Positive electrode active material for sodium secondary batteries, positive electrode for sodium secondary batteries, and sodium secondary batteries

Abstract

To provide a positive electrode active material for sodium secondary batteries that exhibits high capacity and high durability, with an initial charge capacity greater than the initial discharge capacity. [Solution] Formula Na x Mn 1-y-z M1 y M2 z O2(1) [In the formula, M1 represents Fe or Ni. M2 represents Co, Ti, Mg, Cu, or Al. x represents 0.8 to 4, y represents 0.05 to 0.35, and z represents 0 to 0.1.] A positive electrode active material for sodium secondary batteries comprising a composite metal oxide represented by [formula], wherein the initial charge capacity is greater than the initial discharge capacity.

Description

[Technical Field]

[0001] The present invention relates to sodium secondary batteries, and more particularly to positive electrode active materials for sodium secondary batteries, including transition metal oxides. [Background technology]

[0002] Sodium-based secondary batteries can be constructed using readily available and inexpensive materials, and their practical application is expected to enable the mass supply of large-scale power sources.

[0003] Patent Document 1 describes a positive electrode active material for a sodium secondary battery, comprising a sodium-containing composite metal oxide with manganese as the main component. In the examples, composite metal oxides such as systems combining manganese with nickel, cobalt, titanium, magnesium, or copper, and systems combining manganese and iron are used. The positive electrode active material of Patent Document 1 has the characteristic of being able to take in sodium ions during discharge in excess of the amount released during charging, and as a result the initial discharge capacity becomes larger than the initial charge capacity, and a large discharge capacity is achieved. This makes it possible to obtain a capacity exceeding the theoretical capacity and realize a sodium secondary battery with an energy density equivalent to or greater than that of a lithium secondary battery.

[0004] Patent Document 2 describes a precursor for a positive electrode material of a sodium-ion battery, which is composed of core-shell structure particles consisting of a core and a shell, where the core is formed from a hydroxide containing nickel, iron, and manganese, and the shell is formed from a hydroxide containing nickel or iron and manganese. The positive electrode material obtained from this precursor has an O3 phase structure for the core and a P2 phase structure for the shell. The O3 phase has a large capacity for sodium ions and exhibits a high specific capacity, but it is characterized by large structural changes with repeated charging and discharging, which tends to reduce cycle stability. In contrast, the P2 phase has excellent structural stability and good sodium ion diffusion, and therefore has excellent charge-discharge cycle characteristics, but generally its specific capacity is smaller than that of the O3 phase. In this document, by using a core-shell structure in which the inside of the particle is the O3 phase and the surface is the P2 phase, it is possible to achieve both high capacity from the O3 phase and high structural stability from the P2 phase. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2025-79325 [Patent Document 2] Japanese Patent Publication No. 2025-503978 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] The positive electrode active material described in Patent Document 1 requires the replenishment of sodium ions to achieve maximum discharge capacity, which presents a problem in terms of the high workload required for practical use. Furthermore, the positive electrode material precursor described in Patent Document 2 requires the separate formation of the core and shell portions, which also presents a high workload, and has the problem of insufficient capacity due to its low manganese ratio.

[0007] The present invention solves the aforementioned problems and aims to provide a positive electrode active material for sodium secondary batteries that exhibits high capacity and high durability, with an initial charge capacity greater than the initial discharge capacity. [Means for solving the problem]

[0008] The present invention provides the following embodiments. [1] Formula Na x Mn 1-y-z M1 y M2zO2(1) [In the formula, M1 represents Fe or Ni. M2 represents Co, Ti, Mg, Cu, or Al. x represents 0.8 to 4, preferably 0.90 to 2.6, more preferably 0.96 to 1.8, even more preferably 1.0 to 1.2, and especially preferably about 1.0. y represents 0.05 to 0.35, preferably 0.11 to 0.29, more preferably 0.15 to 0.23, and especially preferably about 0.20. z represents 0 to 0.1, preferably 0.02 to 0.08, more preferably 0.04 to 0.06, and especially preferably about 0.05.] A positive electrode active material for sodium secondary batteries comprising a composite metal oxide represented by [formula], wherein the initial charge capacity is greater than the initial discharge capacity.

[0009] [2] The positive electrode active material for a sodium secondary battery according to embodiment 1, wherein the composite metal oxide has an O3 phase structure.

[0010] [3] The positive electrode active material for a sodium secondary battery according to embodiment 2, wherein the composite metal oxide further has an O'3 phase structure.

[0011] [4] A positive electrode active material for a sodium secondary battery according to embodiment 2 or 3, wherein the composite metal oxide further has a P2 phase structure and a P'2 phase structure.

[0012] [5] A positive electrode active material for a sodium secondary battery according to any of embodiments 2 to 4, wherein different types of phase structures are substantially uniformly distributed and coexist within it.

[0013] [6] A positive electrode active material for a sodium secondary battery according to any of embodiments 1 to 5, wherein the composite metal oxide comprises a product calcined in an inert gas-containing atmosphere of a mixture of a precipitate from an aqueous solution of the metal to be oxidized and a sodium compound.

[0014] [7] The positive electrode active material for a sodium secondary battery according to any one of aspects 1 to 5, wherein the composite metal oxide comprises a product obtained by calcining a mixture of a precipitate from an aqueous solution of a metal to be oxidized and a sodium compound in an inert gas- and oxygen-containing atmosphere.

[0015] [8] The positive electrode active material for a sodium secondary battery according to any one of aspects 1 to 7, which has a coating layer containing at least one element selected from the group consisting of carbon, nickel, iron and phosphorus on a surface thereof.

[0016] [9] A positive electrode for a sodium secondary battery, comprising the positive electrode active material for a sodium secondary battery according to any one of aspects 1 to 8.

