Positive electrode active material for sodium-ion battery, method for manufacturing the same, and sodium-ion battery

Abstract

Disclosed is a positive electrode active material for sodium-ion batteries having a high capacity. The positive electrode active material for sodium-ion batteries of the present disclosure contains a Na-containing composite oxide. The Na-containing composite oxide has a crystal structure in which Na x Fe y M 1‑y O z (3.0

Description

Technical Field

[0001] The present application discloses a positive electrode active material for a sodium ion battery, a manufacturing method thereof, and a sodium ion battery. Background Art

[0002] As a positive electrode active material for a sodium ion battery, a material composed of a non-rare metal material and having high performance is required. Patent Document 1 discloses a positive electrode active material for a sodium secondary battery represented by the general formula Na5Mn

[0008] ,

[0014] ,

[0013] , z , y ,

[0012] ,

[0007] ,

[0011] ,

[0006] ,

[0010] ,

[0016] ,

[0009] ,

[0015] , , , 1-y , , x , , , , , , , , Fe x O4 (0 < x ≤ 0.2). Furthermore, Patent Document 2 discloses a sodium-containing composite oxide having a predetermined chemical composition as a precursor of a positive electrode active material for a lithium ion battery.

[0003] Prior Art Documents

[0004] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2014-164860

[0005] Patent Document 2: Japanese Unexamined Patent Application Publication No. 2009-184844 Summary of the Invention

[0006] There is room for improvement in the capacity of existing positive electrode active materials for sodium ion batteries.

[0007] As a means for solving the above problems, the present application discloses the following multiple solutions.

[0008] <Solution 1>

[0009] A positive electrode active material for a sodium ion battery, comprising a Na-containing composite oxide,

[0010] The chemical composition of the Na-containing composite oxide is represented by the following formula:

[0011] Na x Fe y M 1-y O z

[0012] where 3.0 < x < 9.0, 0 < y ≤ 1.0, 3.0 < z < 6.0, M is one or more selected from Mn, P, Cr, Sc, Ti, V, Cr, Co, Ni, Cu, Zn, and Ga, and

[0013] the crystal structure of the Na-containing composite oxide belongs to the space group Pn21a.

[0014] <Solution 2>

[0015] The positive electrode active material for a sodium ion battery according to Solution 1,

[0016] The chemical composition of the Na-containing composite oxide is represented by the following formula:

[0017] Na x Fe y Mn 1-y O z

[0018] where 3.0 < x < 6.0, 0.5 < y ≤ 1.0, 3.0 < z < 4.5.

[0019] <Solution 3>

[0020] A method for manufacturing a positive electrode active material for a sodium ion battery, comprising the following steps:

[0021] Obtaining a mixture containing a Na-containing oxide and an Fe-containing oxide; and

[0022] Obtaining a Na-containing composite oxide by imparting energy to the mixture,

[0023] The ratio of the Na-containing oxide and the Fe-containing oxide in the mixture is adjusted such that the Na-containing composite oxide has a chemical composition represented by the following formula:

[0024] Na x Fe y M 1-y O z

[0025] where 3.0 < x < 9.0, 0 < y ≤ 1.0, 3.0 < z < 6.0, and M is one or more selected from Mn, P, Cr, Sc, Ti, V, Cr, Co, Ni, Cu, Zn, and Ga,

[0026] The energy imparted to the mixture is adjusted such that the Na-containing composite oxide has a crystal structure belonging to the space group Pn21a.

[0027] <Solution 4>

[0028] The method for manufacturing a positive electrode active material for a sodium ion battery according to Solution 3, comprising the following steps:

[0029] Obtaining the Na-containing composite oxide by imparting mechanical energy through mechanical grinding of the mixture,

[0030] After the mechanical grinding, firing of the Na-containing composite oxide is not performed.

[0031] <Solution 5>

[0032] A sodium ion battery having a positive electrode active material layer, an electrolyte layer, and a negative electrode active material layer,

[0033] The positive electrode active material layer contains the positive electrode active material for a sodium ion battery according to Scheme 1 or 2.

[0034] The positive electrode active material for a sodium ion battery of the present disclosure has a high capacity. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 An example of a method for manufacturing a positive electrode active material for a sodium ion battery is shown.

[0036] Figure 2 An example of the configuration of a sodium ion battery is schematically shown.

