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

Abstract

To disclose a positive electrode active material which has an O2 type structure and a large capacity.SOLUTION: A positive electrode active material according to the present disclosure contains Li-containing oxide particles. The Li-containing oxide particles have an O2 type structure and have a predetermined chemical composition. A particle size D50 of the Li-containing oxide particles is more than 0 μm and 3.0 μm or less, and a particle size D90 of the Li-containing oxide particles is 2.0 μm or more and 6.0 μm or less. The X-ray diffraction pattern of the Li-containing particles satisfies 0≤I2 / I1≤0.5. Here, the I1 represents the X-ray diffraction peak intensity derived from the (002) plane of the O2 type structure, and the I2 represents the X-ray diffraction peak intensity derived from the (003) plane of an O3 type structure.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present application discloses a positive electrode active material, a method for producing the positive electrode active material, and a lithium ion secondary battery. [Background technology]

[0002] Positive electrode active materials having an O2-type structure are known. As disclosed in Patent Document 1, a positive electrode active material having an O2-type structure is obtained by ion-exchanging at least a portion of Na in a sodium-containing oxide having a P2-type structure with Li. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-092824 Summary of the Invention [Problem to be solved by the invention]

[0004] Conventional positive electrode active materials with an O2-type structure have room for improvement in terms of capacity. [Means for solving the problem]

[0005] The present application discloses the following aspects as means for solving the above problems. <Aspect 1> A positive electrode active material comprising Li-containing oxide particles, the Li-containing oxide particles have an O2 type structure, The Li-containing oxide particles a Na b Mn x-p Ni y-q Co z-r M p+q+rO2 (where 0 < a ≤ 1.00, 0 ≤ b < 0.01, x + y + z = 1, and 0 ≤ p + q + r < 0.17, and the element M is at least one selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W). It has a chemical composition represented by The particle size D50 of the Li-containing oxide particles is more than 0 μm and 3.0 μm or less, The particle size D90 of the Li-containing oxide particles is 2.0 μm or more and 6.0 μm or less, The X-ray diffraction pattern of the Li-containing oxide particles satisfies 0 ≤ I2 / I1 ≤ 0.5, I1 is the X-ray diffraction peak intensity derived from the (002) plane of the O2-type structure, I2 is the X-ray diffraction peak intensity derived from the (003) plane of the O3-type structure, Positive electrode active material. <Aspect 2> The positive electrode active material of Aspect 1, <QQ000077>The particle size D10 of the Li-containing oxide particles is more than 0 μm and 2.0 μm or less, Positive electrode active material. <Aspect 3> A method for producing a positive electrode active material, Obtaining precursor particles containing at least one element of Mn, Ni, and Co, Coating the surface of the precursor particles with a Na source to obtain composite particles, Subjecting the composite particles to a main firing to obtain Na-containing oxide particles having a P2-type structure, and [[ID=A]] Contacting the Na-containing oxide particles with an ion exchange material to ion-exchange Na in the Na-containing oxide particles with Li to obtain Li-containing oxide particles having an O2-type structure, including The temperature of the main firing is 700°C or higher and lower than 950°C, The particle size D50 of the Na-containing oxide particles is more than 0 μm and 3.0 μm or less, The particle size D90 of the Na-containing oxide particles is 2.0 μm or more and 6.0 μm or less, The ion exchange temperature is equal to or higher than the melting point of the ion exchange material and equal to or lower than 300°C, The ion exchange time is 30 minutes or more and less than 3 hours. Manufacturing method. <Aspect 4> The manufacturing method of embodiment 3, The particle diameter D10 of the Na-containing oxide particles is more than 0 μm and 2.0 μm or less. Cathode active material. <Aspect 5> A lithium ion secondary battery having a positive electrode active material layer, an electrolyte layer, and a negative electrode active material layer, The positive electrode active material layer includes the positive electrode active material of embodiment 1 or 2. Lithium-ion secondary battery. [Effects of the Invention]

[0006] The positive electrode active material of the present disclosure has an O2 type structure and has a large capacity. [Brief explanation of the drawings]

[0007] [Figure 1] 1 shows an example of a flow of a method for producing a Li-containing oxide having an O2 type structure. [Figure 2] 1 shows a schematic diagram of an example of the configuration of a lithium ion secondary battery. [Figure 3] 1 shows an X-ray diffraction pattern of the positive electrode active material of Example 1. [Figure 4] 1 shows the X-ray diffraction patterns of the positive electrode active materials of Comparative Examples 1 to 3. [Figure 5] 1 shows the X-ray diffraction patterns of the positive electrode active materials of Comparative Examples 4 and 5. DETAILED DESCRIPTION OF THE INVENTION

[0008] 1.Cathode active material A positive electrode active material according to one embodiment includes Li-containing oxide particles. The Li-containing oxide particles have an O2-type structure. The Li-containing oxide particles include Li a Na b Mnx-p Ni y-q Co z-r M p+q+r O₂ (where 0 < a ≤ 1.00, 0 ≤ b < 0.01, x + y + z = 1, and 0 ≤ p + q + r < 0.17, and the element M is at least one selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W). It has a chemical composition represented by the following. The particle size D50 of the Li-containing oxide particles is more than 0 μm and 3.0 μm or less. The particle size D90 of the Li-containing oxide particles is 2.0 μm or more and 6.0 μm or less. The X-ray diffraction pattern of the Li-containing oxide particles satisfies 0 ≤ I2 / I1 ≤ 0.5. Here, I1 is the X-ray diffraction peak intensity derived from the (002) plane of the O₂-type structure, and I2 is the X-ray diffraction peak intensity derived from the (003) plane of the O3-type structure.)