[0017]

[10] Formula Mn 1-y-z M1 y M2 z (2) [In the formula, M1 represents Fe or Ni; M2 represents Co, Ti, Mg, Cu or Al; y represents 0.05 to 0.35, preferably 0.11 to 0.29, more preferably 0.15 to 0.23, particularly preferably about 0.20, and z represents 0 to 0.1, preferably 0.02 to 0.08, more preferably 0.04 to 0.06, particularly preferably about 0.05.]] obtaining a precipitate by contacting an aqueous solution containing each metal of the composition represented by the above formula with a precipitant in the presence of oxygen, and calcining a mixture of the precipitate and a sodium compound in an inert gas-containing atmosphere, A method for producing a positive electrode active material for a sodium secondary battery, comprising the steps of:

[0018]

[11] The method for producing a positive electrode active material for a sodium secondary battery according to aspect 10, wherein the atmosphere is an inert gas- and oxygen-containing atmosphere.

[0019]

[12] A method for producing a positive electrode for a sodium secondary battery, comprising molding an electrode mixture containing the positive electrode active material for a sodium secondary battery obtained by the production method according to aspect 11 into the shape of a positive electrode.

[0020]

[13] A sodium secondary battery having a positive electrode for a sodium secondary battery according to embodiment 9. [Effects of the Invention]

[0021] According to the present invention, it is possible to provide a positive electrode active material for sodium secondary batteries that exhibits high capacity and high durability, with an initial charge capacity greater than the initial discharge capacity.

[0022] One form of the positive electrode active material of the present invention is a layered sodium composite metal oxide mainly composed of manganese, in which an O3 phase structure and a P2 phase structure are substantially uniformly distributed and coexist within the same particle. Substantially uniform distribution and coexistence means a state of coexistence in which there are no clear boundaries between the different phase structures. Such a state of coexistence is intended to not include an internal structure in which clear boundaries exist between different phase structures as a result of intentionally forming multiple phase structures by means such as lamination and coating. Furthermore, the O3 phase structure and the O'3 phase structure can exist continuously as structural changes of the same system, and the same applies to the P2 phase structure and the P'2 phase structure. In the present invention, the coexistence of different phase structures mainly refers to the coexistence of O-system structures and P-system structures.

[0023] When the O3 phase structure and the P2 phase structure are substantially uniformly distributed and coexist within the same particle, the positive electrode active material of the present invention can achieve both high capacity and structural stability, and exhibit a characteristic in which the initial charge capacity is greater than the initial discharge capacity. [Brief explanation of the drawing]

[0024] [Figure 1] This is a schematic exploded view showing the configuration of a sodium secondary battery, which is one embodiment of the present invention. [Figure 2] This is the XRD spectrum of the composite metal oxide prepared in Experimental Example 4. [Figure 3] This is the XRD spectrum of the composite metal oxide prepared in Experimental Example 12. [Figure 4] This is the XRD spectrum of the composite metal oxide prepared in Experimental Example 28. [[Mode for Carrying Out the Invention]]

[0025] The scope of the present invention is not limited to the embodiments described herein, and various modifications can be made without departing from the spirit of the present invention. Further, when a plurality of upper limit values and lower limit values are described for a specific parameter, any upper limit value and lower limit value among these upper limit values and lower limit values can be combined to form a suitable numerical range.

[0026] <Positive Electrode Active Material> The positive electrode active material for a sodium secondary battery of the present invention is represented by the formula Na x Mn 1-y-z M1 y M2 z O2 (1) comprising a composite metal oxide represented by the formula. Formula (1) represents the chemical structure of the positive electrode active material before an initial charge is performed on the sodium secondary battery. In one preferred embodiment, the positive electrode active material consists of the composite metal oxide represented by formula (1). In formula (1), Na represents sodium. Mn represents manganese. M1 represents Fe or Ni. M2 represents Co, Ti, Mg, Cu or Al, preferably Ti, Mg or Al. Further, O represents oxygen.

[0027] The lower limit of the atomic ratio x of Na in the above formula may be 0.80, 0.90, 0.96 or 1.0, and the upper limit of the atomic ratio x may be 4.0, 2.6, 1.8 or 1.2. In one embodiment, the atomic ratio x of Na in the above formula may be 0.80 to 4.0, preferably 0.90 to 2.6, more preferably 0.96 to 1.8, still more preferably 1.0 to 1.2, and particularly preferably about 1.0.

[0028] If the atomic ratio x is less than 0.80, the amount of sodium ions released by the positive electrode active material during the initial charge may be insufficient compared to the amount of sodium ions that the positive electrode active material can absorb during the initial discharge, resulting in insufficient improvement in charge-discharge capacity. On the other hand, if x exceeds 4.0, the presence of excess sodium in the crystal structure of the composite metal oxide may reduce the stability of the layered structure or lead to the formation of subphases such as sodium compounds, resulting in a decrease in charge-discharge capacity and deterioration of cycle characteristics.

[0029] The lower limit of the atomic ratio y of M1 in the above formula may be 0.05, 0.11, or 0.15, and the upper limit of the atomic ratio y may be 0.35, 0.29, or 0.23. In one embodiment, the atomic ratio y of M1 in the above formula may be 0.05 to 0.35, preferably 0.11 to 0.29, more preferably 0.15 to 0.23, and particularly preferably about 0.20. Having the atomic ratio y within the above range improves the charge and discharge capacity of the positive electrode active material.

[0030] The lower limit of the atomic ratio z of M2 in the above formula may be 0, 0.02, or 0.04, and the upper limit of the atomic ratio z may be 0.1, 0.08, or 0.06. In one embodiment, the atomic ratio z of M2 in the above formula may be 0 to 0.1, preferably 0.02 to 0.08, more preferably 0.04 to 0.06, and particularly preferably about 0.05. Although M2 is not an essential component, its inclusion in the above amounts may improve the stability of the crystal structure of the composite metal oxide.

