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

Abstract

To provide a positive electrode active material for a sodium secondary battery, which can realize a sodium secondary battery having a large charge / discharge capacity and an energy density equal to or greater than that of a lithium secondary battery.SOLUTION: A positive electrode active material for a sodium secondary battery, includes a composite metal oxide represented by the formula of NaxMn1-y-zM1yM2zO2 (1), and has a high initial discharge capacity compared to an initial charge capacity. In the formula (1), M1 represents Ni or Fe, M2 represents Co, Ti, Mg, Cu or Al, x is 0.95 or less, y is 0.1 to 0.5, and z is 0 to 0.1. A positive electrode active material for a sodium secondary battery includes a composite metal oxide represented by the formula (1), and has a higher initial discharge capacity than the initial charge capacity.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a sodium secondary battery, and particularly to a positive electrode active material for a sodium secondary battery containing a transition metal oxide.

Background Art

[0002] Sodium secondary batteries can be composed of materials that are abundant in supply and inexpensive, and it is expected that by putting this into practical use, a large amount of large-scale power sources can be supplied. On the other hand, sodium secondary batteries have insufficient practicality due to their low energy density. One of the reasons for the low energy density of sodium secondary batteries is that the capacity of the positive electrode material of sodium secondary batteries is insufficient.

[0003] Patent Document 1 describes a sodium secondary battery positive electrode material composed of a sodium-containing transition metal oxide represented by the composition formula Na x (Mn y Co 1-y-z Ni z )O2, and the composition ratio is characterized by 0.5 < x ≤ 1.0, 0.60 ≤ y ≤ 0.80, 0.05 ≤ z ≤ 0.20, 0.05 ≤ 1 - y - z ≤ 0.25. This sodium secondary battery positive electrode material has a specific composition as described above, and further provides a sodium secondary battery having a higher charge-discharge capacity than conventional sodium secondary batteries by controlling the firing temperature of the raw material mixture and the atmosphere during firing.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] The present invention aims to provide a positive electrode active material for a sodium secondary battery that can realize a sodium secondary battery having a large charge / discharge capacity and an energy density equal to or greater than that of a lithium secondary battery. Conventional positive electrode active materials for sodium secondary batteries still have insufficient charge / discharge capacity to achieve an energy density equal to or greater than that of a lithium secondary battery. [Means for solving the problem]

[0006] The present invention provides the following aspects. [Aspect 1] Formula Na x Mn 1-y-z M1 y M2 z O2(1) [In the formula, M1 represents Ni or Fe, M2 represents Co, Ti, Mg, Cu or Al, x represents 0.95 or less, y represents 0.1 to 0.5, and z represents 0 to 0.1.] The composite metal oxide comprises: A positive electrode active material for sodium secondary batteries that has a higher initial discharge capacity compared to the initial charge capacity.

[0007] [Aspect 2] The positive electrode active material for a sodium secondary battery according to Aspect 1, wherein x is 0.2 to 0.8, preferably 0.3 to 0.7, and more preferably 0.4 to 0.6.

[0008] [Aspect 3] The positive electrode active material for a sodium secondary battery according to Aspect 1 or 2, wherein y is 0.15 to 0.3, preferably 0.15 to 0.25.

[0009] [Aspect 4] The positive electrode active material for a sodium secondary battery according to any one of Aspects 1 to 3, wherein z is 0.01 to 0.08, preferably 0.03 to 0.07.

[0010] [Aspect 5] The positive electrode active material for a sodium secondary battery according to any one of Aspects 1 to 4, wherein the initial discharge capacity is at least 150%, preferably 200 to 400%, and more preferably 250 to 350% higher than the initial charge capacity.

[0011] [Aspect 6] A positive electrode active material for a sodium secondary battery according to any one of Aspects 1 to 5, which has a higher charge / discharge capacity from the second time onwards compared to the initial charge capacity.

[0012] [Embodiment 7] The positive electrode active material for a sodium secondary battery according to any one of Embodiments 1 to 6, having an initial discharge capacity of more than 210 mAh / g, preferably 220 to 280 mAh / g.

[0013] [Embodiment 8] A positive electrode active material for a sodium secondary battery according to any one of Embodiments 1 to 7, comprising a mixture obtained by firing a precipitate from an aqueous solution of a metal to be oxidized and a sodium compound.

[0014] [Aspect 9] After discharging the sodium secondary battery, the formula Na x Mn 1-y-z M1 y M2 z O2(3) [In the formula, x represents a value greater than 1. M1, M2, y, and z are as defined above.] 9. A positive electrode active material for a sodium secondary battery according to any one of Aspects 1 to 8, comprising a composite metal oxide represented by the following formula:

[0015] [Aspect 10] The positive electrode active material for a sodium secondary battery according to Aspect 9, wherein x is greater than 1 and less than or equal to 2.

[0016] [Embodiment 11] A positive electrode for a sodium secondary battery comprising the positive electrode active material for a sodium secondary battery of any one of Embodiments 1 to 10.

[0017] [Embodiment 12] The positive electrode for a sodium secondary battery according to Embodiment 11, which has been subjected to a charging treatment at a voltage exceeding 4.2 V, for example, 4.4 to 4.5 V, based on the sodium electrode potential.

[0018] [Embodiment 13] The positive electrode for a sodium secondary battery according to embodiment 11 or 12, which contains a sodium compound.

[0019] [Aspect 14] The positive electrode for a sodium secondary battery of Aspect 13, wherein the sodium compound is contained in the positive electrode in an amount of 1 to 50 mol %, preferably 3 to 40 mol %, and more preferably 5 to 30 mol %, based on the number of moles of the metal portion to be oxidized in the positive electrode active material.