[0037] DESCRIPTION OF REFERENCE NUMERALS

[0038] 100 Battery

[0039] 10 Positive electrode active material layer

[0040] 20 Electrolyte layer

[0041] 30 Negative electrode active material layer

[0042] 40 Positive electrode current collector

[0043] 50 Negative electrode current collector DETAILED DESCRIPTION

[0044] Hereinafter, a positive electrode active material for a sodium ion battery of the present disclosure, a method for manufacturing the same, and an embodiment of a sodium ion battery will be described. However, the positive electrode active material for a sodium ion battery of the present disclosure, the method for manufacturing the same, and the sodium ion battery are not limited to the embodiments described below.

[0045] 1. Positive electrode active material for a sodium ion battery

[0046] The positive electrode active material for a sodium ion battery according to the embodiment contains a Na-containing composite oxide. The chemical composition of the Na-containing composite oxide is shown below. In addition, the Na-containing composite oxide has a crystal structure belonging to the space group Pn21a.

[0047] Na x Fe y M 1-y O z

[0048] Among them, 3.0 < x < 9.0, 0 < y ≤ 1.0, 3.0 < z < 6.0, and M is one or more selected from Mn, P, Cr, Sc, Ti, V, Cr, Co, Ni, Cu, Zn, and Ga.

[0049] 1.1 Chemical composition

[0050] In this embodiment, the Na-containing composite oxide has the characteristics of Na x Fe y M 1-y O z The chemical composition is indicated. M is selected from one or more of Mn, P, Cr, Sc, Ti, V, Cr, Co, Ni, Cu, Zn, and Ga. Especially when M is Mn, higher capacity is easily ensured.

[0051] In this embodiment, x is greater than 3.0 and less than 9.0, and can be 3.5 or greater, 4.0 or greater, or 4.5 or greater, or less than 8.5, less than 8.0, less than 7.5, less than 7.0, less than 6.5, less than 6.0, less than 6.0, or less than 5.5. In particular, when x is greater than 3.0 and less than 6.0, it is easy to ensure higher capacity.

[0052] In this embodiment, y is greater than 0 and less than 1.0, and can be greater than 0.1, greater than 0.2, greater than 0.3, greater than 0.4, greater than 0.5, greater than 0.5, greater than 0.6, greater than 0.7, greater than 0.8, or greater than 0.9. In this embodiment, even when the transition metal contained in the Na-containing composite oxide is mainly Fe, high capacity can be ensured as a positive electrode active material for sodium-ion batteries. That is, y can also be greater than 0.5 and less than 1.0.

[0053] In this embodiment, z is greater than 3.0 and less than 6.0, and can be greater than 3.3, greater than 3.5, or greater than 3.7, or less than 5.7, less than 5.5, less than 5.3, less than 5.0, less than 4.7, less than 4.5, less than 4.5, or less than 4.3.

[0054] In one embodiment, the transition metal contained in the Na-containing complex oxide may be predominantly Fe, and optionally include Mn. For example, in one embodiment, the Na-containing complex oxide may also have the chemical composition shown below.

[0055] Na x Fe y Mn 1-y O z

[0056] Among them, 3.0 <x<6.0,0.5<y≤1.0,3.0<z<4.5。

[0057] 1.2 Crystal Structure

[0058] In this embodiment, the Na-containing composite oxide has a crystal structure belonging to space group Pn21a. By possessing a specific chemical composition and crystal structure, the Na-containing composite oxide exhibits high capacity. This sodium-containing composite oxide may also have other crystal structures besides those belonging to space group Pn21a. Alternatively, the sodium-containing composite oxide may only have a crystal structure belonging to space group Pn21a (only diffraction peaks from the crystal structure belonging to space group Pn21a are confirmed in the X-ray diffraction pattern). For example, in the X-ray diffraction pattern obtained by X-ray diffraction measurement of this sodium-containing composite oxide using radiation light with a wavelength of 0.0775 nm, at least two peaks, a first peak and a second peak, can be confirmed, and the first peak may be present at q = 0.23 ± 0.02 Å. -1 The second peak can exist at q = 0.38 ± 0.02 Å. -1 .

[0059] 1.3 Other

[0060] As described above, the positive electrode active material for sodium-ion batteries according to the embodiments achieves high capacity by including a Na-containing composite oxide having the specific chemical composition and crystal structure described above. One embodiment of the positive electrode active material may consist solely of the aforementioned Na-containing composite oxide, or it may contain other components (other components) besides the aforementioned Na-containing composite oxide. From the viewpoint of further improving the above-mentioned effects, the proportion of other components in the overall positive electrode active material can be small. For example, based on 100% by mass of the overall positive electrode active material, the content of the aforementioned Na-containing composite 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.

