Positive electrode active material and sodium ion secondary battery

Abstract

Disclosed is a positive electrode active material having a P2-type structure and having a large reversible capacity. This positive electrode active material contains an Na-containing oxide. The Na-containing oxide has a P2 type structure. The Na-containing oxide at least contains at least one element selected from Mn, Ni and Co; na; and O as a constituent element. The X-ray diffraction pattern of the Na-containing oxide satisfies I1 / I2 < = 0.10. The X-ray diffraction pattern of the Na-containing oxide satisfies I1 / I3 < = 0.70. Wherein I1 is an X-ray diffraction peak intensity from a (102) plane of the P2-type structure, I2 is an X-ray diffraction peak intensity from a (002) plane of the P2-type structure, and I3 is an X-ray diffraction peak intensity from a (100) plane of the P2-type structure.

Description

TECHNICAL FIELD

[0001] Disclosed is a positive electrode active material and a sodium-ion secondary battery. BACKGROUND

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

[0003] PRIOR ART DOCUMENTS

[0004] PATENT DOCUMENTS

[0005] Patent Document 1: Japanese Patent Application Publication No. 2012-201588

[0006] Patent Document 2: Japanese Patent Application Publication No. 2017-045600 SUMMARY

[0007] PROBLEMS TO BE SOLVED BY THE INVENTION

[0008] The conventional positive electrode active material having a P2-type structure still has room for improvement in terms of reversible capacity.

[0009] MEANS FOR SOLVING THE PROBLEMS

[0010] As a means for solving the above problems, the present application discloses the following multiple schemes.

[0011] <SCHEME 1>

[0012] A positive electrode active material containing a Na-containing oxide,

[0013] The Na-containing oxide has a P2-type structure,

[0014] The Na-containing oxide contains at least one element among Mn, Ni, and Co; Na; and O as constituent elements,

[0015] The X-ray diffraction pattern of the Na-containing oxide satisfies I1 / I2≤0.10,

[0016] The X-ray diffraction pattern of the Na-containing oxide satisfies I1 / I3≤0.70,

[0017] The I1 is an X-ray diffraction peak intensity from a (102) plane of the P2-type structure,

[0018] The I2 is an X-ray diffraction peak intensity from a (002) plane of the P2-type structure,

[0019] The I3 is an X-ray diffraction peak intensity from a (100) plane of the P2-type structure.

[0020] <Scheme 2>

[0021] The positive electrode active material as described in Scheme 1, wherein the X-ray diffraction pattern of the Na-containing oxide satisfies 0.01 < I1 / I2 < 0.10, and the X-ray diffraction pattern of the Na-containing oxide satisfies 0.01 < I1 / I3 < 0.70.

[0022] <Scheme 3>

[0023] The positive electrode active material as described in Scheme 1 or 2, wherein the X-ray diffraction pattern of the Na-containing oxide satisfies 0.40° < HW1, the HW1 being a half width of an X-ray diffraction peak from a (102) plane of the P2-type structure.

[0024] <Scheme 4>

[0025] The positive electrode active material as described in Scheme 3, wherein the X-ray diffraction pattern of the Na-containing oxide satisfies 0.40° < HW1 < 1.00°

[0026] <Scheme 5>

[0027] The positive electrode active material as described in any one of Schemes 1 to 4, wherein the Na-containing oxide has a chemical composition represented by Na a Mn x- p Ni y-q Co z-r M p+q+r O2, wherein 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 the group consisting of B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W.

[0028] <Scheme 6>

[0029] The positive electrode active material as described in any one of Schemes 1 to 5, wherein the Na-containing oxide contains at least Na, Mn, Ni, Co, and O as constituent elements.

[0030] <Scheme 7>

[0031] The positive electrode active material as described in any one of Schemes 1 to 4, wherein the Na-containing oxide contains at least Na, Mn, Fe, and O as constituent elements.

[0032] <Scheme 8>

[0033] A sodium-ion secondary battery having a positive electrode active material layer, an electrolyte layer, and a negative electrode active material layer, wherein the positive electrode active material layer contains a positive electrode active material described in any one of Schemes 1 to 7.

[0034] Effects of the Invention

[0035] The positive electrode active material of the present disclosure has a large reversible capacity. BRIEF DESCRIPTION OF DRAWINGS

[0036] Figure 1 An example of a flow of a manufacturing method of a Na-containing oxide having a P2-type structure is shown.

[0037] Figure 2 An example of a structure of a sodium-ion secondary battery is shown schematically.

[0038] Figure 3 X-ray diffraction patterns of each of Examples 1 to 3 and Comparative Example 1 are shown.

[0039] Figure 4 X-ray diffraction patterns of each of Example 4 and Comparative Example 2 are shown. DETAILED DESCRIPTION

[0040] 1. Positive electrode active material

[0041] The positive electrode active material of the present embodiment contains a Na-containing oxide. The Na-containing oxide has a P2-type structure. The Na-containing oxide contains at least one of Mn, Ni, and Co and Na and O as constituent elements. The X-ray diffraction pattern of the Na-containing oxide satisfies I1 / I2≤0.10. The X-ray diffraction pattern of the Na-containing oxide satisfies I1 / I3≤0.70. The I1 is the X-ray diffraction peak intensity from the (102) plane of the P2-type structure. The I2 is the X-ray diffraction peak intensity from the (002) plane of the P2-type structure. The I3 is the X-ray diffraction peak intensity from the (100) plane of the P2-type structure.

[0042] 1.1 Crystal structure

[0043] The Na-containing oxide of the present embodiment has at least a P2-type structure (belongs to space group P63mc) as a crystal structure. The Na-containing oxide can have a crystal structure other than the P2-type structure while having the P2-type structure. As the crystal structure other than the P2-type structure, for example, various crystal structures (O2-type structure and the like) formed when Na is intercalated into and deintercalated from the P2-type structure can be given. The Na-containing oxide can have the P2-type structure as a main phase. Depending on the charge and discharge state, the crystal structure of the Na-containing oxide that becomes the main phase can change.

[0044] 1.1.1 X-ray diffraction peak intensity

[0045] The X-ray diffraction pattern of the Na-containing oxide of the present embodiment satisfies I1 / I2 < 0.10 and satisfies I1 / I3 < 0.70. Here, I1 is the X-ray diffraction peak intensity from the (102) plane of the P2-type structure, I2 is the X-ray diffraction peak intensity from the (002) plane of the P2-type structure, and I3 is the X-ray diffraction peak intensity from the (100) plane of the P2-type structure.

[0046] The X-ray diffraction peak intensity from the (102) plane of the conventional P2-type positive electrode active material is large, and the crystallinity in any of the longitudinal direction, the lateral direction, and the oblique direction is excellent. In contrast, the Na-containing oxide of the present embodiment has a small X-ray diffraction peak intensity I1 from the (102) plane compared to the X-ray diffraction peak intensity I2 from the (002) plane and the X-ray diffraction peak intensity I3 from the (100) plane by adopting a condition that has not been used in the past as a synthesis condition. That is, in the Na-containing oxide of the present embodiment, the X-ray diffraction peak intensity of the (102) plane is reduced in the P2-type crystal structure, and the crystallinity in the oblique direction is reduced while maintaining the P2-type structure as a whole. According to the present inventors' insight, the Na-containing oxide having such a P2-type structure has an excellent reversible capacity compared to the conventional P2-type positive electrode active material. Here, the low crystallinity of the (102) plane means that a particular crystal plane of the P2-type crystal structure is introduced with a stacking defect (stacking fault). For the Na-containing oxide of the present embodiment, it is presumed that the stacking defect produces some effect, and thus the reversible capacity as a positive electrode active material is increased.

[0047] In the X-ray diffraction pattern of the Na-containing oxide of the present embodiment, the ratio I1 / I2 of the X-ray diffraction peak intensity I1 from the (102) plane of the P2-type structure to the X-ray diffraction peak intensity I2 from the (002) plane of the P2-type structure satisfies I1 / I2 < 0.10. In addition, in the X-ray diffraction pattern of the Na-containing oxide of the present embodiment, the ratio I1 / I3 of the X-ray diffraction peak intensity I1 from the (102) plane of the P2-type structure to the X-ray diffraction peak intensity I3 from the (100) plane of the P2-type structure satisfies I1 / I3 < 0.70. The above X-ray diffraction pattern can satisfy 0.01 < I1 / I2 < 0.10, 0.02 < I1 / I2 < 0.10, or 0.03 < I1 / I2 < 0.10, and 0.01 < I1 / I3 < 0.70, 0.05 < I1 / I3 < 0.70, 0.10 < I1 / I3 < 0.70, 0.15 < I1 / I3 < 0.70, 0.20 < I1 / I3 < 0.70, or 0.20 < I1 / I3 < 0.65.