[0020] [Aspect 15] The positive electrode for a sodium secondary battery according to Aspect 13 or 14, wherein the sodium compound includes sodium hydroxide or sodium carbonate.

[0021] [Aspect 16] Formula Mn 1-y-z M1 y M2 z (2) [In the formula, M1 represents Ni or Fe, M2 represents Co, Ti, Mg, Cu or Al, x represents 0.95 or less, y represents 0.1 to 0.5, and z represents 0 to 0.1.] contacting an aqueous solution containing the metals in the composition represented by the formula (I) with a precipitating agent in the presence of oxygen to obtain a precipitate; and mixing the precipitate with a sodium compound and calcining the mixture; The method for producing a positive electrode active material for a sodium secondary battery includes the steps of:

[0022] [Embodiment 17] A method for producing 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 production method of embodiment 16 into the shape of a positive electrode.

[0023] [Embodiment 18] A sodium secondary battery having the positive electrode for sodium secondary batteries according to any one of Embodiments 11 to 15.

[0024] [Embodiment 19] A method for producing the sodium secondary battery of Embodiment 18, comprising charging at a voltage exceeding 4.2 V, for example, 4.4 to 4.5 V, based on the sodium electrode potential.

[0025] [Embodiment 20] The positive electrode active material after discharge is represented by the formula Na x Mn 1-y-z M1 y M2 z O2(3) [In the formula, x represents a value greater than 1. M1, M2, y, and z are as defined above.] 20. The sodium secondary battery of embodiment 18 or 19, comprising a composite metal oxide represented by the formula:

[0026] [Aspect 21] The sodium secondary battery of Aspect 20, wherein x is greater than 1 and not greater than 2, preferably 1.05 to 1.7, and more preferably 1.1 to 1.5.

[0027] [Aspect 22] The positive electrode for a sodium secondary battery of Aspect 13 or 14, wherein the sodium compound includes pentasodium ferrate.

[0028] [Aspect 23] The positive electrode for a sodium secondary battery according to Aspect 13 or 14, wherein the sodium compound is supported on the surface of the positive electrode active material.

[0029] [Aspect 24] The positive electrode for a sodium secondary battery of Aspect 23, wherein the sodium compound includes sodium hydroxide, sodium carbonate, or pentasodium ferrate.

[0030] [Aspect 25] A sodium secondary battery having the positive electrode for sodium secondary batteries according to any one of Aspects 22 to 24. [Effects of the Invention]

[0031] According to the present invention, it is possible to provide a positive electrode active material for a sodium secondary battery, which can realize a sodium secondary battery having a large charge / discharge capacity and an energy density equal to or greater than that of a lithium secondary battery. [Brief explanation of the drawings]

[0032] [Figure 1] 1 is an exploded view schematically illustrating the configuration of a sodium secondary battery according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0033] 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. Furthermore, when multiple upper and lower limit values ​​are described for a specific parameter, any upper and lower limit values ​​can be combined to form a suitable numerical range.

[0034] <Cathode 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) The positive electrode active material comprises a composite metal oxide represented by the formula (1). Formula (1) represents the chemical structure of the positive electrode active material before the first charge of a sodium secondary battery. In a preferred embodiment, the positive electrode active material comprises a composite metal oxide represented by the formula (1). In formula (1), Na represents sodium. Mn represents manganese. M1 includes a metal other than an alkali metal element. In one embodiment, M1 includes at least one selected from the group consisting of nickel (Ni) and iron (Fe). M2 includes at least one selected from the group consisting of cobalt (Co), titanium (Ti), magnesium (Mg), copper (Cu), and aluminum (Al). In addition, O represents oxygen.

[0035] In one preferred embodiment of formula (1), M1 is composed of Ni. In another preferred embodiment, M1 is composed of Fe. Preferred examples of combinations of M1 and M2 include Ni and Co, Ni and Ti, Ni and Mg, Ni and Cu, and Ni and Al. Other examples of combinations of M1 and M2 include Fe and Co, Fe and Ti, Fe and Mg, Fe and Cu, and Fe and Al.

[0036] The Na atomic ratio x in the above formula is 0.95 or less, preferably 0.2 to 0.8, more preferably 0.3 to 0.7, and even more preferably 0.4 to 0.6. If x exceeds 0.95, sodium tends to remain in the positive electrode active material, which may result in insufficient charge / discharge capacity.

[0037] The atomic ratio y of the metal M1 in the above formula is 0.1 to 0.5, preferably 0.15 to 0.3, and more preferably 0.15 to 0.25. The Mn atomic ratio 1-yz in the above formula is 0.5 to 0.95, preferably 0.6 to 0.9, and more preferably 0.7 to 0.85. When y and 1-yz are within the above ranges, the discharge capacity of the positive electrode active material is improved.

[0038] The atomic ratio z of M2 in the above formula is 0 to 0.1, preferably 0.01 to 0.08, and more preferably 0.03 to 0.07. M2 is not an essential component, but when it is contained in the above amount, the stability of the crystal structure of the composite metal oxide is improved.

[0039] The positive electrode active material of the present invention has a characteristic that the initial discharge capacity is higher than the initial charge capacity, and the initial discharge capacity is 150% or more, preferably 200 to 400%, and more preferably 250 to 350% higher than the initial charge capacity.

[0040] In one embodiment, the positive electrode active material of the present invention has an initial discharge capacity of 150 to 400 mAh / g, preferably 200 to 300 mAh / g, more preferably 220 to 280 mAh / g, for example, a value higher than 210 mAh / g.

[0041] In another embodiment, the positive electrode active material of the present invention may have an initial discharge capacity of 170 to 270 mAh / g, preferably 180 to 260 mAh / g, and more preferably 190 to 250 mAh / g.