[0061] The positive electrode active material for sodium-ion batteries involved in the embodiments can be solid particles, hollow particles, or particles with pores. The size of the positive electrode active material is not particularly limited. For example, the average particle size of the positive electrode active material can be 0.1 μm or more and 10 μm or less, 1.0 μm or more and 8.0 μm or less, or 2.0 μm or more and 6.0 μm or less. Furthermore, the average particle size mentioned in this application refers to the particle size (D50, median particle size) when the cumulative value in the particle size distribution on a volume basis determined by laser diffraction and scattering method is 50%.

[0062] 2. Manufacturing method of positive electrode active material for sodium-ion batteries

[0063] The Na-containing composite oxide having the above chemical composition and crystal structure can be manufactured, for example, by the following method. As Figure 1 shown, a method for manufacturing a positive electrode active material for a sodium ion battery according to an embodiment includes the following steps:

[0064] Step S1: obtaining a mixture containing a Na-containing oxide and an Fe-containing oxide; and

[0065] Step S2: obtaining a Na-containing composite oxide by applying energy to the mixture.

[0066] Here, in the present embodiment, the ratio of the Na-containing oxide and the Fe-containing oxide in the mixture is adjusted so that the Na-containing composite oxide has the chemical composition shown below, and the energy applied to the mixture is adjusted so that the Na-containing composite oxide has a crystal structure belonging to the space group Pn21a.

[0067] Na x Fe y M 1-y O z

[0068] where 3.0 < x < 9.0, 0 < y ≤ 1.0, 3.0 < z < 6.0, and M is one or more selected from Mn, P, Cr, Sc, Ti, V, Cr, Co, Ni, Cu, Zn, and Ga.

[0069] 2.1 Step S1: Preparation of the mixture

[0070] In Step S1, a mixture is obtained by mixing a Na-containing oxide and an Fe-containing oxide at the above-mentioned predetermined ratio.

[0071] The Na-containing oxide constituting the mixture may, for example, be sodium oxide (Na2O). Alternatively, the Na-containing oxide constituting the mixture may include a composite oxide of Na and M. For example, the Na-containing oxide constituting the mixture may include NaMnO2. In the present embodiment, for example, sodium oxide (Na2O) may be used as the Na-containing oxide to adjust the ratio of sodium oxide (Na2O) in the mixture so that x in the Na-containing composite oxide obtained after Step S2 is greater than 3.0 and less than 9.0, y is greater than 0 and 1.0 or less, and z is greater than 3.0 and less than 6.0.

[0072] The Fe-containing oxide constituting the mixture can be, for example, iron oxide (Fe₂O₃). Alternatively, the Fe-containing oxide constituting the mixture can also be a composite oxide of Na and Fe. For example, the Fe-containing oxide constituting the mixture can also be NaFeO₂. In this embodiment, NaFeO₂ can be used as the Fe-containing oxide, and the ratio of NaFeO₂ in the mixture can be adjusted so that x is greater than 3.0 and less than 9.0, y is greater than 0 and less than 1.0, and z is greater than 3.0 and less than 6.0 in the Na-containing composite oxide obtained after step S2.

[0073] The mixture may also contain oxides containing M in addition to Na-containing oxides and Fe-containing oxides. The M-containing oxide may be, for example, an oxide composed solely of M and O, such as manganese oxide (Mn₂O₃). Alternatively, when sodium oxide (Na₂O) is used as the Na-containing oxide, the M-containing oxide may be a composite oxide of Na and M. For example, the M-containing oxide constituting the mixture may be NaMnO₂. In this embodiment, NaMnO₂ may be used as the M-containing oxide, and the ratio of NaMnO₂ in the mixture may be adjusted so that x is greater than 3.0 and less than 9.0, y is greater than 0 and less than 1.0, and z is greater than 3.0 and less than 6.0 in the Na-containing composite oxide obtained after step S2.

[0074] The mixing ratio of Na-containing oxides and Fe-containing oxides only needs to be adjusted to achieve the above-mentioned chemical composition of the Na-containing composite oxide obtained after process S2. The values ​​of x, y, and z in this chemical composition are as described above and will not be explained here.