[0048] 1.1.2 Half-value width of X-ray diffraction peak

[0049] As described above, the crystallinity of the (102) plane of the P2-type structure of the Na-oxide-containing of the present embodiment is low. For example, the X-ray diffraction pattern of the Na-oxide-containing of the present embodiment can satisfy 0.40°≤ HW1. Here, the HW1 is the half width of the X-ray diffraction peak from the (102) plane of the P2-type structure. The half width HW1 can also satisfy 0.40°≤ HW1≤ 1.00°, 0.45°≤ HW1≤ 1.00°, 0.50°≤ HW1≤ 1.00°, 0.55°≤ HW1≤ 1.00°, or 0.60°≤ HW1≤ 1.00°. The half width of the X-ray diffraction peak from the (002) plane of the P2-type structure and the half width of the X-ray diffraction peak from the (100) plane are not particularly limited.

[0050] 1.1.3 Measurement method of X-ray diffraction peak intensity and half width

[0051] Note that, in the present application, the "X-ray diffraction pattern of the Na-oxide-containing", the "X-ray diffraction peak intensity", and the "half width of the X-ray diffraction peak" are obtained under the following conditions. That is, for the Na-oxide-containing, an X-ray diffraction device (Japan Rigaku Corporation, SmartLab, full-automatic multi-functional X-ray diffraction device) is used to perform a 2θ / θ scan under the conditions of CuKα for the ray source, 45 kV for the tube voltage, 200 mA for the tube current, 0.02° for the step width, and 1° / minute for the scanning speed, and the X-ray diffraction pattern is obtained. In the X-ray diffraction pattern, the X-ray diffraction peak from the (102) plane of the P2-type structure typically appears at 38.9°±0.5° (although it can vary depending on the transition metal composition or the Na amount, etc.). In addition, the X-ray diffraction peak from the (002) plane of the P2-type structure typically appears at 15.9°±0.5° (although it can vary depending on the transition metal composition or the Na amount, etc.). In addition, the X-ray diffraction peak from the (100) plane of the P2-type structure typically appears at 36°±0.5° (although it can vary depending on the transition metal composition or the Na amount, etc.). For the X-ray diffraction pattern, after the background value near the peak is subtracted, the above I1 / I2, I1 / I3 are calculated from the intensity of each X-ray diffraction peak, and in addition, the half width HW1 of the X-ray diffraction peak from the (002) plane is calculated.

[0052] 1.1.4 Crystallite

[0053] The Na-containing oxide of one embodiment can be a single crystal composed of one crystallite or a polycrystal having a plurality of crystallites. For example, the surface of the Na-containing oxide of one embodiment can be composed of a plurality of crystallites. In other words, the Na-containing oxide can have a structure in which a plurality of crystallites are connected to each other on the surface thereof. In the case where the surface of the Na-containing oxide is composed of a plurality of crystallites, a grain boundary exists on the surface. Here, the grain boundary sometimes serves as an entrance for intercalation and an exit for deintercalation. That is, in the case where the Na-containing oxide is a polycrystal having a plurality of crystallites, the following effects can be expected: an effect in which the number of entrances and exits for intercalation and deintercalation increases, and thus the reaction resistance decreases; an effect in which the distance through which sodium ions move becomes short, and thus the diffusion resistance decreases; an effect in which the absolute amount of expansion and contraction during charge and discharge decreases, and thus cracks are less likely to be generated; and the like. The size of the crystallite can be large or small, but it is thought that the size of the crystallite is small, the number of grain boundaries is large, and the above-described advantageous effects are easily exhibited. For example, in the case where the diameter of the crystallite that constitutes the Na-containing oxide is less than 1 μm, higher performance is easily obtained. Note that the "crystallite" or the "diameter of the crystallite" can be found by observing the surface of the Na-containing oxide with a scanning electron microscope (SEM) or a transmission electron microscope (TEM). That is, the surface of the Na-containing oxide is observed, and when a closed region surrounded by a grain boundary is observed, the region is regarded as a "crystallite". The largest Feret diameter of the crystallite is found, and the diameter is regarded as the "diameter of the crystallite". Note that in the case where the Na-containing oxide is composed of a single crystal, the particle itself can be referred to as a crystallite, and the largest Feret diameter of the particle is the "diameter of the crystallite". Alternatively, the diameter of the crystallite can be found by EBSD or XRD. For example, the diameter of the crystallite can be found from the half-value width of a diffraction line of an XRD pattern on the basis of the Scherrer formula. In the case where the diameter of the crystallite of the Na-containing oxide determined by any of the above methods is less than 1 μm, higher performance is easily exhibited. The crystallite that constitutes the Na-containing oxide can have a first surface exposed to the surface of the oxide, and the first surface can be planar. That is, the surface of the Na-containing oxide can have a structure in which a plurality of planes are connected. As described later, in the production of the Na-containing oxide, by growing a crystallite on the surface of a particle until one crystallite is connected to another crystallite, a crystallite having a planar first surface is easily obtained.

[0054] 1.2 Chemical composition

[0055] The Na-containing oxide of one embodiment contains at least one of Mn, Ni, and Co; Na; and O as constituent elements. In particular, in the case where the Na-containing oxide contains at least one of Na; Mn; Ni, and Co, or two or more of them; and O as constituent elements, and in the case where the Na-containing oxide contains Na; Mn; Ni, Co, and O as constituent elements, higher performance is easily obtained. Alternatively, in the case where the Na-containing oxide contains at least Na; Mn; Fe; and O as constituent elements, higher performance is also easily obtained.

[0056] In one embodiment, the Na-containing oxide having the P2-type structure also has a chemical composition represented by the following formula: Na a Mn x-p Ni y-q Co z-r M p+q+r O2(wherein, 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 has such a chemical composition, the P2-type structure is easily maintained. In the above chemical composition, a is greater than 0, can 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 can be 1.00 or less, 0.90 or less, 0.80 or less, or 0.70 or less. Further, x is 0 or more, can be 0.10 or more, 0.20 or more, 0.30 or more, 0.40 or more, or 0.50 or more, and can be 1.00 or less, 0.90 or less, 0.80 or less, 0.70 or less, 0.60 or less, or 0.50 or less. In addition, y is 0 or more, can be 0.10 or more, or 0.20 or more, and can be 1.00 or less, 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. In addition, z is 0 or more, can be 0.10 or more, 0.20 or more, or 0.30 or more, and can be 1.00 or less, can be 0.90 or less, 0.80 or less, 0.70 or less, 0.60 or less, 0.50 or less, 0.40 or less, or 0.30 or less. The element M contributes little to charge and discharge. In this regard, in the above chemical composition, by making p + q + r less than 0.17, a high charge and discharge capacity is easily ensured. p + q + r can 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, by containing the element M, the P2-type structure is easily stabilized. In the above chemical composition, p + q + r is 0 or more, can be 0.01 or more, 0.02 or more, 0.03 or more, 0.04 or more, 0.05 or more, 0.06 or more, 0.07 or more, 0.08 or more, 0.09 or more, or 0.10 or more. The composition of O is approximately 2, but is not necessarily exactly 2.0, and is indefinite.

