Method for producing positive electrode active material for sodium-ion secondary batteries

Abstract

A method for producing a positive electrode active material for sodium-ion secondary batteries that contains an olivine sodium transition metal phosphate compound, the method comprising a step (A) for heating a mixed solution 1 containing a sodium source, a transition metal source, and a phosphoric acid source, wherein when the total transition metal content in the mixed solution 1 is taken as 100 mol%, the manganese content in the mixed solution 1 is 25 mol% or more, and the step (A) is carried out at 200 kPa or less.

Description

Method for manufacturing positive electrode active material for sodium-ion secondary batteries

[0001] This invention relates to a method for producing a positive electrode active material for sodium-ion secondary batteries.

[0002] Examples of technologies related to sodium-ion secondary batteries include those described in Patent Documents 1 to 4.

[0003] Patent Document 1 discloses a positive electrode active material containing a highly crystalline olivine-type phosphate, with the objective of providing such a material. The positive electrode active material contains an olivine-type phosphate represented by a predetermined formula, and the maximum peak in the X-ray diffraction pattern obtained using CuKα rays is the peak of the (031) plane of the olivine-type phosphate, and the full width at half maximum of the peak is 1.5° or less.

[0004] Patent Document 2 aims to provide a transition metal phosphate that can be suitably used as a positive electrode active material for an inexpensive and high-capacity sodium secondary battery, and describes a transition metal phosphate containing Na, P and a transition metal element, wherein the BET specific surface area of ​​the transition metal phosphate is 1 m². ２ / g or more 50m ２ Transition metal phosphates with a concentration of less than or equal to / g are disclosed.

[0005] Patent Document 3 discloses a positive electrode active material that includes sodium phosphate transition metal having an olivine-type structure, characterized in that the sodium atoms are arranged in one direction in the <010> direction without being obstructed by other atoms, with the objective of providing a high-performance positive electrode using abundant and inexpensive sodium as a resource.

[0006] Patent Document 4 discloses a positive electrode active material for a sodium-ion battery that contains an olivine-type sodium iron phosphate compound represented by a predetermined formula obtained by a hydrothermal reaction, with the objective of providing a positive electrode active material for a sodium-ion battery that can obtain a sodium-ion battery exhibiting a large discharge capacity.

[0007] Japanese Patent Publication No. 2009-206085, Japanese Patent Publication No. 2010-018472, Japanese Patent Publication No. 2011-034963, Japanese Patent Publication No. 2017-091755

[0008] A first embodiment of the present invention provides a method for producing a positive electrode active material for sodium-ion secondary batteries, which can produce an olivine-type phosphate transition metal sodium compound with few by-product crystals under mild conditions.

[0009] Furthermore, a second embodiment of the present invention provides a positive electrode active material for a sodium-ion secondary battery that can improve the discharge capacity of the sodium-ion secondary battery.

[0010] Furthermore, a third embodiment of the present invention provides a positive electrode active material for a sodium-ion secondary battery that can improve the discharge capacity of the sodium-ion secondary battery.

[0011] According to a first embodiment of the present invention, a method for producing a positive electrode active material for a sodium-ion secondary battery is provided as shown below.

[0012] [1A] A method for producing a positive electrode active material for a sodium-ion secondary battery containing an olivine-type phosphate transition metal sodium compound, comprising step (A) of heating a mixed solution 1 containing a sodium source, a transition metal source, and a phosphate source, wherein when the total content of the transition metal in the mixed solution 1 is 100 mol%, the manganese content in the mixed solution 1 is 25 mol% or more, and step (A) is performed at 200 kPa or less. [2A] The method for producing a positive electrode active material for a sodium-ion secondary battery according to [1A], wherein step (A) comprises a step (A1) of evaporating the mixed solution 1 to dryness, and further comprises a step (A2) of heating a mixture containing the solids obtained in step (A1) and a carbon source. [3A] The method for producing a positive electrode active material for a sodium-ion secondary battery according to [2A], wherein the heating temperature in step (A2) is 100°C or more and 490°C or less. [4A] The method for producing a positive electrode active material for a sodium-ion secondary battery according to [2A] or [3A], wherein the carbon source comprises one or more selected from the group consisting of sugars, resins, organic acids, organic bases, petroleum pitch, coal pitch, carbon black, carbon fibers, fullerenes, carbon nanofibers, carbon nanotubes, alcohols, and hydrocarbons. [5A] The method for producing a positive electrode active material for a sodium-ion secondary battery according to any one of [1A] to [4A], further comprising a step (B) of preparing the mixed solution 1, wherein step (B) comprises a step (B1) of adding the transition metal source in installments or continuously to the mixed solution containing the sodium source and the phosphoric acid source. [6A] The method for producing a positive electrode active material for a sodium-ion secondary battery according to any one of [1A] to [5A], wherein step (A) is performed under atmospheric pressure. [7A] The method for producing a positive electrode active material for a sodium-ion secondary battery according to any one of [1A] to [6A], which does not include a step of heating at a temperature exceeding 490°C. [8A] A method for producing a positive electrode active material for a sodium ion secondary battery according to any one of [1A] to [7A], wherein when the total content of transition metals in the mixed solution 1 is 100 mol%, the content of manganese in the mixed solution 1 is 99 mol% or less.[9A] A method for producing a positive electrode active material for a sodium-ion secondary battery according to any one of [1A] to [8A], wherein the phosphate source contains ammonium ions. [10A] A method for producing a positive electrode active material for a sodium-ion secondary battery according to any one of [1A] to [9A], wherein when the total content of transition metals in the mixed solution 1 is 100 mol%, the content of iron in the mixed solution 1 is 0 mol% or more and 75 mol% or less. [11A] In the spectrum obtained by X-ray diffraction using CuKα rays as a source, the positive electrode active material for the sodium-ion secondary battery originates from the (111) plane of the olivine-type phosphate transition metal sodium compound, is located at a diffraction angle 2θ = 24.2 ± 0.4°, and has a maximum diffraction intensity of I. Ｏ The diffraction peak is located at a diffraction angle of 2θ = 23.7 ± 0.4° and originates from the (111) plane of the malisite-type sodium phosphate transition metal compound, and the maximum diffraction intensity is I Ｍ The diffraction peak is and the peak intensity ratio obtained from it (I Ｏ / (I Ｏ +I Ｍ A method for producing a positive electrode active material for a sodium ion secondary battery according to any one of [1A] to [10A], wherein the value of )) is 0.5 or more.

[0013] Furthermore, according to a second embodiment of the present invention, a positive electrode active material for a sodium-ion secondary battery, a positive electrode active material layer for a sodium-ion secondary battery, a positive electrode for a sodium-ion secondary battery, and a sodium-ion secondary battery are provided as shown below.