[0042] Furthermore, the positive electrode active material of the present invention can exhibit a charge / discharge capacity that is higher than the above-mentioned initial charge capacity even in the second or subsequent charge / discharge cycles.

[0043] The method for producing a positive electrode active material of the present invention includes 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 precipitant to obtain a precipitate, mixing the precipitate with a sodium compound, and calcining the mixture. In this specification, the precipitate is sometimes referred to as a "precipitate from the aqueous solution of the metal to be oxidized."

[0044] The metal to be oxidized includes manganese and metal M1. M1 includes metals other than 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 include metal M2. M2 includes at least one selected from the group consisting of Co, Ti, Mg, Cu, and Al. Specific examples of when M1 is used alone and when M1 and M2 are used in combination are the same as above.

[0045] The composition ratio of the metal to be oxidized contained in the aqueous solution is preferably expressed by the formula Mn 1-y-z M1 y M2 z (2) In the formula (2), the values ​​of y, z and 1-yz are the same as those defined above.

[0046] An aqueous solution containing the metal 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 a raw material that is difficult to dissolve in water is used, that is, when an oxide, hydroxide, or metal material is used as the raw material, the raw material can be dissolved in an acid such as hydrochloric acid, sulfuric acid, or nitric acid, or in an aqueous solution of these acids, to obtain an aqueous solution containing the metal to be oxidized.

[0047] The precipitant may be one or more compounds selected from the group consisting of LiOH (lithium hydroxide), NaOH (sodium hydroxide), KOH (potassium hydroxide), LiCO (lithium carbonate), NaCO (sodium carbonate), KCO (potassium carbonate), (NH)CO (ammonium carbonate), and (NH)CO (urea). One or more hydrates of the compounds may be used, or the compounds and hydrates may be used in combination. These precipitants are preferably dissolved in water and used in the form of an aqueous solution. The concentration of the compound in the aqueous solution is about 0.5 to 10 mol / L, preferably about 1 to 8 mol / L. NaOH is preferably used as the precipitant, and more preferably, it is dissolved in water to form an aqueous NaOH solution. Aqueous ammonia may also be used as an aqueous solution of the compound, and this may be used in combination with the aqueous solution of the compound.

[0048] Examples of methods for contacting an aqueous solution containing the oxidizable metal with a precipitant include adding a precipitant (including a precipitant in the form of an aqueous solution) to an aqueous solution containing the oxidizable metal, adding an aqueous solution containing the oxidizable metal to a precipitant in the form of an aqueous solution, and adding an aqueous solution containing the oxidizable metal and a precipitant (including a precipitant in the form of an aqueous solution) to water. When contacting an aqueous solution containing the oxidizable metal with a precipitant, it is preferable to contact the oxidizable metal with oxygen. Among the above contact methods, a method in which an aqueous solution containing the oxidizable metal is added to a precipitant in the form of an aqueous solution under atmospheric conditions is preferred.

[0049] In the present invention, the above contact can produce a slurry containing a precipitate. This precipitate contains the metal to be oxidized. The precipitate from the aqueous solution of the metal to be oxidized preferably includes the metal to be oxidized that has been contacted with oxygen in the aqueous solution of the metal to be oxidized.

[0050] Next, the slurry is subjected to solid-liquid separation, and the precipitate is recovered. Any method for solid-liquid separation may be used, but from the viewpoint of operability, a method based on solid-liquid separation, such as filtration, is preferably used. Alternatively, a method of volatilizing the liquid by heating, such as spray drying, may also be used. The recovered precipitate may also be washed, dried, or the like. The precipitate obtained after solid-liquid separation may contain excess precipitant components, and these components can be reduced by washing. Water is preferably used as the washing liquid for washing, but water-soluble organic solvents such as ethanol and acetone may also be used. Drying may be performed by heat drying, or may be performed by air drying, vacuum drying, or the like. When heat drying is performed, the temperature is typically 50 to 300°C, preferably about 100 to 200°C. Washing and drying may be performed two or more times.

[0051] In the present invention, the precipitate obtained as described above is mixed with a sodium compound and calcined to obtain a positive electrode active material. The mixing ratio of the sodium compound and the precipitate is such that the relationship between sodium and the metal to be oxidized satisfies formula (1). Because sodium is easily volatilized when heated, the sodium may be mixed in an amount that is in excess of the relationship of formula (1) relative to the metal to be oxidized.

[0052] The sodium compound may be 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 ferrate, and may also be a hydrate thereof. The mixing method may be either dry mixing or wet mixing, but dry mixing is preferred from the viewpoint of simplicity. Examples of mixing equipment include agitation mixers, V-type mixers, W-type mixers, ribbon mixers, drum mixers, and ball mills.

[0053] The calcination is 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 calcination temperature is too low, sodium tends to remain in the positive electrode active material, resulting in insufficient charge / discharge capacity. If the calcination temperature is too high, the crystal structure of the positive electrode active material may be destroyed. As a result of the calcination, a composite metal oxide of formula (1) is produced.

[0054] The time for maintaining the firing temperature is usually 0.1 to 20 hours, preferably 0.5 to 10 hours. The firing atmosphere can be air, oxygen, nitrogen, argon, or a mixture thereof, but air is preferred from the viewpoint of simplicity.

[0055] The mixture of the precipitate and the sodium compound may be calcined under a vacuum condition where oxygen is blocked. The vacuum condition here generally refers to a vacuum level of about 10^5 Pa to 10^-5 Pa, preferably about 10^2 Pa to 10^-1 Pa, and more preferably about 10^-1 Pa to 10^-5 Pa.

