embodiment of the present disclosure is a lithium ion secondary battery including a positive electrode containing the positive electrode active material of the first or second embodiment described above. The positive electrode includes, for example, a current collector and a positive electrode layer disposed on the current collector, and the positive electrode layer includes the positive electrode active material of the present disclosure. The positive electrode layer may be disposed on one or both sides of the current collector.

[0032] Examples of materials constituting the current collector of the positive electrode include aluminum, aluminum alloy, nickel, titanium, stainless steel, etc. Examples of the shape of the current collector include foil, mesh, etc.

[0033] The positive electrode layer is disposed on the current collector by, for example, applying a slurry of the positive electrode material to one or both sides of the current collector. If necessary, a pressure treatment may be performed to adjust the density of the positive electrode layer. The thickness of the positive electrode layer is not particularly limited and can be selected, for example, from the range of 10 μm to 100 μm.

[0034] The positive electrode material may be in the form of a mixture containing components other than the positive electrode active material, such as a conductive additive, a binder, etc. If necessary, a solvent may be added to the mixture to adjust the viscosity of the mixture.

[0035] Specific examples of the conductive aid include carbon materials such as carbon black (acetylene black, thermal black, furnace black, etc.), carbon nanotubes, and graphite. The conductive material contained in the positive electrode material may be one type alone or two or more types.

[0036] Specific examples of binders include polyvinylidene fluoride (PVDF), polyethylene, polypropylene, polyethylene terephthalate, cellulose, nitrocellulose, carboxymethyl cellulose, polyethylene oxide, polyepichlorohydrin, polyacrylonitrile, styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber (NBR), polyacrylate, and polymethacrylate. The binder contained in the positive electrode material may be one type alone or two or more types.

[0037] A lithium ion secondary battery includes, for example, a positive electrode, a negative electrode, and an electrolyte. The negative electrode includes, for example, a current collector and a negative electrode layer that is disposed on the current collector and contains a negative electrode active material. Examples of types of negative electrode active materials include carbon materials such as graphite, hard carbon, soft carbon, and activated carbon, silicon, metallic lithium, lithium alloys, and lithium titanate (LTO). Examples of materials constituting the negative electrode current collector include copper, copper alloy, nickel, titanium, stainless steel, etc. Examples of the shape of the negative electrode current collector include foil, mesh, etc.

[0038] The electrolyte may be either liquid or solid. As a liquid electrolyte (electrolytic solution), a known electrolyte such as LiPF6 dissolved in an organic solvent can be used without any particular limitation. Specific examples of the organic solvent include cyclic or chain carbonates such as ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), etc. The solvent may be a mixture of two or more solvents, or may be a mixture containing a cyclic carbonate and a chain carbonate. The solvent may contain an additive such as vinylene carbonate (VC). As the solid electrolyte, known solid electrolytes such as sulfide solid electrolytes, oxide solid electrolytes, and halide solid electrolytes can be used without any particular limitation.

[0039] The lithium ion secondary battery may include a separator disposed between the positive electrode and the negative electrode. Examples of the separator include nonwoven fabric, cloth, and microporous film mainly composed of polyolefin such as polyethylene or polypropylene. [Example]

[0040] The present disclosure will be described in more detail below with reference to examples, but the invention of the present disclosure is not limited to these examples.

[0041] <Preparation of positive electrode active material> NiSO4, CoSO4, and MnSO4 were dissolved in ion-exchanged water to obtain a raw material solution with a concentration of 30 mass %. The molar ratios of Ni, Co, and Mn in the raw material solution were as shown in Table 1.

[0042] A NH3 aqueous solution was added to the reaction vessel, and nitrogen substitution was performed while stirring. Next, NaOH was added to the reaction vessel to adjust the aqueous solution to alkaline. While controlling the pH in the reaction vessel to maintain a constant value, the raw material solution and NH3 were added dropwise to precipitate hydroxides of Ni, Co, and Mn. The resulting precipitate was filtered and dispersed in ion-exchanged water. The precipitate dispersed in ion-exchanged water was filtered and dried at 120°C for 16 hours to remove moisture, yielding transition metal hydroxides as precursors of the positive electrode active material.

[0043] Lithium hydroxide as a compound containing Li and strontium oxide as a compound containing Sr were added to the obtained precursor and mixed to obtain a raw material for the positive electrode active material. The amount of lithium hydroxide was adjusted so that the amount of Li was 1 mole per 1 mole of the total of the transition metals (Ni, Co, and Mn) in the precursor. The amount of strontium oxide was adjusted so that the Sr content relative to the total of the transition metals (Ni, Co, and Mn) and Sr in the precursor would be the value shown in Table 1 (mol %).