[0014] [1B] A positive electrode active material for a sodium-ion secondary battery comprising an olivine-type phosphate transition metal sodium compound represented by the following formula (1), wherein in the spectrum obtained by X-ray diffraction of the positive electrode active material for a sodium-ion secondary battery using CuKα rays as the source, the (020) plane of the olivine-type phosphate transition metal sodium compound is located at a diffraction angle 2θ = 28.2 ± 0.4° and the maximum diffraction intensity is I ０２０ The diffraction peak is located at a diffraction angle of 2θ = 16.8 ± 0.4° and originates from the (200) plane of the olivine-type phosphate transition metal sodium compound, and has a maximum diffraction intensity of I２００ The diffraction peak that is, and the diffraction peak that is derived from the (002) plane of the olivine-type transition metal sodium phosphate compound, exists at a position where the diffraction angle 2θ = 36.0 ± 0.4°, and the maximum diffraction intensity is I ００２ Each has a diffraction peak that is, and the peak intensity ratio (I ０２０ / (I ２００ + I ００２ )) has a value of 0.5 or more, which is a positive electrode active material for a sodium ion secondary battery. (1): NaMn ａ Fe ｂ M ｃ PO ４(In formula (1), M represents a transition metal (excluding Mn and Fe), a and b each independently represent a real number between 0 and 1, c represents a real number between 0 and 0.5, and a, b, and c satisfy a + b > 0 and 2a + 2b + (valence of M) × c = 2.) [2B] The positive electrode active material for a sodium-ion secondary battery according to [1B], wherein the olivine-type transition metal phosphate sodium compound contains manganese and iron. [3B] The positive electrode active material for a sodium-ion secondary battery according to [2B], wherein, when the total content of transition metals in the olivine-type transition metal phosphate sodium compound is 100 mol%, the content of manganese in the olivine-type transition metal phosphate sodium compound is 30 mol% or more and less than 100 mol%, and the content of iron in the olivine-type transition metal phosphate sodium compound is greater than 0 mol% and 70 mol% or less. [4B] A positive electrode active material for a sodium-ion secondary battery according to any one of [1B] to [3B], further comprising carbon. [5B] A positive electrode active material for a sodium-ion secondary battery according to [4B], wherein the carbon is present on at least the surface of the positive electrode active material for a sodium-ion secondary battery. [6B] A positive electrode active material for a sodium-ion secondary battery according to any one of [1B] to [5B], wherein the discharge capacity according to the method (Method 1) below is 35 mAh / g or more. (Method 1) The discharge capacity of a half cell prepared by the method (Method for preparing a half cell) below is measured by the method (Method for measuring discharge capacity) below. (Method for preparing half-cells) A positive electrode slurry with a solid content of 53% by mass is prepared by mixing 88.0 parts by mass of the positive electrode active material for sodium-ion secondary batteries, 4.0 parts by mass of carbon black, 2.0 parts by mass of multi-walled carbon nanotubes, and 6.0 parts by mass of polyvinylidene fluoride in N-methyl-2-pyrrolidone. Then, the positive electrode slurry is applied to one side of aluminum foil, with a basis weight of 50 g / m² after drying. ２The material is coated in such a manner, then dried at 80°C for 3 hours, then pressed with a force of 20 kN using a roll press, then cut into a disc shape with a diameter of 10 mm, and dried in a vacuum at 120°C for 15 hours to obtain the positive electrode. The counter electrode is obtained by cutting metallic sodium foil into a disc shape with a diameter of 14 mm and a thickness of 0.5 mm in a glove box under a nitrogen atmosphere. An electrolyte is prepared by dissolving sodium hexafluoride phosphate at a ratio of 1 mol / L in a mixed solvent of ethylene carbonate and diethyl carbonate in a volume ratio of 1:1. Then, a half cell is made in a glove box under a nitrogen atmosphere using the positive electrode, the counter electrode, the electrolyte, and a cellulose nonwoven fabric separator. (Method for measuring discharge capacity) The half cell is charged with a constant current at a temperature of 25°C and a current of 0.01C until the voltage reaches 4.5V, and then the half cell is discharged with a constant current at a temperature of 25°C and a current of 0.01C until the voltage reaches 1.0V. The capacity at this time is measured and defined as the discharge capacity. [7B] The resistance value obtained by the following (Method 2) is 1.0 × 10 １０ A positive electrode active material for a sodium-ion secondary battery described in any of [1B] to [6B] above, having a capacitance of Ω or less. (Method 2) 5.0 g of the positive electrode active material for a sodium-ion secondary battery is subjected to a pressure of 100 kgf / cm². ２ Sample 1 is prepared by pressurizing it for a pressurizing time of 1 minute and shaping it into a 34 mm diameter form, and then measuring the resistance of Sample 1 using a resistivity meter under the conditions of a temperature of 25°C, an applied voltage of 10 V, and a holding time of 30 seconds. [8B] A positive electrode active material layer for a sodium-ion secondary battery, comprising the positive electrode active material for a sodium-ion secondary battery described in any of [1B] to [7B]. [9B] A positive electrode for a sodium-ion secondary battery comprising the positive electrode for a sodium-ion secondary battery described in [8B] and a positive electrode current collector. [10B] A sodium-ion secondary battery comprising the positive electrode for a sodium-ion secondary battery described in [9B], an electrolyte layer, and a negative electrode for a sodium-ion secondary battery.

[0015] Furthermore, according to a third embodiment of the present invention, a positive electrode active material for a sodium-ion secondary battery, a positive electrode active material layer for a sodium-ion secondary battery, a positive electrode for a sodium-ion secondary battery, and a sodium-ion secondary battery are provided as shown below.

[0016] [1C] A positive electrode active material for a sodium-ion secondary battery comprising an olivine-type phosphate transition metal sodium compound represented by the following formula (1), wherein in the spectrum obtained by X-ray diffraction of the positive electrode active material for a sodium-ion secondary battery using CuKα rays as the source, the (111) plane of the olivine-type phosphate transition metal sodium compound is located at a diffraction angle 2θ = 24.2 ± 0.4° and the maximum diffraction intensity is I Ｏ The diffraction peak is located at a diffraction angle of 2θ = 23.7 ± 0.4° and originates from the (111) plane of the malisite-type sodium phosphate transition metal compound, and the maximum diffraction intensity is I Ｍ The diffraction peak is and the peak intensity ratio obtained from it (I Ｍ / (I Ｏ +I Ｍ A positive electrode active material for sodium-ion secondary batteries, wherein the value of )) is 0.50 or less. (1): NaMn ａ Fe ｂ M ｃ PO ４(In formula (1), M represents a transition metal (excluding Mn and Fe), a and b each independently represent a real number between 0 and 1, c represents a real number between 0 and 0.5, and a, b, and c satisfy a + b > 0 and 2a + 2b + (valence of M) × c = 2.) [2C] The positive electrode active material for a sodium-ion secondary battery according to [1C], wherein the olivine-type transition metal phosphate sodium compound contains manganese and iron. [3C] The positive electrode active material for a sodium-ion secondary battery according to [2C], wherein, when the total content of transition metals in the olivine-type transition metal phosphate sodium compound is 100 mol%, the manganese content in the olivine-type transition metal phosphate sodium compound is 30 mol% or more and less than 100 mol%, and the iron content in the olivine-type transition metal phosphate sodium compound is greater than 0 mol% and 70 mol% or less. [4C] A positive electrode active material for a sodium-ion secondary battery according to any one of [1C] to [3C], further comprising carbon. [5C] A positive electrode active material for a sodium-ion secondary battery according to [4C], wherein the carbon is present on at least the surface of the positive electrode active material for a sodium-ion secondary battery. [6C] A positive electrode active material for a sodium-ion secondary battery according to any one of [1C] to [5C], wherein the discharge capacity according to the method (Method 1) below is 35 mAh / g or more. (Method 1) The discharge capacity of a half cell prepared by the method (Method for preparing a half cell) below is measured by the method (Method for measuring discharge capacity) below. (Method for preparing half-cells) A positive electrode slurry with a solid content of 53% by mass is prepared by mixing 88.0 parts by mass of the positive electrode active material for sodium-ion secondary batteries, 4.0 parts by mass of carbon black, 2.0 parts by mass of multi-walled carbon nanotubes, and 6.0 parts by mass of polyvinylidene fluoride in N-methyl-2-pyrrolidone. Then, the positive electrode slurry is applied to one side of aluminum foil, with a basis weight of 50 g / m² after drying. ２The material is coated in such a manner, then dried at 80°C for 3 hours, then pressed with a force of 20 kN using a roll press, then cut into a disc shape with a diameter of 10 mm, and dried in a vacuum at 120°C for 15 hours to obtain the positive electrode. The counter electrode is obtained by cutting metallic sodium foil into a disc shape with a diameter of 14 mm and a thickness of 0.5 mm in a glove box under a nitrogen atmosphere. An electrolyte is prepared by dissolving sodium hexafluoride phosphate at a ratio of 1 mol / L in a mixed solvent of ethylene carbonate and diethyl carbonate in a volume ratio of 1:1. Then, a half cell is made in a glove box under a nitrogen atmosphere using the positive electrode, the counter electrode, the electrolyte, and a cellulose nonwoven fabric separator. (Method for measuring discharge capacity) The half cell is charged with a constant current at a temperature of 25°C and a current of 0.01C until the voltage reaches 4.5V, and then the half cell is discharged with a constant current at a temperature of 25°C and a current of 0.01C until the voltage reaches 1.0V. The capacity at this time is measured and defined as the discharge capacity. [7C] The resistance value according to the following (Method 2) is 1.0 × 10 １０ A positive electrode active material for a sodium-ion secondary battery described in any of [1C] to [6C] above, having a capacitance of Ω or less. (Method 2) 5.0 g of the positive electrode active material for a sodium-ion secondary battery is subjected to a pressure of 100 kgf / cm². ２ Sample 1 is prepared by pressurizing it for a pressurizing time of 1 minute and shaping it into a 34 mm diameter form, and then measuring the resistance of Sample 1 using a resistivity meter under the conditions of a temperature of 25°C, an applied voltage of 10 V, and a holding time of 30 seconds. [8C] A positive electrode active material layer for a sodium-ion secondary battery, comprising the positive electrode active material for a sodium-ion secondary battery described in any of [1C] to [7C]. [9C] A positive electrode for a sodium-ion secondary battery comprising the positive electrode for a sodium-ion secondary battery described in [8C] and a positive electrode current collector. [10C] A sodium-ion secondary battery comprising the positive electrode for a sodium-ion secondary battery described in [9C], an electrolyte layer, and a negative electrode for a sodium-ion secondary battery.

[0017] According to the first embodiment of the present invention, a method for producing a positive electrode active material for a sodium-ion secondary battery is provided, which allows for the production of an olivine-type phosphate transition metal sodium compound with few by-product crystals under mild conditions.

