Positive electrode active material and sodium ion secondary battery

Abstract

To provide a positive electrode active material which has a P2-type structure and a large energy density.SOLUTION: The positive electrode active material of the present disclosure includes Na-containing oxide. The Na-containing oxide particles have a P2-type structure. The Na-containing oxide particles include at least one of Mn, Ni, and Co and includes Na and O as constituent elements. The Na-containing oxide particles have an average particle diameter of at least 2.0 μm. The Na-containing oxide particles have an average aspect ratio of at least 1.0 and 3.0 at largest.SELECTED DRAWING: Figure 6

Description

Technical Field

[0001] This application discloses a positive electrode active material and a sodium-ion secondary battery.

Background Art

[0002] Patent Documents 1 and 2 disclose Na-containing oxides having a P2-type structure and a predetermined chemical composition. The Na-containing oxide having a P2-type structure is used as a positive electrode active material of a sodium-ion secondary battery.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

[0006] The positive electrode active material of this disclosure has excellent gravimetric energy density. [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] A schematic diagram of an example of the configuration of a sodium-ion secondary battery is shown. [Figure 3] The X-ray diffraction patterns of the positive electrode active materials according to Examples 1 to 3 are shown. [Figure 4] This shows the X-ray diffraction pattern of the positive electrode active material related to Comparative Example 1. [Figure 5] This shows a cross-sectional SEM image of the positive electrode active material according to Example 1. [Figure 6] This shows an SEM image of the appearance of the positive electrode active material according to Example 1. [Modes for carrying out the invention]

[0008] 1.Cathode active material 1.1 First form The positive electrode active material according to the first embodiment includes Na-containing oxide particles. The Na-containing oxide particles have a P2-type structure. The Na-containing oxide particles contain, as constituent elements, at least one element from among Mn, Ni, and Co, as well as Na and O. The Na-containing oxide particles have an average particle diameter of 2.0 μm or more. The Na-containing oxide particles have an average aspect ratio of 1.0 to 3.0.

[0009] 1.1.1 Crystal structure The Na-containing oxide particles according to the first embodiment have at least a P2-type structure (belonging to space group P63mc) as a crystalline structure. The Na-containing oxide particles may have a P2-type structure as well as a crystalline structure other than the P2-type structure. Examples of crystalline structures other than the P2-type structure include various crystalline structures (such as the P3-type structure) formed when Na is removed or inserted from the P2-type structure. The Na-containing oxide particles may have a P2-type structure as the main phase. The crystalline structure of the main phase of the Na-containing oxide particles may change depending on the charge and discharge state.

[0010] The Na-containing oxide particles according to the first embodiment may be single crystals consisting of one crystallite, or polycrystalline particles having multiple crystallites. The end faces of the crystallites of the Na-containing oxide particles are considered to serve as the entrance and exit points for intercalation. That is, when the crystallites of the Na-containing oxide particles are small, effects such as a decrease in reaction resistance due to an increase in the number of intercalation entrances and exits, 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 cracks to occur can be expected. For example, the diameter of the crystallites constituting the Na-containing oxide particles may be 0.1 μm to 5.0 μm, 0.5 μm to 4.0 μm, or 1.0 μm to 3.0 μm. The "crystallite" and "crystallite diameter" can be determined by observing the Na-containing oxide particles using a scanning electron microscope (SEM) or transmission electron microscope (TEM). That is, when observing Na-containing oxide particles, if a closed region surrounded by a grain boundary is observed, that region is considered a "crystallite". The maximum Ferret diameter of the crystallite in question is determined and considered as the "crystallite diameter." Note that if the Na-containing oxide particles consist 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 oxide particles tend to exhibit higher performance if the crystallite diameter determined by either method falls within the above range. The crystallites constituting the Na-containing oxide particles may have a first face exposed on the surface of the oxide, and this first face may be planar.

[0011] The Na-containing oxide particles according to the first embodiment have a predetermined average particle diameter (D50) and a predetermined average aspect ratio. In order to achieve such an average particle diameter and average aspect ratio, in this embodiment, a specific process described later is employed during the manufacturing of the Na-containing oxide particles. According to the inventor's knowledge, Na-containing oxide particles obtained through such a specific process tend to have a smaller c-axis length of the P2-type structure than conventional particles. For example, the P2-type structure in the Na-containing oxide particles according to the first embodiment may have a c-axis length of 11.10 Å or less. The P2-type structure may also have a c-axis length of 11.05 Å or more and 11.10 Å or less. The lattice constants (a-axis length, b-axis length, and c-axis length) of the P2-type structure can be determined by full pattern fitting from the X-ray diffraction pattern of the Na-containing oxide particles. The software used is Rigaku's PDXL2. Here, "X-ray diffraction pattern of Na-containing oxide particles" refers to one obtained under the following conditions. Specifically, for Na-containing oxide particles, an X-ray diffraction pattern is obtained by using an X-ray diffractometer (RIGAK, SmartLab fully automated multi-purpose X-ray diffractometer) with CuKα as the radiation source, performing a 2θ / θ scan with a tube voltage of 45kV, tube current of 200mA, step width of 0.02°, and scan speed of 1° / min.

[0012] 1.1.2 Chemical composition The Na-containing oxide particles according to the first embodiment contain, as constituent elements, at least one element from among Mn, Ni, and Co, along with Na and O. The Na-containing oxide particles tend to exhibit higher performance when they contain, in particular, Na, Mn, one or both of Ni and Co, along with O, especially when they contain at least Na, Mn, Ni, Co, and O. Alternatively, the Na-containing oxide particles also tend to exhibit higher performance when they contain at least Na, Mn, Fe, and O as constituent elements. Furthermore, the Na-containing oxide particles according to the first embodiment may contain more than 0.35 moles of Na per mole of O. There is no particular upper limit to the amount of Na relative to O. The Na-containing oxide particles according to the first embodiment may contain more than 0.35 moles and 0.50 moles or more, or 0.38 moles and 0.45 moles or more, of Na per mole of O. Thus, sodium-containing oxide particles that contain more than 0.35 moles of sodium per mole of oxygen tend to have an excellent gravimetric energy density as a positive electrode active material.

[0013] The Na-containing oxide particles in the first form are Na a 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, 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 Na-containing oxide particles have such a chemical composition, the P2-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, 0.60 or more, or 0.70 or more, and is at most 1.00, and may be 0.90 or less. Also, x is 0 or more, and may be greater than 0, 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 less than 1.00, 0.90 or less, 0.80 or less, 0.70 or less, 0.60 or less, 0.50 or less, or less than 0.50. Also, y is 0 or more, and may be greater than 0, 0.10 or more, or 0.20 or more, and is at most 1.00, and may be less than 1.00, 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 greater than 0, 0.10 or more, 0.20 or more, or 0.30 or more, and is at most 1.00, and may be less than 1.00, 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-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 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 indeterminate.