[0009] 1.1 Crystal Structure The Li-containing oxide particles according to one embodiment have, as a crystal structure, at least an O₂-type structure (belonging to the space group P63mc). The Li-containing oxide particles according to one embodiment may have an O₂-type structure and also have a crystal structure other than the O₂-type structure. Examples of the crystal structure other than the O₂-type structure include, for example, the T♯2-type structure (belonging to the space group Cmca) formed when Li is deintercalated / inserted from the O₂-type structure, the O6-type structure (belonging to the space group R-3m, the c-axis length is 2.5 nm or more and 3.5 nm or less, typically 2.9 nm or more and 3.0 nm or less, and is different from the O3-type structure also belonging to the space group R-3m), etc. However, as will be described later, the Li-containing oxide particles according to the embodiment substantially do not contain the O3-type structure or contain it very little even if they do. The Li-containing oxide particles according to one embodiment may have an O₂-type structure as the main phase. The Li-containing oxide particles according to one embodiment may have a T♯2-type structure together with the O₂-type structure. The Li-containing oxide particles according to the embodiment may change the crystal structure that becomes the main phase depending on its charge / discharge state.

[0010] As will be described later, Li-containing oxide particles having an O2-type structure are produced by ion-exchanging Na in Na-containing oxide particles having a P2-type structure with Li. According to the inventors' findings, when the particle diameter of the Na-containing oxide particles is large, Na is more likely to remain in the Li-containing oxide particles after ion exchange. On the other hand, when the particle diameter of the Na-containing oxide particles is small, Na is less likely to remain, but if the ion-exchange time is inappropriate, O3-type structures are more likely to be produced in addition to O2-type structures in the Li-containing oxide particles after ion exchange. According to the inventors' findings, when the Li-containing oxide particles contain a large amount of O3-type structures, the capacity as a positive electrode active material decreases. In other words, the lower the X-ray diffraction peak derived from the O3-type structure relative to the X-ray diffraction peak derived from the O2-type structure, the higher the capacity as a positive electrode active material.

[0011] When an X-ray diffraction pattern of the Li-containing oxide particles according to one embodiment is obtained, the ratio I2 / I1 of the X-ray diffraction peak intensity I1 derived from the (002) plane of the O2-type structure to the X-ray diffraction peak intensity I2 derived from the (003) plane of the O3-type structure is sufficiently small, at 0.5 or less, and therefore the capacity as a positive electrode active material is large. The X-ray diffraction pattern may satisfy 0≦I2 / I1≦0.4, 0≦I2 / I1≦0.3, 0≦I2 / I1≦0.2, 0≦I2 / I1≦0.1, or I2 / I1=0. Note that in the present application, the terms "X-ray diffraction pattern of the Li-containing oxide particles" and "X-ray diffraction peak intensity" refer to those obtained under the following conditions. Specifically, an X-ray diffraction pattern was obtained for the Li-containing oxide particles using an X-ray diffractometer (Rigaku, SmartLab, fully automated multipurpose X-ray diffractometer) with a CuKα source, a tube voltage of 45 kV, a tube current of 200 mA, a step width of 0.02°, and a scan rate of 1° / min. From the X-ray diffraction pattern, the X-ray diffraction peaks originating from the (002) plane of the O2-type structure and the (003) plane of the O3-type structure were identified, and the I2 / I1 ratio was calculated from the intensities of each X-ray diffraction peak after subtracting the background values ​​near the peaks. Note that the positions of the X-ray diffraction peaks originating from the (002) plane of the O2-type structure and the (003) plane of the O3-type structure may vary depending on the Li content and transition metal composition.

[0012] 1.2 Chemical composition The Li-containing oxide particles according to one embodiment include Li a Na b Mn x-p Ni y-q Co z-r M p+q+rO2 (where 0 < a ≤ 1.00, 0 ≤ b < 0.01, x + y + z = 1, and 0 ≤ p + q + r < 0.17, and the element M is at least one selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W). When the Li-containing oxide particles have such a chemical composition, the O2-type structure is likely to be maintained. Also, since the composition ratio b of Na is less than 0.01, the Na remaining in the Li-containing oxide particles is sufficiently reduced, and the reversible capacity as a positive electrode active material increases. In the above chemical composition, a is greater than 0 and may be 0.10 or more, 0.20 or more, 0.30 or more, 0.40 or more, 0.50 or more, or 0.60 or more, and is also 1.00 or less and may be 0.90 or less, 0.80 or less, or 0.70 or less. Also, b is 0.01 or less and may be 0.00. Also, x is 0 or more and may be 0.10 or more, 0.20 or more, 0.30 or more, 0.40 or more, or 0.50 or more, and is also less than 1.00 and may be 0.90 or less, 0.80 or less, 0.70 or less, 0.60 or less, or 0.50 or less. Also, y is 0 or more and may be 0.10 or more or 0.20 or more, and is also less than 1.00 and may be 0.90 or less, 0.80 or less, 0.70 or less, 0.60 or less, 0.50 or less, 0.40 or less, 0.30 or less, or 0.20 or less. Also, z is 0 or more and may be 0.10 or more, 0.20 or more, or 0.30 or more, and is also less than 1.00 and may be 0.90 or less, 0.80 or less, 0.70 or less, 0.60 or less, 0.50 or less, 0.40 or less, or 0.30 or less. The element M has little contribution to charge and discharge. In this regard, in the above chemical composition, since p + q + r is less than 0.17, it is easy to ensure a high capacity. p + q + r may be 0.16 or less, 0.15 or less, 0.14 or less, 0.13 or less, 0.12 or less, 0.11 or less, or 0.10 or less. On the other hand, when the element M is included, the O2-type structure is likely to be stabilized. Also, the presence of the element M does not substantially affect the problem-solving mechanism by the positive electrode active material of the present disclosure. That is, regardless of the presence of the element M, since there is little O3-type structure and little remaining Na in the Li-containing oxide particles, the reversible capacity as a positive electrode active material can be improved.In the above chemical composition, p+q+r is 0 or more, and may be 0.01 or more, 0.02 or more, 0.03 or more, 0.04 or more, 0.05 or more, 0.06 or more, 0.07 or more, 0.08 or more, 0.09 or more, or 0.10 or more. The composition of O is approximately 2, but is not necessarily exactly 2.0 and is indefinite.