[0075] The crystal structures of the Na-containing oxide, Fe-containing oxide, and M-containing oxide constituting the mixture are not particularly limited. In this embodiment, by imparting energy to the mixture in step S2 (described later), a substance with a crystal structure belonging to space group Pn21a is obtained as the final Na-containing composite oxide. That is, before step S2, the crystal structures of the Na-containing oxide, Fe-containing oxide, and M-containing oxide constituting the mixture may belong to a space group different from space group Pn21a.

[0076] 2.2 Process S2: Imparting Energy

[0077] In step S2, by imparting energy to the mixture obtained in step S1, a Na-containing composite oxide with a crystal structure belonging to space group Pn21a is obtained.

[0078] The method of imparting energy to the mixture is not particularly limited, as long as the predetermined crystal structure described above can be obtained. In one embodiment, the mixture may also be sintered. When sintering the mixture, there are no particular limitations on the sintering temperature, sintering time, and sintering atmosphere, as long as the specific crystal structure described above can be obtained. For example, the sintering temperature may be 500°C or higher and 700°C or lower, the sintering time may be 10 hours or higher and 48 hours or lower, and the sintering atmosphere may be an oxygen-containing atmosphere or an inactive gas atmosphere. Alternatively, in one embodiment, the mixture may also be mechanically ground. When mechanically grinding the mixture, there are no particular limitations on the rotation speed, rotation time, and number of repetitions, as long as the specific crystal structure described above can be obtained. An example of the conditions for mechanical grinding is shown in the embodiments described later.

[0079] According to the inventors' novel insights, when the crystallinity of the Na-containing composite oxide obtained after step S2 is moderately low, its capacity as a positive electrode active material can be easily further increased. For example, the manufacturing method according to this embodiment includes a step of obtaining the Na-containing composite oxide by imparting mechanical energy through mechanical milling of the mixture, and preferably, the Na-containing composite oxide is not calcined after the mechanical milling. It is believed that by mechanically milling the mixture and omitting the subsequent calcination, the mixture can be moderately crystallized, and the resulting Na-containing composite oxide has a crystal structure belonging to space group Pn21a, which allows for moderate randomization of the atomic arrangement in the crystal structure, creating a state in which Na ions can easily move. In addition, it is believed that mechanical milling can improve the conductivity of the Na-containing composite oxide, and the redox reactions of both Fe and O can be effectively utilized. That is, it is believed that by using the mechanically milled Na-containing composite oxide directly as a positive electrode active material without calcination, a higher capacity can be obtained.

[0080] 3. Sodium-ion batteries

[0081] Figure 2 This illustration schematically shows the configuration of a sodium-ion battery according to one embodiment. For example... Figure 2 As shown, a sodium-ion battery 100 according to one embodiment has a positive electrode active material layer 10, an electrolyte layer 20, and a negative electrode active material layer 30. Here, the positive electrode active material layer 10 contains the positive electrode active material for sodium-ion batteries according to the above embodiment.

[0082] 3.1 Positive Electrode Active Material Layer

[0083] The positive electrode active material layer 10 includes at least the positive electrode active material described in the above embodiments, and may optionally include an electrolyte, conductive additives, and a binder. Furthermore, the positive electrode active material layer 10 may also contain various other additives. The respective contents of the active material, electrolyte, conductive additives, and binder in the positive electrode active material layer 10 can be appropriately determined according to the target battery performance. For example, with the total positive electrode active material layer 10 (solid component as a whole) as 100% by mass, the content of the 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.

[0084] 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 known solid electrolyte 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. 12 Oxides of Na₂O-11Al₂O₃, etc.; NaBH₄, NaB 10 H 10 NaCB9H 10 NaCB 11 H 12 NaB 12 Cl 12 Such hydrides or borides; Na3PS4, Na3SbS4, Na 2.88 Sb 0.88 W 0.12The electrolyte is selected from at least one of the following: sulfides such as S4; fluorides such as NaPF6 and NaBF4. The solid electrolyte may be in particulate form. A single solid electrolyte may be used alone, or two or more may be used in combination. The electrolyte may contain sodium ions as carrier ions. The electrolyte may be an aqueous electrolyte or a non-aqueous electrolyte. The composition of the electrolyte may be the same as that known for the composition of the electrolyte used in the battery. For example, an electrolyte in which sodium salts are dissolved at a predetermined concentration in a carbonate-based solvent may be used. Examples of carbonate-based solvents include, for example, fluoroethylene carbonate (FEC), ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC). Examples of sodium salts include, for example, NaPF6, NaClO4, NaBF4, NaFSI, NaTFSI, and NaBETI.

