JP7560939B2 - Positive electrode active material for sodium secondary battery, positive electrode for sodium secondary battery, sodium secondary battery - Google Patents

Description

The present invention relates to a positive electrode active material for a sodium secondary battery, a positive electrode for a sodium secondary battery, a sodium secondary battery, and a positive electrode active material intermediate.

In recent years, lithium secondary batteries have come into practical use as secondary batteries, and their applications are expanding. However, the lithium, nickel, and cobalt used in lithium secondary batteries are not abundant resources, and there are concerns that these resources will be depleted in the future.

On the other hand, sodium, which is also an alkali metal, is more abundant than lithium and is an order of magnitude cheaper than lithium. In addition, sodium has a relatively high standard potential, so it is thought that sodium secondary batteries can become high-capacity secondary batteries.

If sodium secondary batteries could be used instead of the current lithium secondary batteries, it would be possible to mass-produce large secondary batteries, such as in-vehicle secondary batteries and secondary batteries for distributed power storage, without having to worry about resource depletion.

Patent Document 1 and Non-Patent Document 1 disclose that a sodium-containing composite metal oxide represented by _NaMnO2 is used as a positive electrode active material.

JP 2006-216509 A

MRS Bulletin Vol. 39, p. 416 Komaba et. al.

However, sodium secondary batteries using the sodium-containing composite metal oxides of Patent Document 1 and Non-Patent Document 1 as the positive electrode active material have low energy density. Therefore, it is difficult to say that they are sufficiently usable for non-aqueous electrolyte secondary batteries.

It is known that there is a close relationship between the energy density of the components of a sodium secondary battery and the performance of the positive electrode active material for the sodium secondary battery used in the sodium secondary battery. Specifically, if the mass energy density of the positive electrode active material for the sodium secondary battery is high, the mass energy density of the resulting sodium secondary battery will be high.

The mass energy density (unit: mWh/g) of a positive electrode active material is expressed as the product of the discharge voltage and discharge capacity per unit mass of the positive electrode active material. In this specification, the mass energy density may be simply referred to as "energy density." In order to obtain a positive electrode active material with a high energy density, it has been necessary to increase the discharge voltage and discharge capacity.

The present invention has been made in consideration of these circumstances, and aims to provide a positive electrode active material for a sodium secondary battery that can provide a sodium secondary battery with both high discharge voltage and high discharge capacity. Another aim of the present invention is to provide a high-performance positive electrode for a sodium secondary battery and a sodium secondary battery that use such a positive electrode active material for a sodium secondary battery. A further aim of the present invention is to provide a positive electrode active material intermediate that can be used to suitably manufacture such a positive electrode active material for a sodium secondary battery.

In order to solve the above problems, one embodiment of the present invention includes the following inventions [1] to [8].
[1] A positive electrode active material for a sodium secondary battery, comprising: active material particles made of a composite metal oxide having an O3 type crystal structure and containing Na, Mn, Ti, and Mg; and a carbon layer covering the surfaces of the active material particles.

[2] The positive electrode active material for a sodium secondary battery according to [1], wherein the composite metal oxide is represented by the following formula (1):
Na _a (M ¹ _w M ² _v Mn _x Ti _y-z Mg _z )O ₂ (1)
(Here, ^M1 represents Co or Ni, ^M2 represents one or more elements selected from the group consisting of Fe, Cu, Mo, and Ca, a is 0.6 or more and 1 or less, w is 0 or more and less than 0.25, v is 0 or more and less than 0.25, w+v is 0 or more and less than 0.25, x is more than 0 and less than 0.98, y is more than 0.02 and 0.5 or less, z is 0.03 or more and less than 0.5, and w+v+x+y=1, y>z.)

[3] A positive electrode active material for a sodium secondary battery according to [2], in which w = 0.

[4] A positive electrode for a sodium secondary battery containing the positive electrode active material described in any one of [1] to [3].

[5] A sodium secondary battery having a positive electrode, a negative electrode, and a non-aqueous electrolyte, the positive electrode having the positive electrode for sodium secondary batteries described in [4].

[6] A positive electrode active material intermediate having an O3 type crystal structure and consisting of a composite metal oxide containing Na, Mn, Ti and Mg.

[7] The positive electrode active material intermediate according to [6], wherein the composite metal oxide is represented by the following formula (1):
Na _a (M ¹ _w M ² _v Mn _x Ti _y-z Mg _z )O ₂ (1)
(Here, ^M1 represents Co or Ni, ^M2 represents one or more elements selected from the group consisting of Fe, Cu, Mo, and Ca, a is 0.6 or more and 1 or less, w is 0 or more and less than 0.25, v is 0 or more and less than 0.25, w+v is 0 or more and less than 0.25, x is more than 0 and less than 0.98, y is more than 0.02 and 0.5 or less, z is 0.03 or more and less than 0.5, and w+v+x+y=1, y>z.)

[8] The positive electrode active material intermediate according to [7], where w = 0.

According to the present invention, it is possible to provide a positive electrode active material for a sodium secondary battery that can provide a sodium secondary battery with both high discharge voltage and high discharge capacity. In addition, according to the present invention, it is possible to provide a high-performance positive electrode for a sodium secondary battery and a sodium secondary battery that use such a positive electrode active material for a sodium secondary battery. Furthermore, according to the present invention, it is possible to provide a positive electrode active material intermediate that can be used to suitably manufacture such a positive electrode active material for a sodium secondary battery.

FIG. 1A is a schematic diagram showing an electrode group of a sodium secondary battery. FIG. 1B is an exploded perspective view of the sodium secondary battery. FIG. 2 is an XRD chart showing the results of powder X-ray diffraction measurement of active material particles 1, 4, and 5. FIG. 3 shows charge/discharge curves of positive electrode active materials 1, 4, and 5.

<Positive electrode active material for sodium secondary batteries>
The positive electrode active material for a sodium secondary battery of this embodiment has an O3 type crystal structure and includes active material particles made of a composite metal oxide containing Na, Mn, Ti, and Mg, and a carbon layer covering the surfaces of the active material particles.

In the following description, the positive electrode active material for sodium secondary batteries may be simply referred to as the "positive electrode active material."

In this specification, "discharge capacity of the positive electrode active material (unit: mAh/g)" refers to the value obtained by dividing the discharge capacity measured for a sodium secondary battery by the mass of the positive electrode active material contained in the positive electrode used in the sodium secondary battery. In other words, "discharge capacity of the positive electrode active material" refers to the discharge capacity per unit mass of the positive electrode active material.

In this specification, "discharge voltage of the positive electrode active material (unit: V)" refers to the discharge voltage measured for a sodium secondary battery.