[0014] The Na-containing oxide particles according to the first embodiment contain Na a Mn x-p Ni y-q Co z-r M p+q+r O2 (where 0 < a ≦ 1.00, 0 < x < 1.00, 0 < y < 0.50, 0 < z < 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.) may have a chemical composition represented by. Further, the Na-containing oxide particles according to the first embodiment contain Na a Mn x-p Ni y-q Co z-r M p+q+r O2 (where 0.70 < a ≦ 1.00, 0.30 < x < 0.60, 0.10 < y < 0.40, 0.10 < z < 0.50, 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.) may have a chemical composition represented by. Conventionally, it has been difficult to produce Na-containing oxide particles having a P2-type structure and such a chemical composition. However, in the present embodiment, as the production conditions of the Na-containing oxide particles having a P2-type structure, by adopting specific conditions described later, Na-containing oxide particles having a P2-type structure and such a chemical composition can be obtained. The Na-containing oxide particles having such a chemical composition have an excellent weight energy density as a positive electrode active material.

[0015] 1.1.3 Shape The Na-containing oxide particles according to the first embodiment may be solid particles, hollow particles, or particles having voids. The Na-containing oxide particles according to the first embodiment have an excellent weight energy density as a positive electrode active material by having the following average aspect ratio and average particle diameter.

[0016] 1.1.3.1 Aspect ratio 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 oxide particles having a P2 type structure are usually plate-like particles with a large aspect ratio, where the crystal growth direction is biased in a specific direction. Specifically, Na-containing oxide particles having a P2 type structure are usually plate-like particles with an average aspect ratio significantly greater than 3.0. Here, the edges of the plate-like particles become the entry and exit points for intercalation. As far as the inventors have confirmed, plate-like particles with a large aspect ratio tend to have a smaller proportion of the particle that contributes to intercalation, and are prone to a decrease in gravimetric energy density.

[0017] In contrast, the Na-containing oxide particles of the first form have suppressed bias in the crystal growth direction and possess an aspect ratio below a certain level. Specifically, the Na-containing oxide particles of the first form have an average aspect ratio of 1.0 to 3.0. The average aspect ratio of the Na-containing oxide particles having a P2-type structure being 3.0 or less makes it easier to increase the gravimetric energy density as a positive electrode active material. The average aspect ratio of the Na-containing oxide particles of the first form may be 1.0 to 2.9, 1.0 to 2.8, 1.0 to 2.7, 1.0 to 2.6, 1.0 to 2.5, or 1.0 to 2.4.

[0018] The average aspect ratio of Na-containing oxide particles is measured as follows: The cross-section of the Na-containing oxide particles (or the cross-section of the positive electrode active material layer if the Na-containing oxide particles are contained in the positive electrode active material layer described later) is observed using a scanning electron microscope (SEM) or transmission electron microscope (TEM) to determine the shape of the Na-containing oxide particles. The largest Ferret diameter in that shape is identified and considered as the "major axis". The largest diameter perpendicular to the "major axis" in that shape is considered as the "minor axis". The ratio of the "major axis" to the "minor axis" (major axis / minor axis) is considered as the "aspect ratio" of the Na-containing oxide particle. The "aspect ratio" is calculated for each Na-containing oxide particle, and the average of these values ​​is considered as the "average aspect ratio".

[0019] 1.1.3.2 Average particle size As described above, in the conventional technology, Na-containing oxide particles having a P2-type structure become plate-like particles with a large aspect ratio, where the crystal growth direction is biased in a specific direction. In the conventional technology, if one attempts to suppress the growth of the P2 phase, the average particle size of the Na-containing oxide particles becomes extremely small, raising concerns about excessive aggregation of particles and potentially resulting in insufficient P2 phase. As a result, it is difficult to secure a sufficient gravimetric energy density in the Na-containing oxide particles of the conventional technology. In contrast, the Na-containing oxide particles of the first embodiment can overcome these problems by having a certain size while maintaining the above-mentioned average aspect ratio. Specifically, the Na-containing oxide particles of the first embodiment have an average particle size of 2.0 μm or more. The average particle size of the Na-containing oxide particles of the first embodiment may be 2.0 μm or more and 5.0 μm or less, 2.0 μm or more and 4.0 μm or less, or 2.0 μm or more and 3.0 μm or less.

[0020] Furthermore, the "average particle diameter" of Na-containing oxide particles refers to the particle diameter (D50, median diameter) at 50% of the cumulative value in the volume-based particle size distribution obtained by laser diffraction and scattering.

[0021] 1.1.4 Others As described above, the positive electrode active material according to the first embodiment includes Na-containing oxide particles having the specific average particle diameter and average aspect ratio described above, and thus has an excellent weight energy density. The positive electrode active material according to the first embodiment may be composed only of the above-described Na-containing oxide particles, or may include, together with the above-described Na-containing oxide particles, other positive electrode active materials (other positive electrode active materials). From the viewpoint of further enhancing the above effect, the proportion of other positive electrode active materials in the entire positive electrode active material may be small. For example, assuming that the total amount of the positive electrode active material is 100% by mass, the content of the above-described Na-containing oxide particles may be 50% by mass or more and 100% by mass or less, 60% by mass or more and 100% by mass or less, 70% by mass or more and 100% by mass or less, 80% by mass or more and 100% by mass or less, 90% by mass or more and 100% by mass or less, 95% by mass or more and 100% by mass or less, or 99% by mass or more and 100% by mass or less.

[0022] 1.2 Second Embodiment The positive electrode active material according to the second embodiment includes Na-containing oxide particles. The Na-containing oxide particles have a P2-type structure. The Na-containing oxide particles are Na a Mn x-p Ni y-q Co z-r M p+q+r O2 (where 0.70 < a ≦ 1.00, 0 < x < 1.00, 0 < y < 0.50, 0 < z < 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).) has a chemical composition represented by

[0023] 1.2.1 Crystal Structure The Na-containing oxide particles of the second embodiment, like the Na-containing oxide particles of the first embodiment, have at least a P2-type structure (belonging to space group P63mc) as their crystal structure. The Na-containing oxide particles 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 (such as the P3-type structure) formed when Na is removed or inserted from the P2-type structure. The Na-containing oxide particles may have a P2-type structure as their main phase. The crystal structure of the main phase of the Na-containing oxide particles may change depending on the charge-discharge state.

