Method for producing sodium-containing oxides

Abstract

To reduce O3 phases in Na-containing oxides having P2 type structure.SOLUTION: A manufacturing method for Na-containing oxides of the present disclosure includes: obtaining a precursor including at least one element selected from Mn, Ni, and Co; coating a surface of the precursor with a Na source to obtain a composite; and firing the composite to obtain Na-containing oxides having P2 type structure, and is characterized by that on firing the composite, a rate of temperature rise A(°C / min) from 300°C to 500°C and a volume B(mm3) of the composite, A / B, is 4.5 or under.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] This application discloses a method for producing a Na-containing oxide.

Background Art

[0002] Patent Document 1 discloses a Na-containing oxide having a P2-type structure and having a chemical composition represented by Na , , , , , y , , , , , ,

[0004] , , , , , ,

[0005] , 1-y , x , , , ,

[0003] , , , , , , , Fe y Mn 1-y O2 (where x is less than 1 and y is greater than or equal to 1 / 3 and less than 2 / 3).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In a Na-containing oxide having a P2-type structure, an O3 phase is likely to be formed together with the P2 phase. In a Na-containing oxide having a P2-type structure, a new technology capable of reducing the O3 phase is required.

Means for Solving the Problems

[0005] As means for solving the above problems, this application discloses the following multiple aspects. <Aspect 1> A method for producing a Na-containing oxide having a P2-type structure, comprising: obtaining a precursor containing at least one element selected from Mn, Ni, and Co; coating the surface of the precursor with a Na source to obtain a composite; and firing the composite to obtain a Na-containing oxide having a P2-type structure. including ​​​​When firing the composite, the ratio A / B of the heating rate A (°C / min) from 300°C to 500°C and the thickness B (mm) of the composite is 4.5 or less. Production method. <Aspect 2> The production method according to Aspect 1, where the heating rate A is 13.5°C / min or less. Production method. <Aspect 3> The production method according to Aspect 1 or 2, where the precursor is spherical particles, the composite is obtained by coating 40 area% or more of the surface of the precursor with the Na source, and the Na-containing oxide having the P2-type structure is spherical particles. Production method. <Aspect 4> The production method according to any one of Aspects 1 to 3, using a transition metal ion and an ion source capable of forming a precipitate in an aqueous solution, and a transition metal compound containing at least one element of Mn, Ni, and Co, and obtaining a precipitate as the precursor by a coprecipitation method. Production method. <Aspect 5> The production method according to any one of Aspects 1 to 4, where the Na-containing oxide having the P2-type structure is Na a Mn x-p Ni y-q Co z-r M p+q+r O2 (where 0 < a ≤ 1.00, 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). Production method. [[ID=​​​​​​ [Brief explanation of the drawing]

[0007] [Figure 1] This shows an example of a method for producing a Na-containing oxide having a P2-type structure. [Figure 2] These are SEM images of the appearance of each of the Na-containing oxides in Examples 1, 2, and 4 and the Comparative Example. [Figure 3] The X-ray diffraction patterns of each of the Na-containing oxides in Examples 1-5 and the Comparative Example are shown. [Figure 4] This shows the relationship between A / B and the peak intensity ratio IP2 / IO3. [Modes for carrying out the invention]

[0008] 1. Method for producing a Na-containing oxide having a P2-type structure As shown in Figure 1, a method for producing a Na-containing oxide having a P2-type structure according to one embodiment includes obtaining a precursor containing at least one element among Mn, Ni, and Co (S1), coating the surface of the precursor with a Na source to obtain a composite (S2), and calcining the composite to obtain a Na-containing oxide having a P2-type structure (S3). Here, during the calcination of the composite, the ratio A / B of the heating rate A (°C / min) from 300°C to 500°C and the thickness B (mm) of the composite is 4.5 or less.

[0009] 1.1 S1 In S1, a precursor containing at least one element from among Mn, Ni, and Co is obtained. The precursor may contain at least Mn and one or both of Ni and Co, or it may contain at least Mn, Ni, and Co. The precursor may be a salt containing at least one element from among Mn, Ni, and Co. For example, the precursor may be at least one of carbonates, sulfates, nitrates, and acetates. 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 types of compounds. The precursor may be in various shapes. For example, the precursor may be particulate, or it may be spherical particles as described later. The particle size of the particles made up of the precursor is not particularly limited.