(Active material particles)
The composite metal oxide constituting the active material particles has an O3 type crystal structure, which is represented by a layered rock salt structure. Therefore, when the positive electrode active material for a sodium secondary battery of this embodiment is used in the positive electrode of a sodium secondary battery, sodium ions are likely to be reversibly inserted and removed from the active material particles in response to charging and discharging of the sodium secondary battery.
The active material particles correspond to the "positive electrode active material intermediate" in the present invention.

In the composite metal oxide constituting the active material particles, some of the sites where tetravalent Ti exists in the crystal are replaced with divalent Mg, as described below. This causes distortion in the crystal structure compared to the crystal of the composite metal oxide that is not replaced with Mg. Therefore, the composite metal oxide constituting the active material particles ideally has an O3 type crystal structure, but other impurity phases may be mixed in. The positive electrode active material for sodium secondary batteries of this embodiment may have a crystal structure different from O3 type in the active material particles, such as a P2 type and/or P3 type crystal structure, within a range that does not impair the effects of the invention.

The fact that the active material particles have an O3 type crystal structure can be confirmed by performing powder X-ray diffraction measurement on the positive electrode active material. A publicly known database may be used to identify the crystal structure.

In XRD (X-Ray Diffraction) spectra using CuKα radiation, composite metal oxides with an O3 type crystal structure have a characteristically large ratio of the intensity of the diffraction peak at a 2θ angle of around 18 degrees, which is due to the stacking direction of the layered crystal structure, to the intensity of the diffraction peak at a 2θ angle of around 41 degrees, which is due to the structure in the intralayer direction.

The "diffraction peak at a 2θ angle of about 18 degrees resulting from the structure in the stacking direction of the layered crystal structure" is, for example, a peak corresponding to the 001 plane or the 003 plane.
The "diffraction peak at a 2θ angle of about 41 degrees resulting from the structure in the intralayer direction" is, for example, a peak corresponding to the 111 or 104 plane.

In this embodiment, the crystal structure of the composite metal oxide is measured using a powder X-ray diffraction measuring device RINT2500TTR type manufactured by Rigaku Corporation under the following conditions unless otherwise specified.

X-ray: CuKα
Voltage-current: 40kV-140mA
Measurement angle range: 2θ = 10 to 90°
Step: 0.02°
Scan speed: 4°/min

The active material particles are preferably a composite metal oxide represented by the following formula (1). When the active material particles are the composite metal oxide shown below, the sodium secondary battery formed using the resulting positive electrode active material has a high discharge voltage and discharge capacity. Therefore, the sodium secondary battery using such a positive electrode active material has a high energy density.

Na _a (M ¹ _w M ² _v Mn _x Ti _y-z Mg _z )O ₂ (1)
(wherein ^M1 represents Co or Ni, ^M2 represents one or more elements selected from the group consisting of Fe, Cu, Mo, and Ca, a is 0.6 or more and 1 or less, and w is 0 or more and less than 0.25, v is 0 or more and less than 0.25, w+v is 0 or more and less than 0.25, x is greater than 0 and less than 0.98, and y is greater than 0.02 0.5 or less, z is 0.03 or more and less than 0.5, and w+v+x+y=1, and y>z.

In the above formula (1), if a is 0.6 or more, the capacity of the sodium secondary battery formed using the obtained positive electrode material is large and the energy density is high. Also, if a is 1 or less, impurities such as sodium carbonate are less likely to be mixed into the obtained positive electrode material, and the resistance of the obtained sodium battery is low and the energy density is high.

In the above formula (1), it is preferable that a is 1 and Na is saturated. When a is 1, the composite metal oxide is likely to have an O3 type crystal structure. Therefore, a sodium secondary battery formed using the obtained positive electrode active material has a large discharge voltage and discharge capacity, and a high energy density.

In the above formula (1), if x is greater than 0 and less than 0.98, the discharge capacity of a sodium secondary battery formed using the resulting positive electrode active material will be large.

In the above formula (1), x is preferably 0.5 or more, more preferably 0.6 or more, and even more preferably 0.7 or more. In addition, in order to increase the discharge capacity of a sodium secondary battery formed using the obtained positive electrode active material, x is preferably 0.95 or less.

The upper and lower limits of x can be combined in any way. In formula (1), x may be 0.5 or more and less than 0.98, 0.6 or more and less than 0.98, or 0.7 or more and less than 0.98. Also, x may be 0.5 or more and less than 0.95, 0.6 or more and less than 0.95, or 0.7 or more and less than 0.95.

In the above formula (1), y is greater than 0.02 and less than 0.5, and z is greater than or equal to 0.03 and less than 0.5, assuming that y>z.

y-z represents the molar ratio of Ti in formula (1). When the molar ratio of Ti in formula (1) is within the range represented by y and z, the sodium secondary battery formed using the resulting positive electrode active material has a high discharge voltage.

Furthermore, in the positive electrode active material of this embodiment, in the composite metal oxide represented by the above formula (1) constituting the active material particles, a portion of Ti is substituted with Mg. That is, in the positive electrode active material of this embodiment, trivalent Mn (Mn ³⁺ ) contained in NaMnO ₂ having a layered rock salt structure is substituted with tetravalent Ti (Ti ⁴⁺ ) and divalent Mg (Mg ²⁺ ).

In the case of such a composition, for example, if Mn is replaced with the same amount of Ti and Mg, the average valence of the replaced Ti and Mg becomes trivalent, the same as Mn. Therefore, the crystal structure remains electrically neutral as before the replacement, and the structure is less prone to Na loss. As a result, the positive electrode active material of this embodiment can increase the discharge capacity compared to when a composite metal oxide substituted with Ti but not substituted with Mg is used for the active material particles.

In the above formula (1), if z is 0.03 or more and less than 0.5, the discharge capacity of a sodium secondary battery formed using the resulting positive electrode active material will be large.

In the above formula (1), z is preferably 0.05 or more. In addition, in order to increase the discharge capacity of a sodium secondary battery formed using the obtained positive electrode active material, z is preferably 0.2 or less, more preferably 0.15 or less, and even more preferably 0.12 or less.

The upper and lower limits of z can be combined in any combination. In formula (1), z may be 0.03 or more and 0.2 or less, 0.03 or more and 0.15 or less, or 0.03 or more and less than 0.12. In addition, z may be 0.05 or more and 0.2 or less, 0.05 or more and 0.15 or less, or 0.05 or more and 0.12 or less.

In the above formula (1), y-z is preferably 0.05 or more. In addition, in order to increase the discharge capacity of a sodium secondary battery formed using the obtained positive electrode active material, y-z is preferably 0.2 or less, more preferably 0.15 or less, and even more preferably 0.12 or less.

The upper and lower limits of y-z can be combined in any combination. In the above formula (1), y-z may be 0.05 or more and 0.2 or less, 0.05 or more and 0.15 or less, or 0.05 or more and 0.12 or less.

In the above formula (1), y-z is preferably equal to or greater than z, and more preferably equal to z.

