Positive electrode active material for sodium secondary battery

Abstract

To provide a positive electrode active material for a sodium ion secondary battery which exhibits a high capacity even when a material which does not contain sodium such as a hard carbon is used as a negative electrode active material.SOLUTION: In a formula (1): Nap1(Mnx1 Niy1M11-x1-y1)O2-Na1+p2(Mnx2M21-x2)1-P2O2 (1) [where M1 represents at least one kind selected from a group consisting of V, Al, Ti, Mg and Fe. M2 represents at least one kind selected from a group consisting of V, Ti, Sn, Nb, and Zr. p1 represents 0.50 to 0.70. x1 represents 0.50 to 0.75. y represents 0.15 to 0.45. p2 represents 0.15 to 0.55. x2 represents 0.80 to 1.00.] A positive electrode active material for a low-crystalline sodium ion secondary battery comprises a composite metal oxide (1) having a composition represented by the formula (1).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a positive electrode active material for a sodium secondary battery and a method for producing the same. [Background technology]

[0002] Lithium-ion secondary batteries have a high energy density and are therefore being used in practical applications as small power sources for devices such as mobile phones and laptops, as well as large power sources for devices such as electric vehicles, and demand for these batteries is expected to continue to grow.

[0003] In lithium-ion secondary batteries, lithium is used as a charge carrier, and lithium and cobalt are components of lithium cobalt oxide (LiCoO2), which is commonly used as a positive electrode material. Lithium and cobalt are rare metals, and their resources are concentrated in South America, China, etc., making the raw material expensive and raising concerns about the stable supply of raw materials.

[0004] To solve this problem, sodium ion secondary batteries are being studied as next-generation secondary batteries that can reduce the amounts of rare metals such as lithium and cobalt used (see, for example, Patent Document 1). Sodium, the charge carrier in sodium secondary batteries, is an abundant and inexpensive material, and so in recent years, there have been expectations for the practical application of sodium ion secondary batteries as secondary batteries with fewer resource constraints.

[0005] The positive electrode active material used in sodium ion secondary batteries is required to have a high operating potential. 2 / 3 (Mn 2 / 3 Ni 1 / 3 )O2 etc. are considered promising.

[0006] However, Na is used as the positive electrode active material. 2 / 3 (Mn 2 / 3 Ni 1 / 3Sodium-ion secondary batteries using O2 are prone to deterioration due to a transition from a P2-type crystal structure to an O2-type crystal structure caused by misalignment of the transition metal oxide layers during the Na insertion / extraction process, and an irreversible phase transition associated with the release of O2 at the end of charging, and have presented issues with their charge / discharge cycle characteristics (durability).

[0007] So, Na 2 / 3 (Mn 2 / 3 Ni 1 / 3 It has been found that when synthesizing )O2, if low crystallinity is achieved by performing a mechanical milling process, a distorted layer structure can be formed, which suppresses the above-mentioned phase transition and improves durability (see, for example, Non-Patent Document 1). [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-225525 [Non-patent literature]

[0009] [Non-Patent Document 1] Batteries and Supercaps, 6(2023)e202200462 Summary of the Invention [Problem to be solved by the invention]

[0010] However, Na as a positive electrode active material 2 / 3 (Mn 2 / 3 Ni 1 / 3When using 02, the number of moles of sodium that can be included is structurally possible to insert up to 1 mole, but due to chemical stability, the Na content is limited to about 0.70 moles when produced by a synthesis process such as solid-state sintering. When a material containing sodium is used as the negative electrode active material, a large amount of sodium can be supplied during charging and discharging even in such cases, and a high-capacity sodium-ion secondary battery can be obtained. However, when a material that does not contain sodium, such as hard carbon, is used as the negative electrode active material, the capacity that can be used for charging and discharging decreases, and the amount of Na 2 / 3 (Mn 2 / 3 Ni 1 / 3 ) Due to the difference in the first coulombic efficiency between O2 and hard carbon, Na 2 / 3 (Mn 2 / 3 Ni 1 / 3 ) The initial coulombic efficiency of O2 cannot be effectively utilized.

[0011] The present invention has been made in consideration of the current state of the prior art described above, and aims to provide a positive electrode active material for a sodium ion secondary battery that exhibits high capacity, is excellent in durability, and can effectively utilize the initial coulombic efficiency. [Means for solving the problem]

[0012] The present inventors have conducted extensive research to achieve the above-mentioned object. 2 / 3 (Mn 2 / 3 Ni 1 / 3 It has been discovered that by combining a sodium manganese nickel-based oxide such as Na3MnO4 with a sodium-excess compound such as Na3MnO4, in which the ratio of sodium to transition metal (Na / TM (transition metal)) exceeds 1, the above-mentioned problems can be solved, and high capacity can be achieved even when a material that does not contain sodium, such as hard carbon, is used as the negative electrode active material. The present invention was completed as a result of further research based on these findings. That is, the present invention includes the following configurations.

[0013] Term 1. General formula (1): Na p1 (Mn x1 Ni y1 M1 1-x1-y1 )O2-Na 1+p2 (Mn x2 M2 1-x2 ) 1-P2 O2(1) [In the formula, M1 represents at least one selected from the group consisting of V, Al, Ti, Mg, and Fe. M2 represents at least one selected from the group consisting of V, Ti, Sn, Nb, and Zr. p1 represents 0.50 to 0.70. x1 represents 0.50 to 0.75. y1 represents 0.15 to 0.45. p2 represents 0.15 to 0.55. x2 represents 0.80 to 1.00.] A composite metal oxide (1) having a composition represented by the formula: A low-crystalline sodium ion secondary battery positive electrode active material comprising:

[0014] Item 2. The low-crystalline sodium ion secondary battery positive electrode active material according to Item 1, having a P2 type crystal structure.

[0015] Item 3. In the composite metal oxide (1), the total amount of the composite metal oxide (1) is 100 mass%, and Na p1 (Mn x1 Ni y1 M1 1-x1-y1 )O2 phase contains 40 to 95 mass % and Na 1+p2 (Mn x2 M2 1-x2 ) 1-P2 Item 3. A low-crystalline positive electrode active material for a sodium ion secondary battery according to Item 1 or 2, containing 5 to 60 mass % of an O2 phase.

[0016] Item 4. The low-crystalline sodium-ion secondary battery positive electrode active material according to any one of Items 1 to 3, wherein the composite metal oxide (1) has peaks at diffraction angles 2θ of at least 16.0°, 20.0°, 23.5°, 32.0°, 33.5°, 64.5°, and 67.0° within an allowable range of ±0.4° in a range of 10 to 90° in an X-ray diffraction diagram using CuKα rays.

[0017] Item 5. The low-crystalline sodium-ion secondary battery positive electrode active material according to any one of Items 1 to 4, wherein in an X-ray diffraction pattern using CuKα radiation, the full width at half maximum of a peak at a diffraction angle 2θ=16.0° is 1.8° or more within a tolerance of ±0.4°.