[0010] In S1, a precipitate as a precursor may be obtained by coprecipitation using an ion source capable of forming a precipitate with transition metal ions in aqueous solution and a transition metal compound containing at least one element from Mn, Ni, and Co. This makes it easier to obtain spherical particles as a precursor. The "ion source capable of forming a precipitate with transition metal ions in aqueous solution" may be at least one selected from, for example, 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 from Mn, Ni, and Co. Specifically, in S1, the precipitate as a precursor may be obtained by preparing solutions of the ion source and the transition metal compound separately and then adding and mixing each solution dropwise. In this case, water may be used as the solvent. Various sodium compounds may be used as the base, and aqueous ammonia may be added to adjust the basicity. In the case of coprecipitation, for example, an aqueous solution of the transition metal compound and an aqueous solution of sodium carbonate are prepared, and the precipitate as a precursor is obtained by adding and mixing each aqueous solution dropwise. Alternatively, precursors can be obtained by the sol-gel method. In particular, the coprecipitation method readily yields spherical particles as precursors.

[0011] As described above, the precursor may be spherical particles. In the present application, the "spherical particles" mean 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 which is defined as follows. Here, S is the projected area of the particle, and L is the perimeter of the projected image of the particle. The circularity of the particles can be determined by observing the appearance of the particles using a scanning electron microscope (SEM), a transmission electron microscope (TEM), or an optical microscope. When it is composed of a plurality of particles exceeding 100, its circularity is measured as an average value, for example, as follows.

[0012] (1) First, measure the particle size distribution of the particles. Specifically, obtain the particle diameter (D10) at an integrated value of 10% and the particle diameter (D90) at an integrated value of 90% in the volume-based particle size distribution by the laser diffraction / scattering method. (2) Regarding the appearance of the particles for which the particle size distribution has been measured, perform image observation using SEM, TEM, or an optical microscope, and arbitrarily extract 100 particles having a circular equivalent diameter (the diameter of a circle having the same area as the projected area of the particle) that is D10 or more and D90 or less among the particles included in the image. (3) For each of the 100 extracted particles, determine the circularity by image processing and regard the average value as the "circularity of the particles".

[0013] In S1, the precursor may contain element M. 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, for example, the function of stabilizing the P2 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 each aqueous solution is added dropwise and mixed to obtain a precursor containing element M along with at least one of Mn, Ni, and Co. Alternatively, in the manufacturing method of this disclosure, element M may not be added in S1, and element M may be doped when Na doping is performed in S2 and S3 described later.

[0014] 1.2 S2 In step S2, the surface of the precursor obtained in step S1 is coated with a Na source to obtain a composite. The Na source may be a salt containing Na, such as a carbonate or 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 surface of the precursor should be determined taking into account the amount of Na lost during subsequent calcination.

[0015] In S2, the coverage rate 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% or more, 50% or more, 60% or more, or 70% or more of the surface of the precursor with the Na source. Here, if the precursor obtained in S1 is spherical particles, and the composite obtained in S2 is obtained by covering 40% or more of the surface of the precursor with the Na source, then in S3 described later, the Na-containing oxide having a P2-type structure tends to become spherical particles. If the coverage rate of the Na source is small, when the composite is calcined, P2-type crystals tend to grow on the surface of the composite, and the Na-containing oxide tends to become plate-like. If the coverage rate of the Na source is large, when the composite is calcined, the crystallites of the P2-type crystals tend to be small, and the Na-containing oxide tends to become spherical particles corresponding to the shape of the precursor.

[0016] In S2, the method for coating the surface of the precursor with the Na source is not particularly limited. As mentioned above, when coating 40% or more of the surface area of ​​the precursor with the Na source, various methods can be used. For example, the rolling flow coating method and the spray drying method can be used. That is, a coating solution in which the Na source is dissolved is prepared, and the coating solution is brought into contact with the surface of the precursor, or dried at the same time as, or after, contact. By adjusting the coating conditions (temperature, time, number of passes, etc.), 40% or more of the surface area of ​​the precursor can be coated with the Na source.