In the above formula (1), ^M1 may be included as an optional metal. In the above formula (1), if w exceeds 0, the discharge capacity may be large. In addition, if w is less than 0.25, a composite metal oxide having an O3 type crystal structure is likely to be generated. From the viewpoint of reducing the amount of expensive elements such as nickel and cobalt used, w is preferably 0.

In the above formula (1), ^M2 may be included as an optional metal. In the above formula (1), if v exceeds 0, the discharge capacity may be large. In addition, if v is less than 0.25, a composite metal oxide having an O3 type crystal structure is likely to be generated.

In the above formula (1), when w, v, x, and y satisfy w+v+x+y=1, the cycle characteristics of the sodium secondary battery formed using the obtained positive electrode active material for sodium secondary batteries are improved.

The term "cycle characteristics" refers to the capacity retention rate when the battery is repeatedly charged and discharged. A secondary battery that has a high capacity retention rate when repeatedly charged and discharged is evaluated as having "good cycle characteristics."

In the above formula (1), when the relationship y>z is satisfied, the discharge capacity of the sodium secondary battery formed using the obtained positive electrode active material for sodium secondary batteries is improved.

(Carbon layer)
The carbon layer covers the surface of the active material particles and is made of one or more materials selected from the group consisting of natural graphite, artificial graphite, cokes, carbon black, acetylene black, ketjen black, carbon nanotubes (CNTs), and carbon nanofibers (CNFs).

When making a positive electrode using a typical positive electrode active material, a known method is to prepare a positive electrode mixture by kneading the positive electrode active material with a binder together with a conductive material such as acetylene black, and then apply it to a current collector. In this case, the positive electrode active material and the conductive material are separated from each other unless they come into contact inside the binder during the kneading or application process.

In contrast, in the positive electrode active material of this embodiment, a carbon layer is provided in advance on the surface of the active material particles made of a composite metal oxide, ensuring contact between the active material particles and the carbon layer. In a positive electrode active material with such a structure, the electrical conduction between the active material particles is improved. In addition, when dispersed in a binder in the positive electrode, the carbon is more uniformly dispersed within the binder. This reduces the resistance between the positive electrode active material and the current collector, improving electrical conduction. As a result, the discharge capacity is improved.

The carbon layer can be formed by mixing active material particles with the material for forming the carbon layer. A ball mill can be used for kneading.

For example, the carbon layer can be formed by placing 10 g of active material particles, 1 g of acetylene black (manufactured by Denka) and 150 g of zirconia balls with a diameter of 5 mm in a 100 mL zirconia bottle, mixing them at 300 rpm for 6 hours using a planetary ball mill, and then sieving out the zirconia balls. The kneading conditions, such as the kneading time and the mixing ratio of the active material particles to the material for forming the carbon layer, can be optimized while checking the physical properties of the positive electrode active material obtained after kneading.

When a carbon layer is formed on the surface of the active material particles using this method, the particle size of the active material particles becomes smaller during the carbon layer formation process. Therefore, when the resulting positive electrode active material is dispersed in a binder in a positive electrode, it disperses more uniformly within the binder, reducing resistance. This improves the discharge capacity.

(others)
The BET specific surface area of the positive electrode active material for a sodium secondary battery of this embodiment is preferably 0.1 m ² /g or more and 5 m ² /g or less. When the BET specific surface area of the positive electrode active material for a sodium secondary battery is within such a range, the energy density tends to be particularly high.

The BET specific surface area of the positive electrode active material for sodium secondary batteries is more preferably 0.3 m ² /g or more, and even more preferably 0.5 m ² /g or more. The BET specific surface area is more preferably 4.5 m ² /g or less, and even more preferably 4 m ² /g or less. These upper and lower limit values of the BET specific surface area can be combined arbitrarily.

When the active material particles of the positive electrode active material for sodium secondary batteries of this embodiment are a composite metal oxide represented by formula (1), in addition to the metals contained in formula (1), metal elements such as Li, K, Ag, Ca, Sr, Ba, Al, Ga, In, V, Cr, Cu, Zn, Sc, Y, Nb, Mo, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Ho, Er, Tm, Yb, and Lu may be contained within a range that does not impair the effects of the invention.

<Method of manufacturing positive electrode active material for sodium secondary battery>
The positive electrode active material for a sodium secondary battery of this embodiment can be produced by producing active material particles made of a composite metal oxide, and then forming a carbon layer on the surface of the active material particles.

(Preparation of active material particles)
The active material particles can be produced by firing raw materials having a composition corresponding to the composite metal oxide.

The raw material for the positive electrode active material is a mixture of metal-containing compounds. By firing the mixture, a composite metal oxide having a layered crystal structure is produced.

Specifically, first, the metal element contained in the composite metal oxide of the target positive electrode active material is prepared as a metal-containing compound that contains the metal element.

Next, the metal-containing compound is weighed out so as to have a composition that corresponds to the metal element ratio in the composite metal oxide contained in the positive electrode active material for sodium secondary batteries, and mixed to obtain a mixture.

The resulting mixture is then fired to obtain active material particles, which are composite metal oxide particles.

For example, a composite metal oxide having a metal element ratio of Na:Mn:Mg:Ti=1:0.9:0.05:0.05 may be used as a preferred composite metal oxide for the active material particles. Such a composite _metal oxide may be produced by weighing out the raw materials _Na2CO3 , _{Mn2O3 ,} Mg(OH) ₂ , and _TiO2 so that the molar ratio of Na:Mn:Mn:Ti is 1:0.9:0.05:0.05, mixing them, and firing the resulting mixture.

Examples of metal-containing compounds used as raw materials for active material particles include oxides, hydroxides, carbonates, nitrates, halides, and oxalates, which can be converted to oxides at high temperatures by combining with the oxygen atoms they contain or with oxygen molecules in the atmosphere.

Examples of sodium compounds among the metal-containing 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, and sodium carbonate, and these compounds may be hydrates.

Among these, sodium carbonate is preferred because it has low hygroscopicity and is easy to handle. Sodium hydroxide is also preferred because it has high reactivity at low temperatures and can be fired at a relatively low firing temperature, which allows for low manufacturing costs.

As the _manganese compound among the metal-containing compounds, either or both of _Mn2O3 and _MnO2 are preferred.

Of the metal-containing compounds, the magnesium compound is preferably either or both of Mg(OH) ₂ and _MgCO3 .

Among the metal-containing compounds, the titanium compound is preferably _TiO2 .

These metal-containing compounds may be hydrates or basic salts.

In addition, among the metal-containing compounds used as raw materials for active material particles, those obtained by the coprecipitation method described below can also be used except for sodium compounds. In the following explanation, metal-containing compounds other than sodium compounds that are obtained by the coprecipitation method will be referred to as "transition metal-containing compounds" for convenience.