[0085] Examples of conductive additives that can be included in the positive electrode active material layer 10 include carbon materials such as fumed carbon fiber (VGCF), acetylene black (AB), Ketjen black (KB), carbon nanotubes (CNT), and carbon nanofibers (CNF); and metallic materials such as nickel, aluminum, and stainless steel. The conductive additives can be in particulate or fibrous form, and their size is not particularly limited. Only one conductive additive can be used alone, or two or more can be used in combination.

[0086] Examples of adhesives that can be included in the positive electrode active material layer 10 include butadiene rubber (BR) based adhesives, butene rubber (IIR) based adhesives, acrylate butadiene rubber (ABR) based adhesives, styrene butadiene rubber (SBR) based adhesives, polyvinylidene fluoride (PVdF) based adhesives, polytetrafluoroethylene (PTFE) based adhesives, and polyimide (PI) based adhesives. Only one type of adhesive can be used alone, or two or more types can be used in combination.

[0087] 3.2 Electrolyte Layer

[0088] The electrolyte layer 20 contains at least an electrolyte. When the sodium-ion battery 100 is a solid-state battery (a battery containing a solid electrolyte, which may partially contain a liquid electrolyte or be a completely solid-state battery without a liquid electrolyte), the electrolyte layer 20 contains a solid electrolyte and may optionally contain a binder, etc. In this case, the content of the solid electrolyte and binder, etc., in the electrolyte layer 20 is not particularly limited. On the other hand, when the battery 100 is an electrolyte battery, the electrolyte layer 20 contains an electrolyte and may also have a separator, etc., for retaining the electrolyte and preventing contact between the positive electrode active material layer 10 and the negative electrode active material layer 30. The thickness of the electrolyte layer 20 is not particularly limited; for example, it may be 0.1 μm or more, or 1 μm or more, or 2 mm or less, or 1 mm or less.

[0089] The electrolyte contained in the electrolyte layer 20 can be appropriately selected from electrolytes exemplified as those that can be included in the positive electrode active material layer 10. Similarly, the adhesive that can be included in the electrolyte layer 20 can be appropriately selected from adhesives exemplified as those that can be included in the positive electrode active material layer 10. Only one type of electrolyte and adhesive can be used individually, or two or more types can be used in combination. The separator can be a separator commonly used in batteries, such as separators made of resins like polyethylene (PE), polypropylene (PP), polyester, and polyamide. The separator can be a single-layer structure or a multi-layer structure. Examples of multi-layer separators include a two-layer PE / PP separator, or a three-layer PP / PE / PP or PE / PP / PE separator. The separator can also be made of nonwoven fabrics such as cellulose nonwoven fabric, resin nonwoven fabric, or glass fiber nonwoven fabric.

[0090] 3.3 Negative Electrode Active Material Layer

[0091] The negative electrode active material layer 30 contains at least a negative electrode active material, and may optionally contain an electrolyte, conductive additives, and a binder. Furthermore, the negative electrode active material layer 30 may also contain various other additives. The respective contents of the negative electrode active material, electrolyte, conductive additives, and binder in the negative electrode active material layer 30 can be appropriately determined according to the target battery performance. For example, with the total solid component of the negative electrode active material layer 30 as 100% by mass, the content of the negative 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 negative electrode active material layer 30 is not particularly limited; for example, it can be a sheet-like negative electrode active material layer with a generally planar surface. The thickness of the negative electrode active material layer 30 is not particularly limited; for example, it can be 0.1 μm or more, 1 μm or more, or less than 2 mm or less than 1 mm.

[0092] As the negative electrode active material, various materials whose potential for intercalation and deintercalation charge compensation ions (charge / discharge potential) is lower than that of the aforementioned positive electrode active material can be used. The negative electrode active material can be, for example, an inorganic negative electrode active material such as metallic sodium, a negative electrode active material composed of organic compounds, or a combination thereof. Only one negative electrode active material can be used alone, or two or more can be used in combination. The shape of the negative electrode active material can be any shape typical of negative electrode active materials used in batteries. For example, the negative electrode active material can also be in particle form. These particles can be primary particles or secondary particles formed by the aggregation of multiple primary particles. The average particle size (D50) of the negative electrode active material particles can be, for example, 1 nm or more, 5 nm or more, or 10 nm or more, and can be 500 μm or less, 100 μm or less, 50 μm or less, or 30 μm or less. Alternatively, the negative electrode active material can also be in sheet form (foil, film) such as sodium foil. That is, the negative electrode active material layer 30 can also be composed of sheets of negative electrode active material.