[0024] The Na-containing oxide particles of the second form may be single crystals consisting of one crystallite, or polycrystalline with multiple crystallites, similar to the Na-containing oxide particles of the first form. As described above, the end faces of the crystallites of the Na-containing oxide particles are considered to be the entrance and exit points for intercalation. That is, when the crystallites of the Na-containing oxide particles are small, effects such as a decrease in reaction resistance due to an increase in the number of intercalation entrances and exits, 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 cracks to occur can be expected. For example, the diameter of the crystallites constituting the Na-containing oxide particles may be 0.1 μm or more and 5.0 μm or less, 0.5 μm or more and 4.0 μm or less, or 1.0 μm or more and 3.0 μm or less. The crystallites constituting the Na-containing oxide particles may have a first face exposed on the surface of the oxide, and this first face may be planar.

[0025] The Na-containing oxide particles according to the second embodiment have a P2-type structure and a predetermined chemical composition. In order to achieve such a crystal structure and chemical composition, in this embodiment, a specific process described later is adopted during the production of the Na-containing oxide particles. According to the findings of the present inventors, the Na-containing oxide particles obtained through such a specific process tend to have a smaller c-axis length of the P2-type structure than before. For example, the P2-type structure in the Na-containing oxide particles according to the second embodiment may have a c-axis length of 11.10 Å or less. The P2-type structure may have a c-axis length of 11.05 Å or more and 11.10 Å or less. The method for measuring lattice constants such as the c-axis length is as described above.

[0026] 1.2.2 Chemical Composition The Na-containing oxide particles according to the second embodiment contain Na a Mn x-p Ni y-q Co z-r M p+q+r O2 (where 0.70 < a ≤ 1.00, 0 < x < 1.00, 0 < y < 0.50, 0 < z < 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). The Na-containing oxide particles according to the second embodiment contain Na a Mn x-p Ni y-q Co z-r M p+q+rIt may have a chemical composition represented by O2 (where 0.70 < a ≤ 1.00, 0.30 < x < 0.60, 0.10 < y < 0.40, 0.10 < z < 0.50, 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). Conventionally, it has been difficult to produce Na-containing oxide particles having a P2-type structure and such a chemical composition. However, in this embodiment, by adopting specific conditions as the production conditions for Na-containing oxide particles having a P2-type structure, it is possible to obtain Na-containing oxide particles having a P2-type structure and such a chemical composition. Na-containing oxide particles having such a chemical composition have a high weight energy density as a positive electrode active material. a is greater than 0.70, may be 0.75 or more or 0.80 or more, and is less than or equal to 1.00, and may be less than or equal to 0.90 or less than 0.90. Also, x is greater than 0, may be 0.10 or more, 0.20 or more, 0.30 or more, more than 0.30, 0.40 or more or 0.50 or more, and is less than 1.00, 0.90 or less, 0.80 or less, 0.70 or less, 0.60 or less, less than 0.60, 0.50 or less or less than 0.50. Also, y is greater than 0, may be 0.10 or more, more than 0.10 or 0.20 or more, and is less than 0.50, 0.45 or less or 0.40 or less. Also, z is greater than 0, may be 0.10 or more, more than 0.10, 0.20 or more or 0.30 or more, and is less than 1.00, 0.90 or less, 0.80 or less, 0.70 or less, 0.60 or less, 0.50 or less, less than 0.50, 0.40 or less or 0.30 or less. p + q + r is greater than or equal to 0, 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, and is less than or equal to 0.17, 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. The composition of O is approximately 2, but is not necessarily exactly 2.0 and is indeterminate.

[0027] 1.2.3 Shape The Na-containing oxide particles according to the second embodiment may be solid particles, hollow particles, or particles with voids. The Na-containing oxide particles according to the second embodiment may have the following average aspect ratio and average particle diameter.

[0028] 1.2.3.1 Aspect Ratio The Na-containing oxide particles of the second form may have an aspect ratio below a certain level, with the bias in the crystal growth direction suppressed. Specifically, the Na-containing oxide particles of the second form may have an average aspect ratio of 1.0 to 3.0. When the average aspect ratio of the Na-containing oxide particles having a P2-type structure is 3.0 or less, the gravimetric energy density as a positive electrode active material tends to increase even further. The average aspect ratio of the Na-containing oxide particles of the second form may be 1.0 to 2.9, 1.0 to 2.8, 1.0 to 2.7, 1.0 to 2.6, 1.0 to 2.5, or 1.0 to 2.4. The method for measuring the "average aspect ratio" is as described above.

[0029] 1.2.3.2 Average particle size The Na-containing oxide particles in the second form may have an average particle diameter of 2.0 μm or more. The average particle diameter of the Na-containing oxide particles in the second form may be 2.0 μm or more and 5.0 μm or less, 2.0 μm or more and 4.0 μm or less, or 2.0 μm or more and 3.0 μm or less. The method for measuring the "average particle diameter" of the Na-containing oxide particles is as described above.

[0030] 1.2.4 Others As described above, the positive electrode active material according to the second form has excellent gravimetric energy density by containing Na-containing oxide particles having the specific chemical composition described above. The positive electrode active material according to the second form may consist only of the above-mentioned Na-containing oxide particles, or it may contain other positive electrode active materials (other positive electrode active materials) together with the above-mentioned Na-containing oxide particles. From the viewpoint of further enhancing the above effect, the proportion of other positive electrode active materials in the total positive electrode active material may be small. For example, with the total positive electrode active material being 100% by mass, the content of the above-mentioned Na-containing oxide particles may be 50% by mass or more and 100% by mass or less, 60% by mass or more and 100% by mass or less, 70% by mass or more and 100% by mass or less, 80% by mass or more and 100% by mass or less, 90% by mass or more and 100% by mass or less, 95% by mass or more and 100% by mass or less, or 99% by mass or more and 100% by mass or less.

[0031] 2. Method for manufacturing positive electrode active material The Na-containing oxide particles according to the first and second embodiments described above can be manufactured, for example, by the following method. As shown in Figure 1, the method for manufacturing Na-containing oxide particles having a P2-type structure according to one embodiment is: S1: To 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: Includes obtaining a Na-containing oxide having a P2-type structure by calcining the composite. Here, S3 is S3-1: The composite material is subjected to pre-calcination at a temperature of 300°C or higher and less than 700°C for a period of 2 hours or more and 10 hours or less. S3-2: Following the pre-firing, the composite is subjected to a main firing at a temperature of 700°C to 1100°C for 30 minutes to 48 hours, and S3-3: Following the main firing, the composite is cooled to air from a temperature T1 of 200°C or higher to a temperature T2 of 100°C or lower.

