JP7848740B2 - Method for producing sodium-containing oxides - Google Patents

Description

This application discloses a method for producing sodium-containing oxides.

Patent Document 1 discloses a Na-containing oxide having a P2-type structure and a chemical composition represented by Na _x Fe _y Mn _1-y O ₂ (where x is less than 1 and y is 1/3 or more and less than 2/3).

Japanese Patent Publication No. 2012-201588

In sodium-containing oxides with a P2-type structure, the O3 phase is easily formed along with the P2 phase. A new technology is needed to reduce the O3 phase in sodium-containing oxides with a P2-type structure.

This application discloses several embodiments as means for solving the above-mentioned problems.
<Appearance 1>
A method for producing a Na-containing oxide having a P2 type structure,
To obtain a precursor containing at least one element among Mn, Ni, and Co,
The surface of the precursor is coated with a Na source to obtain a composite, 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.
Manufacturing method.
<Aspect 2>
A manufacturing method according to embodiment 1,
The heating rate A is 13.5°C/min or less.
Manufacturing method.
<Appearance 3>
A manufacturing method according to embodiment 1 or 2,
The precursor is a spherical particle,
The composite is obtained by coating 40% or more of the surface area of the precursor with the Na source, and the Na-containing oxide having the P2 type structure is spherical particles.
Manufacturing method.
<Appearance 4>
A manufacturing method according to any of embodiments 1 to 3,
The method includes obtaining a precipitate as a precursor 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.
Manufacturing method.
<Aspect 5>
A manufacturing method according to any of embodiments 1 to 4,
The Na-containing oxide having the P2-type structure has a chemical composition represented by Na _a Mn _x-p Ni _y-q Co _z-r M _p+q+r O ₂ (where 0 < a ≤ 1.00, x + y + z = 1, and 0 ≤ p + q + r < 0.17, and 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).
Manufacturing method.

According to the method disclosed herein, the O3 phase can be reduced when producing a Na-containing oxide having a P2-type structure.

This shows an example of a method for producing a Na-containing oxide having a P2-type structure. These are SEM images of the appearance of each of the Na-containing oxides in Examples 1, 2, and 4 and the Comparative Example. The X-ray diffraction patterns of each Na-containing oxide from Examples 1 to 5 and the Comparative Example are shown. This shows the relationship between A/B and the peak intensity ratio I _P2 / _IO3 .

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 from among Mn, Ni, and Co (S1), coating the surface of the precursor with a Na source to obtain a composite (S2), and firing the composite to obtain a Na-containing oxide having a P2-type structure (S3). Here, 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.

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.

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 an 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 an 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-mentioned 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, the precursor can be obtained by the sol-gel method. In particular, the coprecipitation method readily yields spherical particles as the precursor.

As described above, the precursor may be spherical particles. In this application, "spherical particles" means particles with a circularity of 0.80 or higher. The circularity of the particles may be 0.81 or higher, 0.82 or higher, 0.83 or higher, 0.84 or higher, 0.85 or higher, 0.86 or higher, 0.87 or higher, 0.88 or higher, 0.89 or higher, or 0.90 or higher. The circularity of the particles is defined as 4πS/ ^L² , where S is the orthographic area of the particle and L is the perimeter of the orthographic 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), transmission electron microscope (TEM), or optical microscope. If there are more than 100 particles, their circularity can be measured as an average value, for example, as follows.

(1) First, measure the particle size distribution. Specifically, use laser diffraction and scattering to determine the particle diameter at 10% of the cumulative value in the volume-based particle size distribution (D10) and the particle diameter at 90% of the cumulative value (D90).
(2) The appearance of the particles whose particle size distribution has been measured is observed using SEM, TEM, or optical microscope, and 100 particles are randomly selected from the images that have a circle equivalent diameter (the diameter of a circle with the same area as the orthographic area of the particle) that is D10 or greater and D90 or less, as determined in (1).
(3) For each of the 100 extracted particles, the circularity is determined by image processing, and the average value is considered to be the "circularity of the particle".

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 the precursor containing element M is not particularly limited. When obtaining the 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 containing element M are prepared, and the respective aqueous solutions are 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 during the Na-doping calcination in S2 and S3 described later.

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.

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 low, 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 high, 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.

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 simultaneously 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.

In step S2, the precursor may be coated with both the Na source and the M source. For example, in step S2, the precursor obtained in step S1 may be mixed with the 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 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.

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.

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. For example, the heating rate A may be 0.05°C/min or more, or 0.10°C/min or more. Furthermore, 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.