[0018] Furthermore, according to a second embodiment of the present invention, a positive electrode active material for a sodium-ion secondary battery can be provided that can improve the discharge capacity of the sodium-ion secondary battery.

[0019] Furthermore, according to a third embodiment of the present invention, a positive electrode active material for a sodium-ion secondary battery can be provided that can improve the discharge capacity of the sodium-ion secondary battery.

[0020] In this specification, the term "poly(meth)acrylic acid" encompasses both polyacrylic acid and polymethacrylic acid. The same applies to similar terms such as "(meth)acryloyl." For each component in each embodiment, one type may be used, or two or more types may be used in combination. Unless otherwise specified, "A to B" representing a numerical range means A or greater and B or less.

[0021] <<First Embodiment>> The following describes a method for producing a positive electrode active material for a sodium-ion secondary battery according to the first embodiment of the present invention.

[0022] [Method for Manufacturing a Cathode Active Material for Sodium-Ion Secondary Batteries] The method for manufacturing a cathode active material for sodium-ion secondary batteries according to the first embodiment is a method for manufacturing a cathode active material for sodium-ion secondary batteries that contains an olivine-type phosphate transition metal sodium compound. The method for manufacturing a cathode active material for sodium-ion secondary batteries according to the first embodiment also includes a step (A) of heating a mixed solution 1 containing a sodium source, a transition metal source, and a phosphate source. Furthermore, in the method for manufacturing a cathode active material for sodium-ion secondary batteries according to the first embodiment, when the total content of transition metals in the mixed solution 1 is 100 mol%, the manganese content in the mixed solution 1 is 25 mol% or more.

[0023] The method for producing a positive electrode active material for a sodium-ion secondary battery according to the first embodiment, having the above configuration, makes it possible to produce an olivine-type phosphate transition metal sodium compound with few by-product crystals under mild conditions.

[0024] Through our investigations, we have found that olivine-type sodium phosphate transition metal compounds can also be produced by heating a mixed solution containing a sodium source, a transition metal source, and a phosphoric acid source. Further investigations by our inventors revealed for the first time that by setting the manganese content in the mixed solution to a predetermined value or higher, olivine-type sodium phosphate transition metal compounds with fewer by-product crystals can be produced under mild conditions, thus completing the present invention. Here, mild conditions refer to conditions that do not require conditions such as high temperatures of 600°C or higher or high pressures exceeding 200 kPa. By-product crystals are crystals other than the olivine-type sodium phosphate transition metal compound, and examples include crystals of malisite-type sodium phosphate transition metal compounds.

[0025] The following will provide a detailed explanation of the manufacturing method for positive electrode active material for sodium-ion secondary batteries (hereinafter sometimes simply referred to as "positive electrode active material").

[0026] The method for producing the positive electrode active material according to the first embodiment comprises the following step (A). The method for producing the positive electrode active material according to the first embodiment may further comprise the following step (B). Step (A): A step of heating a mixed solution 1 containing a sodium source, a transition metal source, and a phosphoric acid source. Step (B): A step of preparing the mixed solution 1.

[0027] (Step (A)) In step (A) of the first embodiment, a mixed solution 1 containing a sodium source, a transition metal source, and a phosphoric acid source is heated.

[0028] Step (A) of the first embodiment is preferably carried out at 200 kPa or less, more preferably at 180 kPa or less, even more preferably at 160 kPa or less, even more preferably at 140 kPa or less, even more preferably at 120 kPa or less, even more preferably at 110 kPa or less, and even more preferably at atmospheric pressure, from the viewpoint of being able to produce olivine-type transition metal sodium phosphate compounds under milder conditions.

[0029] The heating temperature in step (A) of the first embodiment is preferably less than 600°C, more preferably 590°C or less, even more preferably 550°C or less, even more preferably 500°C or less, even more preferably 450°C or less, even more preferably 400°C or less, even more preferably 350°C or less, even more preferably 300°C or less, even more preferably 250°C or less, and even more preferably 200°C or less. The lower limit of the heating temperature in step (A) of the first embodiment is not particularly limited, but for example it may be 100°C or higher, 125°C or higher, or 150°C or higher. From the viewpoint of being able to produce olivine-type phosphate transition metal sodium compounds under milder conditions, the heating temperature in step (A) of the first embodiment is preferably 100°C or more and less than 600°C, more preferably 100°C or more and 590°C or less, even more preferably 100°C or more and 550°C or less, even more preferably 100°C or more and 500°C or less, even more preferably 100°C or more and 450°C or less, even more preferably 100°C or more and 400°C or less, even more preferably 100°C or more and 350°C or less, even more preferably 100°C or more and 300°C or less, even more preferably 125°C or more and 250°C or less, and even more preferably 150°C or more and 200°C or less.

[0030] Examples of sodium sources include sodium oxide, sodium hydroxide, sodium carbonate, sodium sulfate, sodium nitrate, sodium acetate, sodium halides, sodium oxalates, and sodium alkoxides. The sodium source preferably comprises one or more selected from the group consisting of sodium carbonate and sodium hydroxide.

[0031] Examples of transition metal sources include transition metal oxides, transition metal hydroxides, transition metal oxyhydroxides, transition metal carbonates, transition metal sulfates, transition metal nitrates, transition metal acetates, transition metal halides, transition metal ammonium salts, transition metal oxalates, and transition metal alkoxides. The transition metal source preferably comprises one or more selected from the group consisting of iron(II) chloride, manganese(II) chloride, and nickel(II) chloride, and more preferably comprises one or more selected from the group consisting of iron(II) chloride and manganese(II) chloride.

[0032] Examples of phosphate sources include phosphoric acid, sodium phosphate, transition metal phosphate, and ammonium phosphate. The phosphate source preferably contains ammonium ions. Examples of ammonium phosphate salts include triammonium phosphate, diammonium hydrogen phosphate, and ammonium dihydrogen phosphate. The phosphate source more preferably contains diammonium hydrogen phosphate.

[0033] From the viewpoint of being able to produce an olivine-type phosphate transition metal sodium compound with few by-product crystals under mild conditions, the manganese content in the mixed solution 1 of the first embodiment is 25 mol% or more, preferably 35 mol% or more, more preferably 45 mol% or more, and even more preferably 55 mol% or more, when the total transition metal content in the mixed solution 1 is taken as 100 mol%. From the viewpoint of being able to improve the output of a sodium-ion secondary battery, the manganese content in the mixed solution 1 of the first embodiment is preferably 100 mol% or less, more preferably 99 mol% or less, even more preferably 95 mol% or less, even more preferably 85 mol% or less, even more preferably 75 mol% or less, and even more preferably 65 mol% or less, when the total transition metal content in the mixed solution 1 is taken as 100 mol%. The manganese content in the mixed solution 1 of the first embodiment is preferably 25 mol% to 100 mol%, more preferably 25 mol% to 99 mol%, even more preferably 25 mol% to 95 mol%, even more preferably 35 mol% to 85 mol%, even more preferably 45 mol% to 75 mol%, and even more preferably 55 mol% to 65 mol%, when the total content of transition metals in the mixed solution 1 is 100 mol%.

[0034] From the viewpoint of improving the electromotive force and output of the sodium-ion secondary battery, the iron content in the mixed solution 1 of the first embodiment is preferably 0 mol% to 75 mol%, more preferably 1 mol% to 75 mol%, even more preferably 5 mol% to 75 mol%, even more preferably 15 mol% to 65 mol%, even more preferably 25 mol% to 55 mol%, and even more preferably 35 mol% to 45 mol%.

[0035] From the viewpoint of improving the electromotive force and output of the sodium-ion secondary battery, the nickel content in the mixed solution 1 of the first embodiment is preferably 0 mol% to 50 mol%, more preferably 0 mol% to 25 mol%, even more preferably 0 mol% to 15 mol%, and even more preferably 0 mol% to 5 mol%, when the total content of transition metals in the mixed solution 1 is 100 mol%.

[0036] In step (B) of the first embodiment, a mixed solution 1 containing a sodium source, a transition metal source, and a phosphoric acid source is prepared. Step (B) preferably includes step (B1) of adding the transition metal source to the mixed solution containing the sodium source and the phosphoric acid source in installments or continuously. In step (B1), for example, the solution containing the transition metal source may be continuously added dropwise to the mixed solution containing the sodium source and the phosphoric acid source.

[0037] Furthermore, step (A) of the first embodiment may further comprise the following step (A1). Furthermore, step (A) may further comprise the following step (A2). Step (A1): A step of evaporating the mixed solution 1 to dryness. Step (A2): A step of heating the mixture containing the solids obtained in step (A1) and the carbon source.