[0032] 2.1 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 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. The composition of the precursor may be appropriately determined to correspond to the composition of the final product, a Na-containing oxide.

[0033] 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.

[0034] 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.

[0035] 2.2 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.

[0036] 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. Alternatively, in S2, the composite may be obtained by covering less than 40% or less, 35% or less, or 30% or less of the surface of the precursor with the Na source. If the coverage rate of the Na source is low, P2 type crystals tend to grow on the surface of the composite when the composite is calcined. On the other hand, 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 growth of P2 type crystals tends to be suppressed.

[0037] In S2, the method for coating the surface of the precursor with the Na source is not particularly limited. For example, the surface of the precursor can be coated with the Na source by mixing the precursor and the Na source dry or wet. Alternatively, the surface of the precursor may be coated with the Na source by a rolling flow coating method or a spray drying method. That is, a coating solution in which the Na source is dissolved is prepared, and the coating solution is brought into contact with the surface of the precursor, or dried at the same time as or after contact. The coverage rate of the Na source on the surface of the precursor can be controlled by adjusting the coating conditions (temperature, time, number of passes, etc.).

[0038] 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.

[0039] 2.3 S3 In S3, the composite obtained in S2 is calcined to obtain a Na-containing oxide having a P2-type structure. S3 includes the above-mentioned S3-1, S3-2, and S3-3.

[0040] 2.3.1 S3-1 In S3-1, the composite is pre-fired at a temperature of 300°C to less than 700°C for a period of 2 to 10 hours. In S3-1, the composite may be arbitrarily shaped before pre-fired. Pre-fired is performed at a temperature lower than that of the main firing. If pre-fired is insufficient, the main firing in S3-2 will need to be performed at a high temperature and for a long time in order to generate a sufficient amount of P2 phase, which may lead to abnormal growth of the P2 phase. Also, if pre-fired in S3-1 is insufficient, it may be difficult to dope with a sufficient amount of Na, making it difficult to obtain a P2 phase with a specific chemical composition. By performing sufficient pre-fired, the P2 phase can be appropriately generated in the main firing, the generation of crystalline phases other than the P2 phase can be suppressed, and the shape of the P2-type Na-containing oxide particles can be easily controlled. In other words, in S3-1, by setting the pre-calcination temperature to 300°C or higher and less than 700°C, and the pre-calcination time to 2 hours or higher and less than 10 hours, sufficient pre-calcination can be performed on the composite, and the Na-containing oxide particles having a P2-type structure obtained through S3-2 and S3-3 described below will have a predetermined average aspect ratio and average particle size. Furthermore, by performing sufficient pre-calcination on the composite, a P2-type structure can be appropriately generated in S3-2 and S3-3 while having a predetermined chemical composition. The pre-calcination temperature may be 400°C or higher and less than 700°C, 450°C or higher and less than 700°C, 500°C or higher and less than 700°C, or 550°C or higher and less than 650°C. The pre-calcination time may also be 2 hours or higher and less than 8 hours, 3 hours or higher and less than 8 hours, 4 hours or higher and less than 8 hours, 5 hours or higher and less than 8 hours, or 5 hours or higher and less than 7 hours. The pre-firing atmosphere is not particularly limited and may, for example, be an oxygen-containing atmosphere.

[0041] 2.3.2 S3-2 In S3-2, following the pre-sintering described above, the composite is subjected to main firing at a temperature of 700°C to 1100°C for a period of 30 minutes to 48 hours. In S3-2, the main firing temperature of the composite is 700°C to 1100°C, preferably 800°C to 1000°C. If the main firing temperature is too low, the P2 phase will not be formed, 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-sintering temperature to the main firing temperature are not particularly limited. The main firing time is 30 minutes to 48 hours, as described above. 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, if 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 be formed on its surface. In the method of this disclosure, abnormal growth of P2-type crystals is suppressed by growing P2-type crystals along the surface of the composite. As a result, the shape of the Na-containing oxide particles has a predetermined average aspect ratio and average particle diameter. If the firing time is too short, the formation of the P2 phase will be insufficient. On the other hand, if the firing time is too long, the P2-type crystals will grow excessively, and the predetermined average aspect ratio and average particle diameter cannot be achieved. As far as the inventors have confirmed, when the firing time is 30 minutes or more and 3 hours or less, the Na-containing oxide particles are more likely to have the predetermined average aspect ratio and average particle diameter.

[0042] 2.3.3 S3-3 In step S3-3, following the main firing described above, the composite is rapidly cooled (cooled at a cooling rate of 20°C / min or more) from a temperature T1 of 200°C or higher to a temperature T2 of 100°C or lower. The above pre-firing and main firing are performed, for example, in a heating furnace. In step S3-3, for example, after the main firing of the composite is performed in a heating furnace, it is cooled in the heating furnace to an arbitrary temperature T1 of 200°C or higher, and after reaching that temperature T1, the fired product is removed from the heating furnace and rapidly cooled outside the furnace to an arbitrary temperature T2 of 100°C or lower. Temperature T1 is any temperature of 200°C or higher, and may also be any temperature of 250°C or higher. Temperature T2 is any temperature of 100°C or lower, and may also be any temperature of 50°C or lower, and may also be the cooling completion temperature. In a predetermined temperature range between temperature T1 and temperature T2, moisture easily penetrates between the layers of the P2 type structure due to atomic vibrations, molecular motion, etc. When cooling the composite (Na-containing oxide having a P2-type structure) after the main firing, it is thought that reducing the time spent in the temperature range where moisture easily penetrates (i.e., rapid cooling) will reduce the amount of moisture penetrating into the interlayers of the P2-type structure. In this regard, in step S3-3, when cooling the composite after the main firing, by allowing it to cool in a dry atmosphere outside the furnace from an arbitrary temperature T1 of 200°C or higher to an arbitrary temperature T2 of 100°C or lower, the cooling rate from temperature T1 to temperature T2 becomes high (for example, 20°C / min or higher), making it difficult for moisture to penetrate into the interlayers of the P2-type structure and suppressing the collapse of the P2-type structure. As a result, Na-containing oxide particles having a P2-type structure and a predetermined chemical composition can be obtained.

[0043] By the above method, Na-containing oxide particles according to the first form and Na-containing oxide particles according to the second form can be produced.

[0044] 3. Sodium-ion rechargeable battery The positive electrode active material according to the embodiment includes the above-mentioned specific Na-containing oxide. This positive electrode active material can be used, for example, as the positive electrode active material of a sodium-ion secondary battery. Figure 2 schematically shows the configuration of a sodium-ion secondary battery according to one embodiment. As shown in Figure 2, the sodium-ion secondary battery 100 according to one embodiment has a positive electrode active material layer 10, an electrolyte layer 20, and a negative electrode active material layer 30. Here, the positive electrode active material layer 10 includes the positive electrode active material according to the above embodiment.