Specifically, first, compounds such as chlorides, nitrates, acetates, formates, oxalates, or sulfates of Mn, Ti, and Mg are dissolved in water to obtain a mixed aqueous solution.

The aqueous solution can then be contacted with a precipitant to obtain a precipitate containing the desired transition metal-containing compound.

Among the compounds used as the raw material for the transition metal-containing compound, chlorides or sulfates are preferred. In addition, when a compound that is difficult to dissolve in water, such as an oxide, hydroxide, or metal material, is used as the raw material for the transition metal-containing compound, it is possible to dissolve it in an acid such as hydrochloric acid, sulfuric acid, or nitric acid, or an aqueous solution of these acids, to obtain an aqueous solution.

Examples of precipitants used in the preparation of the transition metal-containing compound include 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). Each of these precipitants may be a hydrate, or the compound and a hydrate may be used in combination.

It is also preferable that these precipitants are dissolved in water and used as an aqueous solution. Hereinafter, the aqueous solution in which the precipitant is dissolved is referred to as the "precipitant aqueous solution."

The concentration of the precipitant in the aqueous precipitant solution is about 0.5 mol/L to 10 mol/L, preferably 1 mol/L to 8 mol/L. The precipitant is preferably KOH or NaOH, and the aqueous precipitant solution is preferably a KOH aqueous solution or a NaOH aqueous solution. Ammonia water can also be used as the aqueous precipitant solution. Ammonia water and other aqueous precipitant solutions may be used in combination.

Examples of methods for contacting the mixed aqueous solution with a precipitant include (1) adding either or both of the precipitant or the precipitant aqueous solution to the mixed aqueous solution, (2) adding the mixed aqueous solution to the precipitant aqueous solution, and (3) adding the mixed aqueous solution and either or both of the precipitant or the precipitant aqueous solution to water.

In the methods (1) to (3), it is preferable that stirring is performed. Among the above methods, the method (2) of adding the mixed aqueous solution to the precipitant aqueous solution is preferable. With this method, it is easy to maintain the pH of the precipitant aqueous solution during the operation, and it is easy to control the particle size of the resulting precipitate. In the method (2), the pH tends to decrease as the mixed aqueous solution is added to the precipitant aqueous solution, but it is preferable to add the mixed aqueous solution while adjusting the pH to 9 or more, preferably 10 or more. The pH can be adjusted by adding the precipitant aqueous solution.

In the coprecipitation methods (1) to (3), the atmosphere during operation is preferably nitrogen or argon to suppress the formation of impurities.

The above methods (1) to (3) can be used to prepare a precipitate containing a transition metal-containing compound.

When the mixed aqueous solution is mixed with the precipitant, a slurry containing a precipitate is obtained. The resulting slurry is subjected to solid-liquid separation, and the precipitate is collected to obtain a precipitate containing the transition metal-containing compound. Any method may be used for solid-liquid separation. Solid-liquid separation such as filtration is preferred because it is easy to operate. In addition, a method for volatilizing the liquid from the slurry, such as heat drying, air drying, vacuum drying, or spray drying, may also be used.

The recovered precipitate may be washed with a washing solution and then dried. The precipitate obtained after solid-liquid separation may have an excess amount of precipitant adhering thereto, but washing can reduce the amount of adhering precipitant. The washing solution used for washing is preferably water, or a water-soluble organic solvent such as alcohol or acetone, and more preferably water.

Examples of methods for drying the precipitate include heat drying, air drying, and vacuum drying. Heat drying is preferably performed at a temperature of 50°C or higher and 300°C or lower, and more preferably 100°C or higher and 200°C or lower.

These washing and drying steps may be carried out two or more times, with washing with the washing solution and drying of the precipitate counted as one step.

When mixing sodium compounds, manganese compounds, titanium compounds and magnesium compounds, or mixing sodium compounds with transition metal-containing compounds, examples of mixing methods include dry mixing and wet mixing. Among these, dry mixing is preferred because of its simple operation. Examples of mixing devices include agitation mixers, V-type mixers, W-type mixers, ribbon mixers, drum mixers and ball mills.

By firing the mixture obtained by the above method, active material particles made of composite metal oxide can be obtained. The firing temperature of the mixture depends on the type of sodium compound used and can be specified as appropriate.

The firing temperature is preferably 400°C or higher and 1200°C or lower, and more preferably 500°C or higher and 1100°C or lower.

The time for which the firing temperature is maintained is preferably from 0.1 hours to 24 hours, and more preferably from 0.5 hours to 20 hours.

The rate of temperature rise to the firing temperature is preferably 50°C/hour or more and 400°C/hour or less, and the rate of temperature fall from the firing temperature to room temperature is preferably 10°C/hour or more and 400°C/hour or less.

Examples of the atmosphere during firing include air, oxygen, nitrogen, argon, or a mixture of these. Air is preferred from the viewpoint of ease of atmosphere control, and oxygen, nitrogen, argon, or a mixture of these is preferred from the viewpoint of the stability of the sample after firing.

In addition, by using an appropriate amount of a halide such as a fluoride or chloride as the metal-containing compound, it is possible to control the crystallinity of the resulting composite metal oxide and the average particle size of the particles that make up the composite metal oxide.

In addition, a suitable amount of a reaction accelerator (flux) may be added to the mixture during firing. Examples of the flux include NaF, _MnF3 , FeF2, _NiF2 , _CoF2 , _NaCl , _MnCl2 , FeCl2, _{FeCl3 , NiCl2} , _CoCl2 , _Na2CO3 , _NaHCO3 , _NH4Cl , _NH4I , _{B2O3 ,} and _H3BO3 . Metal-containing compounds _{such as} halides may also function as reaction accelerators (fluxes). These fluxes may be hydrates.

The product (composite metal oxide) obtained by the above-mentioned calcination may be pulverized, washed, or classified using an apparatus commonly used in industry, such as a ball mill, jet mill, or vibration mill. By these operations, the particle size of the active material particles may be adjusted. Calcination may be performed two or more times.

The particle surfaces of the obtained active material particles may also be subjected to a surface treatment such as coating with an inorganic substance containing Si, Al, Ti, Y, etc.

Furthermore, after this surface treatment, a heat treatment may be performed. Depending on the temperature of the heat treatment, the BET specific surface area of the particles after the heat treatment may change from the BET specific surface area of the compound before the heat treatment. Therefore, it is also possible to adjust the BET specific surface area of the active material particles by the heat treatment.

(Formation of carbonized layer)
Next, the active material particles obtained as described above are mixed with a material for forming a carbon layer to form a carbon layer on the surface of the active material particles. For example, the carbon layer can be formed by sealing 10 g of the active material particles, 1 g of acetylene black (manufactured by Denka Co., Ltd.) and 150 g of zirconia balls with a diameter of 5 mm in a 100 mL zirconia bottle, mixing them at 300 rpm using a planetary ball mill for 6 hours, and then sieving out the zirconia balls.