[0057] 1.3 Shape

[0058] The P2-type structure is hexagonal, and the diffusion coefficient of Na ions is large, and easy to grow in a specific direction. In particular, as a transition metal element constituting the P2-type structure, it is easy to grow in a specific direction to a plate shape when at least one of Mn, Ni, and Co is contained. Therefore, the Na-containing oxide having the P2-type structure generally becomes a plate-like particle in which the growth direction of the crystal is biased to a specific direction and the aspect ratio is large. The Na-containing oxide of one embodiment can be such a plate-like particle or a spherical particle. When the Na-containing oxide is a spherical particle, the reaction resistance is easy to decrease due to a decrease in crystallite size, and the diffusion resistance in the particle is easy to decrease. Further, it is considered that, in the case of application to a secondary battery or the like, the degree of bending is decreased by the spherical shape, and the sodium ion conduction resistance is decreased. Thus, for example, the rate characteristics are easy to improve, and the reversible capacity is easy to be large. Note that, in this specification, a "spherical particle" refers to a particle with a circularity of 0.80 or more. The circularity of the particle can be 0.81 or more, 0.82 or more, 0.83 or more, 0.84 or more, 0.85 or more, 0.86 or more, 0.87 or more, 0.88 or more, 0.89 or more, or 0.90 or more. The circularity of the particle is calculated by 4πS / L 2 Definition. Here, S is the area of the orthogonal projection of the particle, and L is the circumference of the orthogonal projection image of the particle. The circularity of the particle can be calculated by observing the appearance of the particle with a scanning electron microscope (SEM), a transmission electron microscope (TEM), or an optical microscope. In the case where a plurality of particles are included, the circularity thereof is measured as an average value, for example, as follows.

[0059] (1) First, the particle size distribution of the particles is measured. Specifically, the particle diameter at the cumulative value of 10 % (D10) and the particle diameter at the cumulative value of 90 % (D90) in the particle size distribution on a volume basis are calculated by a laser diffraction / scattering method.

[0060] (2) For the appearance of the particles whose particle size distribution is measured, image observation is performed with an SEM, a TEM, or an optical microscope, and 100 particles each having an equivalent circle diameter (the diameter of a circle having the same area as the orthogonal projection area of the particle) of D10 or more and D90 or less, which are calculated in (1), are arbitrarily extracted from among the particles included in the image.

[0061] (3) For the 100 particles extracted, the circularity is calculated by image processing for each of the particles, and the average value thereof is regarded as the "circularity of the particles".

[0062] The Na-containing oxide of one embodiment can be a solid particle, can be a hollow particle, or can be a particle having a void. The size of the Na-containing oxide particle is not particularly limited, and a small size is more preferable. For example, the average particle diameter (D50) of the Na-containing oxide particle can be greater than or equal to 0.1 μm and less than or equal to 10 μm, greater than or equal to 1.0 μm and less than or equal to 8.0 μm, or greater than or equal to 2.0 μm and less than or equal to 6.0 μm. Note that the average particle diameter (D50) is the particle diameter (D50, median diameter) at which the cumulative value in the particle size distribution on a volume basis is 50 % measured by a laser diffraction / scattering method.

[0063] 1.4 Other

[0064] As described above, the positive electrode active material of one embodiment contains the above-described specific Na-containing oxide and has an excellent reversible capacity. As described above, the Na-containing oxide is introduced into a stacking fault on a specific crystal plane, and the stacking fault can lead to an increase in reversible capacity. The positive electrode active material of one embodiment can be composed of only the above-described Na-containing oxide, or can contain the above-described Na-containing oxide and another positive electrode active material (another positive electrode active material). From the viewpoint of further improving the above-described effect, the proportion of the other positive electrode active material in the entire positive electrode active material can be small. For example, the content of the above-described Na-containing oxide can be greater than or equal to 50 mass % and less than or equal to 100 mass %, greater than or equal to 60 mass % and less than or equal to 100 mass %, greater than or equal to 70 mass % and less than or equal to 100 mass %, greater than or equal to 80 mass % and less than or equal to 100 mass %, greater than or equal to 90 mass % and less than or equal to 100 mass %, greater than or equal to 95 mass % and less than or equal to 100 mass %, or greater than or equal to 99 mass % and less than or equal to 100 mass %, relative to the entire positive electrode active material, which is assumed to be 100 mass %.

[0065] 2. Method for manufacturing positive electrode active material

[0066] The Na-containing oxide of the above-described embodiment can be manufactured, for example, by the following method. As illustrated in FIG. 1, a method for manufacturing the Na-containing oxide having a P2-type structure of one embodiment includes the following steps. Figure 1

[0067] S1: obtaining a precursor containing at least one element of Mn, Ni, and Co;

[0068] S2: covering the surface of the precursor with a Na source to obtain a composite;

[0069] S3: firing the composite to obtain a Na-containing oxide having a P2-type structure.

[0070] In this case, the above-described S3 includes the following steps:

[0071] ​S3-1: The above composite is subjected to a preliminary firing at a temperature of 300°C or higher and less than 700°C for 2 hours or more and 10 hours or less;

[0072] S3-2: After the above preliminary firing, the above composite is subjected to a main firing at a temperature of 700°C or higher and 1100°C or lower for 30 minutes or more and 48 hours or less;

[0073] S3-3: After the above main firing, the above composite is rapidly cooled from a temperature Tl of 200°C or higher to a temperature T2 of 100°C or lower.

[0074] 2.1 S1

[0075] In S1, a precursor containing at least one of Mn, Ni, and Co is obtained. The precursor can contain at least Mn, and one or both of Ni and Co, or can contain at least Mn, Ni, and Co. The precursor can be a salt containing at least one of Mn, Ni, and Co. For example, the precursor can be at least one of a carbonate, a sulfate, a nitrate, and an acetate. Alternatively, the precursor can be a compound other than a salt. For example, the precursor can be a hydroxide. The precursor can also be a hydrate. The precursor can also be a combination of a plurality of compounds. The precursor can be in various shapes. For example, the precursor can be in a granular form, or can be a spherical particle as described later. The particle diameter of the particle composed of the precursor is not particularly limited.

[0076] In S1, a precipitate as the precursor can be obtained by a coprecipitation method using an ion source capable of forming a precipitate with a transition metal ion in an aqueous solution and a transition metal compound containing at least one of Mn, Ni, and Co. Thereby, it is easy to obtain a spherical particle as the precursor. The "ion source capable of forming a precipitate with a transition metal ion in an aqueous solution" can be, for example, at least one selected from the group consisting of sodium carbonate, sodium nitrate, and other sodium salts; sodium hydroxide; and sodium oxide, and the like. The transition metal compound can be the above salt or hydroxide, or the like, containing at least one of Mn, Ni, and Co. Specifically, in S1, after the ion source and the transition metal compound are each prepared as a solution, the respective solutions are dropped and mixed, thereby obtaining a precipitate as the precursor. At this time, as the solvent, for example, water is used. At this time, as the base, various sodium compounds can be used, and in order to adjust the basicity, an aqueous ammonia solution or the like can be added. In the case of the coprecipitation method, for example, an aqueous solution of the transition metal compound and an aqueous solution of sodium carbonate are prepared, and the respective aqueous solutions are dropped and mixed, thereby obtaining a precipitate as the precursor. Alternatively, the precursor can be obtained by a sol-gel method. In particular, by the coprecipitation method, it is easy to obtain a spherical particle as the precursor.

[0077] In S1, the precursor can contain an element M. The element M is at least one selected from the group consisting of 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, a function of stabilizing the P2-type structure. The method for obtaining the precursor containing the element M is not particularly limited. When the precursor is obtained in S1 by a coprecipitation method, 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 the element M are prepared, and the respective aqueous solutions are added dropwise, whereby a precursor containing at least one of Mn, Ni, and Co and the element M is obtained. Alternatively, in the manufacturing method of the present disclosure, the element M can also not be added in S1, but can be doped when Na-doped firing is performed in S2 and S3 described later.

[0078] 2.2 S2

[0079] In S2, the surface of the precursor obtained in S1 is covered with a Na source, whereby a composite is obtained. The Na source can be a salt containing Na such as a carbonate or a nitrate, or a compound other than a salt such as sodium oxide or sodium hydroxide. In S2, the amount of the Na source covering the surface of the precursor can be determined in consideration of the amount of Na to be lost at the time of firing later.

[0080] In S2, the coverage of the Na source on the surface of the precursor is not particularly limited. For example, in S2, the above-described composite can be obtained by covering the above-described precursor surface with the above-described Na source by 40% or more, 50% or more, 60% or more, or 70% or more in area. Here, in the case where the precursor obtained in S1 is a spherical particle and the composite obtained in S2 is obtained by covering the above-described precursor surface with the above-described Na source by 40% or more in area, in S3 described later, the Na-containing oxide having the P2-type structure is easily a spherical particle. When the coverage of the Na source is small, at the time of firing the composite, P2-type crystals are easily abnormally grown on the surface of the composite, and the Na-containing oxide is easily a plate-like shape. When the coverage of the Na source is large, at the time of firing the composite, the crystallite of the P2-type crystal is easily small, and the Na-containing oxide is easily a spherical particle corresponding to the shape of the precursor.