[0093] Examples of electrolytes that can be included in the negative electrode active material layer 30 include the solid electrolytes, electrolyte solutions, or combinations thereof described above. Examples of conductive additives that can be included in the negative electrode active material layer 30 include the carbon materials or the metal materials described above. Examples of binders that can be included in the negative electrode active material layer 30 include, for example, binders exemplified as those included in the positive electrode active material layer 10 described above. Only one electrolyte and one binder may be used individually, or two or more may be used in combination.

[0094] 3.4 Other Structures

[0095] like Figure 2 As shown, the sodium-ion battery 100 may also 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 current collector commonly used as a positive electrode current collector in a battery. Furthermore, as... Figure 2 As shown, the sodium-ion battery 100 may also 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 current collector typical of a battery's negative electrode current collector. Furthermore, based on the above structure, the sodium-ion battery 100 may also include structures obvious to a battery, such as tabs or terminals. The sodium-ion battery 100 may also be a battery in which the above structures are housed within an outer casing. The outer casing can be any known outer casing for batteries. Additionally, multiple sodium-ion batteries 100 may be arbitrarily electrically connected and arbitrarily stacked to form a battery pack. In this case, the battery pack may also be housed inside a known battery casing. Examples of shapes for the sodium-ion battery 100 include coin-shaped, laminated, cylindrical, and square types. The sodium-ion battery 100 may also be a rechargeable battery.

[0096] 4. Battery manufacturing methods

[0097] Sodium-ion battery 100 can be manufactured using known methods other than the specific positive electrode active material described above. For example, it can be manufactured as described below. However, the manufacturing method of sodium-ion battery 100 is not limited to the following methods; for example, the layers can also be formed by dry forming or the like.

[0098] (1) A slurry for the positive electrode layer is obtained by dispersing the positive electrode active material, which constitutes the positive electrode active material layer, in a solvent. There is no particular limitation on the solvent used in this case, and water or various organic solvents can be used. The slurry for the positive electrode layer is applied to the surface of the positive electrode current collector using a doctor blade or the like, and then dried, thereby forming a positive electrode active material layer on the surface of the positive electrode current collector, and thus producing a positive electrode.

[0099] (2) A slurry for the negative electrode layer is obtained by dispersing the negative electrode active material, which constitutes the negative electrode active material layer, in a solvent. There are no particular limitations on the solvent used in this case; water or various organic solvents can be used. The slurry for the negative electrode layer is applied to the surface of the negative electrode current collector using a doctor blade or the like, and then dried, thereby forming a negative electrode active material layer on the surface of the negative electrode current collector, thus producing a negative electrode. Alternatively, a sheet or foil of metallic active material can also be used as the negative electrode active material layer.

[0100] (3) Stack the layers by sandwiching the electrolyte layer (solid electrolyte layer or membrane) between the negative electrode and the positive electrode to obtain a laminate having a negative current collector, a negative active material layer, an electrolyte layer, a positive active material layer and a positive current collector in sequence. Install terminals and other components on the laminate as needed.

[0101] (4) The laminate is housed in the battery casing. In the case of an electrolyte battery, the battery casing is filled with electrolyte, the laminate is immersed in the electrolyte, and the laminate is sealed in the battery casing, thereby producing a secondary battery. Furthermore, in the case of an electrolyte battery, the electrolyte can also be included in the negative electrode active material layer, the separator, and the positive electrode active material layer in the above (3) stage.

[0102]

Example

[0103] As described above, one embodiment of the positive electrode active material for sodium-ion batteries has been explained, but the technology of this disclosure can be modified in various ways beyond the above embodiment without departing from its spirit. Hereinafter, embodiments are shown to further illustrate the technology of this disclosure in detail, but the technology of this disclosure is not limited to the following embodiments.

[0104] 1. Manufacturing of positive electrode active material

[0105] 1.1 Example 1-1

[0106] In stoichiometric proportions, it becomes Na₄FeO 3.5 Na₂O and NaFeO₂ were mixed in a manner consistent with their chemical composition to obtain a 0.4 g mixture (powder). This mixture, along with 10 g of 3 mm φ zirconia balls, was placed in a 45 mL zirconia container and sealed. The sealed zirconia container was then mounted on a planetary ball mill (Fritsch PL-7) and mechanically ground at 600 rpm for 50 hours to obtain a Na-containing composite oxide as the positive electrode active material. The chemical composition of this Na-containing composite oxide remained unchanged compared to before mechanical grinding. Furthermore, the crystal structure of this Na-containing composite oxide was confirmed by X-ray diffraction, and the crystal structure belonged to space group Pn₂₁a.