[0013] 1.3 Particle size The particle diameter D50 of the Li-containing oxide particles according to one embodiment is greater than 0 μm and less than 3.0 μm, and the particle diameter D90 is greater than 2.0 μm and less than 6.0 μm. Li-containing oxide particles having such D50 and D90 tend to have a low residual Na content (i.e., b in the chemical composition is less than 0.01). The particle diameter D50 of the Li-containing oxide particles may be 0.2 μm to 2.8 μm, 0.4 μm to 2.6 μm, 0.6 μm to 2.4 μm, 0.8 μm to 2.2 μm, or 1.0 μm to 2.0 μm. The particle diameter D90 of the Li-containing oxide particles may be 2.3 μm to 5.5 μm, 2.5 μm to 5.0 μm, 2.7 μm to 4.5 μm, or 2.9 μm to 4.0 μm. Furthermore, the particle diameter D10 of the Li-containing oxide particles according to one embodiment may be greater than 0 μm and equal to or less than 2.0 μm. The particle diameter D10 may be 0.1 μm or more and 1.9 μm or less, 0.2 μm or more and 1.8 μm or less, 0.3 μm or more and 1.7 μm or less, 0.4 μm or more and 1.6 μm or less, or 0.5 μm or more and 1.5 μm or less. In the present application, the term "particle diameter D50" refers to the particle diameter (median diameter) at 50% cumulative total in a volume-based particle size distribution determined by a laser diffraction / scattering method, the term "particle diameter D90" refers to the particle diameter at 90% cumulative total in a volume-based particle size distribution determined by a laser diffraction / scattering method, and the term "particle diameter D10" refers to the particle diameter at 10% cumulative total in a volume-based particle size distribution determined by a laser diffraction / scattering method.

[0014] 1.4 Shape As described below, Li-containing oxide particles having an O2-type structure can be obtained by substituting Li for Na in Na-containing oxide particles having a P2-type structure. The P2-type structure is a hexagonal crystal system with a large diffusion coefficient of Na ions, which facilitates crystal growth in a specific direction. In particular, when at least one of Mn, Ni, and Co is included as a transition metal element constituting the P2-type structure, plate-like crystal growth in a specific direction is facilitated. Therefore, the Na-containing oxide having the P2-type structure is likely to become plate-like particles with a large aspect ratio, in which the crystal growth direction is biased in a specific direction. The Li-containing oxide according to one embodiment may be obtained based on such plate-like Na-containing oxide particles, or may be obtained based on spherical Na-containing oxide particles as described below. That is, the shape of the Li-containing oxide may be plate-like or spherical. When the Li-containing oxide is spherical, the reaction resistance decreases due to the reduction in crystallite size, and the diffusion resistance inside the particles tends to decrease. Furthermore, when the Li-containing oxide is applied to a secondary battery or the like, the degree of curvature is reduced due to the spheroidization, which is thought to reduce the lithium ion conduction resistance. This, for example, improves the rate characteristics and tends to increase the reversible capacity. In the present application, "spherical particles" refers to particles having a circularity of 0.80 or more. The circularity of the particles may be 0.81 or more, 0.82 or more, 0.83 or more, 0.84 or more, 0.85 or more, 0.86 or more, 0.87 or more, 0.88 or more, 0.89 or more, or 0.90 or more. The circularity of the particles is 4πS / L. 2 where S is the orthogonal projected area of ​​the particle, and L is the perimeter of the orthogonal projected image of the particle. The circularity of a particle can be determined by observing the appearance of the particle using a scanning electron microscope (SEM), a transmission electron microscope (TEM), or an optical microscope. The circularity of multiple particles can be measured as an average value as follows: (1) First, the particle size distribution of the particles is measured. Specifically, the particle diameter at 10% of the cumulative value (D10) and the particle diameter at 90% of the cumulative value (D90) in the volume-based particle size distribution are determined by laser diffraction / scattering. (2) The appearance of the particles whose particle size distribution has been measured is observed by image observation using an SEM, TEM, or optical microscope, and 100 particles having a circle-equivalent diameter (the diameter of a circle having the same area as the orthogonal projection area of ​​the particle) equal to or greater than D10 and equal to or less than D90 as determined in (1) are randomly selected from the particles contained in the image. (3) The circularity of each of the 100 extracted particles is determined by image processing, and the average value is regarded as the "circularity of the particle."

[0015] 1.5 Other As described above, the positive electrode active material according to one embodiment has a large capacity due to the inclusion of the specific Li-containing oxide particles. From the viewpoint of further improving the capacity of the positive electrode active material, the content of the Li-containing oxide particles contained in the positive electrode active material may be 50% by mass to 100% by mass, 60% by mass to 100% by mass, 70% by mass to 100% by mass, 80% by mass to 100% by mass, 90% by mass to 100% by mass, 95% by mass to 100% by mass, or 99% by mass to 100% by mass, where 100% by mass is the total positive electrode active material. The positive electrode active material may be made of the specific Li-containing oxide particles.

[0016] 2. Manufacturing method of positive electrode active material The Li-containing oxide particles according to the above embodiment can be produced, for example, by the following method. As shown in FIG. 1, the method for producing a positive electrode active material according to one embodiment includes the following steps: S1: Obtaining precursor particles containing at least one element of Mn, Ni and Co; S2: Coating the surface of the precursor particles with a Na source to obtain composite particles; S3: subjecting the composite particles to a main calcination process to obtain Na-containing oxide particles having a P2 type structure; and S4: bringing the Na-containing oxide particles into contact with an ion exchange material to ion-exchange Na in the Na-containing oxide particles with Li, thereby obtaining Li-containing oxide particles having an O2-type structure. The temperature of the main firing is 700°C or higher and lower than 950°C, The particle diameter D50 of the Na-containing oxide particles is more than 0 μm and 3.0 μm or less, The particle diameter D90 of the Na-containing oxide particles is 2.0 μm or more and 6.0 μm or less, The ion exchange temperature is equal to or higher than the melting point of the ion exchange material and equal to or lower than 300°C, The ion exchange time is 30 minutes or more and less than 3 hours.