[0081] In S2, the method for covering the above-described precursor surface with the Na source is not particularly limited. As described above, when the above-described precursor surface is covered with the Na source by 40% or more in area, as the method, various methods can be given. For example, a flow reverse coating method or a spray drying method can be given. That is, a coating solution in which the Na source is dissolved is prepared, and drying is performed while or after the coating solution is brought into contact with the surface of the precursor. By adjusting the coating conditions (temperature, time, number of times, etc.), the surface of the precursor can be covered with the Na source by 40% or more in area.

[0082] In S2, the precursor obtained in S1, the Na source, and the 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 can be mixed to obtain a composite. The M source can be, for example, a salt such as a carbonate or a sulfate containing the element M, or a compound other than a salt such as an oxide or a hydroxide. The amount of the M source relative to the precursor can be determined based on the chemical composition of the Na-containing oxide after firing.

[0083] 2.3 S3

[0084] In S3, the above-described composite obtained in S2 is fired to obtain a Na-containing oxide having a P2-type structure. S3 includes the above-described S3-1, S3-2, and S3-3.

[0085] 2.3.1 S3-1

[0086] In S3-1, the above-described composite is subjected to a preliminary firing at a temperature of 300°C or higher and less than 700°C for 2 hours or more and 10 hours or less. In S3-1, the preliminary firing can be performed on the basis of the above-described composite being arbitrarily shaped. The preliminary firing is performed at a temperature lower than the main firing. If the preliminary firing in S3-1 is insufficient, the generation of the P2 phase can be insufficient in the Na-containing oxide finally obtained. In S3-1, the preliminary firing temperature is 300°C or higher and less than 700°C, and the preliminary firing time is 2 hours or more and 10 hours or less, whereby the composite can be subjected to sufficient preliminary firing, and the X-ray diffraction pattern of the Na-containing oxide having a P2-type structure obtained through the subsequent S3-2 and S3-3 is likely to satisfy the above-described I1 / I2 and I1 / I3. The preliminary firing temperature can 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, 550°C or higher and less than 700°C, or 550°C or higher and less than 650°C. In addition, the preliminary firing time can be 2 hours or more and 8 hours or less, 3 hours or more and 8 hours or less, 4 hours or more and 8 hours or less, 5 hours or more and 8 hours or less, or 5 hours or more and 7 hours or less. The preliminary firing atmosphere is not particularly limited, and can be, for example, an oxygen-containing atmosphere.

[0087] 2.3.2 S3-2

[0088] In S3-2, after the pre-baking described above, the composite is subjected to main baking at a temperature of 700°C or higher and 1100°C or lower for 30 minutes or longer and 10 hours or shorter. In S3-2, the main baking temperature of the composite is 700°C or higher and 1100°C or lower, and is preferably 800°C or higher and 1000°C or lower. If the main baking temperature is too low, the P2 phase cannot be generated, and if the main baking temperature is too high, the O3 phase or the like is easily generated instead of the P2 phase. The temperature rising conditions from the pre-baking temperature to the main baking temperature are not particularly limited. The main baking time is not particularly limited, and can be, for example, 30 minutes or longer and 48 hours or shorter. However, the shape of the Na-containing oxide can be controlled by the main baking time. As described above, in the method of the present disclosure, in the case where the coverage of the Na source in the composite is 40% or more, when the composite is baked, the P2-type crystal with small crystallites is easily formed on the surface thereof. In the method of the present disclosure, by growing the P2-type crystallites along the surface of the particles, one P2-type crystallite is connected to another P2-type crystallite, and thus the shape of the Na-containing oxide corresponds to the shape of the precursor. For example, when the precursor is a spherical particle, the Na-containing oxide can also be a spherical particle. If the main baking time is too short, the generation of the P2 phase becomes insufficient. On the other hand, if the main baking time is too long, the P2 phase excessively grows to become a plate-like particle instead of a spherical shape. It has been confirmed only by the present inventors that when the main baking time is 30 minutes or longer and 3 hours or shorter, the spherical particle of the Na-containing oxide is easily obtained. The Na-containing oxide obtained after the main baking can have a structure in which a plurality of crystallites exist on the surface and the crystallites are connected to each other.

[0089] 2.3.3 S3-3

[0090] In S3-3, after the above main firing, the above composite is rapidly cooled from a temperature T1 of 200°C or higher to a temperature T2 of 100°C or lower (cooling is performed at a rate of 20°C / min or higher). The above preliminary firing and main firing are performed, for example, in a heating furnace. In process S3-3, after the main firing of the composite is performed in the heating furnace, the composite is cooled to an arbitrary temperature T1 of 200°C or higher in the heating furnace, and after reaching the temperature T1, the fired product is taken out of the heating furnace, and rapid cooling to an arbitrary temperature T2 of 100°C or lower is performed outside the furnace. The temperature T1 can be an arbitrary temperature of 200°C or higher, or an arbitrary temperature of 250°C or higher. The temperature T2 can be an arbitrary temperature of 100°C or lower, or an arbitrary temperature of 50°C or lower, or a cooling end temperature. In a prescribed temperature range between the temperature T1 and the temperature T2, moisture easily invades the interlayer of the P2-type structure due to atomic vibration, molecular motion, and the like. When the composite after the main firing (Na-containing oxide having a P2-type structure) is cooled, it is considered that by setting the time in such a temperature range where moisture easily invades to a short time (i.e., rapid cooling), the amount of moisture invasion into the interlayer of the P2-type structure is reduced. In this regard, in process S3-3, when the composite after the main firing is cooled, by performing cooling from an arbitrary temperature T1 of 200°C or higher to an arbitrary temperature T2 of 100°C or lower in a dry atmosphere outside the furnace, the cooling rate between the temperature T1 and the temperature T2 becomes rapid (for example, 20°C / min or higher), and moisture is difficult to invade the interlayer of the P2-type structure, and collapse of the P2-type structure and the like can be suppressed. As a result, the Na-containing oxide having a P2-type structure obtained through S3-3 easily satisfies the above I1 / I2 and I1 / I3.

[0091] By the above method, a Na-containing oxide having a P2-type structure, having a prescribed chemical composition, and satisfying the above I1 / I2 and I1 / I3 and the like can be manufactured.

[0092] 3. Sodium-ion secondary battery

[0093] The positive electrode active material of the present embodiment contains the above specific Na-containing oxide. This positive electrode active material can be used, for example, as a positive electrode active material of a sodium-ion secondary battery. Figure 2 The structure of a sodium-ion secondary battery of one embodiment is schematically shown. As shown in Figure 2 The sodium-ion secondary battery 100 of one embodiment has a positive electrode active material layer 10, an electrolyte layer 20, and a negative electrode active material layer 30. Here, the positive electrode active material layer 10 contains the positive electrode active material of the above embodiment.

[0094] 3.1 Positive electrode active material layer

[0095] The positive electrode active material layer 10 contains at least the positive electrode active material of the above-described embodiments, and may optionally contain electrolyte, conductive additives, and binders. 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 additives, and binders in the positive electrode active material layer 10 can be appropriately determined according to the target battery performance. For example, when the total solid component of the positive electrode active material layer 10 is set to 100% by mass, the content of the positive electrode active material can be 40% or more by mass, 50% or more by mass, or 60% or more by mass, or less than 100% by mass or less than 90% by mass. The shape of the positive electrode active material layer 10 is not particularly limited; for example, it can be a sheet-like positive electrode active material layer 10 with a generally planar surface. The thickness of the positive electrode active material layer 10 is not particularly limited; for example, it can be 0.1 μm or more, or 1 μm or more, or less than 2 mm or less than 1 mm.

[0096] 3.1.1 Positive electrode active material

[0097] Regarding the positive electrode active material, as described above, the positive electrode active material contains a Na-containing oxide having a P2-type structure. As constituent elements, it contains at least one element selected from Mn, Ni, and Co, and Na and O, satisfying the I1 / I2 and I1 / I3 relationships. As described above, the positive electrode active material may consist solely of the aforementioned Na-containing oxide, or it may contain the aforementioned Na-containing oxide along with other positive electrode active materials (other positive electrode active materials). From the viewpoint of further improving the technical effect of this disclosure, the proportion of other positive electrode active materials in the overall positive electrode active material can be small. For example, with the overall positive electrode active material as 100% by mass, the content of the aforementioned Na-containing oxide can 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.