[0038] In step (A1) of the first embodiment, the mixed solution 1 is evaporated to dryness. This process yields a solid component. The solid component preferably contains an olivine-type phosphate transition metal sodium compound.

[0039] Step (A1) of the first embodiment is preferably carried out at 200 kPa or less, more preferably at 180 kPa or less, even more preferably at 160 kPa or less, even more preferably at 140 kPa or less, even more preferably at 120 kPa or less, even more preferably at 110 kPa or less, and even more preferably at atmospheric pressure, from the viewpoint of being able to produce olivine-type transition metal sodium phosphate compounds under milder conditions.

[0040] The heating temperature in step (A1) of the first embodiment is preferably 100°C or more and less than 600°C, more preferably 100°C or more and 590°C or less, even more preferably 100°C or more and 550°C or less, even more preferably 100°C or more and 500°C or less, even more preferably 100°C or more and 450°C or less, even more preferably 100°C or more and 400°C or less, even more preferably 100°C or more and 350°C or less, even more preferably 100°C or more and 300°C or less, even more preferably 125°C or more and 250°C or less, and even more preferably 150°C or more and 200°C or less, from the viewpoint of being able to produce olivine-type phosphate transition metal sodium compounds under milder conditions.

[0041] Step (A1) of the first embodiment may further include a step of washing the solid obtained by evaporation to dryness before step (A2). In the washing step, for example, water-soluble impurities in the solid can be removed by washing the solid with pure water.

[0042] In step (A2) of the first embodiment, the mixture containing the solids obtained in step (A1) and the carbon source is heated. Note that step (A2) may be performed together with step (A1) instead of after it. In this case, step (A2) may be performed by adding the carbon source during the evaporation of the mixed solution 1 to dryness.

[0043] Step (A2) of the first embodiment is preferably carried out at 200 kPa or less, more preferably at 180 kPa or less, even more preferably at 160 kPa or less, even more preferably at 140 kPa or less, even more preferably at 120 kPa or less, even more preferably at 110 kPa or less, and even more preferably under atmospheric pressure, from the viewpoint of being able to produce a positive electrode active material with few by-product crystals under mild conditions.

[0044] The heating temperature in step (A2) of the first embodiment is preferably less than 600°C, more preferably 550°C or less, even more preferably 500°C or less, even more preferably 490°C or less, even more preferably 480°C or less, even more preferably 470°C or less, even more preferably 460°C or less, even more preferably 455°C or less, and even more preferably 450°C or less, from the viewpoint of suppressing the decrease of olivine-type phosphate transition metal sodium compounds. The heating temperature in step (A2) of the first embodiment is preferably 100°C or higher, more preferably 150°C or higher, even more preferably 200°C or higher, even more preferably 250°C or higher, even more preferably 300°C or higher, even more preferably 350°C or higher, and even more preferably 400°C or higher, from the viewpoint of shortening the time required for step (A2). The heating temperature in step (A2) of the first embodiment is preferably 100°C or more and less than 600°C, more preferably 150°C or more and 550°C or less, even more preferably 200°C or more and 500°C or less, even more preferably 250°C or more and 490°C or less, even more preferably 300°C or more and 480°C or less, even more preferably 350°C or more and 470°C or less, even more preferably 400°C or more and 460°C or less, even more preferably 400°C or more and 455°C or less, and even more preferably 400°C or more and 450°C or less, from the viewpoint of suppressing the decrease of olivine-type phosphate transition metal sodium compound and shortening the time required for step (A2).

[0045] The heating time in step (A2) of the first embodiment is preferably 1 hour or more, more preferably 2 hours or more, and even more preferably 3 hours or more.

[0046] The carbon source preferably comprises one or more selected from the group consisting of sugars, resins, organic acids, organic bases, petroleum pitch, coal pitch, carbon black, carbon fibers, fullerenes, carbon nanofibers, carbon nanotubes, alcohols, and hydrocarbons, more preferably comprising sugars, even more preferably glucose, and even more preferably D(+)-glucose. Examples of sugars include glucose, fructose, sucrose, saccharose, dextrin, starch, cellulose, or derivatives thereof. Examples of resins include polyethylene glycol, polyvinyl alcohol, polyethylene, polypropylene, polyvinyl chloride, phenolic resin, epoxy resin, furan resin, poly(meth)acrylic acid, poly(meth)acrylonitrile, polyglycol, polyaniline, etc. Examples of organic acids include carboxylic acids, carboxylic acid derivatives, etc. Examples of organic bases include pyridine, triethylamine, or derivatives thereof. Examples of carbon blacks include acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, etc. Examples of carbon nanotubes include single-walled carbon nanotubes and multi-walled carbon nanotubes. Examples of hydrocarbons include hydrocarbon gases. Examples of hydrocarbon gases include acetylene gas and propylene gas.

[0047] In step (A2), the amount of carbon source added is preferably 0.1% to 50% by mass, more preferably 1% to 40% by mass, and even more preferably 10% to 35% by mass, when the total amount of positive electrode active material and carbon source added is taken as 100% by mass, from the viewpoint of further improving the discharge capacity of the sodium-ion secondary battery.

[0048] The method for producing the positive electrode active material of the first embodiment preferably does not include a step of heating at a temperature above 600°C, more preferably above 550°C, even more preferably above 500°C, even more preferably above 490°C, even more preferably above 480°C, even more preferably above 470°C, even more preferably above 460°C, even more preferably above 455°C, and even more preferably above 450°C, from the viewpoint of being able to improve the yield of the olivine-type phosphate transition metal sodium compound.

[0049] According to the method for producing the positive electrode active material of the first embodiment, the heating temperature and pressure values ​​during the production process of the positive electrode active material can be reduced compared to conventional methods. Therefore, according to the method for producing the positive electrode active material of the first embodiment, an olivine-type sodium phosphate transition metal compound with few by-product crystals can be produced under mild conditions.

[0050] (Raw material solution for positive electrode active material for sodium-ion secondary battery) The raw material solution for the positive electrode active material for sodium-ion secondary battery of the first embodiment is a raw material solution used in the method for manufacturing the positive electrode active material of the first embodiment. The composition and preferred composition of the raw material solution of the first embodiment are the same as those of the mixed solution 1 described above. According to the raw material solution of the first embodiment, the heating temperature and pressure values ​​during the manufacturing process of the positive electrode active material can be reduced compared to conventional raw material solutions. Therefore, according to the raw material solution of the first embodiment, an olivine-type phosphate transition metal sodium compound with few by-product crystals can be manufactured under mild conditions.

[0051] (Positive electrode active material) The positive electrode active material of the first embodiment includes an olivine-type phosphate transition metal sodium compound, from the viewpoint of being able to improve the discharge capacity of the sodium-ion secondary battery.

[0052] (Olivine-type sodium phosphate transition metal compound) The olivine-type sodium phosphate transition metal compound of the first embodiment includes an olivine-type sodium phosphate transition metal compound represented by the following formula (1), from the viewpoint of improving the discharge capacity of sodium-ion secondary batteries. (1): NaMn ａ Fe ｂ M ｃ PO ４

[0053] In equation (1), M represents a transition metal (excluding Mn and Fe). a and b are independent real numbers between 0 and 1. c is a real number between 0 and 0.5. Furthermore, a, b, and c satisfy a + b > 0 and 2a + 2b + (valence of M) × c = 2. The composition of the olivine-type transition metal sodium compound represented by equation (1) can be identified, for example, by elemental analysis using energy-dispersive X-ray (EDX) spectroscopy. More specifically, the composition of the olivine-type transition metal sodium compound represented by equation (1) can be identified by measuring the content of each component (sodium, manganese, iron, transition metal M, phosphorus, and oxygen) in the olivine-type transition metal sodium compound using EDX spectroscopy and calculating their mol ratios.

[0054] In formula (1), examples of transition metal M include scandium, titanium, vanadium, chromium, cobalt, nickel, copper, yttrium, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, palladium, silver, and zinc.

[0055] The olivine-type phosphate transition metal sodium compound of the first embodiment contains manganese, from the viewpoint of reducing the cost of raw materials for sodium-ion secondary batteries. In formula (1), a is preferably 0.1 to 1.0, more preferably 0.2 to 0.9, even more preferably 0.3 to 0.8, and even more preferably 0.4 to 0.7.

[0056] The olivine-type phosphate transition metal sodium compound of the first embodiment preferably contains iron from the viewpoint of reducing the cost of raw materials for sodium-ion secondary batteries. In formula (1), b is preferably 0.1 to 1.0, more preferably 0.2 to 0.8, even more preferably 0.3 to 0.7, and even more preferably 0.35 to 0.6.

[0057] The olivine-type phosphate transition metal sodium compound of the first embodiment preferably contains manganese and iron, from the viewpoint of reducing the cost of raw materials for sodium-ion secondary batteries.