[0045] 3.1 Cathode active material layer The positive electrode active material layer 10 contains at least the positive electrode active material according to the above embodiment, and may optionally contain an electrolyte, a conductive additive, and a binder. Furthermore, the positive electrode active material layer 10 may also contain various other additives. The respective contents of the positive electrode active material, electrolyte, conductive additive, and binder in the positive electrode active material layer 10 can be appropriately determined according to the desired battery performance. For example, with the entire positive electrode active material layer 10 (total solid content) as 100% by mass, the content of the positive electrode active material may be 40% by mass or more, 50% by mass or more, or 60% by mass or more, or 100% by mass or less, or 90% by mass or less. The shape of the positive electrode active material layer 10 is not particularly limited, and for example, it may be a sheet-shaped positive electrode active material layer 10 having a substantially flat surface. The thickness of the positive electrode active material layer 10 is not particularly limited, and for example, it may be 0.1 μm or more, or 1 μm or more, or 2 mm or less, or 1 mm or less.

[0046] 3.1.1 Cathode active material The positive electrode active material is as described above. That is, the positive electrode active material includes Na-containing oxide particles according to the first form and / or Na-containing oxide particles according to the second form. As described above, the positive electrode active material may consist only of the above-mentioned Na-containing oxide particles, or it may include other positive electrode active materials (other positive electrode active materials) together with the above-mentioned Na-containing oxide particles. From the viewpoint of further enhancing the effects of the technology of this disclosure, the proportion of other positive electrode active materials in the total positive electrode active material may be small. For example, with the total positive electrode active material being 100% by mass, the content of the above-mentioned Na-containing oxide particles may be 50% by mass or more and 100% by mass or less, 60% by mass or more and 100% by mass or less, 70% by mass or more and 100% by mass or less, 80% by mass or more and 100% by mass or less, 90% by mass or more and 100% by mass or less, 95% by mass or more and 100% by mass or less, or 99% by mass or more and 100% by mass or less.

[0047] 3.1.2 Electrolytes The electrolyte contained in the positive electrode active material layer 10 may be a solid electrolyte, a liquid electrolyte (electrolyte solution), or a combination thereof. For the solid electrolyte, any known solid electrolyte for sodium-ion secondary batteries may be used. The solid electrolyte may be an inorganic solid electrolyte or an organic polymer electrolyte. In particular, inorganic solid electrolytes have excellent ionic conductivity and heat resistance. An example of an inorganic solid electrolyte is Na3Zr2PSi2O 12 Oxides such as Na2O-11Al2O3; NaBH4, NaB 10 H 10 NaCB9H 10 NaCB 11 H 12 NaB 12 Cl 12 These include hydrides and borides; Na3PS4, Na3SbS4, Na 2.88 S 0.88 W 0.12Examples include at least one selected from sulfides such as S4 and fluorides such as NaPF6 and NaBF4. The solid electrolyte may be in particulate form, for example. The solid electrolyte may be used alone or in combination of two or more types. The electrolyte may contain, for example, sodium ions as carrier ions. The electrolyte may be an aqueous electrolyte or a non-aqueous electrolyte. The composition of the electrolyte may be the same as that known for the electrolyte of sodium-ion secondary batteries. For example, as the electrolyte, a solution of sodium salt dissolved in a carbonate-based solvent at a predetermined concentration can be used. Examples of carbonate-based solvents include fluoroethylene carbonate (FEC), ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC). Examples of sodium salts include NaPF6.

[0048] 3.1.3 Conductive additives Examples of conductive additives that may be included in the positive electrode active material layer 10 include carbon materials such as vapor-processed carbon fiber (VGCF), acetylene black (AB), Ketjenblack (KB), carbon nanotubes (CNT), and carbon nanofibers (CNF); and metallic materials such as nickel, aluminum, and stainless steel. The conductive additive may be in the form of parts or fibers, and its size is not particularly limited. One type of conductive additive may be used alone, or two or more types may be used in combination.

[0049] 3.1.4 Binder Examples of binders that may be included in the positive electrode active material layer 10 include butadiene rubber (BR) binders, butylene rubber (IIR) binders, acrylate butadiene rubber (ABR) binders, styrene butadiene rubber (SBR) binders, polyvinylidene fluoride (PVdF) binders, polytetrafluoroethylene (PTFE) binders, and polyimide (PI) binders. A single binder may be used alone, or two or more binders may be used in combination.

[0050] 3.2 Electrolyte layer The electrolyte layer 20 contains at least an electrolyte. If the sodium-ion secondary battery 100 is a solid-state battery (a battery containing a solid electrolyte, which may also contain a liquid electrolyte in part, or it may be an all-solid-state battery that does not contain a liquid electrolyte), the electrolyte layer 20 contains a solid electrolyte and may further optionally contain a binder or the like. In this case, the content of the solid electrolyte and the binder or the like in the electrolyte layer 20 is not particularly limited. On the other hand, if the sodium-ion secondary battery 100 is an electrolyte battery, the electrolyte layer 20 contains an electrolyte and may further have a separator or the like to hold the electrolyte and prevent contact between the positive electrode active material layer 10 and the negative electrode active material layer 30. The thickness of the electrolyte layer 20 is not particularly limited and may be, for example, 0.1 μm or more or 1 μm or more, or 2 mm or less or 1 mm or less.

[0051] The electrolyte included in the electrolyte layer 20 may be appropriately selected from among the examples of electrolytes that may be included in the positive electrode active material layer 10 described above. Similarly, the binder included in the electrolyte layer 20 may be appropriately selected from among the examples of binders that may be included in the positive electrode active material layer 10 described above. The electrolyte and binder may each be used individually or in combination of two or more types. The separator may be any separator commonly used in sodium-ion secondary batteries, such as those made of polyethylene (PE), polypropylene (PP), polyester, and polyamide resins. The separator may have a single-layer structure or a multi-layer structure. Examples of multi-layer separators include a PE / PP two-layer separator, or a PP / PE / PP or PE / PP / PE three-layer separator. The separator may also be made of a nonwoven fabric such as cellulose nonwoven fabric, resin nonwoven fabric, or glass fiber nonwoven fabric.