[0018] Item 6. A method for producing a low-crystalline sodium ion secondary battery positive electrode active material according to any one of items 1 to 5, A step of subjecting a raw material of a positive electrode active material having a composition represented by the general formula (1) to a mechanical milling treatment. A manufacturing method comprising:

[0019] Item 7. The positive electrode active material raw material is a step of heating the mixture having the composition represented by the general formula (1) Item 7. The method according to Item 6, wherein the compound is obtained by

[0020] Item 8. The method according to Item 7, wherein the heating temperature in the heating step is 600 to 1000°C.

[0021] Item 9. A positive electrode for a sodium ion secondary battery, comprising the low-crystalline positive electrode active material for a sodium ion secondary battery according to any one of items 1 to 5.

[0022] Item 10. A sodium ion secondary battery comprising the positive electrode for sodium secondary batteries according to Item 9.

[0023] Item 11. Further, a negative electrode for a sodium secondary battery is provided, Item 11. The sodium ion secondary battery according to Item 10, wherein the negative electrode active material for a sodium ion secondary battery contained in the negative electrode for a sodium secondary battery does not contain sodium.

[0024] Item 12. An electrical device using the sodium ion secondary battery according to item 10 or 11. [Effects of the Invention]

[0025] According to the present invention, Na 2 / 3 (Mn 2 / 3 Ni1 / 3 By combining sodium manganese nickel oxides such as )O2 with sodium-rich compounds such as Na3MnO4, a positive electrode active material for sodium ion secondary batteries can be obtained that exhibits high capacity, excellent durability, and can effectively utilize the initial coulombic efficiency. [Brief explanation of the drawings]

[0026] [Figure 1] FIG. 1 is a schematic diagram illustrating a P2 type crystal structure. [Figure 2] An example of a transmission electron microscope (TEM) image of the Nap1(Mnx1Niy1M11-x1-y1)O2 phase is shown. [Figure 3] 1 is an X-ray diffraction pattern of the highly crystalline positive electrode active material raw materials obtained in Synthesis Examples 1 to 4. FIG. [Figure 4] FIG. 2 is an X-ray diffraction pattern of the highly crystalline positive electrode active material raw material obtained in Synthesis Examples 3 and 5. [Figure 5] 1 is an X-ray diffraction pattern of a highly crystalline positive electrode active material raw material obtained in Synthesis Example 3 and a low-crystalline positive electrode active material obtained in Example 3. FIG. [Figure 6] 1 shows the results of X-ray absorption near edge structure (XANES) measurements of the highly crystalline positive electrode active material obtained in Synthesis Example 3 and the low crystalline positive electrode active material obtained in Example 3. [Figure 7] The results of transmission electron microscope (TEM) observation of the low-crystalline positive electrode active material obtained in Example 3 are shown. Each figure shows (a) an HAADF image, and (b) elemental mapping of Na, (c) Mn, (d) Ni, and (e) V, respectively. In each figure, the scale bar indicates 60 nm. [Figure 8] 1 shows charge-discharge curves of a sodium ion secondary battery (half cell) using the highly crystalline positive electrode active material raw material obtained in Synthesis Example 3. [Figure 9] 1 shows charge / discharge curves of a sodium ion secondary battery (half cell) using the low-crystalline positive electrode active material obtained in Example 5. [Figure 10] The charge / discharge curves of highly crystalline Na2 / 3Mn2 / 3Ni1 / 3O2 are shown. DETAILED DESCRIPTION OF THE INVENTION

[0027] In this specification, the term "containing" is a concept that encompasses all of "comprise," "consist essentially of," and "consist only of."

[0028] In addition, in this specification, when a numerical range is expressed as "A to B," it means A or more and B or less.

[0029] 1. Cathode active material for sodium secondary batteries The positive electrode active material for a sodium secondary battery of the present invention is a compound represented by the general formula (1): Na p1 (Mn x1 Ni y1 M1 1-x1-y1 )O2-Na 1+p2 (Mn x2 M2 1-x2 ) 1-P2 O2(1) [In the formula, M1 represents at least one selected from the group consisting of V, Al, Ti, Mg, and Fe. M2 represents at least one selected from the group consisting of V, Ti, Sn, Nb, and Zr. p1 represents 0.50 to 0.70. x1 represents 0.50 to 0.75. y1 represents 0.15 to 0.45. p2 represents 0.15 to 0.55. x2 represents 0.80 to 1.00.] A composite metal oxide (1) having a composition represented by the formula: Contains:

[0030] (1-1) Complex metal oxides (1) In the present invention, the composite metal oxide (1) used as the positive electrode active material for a sodium ion secondary battery is represented by the general formula (1): Na p1 (Mn x1 Ni y1 M1 1-x1-y1 )O2-Na 1+p2 (Mn x2 M2 1-x2 ) 1-P2 O2(1) [In the formula, M1 represents at least one selected from the group consisting of V, Al, Ti, Mg, and Fe. M2 represents at least one selected from the group consisting of V, Ti, Sn, Nb, and Zr. p1 represents 0.50 to 0.70. x1 represents 0.50 to 0.75. y1 represents 0.15 to 0.45. p2 represents 0.15 to 0.55. x2 represents 0.80 to 1.00.] It has a composition represented by the formula:

[0031] Such a complex metal oxide (1) is 2 / 3 (Mn 2 / 3 Ni 1 / 3 By combining sodium manganese nickel oxides such as Na3MnO4 with sodium-rich compounds such as Na3MnO4, it is possible to obtain a positive electrode active material for sodium ion secondary batteries that exhibits high capacity even when a material that does not contain sodium, such as hard carbon, is used as the negative electrode active material, and that also has excellent charge-discharge cycle characteristics (durability). 1+p2 (Mn x2 M2 1-x2 ) 1-P2 Although it is difficult to synthesize O2) as a simple substance, by intentionally incorporating O2) as described in the manufacturing method of the present invention described below, it is possible to increase the sodium content in the positive electrode active material for a sodium ion secondary battery, and the positive electrode active material for a sodium ion secondary battery exhibits high capacity even when a material that does not contain sodium, such as hard carbon, is used as the negative electrode active material, and also has excellent charge-discharge cycle characteristics (durability).

[0032] In the general formula (1), M1 is Na p1 (Mn x1 Ni y1 M1 1-x1-y1 )O2 phase, and specific examples thereof include V, Al, Ti, Mg, and Fe. Among these, V, Al, and the like are preferred from the viewpoints of capacity, charge-discharge cycle characteristics (durability), operating potential, and the like.