[0058] From the viewpoint of improving the electromotive force of the sodium ion secondary battery, the manganese content in the olivine-type transition metal sodium phosphate compound of the first embodiment is preferably 30 mol% or more, more preferably 35 mol% or more, even more preferably 40 mol% or more, even more preferably 45 mol% or more, even more preferably 50 mol% or more, and even more preferably 55 mol% or more, when the total content of transition metals in the olivine-type transition metal sodium phosphate compound is taken as 100 mol%. From the viewpoint of improving the output of the sodium ion secondary battery, the manganese content in the olivine-type transition metal sodium phosphate compound of the first embodiment is preferably less than 100 mol%, more preferably 90 mol% or less, even more preferably 80 mol% or less, even more preferably 75 mol% or less, even more preferably 70 mol% or less, and even more preferably 65 mol% or less, when the total content of transition metals in the olivine-type transition metal sodium phosphate compound is taken as 100 mol%. From the viewpoint of improving the electromotive force and output of the sodium ion secondary battery, the manganese content in the olivine-type transition metal sodium phosphate compound of the first embodiment is preferably 30 mol% or more and less than 100 mol%, more preferably 35 mol% or more and 90 mol%, even more preferably 40 mol% or more and 80 mol%, even more preferably 45 mol% or more and 75 mol%, even more preferably 50 mol% or more and 70 mol%, and even more preferably 55 mol% or more and 65 mol%, when the total content of transition metals in the olivine-type transition metal sodium phosphate compound is taken as 100 mol%.

[0059] From the viewpoint of improving the output of the sodium-ion secondary battery, the iron content in the olivine-type sodium phosphate transition metal compound of the first embodiment is preferably more than 0 mol%, more preferably 10 mol% or more, even more preferably 20 mol% or more, even more preferably 25 mol% or more, even more preferably 30 mol% or more, and even more preferably 35 mol% or more, when the total content of transition metals in the olivine-type sodium phosphate transition metal compound is taken as 100 mol%. From the viewpoint of improving the electromotive force of the sodium-ion secondary battery, the iron content in the olivine-type sodium phosphate transition metal compound of the first embodiment is preferably 70 mol% or less, more preferably 65 mol% or less, even more preferably 60 mol% or less, even more preferably 55 mol% or less, even more preferably 50 mol% or less, and even more preferably 45 mol% or less, when the total content of transition metals in the olivine-type sodium phosphate transition metal compound is taken as 100 mol%. From the viewpoint of improving the electromotive force and output of the sodium ion secondary battery, the iron content in the olivine-type phosphate transition metal sodium compound of the first embodiment is preferably more than 0 mol% and 70 mol% or less, more preferably 10 mol% to 65 mol%, even more preferably 20 mol% to 60 mol%, even more preferably 25 mol% to 55 mol%, even more preferably 30 mol% to 50 mol%, and even more preferably 35 mol% to 45 mol%.

[0060] From the viewpoint of reducing the cost of raw materials for sodium-ion secondary batteries, the total manganese and iron content in the olivine-type phosphate transition metal sodium compound of the first embodiment is preferably 50 mol% to 100 mol%, more preferably 60 mol% to 100 mol%, even more preferably 70 mol% to 100 mol%, even more preferably 80 mol% to 100 mol%, even more preferably 90 mol% to 100 mol%, and even more preferably 95 mol% to 100 mol%.

[0061] From the viewpoint of improving the electromotive force and output of the sodium ion secondary battery, the nickel content in the olivine-type phosphate transition metal sodium compound of the first embodiment is preferably 0 mol% to 50 mol%, more preferably 0 mol% to 25 mol%, even more preferably 0 mol% to 15 mol%, and even more preferably 0 mol% to 5 mol%, when the total transition metal content in the olivine-type phosphate transition metal sodium compound is set to 100 mol%.

[0062] The positive electrode active material of the first embodiment preferably further contains carbon, from the viewpoint of reducing the electrical resistance of the sodium-ion secondary battery. More preferably, the carbon is present on at least the surface of the positive electrode active material. Even more preferably, the carbon covers at least a portion of the surface of the positive electrode active material.

[0063] From the viewpoint of improving the discharge capacity of the sodium-ion secondary battery, the carbon content in the positive electrode active material of the first embodiment is preferably 0.1% to 40% by mass, more preferably 1% to 30% by mass, and even more preferably 3% to 25% by mass, when the content of the positive electrode active material is 100% by mass.

[0064] In the first embodiment, a known shape of the positive electrode active material can be adopted. The positive electrode active material is preferably in particulate form.

[0065] The following describes the physical properties of the positive electrode active material in the first embodiment.

[0066] In the positive electrode active material of the first embodiment, in the spectrum obtained by X-ray diffraction using CuKα rays as the radiation source, the (111) plane of the olivine-type phosphate transition metal sodium compound is located at a diffraction angle 2θ = 24.2 ± 0.4° and the maximum diffraction intensity is I Ｏ The diffraction peak that is defined as diffraction peak O is defined as diffraction peak O. Furthermore, in the spectrum obtained by X-ray diffraction using CuKα rays as the source for the positive electrode active material of the first embodiment, the diffraction peak originates from the (111) plane of the maliscite-type sodium phosphate transition metal compound, is located at a diffraction angle 2θ = 23.7 ± 0.4°, and has a maximum diffraction intensity of I Ｍ Let the diffraction peak be diffraction peak M.

[0067] Here, if two peaks are observed in the diffraction angle 2θ range of 23.8° to 24.1°, the higher-angle peak can be identified as diffraction peak O, and the lower-angle peak as diffraction peak M. Also, if one peak is observed in the diffraction angle 2θ range of 23.8° to 24.1°, the peak can be identified using the following procedure. First, if a peak located at diffraction angle 2θ = 16.8 ± 0.4°, originating from the (200) plane of the olivine-type sodium phosphate transition metal compound, is not observed, it means that the olivine-type sodium phosphate transition metal compound is not present, or the content of the olivine-type sodium phosphate transition metal compound is extremely low. Therefore, the peak observed in the diffraction angle 2θ range of 23.8° to 24.1° is a peak originating from the malisite-type sodium phosphate transition metal compound, and can be identified as diffraction peak M. On the other hand, if a peak originating from the (021) plane of the mallisite-type sodium phosphate transition metal compound, located at a diffraction angle of 2θ = 26.2 ± 0.4°, is not observed, it means that the mallisite-type sodium phosphate transition metal compound is not present, or that its content is extremely low. Therefore, peaks observed in the diffraction angle range of 2θ between 23.8° and 24.1° are peaks originating from the olivine-type sodium phosphate transition metal compound and can be identified as diffraction peak O.

[0068] In the positive electrode active material of the first embodiment, the peak intensity ratio (I) obtained from diffraction peak O and diffraction peak M is Ｏ / (I Ｏ +I Ｍ The value of (I) is preferably 0.5 or higher, more preferably 0.6 or higher, even more preferably 0.7 or higher, even more preferably 0.8 or higher, and even more preferably 0.9 or higher, from the viewpoint of improving the discharge capacity of the sodium-ion secondary battery. Ｏ / (I Ｏ +I Ｍ There is no particular upper limit to the value of ), but for example, it should be 1.0 or less.

[0069] Although the first embodiment of the present invention has been described above, these are merely examples of the first embodiment, and various other configurations can be adopted. Furthermore, the present invention is not limited to the first embodiment described above, and modifications, improvements, etc., that do not impair the effects of the present invention are included in the present invention.

[0070] <<Second Embodiment>> The following describes the positive electrode active material for a sodium-ion secondary battery, the positive electrode active material layer for a sodium-ion secondary battery, the positive electrode for a sodium-ion secondary battery, and the sodium-ion secondary battery according to the second embodiment of the present invention.

[0071] [Positive electrode active material for sodium-ion secondary batteries] The positive electrode active material for sodium-ion secondary batteries of the second embodiment (hereinafter sometimes simply referred to as "positive electrode active material") contains an olivine-type phosphate transition metal sodium compound represented by the following formula (1): (1): NaMn ａ Fe ｂ M ｃ PO ４In equation (1), M represents a transition metal (excluding Mn and Fe), a and b are independent real numbers between 0 and 1, c is a real number between 0 and 0.5, and a, b, and c satisfy a + b > 0 and 2a + 2b + (valence of M) × c = 2. Furthermore, in the spectrum obtained by X-ray diffraction of the positive electrode active material for the sodium-ion secondary battery using CuKα rays as the source, the (020) plane of the olivine-type phosphate transition metal sodium compound is located at a diffraction angle 2θ = 28.2 ± 0.4° and the maximum diffraction intensity is I ０２０ The diffraction peak is located at a diffraction angle of 2θ = 16.8 ± 0.4° and originates from the (200) plane of the olivine-type phosphate transition metal sodium compound, and the maximum diffraction intensity is I ２００ The diffraction peak is located at a diffraction angle of 2θ = 36.0 ± 0.4° and originates from the (002) plane of the olivine-type phosphate transition metal sodium compound, and has a maximum diffraction intensity of I ００２ It has diffraction peaks that are and respectively. Furthermore, in the positive electrode active material for sodium ion secondary battery of the second embodiment, the peak intensity ratio (I ０２０ / (I ２００ +I ００２ The value of )) is 0.5 or greater.