[0052] 3.3 Negative electrode active material layer The negative electrode active material layer 30 contains at least negative electrode active material, and may optionally also contain an electrolyte, a conductive additive, and a binder. Furthermore, the negative electrode active material layer 30 may also contain various other additives. The respective content of the negative electrode active material, electrolyte, conductive additive, and binder in the negative electrode active material layer 30 can be appropriately determined according to the desired battery performance. For example, with the entire negative electrode active material layer 30 (total solid content) as 100% by mass, the content of the negative electrode active material may be 40% by mass or more, 50% by mass or more, 60% by mass or more, or 100% by mass or less, or 90% by mass or less. The shape of the negative electrode active material layer 30 is not particularly limited, and for example, it may be a sheet-like negative electrode active material layer having a substantially flat surface. The thickness of the negative electrode active material layer 30 is not particularly limited, and for example, it may be 0.1 μm or more, 1 μm or more, 2 mm or less, or 1 mm or less.

[0053] As the negative electrode active material, various materials can be used whose potential for intercalating and releasing sodium ions (charge / discharge potential) is lower than that of the positive electrode active material described above. The negative electrode active material may be an inorganic negative electrode active material such as metallic sodium, an organic negative electrode active material, or a combination thereof. The negative electrode active material may be used alone or in combination of two or more types. The shape of the negative electrode active material may be a shape common to negative electrode active materials of batteries. For example, the negative electrode active material may be particulate. The negative electrode active material particles may be primary particles or secondary particles formed by the aggregation of multiple primary particles. The average particle diameter (D50) of the negative electrode active material particles may be, for example, 1 nm or more, 5 nm or more, or 10 nm or more, or 500 μm or less, 100 μm or less, 50 μm or less, or 30 μm or less. Alternatively, the negative electrode active material may be in the form of a sheet (foil or film) such as sodium foil. In other words, the negative electrode active material layer 30 may consist of a sheet of negative electrode active material.

[0054] The electrolyte that may be included in the negative electrode active material layer 30 is the solid electrolyte, electrolyte solution, or a combination thereof as described above. The conductive additive that may be included in the negative electrode active material layer 30 is the carbon material or metal material described above as described above. The binder that may be included in the negative electrode active material layer 30 can be appropriately selected from, for example, the binders exemplified as those that may be included in the positive electrode active material layer 10 described above. The electrolyte and binder may each be used individually or in combination of two or more types.

[0055] 3.4 Positive electrode current collector As shown in Figure 2, the sodium-ion secondary battery 100 may include a positive electrode current collector 40 that contacts the positive electrode active material layer 10. Any of the commonly used positive electrode current collectors for batteries can be used for the positive electrode current collector 40. Furthermore, the positive electrode current collector 40 may be in the form of foil, plate, mesh, perforated metal, or foam. The positive electrode current collector 40 may be composed of metal foil or metal mesh. Metal foil, in particular, offers superior handling. The positive electrode current collector 40 may consist of multiple foils. Examples of metals constituting the positive electrode current collector 40 include Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, and stainless steel. In particular, from the viewpoint of ensuring oxidation resistance, the positive electrode current collector 40 may contain Al. The positive electrode current collector 40 may have some kind of coating layer on its surface for purposes such as adjusting resistance. Furthermore, the positive electrode current collector 40 may be a metal foil or a substrate on which the above-mentioned metal is plated or deposited. Also, if the positive electrode current collector 40 consists of multiple metal foils, there may be some kind of layer between the multiple metal foils. The thickness of the positive electrode current collector 40 is not particularly limited. For example, it may be 0.1 μm or more or 1 μm or more, or 1 mm or less or 100 μm or less.

[0056] 3.5 Negative electrode current collector As shown in Figure 2, the sodium-ion secondary battery 100 may include a negative electrode current collector 50 that contacts the negative electrode active material layer 30. Any of the commonly used negative electrode current collectors for batteries can be used for the negative electrode current collector 50. Furthermore, the negative electrode current collector 50 may be in the form of foil, plate, mesh, perforated metal, or foam. The negative electrode current collector 50 may be a metal foil or metal mesh, or a carbon sheet. Metal foil, in particular, offers superior handling. The negative electrode current collector 50 may consist of multiple foils or sheets. Examples of metals constituting the negative electrode current collector 50 include Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, and stainless steel. In particular, from the viewpoint of ensuring reduction resistance, the negative electrode current collector 50 may contain at least one metal selected from Cu, Ni, and stainless steel. The negative electrode current collector 50 may have some kind of coating layer on its surface for the purpose of adjusting resistance, etc. The negative electrode current collector 50 may also be a metal foil or substrate on which the above-mentioned metal is plated or deposited. Furthermore, if the negative electrode current collector 50 consists of multiple metal foils, there may be some kind of layer between the multiple metal foils. The thickness of the negative electrode current collector 50 is not particularly limited. For example, it may be 0.1 μm or more, 1 μm or more, 1 mm or less, or 100 μm or less.

[0057] 3.6 Other matters In addition to the above configuration, the sodium-ion secondary battery 100 may also have obvious components for a secondary battery, such as tabs and terminals. The sodium-ion secondary battery 100 may have each of the above components housed inside an outer casing. Any known battery casing can be used. Furthermore, multiple batteries 100 may be electrically connected as desired and stacked as desired to form a battery pack. In this case, the battery pack may be housed inside a known battery case. The sodium-ion secondary battery 100 may also have other obvious components such as necessary terminals. Examples of shapes for the sodium-ion secondary battery 100 include coin-type, laminate-type, cylindrical, and prismatic types.

[0058] The sodium-ion secondary battery 100 can be manufactured by applying known methods, except for the use of the specific positive electrode active material described above. For example, it can be manufactured as follows. However, the manufacturing method of the sodium-ion secondary battery 100 is not limited to the following method, and each layer may be formed by, for example, dry molding. (1) A slurry for the positive electrode layer is obtained by dispersing the positive electrode active material and other materials constituting the positive electrode active material layer in a solvent. The solvent used in this case is not particularly limited, and water or various organic solvents can be used. The slurry for the positive electrode layer is applied to the surface of the positive electrode current collector using a doctor blade or the like, and then dried to form a positive electrode active material layer on the surface of the positive electrode current collector, which serves as the positive electrode. (2) The negative electrode active material and other materials constituting the negative electrode active material layer are dispersed in a solvent to obtain a slurry for the negative electrode layer. The solvent used in this case is not particularly limited, and water or various organic solvents can be used. The slurry for the negative electrode layer is applied to the surface of the negative electrode current collector using a doctor blade or the like, and then dried to form a negative electrode active material layer on the surface of the negative electrode current collector, which serves as the negative electrode. (3) The layers are stacked so that an electrolyte layer (solid electrolyte layer or separator) is sandwiched between the negative electrode and the positive electrode, and a laminate is obtained having the negative electrode current collector, negative electrode active material layer, electrolyte layer, positive electrode active material layer and positive electrode current collector in this order. Other members such as terminals are attached to the laminate as needed. (4) The laminate is housed in a battery case, and in the case of an electrolyte battery, the battery case is filled with electrolyte, the laminate is immersed in the electrolyte, and the laminate is sealed inside the battery case to form a secondary battery. In the case of an electrolyte battery, the negative electrode active material layer, separator and positive electrode active material layer may be made to contain the electrolyte at the stage of (3) above.