[0033] In the general formula (1), p1 is Na in the positive electrode active material for a sodium ion secondary battery. p1 (Mn x1 Ni y1 M1 1-x1-y1 The sodium content in the O2 phase is 0.50 to 0.70, preferably 0.65 to 0.68. If p1 is less than 0.50, the sodium content in the positive electrode active material for a sodium ion secondary battery is low, resulting in poor capacity. On the other hand, if p1 exceeds 0.70, defects occur in the transition metal phase, resulting in poor chemical stability.

[0034] In the general formula (1), x1 is Na in the positive electrode active material for a sodium ion secondary battery. p1 (Mn x1 Ni y1 M1 1-x1-y1 The manganese content in the O2 phase is 0.50 to 0.75, preferably 0.55 to 0.70. If x1 is less than 0.50 or exceeds 0.75, it is not preferable because another structure with a low operating potential becomes stable.

[0035] In the general formula (1), y1 is the Na in the positive electrode active material for a sodium ion secondary battery. p1 (Mn x1 Ni y1 M1 1-x1-y1 The nickel content in the O2 phase is 0.15 to 0.45, preferably 0.20 to 0.35. If y1 is less than 0.15, the proportion of Ni that contributes to oxidation-reduction is small, which causes a decrease in capacity. If y1 exceeds 0.45, a different structure with a low operating potential becomes stable, which is not preferable.

[0036] In the general formula (1), M2 is Na 1+p2 (Mn x2 M2 1-x2 ) 1-P2 The term "O2" refers to other transition metal elements that may be present in the O2 phase, specifically V, Ti, Sn, Nb, Zr, etc. By including these elements, the Na content in the positive electrode active material for sodium ion secondary batteries can be reduced. 1+p2 (Mn x2 M2 1-x2 ) 1-P2It is easy to stabilize the O2 phase. Among them, V, Ti, etc., which can be oxidized and reduced, are preferable from the viewpoint of capacity.

[0037] In the general formula (1), p2 is Na in the positive electrode active material for a sodium ion secondary battery. 1+p2 (Mn x2 M2 1-x2 ) 1-P2 The sodium content in the O2 phase is 0.15 to 0.55, preferably 0.33 to 0.50. If p2 is less than 0.15, it is difficult to form a stable structure of the sodium-rich phase, resulting in poor charge-discharge cycle characteristics (durability). On the other hand, if p2 exceeds 0.55, it is difficult to form a stable structure of the sodium-rich phase, and the amount of Na insertion / desorption during charge / discharge is too large, resulting in poor durability.

[0038] In the general formula (1), x2 is Na in the positive electrode active material for a sodium ion secondary battery. 1+p2 (Mn x2 M2 1-x2 ) 1-P2 The content ratio of other transition metals in the O2 phase is 0.80 to 1.00, preferably 0.85 to 0.99. If x2 is less than 0.80, the amount of Mn that is oxidized and reduced during charge and discharge is too small, resulting in a decrease in capacity.

[0039] It is preferable that such a composite metal oxide (1) has a P2 type crystal structure, and in particular, the P2 type crystal structure is the main phase. This P2 type crystal structure is formed by the fact that Na p1 (Mn x1 Ni y1 M1 1-x1-y1 As shown in Figure 1, the sodium ion occupies the central site of a triangular prism structure surrounded by oxygen ions, forming a two-layer repeating stacked structure. This P2-type crystal structure can particularly improve charge-discharge cycle characteristics (durability).

[0040] The P2 type crystal structure of the composite metal oxide (1) is, for example, as shown in the transmission electron microscope image (TEM image) in FIG. 2, p1 (Mnx1 Ni y1 M1 1-x1-y1 It is preferable that the 02 phase has a distorted layer structure. Such a distorted layer structure can be formed by subjecting a highly crystalline cathode active material raw material to mechanical milling to reduce crystallinity, as in the manufacturing method of the present invention described later. This causes the layer structure of the P2 type crystal structure to partially collapse, forming a distorted layer structure. p1 (Mn x1 Ni y1 M1 1-x1-y1 )O2 alone, but even in complex metal oxides (1) combined with sodium-rich compounds such as Na3MnO4, Na p1 (Mn x1 Ni y1 M1 1-x1-y1 )O2 phase has a similar crystal structure.

[0041] Whether or not the P2 type crystal structure is the main phase in the composite metal oxide (1) is confirmed by X-ray diffraction measurement of the positive electrode active material for sodium ion secondary batteries (composite metal oxide (1)). The amount of the P2 type crystal structure is not particularly limited, but the amount of Na, based on the entire positive electrode active material for sodium ion secondary batteries (composite metal oxide (1)) (100 mass%), is p1 (Mn x1 Ni y1 M1 1-x1-y1 The amount of the O2 phase (P2 type crystal structure) present is preferably 40 to 95 mass %, more preferably 45 to 93 mass %.

[0042] The positive electrode active material for a sodium secondary battery of the present invention is generally a sodium salt having a P2 type structure. p1 (Mn x1 Ni y1 M1 1-x1-y1 )O2 phase and Na with various structures 1+p2 (Mn x2 M2 1-x2 ) 1-P2 It can have an O2 phase. 1+p2 (Mn x2 M2 1-x2 ) 1-P2The amount of the O2 phase is not particularly limited, but is based on the entire positive electrode active material for a sodium ion secondary battery (composite metal oxide (1)) (100 mass%). 1+p2 (Mn x2 M2 1-x2 ) 1-P2 The amount of the O2 phase present is preferably 5 to 60 mass %, more preferably 7 to 55 mass %.

[0043] On the other hand, as described above, the positive electrode active material for a sodium secondary battery of the present invention is usually made of Na having a P2 type crystal structure. p1 (Mn x1 Ni y1 M1 1-x1-y1 )O2 phase and Na with various structures 1+p2 (Mn x2 M2 1-x2 ) 1-P2 The positive electrode active material for a sodium ion secondary battery (composite metal oxide (1)) may contain an O2 phase, but may also have other crystal structures other than these as long as the effects of the present invention are not impaired. Specifically, a layered crystal structure called an O3 type or P3 type, in which the molar ratio of Na to TM (transition metal) (Na / TM (transition metal)) is less than 1, may be contained in an amount of about 10 mass% or less, particularly 5 mass% or less, based on the entire positive electrode active material for a sodium ion secondary battery (composite metal oxide (1)) (100 mass%).

[0044] The positive electrode active material for a sodium secondary battery of the present invention having the above-mentioned crystal structure usually has an X-ray diffraction pattern of Na p1 (Mn x1 Ni y1 M1 1-x1-y1 )O2 phase-derived P2 type crystal structure and Na 1+p2 (Mn x2 M2 1-x2 ) 1-P2 Specifically, the positive electrode active material for a sodium secondary battery of the present invention preferably has peaks at diffraction angles 2θ of at least 16.0°, 20.0°, 23.5°, 32.0°, 33.5°, 64.5°, and 67.0° within a range of diffraction angles 2θ of 10° to 90° in an X-ray diffraction diagram using CuKα rays, within an allowable range of ±0.4° (particularly ±0.3°).