By using the above-mentioned positive electrode active material for sodium secondary batteries as the positive electrode active material for sodium secondary batteries, a sodium secondary battery with both a higher discharge voltage and a higher discharge capacity than conventional batteries can be obtained.

<Positive electrode for sodium secondary battery and method for producing same>
The positive electrode for a sodium secondary battery of this embodiment contains the positive electrode active material of this embodiment described above. The positive electrode for a sodium secondary battery can be manufactured by supporting a positive electrode mixture containing a positive electrode active material and a binder on a positive electrode current collector. A conductive material may be further added to the positive electrode mixture.

Conductive materials include carbon materials such as natural graphite, artificial graphite, cokes, and carbon black.

Examples of the binder include thermoplastic resins. Specific examples of thermoplastic resins include fluororesins such as polyvinylidene fluoride (hereinafter sometimes referred to as PVDF), polytetrafluoroethylene (hereinafter sometimes referred to as PTFE), tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymers, hexafluoropropylene-vinylidene fluoride copolymers, and tetrafluoroethylene-perfluorovinyl ether copolymers; and polyolefin resins such as polyethylene and polypropylene. Two or more of these may be used.

Examples of the positive electrode current collector include Al, Ni, and stainless steel. Al is preferred because it is easy to process into a thin film and is inexpensive. The shape of the positive electrode current collector can be, for example, a foil, a flat plate, a mesh, a net, a lath, a punched shape, or a combination of these (e.g., a mesh-like flat plate). The surface of the positive electrode current collector may be unevenly formed by etching or embossing.

One example of a method for supporting the positive electrode mixture on the positive electrode current collector is to adhere the positive electrode mixture to the current collector by pressure molding.

It is also possible to use a method in which an organic solvent is further added to the positive electrode mixture to form a positive electrode mixture paste, and this positive electrode mixture paste is applied to a positive electrode current collector and dried to fix the positive electrode mixture to the positive electrode current collector. In this method, the positive electrode mixture may be firmly fixed to the positive electrode current collector by pressing the sheet obtained by fixing the positive electrode mixture to the positive electrode current collector.

Examples of organic solvents used in the positive electrode mixture paste include amine-based solvents such as N,N-dimethylaminopropylamine and diethylenetriamine; ether-based solvents such as tetrahydrofuran; ketone-based solvents such as methyl ethyl ketone; ester-based solvents such as methyl acetate; and amide-based solvents such as dimethylacetamide and N-methyl-2-pyrrolidone (hereinafter sometimes referred to as NMP).

The kneading method for preparing the positive electrode mixture paste is not particularly limited, but the mixer used for kneading is preferably one that has high shear force. Specific examples include a planetary mixer, a kneader, an extrusion type kneader, and a thin film rotary type high-speed mixer.

In terms of the mixing order, the positive electrode active material, conductive material, binder, and solvent may be mixed all at once, or the binder, positive electrode active material, and conductive material may be mixed in this order into the solvent. This order is not particularly limited, and the mixture of the positive electrode active material and conductive material may be added gradually. Also, the solvent and binder may be mixed and dissolved in advance.

Examples of methods for applying the positive electrode mixture paste to the positive electrode current collector include slit die coating, screen coating, curtain coating, knife coating, gravure coating, and electrostatic spraying.
In this manner, the positive electrode of the present embodiment can be manufactured.

The positive electrode for a sodium secondary battery having the above-mentioned configuration contains the positive electrode active material for a sodium secondary battery of the present embodiment described above, and therefore when used to manufacture a sodium secondary battery, it is possible to obtain a sodium secondary battery having both high discharge voltage and high discharge capacity.

<Sodium secondary battery>
The sodium secondary battery of this embodiment has the positive electrode, negative electrode, and non-aqueous electrolyte of this embodiment described above.

(Negative electrode)
The negative electrode can be doped and dedoped with sodium ions at a lower potential than the positive electrode. The negative electrode can be an electrode in which a negative electrode mixture containing a negative electrode material is supported on a negative electrode current collector, or an electrode made of a negative electrode material alone. The negative electrode material can be a carbon material, a chalcogen compound (oxide, sulfide, etc.), a nitride, a metal, or an alloy, and can be doped and dedoped with sodium ions at a lower potential than the positive electrode. These negative electrode materials may be mixed.

Specific examples of negative electrode materials are shown below. Specific examples of carbon materials include graphite such as natural graphite and artificial graphite, cokes, carbon black, pyrolytic carbons, carbon fibers, polymer sinters, and the like, which can be doped and dedoped with sodium ions at a potential lower than that of the positive electrode. These carbon materials, oxides, sulfides, and nitrides may be used in combination and may be either crystalline or amorphous. These carbon materials, oxides, sulfides, and nitrides are mainly supported on a negative electrode current collector and used as a negative electrode. Specific examples of metals include sodium metal, silicon metal, and tin metal. Examples of alloys include sodium alloys such as Na-Al, Na-Ni, and Na-Si; silicon alloys such as Si-Zn; tin alloys such as Sn-Mn, Sn-Co, Sn-Ni, Sn-Cu, and Sn-La; and alloys such as Cu ₂ Sb and La ₃ Ni ₂ Sn ₇ . These metals and alloys are mainly used alone as electrodes (for example, in the form of foil).Furthermore, examples _of oxides include _oxides such as _Li4Ti5O12 .

The negative electrode mixture may contain a binder as necessary. Examples of the binder include thermoplastic resins. Specific examples of thermoplastic resins include PVDF, thermoplastic polyimide, carboxymethyl cellulose, polyethylene, and polypropylene. When the electrolyte does not contain ethylene carbonate, which will be described later, and the negative electrode mixture contains polyethylene carbonate, the cycle characteristics and large current discharge characteristics of the resulting battery may be improved.

Examples of negative electrode current collectors include Cu, Ni, stainless steel, and Al. Cu or Al is preferred because it is difficult to form an alloy with sodium and is easy to process into a thin film. Methods for supporting the negative electrode mixture on the negative electrode current collector include a pressure molding method, as with the positive electrode, and a method in which an organic solvent or the like is further used to obtain a negative electrode mixture paste, the paste is applied to the negative electrode current collector, dried to obtain a sheet, and the obtained sheet is pressed to fix the negative electrode mixture to the current collector.

(Non-aqueous electrolyte)
Examples of non-aqueous electrolytes that can be used in the sodium secondary battery of this embodiment include NaClO ₄ , NaPF ₆ , NaAsF ₆ , NaSbF ₆ , NaBF ₄ , NaCF ₃ SO ₃ , NaN(SO ₂ CF ₃ ) ₂ , lower aliphatic carboxylic acid sodium salt, and NaAlCl _4. A mixture of two or more of these may be used. The electrolyte preferably contains at least one fluorine-containing sodium salt selected from the group consisting of NaPF ₆ , NaAsF ₆ , NaSbF ₆ , NaBF ₄ , NaCF ₃ SO ₃ , and NaN(SO ₂ CF ₃ ) ₂ .
The non-aqueous electrolyte can be dissolved in an organic solvent and used as a non-aqueous electrolytic solution.