[0031] From the viewpoint of improving the charge-discharge capacity of the positive electrode active material, the lower limit of the atomic ratio 1-yz of Mn in the above formula may be 0.55, 0.65, or 0.7, and the upper limit of the atomic ratio 1-yz may be 0.95, 0.9, or 0.85. In one embodiment, the atomic ratio 1-yz of Mn in the above formula may be 0.55 to 0.95, preferably 0.65 to 0.9, more preferably 0.7 to 0.85, and particularly preferably about 0.8.

[0032] The lower limit of the initial charge capacity of the positive electrode active material of the present invention may be 190 mAh / g, 220 mAh / g, or 250 mAh / g, and the upper limit of the charge capacity may be 400 mAh / g, 370 mAh / g, or 330 mAh / g. In one embodiment, the charge capacity of the positive electrode active material may be 190 to 400 mAh / g, typically 220 to 370 mAh / g, and more typically 250 to 330 mAh / g.

[0033] The lower limit of the initial discharge capacity of the positive electrode active material of the present invention may be 185 mAh / g, 200 mAh / g, or 210 mAh / g, and the upper limit of the discharge capacity may be 300 mAh / g, 280 mAh / g, or 250 mAh / g. In one embodiment, the discharge capacity of the positive electrode active material may be 185 to 300 mAh / g, typically 200 to 280 mAh / g, and more typically 210 to 250 mAh / g.

[0034] The lower limit of the remaining capacity percentage of the positive electrode active material of the present invention after 100 charge-discharge cycles may be 61%, 74%, or 78%, and the upper limit of the remaining capacity percentage may be 100%, 95%, or 86%. In one embodiment, the remaining capacity percentage after 100 cycles may be 61-100%, typically 74-95%, and more typically 78-86%.

[0035] The positive electrode active material of the present invention may have a coating layer on its surface. The coating layer suppresses particle damage, adjusts the direct contact area between the positive electrode active material and the electrolyte, reduces oxidation reactions of the electrolyte and positive electrode film formation, and allows for the stable maintenance of the layered positive electrode active material. The coating layer may be composed of commonly used materials. For example, the coating layer may be composed of materials including carbon, nickel, iron, phosphorus, magnesium, aluminum, and mixtures thereof. The coating layer may include a first coating layer containing the aforementioned elements (e.g., nickel, iron, and phosphorus) and a second coating layer containing elements different from the aforementioned elements (e.g., carbon).

[0036] The present invention relates to a method for producing a positive electrode active material, which involves contacting an aqueous solution containing a metal to be oxidized (i.e., a metal element selected from the group consisting of metal elements excluding alkali metal elements) with a precipitating agent to obtain a precipitate, mixing the precipitate with a sodium compound, and calcining the mixture. In this specification, the precipitate may be referred to as "precipitate from aqueous solution of metal to be oxidized".

[0037] The metal to be oxidized includes manganese and metal M1. M1 includes metals excluding alkali metal elements. In one embodiment, M1 includes at least one selected from the group consisting of Ni and Fe. The metal to be oxidized may also include metal M2. M2 includes at least one selected from the group consisting of Co, Ti, Mg, Cu, and Al. Specific examples of using M1 alone and specific examples of using M1 and M2 in combination are the same as described above.

[0038] The composition ratio of the metal to be oxidized in the aqueous solution is preferably, Mn 1-y-z M1 y M2 z (2) The relationship is adjusted to be expressed as follows. Note that the values ​​of y, z, and 1-yz in equation (2) have the same meaning as described above.

[0039] Aqueous solutions containing metals to be oxidized can be obtained by using compounds such as chlorides, nitrates, acetates, formates, and oxalates as raw materials and dissolving them in water. Furthermore, when using raw materials that are poorly soluble in water, for example, oxides, hydroxides, or metallic materials as raw materials, these raw materials can be dissolved in acids such as hydrochloric acid, sulfuric acid, or nitric acid, or in aqueous solutions thereof, to obtain aqueous solutions containing metals to be oxidized.

[0040] As a precipitating agent, one or more compounds selected from the group consisting of LiOH (lithium hydroxide), NaOH (sodium hydroxide), KOH (potassium hydroxide), Li2CO3 (lithium carbonate), Na2CO3 (sodium carbonate), K2CO3 (potassium carbonate), (NH4)2CO3 (ammonium carbonate), and (NH2)2CO (urea) can be used. One or more hydrates of these compounds may also be used, or the compounds and hydrates may be used in combination. Furthermore, it is preferable to dissolve these precipitating agents in water and use them in aqueous solution form. The concentration of the compound in the aqueous solution of the precipitating agent is about 0.5 to 10 mol / L, preferably about 1 to 8 mol / L. Furthermore, it is preferable to use NaOH as a precipitating agent, and more preferably an aqueous solution of NaOH obtained by dissolving it in water. Also, aqueous ammonia can be used as an aqueous solution of the precipitating agent, and this may be used in combination with an aqueous solution of the compound.

[0041] Methods for contacting an aqueous solution containing a metal to be oxidized with a precipitant include adding a precipitant (including an aqueous precipitant) to an aqueous solution containing a metal to be oxidized, adding an aqueous solution containing a metal to be oxidized to an aqueous precipitant, and adding an aqueous solution containing a metal to be oxidized and a precipitant (including an aqueous precipitant) to water. When contacting an aqueous solution containing a metal to be oxidized with a precipitant, it is preferable to bring the metal to be oxidized into contact with oxygen. Among the above contact methods, the method of adding an aqueous solution containing a metal to be oxidized to an aqueous precipitant under an atmospheric environment is preferred.

[0042] In the present invention, a slurry containing a precipitate can be obtained by the above contact. This precipitate contains the metal to be oxidized. The precipitate from the aqueous solution of the metal to be oxidized preferably includes one that has been brought into contact with oxygen in the aqueous solution of the metal to be oxidized.