[0045] The term "low crystallinity" in the present invention will be explained below. In the positive electrode active material for a sodium secondary battery of the present invention, in an X-ray diffraction pattern using CuKα radiation, the full width at half maximum of the peak at 2θ=16.0° is preferably 1.8° or more, and more preferably 1.8 to 2.3°, within an allowable range of ±0.4° (particularly ±0.3°). Note that for a positive electrode active material raw material with a high degree of crystallinity that is not subjected to mechanical milling treatment as in the manufacturing method of the present invention described below, the full width at half maximum of the peak at 2θ=16.0° is approximately 0.21°. Thus, low crystallinity increases the number of sites where sodium ions can stably exist, thereby improving charge-discharge cycle characteristics (durability).

[0046] The composite metal oxide (1) contained in the positive electrode active material for a sodium secondary battery of the present invention contains a compound having a composition represented by the above general formula (1), and its crystal structure is as described above, but it may contain inevitable impurities. Such inevitable impurities are thought to be those derived from the raw materials, and include sodium-containing compounds such as Na2CO3 and NaOH, as well as transition metal oxides and hydroxides containing one or more of Mn, Ni, V, Al, Ti, Mg, Fe, etc., and may be contained in an amount of about 10 mol % or less, particularly 5 mol % or less, and even more particularly 2 mol % or less, within a range that does not impair the effects of the present invention.

[0047] 2. Manufacturing method of positive electrode active material for sodium secondary battery The positive electrode active material for a sodium secondary battery of the present invention may, for example, A step of subjecting a raw material of a positive electrode active material having a composition represented by the general formula (1) to a mechanical milling treatment. The manufacturing method can be obtained by the following steps.

[0048] The positive electrode active material raw material used in this case is, for example, a step of heating the mixture having the composition represented by the general formula (1) The manufacturing method can be obtained by the following steps.

[0049] The production method of the present invention will now be described in detail.

[0050] (2-1) Raw material compound The raw material compounds for obtaining a mixture having a composition represented by general formula (1) may be any compounds that contain various elements such as sodium, manganese, nickel, and oxygen in predetermined proportions in the final mixture, and may include, for example, a sodium-containing compound, a manganese-containing compound, a nickel-containing compound, an oxygen-containing compound, etc. Furthermore, when the positive electrode active material for a sodium secondary battery of the present invention contains an element such as vanadium, aluminum, titanium, magnesium, iron, tin, or zirconium, the raw material compounds may also include a vanadium-containing compound, an aluminum-containing compound, a titanium-containing compound, a magnesium-containing compound, an iron-containing compound, a tin-containing compound, a zirconium-containing compound, etc.

[0051] The types of the sodium-containing compound, manganese-containing compound, nickel-containing compound, and oxygen-containing compound, and, if necessary, vanadium-containing compound, aluminum-containing compound, titanium-containing compound, magnesium-containing compound, iron-containing compound, tin-containing compound, and zirconium-containing compound, are not particularly limited. A mixture of multiple compounds containing one type each of sodium, manganese, nickel, and oxygen, and, if necessary, vanadium, aluminum, titanium, magnesium, iron, tin, zirconium, etc., may be used, or a compound containing sodium, manganese, nickel, and oxygen, and, if necessary, two or more elements selected from vanadium, aluminum, titanium, magnesium, iron, tin, and zirconium may be used as part of the raw material.

[0052] These raw material compounds are preferably compounds that do not contain any metal elements (particularly rare metal elements) other than sodium, manganese, nickel, and, as required, vanadium, aluminum, titanium, magnesium, and iron.

[0053] Specific examples of such raw material compounds include: Examples of sodium-containing compounds include metallic sodium (Na); sodium hydroxide (NaOH); sodium carbonates such as sodium carbonate (Na2CO3) and sodium bicarbonate (NaHCO3). Examples of manganese-containing compounds include metallic manganese (Mn); manganese oxides such as manganese(II) oxide (MnO) and manganese(IV) oxide (MnO2); manganese hydroxides such as manganese(II) hydroxide (Mn(OH)2) and manganese(III) hydroxide (Mn(OH)3); manganese(II) carbonate (MnCO3); oxides containing iron, manganese, and nickel such as iron-manganese-nickel composite oxide; hydroxides containing iron, manganese, and nickel such as iron-manganese-nickel composite hydroxide; and carbonates containing iron, manganese, and nickel such as iron-manganese-nickel composite carbonate. Examples of nickel-containing compounds include metallic nickel (Ni); nickel oxide (NiO); nickel hydroxides such as nickel(I) hydroxide (Ni(OH)) and nickel(II) hydroxide (Ni(OH)2); nickel(II) carbonate (NiCO3); oxides containing iron, manganese, and nickel such as iron-manganese-nickel composite oxide; hydroxides containing iron, manganese, and nickel such as iron-manganese-nickel composite hydroxide; and carbonates containing iron, manganese, and nickel such as iron-manganese-nickel composite carbonate. Oxygen-containing compounds include sodium hydroxide (NaOH); sodium carbonates such as sodium carbonate (Na2CO3) and sodium bicarbonate (NaHCO3); iron oxides such as iron(II) oxide (FeO) and iron(III) oxide (Fe2O3); iron hydroxides such as iron(II) hydroxide (Fe(OH)2) and iron(III) hydroxide (Fe(OH)3); iron carbonates such as iron(II) carbonate (FeCO3) and iron(III) carbonate (Fe2(CO3)2); manganese oxides such as manganese(II) oxide (MnO) and manganese(IV) oxide (MnO2); manganese hydroxides such as manganese(II) hydroxide (Mn(OH)2) and manganese(III) hydroxide (Mn(OH)3); manganese(II) carbonate (MnCO3); nickel oxide (NiO); and nickel(I) hydroxide (Ni(OH)). Examples include nickel hydroxides such as nickel (II) hydroxide (Ni(OH)2); nickel (II) carbonate (NiCO3); vanadium hydroxide (V(OH)2); vanadium pentoxide (VO5); aluminum oxide (Al2O3); aluminum hydroxide (Al(OH)3); aluminum carbonate (Al2(CO3)3); titanium oxide (TiO2); titanium hydroxide (Ti(OH)2); magnesium oxide (MgO); magnesium hydroxide (Mg(OH)2); magnesium carbonate (MgCO3); oxides containing iron, manganese, and nickel such as iron-manganese-nickel composite oxide; hydroxides containing iron, manganese, and nickel such as iron-manganese-nickel composite hydroxide; and carbonates containing iron, manganese, and nickel such as iron-manganese-nickel composite carbonate. Examples of vanadium-containing compounds include metallic vanadium (V); vanadium hydroxide (V(OH)2); vanadium pentoxide (VO5), etc. Examples of aluminum-containing compounds include metallic aluminum (Al); aluminum oxide (Al2O3); aluminum hydroxide (Al(OH)3); aluminum carbonate (Al2(CO3)3), etc. Examples of titanium-containing compounds include metallic titanium (Ti); titanium oxide (TiO2); and titanium hydroxide (Ti(OH)2). Examples of magnesium-containing compounds include metallic magnesium (Mg); magnesium oxide (MgO); magnesium hydroxide (Mg(OH)2); magnesium carbonate (MgCO3); Examples of iron-containing compounds include metallic iron (Fe); iron oxides such as iron (II) oxide (FeO) and iron (III) oxide (Fe2O3); iron hydroxides such as iron (II) hydroxide (Fe(OH)2) and iron (III) hydroxide (Fe(OH)3); iron carbonates such as iron (II) carbonate (FeCO3) and iron (III) carbonate (Fe2(CO3)2); oxides containing iron, manganese, and nickel such as iron-manganese-nickel composite oxide; hydroxides containing iron, manganese, and nickel such as iron-manganese-nickel composite hydroxide; and carbonates containing iron, manganese, and nickel such as iron-manganese-nickel composite carbonate. Examples of tin-containing compounds include metallic tin (Sn); tin oxides such as tin(II) oxide (SnO) and tin(IV) oxide (SnO). Examples of zirconium-containing compounds include metallic zirconium (Zr); and zirconium oxide such as zirconium oxide (ZrO2).