[0059] 4. Method for increasing the gravimetric energy density of sodium-ion secondary batteries The technology of this disclosure also has an aspect as a method for increasing the gravimetric energy density of a sodium-ion secondary battery. That is, the method for increasing the gravimetric energy density of a sodium-ion secondary battery of this disclosure is characterized by using the positive electrode active material of this disclosure in the positive electrode active material layer of the sodium-ion secondary battery.

[0060] 5. Vehicles equipped with sodium-ion secondary batteries As described above, the positive electrode active material of this disclosure has an excellent gravimetric energy density and is suitable as a positive electrode active material for sodium-ion secondary batteries. A sodium-ion secondary battery with such a high gravimetric energy density can be suitably used in at least one type of vehicle selected from, for example, hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and battery electric vehicles (BEVs). In other words, the technology of this disclosure also has an aspect as a vehicle having a sodium-ion secondary battery, wherein the sodium-ion secondary battery has a positive electrode active material layer, an electrolyte layer, and a negative electrode active material layer, and the positive electrode active material layer contains the positive electrode active material of this disclosure. [Examples]

[0061] As described above, one embodiment of the positive electrode active material and sodium-ion secondary battery has been explained, but the technology of this disclosure can be modified in various ways other than the above embodiment without departing from its gist. 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.

[0062] 1. Preparation of positive electrode active material 1.1 Example 1 1.1.1 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 1.2 mol / L to obtain the first solution. In a separate container, Na2CO3 was dissolved in distilled water to a concentration of 1.2 mol / L to obtain the second solution. (2) 1000 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 150 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 120°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. The molar ratio of Mn, Ni, and Co in the precursor particles was Mn:Ni:Co = 4:3:3.

[0063] 1.1.2 Preparation of the complex Precursor particles and Na2CO3, Na 0.9 Mn 0.4 Ni 0.3 Co 0.3 The O2 was weighed to achieve the required composition. The weighed precursor and Na2CO3 were mixed using a mortar and pestle to obtain the complex.

[0064] 1.1.3 Firing of the composite The composite was placed in an alumina crucible and calcined under an atmospheric environment to obtain a Na-containing oxide having a P2-type structure. The calcination conditions were as follows (1) to (7). (1) Place an alumina crucible containing the above composite in a heating furnace in an atmospheric environment. (2) Heat the inside of the furnace from room temperature (25°C) to 600°C in 115 minutes. (3) Maintain the temperature inside the heating furnace at 600°C for 360 minutes to perform pre-firing. (4) After pre-firing, the temperature inside the heating furnace is raised from 600°C to 900°C in 100 minutes. (5) Maintain the temperature inside the furnace at 900°C for 60 minutes to perform the final firing. (6) After the main firing, the temperature inside the furnace is lowered from 900°C to 250°C over 120 minutes. (7) Remove the alumina crucible from the heating furnace at 250°C and allow it to cool in a dry atmosphere outside the furnace, reaching 25°C in 10 minutes.

[0065] By crushing the calcined material, which had been cooled in the atmosphere, using a mortar and pestle under a dry atmosphere, Na-containing oxide particles having a P2-type structure were obtained.

[0066] 1.2 Example 2 Except for changing the composition of the precursor particles and the charging ratio of the precursor particles to Na2CO3, the method is the same as in Example 1. In Example 2, the molar ratio of Mn, Ni, and Co in the precursor particles was set to Mn:Ni:Co=5:3:2. Also, the precursor particles and Na2CO3 were changed to Na 0.8 Mn 0.5 Ni 0.3 Co 0.2 The ingredients were weighed to achieve the required O2 composition.

[0067] 1.3 Example 3 Except for changing the composition of the precursor particles and the charging ratio of the precursor particles to Na2CO3, Example 3 is the same as Example 1. In Example 3, the molar ratio of Mn, Ni, and Co in the precursor particles was set to Mn:Ni:Co=4:2:4. Also, the precursor particles and Na2CO3 were changed to Na 0.8 Mn 0.4 Ni 0.2 Co 0.4 The ingredients were weighed to achieve the required O2 composition.

[0068] 1.4 Comparative Example 1 The composition of the precursor particles and the charging ratio of the precursor particles to Na2CO3 were changed, and fine particles after gas classification were used as the precursor particles, otherwise the process was the same as in Example 1. In Comparative Example 1, the molar ratio of Mn, Ni, and Co in the precursor particles was set to Mn:Ni:Co=5:2:3. Also, the precursor particles and Na2CO3 were changed to Na 0.7 Mn 0.5 Ni 0.2 Co 0.3 The ingredients were weighed to achieve the required O2 composition.

[0069] 2. Evaluation of the positive electrode active material 2.1 Elemental analysis Elemental analysis was performed on each of the positive electrode active materials in Examples 1-3 and Comparative Example 1 to determine their chemical composition. The results are shown in Table 1 below.

[0070] 2.2 Identification of crystal structure by X-ray diffraction measurement X-ray diffraction measurements were performed on each of the positive electrode active materials of Examples 1-3 and Comparative Example 1 using CuKα as the radiation source, and the X-ray diffraction patterns were obtained. Figure 3 shows the X-ray diffraction patterns of Examples 1-3. Figure 4 shows the X-ray diffraction pattern of Comparative Example 1. As shown in Figures 3 and 4, it can be seen that all of the positive electrode active materials of Examples 1-3 and Comparative Example 1 have a P2-type structure belonging to the space group P63mc. Furthermore, the lattice constants (a-axis length, b-axis length, c-axis length) of each P2-type structure of Examples 1-3 and Comparative Example 1 were determined from each X-ray diffraction pattern. The results are shown in Table 1 below.

[0071] 2.3 Measurement of average particle size The average particle size (D50) was measured for each of the positive electrode active materials in Examples 1-3 and Comparative Example 1. The results are shown in Table 1 below.