[0056] The positive electrode active material obtained by firing may or may not be washed with a solvent such as distilled water. If firing is performed in a vacuum state where oxygen is blocked and the material is not washed, the positive electrode active material may contain more sodium ions than those released during the first charge.

[0057] The positive electrode active material obtained as described above may be pulverized using a ball mill or a jet mill, or the pulverization and firing may be repeated two or more times. The positive electrode active material may also be washed or classified as necessary.

[0058] The composition ratio of the obtained positive electrode active material can be identified by, for example, ICP (inductively coupled plasma atomic absorption spectrometry).

[0059] <Positive electrode> The positive electrode of the present invention contains the positive electrode active material. The positive electrode may contain a sodium compound in addition to the positive electrode active material. This allows a larger amount of sodium ions to be supplied to the electrolyte than the sodium ions released by the positive electrode active material during initial charging, further improving the energy density of the sodium secondary battery. The sodium compound used is one that releases sodium ions. Specific examples of the sodium compound are as described above, and preferred sodium compounds include sodium hydroxide, sodium carbonate, and pentasodium ferrate. The sodium compound is contained in the positive electrode in an amount of 1 to 50 mol %, preferably 3 to 40 mol %, and more preferably 5 to 30 mol %, based on the number of moles of the oxidizable metal portion in the positive electrode active material. The "oxidizable metal portion in the positive electrode active material" refers to the oxidizable metal portion represented by formula (2).

[0060] The positive electrode is useful as a positive electrode in a sodium secondary battery, and can be used as the positive electrode of the 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 positive electrode as the positive electrode in the sodium secondary battery.

[0061] The positive electrode of the present invention can be produced by forming an electrode mixture containing the positive electrode active material and, if necessary, a sodium compound into the shape of a positive electrode. The electrode mixture generally contains a binder and, if necessary, a conductive agent. The electrode mixture may be supported on an electrode current collector and then formed.

[0062] The method for producing a positive electrode includes, for example, mixing the positive electrode active material, a binder, optionally a sodium compound, and optionally a conductive material to obtain an electrode mixture, and molding the obtained electrode mixture into the shape of a positive electrode. The electrode mixture may be obtained by spraying a sodium compound solution onto the positive electrode active material, drying the solvent as necessary, and mixing a binder and optionally a conductive material. Alternatively, the electrode mixture may be obtained by preparing a liquid by mixing the positive electrode active material, a sodium compound, and a solvent, drying the solvent as necessary, and mixing a binder and optionally a conductive material. These positive electrode production methods do not require the handling of metallic sodium as a raw material, are highly safe, and can be performed using simple processes and equipment.

[0063] Examples of the conductive agent include carbon materials such as natural graphite, artificial graphite, cokes, and carbon black. Examples of the binder include thermoplastic resins, specifically fluororesins such as polyvinylidene fluoride (hereinafter sometimes referred to as "PVDF"), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymers, hexafluoropropylene-vinylidene fluoride copolymers, and tetrafluoroethylene-perfluorovinyl ether copolymers; and polyolefin resins such as polyethylene and polypropylene.

[0064] When the positive electrode is used as the positive electrode of a secondary battery, Al, Ni, stainless steel, etc. can be used as the electrode current collector.

[0065] Methods for forming the electrode mixture include pressure molding the electrode mixture itself, or forming a paste using an organic solvent, applying the paste to an electrode current collector, drying, and then pressing to adhere it to the electrode current collector. When forming a paste, a slurry containing the electrode active material, conductive agent, binder, and organic solvent is prepared. Examples of organic solvents include amines such as N,N-dimethylaminopropylamine and diethyltriamine; ethers such as ethylene oxide and tetrahydrofuran; ketones such as methyl ethyl ketone; esters such as methyl acetate; and aprotic polar solvents such as dimethylacetamide and N-methyl-2-pyrrolidone. Examples of methods for applying the electrode mixture to an electrode current collector include slit die coating, screen coating, curtain coating, knife coating, gravure coating, and electrostatic spraying.

[0066] <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 absorbing and releasing sodium. Examples of the negative electrode active material include carbon materials such as hard carbon, soft carbon, natural graphite, artificial graphite, cokes, carbon black, pyrolytic carbons, carbon fiber, and fired organic polymer compounds; Maxene, a layered phosphorus compound and 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; and alloys, oxides, halides, and chalcogenides of these elements.

[0067] Carbon materials can be obtained, for example, by carbonizing 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 structure with developed three-dimensional cross-linking, and it is presumed that carbon materials obtained by carbonizing such resins also have a unique structure with developed three-dimensional cross-linking due to this characteristic, and are considered preferable from the viewpoint of increasing charge / discharge capacity.

[0068] Sodium 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 discharging. It is recognized that the discharge capacity of the positive electrode active material of the present invention can be higher than the theoretical capacity corresponding to when x in formula (1) is 1. In such a case, the positive electrode active material after discharging the sodium secondary battery has a capacity corresponding to the theoretical capacity of when x in formula (1) is 1. Na x Mn 1-y-z M1 y M2 z O2(3) [In the formula, x represents a value greater than 1. M1, M2, y, and z are as defined above.] The formula (3) represents the chemical structure of the positive electrode active material after discharging the sodium secondary battery.

[0069] Since the capacity of the positive electrode active material of the present invention increases during the first discharge compared to the first charge, it is necessary to absorb a larger amount of sodium ions during the first discharge than the sodium ions released during the first charge. Therefore, the high charge-discharge capacity inherent to the positive electrode active material can be exhibited by, for example, pre-containing a sodium compound in the positive electrode, increasing the electrolyte concentration of the electrolyte solution to a higher level than conventional methods, or pre-doping the negative electrode with sodium ions.