[0054] In the present invention, the raw material compounds mentioned above may be commercially available products or may be separately synthesized and used.

[0055] The shape of these raw material compounds is not particularly limited, but from the viewpoint of ease of handling, powder is preferable. Also, from the viewpoint of reactivity, the particles are preferably fine, and powder with an average particle size of 1 μm or less is preferable. The average particle size of the raw material compounds is measured by electron microscope observation (SEM).

[0056] The mixture having the composition represented by general formula (1) can be obtained by mixing the necessary materials among the raw material compounds explained above.

[0057] In the mixture having the composition represented by the general formula (1), the content ratio of each element is not particularly limited, but it is preferable to adjust it so as to have the composition represented by the general formula (1).p1 (Mn x1 Ni y1 M1 1-x1-y1 )O2 and Na 1+p2 (Mn x2 M2 1-x2 ) 1-P2 The ratio of Na to O2 is compared to the feed ratio. p1 (Mn x1 Ni y1 M1 1-x1-y1 Therefore, when it is desired to obtain the composite metal oxide (1), the amount of Na in the mixture having the composition represented by the general formula (1) is set to 100 mass % of the total amount of the mixture having the composition represented by the general formula (1). p1 (Mn x1 Ni y1 M1 1-x1-y1 )O2 content is 80 to 95 mass% (especially 85 to 90 mass%), Na 1+p2 (Mn x2 M2 1-x2 ) 1-P2 It is preferable to adjust the O2 content to 5 to 20 mass % (particularly 10 to 15 mass %).

[0058] (2-2) Manufacturing method (heating process) The mixing method for producing a mixture having the composition represented by general formula (1) is not particularly limited, and any method capable of uniformly mixing the raw material compounds can be used. For example, hand mixing, mortar mixing, a method in which each component is dispersed in a solvent and then mixed, a method in which each component is dispersed in a solvent at once and then mixed, etc. can be used.

[0059] The atmosphere during mixing is not particularly limited, but for example, an inert gas atmosphere such as Ar or N2, or an air atmosphere can be used.

[0060] The heating temperature of the mixture having the composition represented by general formula (1) is not particularly limited, but is preferably 600 to 1000°C, more preferably 650 to 950°C, from the viewpoint of being a temperature sufficient to promote the reaction and suppressing evaporation of Na.

[0061] The heating time is not particularly limited as long as the reaction proceeds sufficiently, and is preferably 1 to 48 hours, more preferably 3 to 10 hours, for example.

[0062] (2-3) Manufacturing method (mechanical milling process) Mechanical milling is a method of grinding and mixing raw materials while applying mechanical energy. This method involves applying mechanical impact and friction to the raw materials, resulting in vigorous contact and pulverization of the positive electrode active material raw materials, which then undergoes a reaction. This means that mixing, grinding, and reaction occur simultaneously. Therefore, mechanical milling can sometimes produce a metastable crystal structure that cannot be achieved by conventional heat treatment.

[0063] Specifically, the mechanical milling treatment can be performed by mixing and pulverizing using a mechanical mill such as a ball mill, a bead mill, a rod mill, a vibration mill, a disk mill, a hammer mill, a jet mill, etc. The mechanical milling treatment can also be performed in multiple steps with breaks in between, if necessary.

[0064] Various conditions of the mechanical milling process, such as the rotation speed, energy, temperature, and time, can be set as appropriate, and it is preferable to adjust them so that the full width at half maximum of the peak at a diffraction angle 2θ = 16.0° is 1.8° or more as described above.

[0065] When the mechanical milling process is repeated multiple times, the above conditions can be applied to each mechanical milling process.

[0066] By the mechanical milling treatment described above, the target positive electrode active material for a sodium ion secondary battery of the present invention can be obtained as a fine powder.

[0067] 3. Positive electrode for sodium ion secondary battery and sodium ion secondary battery The positive electrode active material for sodium ion secondary batteries of the present invention can be effectively used as a positive electrode active material for sodium ion secondary batteries by taking advantage of the above-mentioned excellent properties (capacity, charge / discharge cycle characteristics (durability), operating voltage, etc.). In particular, the positive electrode active material for sodium ion secondary batteries of the present invention is a material that contains a sufficient amount of sodium in its structure, and is therefore a material that can be charged and discharged, and further has high capacity, high charge / discharge cycle characteristics (durability), and high operating voltage, and is therefore useful as a positive electrode active material for sodium ion secondary batteries. A sodium ion secondary battery that uses the positive electrode active material for sodium ion secondary batteries of the present invention as a positive electrode active material may be a nonaqueous electrolyte sodium ion secondary battery that uses a nonaqueous solvent-based electrolytic solution as the electrolyte, or may be an all-solid-state sodium ion secondary battery that uses a sodium ion-conductive solid electrolyte.

[0068] The structures of the nonaqueous electrolyte sodium ion secondary battery and the all-solid-state sodium ion secondary battery can be the same as those of known sodium ion secondary batteries, except that the positive electrode active material for sodium ion secondary batteries of the present invention is used as the positive electrode active material.

[0069] For example, a non-aqueous electrolyte sodium ion secondary battery can have the same basic structure as a known non-aqueous electrolyte sodium ion secondary battery, except that the above-described positive electrode active material for sodium ion secondary batteries is used as the positive electrode active material.