[0043] Next, the slurry is separated into solid and liquid components, and the precipitate is recovered. Solid-liquid separation can be performed by any method, but from the viewpoint of operability, methods such as filtration are preferred, and methods that volatilize the liquid component by heating, such as spray drying, may also be used. The recovered precipitate may also be washed and dried. The precipitate obtained after solid-liquid separation may have excess precipitant components attached to it, and these components can be reduced by washing. Water is preferably used as the washing solution, but water-soluble organic solvents such as ethanol and acetone may also be used. Drying can be performed by heat drying, or by forced air drying, vacuum drying, etc. When heat drying is performed, it is usually done at 50 to 300°C, preferably at around 100 to 200°C. Washing and drying may also be performed two or more times.

[0044] In the present invention, a positive electrode active material can be obtained by mixing the precipitate obtained as described above with a sodium compound and calcining the mixture. The mixing ratio of the sodium compound and the precipitate satisfies the relationship between sodium and the metal to be oxidized in formula (1). Since sodium is volatile when heated, the amount of sodium mixed may be in excess of the relationship between sodium and the metal to be oxidized in formula (1).

[0045] Examples of sodium compounds include one or more compounds selected from the group consisting of sodium hydroxide, sodium chloride, sodium nitrate, sodium peroxide, sodium sulfate, sodium bicarbonate, sodium oxalate, sodium carbonate, sodium phosphate, and pentasodium iron salt, and their hydrates may also be used. As for the mixing method, either dry mixing or wet mixing may be used, but dry mixing is preferred from the viewpoint of simplicity. Examples of mixing equipment include agitators, V-type mixers, W-type mixers, ribbon mixers, drum mixers, and ball mills.

[0046] The firing process is typically carried out at a temperature of 400 to 1200°C, preferably 600 to 1000°C, and more preferably 700 to 900°C, depending on the type of sodium compound used. If the firing temperature is too low, sodium tends to remain in the positive electrode active material, resulting in insufficient charge-discharge capacity. If it is too high, the crystal structure of the positive electrode active material may be destroyed. As a result of the firing process, a composite metal oxide of formula (1) is formed.

[0047] The holding time at the firing temperature is usually 0.1 to 20 hours, preferably 0.5 to 10 hours. Furthermore, an atmosphere containing an inert gas is used for firing. This is thought to improve the firing effect, generate an O3 phase structure, and improve the charge-discharge capacity of the positive electrode active material. The atmosphere containing an inert gas may also contain an inert gas and oxygen. This allows for appropriate adjustment of the phase structure of the positive electrode active material, generates an O'3 phase structure, improves the charge-discharge capacity, and also improves the charge-discharge durability. The firing atmosphere may contain only an inert gas, or only an inert gas and oxygen.

[0048] The type of inert gas included in the firing atmosphere is not particularly limited; for example, at least one selected from the group consisting of nitrogen, argon, and helium can be used. A specific example of a preferred inert gas is nitrogen.

[0049] The lower limit of the oxygen content of the firing atmosphere may be 0.10% by volume, 0.20% by volume, 0.50% by volume, or 0.70% by volume, and the upper limit of the oxygen content may be 10% by volume, 6.0% by volume, 3.0% by volume, or 2.0% by volume. In one embodiment, the oxygen content of the firing atmosphere may be 0.10 to 10% by volume, preferably 0.20 to 6.0% by volume, more preferably 0.50 to 3.0% by volume, even more preferably 0.70 to 2.0% by volume, and particularly preferably about 1.0% by volume. When the oxygen content of the firing atmosphere is within the above range, the charge and discharge capacity of the positive electrode active material tends to increase and its durability tends to improve.

[0050] After calcining the mixture of the precipitate and the sodium compound, the resulting calcined product may be further calcined in an oxygen-containing atmosphere. This allows for appropriate adjustment of the phase structure of the positive electrode active material, generating P2 phase and P'2 phase structures, which is thought to improve charge-discharge durability. As the oxygen-containing atmosphere, for example, air or an atmosphere containing the aforementioned inert gas and oxygen can be used. The conditions for such post-calcination are adjusted as appropriate while confirming the effect, but it can usually be carried out at a temperature of 300 to 800°C, preferably 400 to 700°C, more preferably 500 to 650°C, for 0.5 to 10 hours, preferably 1 to 5 hours, and more preferably about 3 hours.

[0051] The positive electrode active material obtained by calcination may or may not be washed with a solvent such as distilled water. Furthermore, the positive electrode active material obtained as described above may be crushed using a ball mill or jet mill, or the crushing and calcination process may be repeated two or more times. In addition, the positive electrode active material may be washed or classified as needed.

[0052] Furthermore, the composition ratio of the metals to be oxidized in the positive electrode active material can be identified, for example, by ICP (plasma atomic absorption spectrometry). The composition ratio of the metals to be oxidized in the obtained positive electrode active material will match the composition ratio of the metals to be oxidized in the raw materials.

[0053] When forming a coating layer on the surface of a positive electrode active material, the coating layer formation process can be carried out by contacting the positive electrode active material particles with a compound or dispersion containing the aforementioned element, followed by drying and, if necessary, heat treatment. Examples of such methods include spray coating, immersion (dip coating), spray drying, and solution coating.

[0054] Specifically, a coating layer may be formed by immersing positive electrode active material particles in a solution or dispersion containing the aforementioned elements, then removing and drying them, or by spraying the solution or dispersion onto the positive electrode active material particles, then drying and, if necessary, firing them to form the coating layer.

[0055] <Positive electrode> The positive electrode of the present invention contains the positive electrode active material. The positive electrode may also contain a sodium compound in addition to the positive electrode active material.

[0056] The aforementioned positive electrode is useful as the positive electrode in a sodium secondary battery and can be used as the positive electrode in such a battery. From the viewpoint of obtaining a sodium secondary battery that provides a larger potential difference, i.e., a sodium secondary battery with a higher energy density, it is preferable to use the aforementioned positive electrode as the positive electrode in a sodium secondary battery.