[0017] 2.1 S1 In step S1, precursor particles containing at least one element selected from Mn, Ni, and Co are obtained. The precursor particles may contain at least Mn and one or both of Ni and Co, or may contain at least Mn, Ni, and Co. The precursor particles may be a salt containing at least one element selected from Mn, Ni, and Co. For example, the precursor particles may be at least one of carbonate, sulfate, nitrate, and acetate. Alternatively, the precursor particles may be a compound other than a salt. For example, the precursor particles may be a hydroxide. The precursor particles may be a hydrate. The precursor particles may be a combination of multiple compounds. The particle size of the precursor particles is not particularly limited. In step S1, a precipitate may be obtained as the precursor particles by coprecipitation using an ion source capable of forming a precipitate in an aqueous solution with transition metal ions and a transition metal compound containing at least one element selected from Mn, Ni, and Co. The "ion source capable of forming a precipitate in an aqueous solution with transition metal ions" may be, for example, at least one selected from sodium salts such as sodium carbonate and sodium nitrate, sodium hydroxide, and sodium oxide. The transition metal compound may be the above salts or hydroxides containing at least one element selected from Mn, Ni, and Co. Specifically, in S1, the ion source and the transition metal compound may be prepared as solutions, and the solutions may be added dropwise and mixed to obtain a precipitate as precursor particles. In this case, for example, water is used as the solvent. Various sodium compounds may be used as bases, and aqueous ammonia may be added to adjust the basicity. In the case of the coprecipitation method, for example, an aqueous solution of a transition metal compound and an aqueous solution of sodium carbonate are prepared, and the respective solutions are added dropwise and mixed to obtain spherical precursor particles. Alternatively, precursor particles can be obtained by a sol-gel method. In S1, the precursor particles may contain element M. The element M is at least one selected from B, Mg, Al, K, Ca, Ti, V, Cr, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W. These elements M have the function of stabilizing, for example, the P2 type structure or the O2 type structure.The method for obtaining a precursor containing element M is not particularly limited. When obtaining a precursor by coprecipitation in S1, for example, an aqueous solution of a transition metal compound containing at least one of Mn, Ni, and Co, an aqueous solution of sodium carbonate, and an aqueous solution of a compound of element M are prepared, and the respective aqueous solutions are added dropwise and mixed to obtain a precursor containing element M together with at least one of Mn, Ni, and Co. Alternatively, in the production method of the present disclosure, element M may not be added in S1, and element M may be doped when Na-doping calcination is performed in S2 and S3 described below.

[0018] 2.2 S2 In step S2, the surfaces of the precursor particles obtained in step S1 are coated with a Na source to obtain composite particles. The Na source may be a salt containing Na, such as a carbonate or a nitrate, or a compound other than a salt, such as sodium oxide or sodium hydroxide. In step S2, the amount of Na source coated on the surfaces of the precursor particles may be determined taking into account the amount of Na lost during subsequent calcination. In step S2, the coverage rate of the Na source relative to the surface of the precursor is not particularly limited. For example, in step S2, the composite particles may be obtained by coating 40 area% or more, 50 area% or more, 60 area% or more, or 70 area% or more of the surface of the precursor particles with the Na source. If the coverage rate of the Na source is low, when the composite particles are calcined, P2 type crystals tend to grow in a specific direction on the surface of the composite particles, and the Na-containing oxide tends to become plate-like. If the coverage rate of the Na source is high, when the composite particles are calcined, the crystallites of the P2 type crystals tend to become small, and the shape of the Na-containing oxide particles tends to correspond to that of the precursor particles. In S2, the method for coating the surfaces of the precursor particles with the Na source is not particularly limited. As described above, when 40% or more of the surface area of ​​the precursor particles is coated with the Na source, examples of the method include a tumbling fluidized coating method and a spray drying method. That is, a coating solution in which the Na source is dissolved is prepared, and the coating solution is brought into contact with the surfaces of the precursor particles, and then the solution is dried simultaneously with or after the contact. By adjusting the coating conditions (temperature, time, number of times, etc.), it is possible to coat 40% or more of the surface area of ​​the precursor particles with the Na source. In S2, the precursor particles may be coated with an M source together with the Na source. For example, in S2, the precursor particles obtained in S1 may be mixed with a Na source and an M source containing the element M to obtain composite particles. The M source may be, for example, a salt containing the element M, such as a carbonate or sulfate, or a compound other than a salt, such as an oxide or hydroxide.

[0019] 2.3 S3 In S3, the composite particles obtained in S2 are subjected to a main calcination process to obtain Na-containing oxide particles having a P2 type structure. S3 may include the following S3-1 to S3-3.

[0020] In step S3-1, the composite particles are pre-baked at a temperature of 300°C or higher but lower than 700°C for 2 hours or higher but 10 hours or lower. The pre-baking temperature may be 400°C or higher but lower than 700°C, 450°C or higher but lower than 700°C, 500°C or higher but lower than 700°C, 550°C or higher but lower than 700°C, or 550°C or higher but lower than 650°C. The pre-baking time may be 2 hours or higher but lower than 8 hours, 3 hours or higher but lower than 8 hours, 4 hours or higher but lower than 8 hours, 5 hours or higher but lower than 8 hours, or 5 hours or higher but lower than 7 hours. The pre-baking atmosphere is not particularly limited, and may be, for example, an oxygen-containing atmosphere.

[0021] In S3-2, following the pre-calcination, the composite particles are subjected to main calcination. The main calcination temperature is 700°C or higher but lower than 950°C, and may be 800°C or higher but 920°C or lower. If the main calcination temperature is too low, the P2 phase will not be formed. If the main calcination temperature is too high, Na is likely to remain after the ion exchange in S4. The temperature rise conditions from the pre-calcination temperature to the main calcination temperature are not particularly limited. The main calcination time is not particularly limited and may be, for example, 30 minutes to 48 hours, 30 minutes to 24 hours, 30 minutes to 10 hours, or 30 minutes to 3 hours. However, the shape of the Na-containing oxide can be controlled by the main calcination time. As described above, in the method of the present disclosure, if the coverage of the Na source on the composite particles is 40 area % or higher, P2-type crystals with small crystallites are likely to form on the surface of the composite particles when the composite particles are calcined. In the method of the present disclosure, the P2 type crystals are grown along the surface of the particles so as to connect one P2 type crystallite to another, thereby making the shape of the Na-containing oxide particles correspond to the shape of the precursor particles. For example, if the precursor particles are spherical, the Na-containing oxide particles may also be spherical. If the firing time is too short, the P2 phase is not sufficiently generated. On the other hand, if the firing time is too long, the P2 phase grows, resulting in plate-like particles rather than spherical ones. As far as the inventors have confirmed, if the firing time is 30 minutes or more and 3 hours or less, the Na-containing oxide particles are likely to become spherical. The Na-containing oxide particles obtained after firing may have a structure in which multiple crystallites are present on the surface and the crystallites are connected to each other.