[0070] The positive electrode can be produced by using the above-described positive electrode active material for sodium secondary batteries as the positive electrode active material, mixing it with a conductive agent and a binder to prepare a positive electrode mixture, and supporting the resulting mixture on a positive electrode current collector made of aluminum, nickel, stainless steel, carbon cloth, etc. As the conductive agent, for example, carbon materials such as graphite, cokes, carbon black, and acicular carbon can be used.

[0071] For the negative electrode, both sodium-containing and sodium-free materials can be used as the negative electrode active material. For example, any material that reacts with sodium, such as non-graphitizable carbon (hard carbon), graphitizable carbon (soft carbon), sodium metal, tin, and alloys containing these, can be used. In particular, the positive electrode active material for a sodium-ion secondary battery of the present invention contains a sufficient amount of sodium, so that a sufficiently high capacity can be obtained even when a sodium-free material is used as the negative electrode active material. This demonstrates the superiority of the positive electrode active material for a sodium-ion secondary battery of the present invention when a sodium-free material is used as the negative electrode active material. These negative electrode active materials can also be supported on a negative electrode current collector made of aluminum, copper, nickel, stainless steel, carbon, or the like, using a conductive agent, binder, or the like, as needed, to produce a negative electrode.

[0072] The separator may be made of a material such as a polyolefin resin, such as polyethylene or polypropylene; a fluororesin; nylon; aromatic aramid; or inorganic glass, and may be in the form of a porous film, nonwoven fabric, or woven fabric.

[0073] As the solvent for the non-aqueous electrolyte, any solvent known as a solvent for non-aqueous solvent-based secondary batteries, such as carbonates, ethers, nitriles, and sulfur-containing compounds, can be used.

[0074] In addition, all-solid-state sodium ion secondary batteries can also have the same structure as known all-solid-state sodium ion secondary batteries, except that the positive electrode active material for sodium ion secondary batteries of the present invention is used as the positive electrode active material.

[0075] In this case, examples of the electrolyte include polymer-based solid electrolytes such as polyethylene oxide-based polymer compounds, polymer compounds containing at least one of polyorganosiloxane chains and polyoxyalkylene chains, as well as sulfide-based solid electrolytes and oxide-based solid electrolytes.

[0076] The positive electrode of an all-solid-state sodium ion secondary battery can be produced, for example, by using the positive electrode active material for sodium ion secondary batteries of the present invention as the positive electrode active material, and supporting a positive electrode mixture containing a conductive agent, a binder, a solid electrolyte, etc. on a positive electrode current collector made of titanium, aluminum, nickel, stainless steel, etc. As with non-aqueous solvent secondary batteries, for example, carbon materials such as graphite, cokes, carbon black, and acicular carbon can be used as the conductive agent.

[0077] There are no particular limitations on the shape of the non-aqueous electrolyte sodium ion secondary battery and the all-solid-state sodium ion secondary battery, and they may be cylindrical, prismatic, or the like. [Example]

[0078] EXAMPLES The present invention will be specifically explained below by showing examples and comparative examples, but it goes without saying that the present invention is not limited to these examples.

[0079] [Synthesis Example 1: Positive electrode active material raw material (V 5 mol%; calcined at 850°C for 3 hours, then at 600°C for 18 hours)] Commercially available sodium hydroxide (NaOH), manganese oxide (Mn2O3), nickel hydroxide (Ni(OH)2), and vanadium trioxide (V2O3) were weighed out to a molar ratio of 137:70:23:7 to prepare a raw material mixture.

[0080] Next, the obtained raw material mixture was heated to 850°C in the atmosphere using an electric furnace, and after being held at 850°C for 3 hours, was cooled to 300°C in the furnace. The fired product was removed from the furnace at a temperature of 300°C and cooled to room temperature (20°C) in a dry chamber (chamber temperature 20°C) with a dew point temperature of -50°C. 2 / 3 (Mn 2 / 3 Ni 1 / 3 ) 0.95 V 0.05 O2-xNa3Mn 0.95 V 0.05 A material presumed to be O4 was obtained.

[0081] [Synthesis Example 2: Positive electrode active material raw material (V 5 mol%; 850°C for 10 hours)] The obtained raw material mixture was heated to 850°C in the air using an electric furnace, held at 850°C for 10 hours, and then cooled to 300°C in the furnace. 2 / 3 (Mn 2 / 3 Ni 1 / 3 ) 0.95 V 0.05 O2-xNa3Mn 0.95 V 0.05 A material presumed to be O4 was obtained.

[0082] [Synthesis Example 3: Positive electrode active material raw material (V 5 mol%; 850°C for 3 hours)] The obtained raw material mixture was heated to 850°C in the air using an electric furnace, held at 850°C for 3 hours, and then cooled to 300°C in the furnace. 2 / 3 (Mn 2 / 3 Ni 1 / 3 ) 0.95 V 0.05 O2-xNa3Mn 0.95 V 0.05 A material presumed to be O4 was obtained.

[0083] [Synthesis Example 4: Positive electrode active material raw material (V 5 mol%; 700°C for 3 hours)] The obtained raw material mixture was heated to 700°C in the air using an electric furnace, held at 700°C for 3 hours, and then cooled to 300°C in the furnace. 2 / 3 (Mn 2 / 3 Ni 1 / 3 ) 0.95 V 0.05 O2-xNa3Mn 0.95 V 0.05 A material presumed to be O4 was obtained.

[0084] [Synthesis Example 5: Positive electrode active material raw material (V 10 mol%; 850°C for 3 hours)] Commercially available sodium hydroxide (NaOH), manganese oxide (Mn2O3), nickel hydroxide (Ni(OH)2), and vanadium pentoxide (V2O5) were weighed out to a molar ratio of 137:70:23:7 to prepare a raw material mixture.

[0085] Next, the obtained raw material mixture was heated to 850°C in the air using an electric furnace, and after being held at 850°C for 3 hours, was cooled to 300°C in the furnace. The fired product was removed from the furnace at a temperature of 300°C and cooled to room temperature (20°C) in a dry chamber (chamber temperature 20°C) with a dew point temperature of -50°C. 2 / 3 (Mn 2 / 3 Ni 1 / 3 ) 0.90 V 0.10 O2-xNa3Mn 0.90 V 0.10 A material presumed to be O4 was obtained.

[0086] [Example 1: Positive electrode active material (V 5 mol%; baked at 850°C for 3 hours, then baked at 600°C for 18 hours)] 1 g of the highly crystalline positive electrode active material raw material obtained in Synthesis Example 1 was taken and placed in a ball mill with an internal volume of 80 mL together with 40 g of zirconium oxide balls with a diameter of 4 mm. Then, the mixture was mixed at a rotation speed of 500 rpm for 2 hours, and the highly crystalline positive electrode active material raw material was subjected to mechanical milling treatment, and the composition of the low crystalline positive electrode active material of Example 1 was found to be Na 2 / 3 (Mn 2 / 3 Ni 1 / 3 ) 0.95 V 0.05 O2-xNa3Mn 0.95 V 0.05 A material presumed to be O4 was obtained.

