Method for producing positive electrode active material

Abstract

To disclose a method for manufacturing an O2 positive electrode active material with an excellent rate characteristic.SOLUTION: The method for manufacturing a positive electrode active material with an O2-type structure of the present disclosure includes the steps of: obtaining a precursor by a coprecipitation technique; reacting the precursor and an Na source with each other and obtaining an Na-containing oxide with a P2-type structure; and ion-exchanging at least a part of Na of the Na-containing oxide with Li and obtaining a Li-containing oxide with an O2-type structure. The power for agitation at the time of coprecipitation is in the range of 0.020 W and 0.200 W, both inclusive.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present application discloses a method for producing a positive electrode active material. [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. 2014-186937 Summary of the Invention [Problem to be solved by the invention]

[0004] There is room for improvement in the rate characteristics of positive electrode active materials having an O2 structure. [Means for solving the problem]

[0005] The present application discloses the following aspects as means for solving the above problems. <Aspect 1> A method for producing a positive electrode active material having an O2 type structure, obtaining the precursor by coprecipitation; reacting the precursor with a Na source to obtain a Na-containing oxide having a P2 type structure; and ion-exchanging at least a portion of the Na in the Na-containing oxide with Li to obtain a Li-containing oxide having an O2-type structure; Including, The stirring power required during coprecipitation is 0.020 W or more and 0.200 W or less. Manufacturing method. <Aspect 2> The manufacturing method of embodiment 1, The dropping rate during coprecipitation is 4 ml / min or more and 20 ml / min or less. Manufacturing method. <Aspect 3> The production method of aspect 1 or 2, The precursor contains at least one element of Mn, Ni, and Co. Manufacturing method. <Aspect 4> In the production method of any one of aspects 1 to 3, obtaining a precipitate as the precursor by the coprecipitation method using an ion source capable of forming a precipitate in an aqueous solution together with transition metal ions and a transition metal compound containing at least one element selected from Mn, Ni, and Co. Manufacturing method. <Aspect 5> In the production method of any one of aspects 1 to 4, Coating the surface of the precursor with a Na source to obtain a composite; and calcining the composite to obtain the Na-containing oxide; Including, the precursor is a spherical particle, The composite is obtained by covering 40% or more by area of ​​the surface of the precursor with the Na source; and The Na-containing oxide is a spherical particle. Manufacturing method. [Effects of the Invention]

[0006] According to the method of the present disclosure, a positive electrode active material having an O2 type structure and having excellent rate characteristics can be produced. [Brief explanation of the drawings]

[0007] [Figure 1] 1 shows an example of a flow of a method for producing a Na-containing oxide having an O2 type structure. [Figure 2] 1 shows SEM photographs of the appearance of precursors of Examples 1 and 2 and Comparative Examples 1 to 3. [Figure 3]1 shows the relationship between the circularity of the precursor and the stirring power required during coprecipitation. [Figure 4] 1 shows SEM photographs of the appearance of the positive electrode active materials of Examples 1 and 2 and Comparative Examples 1 to 3. DETAILED DESCRIPTION OF THE INVENTION

[0008] 1, a method for producing a cathode active material having an O2-type structure according to one embodiment includes obtaining a precursor by coprecipitation (S1), reacting the precursor with a Na source to obtain a Na-containing oxide having a P2-type structure (S2), and ion-exchanging at least a portion of the Na in the Na-containing oxide with Li to obtain a Li-containing oxide having an O2-type structure (S3). The stirring power required during the coprecipitation is 0.020 W or more and 0.200 W or less.

[0009] 1.S1 In step S1, a precursor is obtained by coprecipitation. The chemical composition of the precursor is not particularly limited and depends on the chemical composition of the Na-containing oxide or Li-containing oxide described below. The precursor may contain at least one element selected from Mn, Ni, and Co, or may contain at least Mn and one or both of Ni and Co, or may contain at least Mn, Ni, and Co. The precursor may be a salt containing at least one element selected from Mn, Ni, and Co. For example, the precursor may be at least one of carbonate, sulfate, nitrate, and acetate. Alternatively, the precursor may be a compound other than a salt. For example, the precursor may be a hydroxide. The precursor may be a hydrate. The precursor may be a combination of multiple compounds. The precursor may be, for example, particulate, or may be spherical as described below. The particle size of the precursor particles is not particularly limited.