[0070] When the electrolyte concentration of the electrolytic solution is made higher than conventionally, or when sodium ions are doped in advance into the negative electrode, the electrolyte concentration of the electrolytic solution or the amount of sodium doped into the negative electrode is preferably set so that the equivalent ratio of sodium to the metal to be oxidized is 1:1, or so that the sodium is in excess of the equivalent ratio of 1:1 relative to the metal to be oxidized, based on the entire sodium secondary battery.

[0071] The method for doping the negative electrode with sodium is not particularly limited, and examples thereof include (i) a method in which metallic sodium is attached to a portion of the negative electrode current collector where there is no negative electrode material, and a local cell is formed by injecting the sodium, and sodium is doped into the negative electrode active material; (ii) a method in which metallic sodium is formed on the negative electrode material by vapor deposition or sputtering, and sodium is doped into the negative electrode material by a solid-phase reaction; (iii) a method in which sodium is electrochemically doped into the negative electrode in an electrolyte before the battery is constructed; and (iv) a method in which metallic sodium is added and mixed during the production of the composite powder used in the present invention, and sodium is doped into the negative electrode material.

[0072] From the viewpoint of improving the energy density of a sodium secondary battery, the amount of sodium doped into the negative electrode is 60% by weight or less, preferably 1 to 40% by weight, and more preferably 5 to 30% by weight, based on the negative electrode active material. If the amount of sodium doped exceeds 60% by weight based on the negative electrode active material, the energy density of the entire cell may decrease, which is not preferable.

[0073] The negative electrode mixture contains a negative electrode active material and, as necessary, the binder, the conductive agent, etc. Examples of the negative electrode current collector include Al, Cu, Ni, and stainless steel. Al or Cu is preferred because it is difficult to form an alloy with sodium and is easy to process into a thin film. The method for supporting the negative electrode mixture on the negative electrode current collector is the same as that for the positive electrode described above, and examples include pressure molding, applying a paste using a solvent or the like to the negative electrode current collector, drying it, and then pressing it to adhere it.

[0074] <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 stacking or winding a positive electrode, a separator, and a negative electrode in this order to obtain an electrode group, storing the electrode group in a container such as a battery can, and impregnating the electrode group with an electrolytic solution comprising an organic solvent containing an electrolyte. The positive electrode or negative electrode may be formed by supporting an electrode mixture on an electrode current collector.

[0075] Examples of the shape of the electrode group include a shape in which the cross section of the electrode group cut perpendicular to the winding axis is a circle, an ellipse, a rectangle, a rectangle with rounded corners, or the like.

[0076] The shape of the battery may be, for example, a paper type, a coin type, a cylindrical type, or a square type.

[0077] Fig. 1 is an exploded view showing a schematic configuration of a sodium secondary battery according to one embodiment of the present invention. The sodium secondary battery shown in Fig. 1 is a coin-type sodium secondary battery, and is composed of an electrode plate assembly using the positive electrode 4, an electrolyte (not shown), and a battery case that houses these. The electrode plate assembly is composed of a sheet-shaped positive electrode 4, a sheet-shaped negative electrode 6, and a sheet-shaped separator 5 that insulates the positive electrode 4 from the negative electrode 6.

[0078] These are stacked together with a leaf spring 2, a spacer 3, a gasket 7, etc., which are commonly used in the field of sodium secondary batteries, and housed in battery cases 1, 8.

[0079] Separators that can be used in sodium secondary batteries include materials in the form of porous films, nonwoven fabrics, woven fabrics, and the like, made of polyolefin resins such as polyethylene and polypropylene, fluororesins, and nitrogen-containing aromatic polymers. Monolayer or laminated separators using two or more of these materials may also be used. Examples of separators include those described in JP-A Nos. 2000-30686 and 10-324758. The thickness of the separator is preferably as thin as possible while maintaining mechanical strength, as this increases the volumetric energy density and reduces the internal resistance of the battery. The thickness of the separator is generally preferably about 5 to 200 μm, more preferably about 5 to 40 μm. From the viewpoint of ion permeability, the separator preferably has an air permeability of 50 to 300 seconds / 100 cc, and even more preferably 50 to 200 seconds / 100 cc, as measured by the Gurley method. The porosity of the separator is usually 30 to 80% by volume, and preferably 40 to 70% by volume. The separator may be a laminate of separators with different porosities.

[0080] In the electrolyte solution that can be used in the sodium secondary battery, examples of organic solvents include 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 carbonates such as 1,2-di(methoxycarbonyloxy)ethane; 1,2-dimethoxyethane, 1,3-dimethoxypropane, pentafluoropropyl methyl ether, and 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-propane sultone; or organic solvents such as those mentioned above that further contain fluorine substituents can be used. Typically, two or more of these organic solvents are mixed and used as the organic solvent.

[0081] The electrolyte of the electrolytic solution is not particularly limited, but salts commonly used in sodium secondary batteries can be used, such as NaPF6, NaBF4, NaClO4, NaTiF4, NaVF5, NaAsF, NaSbF6, NaCF3SO3, Na(C2F5SO2)2N, NaB(C2O4)2, NaB 10 Cl 10 , NaB 12 Cl 12 , NaCF3COO, Na2S2O4, NaNO3, Na2SO4, NaPF3(C2F5)3, NaB(C6F5)4, and Na(CF3SO2)3C can be used. Note that one of the above salts may be used alone, or two or more may be used in combination.