[0107] 1.2 Examples 1-2

[0108] Na₂O and NaFeO₂ were mixed in a stoichiometric ratio to obtain 0.4 g of a mixture (powder). This mixture was mechanically ground under the same conditions as in Examples 1-1 to obtain a Na-containing composite oxide, which serves as the positive electrode active material. The chemical composition of this Na-containing composite oxide remained unchanged compared to before mechanical grinding. Furthermore, the crystal structure of this Na-containing composite oxide was confirmed by X-ray diffraction, and the crystal structure belonged to space group Pn₂₁a.

[0109] 1.3 Examples 1-3

[0110] Na₂O and NaFeO₂ were mixed in a stoichiometric ratio to obtain Na₅FeO₄, yielding 0.4 g of a mixture (powder). This mixture was shaped into granules. The granules were placed in a carbon crucible and calcined at 600°C for 20 hours under an Ar atmosphere to obtain a Na-containing composite oxide, which serves as the positive electrode active material. The chemical composition of this Na-containing composite oxide remained unchanged compared to that before mechanical grinding. Furthermore, X-ray diffraction confirmed the crystal structure of this Na-containing composite oxide, which belongs to space group Pn₂₁a.

[0111] 1.4 Comparative Example 1-1

[0112] Na₂O and NaFeO₂ were mixed in a stoichiometric ratio to obtain 0.4 g of a mixture (powder). This mixture was mechanically ground under the same conditions as in Examples 1-1 to obtain a Na-containing composite oxide, which serves as the positive electrode active material. The chemical composition of this Na-containing composite oxide remained unchanged compared to before mechanical grinding. Furthermore, the crystal structure of this Na-containing composite oxide was confirmed by X-ray diffraction, and the crystal structure belonged to space group Pn₂₁a.

[0113] 1.5 Comparative Examples 1-2

[0114] Na₂O and NaFeO₂ were mixed in a stoichiometric ratio to obtain 0.4 g of a mixture (powder). This mixture was mechanically ground under the same conditions as in Examples 1-1 to obtain a Na-containing composite oxide, which serves as the positive electrode active material. The chemical composition of this Na-containing composite oxide remained unchanged compared to before mechanical grinding. Furthermore, the crystal structure of this Na-containing composite oxide was confirmed by X-ray diffraction, and the crystal structure belonged to space group Pn₂₁a.

[0115] 1.6 Example 2-1

[0116] In stoichiometry, it becomes Na5Fe 0.75 Mn 0.25 Na₂O, NaFeO₂, and NaMnO₂ were mixed in a manner consistent with the chemical composition of O₄ to obtain 0.4 g of a mixture (powder). This mixture was mechanically ground under the same conditions as in Examples 1-1 to obtain a Na-containing composite oxide as the positive electrode active material. The chemical composition of this Na-containing composite oxide remained unchanged compared to before mechanical grinding. Furthermore, the crystal structure of this Na-containing composite oxide was confirmed by X-ray diffraction, and the crystal structure belonged to space group Pn₂₁a.

[0117] 1.7 Example 2-2

[0118] In stoichiometry, it becomes Na5Fe 0.5 Mn 0.5 Na₂O, NaFeO₂, and NaMnO₂ were mixed in a manner consistent with the chemical composition of O₄ to obtain 0.4 g of a mixture (powder). This mixture was mechanically ground under the same conditions as in Examples 1-1 to obtain a Na-containing composite oxide as the positive electrode active material. The chemical composition of this Na-containing composite oxide remained unchanged compared to before mechanical grinding. Furthermore, the crystal structure of this Na-containing composite oxide was confirmed by X-ray diffraction, and the crystal structure belonged to space group Pn₂₁a.

[0119] 1.8 Comparative Example 2-1

[0120] Na₂O and NaMnO₂ were mixed in a stoichiometric ratio to obtain 0.4 g of a mixture (powder). This mixture was mechanically ground under the same conditions as in Examples 1-1 to obtain a Na-containing composite oxide, which serves as the positive electrode active material. The chemical composition of this Na-containing composite oxide remained unchanged compared to before mechanical grinding. Furthermore, the crystal structure of this Na-containing composite oxide was confirmed by X-ray diffraction, and the crystal structure belonged to space group Pn₂₁a.