[0010] In S1, the precursor precipitate may be obtained by coprecipitation using an ion source capable of forming a precipitate in aqueous solution with transition metal ions and a transition metal compound containing at least one element selected from Mn, Ni, and Co. This facilitates the production of spherical particles as the precursor. The "ion source capable of forming a precipitate in 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 salt or hydroxide 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 separate solutions, and the solutions may be added dropwise and mixed to obtain the precursor precipitate. In this case, for example, water is used as the solvent. In this case, various sodium compounds may be used as bases, and aqueous ammonia or the like may be added to adjust the basicity. In S1, for example, an aqueous solution of a transition metal compound containing at least one element selected from Mn, Ni, and Co, and an aqueous solution of sodium carbonate are prepared, and each aqueous solution is added dropwise and mixed to obtain a precipitate as a precursor by a coprecipitation method.

[0011] According to the findings of the present inventors, when a precursor is obtained by a coprecipitation method, spherical particles as the precursor can be obtained more efficiently by controlling the dropping speed of the aqueous solution and appropriately controlling the pH, etc. In this regard, in S1, it is preferable that the dropping speed during coprecipitation is 4 ml / min or more and 20 ml / min or less.

[0012] As described above, the precursor may be spherical particles. In this 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 may be 4πS / L. 2where 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. When a particle is made up of more than 100 particles, the circularity is measured, for example, as an average value as follows.

[0013] (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) of not less than D10 and not more 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."

[0014] In step S1, the precursor may contain element M. The element M is at least one element selected from B, Mg, Al, K, Ca, Ti, V, Cr, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W. The element M, for example, functions to stabilize the P2 structure. The method for obtaining a precursor containing element M is not particularly limited. When obtaining a precursor by coprecipitation in step 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 then these aqueous solutions are added dropwise and mixed to obtain a precursor containing element M together with at least one element of Mn, Ni, and Co. Alternatively, in the manufacturing method of the present disclosure, element M may not be added in step S1, and element M may be doped during the Na-doping calcination in steps S2 and S3 described below.

[0015] In S1, it is important that the stirring power required during the co-precipitation (the stirring power required during the dropwise addition of the solution) is 0.020 W or more and 0.200 W or less. By ensuring that the stirring power required during the co-precipitation is within the specified range, it becomes easier to obtain a precursor of appropriate size and spherical shape. The stirring power required may be 0.030 W or more, 0.040 W or more, or 0.050 W or more, or 0.180 W or less, 0.160 W or less, 0.140 W or less, 0.120 W or less, or 0.100 W or less.

[0016] "Required mixing power (hereinafter, P)" refers to the work done by the mixing blade on the solution per unit time (power: P(W)). The relationship for calculating power is publicly known (power = force x speed), and the required mixing power P can similarly be calculated as "required mixing power P = force applied to the mixer x rotational speed of the mixer." For example, the required mixing power P can be calculated from the following formula (I) described in Non-Patent Document 1 (Kamei Noboru, Hiraoka Setsuro, Kato Yoshito, Tada Yutaka, Shida Yuki, Lee Young-se, Yamaguchi Takao, and Ko Seung-tae; Chemical Engineering Journal, 21, 41-48 (1995)). P=N p ρn 3 d 5 (I)

[0017] The above formula (I) can be interpreted as follows. Required mixing power P = force applied to the mixer × rotation speed of the mixer v Force acting on the agitator = density of the solution ρ × cross-sectional area of ​​the agitator A × (rotational speed of the agitator v) 2 Cross-sectional area of ​​the agitator A ∝ (impeller diameter d) 2 Agitator rotation speed v = blade diameter d × rotation speed n Combining these gives Mixing power required P∝ρd 2 (nd) 2 (nd)=ρn 3 d 5 (I') Multiplying the above formula (I') by the power number Np as a proportionality constant yields the above formula (I). The mixing power P is well known in the art, and the formulas and factors used to calculate the mixing power P are also well known. For the factors used to calculate the mixing power P (e.g., power number Np), see, for example, Non-Patent Document 2 (Edited by the Society of Chemical Engineers; Fundamentals and Applications of the Latest Mixing Technology (Advances in Chemical Engineering 42), Sankei Publishing (2008), 1. Fundamentals), Non-Patent Document 3 (Edited by the Society of Chemical Engineers; Chemical Engineering Handbook, Revised Seventh Edition, Maruzen (2011), I-6, Mixing and Mixing), and Non-Patent Document 4 (Sadato Kato; Calculation Methods and Experimental Methods for the Operation and Design of Mixing Vessels, Revised and Expanded Edition, Information Organization (2015), Chapter 5, Power Characteristics). For example, the formula is as follows:

[0018]

number

[0019]

number

[0020] In the above formula, n p : Number of blades (pieces), D: Diameter of reactor (m), d: Diameter of blade (m), b: Width of stirring blade (m), θ: Inclination angle of stirring blade (deg.), H: Height of liquid surface (m), ρ: Liquid density (kg / m 3 ), n: Blade rotation speed (1 / s), μ: Liquid viscosity (Pa s), π: Pi, B w :Baffle plate width (m), n B : Number of baffle plates (pieces).

[0021] 2.S2 In S2, the precursor obtained in S1 is reacted with a Na source to obtain a Na-containing oxide having a P2-type structure. In one embodiment, S2 may include coating the surface of the precursor with a Na source to obtain a composite (S2-1), and calcining the composite to obtain the Na-containing oxide (S2-2).

[0022] 2.1 S2-1 In S2-1, the surface of the precursor is coated with a sodium source to obtain a composite. The sodium source may be a salt containing sodium, such as a carbonate or nitrate, or a compound other than a salt, such as sodium oxide or sodium hydroxide. In S2-1, the amount of sodium source coated on the surface of the precursor may be determined taking into account the amount of sodium lost during subsequent calcination.

[0023] In S2-1, the coverage of the Na source on the surface of the precursor is not particularly limited. For example, in S2, the composite may be obtained by covering 40 area% or more, 50 area% or more, 60 area% or more, or 70 area% or more of the surface of the precursor with the Na source. Here, if the precursor obtained in S1 is a spherical particle and the composite obtained in S2-1 is obtained by covering 40 area% or more of the surface of the precursor with the Na source, the Na-containing oxide having a P2 structure is likely to become spherical particles in S2-2 described below. If the coverage of the Na source is low, when the composite is fired, P2 crystals are likely to grow abnormally on the surface of the composite, and the Na-containing oxide is likely to become plate-like. If the coverage of the Na source is high, when the composite is fired, the crystallites of the P2 crystals are likely to become small, and the Na-containing oxide is likely to become spherical particles corresponding to the shape of the precursor.

[0024] In S2-1, the method for coating the surface of the precursor with the Na source is not particularly limited. As described above, when 40% or more of the area of ​​the precursor surface is to be coated with the Na source, various methods can be used. For example, a tumbling fluidized coating method or a spray drying method can be used. That is, a coating solution in which a Na source is dissolved is prepared, and the coating solution is brought into contact with the surface of the precursor, and then the precursor is dried at the same time as or after the contact. By adjusting the coating conditions (temperature, time, number of times, etc.), 40% or more of the area of ​​the precursor surface can be coated with the Na source.

[0025] In S2-1, the precursor may be coated with an M source together with a Na source. For example, in S2-1, the precursor obtained in S1 may be mixed with a Na source and an M source containing at least one element M selected from B, Mg, Al, K, Ca, Ti, V, Cr, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W to obtain a composite. The M source may be, for example, a salt containing the element M, such as a carbonate or a sulfate, or a compound other than a salt, such as an oxide or a hydroxide. The amount of the M source relative to the precursor may be determined depending on the chemical composition of the Na-containing oxide after firing.

[0026] 2.2 S2-2 In S2-2, the composite obtained in S2-1 is fired to obtain a Na-containing oxide having a P2 type structure. In S2-2, the composite may be optionally shaped, optionally pre-fired, and then fired.

[0027] The pre-firing of the composite may be carried out at a temperature equal to or lower than that of the main firing. For example, the pre-firing may be carried out at a temperature lower than 700°C. The pre-firing time is not particularly limited. The pre-firing atmosphere is also not particularly limited. The pre-firing atmosphere may be the same as or different from the main firing atmosphere.

[0028] The composite may be sintered at a temperature of, for example, 700°C or higher and 1100°C or lower. Preferably, it is 800°C or higher and 1000°C or lower. If the sintering temperature is too low, Na doping will not occur, and if the sintering temperature is too high, phases other than the P2 phase are likely to form. The temperature rise conditions from the pre-sintering temperature to the sintering temperature are not particularly limited.