[0082] Among these, sodium hexafluorophosphate (NaPF6), sodium perchlorate (NaClO4), sodium tetrafluoroborate (NaBF4), etc. are preferably used, and NaPF6 is particularly preferably used. By using NaPF6 as a salt, the discharge capacity and cycle life of the positive electrode are improved, and the effect of improving the cycle life of the negative electrode is enhanced.

[0083] From the viewpoint of improving the energy density of a sodium secondary battery, the electrolyte concentration of the electrolytic 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.

[0084] Sodium secondary batteries are operated at a voltage of 4.2 V or less based on the sodium electrode potential. Operating at a higher voltage may result in problems such as exceeding the oxidation resistance pressure of the electrolyte solvent, resulting in reduced cycle durability. On the other hand, it has been revealed that exposing the positive electrode of the present invention to a high voltage further increases the energy density of sodium secondary batteries. In other words, from the perspective 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 4.2 V or more.

[0085] 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 4.2 V or more, for example, 4.4 to 4.5 V, based on the sodium electrode potential, at least once. This can further increase 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 to 25%, preferably 10 to 15%. After the positive electrode active material has been subjected to a high voltage, the sodium secondary battery can be operated at a rated voltage of, for example, 4.2 V or less, thereby preventing deterioration of cycle characteristics due to high voltage while maintaining improved performance. [Example]

[0086] The present invention will be explained in more detail with reference to the following examples, but the present invention is not limited to these examples. However, Example 12 is a reference example.

[0087] <Examples 1 to 17> (1) Synthesis of complex metal oxides A metal solution was prepared by dissolving manganese chloride, cobalt chloride, nickel chloride, iron chloride, titanium trichloride, magnesium chloride, and copper chloride in purified water at a predetermined ratio. A sodium hydroxide solution was prepared by dissolving 10 g of sodium hydroxide in 50 ml of purified water. The metal solution was added dropwise to the sodium hydroxide solution under atmospheric conditions to form a precipitate, which was then exposed to oxygen. The precipitate was filtered from the slurry and washed with purified water.

[0088] The precipitate and sodium hydroxide were weighed out in amounts such that the metal and sodium contained in the precipitate were in an equivalent ratio of 1:1, and pure water was added and mixed uniformly. The resulting slurry was dried at 120°C. The resulting powder was pre-fired in air at 600°C for 5 hours and crushed in a mortar. The resulting powder was pre-fired at 750°C for 5 hours, crushed in a mortar, washed with water, and dried. The resulting powder was then fired in air at 900°C, crushed in a mortar, washed with water, and dried. In this way, a composite metal oxide was synthesized. The metal compounding ratio (molar ratio) is shown in Table 1.

[0089] The composition ratios of the composite metal oxides synthesized in Examples 13 to 15 and 17 were measured by ICP (plasma atomic absorption spectrometry). 0.45 Mn 0.8 Ni 0.15 Co 0.05 O2 (Example 13), Na 0.55 Mn 0.8 Ni 0.15 Ti 0.05 O2 (Example 14), Na 0.60 Mn 0.8 Ni 0.15 Mg 0.05 O2 (Example 15) and Na 0.40 Mn 0.8 Ni 0.15 Cu 0.05 O2 (Example 17).

[0090] (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, "F104" (trade name) manufactured by Daikin Industries, Ltd.) as the binder were weighed out to a weight ratio of electrode active material:conductive material:binder = 8:1:1, and then placed in a bottle and shaken to mix. The mixture was crushed in a mortar while applying shear force to form a sheet with a thickness of approximately 100 μm. The resulting sheet was punched out to a diameter of 13 mm to obtain a positive electrode or a negative electrode.

[0091] (4) Preparation of Na-doped anode A metallic sodium foil having a thickness of 20 μm and a diameter of 5 mm was pressed onto the dried negative electrode disk in a glove box.

[0092] (5) Fabrication of sodium secondary batteries The resulting positive and negative electrodes, electrolyte, separator, gasket, case, and other commonly used components were prepared and used to assemble a sodium secondary battery cell. The weight ratio of the positive and negative electrodes was adjusted to 1:0.8. An exploded perspective view of the cell is shown in Figure 1.

[0093] (Electrolyte A) An electrolyte containing NaPF6 at a specified concentration in a 1:1 mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC).

[0094] (Electrolyte B) An electrolyte containing NaClO4 at a specified concentration in a 1:1 mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC).

[0095] Next, a charge / discharge test was performed on the cell under the following conditions using a charge / discharge tester to determine its characteristics. The measurement items and results are shown in Tables 1 and 2.

[0096] Current: 0.lmAh Mode: CCCVCC(0.05C cutoff)CC Temperature: 35℃ Positive voltage range: 1.25~4.2V

[0097] Example 18 Using a charge / discharge test device, the sodium secondary battery of Example 1 was subjected to one cycle of charge / discharge under the following conditions.

[0098] Current: 0.lmAh Mode: CCCVCC(0.05C cutoff)CC Temperature: 35℃ Positive voltage range: 1.25~4.4V

[0099] Thereafter, the sodium secondary batteries that had been subjected to high voltage were subjected to determination of battery characteristics in the same manner as in Example 1. The measurement items and measurement results are shown in Tables 1 and 2.

[0100] Example 19 A composite metal oxide was synthesized in the same manner as in Example 1, except that sodium carbonate was used instead of sodium hydroxide, and a sodium secondary battery was fabricated and its characteristics were tested. The results are shown in Tables 1 and 2.

[0101] <Comparative Example 1> A sodium secondary battery was fabricated in the same manner as in Example 1, except that a composite metal oxide of Mn:Ni:Fe=1:1:1 (manufactured by Beijing Dangsheng Co., Ltd.) was used as the positive electrode active material, and its characteristics were tested. The results are shown in Tables 1 and 2.