[0087] [Example 2: Positive electrode active material (V 5 mol%; 850°C for 10 hours)] The composition of the low-crystalline positive electrode active material of Example 2 was the same as that of Example 1, except that the high-crystalline positive electrode active material obtained in Synthesis Example 2 was used instead of the high-crystalline positive electrode active material obtained in Synthesis Example 1. 2 / 3 (Mn 2 / 3 Ni 1 / 3 ) 0.95 V 0.05O2-xNa3Mn 0.95 V 0.05 A material presumed to be O4 was obtained.

[0088] [Example 3: Positive electrode active material raw material (V 5 mol%; 850°C for 3 hours)] The composition of the low-crystalline positive electrode active material raw material in Example 3 was the same as in Example 1, except that the high-crystalline positive electrode active material raw material obtained in Synthesis Example 3 was used instead of the high-crystalline positive electrode active material raw material obtained in Synthesis Example 1. 2 / 3 (Mn 2 / 3 Ni 1 / 3 ) 0.95 V 0.05 O2-xNa3Mn 0.95 V 0.05 A material presumed to be O4 was obtained.

[0089] [Example 4: Positive electrode active material raw material (V 5 mol%; 700°C for 3 hours)] The composition of the low-crystalline positive electrode active material raw material in Example 4 was the same as in Example 1, except that the high-crystalline positive electrode active material raw material obtained in Synthesis Example 4 was used instead of the high-crystalline positive electrode active material raw material obtained in Synthesis Example 1. 2 / 3 (Mn 2 / 3 Ni 1 / 3 ) 0.95 V 0.05 O2-xNa3Mn 0.95 V 0.05 A material presumed to be O4 was obtained.

[0090] [Example 5: Positive electrode active material raw material (V 10 mol%; 850°C for 3 hours)] The composition of the low-crystalline positive electrode active material raw material in Example 5 was the same as in Example 1, except that the high-crystalline positive electrode active material raw material obtained in Synthesis Example 5 was used instead of the high-crystalline positive electrode active material raw material obtained in Synthesis Example 1. 2 / 3 (Mn 2 / 3 Ni 1 / 3 ) 0.90 V 0.10 O2-xNa3Mn 0.90 V 0.10 A material presumed to be O4 was obtained.

[0091] [Experimental Example 1: X-ray diffraction measurement (part 1)] X-ray structural diffraction (XRD) was measured using CuKα radiation for the raw materials for the positive electrode active material for sodium ion secondary batteries obtained in Synthesis Examples 1 to 5. The results are shown in Figures 3 and 4. As a result, in all samples, peaks were observed at diffraction angles 2θ of 16.0°, 20.0°, 23.5°, 32.0°, 33.5°, 64.5°, and 67.0° in the range of 10 to 90°. 2 / 3 (Mn 2 / 3 Ni 1 / 3 ) 0.95 V 0.05 O2 phase or Na 2 / 3 (Mn 2 / 3 Ni 1 / 3 ) 0.90 V 0.10 O2 phase and Na3Mn 0.95 V 0.05 O4 phase or Na3Mn 0.90 V 0.10 It can be seen that the O4 phase and the O4 phase are combined, and that both peaks are present. In addition, the full width at half maximum of the peak at a diffraction angle 2θ = 16.0° is 0.296° in Synthesis Example 1, 0.18° in Synthesis Example 2, 0.27° in Synthesis Example 3, 0.25° in Synthesis Example 4, and 0.20° in Synthesis Example 5, which indicates that all of these are highly crystalline materials.

[0092] In addition, by Rietveld analysis, Na 2 / 3 (Mn 2 / 3 Ni 1 / 3 ) 0.95 V 0.05 O2 phase or Na 2 / 3 (Mn 2 / 3 Ni 1 / 3 ) 0.90 V 0.10 O2 phase and Na3Mn 0.95 V 0.05 O4 phase or Na3Mn 0.90 V 0.10 The content ratio of the O4 phase was evaluated, and the results are shown in Table 1.

[0093] [Table 1]

[0094] As a result of the above, it was found that the sodium-rich phase (Na3Mn 0.95 V 0.05 O4 phase or Na3Mn 0.90 V 0.10 The materials heated at 850°C had different sodium-rich phases (Na3Mn 0.95 V 0.05 O4 phase or Na3Mn 0.90 V 0.10 O4 phase), and the sodium content in the positive electrode active material for sodium-ion secondary batteries was particularly increased. On the other hand, even if the amount of vanadium added was changed, the sodium-excess phase (Na3Mn 0.95 V 0.05 O4 phase or Na3Mn 0.90 V 0.10 The content of O4 phase did not change significantly.

[0095] In addition, since Na3VO4 is known to be stable by itself, the addition of vanadium can produce a sodium-rich phase (Na3Mn 0.95 V 0.05 O4 phase or Na3Mn 0.90 V 0.10 It is believed that this stabilized the O4 phase.

[0096] On the other hand, X-ray diffraction (XRD) using CuKα radiation was measured for the raw material of the positive electrode active material for a sodium ion secondary battery obtained in Example 5. The results for Synthesis Example 5 and Example 5 are shown in Figure 5. The peaks were broadened, and Na 2 / 3 (Mn 2 / 3 Ni 1 / 3 ) 0.95 V 0.05 O2 phase or Na 2 / 3 (Mn 2 / 3 Ni 1 / 3 ) 0.90 V 0.10 O2 phase and Na3Mn 0.95 V 0.05 O4 phase or Na3Mn 0.90 V 0.10 It is difficult to understand the composite state of the O4 phase. In addition, the full width at half maximum of the peak at a diffraction angle 2θ=16.0° is 5.1° in Example 5, which indicates that the material is of low crystallinity.

[0097] As described above, mechanical milling reduces the crystallinity, making it difficult to distinguish between the two phases. However, because the composition is not changed by mechanical milling, it can be understood that the content ratio of the two phases in the low-crystalline positive electrode active material is the same as that of the high-crystalline positive electrode active material before mechanical milling. Therefore, the content ratio measured before mechanical milling (high-crystalline positive electrode active material) is treated as the content ratio of the two phases in the low-crystalline positive electrode active material.

[0098] Next, the V K-edge X-ray absorption near edge structure (XANES) measurement was performed on the highly crystalline cathode active material obtained in Synthesis Example 3 and the low-crystalline cathode active material obtained in Example 3. The results are shown in Figure 6. As a result, both before and after mechanical milling, a peak was observed at approximately 5.468 keV, indicating that tetrahedral vanadium was present in both cases, and that Na3Mn 0.90 V 0.10 It can be seen that the O4 phase exists in the low-crystalline positive electrode active material. In other words, vanadium is present in the structure both before and after mechanical milling, confirming that the composition remains unchanged by mechanical milling.