[0098] 3.1.2 Electrolytes

[0099] The electrolyte that can be included in the positive electrode active material layer 10 can be a solid electrolyte, a liquid electrolyte (electrolyte), or a combination thereof. A solid electrolyte known as a solid electrolyte used in sodium-ion secondary batteries can be used. The solid electrolyte can be an inorganic solid electrolyte or an organic polymer electrolyte. In particular, inorganic solid electrolytes have excellent ion conductivity and heat resistance. Examples of inorganic solid electrolytes include Na3Zr2PSi2O. 12oxides such as Na2O-11Al2O3; NaBH4, NaB 10 H 10 , NaCB9H 10 , NaCB 11 H 12 , NaB 12 Cl 12 hydrogen compounds or boron compounds; Na3PS4, Na3SbS4, Na 2.88 Sb 0.88 W 0.12 S4sulfides; NaPF6, NaBF4fluorides, and the like. The solid electrolyte may, for example, be in a particulate form. The solid electrolyte can be used alone with only one kind or in combination with two or more kinds. The electrolytic solution may, for example, contain sodium ions as carrier ions (carriers). The electrolytic solution can be an aqueous electrolytic solution or a non-aqueous electrolytic solution. The composition of the electrolytic solution can be the same as a publicly known composition as the composition of the electrolytic solution of a sodium-ion secondary battery. For example, as the electrolytic solution, an electrolytic solution in which a sodium salt is dissolved in a carbonate-based solvent at a prescribed concentration can be used. As the carbonate-based solvent, for example, fluorine ethylene carbonate (FEC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and the like can be cited. As the sodium salt, for example, NaPF6and the like can be cited.

[0100] 3.1.3 Conductive aid

[0101] As the conductive aid that can be contained in the positive electrode active material layer 10, for example, carbon materials such as vapor-grown carbon fiber (VGCF), acetylene black (AB), ketjen black (KB), carbon nanotube (CNT), or carbon nanofiber (CNF); and metal materials such as nickel, aluminum, and stainless steel can be cited. The conductive aid may, for example, be in a particulate form or a fibrous form, and the size thereof is not particularly limited. The conductive aid can be used alone with only one kind or in combination with two or more kinds.

[0102] 3.1.4 Binder

[0103] As the binder that can be contained in the positive electrode active material layer 10, for example, butadiene rubber (BR)-based binders, butyl rubber (IIR)-based binders, acrylate butadiene rubber (ABR)-based binders, styrene butadiene rubber (SBR)-based binders, polyvinylidene fluoride (PVdF)-based binders, polytetrafluoroethylene (PTFE)-based binders, polyimide (PI)-based binders, and the like can be cited. The binder can be used alone with only one kind or in combination with two or more kinds.

[0104] 3.2 Electrolyte layer

[0105] The electrolyte layer 20 contains at least an electrolyte. In the case where the sodium-ion secondary battery 100 is a solid battery (a battery containing a solid electrolyte, and a part of which can use a liquid electrolyte, or a full solid battery which does not contain a liquid electrolyte), the electrolyte layer 20 contains a solid electrolyte, and can optionally contain a binder and the like. In this case, the content of the solid electrolyte and the binder and the like in the electrolyte layer 20 is not particularly limited. On the other hand, in the case where the sodium-ion secondary battery 100 is an electrolytic solution battery, the electrolyte layer 20 contains an electrolytic solution, and can have a separator and the like for preventing the positive electrode active material layer 10 from contacting the negative electrode active material layer 30 while the electrolytic solution is held. The thickness of the electrolyte layer 20 is not particularly limited, and can be, for example, 0.1 μm or more or 1 μm or more, and can be 2 mm or less or 1 mm or less.

[0106] As the electrolyte contained in the electrolyte layer 20, an electrolyte can be appropriately selected from among the electrolytes exemplified as the electrolyte which can be contained in the positive electrode active material layer 10 described above. In addition, as the binder which can be contained in the electrolyte layer 20, a binder can be appropriately selected from among the binders exemplified as the binder which can be contained in the positive electrode active material layer 10 described above. The electrolyte or the binder can be used alone only one kind, or two or more kinds can be used in combination. The separator can be any separator generally used in a sodium-ion secondary battery, and examples thereof include a separator composed of a resin such as polyethylene (PE), polypropylene (PP), polyester, and polyamide, and the like. The separator can be a single layer structure, or a multilayer structure. As the separator of the multilayer structure, for example, a separator of a double layer structure of PE / PP, or a separator of a triple layer structure of PP / PE / PP or PE / PP / PE, and the like can be exemplified. The separator can also be composed of a nonwoven fabric such as a cellulose nonwoven fabric, a resin nonwoven fabric, a glass fiber nonwoven fabric, and the like.

[0107] 3.3 Negative electrode active material layer

[0108] The negative electrode active material layer 30 contains at least a negative electrode active material, and can optionally contain an electrolyte, a conductive aid, a binder, and the like. In addition, the negative electrode active material layer 30 can contain various other additives. The content of each of the negative electrode active material, the electrolyte, the conductive aid, and the binder and the like in the negative electrode active material layer 30 can be appropriately determined depending on the target battery performance. For example, the content of the negative electrode active material can be 40% by mass or more, 50% by mass or more, or 60% by mass or more, and can be 100% by mass or less or 90% by mass or less, based on the entire negative electrode active material layer 30 (the entire solid content). The shape of the negative electrode active material layer 30 is not particularly limited, and for example, the negative electrode active material layer can be a sheet-like negative electrode active material layer having a substantially planar shape. The thickness of the negative electrode active material layer 30 is not particularly limited, and can be, for example, 0.1 μm or more or 1 μm or more, and can be 2 mm or less or 1 mm or less.

[0109] As the negative active material, various substances having a low potential (charge-discharge potential) compared with the above-described positive active material of the present disclosure can be used. The negative active material can be, for example, an inorganic negative active material such as metallic sodium, or a negative active material containing an organic compound, or a combination thereof. The negative active material can be used alone as only one kind, or can be used in combination as two or more kinds. The shape of the negative active material can be a general shape as a negative active material for a battery. For example, the negative active material can be in the form of particles. The negative active material particles can be primary particles, or secondary particles in which a plurality of primary particles are aggregated. The average particle diameter (D50) of the negative active material particles can be, for example, 1 nm or more, 5 nm or more, or 10 nm or more, and can be 500 pm or less, 100 pm or less, 50 pm or less, or 30 pm or less. Alternatively, the negative active material can be in the form of a sheet such as a sodium foil (sheet, foil, film). That is, the negative active material layer 30 can be composed of a sheet of the negative active material.

[0110] As the electrolyte that can be contained in the negative active material layer 30, the above-described solid electrolyte, electrolytic solution, or a combination thereof can be given. As the conductive aid that can be contained in the negative active material layer 30, the above-described carbon material or the above-described metal material can be given. The binder that can be contained in the negative active material layer 30 can be appropriately selected from the binders exemplified as the above-described binder that can be contained in the positive active material layer 10. The electrolyte or the binder can be used alone as only one kind, or can be used in combination as two or more kinds.

[0111] 3.4 Positive electrode current collector

[0112] As Figure 2As shown, the sodium-ion secondary battery 100 may include a positive electrode current collector 40 in contact with the aforementioned positive electrode active material layer 10. The positive electrode current collector 40 can be any type of positive electrode current collector commonly used in batteries. Furthermore, the positive electrode current collector 40 can be in the form of a foil, plate, mesh, perforated metal, or foam. The positive electrode current collector 40 may also be composed of metal foil or metal mesh. Metal foil, in particular, offers excellent operability. The positive electrode current collector 40 may also be composed 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. To adjust resistance, the positive electrode current collector 40 may have a coating on its surface. Alternatively, the positive electrode current collector 40 may be a current collector formed by depositing or vapor-depositing the aforementioned metal onto a metal foil or substrate. Alternatively, when the positive current collector 40 is composed of multiple metal foils, a layer may be formed between the multiple metal foils. There is no particular limitation on the thickness of the positive current collector 40. For example, it can be 0.1 μm or more, or 1 μm or more, or it can be less than 1 mm or less than 100 μm.