[0029] The firing time is not particularly limited and may be, for example, 30 minutes to 48 hours. However, the shape of the Na-containing oxide can be controlled by the firing time. As described above, in the method of the present disclosure, when the coverage of the Na source in the composite is 40 area % or more, small P2-type crystals are likely to form on the surface of the composite when the composite is fired. In this embodiment, the P2-type crystals are grown along the surface of the particles so that one P2-type crystallite is connected to another P2-type crystallite, thereby making the shape of the Na-containing oxide correspond to that of the precursor. For example, if the precursor is spherical particles, the Na-containing oxide can also be spherical particles. If the firing time is too short, Na doping will not occur, and the desired P2-type structure will not be obtained. On the other hand, if the firing time is too long, the P2-type structure will grow excessively, resulting in plate-like particles rather than spherical ones. As far as the inventors have confirmed, spherical particles of Na-containing oxide are likely to be obtained when the firing time is 30 minutes to 3 hours. The Na-containing oxide obtained after the main firing may have a structure in which a plurality of crystallites are present on the surface and the crystallites are connected to each other.

[0030] The firing atmosphere is not particularly limited, and may be, for example, an oxygen-containing atmosphere such as air atmosphere or an inert gas atmosphere.

[0031] 3.S3 In S3, at least a portion of the Na in the Na-containing oxide obtained in S2 is ion-exchanged with Li to obtain a Li-containing oxide having an O2 structure. In S3, the Na-containing oxide is brought into contact with an ion exchange material, whereby Na in the Na-containing oxide particles can be ion-exchanged with Li.

[0032] Examples of ion exchange materials include mixtures of lithium halides and other lithium salts (e.g., molten salts). The lithium halides constituting the molten salt are preferably at least one of lithium chloride, lithium bromide, and lithium iodide. The other lithium salts constituting the molten salt are preferably lithium nitrate. By using a molten salt, the melting point becomes lower than when lithium halides or other lithium salts are used alone, enabling ion exchange at lower temperatures. The temperature for ion exchange is from the melting point of the ion exchange material to 300°C. If the ion exchange temperature is too high, the stable O3-type structure is likely to be formed rather than the 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 30 minutes or more and less than 3 hours. If the ion exchange time is too short, the amount of residual Na in the Li-containing oxide particles becomes large. On the other hand, if the ion exchange time is too long, the O3-type structure and the like are likely to be formed together with the O2-type structure.

[0033] 4. Cathode active material with O2-type structure By the above steps S1 to S3, a Li-containing oxide having an O2-type structure can be produced as a positive electrode active material. Hereinafter, a Li-containing oxide according to one embodiment will be described.

[0034] 4.1 Crystal structure The Li-containing oxide according to an embodiment has at least an O2-type structure (belonging to the space group P63mc) as a crystal structure. The Li-containing oxide according to an embodiment has the O2-type structure and may have a crystal structure other than the O2-type structure. Examples of crystal structures other than the O2-type structure include a T#2-type structure (belonging to the space group Cmca) formed when Li is deintercalated from the O2-type structure and an O6-type structure (belonging to the space group R-3m, with a c-axis length of 2.5 nm to 3.5 nm, typically 2.9 nm to 3.0 nm, and different from the O3-type structure also belonging to the space group R-3m). The Li-containing oxide according to an embodiment may have an O2-type structure as a main phase, or may have a structure other than the O2-type structure as a main phase. The Li-containing oxide according to an embodiment may have one or both of the T#2-type structure and the O6-type structure in addition to the O2-type structure. The crystal structure of the Li-containing oxide as a main phase may change depending on its charge / discharge state.