[0102] <Comparative Example 2> Sodium carbonate (Na2CO3) powder with a purity of 99% or higher, manganese (II, III) oxide (Mn3O4), and nickel manganese cobalt hydroxide (NiMnCo)OH2 were weighed out to a molar ratio of Na:(Mn + Co + Ni) = 1:1 and a molar ratio of Mn:Ni:Co = 8:1.5:0.5. These were mixed in a mortar, then filled into an alumina crucible and fired in an electric furnace at 900°C in air for 10 hours. The mixture was then allowed to cool naturally in the electric furnace to obtain a composite metal oxide.

[0103] A sodium secondary battery was fabricated in the same manner as in Example 1, except that the obtained composite metal oxide was used as the positive electrode active material, and the characteristics were tested. The results are shown in Tables 1 and 2.

[0104] <Control Example 1> A lithium secondary battery was fabricated and its characteristics were tested in the same manner as in Example 1, except that a composite metal oxide of Mn:Ni:Co=1:1:1 (manufactured by Beijing Dangsheng Co., Ltd.) was used as the positive electrode active material, graphite (manufactured by BTR Co., Ltd.) was used as the negative electrode, and a 1 mol / L solution of LiPF6 (EC:DEC=1:1) was used as the electrolyte. The results are shown in Tables 1 and 2.

[0105] (Electrolyte C) An electrolyte containing LiPF6 at a specified concentration in a 1:1 mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC).

[0106] <Control Example 2> A lithium secondary battery was fabricated in the same manner as in Example 1, except that LFP (lithium iron phosphate (LiFePO4), manufactured by Gelon) was used as the positive electrode active material, graphite (manufactured by BTR) was used as the negative electrode, and a 1 mol / L solution of LiPF6 (EC:DEC = 1:1) was used as the electrolyte, and its characteristics were tested. The results are shown in Tables 1 and 2.

[0107] <Control Example 3> A lithium secondary battery was fabricated in the same manner as in Comparative Example 2, except that silicon (Si)-doped graphite (manufactured by BTR) was used as the negative electrode, and its characteristics were tested. The results are shown in Tables 1 and 2.

[0108] [Table 1] *1: Phenolic resin carbide *2: Sodium-doped phenolic resin carbide *3: Anode made of metallic sodium, 250 μm thick and 14 mm in diameter

[0109] [Table 2]

[0110] From the measurement results shown in Table 2, it can be seen that the sodium secondary battery using the positive electrode active material of the present invention has an energy density equal to or higher than that of a lithium secondary battery using a ternary material or an LFP lithium secondary battery.

[0111] <Examples 20 to 22> Preparation of sodium carbonate-containing positive electrode The metal portion to be oxidized in the composite metal oxide (positive electrode active material) produced in Example 1 was mixed with sodium carbonate at a predetermined mixing ratio. The resulting mixture, acetylene black (manufactured by Denki Kagaku Kabushiki Kaisha) as a conductive material, and polyfluorotetraethylene (PTFE, "F104" (trade name) manufactured by Daikin Industries, Ltd.) as a binder were weighed out so as to achieve a composition of mixture:conductive material:binder = 8:1:1 (weight ratio), and then placed in a bottle and shaken to mix. The mixture was crushed in a mortar while applying shear force to form a sheet with a thickness of approximately 100 μm. The resulting sheet was punched out to a diameter of 13 mm to obtain a positive electrode.

[0112] A sodium secondary battery was fabricated in the same manner as in Example 1 except that the obtained positive electrode was used, and the characteristics were tested. The results are shown in Table 3.

[0113] Example 23 Sodium carbonate and the composite metal oxide (positive electrode active material) produced in Example 1 were dispersed in ethanol. This dispersion was dried by spray drying to obtain particles with sodium carbonate supported on the surface of the active material. The amount of sodium carbonate mixed was adjusted so that the compounding ratio of the metal portion to be oxidized in the positive electrode active material to the sodium carbonate was a predetermined compounding ratio. A positive electrode was obtained in the same manner as in Example 20, except that the obtained mixture was used.

[0114] A sodium secondary battery was fabricated in the same manner as in Example 1 except that the obtained positive electrode was used, and the characteristics were tested. The results are shown in Table 3.

[0115] [Table 3] *1: Phenolic resin carbide *2: Sodium carbonate was dispersed in ethanol together with the active material of Example 1, and the resulting dispersion was spray-dried to be supported on the surface of the active material.

[0116] Example 24 A sodium secondary battery was fabricated in the same manner as in Example 11 and charged and discharged three times under the same conditions. The charge and discharge capacities were measured during the third charge and discharge. The sodium secondary battery was then disassembled in a discharged state to remove the positive electrode. The positive electrode mixture was washed with ethanol and dissolved in nitric acid. The residue was filtered from the solution to prepare a sample solution, and its composition was analyzed using ICP. The results are shown in Table 4.

[0117] Example 25 A sodium secondary battery was fabricated in the same manner as in Example 17 and charged and discharged three times under the same conditions. The charge and discharge capacities were measured during the third charge and discharge. The sodium secondary battery was then disassembled in a discharged state to remove the positive electrode. The positive electrode mixture was washed with ethanol and dissolved in nitric acid. The residue was filtered from the solution to prepare a sample solution, and its composition was analyzed using ICP. The results are shown in Table 4.

[0118] <Control Example 4> A sodium secondary battery was fabricated in the same manner as in Example 23, except that a composite metal oxide of Mn:Ni:Fe=1:1:1 (manufactured by Beijing Tosho Co., Ltd.) was used as the positive electrode active material, and the charge / discharge generation capacity was measured. The positive electrode active material was then removed from the sodium secondary battery in a discharged state, and its composition was analyzed using ICP. The results are shown in Table 4.