In S3, the thickness B of the composite refers to 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 is the thickness of the composite when the composite is weighed to be uniform in height within the container. For example, the height of the composite inside the container when the container is placed on the bottom surface of the heating 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.

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; it is greater than 0 and may be 0.01 or greater, 0.02 or greater, or 0.03 or greater.

In step S3, the heating rate to reach 300°C is not particularly limited. Similarly, the heating rate from 500°C to the firing temperature is also not particularly limited. In this embodiment, regardless of the heating rate to reach 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.

In step S3, the composite material may be optionally molded, pre-fired, and then fired. The pre-fired composite material may be fired at a temperature lower than or equal to the firing temperature. For example, pre-fired firing can be performed at a temperature between 500°C and 700°C. The pre-fired firing time is not particularly limited. Furthermore, the pre-fired atmosphere is not particularly limited. The pre-fired atmosphere may be the same as or different from the firing atmosphere.

In step 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.

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 easily formed 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 multiple crystallites on its surface, and the crystallites may be linked together to form a structure.

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.

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

2.1 Chemical Composition The Na-containing oxide according to one embodiment contains, as constituent elements, at least one element from among Mn, Ni, and Co, 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, a Na-containing oxide having a P2-type structure may have a chemical composition represented by Na _a Mn _x-p Ni _y-q Co _z-r M _p+q+r O ₂ (where 0 < a ≤ 1.00, x + y + z = 1, and 0 ≤ p + q + r < 0.17, and 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 a Na-containing oxide has such a chemical composition, the P2-type structure is easily 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 may be 1.00 or less and may be 0.90 or less, 0.80 or less, or 0.70 or less. Also, x is greater than or equal to 0 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 may be 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 greater than or equal to 0 and may be 0.10 or more or 0.20 or more, and may be 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. Furthermore, z is 0 or greater, and may be 0.10 or greater, 0.20 or greater, or 0.30 or greater, and may be 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. Element M has a small contribution to charging and discharging. In this respect, in the above chemical composition, a high charge-discharge capacity is easily secured when p+q+r is less than 0.17. 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, the inclusion of element M makes it easier to stabilize the P2 type structure. In the above chemical composition, p + q + r is 0 or greater, and may be 0.01 or greater, 0.02 or greater, 0.03 or greater, 0.04 or greater, 0.05 or greater, 0.06 or greater, 0.07 or greater, 0.08 or greater, 0.09 or greater, or 0.10 or greater. The O composition is approximately 2, but is not necessarily exactly 2.0 and is indeterminate.

2.2 Crystal Structure The Na-containing oxide according to one embodiment has at least a P2-type structure (belonging to space group P63 /m mc) as its crystal structure. The Na-containing oxide may have a P2-type structure as well as other crystal structures. 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 according to this embodiment may have a P2-type structure as its main phase. For example, the X-ray diffraction pattern of the Na-containing oxide according to this embodiment may satisfy 0.5 ≤ I _P2 / I _O3 ≤ 20.0. Here, I _P2 is the X-ray diffraction peak intensity originating from the (102) plane of the P2-type structure. I _O3 is the X-ray diffraction peak intensity originating from the (015) plane of the O3-type structure. _IP2 / _IO3 may be 1.0 or greater, 2.0 or greater, 3.0 or greater, or 4.0 or greater, and may be 18.0 or less, 16.0 or less, 14.0 or less, 12.0 or less, 10.0 or less, 9.0 or less, 8.0 or less, or 7.0 or less.

The X-ray diffraction peak intensity I _P2 originating from the (102) plane of the P2-type structure is identified as follows.
(1) In the X-ray diffraction pattern using CuKα as the source, the average value _I35 of the diffraction peak intensity at 35.0° ± 0.2° is determined as the background.
(2) Determine the maximum value _I39 of the diffraction peak intensity between 39.23° and 40.05° in the X-ray diffraction pattern.
(3) Let I _P2 be the value obtained by subtracting I ₃₅ from I ₃₉ (I _P2 = I ₃₉ - I ₃₅ ).

The X-ray diffraction peak intensity _I₀O₃ originating from the (015) plane of the O₃ type structure is identified as follows.
(1) In the X-ray diffraction pattern using CuKα as the source, the average value _I35 of the diffraction peak intensity at 35.0° ± 0.2° is determined as the background.
(2) Determine the maximum value I ₄₅ of the diffraction peak intensity between 45.35° and 45.65° in the X-ray diffraction pattern.
(3) Let I _O3 be the value obtained by subtracting I ₃₅ from I ₄₅ (I _O3 = I ₄₅ - I ₃₅ ).