[0022] In step S3-3, following the main firing, the composite particles after the main firing are rapidly cooled (cooled at a cooling rate of 20°C / min or more) from a temperature T1 of 200°C or higher to a temperature T2 of 100°C or lower. The pre-firing and main firing are performed, for example, in a heating furnace. In step S3-3, for example, the composite is main fired in a heating furnace, then cooled to a temperature T1 of 200°C or higher in the heating furnace. After reaching temperature T1, the fired product is removed from the heating furnace and rapidly cooled outside the furnace to a temperature T2 of 100°C or lower. Temperature T1 may be any temperature of 200°C or higher, or any temperature of 250°C or higher. Temperature T2 may be any temperature of 100°C or lower, or any temperature of 50°C or lower, or may be the cooling end temperature. In the predetermined temperature range between temperature T1 and temperature T2, moisture is likely to penetrate between the layers of the P2-type structure due to atomic vibration, molecular motion, etc. When cooling the composite particles (Na-containing oxide particles having a P2-type structure) after the sintering process, it is believed that the amount of moisture that penetrates between the layers of the P2-type structure can be reduced by shortening the time spent in this temperature range where moisture easily penetrates (i.e., by rapid cooling). In this regard, in S3-3, when cooling the composite after the sintering process, cooling is performed in a dry atmosphere outside the furnace from a temperature T1 of 200°C or higher to a temperature T2 of 100°C or lower. This results in a high cooling rate (e.g., 20°C / min or higher) from temperature T1 to temperature T2, which makes it difficult for moisture to penetrate between the layers of the P2-type structure and prevents the collapse of the P2-type structure. As a result, Na can be efficiently ion-exchanged with Li in S4, which makes it easier to reduce the amount of Na remaining after the ion exchange.

[0023] By the above method, it is possible to produce Na-containing oxide particles having a P2 type structure and a predetermined chemical composition. c Mn x-p Ni y-q Co z-r M p+q+rThe Na-containing oxide particles may have a chemical composition represented by the formula (I) (O2). Here, 0.10≦c≦1.00, x+y+z=1, and 0≦p+q+r<0.17. M is at least one element selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W. When the Na-containing oxide particles have such a chemical composition, the P2 structure is easily maintained. In the above chemical composition, c may be 0.10 or more, 0.20 or more, 0.30 or more, 0.40 or more, 0.50 or more, or 0.60 or more, and may be 1.00 or less, 0.90 or less, 0.80 or less, or 0.70 or less. x, y, z, p, q, and r are as described above.

[0024] The Na-containing oxide particles obtained by the above method have a particle size distribution. In this embodiment, Na-containing oxide particles having a predetermined D50 and D90 are obtained by performing air classification on the precursor particles, the composite particles, or the Na-containing oxide particles, and then the Na-containing oxide particles are used in S4 described below. This results in Li-containing oxide particles having a predetermined D50 and D90. Specifically, the particle diameter D50 of the Na-containing oxide particles used in S4 is greater than 0 μm and less than or equal to 3.0 μm, and the particle diameter D90 is 2.0 μm to 6.0 μm. The particle diameter D50 may be 0.2 μm to 2.8 μm, 0.4 μm to 2.6 μm, 0.6 μm to 2.4 μm, 0.8 μm to 2.2 μm, or 1.0 μm to 2.0 μm. The particle diameter D90 may be 2.3 μm or more and 5.5 μm or less, 2.5 μm or more and 5.0 μm or less, 2.7 μm or more and 4.5 μm or less, or 2.9 μm or more and 4.0 μm or less. The particle diameter D10 of the Na-containing oxide particles may be more than 0 μm and 2.0 μm or less. The particle diameter D10 may be 0.1 μm or more and 1.9 μm or less, 0.2 μm or more and 1.8 μm or less, 0.3 μm or more and 1.7 μm or less, 0.4 μm or more and 1.6 μm or less, or 0.5 μm or more and 1.5 μm or less.

[0025] 2.4 S4 In step S4, the Na-containing oxide particles obtained in step S3 are brought into contact with an ion exchange material to ion-exchange Na in the Na-containing oxide particles with Li, thereby obtaining Li-containing oxide particles having an O2-type structure. Examples of the ion exchange material include a mixture of lithium halide and other lithium salts (e.g., molten salt). The lithium halide constituting the molten salt is preferably at least one of lithium chloride, lithium bromide, and lithium iodide. The other lithium salt constituting the molten salt is preferably lithium nitrate. The use of a molten salt lowers the melting point compared to the use of lithium halide or other lithium salts alone, enabling ion exchange at lower temperatures. The ion exchange temperature is between the melting point of the ion exchange material and 300°C. If the ion exchange temperature is too high, a stable O3-type structure is likely to be formed rather than an O2-type structure. On the other hand, from the viewpoint of shortening the ion exchange time, it is preferable that the ion exchange temperature be as high as possible. The ion exchange time is between 30 minutes and 3 hours. If the ion exchange time is too short, the amount of residual Na in the Li-containing oxide particles increases, whereas if the ion exchange time is too long, O3-type structures and the like tend to form along with O2-type structures.