Examples of the organic solvent in the nonaqueous electrolyte include:
Carbonates such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, isopropyl methyl carbonate, vinylene carbonate, 4-trifluoromethyl-1,3-dioxolan-2-one, and 1,2-di(methoxycarbonyloxy)ethane;
Ethers such as 1,2-dimethoxyethane, 1,3-dimethoxypropane, pentafluoropropyl methyl ether, 2,2,3,3-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;
It is also possible to use an organic solvent having a fluorine substituent, which is obtained by further introducing a fluorine substituent into the above organic solvent.

It is preferable that part of the organic solvent in the non-aqueous electrolyte solution contains an organic solvent having a fluorine substituent.

Examples of organic solvents having a fluorine substituent include 4-fluoro-1,3-dioxolan-2-one (hereinafter sometimes referred to as FEC or fluoroethylene carbonate), trans- or cis-4,5-difluoro-1,3-dioxolan-2-one (hereinafter sometimes referred to as DFEC or difluoroethylene carbonate), etc.

The preferred organic solvent having a fluorine substituent is 4-fluoro-1,3-dioxolan-2-one.

These organic solvents having fluorine substituents may be used alone, but are preferably used as a mixed solvent in combination with an organic solvent not having a fluorine substituent. When an organic solvent having a fluorine substituent is included as part of the organic solvent in the nonaqueous electrolyte, the ratio of the organic solvent having a fluorine substituent to the entire nonaqueous electrolyte is in the range of 0.01% by volume to 10% by volume, preferably 0.1% by volume to 8% by volume, and more preferably 0.5% by volume to 5% by volume.

The non-aqueous electrolyte can also be used in a state where the non-aqueous electrolyte is held in a polymer compound, i.e., as a gel electrolyte.

In addition, a solid electrolyte can also be used as the nonaqueous electrolyte that can be used in the sodium secondary battery of this embodiment.

As the solid electrolyte, for example, a polymer electrolyte such as a polyethylene oxide-based polymer compound, a polymer compound containing at least one of a polyorganosiloxane chain or a polyoxyalkylene chain can be used. In addition, if a sulfide electrolyte such as Na ₂ S-SiS ₂ , Na ₂ S-GeS ₂ , Na ₂ S-P ₂ S ₅ , or Na ₂ S-B ₂ S ₃ , or an inorganic compound electrolyte containing a sulfide such as Na ₂ S-SiS ₂ -Na ₃ PO ₄ or Na ₂ S-SiS ₂ -Na ₂ SO ₄ , or a NASICON type electrolyte such as NaZr ₂ (PO ₄ ) ₃ is used as the solid electrolyte, safety may be further improved.

In addition, when a solid electrolyte is used in the sodium secondary battery of this embodiment, the solid electrolyte may also function as a separator, which will be described later. In that case, a separator may not be required.

(Separator)
The sodium secondary battery of the present embodiment may have a separator disposed between the positive electrode and the negative electrode. Examples of the form of the separator include a porous film, a nonwoven fabric, and a woven fabric.

Materials for forming the separator include, for example, polyolefin resins such as polyethylene and polypropylene, fluororesins, and nitrogen-containing aromatic polymers. In addition, a single-layer or laminated separator may be formed using two or more of these materials.

Examples of the separator include those described in JP-A-2000-30686 and JP-A-10-324758.

The thinner the separator, the higher the volumetric energy density of the battery and the lower the internal resistance, while still maintaining mechanical strength. In general, the thickness of the separator is preferably 5 μm or more and 200 μm or less, and more preferably 5 μm or more and 40 μm or less.

The separator preferably has a porous film containing a thermoplastic resin. In a sodium secondary battery, when an abnormal current flows in the battery due to a short circuit between the positive and negative electrodes, it is preferable to cut off the current and prevent excessive current from flowing (shutdown).

When the separator has a porous film containing a thermoplastic resin, shutdown occurs when the separator at the short-circuited location becomes overheated due to a short circuit and exceeds a pre-expected (normal) operating temperature, causing the porous film in the separator to soften or melt and block the micropores. It is preferable that the separator has high heat resistance so that it can maintain the shutdown state without being ruptured by the temperature even if the temperature inside the battery rises to a certain high level after shutdown.

By using a separator made of a laminated porous film in which a heat-resistant porous layer containing a heat-resistant resin and a porous film containing a thermoplastic resin are laminated, it is possible to better prevent thermal rupture. Here, the heat-resistant porous layer may be laminated on both sides of the porous film.

(Method of manufacturing sodium secondary battery)
1A and 1B are schematic diagrams showing an example of a sodium secondary battery of this embodiment. Fig. 1A is a schematic diagram showing an electrode group of the sodium secondary battery. Fig. 1B is an exploded perspective view of the sodium secondary battery. The cylindrical sodium secondary battery 10 of this embodiment is manufactured as follows.

First, as shown in FIG. 1A, a pair of strip-shaped separators 1, a strip-shaped positive electrode 2 having a lead 21 at one end, and a strip-shaped negative electrode 3 having a lead 31 at one end are stacked in the order of separator 1, positive electrode 2, separator 1, negative electrode 3, and then wound to form an electrode group 4.

Next, as shown in FIG. 1B, the electrode group 4 and an insulator (not shown) are placed in the battery can 5, the bottom of the can is sealed, the electrode group 4 is impregnated with an electrolyte solution 6, and the electrolyte is disposed between the positive electrode 2 and the negative electrode 3. Furthermore, the top of the battery can 5 is sealed with a top insulator 7 and a sealing body 8, thereby manufacturing a sodium secondary battery 10.

Examples of the shape of the electrode group 4 include a cylindrical shape such that the cross-sectional shape when the electrode group 4 is cut perpendicular to the winding axis is a circle, an ellipse, a rectangle, or a rectangle with rounded corners.

The shape of a sodium secondary battery having such an electrode group 4 can be any shape specified by IEC 60086, a standard for batteries established by the International Electrotechnical Commission (IEC), or JIS C 8500. Examples of shapes include cylindrical and rectangular shapes.

Furthermore, the sodium secondary battery is not limited to the above-mentioned wound type configuration, but may be a laminated type configuration in which a laminated structure of a positive electrode, a separator, a negative electrode, and a separator is repeatedly stacked. Examples of laminated sodium secondary batteries include so-called coin type batteries, button type batteries, and paper type (or sheet type) batteries.

The sodium secondary battery configured as described above has a positive electrode for a sodium secondary battery according to the present embodiment, and therefore has high discharge voltage and discharge capacity.