[0017] In S2, the precursor may be coated with an M source along with a Na source. For example, in S2, a composite may be obtained by mixing the precursor obtained in S1 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. The M source may be a salt containing element M, such as a carbonate or sulfate, or a compound other than a salt, such as an oxide or hydroxide. The amount of the M source relative to the precursor should be determined according to the chemical composition of the Na-containing oxide after calcination.

[0018] 1.3 S3 In S3, the composite obtained in S2 is calcined to obtain a Na-containing oxide having a P2-type structure. In S3, when calcining the composite, the ratio A / B of the heating rate A (°C / min) from 300°C to 500°C and the thickness B (mm) of the composite is 4.5 or less. According to the inventor's findings, when calcining the composite, by controlling the heating rate A from 300°C to 500°C to a predetermined low speed or less, depending on the thickness B of the composite, the O3 phase is easily reduced regardless of the scale, and a Na-containing oxide mainly composed of the P2 phase is easily obtained.

[0019] In S3, the heating rate A only needs to satisfy the above ratio A / B. According to the inventor's findings, the O3 phase is more easily reduced when the heating rate A is 13.5°C / min or less. The lower limit of the heating rate A is not particularly limited. The heating rate A may be, for example, 0.05°C / min or more, or 0.10°C / min or more. Also, the heating rate A may be 12.0°C / min or less, 11.0°C / min or less, 10.0°C / min or less, 9.0°C / min or less, 8.0°C / min or less, or 7.0°C / min or less. Note that the heating rate A refers to the target heating rate set in the heating device. In the case of a heating furnace, it is the heating rate based on the indicated temperature of the heating furnace.

[0020] In S3, the thickness B of the composite is the thickness of the composite inside a container when the container containing the composite is placed inside a heating device such as a heating furnace, and refers to the thickness of the composite when the composite is weighed so that it is at a uniform height inside the container. For example, the height of the composite inside the container when the container is placed on the bottom surface of the furnace is "thickness B". The thickness B of the composite may be, for example, 1 mm or more and 100 mm or less, or 1 mm or more and 50 mm or less.

[0021] In S3, the ratio A / B is 4.5 or less. The ratio A / B may also be 4.0 or less, 3.5 or less, 3.0 or less, 2.5 or less, 2.0 or less, 1.5 or less, 1.0 or less, or 0.5 or less. The lower limit of the ratio A / B is not particularly limited and is greater than 0, and may be 0.01 or greater, 0.02 or greater, or 0.03 or greater.

[0022] In S3, the heating rate up to 300°C is not particularly limited. Similarly, the heating rate from 500°C to the firing temperature is not particularly limited. In this embodiment, regardless of the heating rate up to 300°C or the heating rate from 500°C to the firing temperature, the O3 phase can be reduced by controlling the heating rate from 300°C to 500°C, as described above.

[0023] In step S3, the composite material may be optionally shaped, optionally pre-fired, and then the main firing may be performed. The pre-firing of the composite material may be performed at a temperature lower than or equal to the main firing temperature. For example, pre-firing may be performed at a temperature between 500°C and 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.

[0024] In S3, the main firing of the composite may be carried out at a temperature of, for example, 700°C to 1100°C. Preferably, it is 800°C to 1000°C. If the main firing temperature is too low, Na doping will not occur, and if the main firing temperature is too high, the O3 phase or the like is likely to be formed instead of the P2 phase. The heating conditions from the pre-firing temperature to the main firing temperature are not particularly limited.

[0025] The main firing time is not particularly limited and may be, for example, 30 minutes or more and 48 hours or less. However, the shape of the Na-containing oxide can be controlled by the main firing time. As described above, in the method of this disclosure, when the coverage rate of the Na source in the composite is 40 area % or more, when the composite is fired, small P2-type crystals are likely to form on its surface. In this embodiment, by linking one P2-type crystallite with another P2-type crystallite and growing P2-type crystals along the surface of the particles, the shape of the Na-containing oxide corresponds to the shape of the precursor. For example, if the precursor is a spherical particle, the Na-containing oxide may also become a spherical particle. If the main firing time is too short, Na doping will not occur, and it will be difficult to obtain the desired P2-type structure. On the other hand, if the main firing time is too long, the P2-type structure will grow, resulting in plate-like particles instead of spherical ones. As far as the inventors have confirmed, spherical particles of Na-containing oxide are easily obtained when the main firing time is 30 minutes or more and 3 hours or less. The Na-containing oxide obtained after the final calcination may have a structure in which multiple crystallites exist on the surface and are linked to each other.