[0119] [Table 4]

[0120] From the experimental results shown in Table 4, it can be understood that the discharge capacity of a sodium secondary battery using the positive electrode active material of the present invention can be increased beyond the theoretical capacity, and that the positive electrode active material after discharge has the chemical structure shown in formula (3).

[0121] Example 26 A composite metal oxide was synthesized in the same manner as in Example 1, except that manganese chloride and iron chloride were used in amounts such that the molar ratio of Mn to Fe was 8:2.

[0122] The composite metal oxide (positive electrode active material) and Na5FeO4 were dispersed in ethanol in a molar ratio of 1:0.1. This dispersion was dried by spray drying to obtain particles with Na5FeO4 supported on the active material surface. A positive electrode was obtained in the same manner as in Example 20, except that the obtained mixture was used.

[0123] A sodium secondary battery was fabricated in the same manner as in Example 1 except that the obtained positive electrode was used, and the characteristics were tested. The results are shown in Table 5.

[0124] Example 27 A metal solution was prepared by dissolving manganese chloride and iron chloride in pure water in amounts such that the molar ratio of Mn to Fe was 8:2. A sodium hydroxide solution was prepared by dissolving 10 g of sodium hydroxide in 50 ml of pure water. The metal solution was added dropwise to the sodium hydroxide solution under atmospheric conditions to form a precipitate, which was then exposed to oxygen. The precipitate was filtered from the slurry and washed with pure water.

[0125] The precipitate and sodium hydroxide were weighed in amounts such that the metal and sodium contained in the precipitate were in an equivalent ratio of 1:1, and pure water was added and mixed uniformly. The resulting slurry was dried at 120°C. The resulting powder was pre-fired at 600°C for 5 hours under vacuum and crushed in a mortar. The resulting powder was pre-fired at 750°C for 5 hours and crushed in a mortar. The resulting powder was finally fired at 900°C under vacuum, crushed in a mortar, and dried. In this way, a composite metal oxide was synthesized. The metal compounding ratio (molar ratio) is shown in Table 1. A composite metal oxide was synthesized in the same manner as in Example 1, except for using ammonium hydroxide.

[0126] A positive electrode was obtained in the same manner as in Example 20, except that the produced composite metal oxide (positive electrode active material) was used.

[0127] A sodium secondary battery was fabricated in the same manner as in Example 1 except that the obtained positive electrode was used, and the characteristics were tested. The results are shown in Table 5.

[0128] [Table 5]

[0129] *1: Phenolic resin carbide *2: Na5FeO4 was dispersed in ethanol together with the positive electrode active material, and this dispersion was spray-dried to support it on the surface of the positive electrode active material. [Explanation of symbols]

[0130] 1, 8 cases 2 leaf springs 3 spacers 4 Positive electrode 5 Separator 6 negative electrode 7 Gasket

Claims

1. formula <h2 style=";text-align:left;direction:ltr">Na<h2 style=";text-align:left;direction:ltr"> ｘ <h2 style=";text-align:left;direction:ltr"> 4Mn<h2 style=";text-align:left;direction:ltr"> １－ｙ-z <h2 style=";text-align:left;direction:ltr"> 11<h2 style=";text-align:left;direction:ltr"> ｙ <h2 style=";text-align:left;direction:ltr"> 12<h2 style=";text-align:left;direction:ltr"> ｚ <h2 style=";text-align:left;direction:ltr"> O<h2 style=";text-align:left;direction:ltr"> ２ <h2 style=";text-align:left;direction:ltr"> (1) [In the formula, M1 represents Ni or Fe, M2 represents Co, Ti, Mg, Cu, or Al, x represents 0.6 or less, y represents 0.1 to 0.3, and z represents 0 to 0.1.] A positive electrode active material for a sodium secondary battery, comprising a composite metal oxide represented by the formula:

2. 2. The positive electrode active material for a sodium secondary battery according to claim 1, wherein x is 0.2 to 0.

6.

3. 3. The positive electrode active material for a sodium secondary battery according to claim 2, wherein y is 0.15 to 0.

3.

4. The positive electrode active material for a sodium secondary battery according to claim 3, wherein z is 0.01 to 0.

08.

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

6. The positive electrode for a sodium secondary battery according to claim 5 , comprising a sodium compound.

7. 7. The positive electrode for a sodium secondary battery according to claim 6, wherein the sodium compound is contained in the positive electrode in an amount of 1 to 50 mol % based on the number of moles of the metal portion to be oxidized in the positive electrode active material.

8. The positive electrode for a sodium secondary battery according to claim 6 , wherein the sodium compound includes sodium hydroxide or sodium carbonate.

9. 7. The positive electrode for a sodium secondary battery according to claim 6, wherein the sodium compound includes pentasodium ferrate.

10. 7. The positive electrode for a sodium secondary battery according to claim 6, wherein the sodium compound is supported on the surface of the positive electrode active material.

11. The positive electrode for a sodium secondary battery according to claim 10 , wherein the sodium compound includes sodium hydroxide, sodium carbonate, or pentasodium ferrate.

12. A sodium secondary battery comprising the positive electrode for sodium secondary batteries according to claim 5.

13. A sodium secondary battery comprising the positive electrode for sodium secondary batteries according to claim 6.

14. A sodium secondary battery comprising the positive electrode for sodium secondary batteries according to claim 7.

15. A sodium secondary battery comprising the positive electrode for sodium secondary batteries according to claim 10.

16. The method for producing a sodium secondary battery according to claim 12, comprising charging at a voltage exceeding 4.2 V based on a sodium electrode potential.

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