<Applications of sodium secondary batteries>
The sodium secondary battery of the present embodiment has both high discharge voltage and high discharge capacity and high energy density, and is therefore suitable as a small battery that is a power source for small devices such as mobile phones, portable audio devices, and notebook computers; a power source for transportation devices such as automobiles, motorcycles, electric chairs, forklifts, trains, airplanes, ships, spacecraft, and submarines; a power source for machines such as tillers; an outdoor power source for camping purposes; and a medium- to large-sized battery that is a mobile battery such as an outdoor/indoor power source for vending machines.

In addition, since the sodium secondary battery of this embodiment uses raw materials that are abundantly supplied and inexpensive, it is suitable as a medium- to large-sized stationary battery, such as an outdoor/indoor power source for factories, homes, etc.; a load-leveling power source for various power generation such as a solar battery charger and a wind power generation charger; a power source for low-temperature environments such as in refrigerated or frozen warehouses and extremely cold areas; a power source for high-temperature environments such as deserts; and a power source for space environments such as space stations.

The above describes preferred embodiments of the present invention with reference to the attached drawings, but it goes without saying that the present invention is not limited to these examples. The shapes and combinations of the components shown in the above examples are merely examples, and various modifications can be made based on design requirements, etc., without departing from the spirit of the present invention.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples unless the gist of the invention is changed.

<Evaluation method>
In this example, the produced composite metal oxide was measured by the following method. In addition, a sodium secondary battery was produced by using the produced positive electrode active material by the following method and evaluated.

(1) Powder X-ray Diffraction Measurement Measurement of the crystal structure of the composite metal oxide was performed using a powder X-ray diffractometer RINT2500TTR type manufactured by Rigaku Corporation under the following conditions unless otherwise specified.

X-ray: CuKα
Voltage-current: 40kV-140mA
Measurement angle range: 2θ = 10 to 90°
Step: 0.02°
Scan speed: 4°/min

(2) Preparation of Positive Electrode For Examples 1 and 2 and Comparative Examples 2 and 3 produced by the methods described below, the positive electrode active material, acetylene black (manufactured by Denka Co., Ltd.) as a conductive material, and PVDF (manufactured by Kureha Corporation) as a binder were weighed out so as to have a composition of positive electrode active material:conductive material:binder=85:5:10 (mass ratio).
For Comparative Example 1 produced by a method described later, the positive electrode active material, acetylene black (manufactured by Denka Co., Ltd.) as a conductive material, and PVDF (manufactured by Kureha Corporation) as a binder were weighed out to achieve a composition of positive electrode active material:conductive material:binder=80:10:10 (mass ratio).
Thereafter, the positive electrode active material and acetylene black were first thoroughly mixed in an agate mortar, N-methyl-2-pyrrolidone (NMP: manufactured by Tokyo Chemical Industry Co., Ltd.) was added to this mixture, and PVDF was further added, followed by mixing in the agate mortar until the mixture was homogenous, thereby obtaining a positive electrode mixture paste.

The obtained positive electrode mixture paste was applied to a 40 μm-thick aluminum foil current collector with an applicator to a thickness of 100 μm. The current collector to which the positive electrode mixture paste was applied was placed in a dryer and dried while removing the NMP to obtain an electrode sheet. This electrode sheet was punched out to a diameter of 1.5 cm using an electrode punching machine and then pressed with a hand press to obtain a positive electrode containing the positive electrode active material.

(3) Preparation of Battery The above-mentioned positive electrode was placed with the aluminum foil facing down in the recess of the lower part of a coin cell (manufactured by Hosen Co., Ltd.), and this was combined with 1 M NaPF ₆ /propylene carbonate as an electrolyte, a polypropylene porous film (thickness 20 μm) as a separator, and metallic sodium (manufactured by Aldrich Co.) as a negative electrode to prepare a sodium secondary battery. The battery was assembled in a glove box with an argon atmosphere.

Example 1 (Na:Mn:Ti:Mg=1:0.9:0.05:0.05)
As metal-containing compounds, sodium carbonate ( _{Na2CO3 :} manufactured by Wako Pure Chemical Industries, Ltd.: purity 99.8%), manganese (IV) oxide ( _MnO2 : manufactured by Kojundo Chemical Laboratory Co., Ltd.: purity 99.9%), titanium oxide ( _TiO2 : manufactured by Kojundo Chemical Laboratory Co., Ltd.: purity 99.9%) and magnesium hydroxide (Mg(OH) ₂ : manufactured by Wako Pure Chemical Industries, Ltd.: purity 99.8%) were used and weighed out so that the molar ratio of Na:Mn:Ti:Mg was 1.00:0.90:0.05:0.05, and mixed in a wet ball mill at 300 rpm for 5 hours to obtain a mixture of metal-containing compounds.

The obtained mixture of metal-containing compounds was subjected to solid-liquid separation, and the wet cake was dried and then placed in a metal mold to form pellets. The obtained pellets were then placed on an alumina firing boat and fired in an electric furnace at 1000°C in an argon atmosphere for 10 hours, thereby firing the mixture. After firing the mixture, the fired product was cooled to room temperature to obtain active material particles 1, which are particles of a composite metal oxide. Active material particles 1 correspond to the "positive electrode active material" in this invention.

Figure 2 is an XRD chart showing the results of powder X-ray diffraction measurement of active material particle 1 and active materials 4 and 5 described below. Figure 2 shows XRD charts in the range of 10° to 100° from the measurement results in the above-mentioned measurement angle range.

Powder X-ray diffraction measurements revealed that the crystal structure of active material particle 1 belongs to the O3 type crystal structure, which is a layered type.

Next, 10 g of active material particles, 1 g of acetylene black (manufactured by Denka Co., Ltd.), and 150 g of zirconia balls with a diameter of 5 mm were placed in a 100 mL zirconia bottle and mixed at 300 rpm for 6 hours using a planetary ball mill. The zirconia balls were then sieved out to obtain positive electrode active material 1, in which a carbon layer was formed on the surface of the active material particles.

Example 2 (Na:Mn:Ti:Mg=1:0.8:0.1:0.1)
Active material particles 2, which are composite metal oxide particles, were obtained in the same manner as in Example 1, except that a metal-containing compound was used in an amount such that the molar ratio of Na:Mn:Ti:Mg was 1.00:0.80:0.10:0.10.

The results of powder X-ray diffraction measurements showed that the crystal structure of active material particle 2 was an O3 type crystal structure, which is a layered type.

Furthermore, positive electrode active material 2 was obtained in the same manner as in Example 1, except that active material particles 2 were used.