[0044] The raw material for the positive electrode active material was subjected to a heat treatment at 620° C. for 2 hours, followed by a heat treatment at 850° C. for 5 hours. Through these steps, a positive electrode active material was obtained.

[0045] <Battery evaluation> A positive electrode active material (88 parts by mass), acetylene black (10 parts by mass) as a conductive material, and polyvinylidene fluoride (2 parts by mass) as a binder were mixed, and the viscosity was adjusted with a solvent to obtain a positive electrode mixture. The positive electrode mixture was coated on aluminum foil and dried at 80°C for 5 minutes to obtain a positive electrode. A laminated test battery was fabricated using an electrode assembly formed by laminating the obtained positive electrode, a separator (a polyethylene microporous film), and a negative electrode containing graphite as an active material in this order, and an electrolyte solution. As the electrolyte, a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in which LiPF6 (concentration: 1 M) was dissolved (the volume ratio of EC / DMC / EMC was 3 / 4 / 3) was used.

[0046] For the evaluation battery, the discharge capacity when performing CCCV charging (1.5 to 4.1 V) at a current value of 0.1 C was measured and used as the initial capacity. Next, charge and discharge were performed 50 cycles under the conditions of 1 C, 1.5 to 4.1 V, and the discharge capacity after 50 cycles was measured. The capacity retention rate of the battery was determined by the following formula. The results are shown in Table 1. Capacity retention rate (%) = (Discharge capacity after 50 cycles / Initial capacity) × 100 The battery of Comparative Example 1 was produced in the same manner as the evaluation battery of the Example, except that strontium oxide was not mixed in the raw material of the positive electrode active material and the heat treatment was carried out in one step at 700 °C for 10 hours.

[0047] <XRD analysis> The X-ray diffraction patterns of the positive electrode active materials obtained in Examples 1 to 5 were acquired using a fully automatic multi-purpose X-ray diffractometer SmartLab manufactured by Rigaku Holdings Co., Ltd. As a result, peaks corresponding to a composite oxide belonging to R-3m as the composite oxide constituting the first phase and peaks corresponding to Sr2MnO4 as the composite oxide constituting the second phase were observed in the XRD patterns, respectively. From the above results, it was found that the positive electrode active materials obtained in Examples 1 to 5 contain a specific composite oxide constituting the first phase and a specific composite oxide constituting the second phase.

[0048]

Table 1

[0049] As shown in Table 1, the batteries of Examples 1 to 5, which were fabricated using a cathode active material containing a specific composite oxide constituting the first phase and a specific composite oxide constituting the second phase, had a higher capacity retention rate after repeated charge and discharge, and degradation was more suppressed, compared to the battery fabricated using the cathode active material of Comparative Example 1, which did not contain the specific composite oxide constituting the second phase. The reason for this is not clear, but it is thought that, for example, while the composite oxide constituting the first phase functions as an active material, the composite oxide constituting the second phase inhibits the reaction between the cathode active material and the electrolyte near the particle surface.

Claims

1. The first phase is made of a composite oxide belonging to R-3m, and the second phase is made of a composite oxide belonging to a tetragonal or hexagonal crystal, The composite oxide belonging to R-3m contains at least one selected from Ni, Co, and Mn, and Li, The composite oxide belonging to a tetragonal or hexagonal crystal system contains Sr and at least one element selected from Li, Ni, Co, and Mn.

2. It shows an X-ray diffraction pattern including a peak corresponding to a composite oxide attributable to R-3m and a peak corresponding to a composite oxide attributable to a tetragonal or hexagonal crystal; The composite oxide belonging to R-3m contains at least one selected from Ni, Co, and Mn, and Li, The composite oxide belonging to a tetragonal or hexagonal crystal system contains Sr and at least one element selected from Li, Ni, Co, and Mn.

3. A positive electrode active material as described in claim 1, wherein the proportion of Sr in the total of Ni, Co, Mn, Li and Sr among the elements constituting the positive electrode active material is 0.01 mol% or more and 10 mol% or less.

4. A positive electrode active material as described in claim 2, wherein the proportion of Sr in the total of Ni, Co, Mn, Li and Sr among the elements constituting the positive electrode active material is 0.01 mol% or more and 10 mol% or less.

5. 5. The method for producing a positive electrode active material according to claim 1, comprising a step of heating a mixture containing a compound containing at least one selected from Ni, Co, and Mn, a compound containing Li, and a compound containing Sr.

6. 6. The method for producing a positive electrode active material according to claim 5, wherein the step of heating the mixture includes a step of heating the mixture at a temperature A and a step of heating the mixture at a temperature B higher than the temperature A, in this order.

7. A lithium ion secondary battery comprising a positive electrode containing the positive electrode active material according to any one of claims 1 to 4 and an electrolyte solution.

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