[0057] The positive electrode of the present invention can be manufactured by molding an electrode mixture containing the positive electrode active material and, optionally, a sodium compound, into the shape of a positive electrode. The electrode mixture generally contains a binder and, optionally, a conductive agent. The electrode mixture may also be molded on an electrode current collector.

[0058] The method for manufacturing the positive electrode includes, for example, mixing the positive electrode active material, a binder, a sodium compound if necessary, and a conductive material to obtain an electrode mixture, and molding the obtained electrode mixture into the shape of a positive electrode.

[0059] Examples of conductive agents include natural graphite, artificial graphite, coke, and carbon materials such as carbon black. Examples of binders include thermoplastic resins, specifically polyvinylidene fluoride (hereinafter sometimes referred to as "PVDF"), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymers, hexafluoropropylene-vinylidene fluoride copolymers, tetrafluoroethylene-perfluorovinyl ether copolymers, and other fluororesins; as well as polyolefin resins such as polyethylene and polypropylene.

[0060] When the aforementioned positive electrode is used as the positive electrode in a secondary battery, materials such as Al, Ni, and stainless steel can be used as the electrode current collector.

[0061] Methods for forming the electrode mixture include pressure molding, or paste formation using an organic solvent, coating onto the electrode current collector, drying, and then pressing to fix it to the electrode current collector. When paste formation is used, a slurry consisting of electrode active material, conductive agent, binder, and organic solvent is prepared. Examples of organic solvents include amine solvents such as N,N-dimethylaminopropylamine and diethyltriamine; ether solvents such as ethylene oxide and tetrahydrofuran; ketone solvents such as methyl ethyl ketone; ester solvents such as methyl acetate; and aprotic polar solvents such as dimethylacetamide and N-methyl-2-pyrrolidone. Methods for coating the electrode mixture onto the electrode current collector include, for example, slit die coating, screen coating, curtain coating, knife coating, gravure coating, and electrostatic spraying.

[0062] <Negative electrode> The negative electrode used in the present invention may be any negative electrode used in a sodium-ion battery, and may be an electrode containing sodium metal or a negative electrode active material capable of intercalating and releasing sodium. The negative electrode active material may be, for example, carbon materials such as hard carbon, soft carbon, natural graphite, artificial graphite, coke, carbon black, pyrolysis carbons, carbon fibers, and calcined organic polymer compounds; layered phosphorus compounds and Maxene, a composite oxide of phosphorus and titanium; one or more elements selected from Mg, Al, Si, P, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Y, Zr, Nb, Mo, Pd, Ag, Cd, In, Sn, Sb, W, Pb, and Bi; alloys or oxides, halides, chalcogenides using these elements, etc.

[0063] Carbon materials can be obtained by carbonizing, for example, phenolic resins (novolac-type phenolic resins, resol-type phenolic resins, etc.), epoxy resins (bisphenol-type epoxy resins, novolac-type epoxy resins, etc.), aniline resins, bismaleimide resins, benzoxazine resins, etc. Among these, phenolic resins are characterized by a well-developed three-dimensional crosslinking structure, and it is presumed that the carbon materials obtained by carbonizing these resins also have a unique, well-developed three-dimensional crosslinking structure derived from this characteristic, making them preferable from the viewpoint of increasing charge and discharge capacity.

[0064] Sodium-based secondary batteries have the property of releasing sodium ions from the positive electrode active material during charging and incorporating sodium ions into the positive electrode active material during discharge. For example, in the composite metal oxide described in Patent Document 1, which combines manganese with a manganese ratio of 6, 7, or 8 or higher, iron or nickel, and cobalt, titanium, magnesium, or copper, the amount of sodium ions released during the initial charge is insufficient to meet the discharge capacity. Therefore, the positive electrode for sodium-based secondary batteries needed to be pre-replenished with the insufficient amount of sodium ions to increase its discharge capacity.

[0065] The positive electrode active material of the present invention exhibits the characteristic that its initial charge capacity is greater than its initial discharge capacity. Therefore, in the positive electrode for sodium secondary batteries containing the composite metal oxide of the present invention, a large discharge capacity can be achieved from the first use without the need to replenish sodium ions.

[0066] <Sodium secondary battery> The sodium secondary battery of the present invention has the positive electrode and the negative electrode. The sodium secondary battery of the present invention can be manufactured by obtaining an electrode group by stacking or winding the positive electrode, separator and negative electrode in this order, housing this electrode group in a container such as a battery case, and impregnating the electrode group with an electrolyte solution consisting of an organic solvent containing an electrolyte. The positive electrode or negative electrode may be an electrode current collector on which an electrode mixture is supported.

[0067] Examples of electrode group shapes include those in which the cross-section obtained by cutting the electrode group perpendicular to the winding axis is a circle, ellipse, rectangle, or a rectangle with rounded corners.

[0068] In addition, examples of battery shapes include paper-type, coin-type, cylindrical, and prismatic types.

[0069] Figure 1 is a schematic exploded view showing the configuration of a sodium secondary battery, which is one embodiment of the present invention. The sodium secondary battery shown in Figure 1 is a coin-type sodium secondary battery and consists of an electrode group using the positive electrode 4, an electrolyte (not shown), and a battery case that houses these. The electrode group consists of a sheet-shaped positive electrode 4, a sheet-shaped negative electrode 6, and a sheet-shaped separator 5 that insulates the space between the positive electrode 4 and the negative electrode 6.

[0070] These are stacked together with leaf springs 2, spacers 3, and gaskets 7, which are commonly used in the field of sodium secondary batteries, and housed inside battery cases 1 and 8.