[0035] The Li-containing oxide according to one embodiment may be polycrystalline having a plurality of crystallites. For example, the surface of the Li-containing oxide according to one embodiment may be composed of a plurality of crystallites. In other words, the Li-containing oxide may have a structure in which a plurality of crystallites are connected to each other on its surface. When the surface of the Li-containing oxide is composed of a plurality of crystallites, crystal grain boundaries are present on the surface. Here, the crystal grain boundaries may serve as inlets and outlets for intercalation. That is, when the Li-containing oxide is polycrystalline having a plurality of crystallites, the effects of increasing the number of inlets and outlets for intercalation, thereby reducing reaction resistance, shortening the migration distance of lithium ions, thereby reducing diffusion resistance, and reducing the absolute amount of expansion and contraction during charge and discharge, thereby making cracking less likely to occur, can be expected. The crystallite size may be large or small, but it is believed that smaller crystallite size increases the number of crystal grain boundaries, making the above-mentioned advantageous effects more likely to be exhibited. For example, when the diameter of the crystallites constituting the Li-containing oxide is less than 1 μm, higher performance is likely to be obtained. The "crystallite" and "crystallite diameter" can be determined by observing the surface of a Li-containing oxide using a scanning electron microscope (SEM) or a transmission electron microscope (TEM). Specifically, when observing the surface of a Li-containing oxide and observing a single closed region surrounded by a grain boundary, the region is considered a "crystallite." The maximum Feret diameter of the crystallite is determined and considered to be the "crystallite diameter." If the Li-containing oxide is composed of a single crystal, the particle itself can be considered a single crystallite, and the maximum Feret diameter of the particle is the "crystallite diameter." Alternatively, the crystallite diameter can be determined by EBSD or XRD. For example, the crystallite diameter can be determined based on the Scherrer equation from the half-width of the diffraction line in the XRD pattern. Li-containing oxides with crystallite diameters of less than 1 μm, as determined by either method, tend to exhibit higher performance.

[0036] 4.2 Chemical composition The Li-containing oxide according to one embodiment may contain, as constituent elements, at least one element selected from Mn, Ni, and Co, Li, and O. In particular, when the constituent elements contain at least Li, Mn, one or both of Ni and Co, and O, higher performance is likely to be obtained when the constituent elements contain at least Li, Mn, Ni, Co, and O. The Li-containing oxide according to one embodiment may contain, as constituent elements, Li, Mn, Ni, Co, and O. a Na b Mn x-p Ni y-q Co z-r M p+q+rIt may have a chemical composition represented by O2 (where 0 < a ≤ 1.00, 0 ≤ b ≤ 0.20, 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 has such a chemical composition, the O2-type structure is likely to be maintained. 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 at most 1.00, and may be 0.90 or less, 0.80 or less, or 0.70 or less. In the above chemical composition, b is 0 or more, and may be 0.01 or more, 0.02 or more, or 0.03 or more, and is at most 0.20, and may be 0.15 or less, or 0.10 or less. 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 at most 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 at most 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 at most 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, when p + q + r is less than 0.17, it is easy to ensure a high charge and discharge 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. 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 variable.

[0037] 4.3 Shape The P2-type structure before ion exchange 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, a Na-containing oxide having a P2-type structure typically becomes a plate-like particle with a large aspect ratio, in which the crystal growth direction is biased in a specific direction. When ion exchange is performed using such plate-like particles, a Li-containing oxide having an O2-type structure also becomes plate-like. In contrast, as described above, the Na-containing oxide according to one embodiment may be a spherical particle, and the Li-containing oxide after ion exchange may also be a spherical particle. When the Li-containing oxide is a spherical particle, the reaction resistance decreases due to the reduction in crystallite size, and the diffusion resistance inside the particle tends to decrease. Furthermore, when applied to a secondary battery or the like, the spheroidization is thought to reduce the degree of curvature and reduce the lithium ion conduction resistance. This, for example, tends to improve rate characteristics and increase reversible capacity. The Li-containing oxide according to one embodiment may be a solid particle, a hollow particle, or a particle having voids. The size of the Li-containing oxide particles is not particularly limited, but a smaller size is considered to be advantageous. For example, the average particle diameter (D50) of the Li-containing oxide particles may be 0.1 μm or more and 10 μm or less, 1.0 μm or more and 8.0 μm or less, or 2.0 μm or more and 6.0 μm or less. The average particle diameter (D50) is the particle diameter (D50, median diameter) at 50% cumulative value in a volume-based particle size distribution determined by a laser diffraction / scattering method. [Example]

[0038] As described above, one embodiment of the method for producing a positive electrode active material (Li-containing oxide) having an O2-type structure has been described, but the production method of the present disclosure can be modified in various ways other than the above embodiment without departing from the gist of the present disclosure. 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.