[0113] 3.5 negative current collector

[0114] like Figure 3 As shown, the sodium-ion secondary battery 100 may include a negative electrode current collector 50 in contact with the aforementioned negative electrode active material layer 30. The negative electrode current collector 50 can be any type of negative electrode current collector commonly used in batteries. Furthermore, the negative electrode current collector 50 can be in the form of foil, plate, mesh, perforated metal, or foam. The negative electrode current collector 50 can be a metal foil or metal mesh, or it can be a carbon sheet. Metal foil, in particular, offers excellent operability. The negative electrode current collector 50 may also be composed 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. Especially 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. To adjust resistance, the negative electrode current collector 50 may have a coating on its surface. Alternatively, the negative current collector 50 can also be a current collector formed by depositing or vapor-depositing the aforementioned metal onto a metal foil or substrate. Furthermore, when the negative current collector 50 is composed of multiple metal foils, a layer may be formed between these multiple metal foils. There is no particular limitation on the thickness of the negative current collector 50. For example, it can be 0.1 μm or more, or 1 μm or more, or it can be 1 mm or less, or 100 μm or less.

[0115] 3.6 Other matters

[0116] The sodium-ion secondary battery 100 can have, in addition to the above-described structure, a tab or a terminal, and the like as a structure apparent to a secondary battery. The sodium-ion secondary battery 100 can house the above-described structures inside an outer packaging body. The outer packaging body can employ any outer packaging body known as an outer packaging body of a battery. In addition, a plurality of batteries 100 can be electrically connected as desired, and can be overlapped and made into a battery pack as desired. In this case, the battery pack can be housed inside a battery case known as such. In addition to this, the sodium-ion secondary battery 100 can have a necessary terminal and the like as a structure apparent to a secondary battery. As the shape of the sodium-ion secondary battery 100, for example, a coin type, a laminate type, a cylindrical type, a square type, and the like can be given.

[0117] The sodium-ion secondary battery 100 can be manufactured using a known method, in addition to the above-described specific positive electrode active material. 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 can be formed by dry molding or the like, for example.

[0118] (1) The positive electrode active material and the like that constitute the positive electrode active material layer are dispersed in a solvent to obtain a slurry for a positive electrode layer. As the solvent used at this time, there is no particular limitation, and water or various organic solvents can be used. The slurry for a positive electrode layer is applied to the surface of the positive electrode current collector using a doctor blade or the like, and then dried, whereby the positive electrode active material layer is formed on the surface of the positive electrode current collector, and a positive electrode is manufactured.

[0119] (2) The negative electrode active material and the like that constitute the negative electrode active material layer are dispersed in a solvent to obtain a slurry for a negative electrode layer. As the solvent used at this time, there is no particular limitation, and water or various organic solvents can be used. The slurry for a negative electrode layer is applied to the surface of the negative electrode current collector using a doctor blade or the like, and then dried, whereby the negative electrode active material layer is formed on the surface of the negative electrode current collector, and a negative electrode is manufactured.

[0120] (3) The layers are stacked in a manner that the electrolyte layer (solid electrolyte layer or separator) is interposed between the negative electrode and the positive electrode, to obtain a laminate having a negative electrode current collector, a negative electrode active material layer, an electrolyte layer, a positive electrode active material layer, and a positive electrode current collector in this order. Other components such as a terminal are attached to the laminate as necessary.

[0121] (4) The laminate is housed in a battery case, and in the case of an electrolytic solution battery, an electrolytic solution is filled in the battery case so that the laminate is immersed in the electrolytic solution, and the laminate is sealed in the battery case, whereby a secondary battery is manufactured. Note that in the case of an electrolytic solution battery, the negative electrode active material layer, the separator, and the positive electrode active material layer can contain an electrolytic solution at the stage of the above-described (3).

[0122] 4. Method for increasing the reversible capacity of a sodium-ion secondary battery

[0123] The technology of the present disclosure also has an aspect as a method of increasing the reversible capacity of a sodium-ion secondary battery. That is, the method of the present disclosure for increasing the reversible capacity of a sodium-ion secondary battery is characterized by using the above-described positive electrode active material of the present disclosure in a positive electrode active material layer of a sodium-ion secondary battery.

[0124] 5. Vehicle having a sodium-ion secondary battery

[0125] As described above, the positive electrode active material of the present disclosure is excellent in reversible capacity and is suitable as a positive electrode active material of a sodium-ion secondary battery. Thus, a sodium-ion secondary battery having a large reversible capacity can be applied, for example, to at least one vehicle selected from a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), and an electric vehicle (BEV). That is, one aspect of the technology of the present disclosure is a vehicle having a sodium-ion secondary battery having a positive electrode active material layer, an electrolyte layer, and a negative electrode active material layer, the positive electrode active material layer containing the above-described positive electrode active material of the present disclosure.

[0126] Embodiment

[0127] As described above, an embodiment of the positive electrode active material and the like, such as a sodium-ion secondary battery, is described, but various modifications can be made in addition to the above-described embodiment within the scope of the gist thereof. Hereinafter, an embodiment is shown, and the technology of the present disclosure is described in more detail, but the technology of the present disclosure is not limited to the following embodiment.

[0128] 1. Production of positive electrode active material

[0129] 1.1 Embodiment 1

[0130] 1.1.1 Production of precursor

[0131] (1) MnSO4·5H2O, NiSO4·6H2O, and CoSO4·7H2O were weighed so as to have a target composition ratio, dissolved in distilled water so as to have a concentration of 1.2 mol / L, and a first solution was obtained. In addition, Na2CO3 was dissolved in distilled water so as to have a concentration of 1.2 mol / L in another container, and a second solution was obtained.

[0132] (2) 1000 mL of pure water was added to a reaction container (with a baffle), and 500 mL of the first solution and 500 mL of the second solution were separately added thereto at a rate of about 4 mL / min.

[0133] (3) After the addition was completed, stirring was performed at a stirring rate of 150 rpm at room temperature for 1 hour, and a product was obtained.

[0134] (4) The product was washed with pure water, and solid-liquid separation was performed with a centrifugal separator, and a precipitate was recovered.

[0135] (5) The obtained precipitate was dried at 120°C overnight, and after pulverization in a mortar, fine particles were removed by air classification to obtain precursor particles. The precursor particles were a composite salt containing Mn, Ni, and Co, and were spherical particles having a circularity of 0.86.

[0136] 1.1.2 Production of composite

[0137] (1) Na2CO3 and distilled water were weighed so as to be 1150 g / L, and then stirred using a stirrer until completely dissolved, whereby an aqueous Na2CO3 solution was produced.

[0138] (2) The above-mentioned aqueous Na2CO3 solution and the above-mentioned precursor particles were weighed and mixed in such a manner that the composition after firing described later is Na2CO3 : Mn : Ni : Co = 0.5 : 0.5 : 0.5, whereby a slurry was obtained. 0.7 Mn 0.5 Ni 0.2 Co 0.3 O2. The slurry was obtained.

[0139] (3) The above-mentioned slurry was air-dried by spray drying to obtain a composite. Specifically, using a spray drying device DL410, the above-mentioned slurry was air-dried under conditions in which the slurry delivery rate was 30 mL / min, the inlet temperature was 200°C, the circulating air volume was 0.8 m 3 / minute, and the spray pressure was 0.3 MPa, the surface of the precursor particles was covered with Na2CO3, and a composite was obtained. In this composite, 77 area% of the surface of the precursor particles was covered with Na2CO3.

[0140] 1.1.3 Firing of composite

[0141] The composite was placed in an alumina crucible, and firing was performed under an atmospheric air atmosphere to obtain a Na-containing oxide having a P2-type structure. The firing conditions are described below in (1) to (7).

[0142] (1) An alumina crucible containing the above-mentioned composite was placed in a heating furnace under an atmospheric air atmosphere.

[0143] (2) The inside of the heating furnace was raised from room temperature (25°C) to 600°C over 115 minutes.

[0144] (3) The inside of the heating furnace was maintained at 600°C for 360 minutes to perform pre-firing.

[0145] (4) After pre-firing, the inside of the heating furnace was raised from 600°C to 950°C over 140 minutes.

[0146] (5) The inside of the heating furnace was maintained at 950°C for 60 minutes to perform main firing.

[0147] (6) After the main firing, the temperature in the furnace was decreased from 950°C to 250°C in 140 minutes.

[0148] (7) The alumina crucible was taken out of the furnace at 250°C and left to cool in a dry atmosphere outside the furnace, reaching 25°C in 10 minutes.