<Comparative Example 1> (Na:Mn=0.7:1.0)
The examples were prepared without using _TiO2 as a Ti source or Mg(OH) ₂ as a Mg source, and with using a metal-containing compound in an amount such that the molar ratio of Na:Mn was 0.70:1.00. In Comparative Example 1, the composite metal oxide particles were obtained in the same manner as in Comparative Example 1. In Comparative Example 1, the composite metal oxide particles were used as positive electrode active material 3.

The results of powder X-ray diffraction measurements showed that the crystal structure of the positive electrode active material 3 was a P2 type crystal structure, which is a layered type.

<Comparative Example 2> (Na:Mn=1:1)
The examples were prepared without using _TiO2 as a Ti source or Mg(OH) ₂ as a Mg source, and with using a metal-containing compound in an amount such that the molar ratio of Na:Mn was 1.00:1.00. Active material particles 4, which are composite metal oxide particles, were obtained in the same manner as in Example 1.

As shown in Figure 2, the results of the powder X-ray diffraction measurement showed that the crystal structure of active material particle 4 belongs to the O3 type crystal structure, which is a layered type.

Powder X-ray diffraction measurements revealed that the crystal structure of the positive electrode active material 4 belongs to the O3 type crystal structure, which is a layered type.

Furthermore, positive electrode active material 4 was obtained in the same manner as in Example 1, except that active material particles 4 were used.

<Comparative Example 3> (Na:Mn:Ti=0.8:0.8:0.2)
Example 1 was repeated except that no Mg(OH) ₂ was used as the Mg source, and metal-containing compounds were used in an amount such that the molar ratio of Na:Mn:Ti was 0.80:0.80:0.20. In the same manner as above, active material particles 5, which are composite metal oxide particles, were obtained.

As shown in FIG. 2, the results of the powder X-ray diffraction measurement showed that the crystal structure of active material particle 5 belongs to the O3 type crystal structure, which is one of the layered types.

Furthermore, positive electrode active material 5 was obtained in the same manner as in Example 1, except that active material particles 5 were used.

<Evaluation of sodium secondary batteries>
The sodium secondary batteries using the positive electrode active materials of Examples 1 and 2 and Comparative Examples 1 to 3 were evaluated as follows.

(10 times discharge capacity)
The prepared battery was subjected to a charge/discharge test under the following conditions: When repeated charge/discharge was performed in the cycle test, charging and discharging were repeated under the following conditions.
Charging conditions: CC (constant current) charging was performed from the rest potential to 4.5 V at a rate of 0.2 C (a rate at which the battery is fully charged in 5 hours).
Discharge conditions: CC (constant current) discharge was performed at a rate of 0.2 C (a rate at which the battery is completely discharged in 5 hours), and cut off at a voltage of 1.0 V.

Each charge and discharge under the above conditions was counted as one cycle, and the 10th discharge capacity was measured after 10 charge and discharge cycles. The value obtained was the 10th discharge capacity.

(Average discharge voltage)
The charging and discharging were performed under the above conditions, and the voltage during discharging was measured. The charging and discharging were repeated 10 times, and the voltage during discharging was measured 10 times. The arithmetic mean value of the obtained discharge voltages was defined as the average discharge voltage.

The evaluation results are shown in Tables 1 and 2 and Figure 3. Figure 3 shows the charge/discharge curves of positive electrode active materials 2, 4, and 5.

The "Mn ratio," "Ti ratio," and "Mg ratio" in the table respectively indicate the molar content ratio of each element per mole of the composition formula of the composite metal oxide that constitutes the active material particles.

As for Comparative Example 1, as mentioned above, the crystal structure is P2 type, and there is a comparative problem with the 10-cycle discharge capacity, so it is not listed.

As a result of the evaluation, the sodium secondary batteries using the positive electrode active materials of Examples 1 and 2 showed higher 10-cycle discharge capacity and average discharge voltage than the sodium secondary batteries using the positive electrode active materials of Comparative Examples 1 to 3.

The positive electrode active material of Example 1 had an initial coulombic efficiency (= discharge capacity/charge capacity x 100 (%)) of approximately 94%, confirming its high practicality as a positive electrode material for secondary batteries.

<Effect of carbon layer>
(Reference Example 1)
A sodium secondary battery having active material particle 1 as a positive electrode active material was fabricated in the same manner as in the above “(2) Fabrication of positive electrode” and “(3) Fabrication of battery”, except that active material particle 1 was used instead of positive electrode active material 1.

(Reference Example 2)
A sodium secondary battery having active material particles 2 as a positive electrode active material was fabricated in the same manner as in the above “(2) Fabrication of positive electrode” and “(3) Fabrication of battery”, except that active material particles 2 were used instead of positive electrode active material 2.

<Evaluation of sodium secondary batteries>
The sodium secondary batteries using the positive electrode active materials of Examples 1 and 2 and the sodium secondary batteries using the active material particles of Reference Examples 1 and 2 as the positive electrode active material were evaluated as follows.

(Initial discharge capacity)
The produced battery was charged and discharged under the above-mentioned conditions, and the discharge capacity at the first discharge was measured as the first discharge capacity.

The evaluation results are shown in Table 3.

As shown in Table 3, it was found that the active material particles 1 and 2 used in Reference Examples 1 and 2 functioned as active material particles for a sodium secondary battery.

Furthermore, when comparing the results of Reference Examples 1 and 2 with the results of Examples 1 and 2, it was found that the initial discharge capacity increased when a carbon layer was provided.

These results confirm the usefulness of this invention.

1...separator, 2...positive electrode, 3...negative electrode, 4...electrode group, 5...battery can, 6...electrolyte, 7...top insulator, 8...sealing body, 10...sodium secondary battery, 21...lead, 31...lead

Claims (5)

active material particles made of a composite metal oxide having an O3 type crystal structure and containing Na, Mn, Ti and Mg;
a carbon layer covering the surface of the active material particle;
The carbon layer is formed of acetylene black,
The composite metal oxide is a positive electrode active material for a sodium secondary battery represented by the following formula (1).
Na _a (M ¹ _w M ² _v Mn _x Ti _y-z Mg _z )O ₂ (1)
(Here, ^M1 represents Co or Ni, ^M2 represents one or more elements selected from the group consisting of Fe, Cu, Mo, and Ca, a is 0.6 or more and 1 or less, w is 0 or more and less than 0.25, v is 0 or more and less than 0.25, w+v is 0 or more and less than 0.25, x is 0.7 or more and less than 0.98, y is more than 0.02 and 0.5 or less, z is 0.03 or more and less than 0.5, and w+v+x+y=1, y>z.) The positive electrode active material for a sodium secondary battery according to claim 1, wherein w = 0. The positive electrode active material for a sodium secondary battery according to claim 2 , wherein v=0. A positive electrode for a sodium secondary battery, comprising the positive electrode active material according to claim 1 . A sodium secondary battery having a positive electrode, a negative electrode and a non-aqueous electrolyte, wherein the positive electrode has the positive electrode for sodium secondary batteries according to claim 4 .

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