[0039] 1. Preparation of positive electrode active material 1.1 Precursor preparation (1) MnSO4·5H2O, NiSO4·6H2O, and CoSO4·7H2O were weighed to obtain the desired composition ratio and dissolved in distilled water to obtain the desired concentration to obtain solution 1. In a separate container, Na2CO3 was dissolved in distilled water to obtain the desired concentration to obtain solution 2. (1-1) In Example 1 and Comparative Examples 1 to 3, the concentrations of the first and second solutions were both set to 1.2 mol / L. (1-2) In Example 2, the concentrations of the first and second solutions were both set to 2.0 mol / L. (2) Pure water was placed in a reaction vessel, and the first solution and the second solution were added dropwise thereto at a predetermined rate. (2-1) In Example 1 and Comparative Examples 1 to 3, 1000 mL of pure water was placed in a 4-L reaction vessel, and 500 mL of the first solution and 500 mL of the second solution were added dropwise at a rate of 4.17 mL / min. The stirring power required during the addition was changed by changing the design of the stirring blades in the reaction vessel and the stirring rotation speed. Specifically, the stirring power required for Example 1 was 0.086 W, the stirring power required for Comparative Example 1 was 0.015 W, the stirring power required for Comparative Example 2 was 0.203 W, and the stirring power required for Comparative Example 3 was 0.685 W. (2-2) In Example 2, 2500 mL of pure water was placed in a 9 L reaction vessel, and 2250 mL of the first solution and 2250 mL of the second solution were added dropwise at a rate of 18.75 mL / min. The design of the stirring blades in the reaction vessel and the stirring speed were adjusted so that the stirring power required during the dropping was 0.054 W. (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 fine particles were removed by air classification to obtain a precursor.

[0040] 1.2 Appearance observation using SEM FIG. 2 shows SEM photographs of the appearance of the precursors of Examples 1 and 2 and Comparative Examples 1 to 3. FIG. 3 shows the relationship between the circularity of the precursor and the required stirring power. As shown in FIGS. 2 and 3, spherical precursors with a circularity of 0.80 or more were obtained in Examples 1 and 2, whereas non-spherical precursors with small particle diameters and a circularity of 0.60 or less were obtained in Comparative Examples 1 to 3. It is believed that the required stirring power for the liquid-phase synthesis of the precursor was too small, resulting in insufficient stirring and inhibiting the growth of precursor particles. It is also believed that the required stirring power for the liquid-phase synthesis of the precursor was too large, resulting in excessive stirring and causing the precursor particles to collapse.

[0041] 1.3 Preparation of the complex (1) Na2CO3 as a Na source and the above precursor are mixed, and the composition after calcination described below is Na 0.7 Mn 0.5 Ni 0.2 Co 0.3 It was weighed to be O2. (2) The weighed Na source and precursor were mixed by spray drying. Specifically, the weighed Na source and precursor were added to a solvent, and the dispersion solution in which the Na source was dissolved and the precursor was dispersed was spray dried. The spray drying temperature was 200°C, and the spray pressure was 0.3 MPa. A composite in which 75% or more of the surface area of ​​the precursor was covered with the Na source was obtained by spray drying.

[0042] 1.4 Firing of the composite The composite was placed in an alumina crucible and fired in an air atmosphere under the following conditions (1) to (5). (1) The alumina crucible containing the above composite is placed in a heating furnace. (2) The temperature inside the heating furnace is increased from room temperature to 600°C. (3) The temperature in the heating furnace is maintained at 600°C for 6 hours to perform pre-firing (pre-firing). (4) The temperature inside the heating furnace is increased from 600°C to 900°C. (5) The temperature in the heating furnace is maintained at 900°C for 1 hour to carry out the main firing. (6) After the main firing, the temperature inside the heating furnace is lowered from 900°C to 250°C over 2 hours and 20 minutes. (7) At 250°C, the alumina crucible is removed from the heating furnace and allowed to cool in the air.

[0043] The fired product after being allowed to cool in the air was pulverized in a mortar in a dry atmosphere to obtain Na-containing oxide particles having a P2 type structure. 0.7 Mn 0.5 Ni 0.2 Co 0.3 It had a chemical composition indicated by O2.

[0044] 1.5 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 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 a positive electrode active material.

[0045] 2. Evaluation of positive electrode active material 2.1 Identification of crystal structure by X-ray diffraction measurement For each of the positive electrode active materials of Examples 1 and 2 and Comparative Examples 1 to 3, X-ray diffraction measurements were performed using CuKα as a radiation source to obtain X-ray diffraction patterns and identify the crystal structure. As a result, all of the positive electrode active materials were found to have an O2-type crystal structure.