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 exhibit 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). Specifically, when observing the surface of a Na-containing oxide, if a closed region surrounded by grain boundaries is observed, this region is considered a "crystallite." The maximum Ferret diameter of this crystallite is determined and considered 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 in 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.

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.

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. Note that 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.

2.4 Supplementary Information 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 satisfies 0.5 ≤ I _P2 / I _O3 ≤ 20.0.
(4) The Na-containing oxide is a spherical particle.

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 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.

As described above, one embodiment of a method for producing a Na-containing oxide having a P2-type structure has been explained. However, the manufacturing method of this disclosure can be modified in various ways other than the above embodiment without departing from its essence. 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.

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 addition, _Na₂CO₃ was dissolved in distilled water to a concentration of 1.6 _mol /L in a separate container 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 to it at a rate of approximately 4 mL/min each.
(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.

1.2 Preparation of the composite (1) _Na₂CO₃ as the Na source and the above-mentioned precursor _particles were weighed so that the composition after calcination, as described later, would be _{Na₀.7 Mn₀.5 Ni₀.2 Co₀.3 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.

1.3 Firing of the composite 1 g of the composite was placed in an alumina crucible and fired in an air atmosphere. The conditions for the firing process 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 heating furnace at 900°C for 1 hour to perform the final firing.
(4) After the main firing, the temperature inside the heating 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.

Na-containing oxide particles having a P2-type structure were obtained by crushing the calcined material, which had been cooled to air, using a mortar and pestle in a dry atmosphere. These Na-containing oxide particles had a chemical composition represented by Na _0.7 , Mn _0.5 , Ni _0.2 , Co _0.3, and O ₂ .

2. Evaluation of Na-containing oxide particles 2.1 Visual observation by SEM Figure 2 shows SEM images of the appearance of the Na-containing oxides for each of Examples 1, 2, 4 and the Comparative Example. As is clear from Figure 2, the Na-containing oxides for each of Examples 1, 2, 4 and the Comparative Example are spherical particles with a circularity of 0.80 or more. 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 for Examples 3 and 5 had a similar morphology.

2.2 Determination of Crystal Structure by X-ray Diffraction Measurement For each of the Na-containing oxides in Examples 1 to 5 and the Comparative Example, X-ray diffraction measurements were performed using CuKα as the radiation source to obtain X-ray diffraction patterns. From these X-ray diffraction patterns, the ratio _I₁P₂ / _I₂O₃ , between the diffraction peak intensity _I₁P₂ originating from the (102) plane of the P2-type structure and the diffraction peak intensity _I₁O₃ originating from the (015) plane of the O3-type structure, was determined. Figure 3 shows the X-ray diffraction patterns of each of the Na-containing oxides in Examples 1 to 5 and the Comparative Example. As shown in Figure 3, the Na-containing oxide in the Comparative Example had a small ratio _I₁P₂ / _I₂O₃ between the diffraction peak intensity _I₁P₂ originating from the _P2 -type structure and the diffraction peak intensity I₁O₃ originating from the O3-type structure, meaning it contained a large amount of the O3 phase. In contrast, the Na-containing oxides in Examples 1 to 5 had a large ratio _I₁P₂ / _I₂O₃ , indicating a significantly reduced amount of the O3 phase compared to the Comparative Example. In Examples 1 to 5, it is believed that the O3 phase was reduced because the heating rate during the main firing was slow. The inventors have investigated further and found that the amount of the 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 to the thickness B (mm) of the composite is 4.5 or less, I _P2 /I _O3 becomes a significantly large value, meaning that the O3 phase is significantly reduced.

3. Supplementary Information In the above examples, the case in which the precursor is obtained by coprecipitation is illustrated, but the precursor can also be obtained by other methods. Also, in the above examples, the case in which the composite is obtained by coating the surface of the precursor with a Na source by spray drying is illustrated, but the composite can also be obtained by other methods. Also, in the above examples, a Na-containing oxide having a P2-type structure with a predetermined chemical composition is 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. Also, 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. Also, in the above examples, the case in which spherical precursors are used to obtain spherical Na-containing oxide particles is illustrated, 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.

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.

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 element from among Mn, Ni, and Co, as well as Na and O.
(2) It has a P2 type structure.
(3) It has an X-ray diffraction pattern that satisfies 0.5 ≤ I _P2 / I _O3 ≤ 20.0.
(4) It is a spherical particle.

Claims (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 can be 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.