[0072] The positive electrode active material for sodium-ion secondary batteries of the second embodiment, having the above-described configuration, can provide a positive electrode active material for sodium-ion secondary batteries that can improve the discharge capacity of the sodium-ion secondary battery.

[0073] The reason for this is not entirely clear, but the following reasons can be inferred. Through our investigations, we have found that conventional positive electrode active materials for sodium-ion secondary batteries have a crystal structure that makes it difficult for sodium ions to diffuse, resulting in high sodium ion diffusion resistance compared to positive electrode active materials for lithium-ion secondary batteries. When sodium ion diffusion resistance is high, it is difficult to obtain sufficient performance in terms of battery characteristics such as discharge capacity in sodium-ion secondary batteries. Further investigation by our inventors revealed that the intensity of the X-ray diffraction spectrum of the (020) plane of the olivine-type phosphate transition metal sodium compound is related to the ease of sodium ion diffusion (ease of insertion and removal). This relationship is thought to be because the direction along the (020) plane of the olivine-type phosphate transition metal sodium compound is the direction involved in sodium ion insertion and removal. Based on the above findings, our inventors further investigated and found that the above peak intensity ratio (I ０２０ / (I ２００ +I ００２ The present invention was completed by discovering for the first time that the discharge capacity of a sodium-ion secondary battery can be improved by having a certain value or higher.

[0074] (Olivine-type sodium phosphate transition metal compound) The positive electrode active material of the second embodiment includes an olivine-type sodium phosphate transition metal compound, from the viewpoint of being able to improve the discharge capacity of the sodium-ion secondary battery.

[0075] The olivine-type transition metal sodium phosphate compound of the second embodiment includes an olivine-type transition metal sodium phosphate compound represented by the following formula (1), from the viewpoint of improving the discharge capacity of sodium-ion secondary batteries. (1): NaMn ａ Fe ｂ M ｃ PO ４

[0076] In equation (1), M represents a transition metal (excluding Mn and Fe). a and b are independent real numbers between 0 and 1. c is a real number between 0 and 0.5. Furthermore, a, b, and c satisfy a + b > 0 and 2a + 2b + (valence of M) × c = 2. The composition of the olivine-type transition metal sodium compound represented by equation (1) can be identified, for example, by elemental analysis using energy-dispersive X-ray (EDX) spectroscopy. More specifically, the composition of the olivine-type transition metal sodium compound represented by equation (1) can be identified by measuring the content of each component (sodium, manganese, iron, transition metal M, phosphorus, and oxygen) in the olivine-type transition metal sodium compound using EDX spectroscopy and calculating their mol ratios.

[0077] In formula (1), examples of transition metal M include scandium, titanium, vanadium, chromium, cobalt, nickel, copper, yttrium, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, palladium, silver, and zinc.

[0078] The olivine-type phosphate transition metal sodium compound of the second embodiment preferably contains manganese, from the viewpoint of reducing the cost of raw materials for sodium-ion secondary batteries. In formula (1), a is preferably 0.1 to 1.0, more preferably 0.2 to 0.9, even more preferably 0.3 to 0.8, and even more preferably 0.4 to 0.7.

[0079] The olivine-type phosphate transition metal sodium compound of the second embodiment preferably contains iron, from the viewpoint of reducing the cost of raw materials for sodium-ion secondary batteries. In formula (1), b is preferably 0.1 to 1.0, more preferably 0.2 to 0.8, even more preferably 0.3 to 0.7, and even more preferably 0.35 to 0.6.

[0080] The olivine-type phosphate transition metal sodium compound of the second embodiment preferably contains manganese and iron, from the viewpoint of reducing the cost of raw materials for sodium-ion secondary batteries.

[0081] From the viewpoint of improving the electromotive force of the sodium-ion secondary battery, the manganese content in the olivine-type transition metal sodium phosphate compound of the second embodiment is preferably 30 mol% or more, more preferably 35 mol% or more, even more preferably 40 mol% or more, even more preferably 45 mol% or more, even more preferably 50 mol% or more, and even more preferably 55 mol% or more, when the total content of transition metals in the olivine-type transition metal sodium phosphate compound is taken as 100 mol%. From the viewpoint of improving the output of the sodium-ion secondary battery, the manganese content in the olivine-type transition metal sodium phosphate compound of the second embodiment is preferably less than 100 mol%, more preferably 90 mol% or less, even more preferably 80 mol% or less, even more preferably 75 mol% or less, even more preferably 70 mol% or less, and even more preferably 65 mol% or less, when the total content of transition metals in the olivine-type transition metal sodium phosphate compound is taken as 100 mol%. In the second embodiment, the manganese content in the olivine-type phosphate transition metal sodium compound is preferably 30 mol% or more and less than 100 mol%, more preferably 35 mol% or more and 90 mol%, even more preferably 40 mol% or more and 80 mol%, even more preferably 45 mol% or more and 75 mol%, even more preferably 50 mol% or more and 70 mol%, and even more preferably 55 mol% or more and 65 mol%, when the total transition metal content in the olivine-type phosphate transition metal sodium compound is taken as 100 mol%.

[0082] From the viewpoint of improving the output of the sodium-ion secondary battery, the iron content in the olivine-type sodium phosphate transition metal compound of the second embodiment is preferably more than 0 mol%, more preferably 10 mol% or more, even more preferably 20 mol% or more, even more preferably 25 mol% or more, even more preferably 30 mol% or more, and even more preferably 35 mol% or more, when the total content of transition metals in the olivine-type sodium phosphate transition metal compound is taken as 100 mol%. From the viewpoint of improving the electromotive force of the sodium-ion secondary battery, the iron content in the olivine-type sodium phosphate transition metal compound of the second embodiment is preferably 70 mol% or less, more preferably 65 mol% or less, even more preferably 60 mol% or less, even more preferably 55 mol% or less, even more preferably 50 mol% or less, and even more preferably 45 mol% or less, when the total content of transition metals in the olivine-type sodium phosphate transition metal compound is taken as 100 mol%. In the second embodiment, the iron content in the olivine-type phosphate transition metal sodium compound is preferably greater than 0 mol% and less than or equal to 70 mol%, more preferably between 10 mol% and 65 mol%, even more preferably between 20 mol% and 60 mol%, even more preferably between 25 mol% and 55 mol%, even more preferably between 30 mol% and 50 mol%, and even more preferably between 35 mol% and 45 mol%, when the total transition metal content in the olivine-type phosphate transition metal sodium compound is taken as 100 mol%.

[0083] In the second embodiment, the total manganese and iron content in the olivine-type phosphate transition metal sodium compound is preferably 50 mol% to 100 mol%, more preferably 60 mol% to 100 mol%, even more preferably 70 mol% to 100 mol%, even more preferably 80 mol% to 100 mol%, even more preferably 90 mol% to 100 mol%, and even more preferably 95 mol% to 100 mol%.

[0084] The positive electrode active material of the second embodiment preferably further contains carbon, from the viewpoint of reducing the electrical resistance of the sodium-ion secondary battery. The carbon is more preferably present on at least the surface of the positive electrode active material. More preferably, the carbon covers at least a portion of the surface of the positive electrode active material.

[0085] In the second embodiment, the carbon content in the positive electrode active material is preferably 0.1% to 40% by mass, more preferably 1% to 30% by mass, and even more preferably 3% to 25% by mass, when the content of the positive electrode active material is 100% by mass, from the viewpoint of further improving the discharge capacity of the sodium-ion secondary battery.

[0086] In the second embodiment, a known shape of the positive electrode active material can be adopted. The positive electrode active material is preferably in particulate form.

[0087] The following describes the physical properties of the positive electrode active material in the second embodiment.