[0026] 3. Lithium-ion secondary batteries FIG. 2 schematically shows the configuration of a lithium-ion secondary battery according to one embodiment. As shown in FIG. 2, a lithium-ion secondary battery 100 according to one embodiment has a positive electrode active material layer 10, an electrolyte layer 20, and a negative electrode active material layer 30. Here, the positive electrode active material layer 10 contains the positive electrode active material according to the above embodiment. As shown in FIG. 2, the lithium-ion secondary battery 100 may include a positive electrode current collector 40 and a negative electrode current collector 50. The configuration of the lithium-ion secondary battery 100 other than the positive electrode active material is the same as that of a conventional battery, and may employ, for example, the configuration described in Patent Document 1 (JP 2010-092824 A). [Example]

[0027] As described above, one embodiment of the positive electrode active material, the method for manufacturing the positive electrode active material, and the lithium ion secondary battery has been described, but the technology of the present disclosure can be modified in various ways other than the above embodiment without departing from the gist thereof. Below, the technology of the present disclosure will be described in more detail with reference to examples, but the technology of the present disclosure is not limited to the following examples.

[0028] 1. Preparation of positive electrode active material 1.1 Example 1 1.1.1 Preparation of precursor particles (1) MnSO4·5H2O, NiSO4·6H2O, and CoSO4·7H2O were weighed to achieve the desired composition ratio and dissolved in distilled water to a concentration of 1.2 mol / L to obtain solution 1. In a separate container, Na2CO3 was dissolved in distilled water to a concentration of 1.2 mol / L to obtain solution 2. (2) 1000 mL of pure water was placed in a reaction vessel (with a baffle plate), and 500 mL of the first solution and 500 mL of the second solution were added dropwise thereto at a rate of about 4 mL / min. (3) After the dropwise addition was completed, the mixture was stirred at room temperature for 1 hour at a stirring speed of 150 rpm to obtain a product. (4) The product was washed with pure water, and the solid was separated into liquid using a centrifuge to recover the precipitate. (5) The obtained precipitate was dried overnight at 120°C, crushed in a mortar, and then separated into coarse particles and fine particles by air classification. Both the coarse particles and the fine particles were composite salts containing Mn, Ni, and Co. In Example 1, the above-mentioned fine particles were used as precursor particles.

[0029] 1.1.2 Preparation of composite particles The precursor particles and Na2CO3 were mixed to form a mixture of Na2CO3 and Na2CO3. 0.7 Mn 0.5 Ni 0.2 Co 0.3 The surfaces of the precursor particles were coated with Na2CO3, and composite particles were obtained.

[0030] 1.1.3 Sintering of composite particles The composite particles were placed in an alumina crucible and fired in an air atmosphere to obtain a Na-containing oxide having a P2 structure under the firing conditions (1) to (7) below. (1) The alumina crucible containing the composite particles is placed in a heating furnace in an air atmosphere. (2) The temperature inside the heating furnace is raised from room temperature (25°C) to 600°C in 115 minutes. (3) The temperature in the heating furnace is maintained at 600°C for 360 minutes to perform pre-baking. (4) After the preliminary firing, the temperature inside the heating furnace is increased from 600°C to 900°C in 60 minutes. (5) The temperature in the heating furnace is kept at 900°C for 60 minutes to carry out the main firing. (6) After the main firing, the temperature in the heating furnace is lowered from 900°C to 250°C over 130 minutes. (7) At 250°C, the alumina crucible is removed from the heating furnace and allowed to cool in a dry atmosphere outside the furnace.

[0031] The fired product after cooling was pulverized in a mortar in a dry atmosphere to obtain Na-containing oxide particles A having a P2 structure. The chemical composition and particle diameters D10, D50, and D90 of the Na-containing oxide particles A are as shown in Table 1 below.

[0032] 1.1.4 Ion exchange (1) LiNO3 and LiCl were weighed out to a molar ratio of 50:50, and mixed with the above-mentioned Na-containing oxide particles A in a molar ratio that was 10 times the minimum Li amount required for ion exchange to obtain a mixture. (2) Using an alumina crucible, ion exchange was carried out in an air atmosphere at 280°C for 1 hour to obtain a product containing Li-containing oxide particles. (3) The salt remaining in the product was washed with pure water, and solid-liquid separation was carried out by vacuum filtration to obtain a precipitate. (4) The obtained precipitate was dried overnight at 120° C. to obtain the positive electrode active material according to Example 1.

[0033] 1.2 Comparative Example 1 1.2.1 Preparation of precursor particles and composite particles Precursor particles and composite particles were obtained in the same manner as in Example 1.

[0034] 1.2.2 Sintering of composite particles The composite particles were placed in an alumina crucible and fired in an air atmosphere to obtain Na-containing oxide particles having a P2 structure under the firing conditions (1) to (7) below. (1) The alumina crucible containing the composite particles is placed in a heating furnace in an air atmosphere. (2) The temperature inside the heating furnace is raised from room temperature (25°C) to 600°C in 115 minutes. (3) The temperature in the heating furnace is maintained at 600°C for 360 minutes to perform pre-baking. (4) After the preliminary firing, the temperature inside the heating furnace is increased from 600°C to 950°C in 70 minutes. (5) The temperature in the heating furnace is kept at 950°C for 60 minutes to carry out the main firing. (6) After the main firing, the temperature in the heating furnace is lowered from 950°C to 250°C over 140 minutes. (7) At 250°C, the alumina crucible is removed from the heating furnace and allowed to cool in a dry atmosphere outside the furnace.

[0035] The fired product after cooling was pulverized in a mortar in a dry atmosphere to obtain Na-containing oxide particles B having a P2 structure. The chemical composition and particle diameters D10, D50, and D90 of the Na-containing oxide particles B are shown in Table 1 below.

[0036] 1.2.3 Ion exchange Ion exchange was carried out under the same conditions as in Example 1, except that the Na-containing oxide particles B were used instead of the Na-containing oxide particles A, to obtain a positive electrode active material according to Comparative Example 1.

[0037] 1.3 Comparative Example 2 1.3.1 Preparation of precursor particles and composite particles Precursor particles and composite particles were obtained in the same manner as in Example 1.