[0071] As separators that can be used in sodium secondary batteries, for example, materials having the form of porous films, nonwoven fabrics, woven fabrics, etc., made of materials such as polyethylene, polyolefin resins such as polypropylene, fluororesins, and nitrogen-containing aromatic polymers can be used. Alternatively, single-layer or laminated separators using two or more of these materials may be used. Examples of separators include those described in Japanese Patent Publication No. 2000-30686 and Japanese Patent Publication No. 10-324758. The thickness of the separator is preferably as thin as possible while maintaining mechanical strength, in that it increases the volumetric energy density of the battery and reduces internal resistance. Generally, the thickness of the separator is preferably about 5 to 200 μm, and more preferably about 5 to 40 μm. From the viewpoint of ion permeability, the air permeability of the separator, as measured by the Gurley method, is preferably 50 to 300 seconds / 100 cc, and more preferably 50 to 200 seconds / 100 cc. Furthermore, the porosity of the separator is typically 30 to 80 volume%, preferably 40 to 70 volume%. The separator may also be made by laminating separators with different porosities.

[0072] In electrolytes usable in sodium secondary batteries, organic solvents include, for example, carbonates such as propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate, isopropyl methyl carbonate, vinylene carbonate, 4-trifluoromethyl-1,3-dioxolan-2-one, and 1,2-di(methoxycarbonyloxy)ethane; 1,2-dimethoxyethane, 1,3-dimethoxypropane, pentafluoropropyl methyl ether, 2,2,3 Ethers such as 3-tetrafluoropropyl difluoromethyl ether, tetrahydrofuran, and 2-methyltetrahydrofuran; esters such as methyl formate, methyl acetate, and γ-butyrolactone; nitriles such as acetonitrile and butyronitrile; amides such as N,N-dimethylformamide and N,N-dimethylacetamide; carbamates such as 3-methyl-2-oxazolidone; sulfur-containing compounds such as sulfolane, dimethyl sulfoxide, and 1,3-propanesalton; or the above organic solvents to which fluorine substituents have been further introduced can be used. Typically, two or more of these are used as the organic solvent in mixture form.

[0073] The electrolyte in the electrolyte solution is not particularly limited, but salts commonly used in sodium secondary batteries can be used. For example, NaPF6, NaBF4, NaClO4, NaTiF4, NaVF5, NaAsF, NaSbF6, NaCF3SO3, Na(C2F5SO2)2N, NaB(C2O4)2, NaB 10 Cl 10 NaB 12 Cl 12 Salts such as NaCF3COO, Na2S2O4, NaNO3, Na2SO4, NaPF3(C2F5)3, NaB(C6F5)4, and Na(CF3SO2)3C can be used. One of the above salts may be used alone, or two or more may be used in combination.

[0074] Of these, sodium hexafluoride phosphate (NaPF6), sodium perchlorate (NaClO4), and sodium tetraborate (NaBF4) are preferably used, with NaPF6 being particularly preferred. Using NaPF6 as a salt improves the discharge capacity and cycle life of the positive electrode, and enhances the effect of improving the cycle life of the negative electrode.

[0075] From the viewpoint of improving the energy density of sodium secondary batteries, the electrolyte concentration of the electrolyte solution is 1.0 mol / kg or more, preferably 1.3 to 1.5 mol / kg, and more preferably 1.2 to 1.3 mol / kg.

[0076] Sodium secondary batteries are operated at a voltage of 4.2V or less relative to the sodium electrode potential. Operating at a higher voltage may reduce cycle durability due to problems such as exceeding the oxidation resistance of the electrolyte solvent. On the other hand, it has become clear that the energy density of the sodium secondary battery can be further increased by exposing the positive electrode of the present invention to a high voltage. In other words, from the viewpoint of increasing capacity, it is preferable to temporarily expose the positive electrode active material or positive electrode of the present invention to a high voltage of 4.2V or higher.

[0077] Specifically, when charging the sodium secondary battery, positive electrode active material, or positive electrode of the present invention, it is preferable to apply a voltage of 4.2V or higher, for example 4.4~4.6V, relative to the sodium electrode potential, at least once. This further increases the energy density of the sodium secondary battery and the charge / discharge capacity of the positive electrode active material or positive electrode. The rate of increase in the charge / discharge capacity (mAh / g) of the positive electrode active material or positive electrode is 5~25%, preferably 10~15%. After exposing the positive electrode active material to high voltage, operating the sodium secondary battery at a rated voltage of, for example, 4.2V or lower can prevent deterioration of cycle characteristics due to high voltage while maintaining improved performance. [Examples]

[0078] The present invention will be further described by the following examples, but the present invention is not limited thereto.

[0079] <Experimental Examples 1-11> (1) Preparation of composite metal oxides A metal solution was prepared by dissolving manganese sulfate and iron sulfate in pure water in amounts that provided a predetermined molar ratio of manganese and iron. The metal solution was added dropwise to an aqueous sodium hydroxide solution adjusted to pH 10 under atmospheric pressure to form a precipitate, which was then brought into contact with oxygen. The solution's pH was maintained at pH 10 during this process. The precipitate was filtered from the slurry and washed with pure water.

[0080] The precipitate and sodium hydroxide were weighed in amounts that resulted in a predetermined molar ratio of metal to sodium in the precipitate, and pure water was added and mixed uniformly. The resulting slurry was dried at 120°C. The resulting powder was heated from room temperature to 750°C over 2 hours under a predetermined atmosphere and maintained for 5 hours, then allowed to cool naturally, crushed in a mortar, washed with water, and dried to prepare a composite metal oxide.

[0081] In Experimental Examples 1 and 2, nitrogen was used as the atmosphere during firing, while in Experimental Examples 3 to 11, nitrogen containing a predetermined volume percent of oxygen at room temperature was used.

[0082] The prepared composite metal oxide was placed in a powder X-ray diffractometer, and the XRD spectrum was measured using Cu-Kα rays. In the XRD spectrum, the horizontal axis represents the angle of incidence (2θ), and the vertical axis represents the diffraction intensity (cps).