[0088] In the second embodiment, the positive electrode active material, in the spectrum obtained by X-ray diffraction using CuKα rays as a source, originates from the (020) plane of the olivine-type phosphate transition metal sodium compound, is located at a diffraction angle 2θ = 28.2 ± 0.4°, and has a maximum diffraction intensity of I ０２０It has a diffraction peak A. Furthermore, in the spectrum obtained by X-ray diffraction using CuKα rays as the source, the positive electrode active material originates from the (200) plane of the olivine-type phosphate transition metal sodium compound, is located at a diffraction angle 2θ = 16.8 ± 0.4°, and has a maximum diffraction intensity of I ２００ It has a diffraction peak B. Furthermore, in the spectrum obtained by X-ray diffraction using CuKα rays as the source, the positive electrode active material originates from the (002) plane of the olivine-type phosphate transition metal sodium compound, is located at a diffraction angle 2θ = 36.0 ± 0.4°, and has a maximum diffraction intensity of I ００２ It has diffraction peak C. In other words, the positive electrode active material has diffraction peaks A, B, and C in the spectrum obtained by X-ray diffraction using CuKα rays as the source.

[0089] Peak intensity ratio of positive electrode active material (I ０２０ / (I ２００ +I ００２ The value of (I) is, from the viewpoint of improving the discharge capacity of the sodium-ion secondary battery, 0.5 or more, preferably 0.6 or more, more preferably 0.7 or more, even more preferably 0.8 or more, even more preferably 0.9 or more, even more preferably 1.0 or more, even more preferably 1.1 or more, even more preferably 1.2 or more, and even more preferably 1.3 or more. Peak intensity ratio of positive electrode active material (I ０２０ / (I ２００ +I ００２ The upper limit of the value of )) is not particularly limited, but for example it may be 3.0 or less, 2.5 or less, 2.0 or less, 1.8 or less, or 1.5 or less. Peak intensity ratio of positive electrode active material (I ０２０ / (I ２００ +I ００２The value of )) is preferably 0.5 to 3.0, more preferably 0.6 to 2.5, even more preferably 0.7 to 2.5, even more preferably 0.8 to 2.0, even more preferably 0.9 to 2.0, even more preferably 1.0 to 1.8, even more preferably 1.1 to 1.8, even more preferably 1.2 to 1.8, and even more preferably 1.3 to 1.5.

[0090] The discharge capacity of the positive electrode active material according to the second embodiment by the following (Method 1) is preferably 35 mAh / g or more, more preferably 40 mAh / g or more, even more preferably 45 mAh / g or more, even more preferably 50 mAh / g or more, even more preferably 75 mAh / g or more, even more preferably 90 mAh / g or more, even more preferably 100 mAh / g or more, and even more preferably 110 mAh / g or more. The upper limit of the discharge capacity of the positive electrode active material is not particularly limited, but for example it may be 300 mAh / g or less, 250 mAh / g or less, or 200 mAh / g or less. From the viewpoint of further improving the discharge capacity of the sodium-ion secondary battery, the discharge capacity of the positive electrode active material by the following method (Method 1) is preferably 35 mAh / g or more and 300 mAh / g or less, more preferably 40 mAh / g or more and 250 mAh / g or less, even more preferably 45 mAh / g or more and 200 mAh / g or less, even more preferably 50 mAh / g or more and 200 mAh / g or less, even more preferably 75 mAh / g or more and 200 mAh / g or less, even more preferably 90 mAh / g or more and 200 mAh / g or less, even more preferably 100 mAh / g or more and 200 mAh / g or less, and even more preferably 110 mAh / g or more and 200 mAh / g or less.

[0091] (Method 1) Half cells are prepared according to the method described below (Method for preparing half cells). The discharge capacity of the prepared half cells is measured according to the method described below (Method for measuring discharge capacity). More specifically, the method for measuring the half cells and the method for measuring the discharge capacity can be the method described in the examples.

[0092] (Method for preparing half-cells) A cathode slurry with a solid content of 53% by mass is prepared by mixing 88.0 parts by mass of cathode active material, 4.0 parts by mass of carbon black, 2.0 parts by mass of multi-walled carbon nanotubes, and 6.0 parts by mass of polyvinylidene fluoride in N-methyl-2-pyrrolidone. Next, the cathode slurry is applied to one side of aluminum foil, with a basis weight of 50 g / m² after drying. ２ The material is coated in the manner described above. Next, it is dried at 80°C for 3 hours. Then, it is pressed with a force of 20kN using a roll press. Next, it is cut into a disc shape with a diameter of 10 mm and dried in a vacuum at 120°C for 15 hours to obtain the positive electrode. In addition, a counter electrode is obtained by cutting metallic sodium foil into a disc shape with a diameter of 14 mm and a thickness of 0.5 mm in a glove box under a nitrogen atmosphere. An electrolyte is prepared by dissolving sodium hexafluoride phosphate at a ratio of 1 mol / L in a mixed solvent of ethylene carbonate and diethyl carbonate in a volume ratio of 1:1. Next, a half cell is made in a glove box under a nitrogen atmosphere using the positive electrode, counter electrode, electrolyte, and cellulose nonwoven fabric separator. (Method for measuring discharge capacity) The half cell is charged with a constant current at a temperature of 25°C and a current of 0.01C until the voltage reaches 4.5V. Next, the capacity of the half-cell is measured when it is discharged at a constant current of 25°C and 0.01C until the voltage reaches 1.0V, and this capacity is defined as the discharge capacity.

[0093] The resistance value of the positive electrode active material in the second embodiment, determined by the following method (Method 2), is preferably 1.0 × 10⁻¹⁰, from the viewpoint of further improving the discharge capacity of the sodium-ion secondary battery. １０ It is less than or equal to Ω, and more preferably 5.0 × 10 ９ It is less than or equal to Ω, and more preferably 2.0 × 10⁻¹⁰. ９ It is less than or equal to Ω, and more preferably 1.0 × 10⁻⁶. ９ It is less than or equal to Ω, and more preferably 1.0 × 10⁻⁶.８ is Ω or less, more preferably 1.0×10 ７ is Ω or less, more preferably 5.0×10 ６ is Ω or less, more preferably 4.5×10 ６ is Ω or less. The lower limit of the resistance value of the positive electrode active material of the second embodiment is not particularly limited. For example, it is 1.0×10 ―３ Ω or more, and may be 1.0×10 ―２ Ω or more, may be 1.0×10 ―１ Ω or more, may be 1.0×10 ０ Ω or more, may be 1.0×10 １ Ω or more. The resistance value of the positive electrode active material of the second embodiment by the following (Method 2) is preferably 1.0×10 ―３ Ω or more and 1.0×10 １０ Ω or less, more preferably 1.0×10 ―３ Ω or more and 5.0×10 ９ Ω or less, more preferably 1.0×10 ―３ Ω or more and 2.0×10 ９ Ω or less, more preferably 1.0×10 ―３ Ω or more and 1.0×10 ９ Ω or less, more preferably 1.0×10 ―３ Ω or more and 1.0×10 ８ Ω or less, more preferably 1.0×10 ―３ Ω or more and 1.0×10 ７ Ω or less, more preferably 1.0×10 ―３ Ω or more and 5.0×10 ６ Ω or less, more preferably 1.0×10 ―３ Ω or more and 4.5×10 ６ Ω or less.

[0094] (Method 2) 5.0 g of the positive electrode active material is pressed under the conditions of a pressure of 100 kgf / cm ２ and a pressing time of 1 minute to form a sample 1 into a shape with a diameter of 34 mm. Then, the resistance value of the sample 1 is measured using a resistivity meter under the conditions of a temperature of 25°C, an applied voltage of 10 V, and a holding time of 3 seconds. Note that, more specifically, the method described in the examples can be adopted as the method for measuring the resistance value of the positive electrode active material.

[0095] <Method for producing positive electrode active material> The method for producing positive electrode active material according to the second embodiment comprises the following step (A). The method for producing positive electrode active material according to the second embodiment may further comprise the following step (B). Step (A): A step of heating a mixed solution 1 containing a sodium source, a transition metal source, and a phosphoric acid source. Step (B): A step of preparing the mixed solution 1. More specifically, the method for producing positive electrode active material can be the method described in the examples.

[0096] (Step (A)) In step (A), a mixed solution 1 containing a sodium source, a transition metal source, and a phosphoric acid source is heated.

[0097] Step (A) is preferably carried out at a pressure of 200 kPa or less, and more preferably at atmospheric pressure. The heating temperature in step (A) is preferably 100°C or more and less than 600°C, and more preferably 100°C or more and 450°C or less.

[0098] Examples of sodium sources include sodium oxide, sodium hydroxide, sodium carbonate, sodium sulfate, sodium nitrate, sodium acetate, sodium halides, sodium oxalates, and sodium alkoxides. The sodium source preferably comprises one or more selected from the group consisting of sodium carbonate and sodium hydroxide.

[0099] Examples of transition metal sources include transition metal oxides, transition metal hydroxides, transition metal oxyhydroxides, transition metal carbonates, transition metal sulfates, transition metal nitrates, transition metal acetates, transition metal halides, transition metal ammonium salts, transition metal oxalates, and transition metal alkoxides. The transition metal source preferably comprises one or more selected from the group consisting of iron(II) chloride and manganese(II) chloride.