[0149] The fired product after cooling was pulverized using a mortar in a dry atmosphere, thereby obtaining Na-containing oxide particles having a P2-type structure.

[0150] 1.2 Example 2

[0151] The same operation as in Example 1 was performed to obtain a composite. The obtained composite was put in an alumina crucible and fired in an atmospheric atmosphere, and the fired product after cooling was pulverized in a dry atmosphere to obtain Na-containing oxide having a P2-type structure. The firing conditions are described below in (1) to (7).

[0152] (1) An alumina crucible containing the above composite was set in a heating furnace in an atmospheric atmosphere.

[0153] (2) The temperature in the furnace was increased from room temperature (25°C) to 600°C in 115 minutes.

[0154] (3) The temperature in the furnace was maintained at 600°C for 360 minutes to perform pre-firing.

[0155] (4) After the pre-firing, the temperature in the furnace was increased from 600°C to 925°C in 130 minutes.

[0156] (5) The temperature in the furnace was maintained at 925°C for 60 minutes to perform main firing.

[0157] (6) After the main firing, the temperature in the furnace was decreased from 925°C to 250°C in 135 minutes.

[0158] (7) The alumina crucible was taken out of the furnace at 250°C and left to cool in a dry atmosphere outside the furnace, reaching 25°C in 10 minutes.

[0159] 1.3 Example 3

[0160] The same operation as in Example 1 was performed to obtain a composite. The obtained composite was put in an alumina crucible and fired in an atmospheric atmosphere, and the fired product after cooling was pulverized in a dry atmosphere to obtain Na-containing oxide having a P2-type structure. The firing conditions are described below in (1) to (7).

[0161] (1) An alumina crucible containing the above composite was set in a heating furnace in an atmospheric atmosphere.

[0162] (2) The temperature in the furnace was increased from room temperature (25°C) to 600°C in 115 minutes.

[0163] (3) The heating furnace was maintained at 600°C for 360 minutes to perform pre-firing.

[0164] (4) After the pre-firing, the heating furnace was heated from 600°C to 900°C in 120 minutes.

[0165] (5) The heating furnace was maintained at 900°C for 60 minutes to perform main firing.

[0166] (6) After the main firing, the heating furnace was cooled from 900°C to 250°C in 130 minutes.

[0167] (7) The alumina crucible was taken out of the heating furnace at 250°C and left to cool in a dry atmosphere outside the furnace, reaching 25°C in 10 minutes.

[0168] 1.4 Comparative Example 1

[0169] The precursor particles were obtained by the same operation as in Example 1. The precursor particles and Na2CO3 were weighed so as to become a composition of P2-type Na 0.7 Mn 0.5 Ni 0.2 Co 0.3 O2. The weighed precursor and Na2CO3 were mixed by using a mortar to obtain a composite. In the composite, 24 area% of the surface of the precursor particles was covered with Na2CO3. The obtained composite was put into an alumina crucible and fired under an atmospheric atmosphere, and the fired product after being left to cool was pulverized in a dry atmosphere to obtain a Na-containing oxide having a P2-type structure. The firing conditions are described below in (1) to (7).

[0170] (1) An alumina crucible containing the above composite was set in a heating furnace under an atmospheric atmosphere.

[0171] (2) The heating furnace was heated from room temperature (25°C) to 600°C in 115 minutes.

[0172] (3) The heating furnace was maintained at 600°C for 360 minutes to perform pre-firing.

[0173] (4) After the pre-firing, the heating furnace was heated from 600°C to 900°C in 120 minutes.

[0174] (5) The heating furnace was maintained at 900°C for 720 minutes to perform main firing.

[0175] (6) After the main firing, the heating furnace was cooled from 900°C to 250°C in 130 minutes.

[0176] (7) The alumina crucible was taken out of the heating furnace at 250°C and left to cool in a dry atmosphere outside the furnace, reaching 25°C in 10 minutes.

[0177] 1.5 Example 4

[0178] 1.5.1 Preparation of precursor

[0179] (1) MnSO4-5H2O and FeSO4-7H2O were weighed so as to have a target composition ratio, dissolved in distilled water so as to have a concentration of 1.2 mol / L, and a first solution was obtained. Separately, Na2CO3 was dissolved in distilled water so as to have a concentration of 1.2 mol / L, and a second solution was obtained.

[0180] (2) 1000 mL of pure water was added to a reaction vessel (with a baffle), and 500 mL of the first solution and 500 mL of the second solution were separately added dropwise thereto at a rate of about 4 mL / min.

[0181] (3) After the dropwise addition was completed, stirring was performed at a stirring rate of 150 rpm at room temperature for 1 hour, and a product was obtained.

[0182] (4) The product was washed with pure water, and solid-liquid separation was performed using a centrifugal separator, and a precipitate was recovered.

[0183] (5) The obtained precipitate was dried at 120°C overnight, pulverized in a mortar, and particulates were removed by air classification, and a precursor particle was obtained. The precursor particle was a composite salt containing Mn and Fe, and was a spherical particle having a circularity of 0.86.

[0184] 1.5.2 Preparation of composite

[0185] (1) Na2CO3 and distilled water were weighed so as to have a concentration of 1150 g / L, and a stirrer was used to stir until complete dissolution, and thus an aqueous Na2CO3 solution was prepared.

[0186] (2) The above-described aqueous Na2CO3 solution and the above-described precursor particle were weighed and mixed in such a manner that the composition after firing described later was Na 0.7 Mn 0.5 Fe 0.5 O2, and thus a slurry was obtained.

[0187] (3) The above-described slurry was air-dried by spray drying, and a composite was obtained. Specifically, using a spray drying device DL410, the above-described slurry was air-dried under conditions in which the slurry delivery rate was 30 mL / min, the inlet temperature was 200°C, the circulating air volume was 0.8 m 3 / min, and the spray pressure was 0.3 MPa, and the surface of the precursor particle was covered with Na2CO3, and thus a composite was obtained. In this composite, 77 area% of the surface of the precursor particle was covered with Na2CO3.

[0188] 1.5.3 Firing of the composite

[0189] The composite was put into an alumina crucible and fired under an atmospheric atmosphere. The fired product after being left to cool was pulverized in a dry atmosphere to obtain a Na-containing oxide having a P2-type structure. The firing conditions were the same as the conditions (1) to (7) of Example 3. That is, in-furnace temperature increase (temperature increase from 25°C to 600°C in 115 minutes), in-furnace preliminary firing (holding at 600°C for 360 minutes), in-furnace temperature increase (temperature increase from 600°C to 900°C in 120 minutes), in-furnace main firing (holding at 900°C for 60 minutes), in-furnace cooling (temperature decrease from 900°C to 250°C in 130 minutes), and out-of-furnace temperature decrease (temperature decrease from 250°C to 25°C in 10 minutes) were performed to obtain a Na-containing oxide having a P2-type structure.

[0190] 1.6 Comparative Example 2

[0191] The precursor particles were obtained by the same operation as in Example 4. The precursor particles and Na2CO3 were weighed so as to make the composition of the precursor particles and Na2CO3 be Na 0.7 Mn 0.5 Fe 0.5 O2. The weighed precursor and Na2CO3 were mixed by using a mortar to obtain a composite. In the composite, 26 area% of the surface of the precursor particles was covered with Na2CO3. The obtained composite was subjected to in-furnace temperature increase (temperature increase from 25°C to 600°C in 115 minutes), in-furnace preliminary firing (holding at 600°C for 360 minutes), in-furnace temperature increase (temperature increase from 600°C to 900°C in 120 minutes), in-furnace main firing (holding at 900°C for 720 minutes), in-furnace cooling (temperature decrease from 900°C to 250°C in 130 minutes), and out-of-furnace temperature decrease (temperature decrease from 250°C to 25°C in 10 minutes) under the same conditions (1) to (7) as in Comparative Example 1. The fired product after being left to cool was pulverized in a dry atmosphere to obtain a Na-containing oxide having a P2-type structure.

[0192] 2. Evaluation of the positive electrode active material

[0193] 2.1 Elemental analysis

[0194] The elemental analysis was performed on each of the positive electrode active materials of Examples 1 to 4 and Comparative Examples 1 to 2 to determine the chemical composition. The results are shown in Table 1 below.