[0046] 2.2 Appearance observation by SEM FIG. 4 shows SEM photographs of the appearance of the positive electrode active materials according to Examples 1 and 2 and Comparative Examples 1 to 3. From FIG. 4, it can be seen that the positive electrode active materials according to Examples 1 and 2 are spherical particles with a circularity of 0.80 or more, while the positive electrode active materials according to Comparative Examples 1 to 3 are non-spherical particles. It can also be seen that the surfaces of the positive electrode active materials according to Examples 1 and 2 are composed of multiple crystallites, and the diameter of the crystallites is less than 1 μm. On the other hand, it can be seen that the positive electrode active materials according to Comparative Examples 1 to 3 have a structure in which plate-like single crystals are aggregated to form secondary particles, or a structure in which spherical particles are bonded to each other to form secondary particles. As described above, in Comparative Examples 1 to 3, the Na-containing oxide was synthesized using a precursor having a small particle size and a non-spherical shape. Therefore, it is believed that the Na-containing oxide was also non-spherical, and as a result, the positive electrode active material was also non-spherical. It should be noted that the positive electrode active material according to Comparative Example 1 was composed of a large amount of plate-like primary particles aggregated, and therefore the circularity could not be measured.

[0047] 3. Preparation of evaluation cells Coin cells were fabricated using the positive electrode active materials of Examples 1 and 2 and Comparative Examples 1 to 3. The coin cells were fabricated according to the following procedure. (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.

[0048] 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 2.0-4.8 V in a thermostatic chamber maintained at 25°C, and the discharge capacity was measured. Each coin cell was also charged at 0.1 C in a voltage range of 2.0-4.8 V in a thermostatic chamber maintained at 5°C, and discharged at 0.5 C, 1 C, or 3 C, and the discharge capacity at each rate was measured. The ratio of the discharge capacity at each rate to the discharge capacity at 0.1 C was calculated to evaluate the rate characteristics. The results are shown in Table 1 below.

[0049] [Table 1]

[0050] As shown in Table 1, the coin cells of Examples 1 and 2 have a larger discharge capacity than the coin cells of Comparative Examples 1 to 3, and the difference between the capacity during low-rate discharge and the capacity during high-rate discharge is small, meaning that a large capacity can be maintained even during high-rate discharge.

[0051] 4. Supplementary Information In the above examples, the surface of the precursor is coated with a Na source by spray drying to obtain a composite, but the composite can also be obtained by other methods. In the above examples, the Na-containing oxide having a P2-type structure and the positive electrode active material having an O-type structure are exemplified as having a predetermined chemical composition, but the chemical composition is not limited thereto, and various chemical compositions can be adopted. Furthermore, the positive electrode active material may be doped with an element M other than Mn, Ni, and Co. The element M is as described in the embodiments.

[0052] 5. Summary As described above, a method for producing a positive electrode active material having an O2 type structure, which includes the following steps S1 to S3, can produce a positive electrode active material with excellent rate characteristics. S1: Obtaining a precursor by coprecipitation, wherein the stirring power required during coprecipitation is 0.020 W or more and 0.200 W or less. S2: Reacting the precursor with a Na source to obtain a Na-containing oxide having a P2 type structure. S3: At least a portion of the Na in the Na-containing oxide is ion-exchanged with Li to obtain a Li-containing oxide having an O2-type structure.

Claims

1. A method for producing a positive electrode active material having an O2 type structure, comprising: obtaining the precursor by coprecipitation; reacting the precursor with a Na source to obtain a Na-containing oxide having a P2 type structure; and ion-exchanging at least a portion of the Na in the Na-containing oxide with Li to obtain a Li-containing oxide having an O2-type structure; Including, The stirring power required during coprecipitation is 0.020 W or more and 0.200 W or less. Manufacturing method.

2. The method of claim 1, The dropping rate during coprecipitation is 4 ml / min or more and 20 ml / min or less. Manufacturing method.

3. The method of claim 1, The precursor comprises at least one element of Mn, Ni, and Co; Manufacturing method.

4. The method of claim 1, obtaining a precipitate as the precursor by the coprecipitation method using an ion source capable of forming a precipitate in an aqueous solution together with transition metal ions and a transition metal compound containing at least one element selected from Mn, Ni, and Co. Manufacturing method.

5. The manufacturing method according to any one of claims 1 to 4, Coating the surface of the precursor with a Na source to obtain a composite; and calcining the composite to obtain the Na-containing oxide; Including, the precursor is a spherical particle, The composite is obtained by covering 40 area % or more of the surface of the precursor with the Na source; and The Na-containing oxide is a spherical particle. Manufacturing method.

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