[0026] The firing atmosphere in S3 is not particularly limited and may be, for example, an oxygen-containing atmosphere such as air or an inert gas atmosphere.

[0027] 2. Na-containing oxides having a P2-type structure By the above method, a Na-containing oxide having a P2-type structure and reduced O3 phase can be produced. A Na-containing oxide according to one embodiment will be described below.

[0028] 2.1 Chemical composition In one embodiment, the Na-containing oxide contains, as constituent elements, at least one element from among Mn, Ni, and Co, as well as Na and O. In particular, when the constituent elements include at least Na, Mn, one or both of Ni and Co, and O, higher performance is more easily obtained when the constituent elements include at least Na, Mn, Ni, Co, and O. In one embodiment, the Na-containing oxide having a P2 type structure is Na a Mnx-p Ni y-q Co z-r M p+q+r O₂ (where 0 < a ≤ 1.00, 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 may have a chemical composition represented by this. When the Na-containing oxide has such a chemical composition, it is easy to maintain the P2-type structure. 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 1.00 or less, and may be 0.90 or less, 0.80 or less, or 0.70 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 1.00 or less, 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 1.00 or less, 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 1.00 or less, 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 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, since the element M is contained, the P2-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 indefinite.

[0029] 2.2 Crystal structure According to one embodiment, the Na-containing oxide has a crystal structure of at least a P2 type structure (space group P63 / m It has (belonging to mc). The Na-containing oxide has a P2-type structure and may also have crystal structures other than the P2-type structure. Examples of crystal structures other than the P2-type structure include various crystal structures formed when Na is removed or inserted from a P2-type structure. In this embodiment, the Na-containing oxide has a P2-type structure, while having few O3-type structures. The Na-containing oxide in this embodiment may have a P2-type structure as its main phase. For example, the X-ray diffraction pattern of the Na-containing oxide in this embodiment is 0.5 ≤ I P2 / I O3 It may also satisfy ≤20.0. Here, I P2 This is the X-ray diffraction peak intensity originating from the (102) plane of the P2 type structure. Also, I O3 This is the X-ray diffraction peak intensity originating from the (015) plane of the O3 type structure. P2 / I O3 It may be 1.0 or higher, 2.0 or higher, 3.0 or higher, or 4.0 or higher, and may be 18.0 or lower, 16.0 or lower, 14.0 or lower, 12.0 or lower, 10.0 or lower, 9.0 or lower, 8.0 or lower, or 7.0 or lower.

[0030] X-ray diffraction peak intensity I originating from the (102) plane of the P2-type structure P2 It is identified as follows: (1) In the X-ray diffraction pattern using CuKα as the source, the average value of the diffraction peak intensity at 35.0°±0.2° is used as the background. 35 We seek. (2) The maximum value of the diffraction peak intensity between 39.23° and 40.05° in the X-ray diffraction pattern I 39 We seek. (3)I 39 From I 35 The value obtained by subtracting I P2 (I P2 =I 39 -I 35 ).

[0031] X-ray diffraction peak intensity I originating from the (015) plane of the O3 type structure O3 It is identified as follows: (1) In the X-ray diffraction pattern using CuKα as the source, the average value of the diffraction peak intensity at 35.0°±0.2° is used as the background. 35 We seek. (2) The maximum value of the diffraction peak intensity between 45.35° and 45.65° in the X-ray diffraction pattern I 45 We seek. (3)I 45 From I 35 The value obtained by subtracting I O3 (I O3 =I 45 -I 35 ).