[0038] 1.3.2 Sintering of composite particles The composite particles were placed in an alumina crucible and fired in an air atmosphere to obtain a Na-containing oxide having a P2 structure under the firing conditions (1) to (7) below. (1) The alumina crucible containing the composite particles is placed in a heating furnace in an air atmosphere. (2) The temperature inside the heating furnace is raised from room temperature (25°C) to 600°C in 115 minutes. (3) The temperature in the heating furnace is maintained at 600°C for 360 minutes to perform pre-baking. (4) After the preliminary firing, the temperature inside the heating furnace is increased from 600°C to 1000°C in 80 minutes. (5) The temperature in the heating furnace is maintained at 1000°C for 60 minutes to carry out the main firing. (6) After the main firing, the temperature in the heating furnace is lowered from 1000°C to 250°C over 150 minutes. (7) At 250°C, the alumina crucible is removed from the heating furnace and allowed to cool in a dry atmosphere outside the furnace.

[0039] The fired product after cooling was pulverized in a mortar in a dry atmosphere to obtain Na-containing oxide particles C having a P2 type structure. The chemical composition and particle diameters D10, D50, and D90 of the Na-containing oxide particles C are as shown in Table 1 below.

[0040] 1.3.3 Ion exchange Ion exchange was carried out under the same conditions as in Example 1, except that the Na-containing oxide particles C were used instead of the Na-containing oxide particles A, to obtain a positive electrode active material according to Comparative Example 2.

[0041] 1.4 Comparative Example 3 1.4.1 Preparation of precursor particles and composite particles Precursor particles and composite particles were obtained in the same manner as in Example 1, except that the above-mentioned coarse particles were used as precursor particles.

[0042] 1.4.2 Sintering of composite particles The composite particles were placed in an alumina crucible and fired in an air atmosphere to obtain a Na-containing oxide having a P2 structure under the firing conditions (1) to (7) below. (1) The alumina crucible containing the composite particles is placed in a heating furnace in an air atmosphere. (2) The temperature inside the heating furnace is raised from room temperature (25°C) to 600°C in 115 minutes. (3) The temperature in the heating furnace is maintained at 600°C for 360 minutes to perform pre-baking. (4) After the preliminary firing, the temperature inside the heating furnace is increased from 600°C to 1000°C in 80 minutes. (5) The temperature in the heating furnace is maintained at 1000°C for 1440 minutes to carry out the main firing. (6) After the main firing, the temperature in the heating furnace is lowered from 1000°C to 250°C over 150 minutes. (7) At 250°C, the alumina crucible is removed from the heating furnace and allowed to cool in a dry atmosphere outside the furnace.

[0043] The fired product after cooling was pulverized in a mortar in a dry atmosphere to obtain Na-containing oxide particles D having a P2 type structure. The chemical composition and particle diameters D10, D50, and D90 of the Na-containing oxide particles D are as shown in Table 1 below.

[0044] 1.4.3 Ion exchange Ion exchange was carried out under the same conditions as in Example 1, except that Na-containing oxide particles D were used instead of Na-containing oxide particles A, to obtain a positive electrode active material according to Comparative Example 3.

[0045] 1.5 Comparative Example 4 1.5.1 Preparation of precursor particles and composite particles, and calcination of composite particles Precursor particles and composite particles were obtained in the same manner as in Example 1, and then the composite particles were fired and pulverized under the same conditions as in Example 1 to obtain Na-containing oxide particles E having a P2 type structure. The chemical composition and particle diameters D10, D50, and D90 of the Na-containing oxide particles E are as shown in Table 1 below.

[0046] 1.5.2 Ion exchange Ion exchange was performed under the same conditions as in Example 1, except that Na-containing oxide particles E were used instead of Na-containing oxide particles A and the ion exchange time was changed from 1 hour to 3 hours, thereby obtaining a positive electrode active material according to Comparative Example 4.

[0047] 1.6 Comparative Example 5 1.6.1 Preparation of precursor particles and composite particles, and calcination of composite particles Precursor particles and composite particles were obtained in the same manner as in Comparative Example 3, and then the composite particles were fired and pulverized under the same conditions as in Comparative Example 3 to obtain Na-containing oxide particles F having a P2 type structure. The chemical composition and particle diameters D10, D50, and D90 of the Na-containing oxide particles F are as shown in Table 1 below.

[0048] 1.6.2 Ion exchange Ion exchange was performed under the same conditions as in Example 1, except that Na-containing oxide particles F were used instead of Na-containing oxide particles A and the ion exchange time was changed from 1 hour to 3 hours, to obtain a positive electrode active material according to Comparative Example 5.

[0049] 2. Evaluation of positive electrode active material 2.1 Elemental analysis The chemical composition was determined by elemental analysis for each of the positive electrode active materials of Example 1 and Comparative Examples 1 to 5. The results are shown in Table 2 below.

[0050] 2.2 Particle size distribution measurement The particle size distribution was measured to identify the particle diameters D10, D50, and D90 for each of the positive electrode active materials of Example 1 and Comparative Examples 1 to 5. The results are shown in Table 2 below.

[0051] 2.2 Identification of crystal structure by X-ray diffraction measurement X-ray diffraction measurements were performed using CuKα as a radiation source to obtain X-ray diffraction patterns for each of the positive electrode active materials of Example 1 and Comparative Examples 1 to 5. Figures 3 to 5 show the X-ray diffraction patterns for each of Example 1 and Comparative Examples 1 to 5. As shown in Figures 3 to 5, it can be seen that all of the positive electrode active materials of Example 1 and Comparative Examples 1 to 5 have an O2-type structure. Table 3 below shows the crystalline structure contained in each of the positive electrode active materials of Example 1 and Comparative Examples 1 to 5, as well as the ratio I2 / I1 of the X-ray diffraction peak intensity I2 derived from the (003) plane of the O3-type structure to the X-ray diffraction peak intensity I1 derived from the (002) plane of the O2-type structure.