[0083] Figure 2 shows the XRD spectrum of the composite metal oxide prepared in Experimental Example 4. Peak patterns originating from the O3 phase structure can be observed around 2θ = approximately 16.8° and approximately 43.8–44.2°.

[0084] In layered sodium composite metal oxides mainly composed of manganese, a partially oxidized O'3 phase structure may be formed as the oxidation state of manganese changes. In this case, diffraction peaks originating from the O3 and O'3 phase structures may be observed superimposed, and as a result, the peak position corresponding to the O3 phase may appear to be shifted to a lower angle. Such superposition of O3 and O'3 phase structures can also occur in the composite metal oxide of the present invention.

[0085] Table 2 shows the types of crystal structures detected in each prepared composite metal oxide. The meaning of the symbols in the table is explained below.

[0086] [Table 1]

[0087] (2) Preparation of positive and negative electrodes A composite metal oxide as the positive electrode active material or a phenolic resin carbide as the negative electrode active material, acetylene black (manufactured by Denki Kagaku Co., Ltd.) as the conductive material, and polyfluorotetraethylene (PTFE, Daikin Corporation's "F104" (product name)) as the binder were weighed out in a composition of electrode active material:conductive material:binder = 8:1:1 (by weight ratio), placed in a bottle, and shaken. The mixture was then ground in a mortar under shearing force to form a sheet approximately 100 μm thick. The resulting sheet was punched out to a diameter of 13 mm to obtain either the positive or negative electrode.

[0088] (3) Fabrication of sodium secondary battery The obtained positive and negative electrodes, electrolyte, separator, gasket, and case, along with other commonly used components, were prepared, and a sodium secondary battery cell was assembled. The weight ratio of the positive to negative electrodes was adjusted to 1:0.8. A perspective view of the exploded cell is shown in Figure 1.

[0089] (Electrolyte A) An electrolyte containing NaPF6 at a predetermined concentration in a 1:1 mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC).

[0090] (Electrolyte B) An electrolyte containing NaClO4 at a predetermined concentration in a 1:1 mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC).

[0091] Next, the cells were subjected to charge-discharge tests under the following conditions using a charge-discharge testing device to determine their characteristics. The measurement items and results are shown in Table 2.

[0092] Current: 0.lmAh Mode: CCCVCC (0.05C cutoff) CC Temperature: 35℃ Positive voltage range: 1.25~4.2V

[0093] [Table 2]

[0094] <Experimental Examples 12-16> A composite metal oxide prepared in the same manner as in Experimental Example 4, after being calcined under a predetermined atmosphere, was further calcined in air at 600°C for 1 to 5 hours, except that it was prepared in the same manner as in Experimental Example 1, and its XRD spectrum was measured. Figure 3 shows the XRD spectrum of the composite metal oxide prepared in Experimental Example 12. In the composite metal oxide shown in Figure 3, where the O3 phase structure and P2 phase structure are uniformly distributed and coexist, a peak originating from the P2 phase structure around 2θ = approximately 15.5° and a peak originating from the O3 phase structure around 2θ = approximately 16.8° are observed in close proximity, and it can be confirmed that a peak originating from the O3 phase structure around 2θ = approximately 43.8 to 44.2° and a peak originating from the P2 phase structure around 2θ = approximately 44.8 to 45.3° are observed together.

[0095] In layered sodium composite metal oxides mainly composed of manganese, a partially oxidized O'3 phase structure may be formed as the oxidation state of manganese changes. In this case, diffraction peaks originating from the O3 and O'3 phase structures may be observed superimposed, and as a result, the peak position corresponding to the O3 phase may appear to be shifted to a lower angle. Such superposition of O3 and O'3 phase structures can also occur in the composite metal oxide of the present invention.

[0096] A sodium secondary battery was fabricated in the same manner as in Experimental Example 1, except that the obtained composite metal oxide was used as the positive electrode active material, and its characteristics were tested. The results are shown in Table 3.

[0097] <Experimental Example 17 or 18> In charge-discharge tests of sodium secondary battery cells prepared in the same manner as in Experimental Example 15, the characteristics were tested in the same manner except that the voltage during the initial charge was changed to 4.4V or 4.6V. The results are shown in Table 3.

[0098] <Experimental Examples 19-21> A composite metal oxide was prepared in the same manner as in Experimental Example 4, except that the amount of iron sulfate was reduced when preparing the metal solution, and instead, a compound containing titanium, aluminum, or magnesium was added in an amount that resulted in a heterogeneous metal content of 5 mol%. Specifically, titanium sulfate (TiOSO4), aluminum sulfate (Al2(SO4)3), or magnesium sulfate (MgSO4) was used as the compound. A sodium secondary battery was fabricated in the same manner as in Experimental Example 1, except that the obtained composite metal oxide was used as the positive electrode active material, and its characteristics were tested. The results are shown in Table 3.

[0099] [Table 3]

[0100] <Experimental Example 22> A composite metal oxide, prepared in the same manner as in Experimental Example 1, was added to an aqueous sucrose solution, uniformly mixed, and dried to deposit an amount of sucrose on the surface of the composite metal oxide particles such that the carbon content was approximately 5 mol% of the total. The sucrose-coated composite metal oxide particles were heat-treated in an inert atmosphere to carbonize the sucrose, forming a carbon coating layer on the surface of the composite metal oxide particles. A sodium secondary battery was fabricated in the same manner as in Experimental Example 1, except that the obtained composite particles were used as the positive electrode active material, and its characteristics were tested. The results are shown in Table 4.