[0032] The Na-containing oxide according to one embodiment may be a single crystal consisting of one crystallite, or a polycrystalline material having multiple crystallites. For example, the surface of the Na-containing oxide according to one embodiment may be composed of multiple crystallites. In other words, the Na-containing oxide may have a structure on its surface in which multiple crystallites are linked to each other. When the surface of the Na-containing oxide is composed of multiple crystallites, grain boundaries will exist on the surface. Here, grain boundaries may serve as entry and exit points for intercalation. That is, if the Na-containing oxide is a polycrystalline material having multiple crystallites, effects such as a decrease in reaction resistance due to an increase in the number of entry and exit points for intercalation, a decrease in diffusion resistance due to a shorter movement distance of sodium ions, and a reduction in the absolute amount of expansion and contraction during charging and discharging, making it less likely for cracking to occur can be expected. The size of the crystallites may be large or small, but it is thought that a smaller crystallite size results in more grain boundaries, making it easier to achieve the above-mentioned advantageous effects. For example, if the diameter of the crystallites constituting the Na-containing oxide is less than 1 μm, higher performance is more likely to be obtained. Furthermore, "crystallites" and "crystallite diameters" can be determined by observing the surface of a Na-containing oxide using a scanning electron microscope (SEM) or transmission electron microscope (TEM). That is, when observing the surface of a Na-containing oxide, if a closed region surrounded by grain boundaries is observed, that region is considered a "crystallite." The maximum Ferret diameter of that crystallite is determined and considered to be the "crystallite diameter." If the Na-containing oxide consists of a single crystal, the particle itself can be considered a single crystallite, and the maximum Ferret diameter of that particle is the "crystallite diameter." Alternatively, the crystallite diameter can also be determined by EBSD or XRD. For example, the crystallite diameter can be determined from the full width at half maximum of the diffraction lines of the XRD pattern based on Scherrer's formula. Na-containing oxides tend to exhibit higher performance when the crystallite diameter, as determined by either method, is less than 1 μm.

[0033] 2.3 Shape The P2 type structure is hexagonal, has a large diffusion coefficient for Na ions, and is prone to 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, it is prone to plate-like crystal growth in a specific direction. Therefore, Na-containing transition metal oxides having a P2 type structure usually become plate-like particles with a large aspect ratio, where the crystal growth direction is biased in a specific direction. In contrast, the Na-containing oxide according to one embodiment may be spherical particles, as described above. When the Na-containing oxide is spherical, as described above, the reaction resistance decreases due to the reduction in crystallite size, and the diffusion resistance inside the particles tends to decrease. Furthermore, when applied to secondary batteries, etc., it is thought that the curvature is reduced by spheroidization, and the sodium ion conduction resistance decreases. As a result, for example, the rate characteristics are improved and the reversible capacity tends to increase.

[0034] The Na-containing oxide according to one embodiment may be solid particles, hollow particles, or particles with voids. The size of the Na-containing oxide particles is not particularly limited, but smaller sizes are considered advantageous. For example, the average particle diameter (D50) of the Na-containing oxide particles may be 0.1 μm to 10 μm, 1.0 μm to 8.0 μm, or 2.0 μm to 6.0 μm. The average particle diameter (D50) is the particle diameter (D50, median diameter) at 50% of the cumulative value in the volume-based particle size distribution obtained by laser diffraction-scattering.

[0035] 2.4 Supplement In summary, the Na-containing oxide according to one embodiment may, for example, have the following configurations (1) to (4). (1) The Na-containing oxide comprises, as constituent elements, at least one element from among Mn, Ni, and Co, as well as Na and O. (2) The Na-containing oxide has a P2-type structure. (3) The X-ray diffraction pattern of the Na-containing oxide is 0.5 ≤ I P2 / I O3 It satisfies ≤ 20.0. (4) The Na-containing oxide is a spherical particle.

[0036] 3.Applications The Na-containing oxide having a P2-type structure produced by the above method can be used, for example, as a positive electrode active material in a sodium-ion battery. A sodium-ion battery according to one embodiment comprises a positive electrode active material layer, an electrolyte layer, and a negative electrode active material layer, wherein the positive electrode active material layer contains the Na-containing oxide having a P2-type structure as described above as the positive electrode active material. A sodium-ion battery can have the same configuration as conventional batteries, except that it contains the above-described specific positive electrode active material. [Examples]

[0037] As described above, one embodiment of a method for producing a Na-containing oxide having a P2-type structure has been explained, but the method of production described herein can be modified in various ways other than the above embodiment without departing from the gist of the invention. The technology of this disclosure will be described in more detail below with reference to examples, but the technology of this disclosure is not limited to the following examples.