[0099] [Experimental Example 2: Electron microscope observation] The low-crystalline positive electrode active material obtained in Example 3 was observed with a transmission electron microscope (TEM). The results are shown in Figure 7. In Figure 7, each figure shows (a) an HAADF image, (b) elemental mapping of Na, (c) Mn, (d) Ni, and (e) V. In each figure, the scale bar indicates 60 nm.

[0100] [Experimental Example 3: Charge / Discharge Test (Half-Cell Part 1)] A sodium ion secondary battery was fabricated by the following method using the highly crystalline positive electrode active material raw material obtained in Synthesis Example 3 and the low-crystalline positive electrode active material for sodium ion secondary batteries obtained in Example 5, and a charge-discharge test was performed at 30°C.

[0101] The electrode composition was prepared by mixing 84% by mass of the low-crystalline positive electrode active material obtained in Examples 3 and 5, 8% by mass of acetylene black (AB), and 8% by mass of polytetrafluoroethylene (PTFE) binder to prepare a mixture, which was then tightly bonded to an aluminum mesh and then heat-treated (under reduced pressure at 220°C for 10 hours or more). A coin cell (CR2032) was fabricated using a metallic sodium foil having a capacity approximately 50 times the calculated capacity of the test electrode as the counter electrode, a polypropylene microporous membrane as the separator, and 1 mol / L NaPF6 (ethylene carbonate (EC):diethyl carbonate (DEC) = 1:1 (volume ratio)) as the electrolyte.

[0102] Charge / discharge test: charge / discharge range is 1.5~4.3V vs Na / Na + The battery was subjected to 5 cycles of constant current charging and discharging at a current density of 10 mA / g. The results are shown in Figures 8 and 9.

[0103] As a result, in Example 3, the initial coulombic efficiency was 89% and the initial charge capacity was 201 mAh / g, and in Example 5, the initial coulombic efficiency was 96% and the initial charge capacity was 224 mAh / g. 0.95 V 0.05 O4 phase or Na3Mn 0.90 V 0.10 By combining the crystalline phase with the crystalline phase (O4), the initial sodium content was increased, resulting in a high capacity. Note that the highly crystalline sample obtained in Synthesis Example 3, which was not subjected to mechanical milling, showed a large initial charge / discharge capacity, but a small discharge capacity.

[0104] As a comparative example, highly crystalline Na 2 / 3 Mn 2 / 3 Ni 1 / 3 The charge-discharge curve for O2 is shown in Figure 10. The initial charge capacity was 110 mAh / g, while the discharge capacity was 204 mAh / g, meaning the initial charge-discharge efficiency was 53.9%. Also, the discharge capacity at the fifth cycle was 167 mAh / g, indicating a significant capacity degradation within several cycles.

[0105] In addition, highly crystalline Na2 / 3 Mn 2 / 3 Ni 1 / 3 O2 was synthesized as follows.

[0106] Commercially available sodium hydroxide (NaOH), manganese oxide (Mn2O3), and nickel hydroxide (Ni(OH)2) were weighed out to a molar ratio of 67:33:33 to prepare a raw material mixture.

[0107] Next, the obtained raw material mixture was heated to 850°C in the atmosphere using an electric furnace, held at 850°C for 3 hours, and then cooled to 300°C in the furnace. The fired product was removed from the furnace at a temperature of 300°C and cooled to room temperature (20°C) in a dry chamber (chamber temperature 20°C) with a dew point temperature of -50°C.

Claims

1. General formula (1): Na ｐ１ (Mn ｘ１ Ni ｙ１ M1 １－ｘ１－ｙ１ )O ２ - １＋ｐ２ (Mn ｘ２ M2 １－ｘ２ ) １－Ｐ２ O ２ (1) [In the formula, M1 represents at least one element selected from the group consisting of V, Al, Ti, Mg, and Fe. M2 represents at least one element selected from the group consisting of V, Ti, Sn, Nb, and Zr. p1 represents 0.50 to 0.

70. x1 represents 0.50 to 0.

75. y1 represents 0.15 to 0.

45. p2 represents 0.15 to 0.

55. x2 represents 0.80 to 1.00.] A composite metal oxide (1) having a composition represented by the formula: A low-crystalline sodium ion secondary battery positive electrode active material comprising:

2. The low-crystalline positive electrode active material for a sodium ion secondary battery according to claim 1 , having a P2 type crystal structure.

3. In the composite metal oxide (1), the total amount of the composite metal oxide (1) is taken as 100 mass%, and Na ｐ１ (Mn ｘ１ Ni ｙ１ M1 １－ｘ１－ｙ１ ) O ２ phase in an amount of 40 to 95 mass %, and Na １＋ｐ２ (Mn ｘ２ M2 １－ｘ２ ) １－Ｐ２ O ２ The low-crystalline sodium ion secondary battery positive electrode active material according to claim 1, containing 5 to 60 mass% of the phase.

4. In the composite metal oxide (1), in the range of 10 to 90° in an X-ray diffraction diagram using CuKα rays, the diffraction angle 2θ has peaks at at least 16.0 °, 20.0 °, 23.5 °, 32.0 °, 33.5 °, 64.5 °, and 67.0 ° within an allowable range of ±0.4 °. The low-crystalline sodium ion secondary battery positive electrode active material according to claim 1.

5. 2. The low-crystalline sodium ion secondary battery positive electrode active material according to claim 1, wherein the full width at half maximum of a peak at a diffraction angle 2θ = 16.0° is 1.8° or more within a tolerance range of ±0.4° in an X-ray diffraction pattern using CuKα rays.

6. A method for producing the low-crystalline sodium ion secondary battery positive electrode active material according to any one of claims 1 to 5, A step of subjecting a raw material of a positive electrode active material having a composition represented by the general formula (1) to a mechanical milling treatment. A manufacturing method comprising:

7. The positive electrode active material raw material is a step of heating the mixture having the composition represented by the general formula (1) The method according to claim 6, wherein the compound is obtained by

8. The method according to claim 7, wherein the heating temperature in the heating step is 600 to 1000°C.

9. A positive electrode for a sodium ion secondary battery, comprising the low-crystalline positive electrode active material for a sodium ion secondary battery according to any one of claims 1 to 5.

10. A sodium ion secondary battery comprising the positive electrode for a sodium secondary battery according to claim 9.

11. Further, the device includes a negative electrode for a sodium secondary battery, The sodium ion secondary battery according to claim 10, wherein the negative electrode active material for a sodium ion secondary battery contained in the negative electrode for a sodium secondary battery does not contain sodium.

12. An electrical device using the sodium ion secondary battery according to claim 10.

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