[0101] <Experimental Examples 23 and 24> A composite metal oxide, prepared in the same manner as in Experimental Example 1, was added to an aqueous solution of a nickel compound or iron compound, mixed uniformly, and dried to deposit nickel or iron in an amount that resulted in a total nickel or iron content of approximately 5 mol%, onto the surface of the composite metal oxide particles. This was then heat-treated in an inert atmosphere to form a nickel or iron coating layer 1 on the surface of the composite metal oxide particles. The resulting composite particles were added to an aqueous sucrose solution, mixed uniformly, and dried to deposit sucrose in an amount that resulted in a total carbon content of approximately 5 mol%, onto the surface of the composite metal oxide particles. This was then heat-treated in an inert atmosphere to carbonize the sucrose, forming a carbon coating layer 2 on the surface of the composite particles. A sodium secondary battery was prepared in the same manner as in Experimental Example 1, except that the resulting composite particles were used as the positive electrode active material, and its characteristics were tested. The results are shown in Table 4.

[0102] <Experimental Examples 25 and 26> A composite metal oxide, prepared in the same manner as in Experimental Example 1, was added to an aqueous solution of an iron compound or a phosphorus compound, uniformly mixed, and dried to deposit an amount of iron or phosphorus, resulting in an iron or phosphorus content of approximately 5 mol% of the total, onto the surface of the composite metal oxide particles. This was then heat-treated in an inert atmosphere to form an iron or phosphorus coating layer on the surface of the composite metal oxide particles. A sodium secondary battery was fabricated in the same manner as in Experimental Example 1, except that the obtained composite particles were used as the positive electrode active material, and its characteristics were tested. The results are shown in Table 4.

[0103] <Experimental Example 27> A composite metal oxide was prepared in the same manner as in Experimental Example 1, except that the precipitate, which was filtered from the metal solution slurry and dried, was calcined under vacuum instead of a nitrogen atmosphere. A sodium secondary battery was constructed in the same manner as in Experimental Example 1, except that the obtained composite metal oxide was used as the positive electrode active material, and its characteristics were tested. The results are shown in Table 4.

[0104] <Experimental Example 28> A composite metal oxide was prepared in the same manner as in Experimental Example 1, except that the precipitate, which was filtered from the metal solution slurry and dried, was calcined under an atmospheric atmosphere instead of a nitrogen atmosphere, and the XRD spectrum was measured. Figure 4 shows the XRD spectrum of the composite metal oxide prepared in Experimental Example 28. Peak patterns originating from the P2 phase structure can be observed around 2θ = approximately 15.5° and approximately 31.3°.

[0105] A sodium secondary battery was fabricated in the same manner as in Experimental Example 1, except that the obtained composite metal oxide was used as the positive electrode active material, and its characteristics were tested. The results are shown in Table 4.

[0106] [Table 4] [Explanation of Symbols]

[0107] 1. 8 cases 2 leaf springs 3 Spacers 4 Positive electrode 5 Separators 6 negative electrode 7 Gasket

Claims

1. formula Na ｘ Mn １－ｙ－ｚ M11 ｙ M2 ｚ O ２ (1) [In the formula, M1 represents Fe or Ni. M2 represents Co, Ti, Mg, Cu, or Al. x represents 0.8 to 4, y represents 0.05 to 0.35, and z represents 0 to 0.1.] It comprises a composite metal oxide represented by, The aforementioned composite metal oxide is given by the formula Mn 1-y-z M1 y M2 z (2) [In the formula, M1 represents Fe or Ni. M2 represents Co, Ti, Mg, Cu, or Al. y represents 0.05 to 0.35, and z represents 0 to 0.1.] The mixture comprises a mixture of an oxide-to-oxidize metal composition and a sodium compound contained in an aqueous solution containing each metal in the specified composition ratio, and is calcined under an inert gas-containing atmosphere. A positive electrode active material for a sodium secondary battery, wherein, when the inert gas-containing atmosphere contains oxygen, the upper limit of the oxygen content is 10% by volume.

2. The composite metal oxide has an O3 phase structure, wherein the positive electrode active material for a sodium secondary battery is as described in claim 1.

3. The composite metal oxide further has an O'3 phase structure, as described in claim 2, for a positive electrode active material for a sodium secondary battery.

4. The positive electrode active material for a sodium secondary battery according to claim 3, wherein the composite metal oxide further has a P2 phase structure and a P'2 phase structure.

5. The positive electrode active material for a sodium secondary battery according to any one of claims 2 to 4, wherein different types of phase structures are substantially uniformly distributed and coexist within it.

6. The positive electrode active material for a sodium secondary battery according to any one of claims 1 to 4, wherein the composite metal oxide includes a product calcined in an inert gas and oxygen-containing atmosphere of a mixture of the metal to be oxidized composition and a sodium compound.

7. A positive electrode active material for a sodium secondary battery according to any one of claims 1 to 4, having a coating layer on its surface containing at least one element selected from the group consisting of carbon, nickel, iron, and phosphorus.

8. A positive electrode for a sodium secondary battery comprising the positive electrode active material for a sodium secondary battery described in any one of claims 1 to 4.

9. formula Mn １－ｙ－ｚ M11 ｙ M2 ｚ (2) [In the formula, M1 represents Fe or Ni. M2 represents Co, Ti, Mg, Cu, or Al. y represents 0.05 to 0.35, and z represents 0 to 0.1.] To obtain a metal oxide composition contained in an aqueous solution containing each metal in the following composition ratios, and The mixture of the metal to be oxidized and the sodium compound is fired in an inert gas-containing atmosphere. A method for producing a positive electrode active material for a sodium secondary battery, comprising: A method for producing a positive electrode active material for a sodium secondary battery, wherein, when the inert gas-containing atmosphere contains oxygen, the upper limit of the oxygen content is 10% by volume.

10. The method for producing a positive electrode active material for a sodium secondary battery according to claim 9, wherein the inert gas-containing atmosphere is an inert gas and oxygen-containing atmosphere.

11. A method for manufacturing a positive electrode for a sodium secondary battery, comprising molding an electrode mixture containing a positive electrode active material for a sodium secondary battery obtained by the manufacturing method of claim 10 into the shape of a positive electrode.

12. A sodium secondary battery having the positive electrode for a sodium secondary battery as described in claim 8.

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