[0038] 1. Preparation of Na-containing oxides having a P2-type structure 1.1 Preparation of Precursors (1) MnSO4·5H2O, NiSO4·6H2O, and CoSO4·7H2O were weighed to the desired composition ratio and dissolved in distilled water to a concentration of 1.6 mol / L to obtain the first solution. In a separate container, Na2CO3 was dissolved in distilled water to a concentration of 1.6 mol / L to obtain the second solution. (2) 850 mL of pure water was placed in a reaction vessel (with baffles), and 500 mL of the first solution and 500 mL of the second solution were added dropwise, each at a rate of approximately 4 mL / min. (3) After the dropwise addition was complete, the mixture was stirred at room temperature at a stirring speed of 160 rpm for 1 hour to obtain the product. (4) The product was washed with pure water, and solid-liquid separation was performed using a centrifuge to recover the precipitate. (5) The obtained precipitate was dried overnight at 150°C, ground in a mortar, and then fine particles were removed by air classification to obtain precursor particles. The precursor particles were a complex salt containing Mn, Ni, and Co, and were spherical particles with a circularity of 0.80 or more.

[0039] 1.2 Composite fabrication (1) Na2CO3 as the Na source and the above precursor particles, the composition after calcination described later is Na 0.7 Mn 0.5 Ni 0.2 Co 0.3 The amount was weighed to be equal to O2. (2) The weighed Na source and precursor particles were mixed by spray drying. Specifically, the weighed Na source and precursor particles were added to a solvent, and the dispersion solution in which the Na source was dissolved and the precursor particles were dispersed was spray dried. The spray drying temperature was 200°C and the spray pressure was 0.3 MPa. By spray drying, a composite was obtained in which 75% of the surface area of ​​the precursor particles was coated with the Na source.

[0040] 1.3 Firing of the composite One g of the composite was placed in an alumina crucible and fired in an air atmosphere. The firing conditions were as follows (1) to (5). (1) Place the alumina crucible containing the above composite material in the heating furnace. Here, the thickness B of the composite material is 3 mm. (2) The temperature inside the heating furnace is raised from room temperature to 900°C at a predetermined heating rate A. The relationship between the heating rate A and the thickness B is as shown in Table 1 below. (3) Maintain the temperature inside the furnace at 900°C for 1 hour to perform the final firing. (4) After the main firing, the temperature inside the furnace is lowered from 900°C to 250°C over 4 hours. (5) Remove the alumina crucible from the heating furnace at 250°C and allow it to cool in the air.

[0041] By crushing the calcined material, which had been cooled in the atmosphere, using a mortar and pestle in a dry atmosphere, Na-containing oxide particles having a P2-type structure were obtained. These Na-containing oxide particles are Na 0.7 Mn 0.5Ni 0.2 Co 0.3 It had a chemical composition represented by O2.

[0042] 2. Evaluation of Na-containing oxide particles 2.1 Visual Observation using SEM Figure 2 shows SEM images of the appearance of the Na-containing oxides in Examples 1, 2, 4, and the Comparative Example. As is clear from Figure 2, the Na-containing oxides in Examples 1, 2, 4, and the Comparative Example are spherical particles with a circularity of 0.80 or greater. Furthermore, it can be seen that the surface of these spherical particles is composed of multiple crystallites, and the crystallite diameter is less than 1 μm. The Na-containing oxides in Examples 3 and 5 had a similar morphology.