[0121] 2. Manufacturing of the positive electrode

[0122] Weigh the above-mentioned positive electrode active material and carbon-based conductive additive, making their mass ratio 75:25 (positive electrode active material: conductive additive). Place the weighed positive electrode active material and conductive additive, along with 10g of 0.8mm diameter zirconia balls, into a 45mL zirconia container and seal it. Install the sealed zirconia container on a planetary ball mill (Fritsch PL-7) and mix at 300rpm for 30 minutes to obtain the positive electrode mixture. Furthermore, the chemical composition and crystal structure of the positive electrode active material in the mixed positive electrode mixture are substantially the same as those of the positive electrode active material before mixing. After mixing the obtained positive electrode mixture with PTFE, press it onto an Al mesh to obtain the positive electrode.

[0123] 3. Battery manufacturing and evaluation

[0124] An evaluation battery was fabricated using the aforementioned positive electrode, sodium metal negative electrode, electrolyte (electrolyte: 1M NaPF6, solvent: PC), and glass fiber separator. The resulting battery was charged and discharged in a constant current mode at a current density of 10 mA / g, and the initial discharge capacity and average voltage were measured.

[0125] 4. Evaluation Results

[0126] Table 1 below shows the manufacturing conditions and chemical composition of each positive electrode active material, as well as the discharge capacity and average voltage of each battery.

[0127] Table 1

[0128]

[0129] As shown in Table 1, it can be said that Na-containing composite oxides that meet the following conditions (A) and (B) have high capacity as positive electrode active materials for sodium-ion batteries.

[0130] (A) The Na-containing composite oxide has the following chemical composition.

[0131] Nax Fe y M 1-y O z

[0132] Among them, 3.0 < x < 9.0, 0 < y ≤ 1.0, 3.0 < z < 6.0, and M is one or more selected from Mn, P, Cr, Sc, Ti, V, Cr, Co, Ni, Cu, Zn, and Ga.

[0133] (B) The Na-containing composite oxide has a crystal structure belonging to the space group Pn-21a.

[0134] Furthermore, in the above embodiments, the case where the element M constituting the Na-containing composite oxide is not included or the case where Mn is used as the element M is illustrated, but the type of the element M is not limited thereto. It is considered that a high capacity as a positive electrode active material can be ensured when the element M is one or more selected from Mn, P, Cr, Sc, Ti, V, Cr, Co, Ni, Cu, Zn, and Ga.

Claims

1. A positive electrode active material for a sodium-ion battery, containing a Na-containing composite oxide, The chemical composition of the Na-containing composite oxide is represented by the following formula: And x Fe y I 1-y Oh z in, 3.0 < x < 9.0, 0 < y ≤ 1.0, 3.0 < z < 6.0, M is one or more selected from Mn, P, Cr, Sc, Ti, V, Cr, Co, Ni, Cu, Zn, and Ga, and The crystal structure of the Na-containing composite oxide belongs to the space group Pn21a.

2. The positive electrode active material for a sodium-ion battery according to claim 1, The chemical composition of the Na-containing composite oxide is represented by the following formula: So x Feb y Mr 1-y O z in, 3.0 < x < 6.0, 0.5 < y ≤ 1.0, 3.0 < z < 4.

5.

3. A method for manufacturing a positive electrode active material for a sodium-ion battery, comprising the following steps: Obtaining a mixture containing a Na-containing oxide and an Fe-containing oxide; and Obtaining a Na-containing composite oxide by imparting energy to the mixture, The ratio of the Na-containing oxide and the Fe-containing oxide in the mixture is adjusted so that the Na-containing composite oxide has a chemical composition represented by the following formula: And x Fe y I 1-y Oh z in, 3.0 < x < 9.0, 0 < y ≤ 1.0, 3.0 < z < 6.0, M is one or more selected from Mn, P, Cr, Sc, Ti, V, Cr, Co, Ni, Cu, Zn, and Ga, The energy imparted to the mixture is adjusted so that the Na-containing composite oxide has a crystal structure belonging to the space group Pn21a.

4. The method for manufacturing a positive electrode active material for a sodium-ion battery according to claim 3, comprising the following steps: Obtaining the Na-containing composite oxide by imparting mechanical energy through mechanical grinding of the mixture, After the mechanical grinding, firing of the Na-containing composite oxide is not performed.

5. A sodium-ion battery having a positive electrode active material layer, an electrolyte layer, and a negative electrode active material layer, The positive electrode active material layer contains the positive electrode active material for a sodium-ion battery according to claim 1 or 2.

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