[0052] 3. Preparation of evaluation cells Coin cells were fabricated using the positive electrode active materials of Examples 1 and 2 and Comparative Examples 1 and 2. The coin cell fabrication procedure was as follows. (1) The positive electrode active material, acetylene black (AB) as a conductive additive, and polyvinylidene fluoride (PVdF) as a binder were weighed out in a mass ratio of positive electrode active material:AB:PVdF = 85:10:5, and dispersed and mixed in N-methyl-2-pyrrolidone to obtain a positive electrode composite slurry. The positive electrode composite slurry was coated onto aluminum foil and vacuum dried overnight at 120°C to obtain a positive electrode, which is a laminate of a positive electrode active material layer and a positive electrode current collector. (2) TDDK-217 (manufactured by Daikin Corporation) was prepared as the electrolyte. (3) Metallic lithium foil was prepared as the negative electrode. (4) A coin cell (CR2032) was fabricated using the positive electrode, electrolyte, and negative electrode.

[0053] 4.Charge-discharge characteristic evaluation Each coin cell was charged and discharged at 0.1 C (1 C = 220 mA / g) in a voltage range of 2.0 to 4.8 V in a thermostatic chamber maintained at 25° C., and the discharge capacity was measured. The results are shown in Table 3 below.

[0054] 5. Evaluation Results Table 1 below shows the firing temperature, chemical composition, and particle diameters D10, D50, and D90 for each of the Na-containing oxide particles A to F used in Example 1 and Comparative Examples 1 to 5. Table 2 below also shows the ion exchange time, chemical composition, and particle diameters D10, D50, and D90 for each of the positive electrode active materials in Example 1 and Comparative Examples 1 to 5. Table 3 below also shows the crystalline structure contained in the positive electrode active material, I2 / I1 determined from the X-ray diffraction pattern, and the discharge capacity of the evaluation cell for each of Example 1 and Comparative Examples 1 to 5.

[0055] [Table 1]

[0056] [Table 2]

[0057] [Table 3]

[0058] As is clear from the results shown in Tables 1 to 3, in Comparative Examples 1 to 3, the sintering temperature during preparation of the Na-containing oxide particles having a P2 structure was too high, so ion exchange after sintering did not proceed sufficiently, and Na remained in the Li-containing oxide particles. In particular, in Comparative Examples 2 and 3, the particle size of the Na-containing oxide particles after sintering was too large, making it difficult for ion exchange to proceed further, and the amount of Na remaining in the Li-containing oxide particles increased. In Comparative Example 5, the particle size of the Na-containing oxide particles after sintering was too large, so Na remained in the Li-containing oxide particles after ion exchange, even though the ion exchange time was extended. In Comparative Example 4, the particle size of the Na-containing oxide particles after sintering was small, but the ion exchange time after sintering was too long, so an O3 phase was formed in the Li-containing oxide after ion exchange. Due to these factors, the Li-containing oxide particles according to Comparative Examples 1 to 5 had lower capacities as positive electrode active materials than the Li-containing oxide particles according to Example 1. In contrast, the Li-containing oxide particles according to Example 1 had a suitable chemical composition, particle size, and crystal structure, and therefore had a high capacity as a positive electrode active material.

[0059] 6. Supplementary Information In the above examples, the case of obtaining precursor particles by the coprecipitation method was exemplified, but the precursor particles can also be obtained by other methods. Further, in the above examples, the case of obtaining composite particles by mixing precursor particles and a Na source (Na2CO3) was exemplified, but the composite particles can also be obtained by other methods. Further, in the above examples, as the Na-containing oxide having a P2-type structure or the Li-containing oxide having an O2-type structure, those having a predetermined chemical composition were exemplified, but the chemical compositions of the Na-containing oxide and the Li-containing oxide are not limited thereto. For example, the Na-containing oxide and the Li-containing oxide may be doped with an element M other than Mn, Ni, and Co. The element M is as described in the embodiment.

[0060] From the above results, it can be said that the Li-containing oxide particles satisfying the following requirements (1) to (5) have a high capacity as a positive electrode active material. (1) The Li-containing oxide particles have an O2-type structure. (2) The Li-containing oxide particles contain Li a Na b Mn x-p Ni y-q Co z-r M p+q+r O2 (where 0 < a ≤ 1.00, 0 ≤ b < 0.01, x + y + z = 1, and 0 ≤ p + q + r < 0.17, and the element M is at least one selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W). (3) The particle size D50 of the Li-containing oxide particles is more than 0 μm and 3.0 μm or less. (4) The particle size D90 of the Li-containing oxide particles is 2.0 μm or more and 6.0 μm or less. (5) The X-ray diffraction pattern of the Li-containing oxide particles satisfies 0 ≤ I2 / I1 ≤ 0.5, where I1 is the X-ray diffraction peak intensity derived from the (002) plane of the O2-type structure, and I2 is the X-ray diffraction peak intensity derived from the (003) plane of the O3-type structure.

Explanation of symbols

[0061] 100 Lithium-ion secondary battery 10 Cathode active material layer 20 Electrolyte layer 30 Negative electrode active material layer 40 Positive electrode current collector 50 Negative electrode current collector

Claims

1. A method for producing a positive electrode active material, obtaining precursor particles comprising at least one element of Mn, Ni and Co; coating the surfaces of the precursor particles with a Na source to obtain composite particles; The composite particles are subjected to a main firing process to obtain Na-containing oxide particles having a P2 type structure; and bringing the Na-containing oxide particles into contact with an ion exchange material to ion-exchange Na in the Na-containing oxide particles with Li, thereby obtaining Li-containing oxide particles having an O2-type structure; The temperature of the main firing is 700°C or higher and lower than 950°C, The particle diameter D50 of the Na-containing oxide particles is more than 0 μm and 3.0 μm or less, The particle diameter D90 of the Na-containing oxide particles is 2.0 μm or more and 6.0 μm or less, The ion exchange temperature is equal to or higher than the melting point of the ion exchange material and equal to or lower than 300°C, The ion exchange time is 30 minutes or more and less than 3 hours. Manufacturing method.

2. The method of claim 1, The particle diameter D10 of the Na-containing oxide particles is more than 0 μm and 2.0 μm or less. Manufacturing method.

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