[0195] 2.2 Determination of crystal structure by X-ray diffraction measurement

[0196] X-ray diffraction measurement was performed on each of the positive electrode active materials of Examples 1 to 4 and Comparative Examples 1 to 2 using CuKα as the radiation source to obtain X-ray diffraction patterns. Figure 4 The X-ray diffraction patterns of each of Examples 1 to 3 and Comparative Example 1 are shown. In addition, Figure 3The respective X-ray diffraction patterns of Example 3 and Comparative Example 2 are shown. As shown in Figure 4 and Figure 3 It is understood that any of the positive electrode active materials of Examples 1 to 4 and Comparative Examples 1 to 2 has a P2-type structure belonging to the space group P63mc. In addition, as shown in Figure 4 It is understood that the positive electrode active materials of Examples 1 to 3 have a reduced X-ray diffraction peak intensity from the (102) plane compared to the positive electrode active material of Comparative Example 1, and have introduced stacking defects in the (102) plane. In addition, as shown in ​ It is understood that the positive electrode active material of Example 4 has a reduced X-ray diffraction peak intensity from the (102) plane compared to the positive electrode active material of Comparative Example 2, and has introduced stacking defects in the (102) plane. In the following Table 1, the ratio I1 / I2 of the X-ray diffraction peak intensity I1 from the (102) plane of the P2-type structure to the X-ray diffraction peak intensity I2 from the (002) plane of the P2-type structure, the ratio I1 / I3 of the X-ray diffraction peak intensity I1 to the X-ray diffraction peak intensity I3 from the (100) plane of the P2-type structure, and the half-width HW1 of the X-ray diffraction peak from the (102) plane of each of the positive electrode active materials of Examples 1 to 4 and Comparative Examples 1 to 2 are shown.

[0197] 3. Production of evaluation battery cell

[0198] A coin-type battery cell was produced using each of the positive electrode active materials of Examples 1 to 4 and Comparative Examples 1 to 2. The production sequence of the coin-type battery cell is described below.

[0199] (1) The positive electrode active material, acetylene black (AB) as a conductive aid, and polyvinylidene fluoride (PVdF) as a binder were weighed so as to have a mass ratio of positive electrode active material: AB: PVdF = 85: 10: 5, and they were mixed and dispersed in N-methyl-2-pyrrolidone to obtain a positive electrode composite slurry. The positive electrode composite slurry was coated on an aluminum foil, and vacuum-dried at 120°C for one night, whereby a laminate of a positive electrode active material layer and a positive electrode current collector, i.e., a positive electrode, was obtained.

[0200] (2) As an electrolyte, an electrolyte in which NaPF6 was dissolved at a concentration of 1M in a solvent in which EC and DEC were mixed at a volume ratio of 1:1 was used.

[0201] (3) As a negative electrode, a metal sodium foil was prepared.

[0202] (4) Using the positive electrode, the electrolyte, and the negative electrode, a coin-type battery cell (CR2032) was produced.

[0203] 4. Charge-discharge characteristic evaluation

[0204] (1) The coin-type battery cell of each of Examples 1 to 3 and Comparative Example 1 was charged and discharged at 0.1 C (1 C = 160 mA / g) in a voltage range of 1.0 to 4.5 V while being kept in a thermostat maintained at 25°C, and the reversible capacity was measured. The results are shown in Table 1 below.

[0205] (2) The coin-type battery cell of each of Example 4 and Comparative Example 2 was charged and discharged at 0.1 C (1 C = 190 mA / g) in a voltage range of 1.0 to 4.3 V while being kept in a thermostat maintained at 25°C, and the reversible capacity was measured. The results are shown in Table 1 below.

[0206] 5. Evaluation results

[0207] For each of Examples 1 to 4 and Comparative Examples 1 to 2, I1 / I2, I1 / I3, and half-peak width HW1 determined from the X-ray diffraction pattern, and the reversible capacity of the battery were shown.

[0208] [Table 1]

[0209] (Table 1)

[0210]

[0211] As is apparent from the results shown in Table 1, the positive electrode active material of Examples 1 to 3, which had relatively small I1 / I2 and I1 / I3 and relatively many stacking defects of the (102) plane of the P2-type structure, had a larger reversible capacity than the positive electrode active material of Comparative Example 1, which had relatively large I1 / I2 and I1 / I3 and relatively few stacking defects of the (102) plane of the P2-type structure. The same was true for Example 4 and Comparative Example 2, in which the transition metal species was different from that of Examples 1 to 3 and Comparative Example 1.

[0212] 6. Supplement

[0213] Note that, in the above-described examples, a case in which the precursor is obtained by the coprecipitation method is exemplified, but the precursor can be obtained by a method other than this. Also, in the above-described examples, a case in which the composite is obtained by coating the surface of the precursor with a Na source by spray drying is exemplified, but the composite can be obtained by a method other than this. Also, in the above-described examples, a Na-containing oxide having a P2-type structure is exemplified as the Na-containing oxide having a prescribed chemical composition, but the chemical composition of the Na-containing oxide is not limited to this. Also, the Na-containing oxide can be doped with an element M other than Mn, Ni, and Co. As for the element M, as described in the embodiments.

[0214] 7. Summary

[0215] As described above, in the positive electrode active material containing the Na-containing oxide, the reversible capacity of the positive electrode active material can be said to increase in a case where the Na-containing oxide satisfies the following requirements (1) to (4).

[0216] (1) The Na-containing oxide has a P2-type structure.

[0217] (2) The Na-containing oxide contains at least one kind of element among Mn, Ni, and Co; Na; and O as constituent elements.

[0218] (3) The X-ray diffraction pattern of the Na-containing oxide satisfies I1 / I2≤0.10.

[0219] (4) The X-ray diffraction pattern of the Na-containing oxide satisfies I1 / I3≤0.70.

[0220] wherein the I1 is an X-ray diffraction peak intensity from a (102) plane of the P2-type structure, the I2 is an X-ray diffraction peak intensity from a (002) plane of the P2-type structure, and the I3 is an X-ray diffraction peak intensity from a (100) plane of the P2-type structure.

[0221] Explanation of Reference Signs

[0222] 100 sodium-ion secondary battery

[0223] 10 positive electrode active material layer

[0224] 20 electrolyte layer

[0225] 30 negative electrode active material layer

[0226] 40 positive electrode current collector

[0227] 50 negative electrode current collector

Claims

1. The positive electrode active material contains sodium oxide. The Na-containing oxide has a P2 type structure. The Na-containing oxide contains at least one element selected from Mn, Ni, and Co; Na and O are constitutive elements. The X-ray diffraction pattern containing Na oxide satisfies I1 / I2≤0.

10. The X-ray diffraction pattern containing Na oxide satisfies I1 / I3≤0.

70. I1 refers to the X-ray diffraction peak intensity from the (102) plane of the P2-type structure. I2 is the X-ray diffraction peak intensity from the (002) plane of the P2-type structure. The I3 is the X-ray diffraction peak intensity from the (100) plane of the P2 type structure.

2. The positive electrode active material according to claim 1, wherein, The X-ray diffraction pattern containing Na oxide satisfies 0.01≤I1 / I2≤0.10 and the X-ray diffraction pattern containing Na oxide satisfies 0.01≤I1 / I3≤0.

70.

3. The positive electrode active material according to claim 1 or 2, wherein, The X-ray diffraction pattern containing Na oxide satisfies 0.40°≤HW1, where HW1 is the half-width of the X-ray diffraction peak from the (102) plane of the P2 type structure.

4. The positive electrode active material according to claim 3, wherein, The X-ray diffraction pattern containing Na oxide satisfies 0.40°≤HW1≤1.00°.

5. The positive electrode active material according to any one of claims 1 to 4, wherein, The Na-containing oxide has Na a Mn x-p Ni y-q Co z-r M p+q+r O2 represents the chemical composition. Wherein, 0 < a ≤ 1.00, x + y + z = 1, and 0 ≤ p + q + r < 0.17, and element M is selected from at least one of B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo and W.

6. The positive electrode active material according to any one of claims 1 to 5, wherein, The Na-containing oxide contains at least Na, Mn, Ni, Co, and O as constituent elements.

7. The positive electrode active material according to any one of claims 1 to 4, wherein, The Na-containing oxide contains at least Na, Mn, Fe and O as constituent elements.

8. A sodium-ion secondary battery, which has 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 positive electrode active material as described in any one of claims 1 to 7.

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