[0072] 2.4 Measurement of the average aspect ratio by SEM observation For each of the positive electrode active materials in Examples 1-3 and Comparative Example 1, the materials were formed into pellets, subjected to CP processing, and then observed in cross-section using FE-SEM to measure the average aspect ratio. The results are shown in Table 1 below. For reference, Figure 5 shows an SEM image of the cross-section of the positive electrode active material from Example 1 after it was formed into pellets. Figure 6 shows the external shape of the positive electrode active material from Example 1.

[0073] 3. Creation of evaluation cells Coin cells were fabricated using the positive electrode active materials of Examples 1-3 and Comparative Example 1. The procedure for fabricating the coin cells is as follows. (1) The positive electrode active material, acetylene black (AB) as a conductive additive, and polyvinylidene fluoride (PVdF) as a binder were weighed 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 mixture slurry. The positive electrode mixture 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) As the electrolyte, a solvent was prepared by mixing EC and DEC in a volume ratio of 1:1, and dissolving NaPF6 in it to a concentration of 1M. (3) A sodium metallic foil was prepared as the negative electrode. (4) A coin cell (CR2032) was fabricated using the positive electrode, electrolyte, and negative electrode.

[0074] 4. Evaluation of charge and discharge characteristics For each coin cell of Examples 1-3 and Comparative Example 1, the capacity was measured by charging and discharging at 0.1C (1C = 160mA / g) in a constant temperature bath maintained at 25°C, within a voltage range of 1.0-4.8V. The results are shown in Table 1 below. (2) For each coin cell of Example 4 and Comparative Example 2, charging and discharging were performed in a constant temperature bath maintained at 25°C at a voltage range of 1.0-4.3V and at 0.1C (1C = 190mA / g), and the initial discharge capacity, average discharge potential, and gravimetric energy density were measured. The results are shown in Table 2 below.

[0075] 5. Evaluation Results For each of Examples 1-3 and Comparative Example 1, the chemical composition of the positive electrode active material, average particle size (D50), average aspect ratio, and lattice constant, as well as the initial discharge capacity, average discharge potential, and gravimetric energy density of the evaluation cell are shown.

[0076] [Table 1]

[0077] [Table 2]

[0078] As is clear from the results shown in Tables 1 and 2, the positive electrode active materials of Examples 1 to 3, in which D50 is relatively large (2.0 μm or more) and average aspect ratio is relatively small (3.0 or less), have a superior gravimetric energy density compared to the positive electrode active material of Comparative Example 1, in which D50 is relatively small (less than 2.0 μm) and average aspect ratio is relatively large (greater than 3.0). Furthermore, the initial discharge capacity and average discharge potential of the positive electrode active materials of Examples 1 to 3 are equivalent to or better than those of Comparative Example 1.

[0079] Furthermore, as is clear from the results shown in Tables 1 and 2, the positive electrode active materials of Examples 1 to 3, which have the predetermined chemical composition, have a superior gravimetric energy density compared to the positive electrode active material of Comparative Example 1, which has a different chemical composition from Examples 1 to 3.

[0080] 6. Supplement In the above examples, the method of obtaining the precursor by coprecipitation was illustrated, but the precursor can also be obtained by other methods. Furthermore, in the above examples, the method 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. Also, 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 thereto. 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.

[0081] 7. Summary As described above, in a positive electrode active material containing a Na-containing oxide, if the Na-containing oxide satisfies the following requirements (1-1) to (1-4), it can be said that the energy density of the positive electrode active material increases. (1-1) The Na-containing oxide particles have a P2-type structure. (1-2) The Na-containing oxide particles include, as constituent elements, at least one element from among Mn, Ni, and Co, as well as Na and O. (1-3) The Na-containing oxide particles have an average particle diameter of 2.0 μm or more. (1-4) The Na-containing oxide particles have an average aspect ratio of 1.0 or more and 3.0 or less.

[0082] Also, in the positive electrode active material containing the Na-containing oxide, when the Na-containing oxide satisfies the following requirements (2-1) and (2-2), it can be said that the energy density of the positive electrode active material increases. (2-1) The Na-containing oxide particles have a P2-type structure. (2-2) The Na-containing oxide particles are Na a Mn x-p Ni y-q Co z-r M p+q+r O2 (where 0.70 < a ≤ 1.00, 0 < x < 1.00, 0 < y < 0.50, 0 < z < 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 has a chemical composition represented by.

Explanation of symbols

[0083] 100 Sodium ion secondary battery 10 Positive electrode active material layer 20 Electrolyte layer 30 Negative electrode active material layer 40 Positive electrode current collector 50 Negative electrode current collector

Claims

1. A positive electrode active material comprising Na-containing oxide particles, The Na-containing oxide particles have a P2-type structure. The Na-containing oxide particles include, as constituent elements, at least one element from among Mn, Ni, and Co, as well as Na and O. The aforementioned Na-containing oxide particles contain more than 0.35 moles of Na per mole of O as constituent elements. The Na-containing oxide particles have a particle diameter D50 of 2.0 μm or more, and the particle diameter D50 is the particle diameter at 50% of the cumulative value in the volume-based particle size distribution determined by laser diffraction and scattering method. The Na-containing oxide particles have an average aspect ratio of 1.73 to 3.

0. Cathode active material.

2. The positive electrode active material according to claim 1, The Na-containing oxide particles are Na ａ Mn ｘ－ｐ Ni ｙ－ｑ Co ｚ－ｒ M ｐ＋ｑ＋ｒ O ２ (Here, 0.70 < 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.) Having the chemical composition shown, Cathode active material.

3. The positive electrode active material according to claim 1, The Na-containing oxide particles are Na ａ Mn ｘ－ｐ Ni ｙ－ｑ Co ｚ－ｒ M ｐ＋ｑ＋ｒ O ２ (where 0.70 < a ≤ 1.00, 0 < x < 1.00, 0 < y < 0.50, 0 < z < 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.) having a chemical composition represented by Cathode active material.

4. The positive electrode active material according to claim 1, The Na-containing oxide particles are Na ａ Mn ｘ－ｐ Ni ｙ－ｑ Co ｚ－ｒ M ｐ＋ｑ＋ｒ O ２ (Hereinafter, 0.70 < a ≤ 1.00, 0.30 < x < 0.60, 0.10 < y < 0.40, 0.10 < z < 0.50, 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.) Having the chemical composition shown above, Cathode active material.

5. A positive electrode active material according to any one of claims 1 to 4, The P2-type structure has a c-axis length of 11.10 Å or less. Cathode active material.

6. A sodium-ion secondary battery, It has a positive electrode active material layer, an electrolyte layer, and a negative electrode active material layer. The positive electrode active material layer comprises the positive electrode active material described in any one of claims 1 to 4. Sodium-ion rechargeable battery.

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