[0043] 2.2 Identification of crystal structure by X-ray diffraction measurement For each of the Na-containing oxides in Examples 1-5 and the Comparative Example, X-ray diffraction measurements were performed using CuKα as the radiation source to obtain the X-ray diffraction pattern, and the diffraction peak intensity I originating from the (102) plane of the P2-type structure was determined from the X-ray diffraction pattern. P2 and diffraction peak intensity I originating from the (015) plane of the O3 type structure O3 Ratio I P2 / I O3 The following was determined. Figure 3 shows the X-ray diffraction patterns of each Na-containing oxide in Examples 1 to 5 and the Comparative Example. As shown in Figure 3, the Na-containing oxide in the Comparative Example has a diffraction peak intensity I that originates from the P2 type structure. P2 and diffraction peak intensity I derived from the O3 type structure O3 Ratio I P2 / I O3 The ratio was small, meaning it contained a large amount of the O3 phase. In contrast, the Na-containing oxides in Examples 1-5 had a ratio of 1 P2 / I O3The amount of O3 phase was significantly reduced compared to the comparative example. In Examples 1 to 5, it is thought that the O3 phase was reduced because the heating rate during the main firing was slow. Upon further investigation by the inventors, it was found that the amount of O3 phase can be controlled by the heating rate from 300°C to 500°C when firing the composite. Specifically, as shown in Table 1 and Figure 4 below, when the ratio A / B of the heating rate A (°C / min) from 300°C to 500°C and the thickness B (mm) of the composite is 4.5 or less, P2 / I O3 This value becomes significantly large, meaning that the O3 phase is easily reduced significantly.

[0044] [Table 1]

[0045] 3. Supplement In the above examples, the case of obtaining the precursor by coprecipitation was illustrated, but the precursor can also be obtained by other methods. Also, in the above examples, the case of obtaining the composite by coating the surface of the precursor with a Na source by spray drying was illustrated, but the composite can also be obtained by other methods. Furthermore, in the above examples, a Na-containing oxide having a P2-type structure with a predetermined chemical composition was illustrated, but the chemical composition of the Na-containing oxide is not limited to this. Various chemical compositions that adopt a P2-type structure can be used. In addition, the Na-containing oxide may be doped with an element M other than Mn, Ni, and Co. The element M is as described in the examples. Furthermore, in the above examples, the case of obtaining spherical Na-containing oxide particles at the end was illustrated by using a spherical precursor, but the precursor and Na-containing oxide are not limited to spherical particles. However, when spherical particles are used, for example, a high effect can be expected as a positive electrode active material for batteries.

[0046] 4. Summary As described above, a method for producing a Na-containing oxide having a P2-type structure, comprising the following steps S1 to S3, can reduce the O3 phase in the Na-containing oxide. That is, it is possible to produce a Na-containing oxide having a P2-type structure and a reduced O3 phase. S1: Obtain a precursor containing at least one element from among Mn, Ni, and Co. S2: The surface of the precursor is coated with a Na source to obtain a composite. S3: By calcining the composite, a Na-containing oxide having a P2-type structure is obtained. Here, during the calcination of the composite, the ratio A / B of the heating rate A (°C / min) from 300°C to 500°C and the thickness B (mm) of the composite is 4.5 or less.

[0047] Furthermore, according to the above manufacturing method, for example, a Na-containing oxide satisfying the following configurations (1) to (4) can be obtained. That is, the Na-containing oxide is (1) The constituent elements include at least one of Mn, Ni, and Co, as well as Na and O. (2) It has a P2 type structure. (3) 0.5 ≤ I P2 / I O3 It has an X-ray diffraction pattern that satisfies ≤20.0. (4) It is a spherical particle.

Claims

[Claim 1] A method for producing a Na-containing oxide having a P2-type structure belonging to the space group P63 / mmc, Using an ion source capable of forming precipitates with transition metal ions in aqueous solution, a transition metal compound containing Mn, a transition metal compound containing Ni, and a transition metal compound containing Co, a precursor containing Mn, Ni, and Co is obtained by coprecipitation. The composite is obtained by coating the surface of the precursor with a Na source using a rolling flow coating method or a spray drying method, and By calcining the aforementioned composite, a Na-containing oxide having a P2-type structure is obtained. Includes, During the firing of the composite, the ratio A / B of the heating rate A (°C / min) from 300°C to 500°C to the thickness B (mm) of the composite is 4.5 or less. The heating rate A is 13.5°C / min or less. The firing temperature of the composite is between 700°C and 1100°C. The main firing time of the composite is 30 minutes or more and 48 hours or less. The Na-containing oxide having the P2-type structure has a chemical composition represented by Na a Mn x Ni y Co z O 2 (where 0 < a ≤ 1.00, 0.30 ≤ x ≤ 0.70, 0.10 ≤ y ≤ 0.30, 0.20 ≤ z ≤ 0.60, and x + y + z = 1). Manufacturing method.

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