[0100] Examples of phosphate sources include phosphoric acid, sodium phosphate, transition metal phosphate, and ammonium phosphate. The phosphate source preferably contains ammonium ions. Examples of ammonium phosphate salts include triammonium phosphate, diammonium hydrogen phosphate, and ammonium dihydrogen phosphate. The phosphate source more preferably contains diammonium hydrogen phosphate.

[0101] In step (B), a mixed solution 1 containing a sodium source, a transition metal source, and a phosphoric acid source is prepared. Preferably, step (B) includes step (B1) of adding the transition metal source to the mixed solution containing the sodium source and the phosphoric acid source in installments or continuously. In step (B1), for example, the solution containing the transition metal source may be added dropwise to the mixed solution containing the sodium source and the phosphoric acid source continuously.

[0102] Furthermore, step (A) may further comprise the following step (A1). Furthermore, step (A) may further comprise the following step (A2). Step (A1): A step of evaporating the mixed solution 1 to dryness. Step (A2): A step of heating the mixture containing the solids obtained in step (A1) and the carbon source.

[0103] In step (A1), the mixed solution 1 is evaporated to dryness. This yields a solid component in step (A1). The solid component preferably contains an olivine-type phosphate transition metal sodium compound.

[0104] Step (A1) is preferably carried out at a pressure of 200 kPa or less, and more preferably at atmospheric pressure. The heating temperature in step (A1) is preferably 100°C or more and less than 600°C, and more preferably 100°C or more and 450°C or less.

[0105] Step (A1) may further include a step of washing the solid obtained by evaporation to dryness before step (A2). In the washing step, for example, water-soluble impurities in the solid can be removed by washing the solid with pure water.

[0106] In step (A2), the mixture containing the solids obtained in step (A1) and the carbon source is heated. Note that step (A2) may be performed together with step (A1) instead of after it. In this case, step (A2) may be performed by adding the carbon source during the evaporation of the mixed solution 1 to dryness.

[0107] Step (A2) is preferably carried out at a pressure of 200 kPa or less, and more preferably at atmospheric pressure. The heating temperature in step (A2) (hereinafter sometimes referred to as "heat treatment temperature") is preferably 100°C or more and less than 600°C, more preferably 100°C or more and 500°C, even more preferably 100°C or more and 490°C, even more preferably 100°C or more and 480°C, even more preferably 100°C or more and 470°C, even more preferably 100°C or more and 460°C, even more preferably 100°C or more and 455°C, and even more preferably 100°C or more and 450°C, from the viewpoint of suppressing the reduction of olivine-type phosphate transition metal sodium compounds and shortening the time required for step (A2).

[0108] The carbon source preferably comprises one or more selected from the group consisting of sugars, resins, organic acids, organic bases, petroleum pitch, coal pitch, carbon black, carbon fibers, fullerenes, carbon nanofibers, carbon nanotubes, alcohols, and hydrocarbons, more preferably comprising sugars, even more preferably glucose, and even more preferably D(+)-glucose. Examples of sugars include glucose, fructose, sucrose, saccharose, dextrin, starch, cellulose, or derivatives thereof. Examples of resins include polyethylene glycol, polyvinyl alcohol, polyethylene, polypropylene, polyvinyl chloride, phenolic resin, epoxy resin, furan resin, poly(meth)acrylic acid, poly(meth)acrylonitrile, polyglycol, polyaniline, etc. Examples of organic acids include carboxylic acids, carboxylic acid derivatives, etc. Examples of organic bases include pyridine, triethylamine, or derivatives thereof. Examples of carbon blacks include acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, etc. Examples of carbon nanotubes include single-walled carbon nanotubes and multi-walled carbon nanotubes. Examples of hydrocarbons include hydrocarbon gases. Examples of hydrocarbon gases include acetylene gas and propylene gas.

[0109] In step (A2), the amount of carbon source added is preferably 0.1% to 50% by mass, more preferably 1% to 40% by mass, and even more preferably 10% to 35% by mass, when the total amount of positive electrode active material and carbon source added is taken as 100% by mass, from the viewpoint of further improving the discharge capacity of the sodium-ion secondary battery.

[0110] The method for producing the positive electrode active material of the second embodiment preferably does not include a step of heating at a temperature above 600°C, more preferably above 550°C, even more preferably above 500°C, even more preferably above 490°C, even more preferably above 480°C, even more preferably above 470°C, even more preferably above 460°C, even more preferably above 455°C, and even more preferably above 450°C, from the viewpoint of improving the yield of the olivine-type phosphate transition metal sodium compound.

[0111] According to the method for producing the positive electrode active material of the second embodiment, the heating temperature and pressure values ​​during the production process of the positive electrode active material can be reduced compared to conventional methods. Therefore, according to the method for producing the positive electrode active material of the second embodiment, olivine-type transition metal sodium phosphate compounds can be produced under mild conditions.

[0112] [Positive electrode active material layer for sodium-ion secondary battery] The positive electrode active material layer for sodium-ion secondary battery of the second embodiment (hereinafter also referred to as the positive electrode active material layer) includes the positive electrode active material of the second embodiment, from the viewpoint of further improving the discharge capacity of the sodium-ion secondary battery.

[0113] In the second embodiment, the content of the positive electrode active material in the positive electrode active material layer is preferably 50% by mass or more and 100% by mass or less, more preferably 60% by mass or more and 100% by mass or less, even more preferably 70% by mass or more and 100% by mass or less, even more preferably 75% by mass or more and 100% by mass or less, even more preferably 80% by mass or more and 100% by mass or less, and even more preferably 85% by mass or more and 100% by mass or less, from the viewpoint of further improving the discharge capacity of the sodium-ion secondary battery, when the total amount of the positive electrode active material layer is 100% by mass or less.

[0114] The positive electrode active material layer of the second embodiment further includes a binder from the viewpoint of further improving the discharge capacity of the sodium-ion secondary battery. Examples of the binder include thermoplastic resins. Examples of thermoplastic resins include fluororesins and olefin resins. Examples of fluororesins include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene / hexafluoropropylene / vinylidene fluoride copolymer, hexafluoropropylene / vinylidene fluoride copolymer, and tetrafluoroethylene / perfluorovinyl ether copolymer. Examples of olefin resins include polyethylene and polypropylene. From the viewpoint of further improving the discharge capacity of the sodium-ion secondary battery, the binder preferably includes a fluororesin and more preferably includes PVDF.

[0115] In the second embodiment, the binder content in the positive electrode active material layer is preferably 0% to 20% by mass, more preferably 0% to 18% by mass, even more preferably 0% to 16% by mass, even more preferably 0% to 14% by mass, even more preferably 0% to 12% by mass, and even more preferably 0% to 10% by mass, when the total amount of the positive electrode active material layer is 100% by mass. The lower limit of the binder content in the positive electrode active material layer is not particularly limited, but for example it may be 0% by mass or more, 1% by mass or more, 2% by mass or more, 3% by mass or more, 4% by mass or more, or 5% by mass or more.

[0116] The positive electrode active material layer of the second embodiment further includes a conductive additive from the viewpoint of improving the output of the sodium-ion secondary battery. Examples of conductive additives include carbon materials. Examples of carbon materials include natural graphite, artificial graphite, coke, carbon black, graphitized carbon fibers, and carbon nanotubes. The carbon material may be crystalline or amorphous. From the viewpoint of improving the output of the sodium-ion secondary battery, the conductive additive preferably includes a carbon material, and more preferably includes one or more selected from the group consisting of carbon black and carbon nanotubes.

[0117] In the second embodiment, the content of the conductive additive in the positive electrode active material layer is preferably 0% by mass or more and 20% by mass or less, more preferably 0% by mass or more and 18% by mass or less, even more preferably 0% by mass or more and 16% by mass or less, even more preferably 0% by mass or more and 14% by mass or less, even more preferably 0% by mass or more and 12% by mass or less, and even more preferably 0% by mass or more and 10% by mass or less, from the viewpoint of further improving the discharge capacity of the sodium-ion secondary battery, when the total amount of the positive electrode active material layer is 100% by mass.

[0118] From the viewpoint of further improving the discharge capacity of the sodium-ion secondary battery, the mass per unit area of ​​the positive electrode active material layer in the second embodiment is preferably 10 g / m² per electrode side. ２ More than 500g / m ２ The following is more preferable: 25 g / m² ２ More than 400g / m ２ The following, and more preferably 40 g / m² ２ More than 300g / m ２ The following applies:

[0119] [Positive electrode for sodium-ion secondary battery] The positive electrode for a sodium-ion secondary battery of the second embodiment (hereinafter also referred to as the positive electrode) comprises a positive electrode active material layer and a positive electrode current collector, from the viewpoint of improving the output of the sodium-ion secondary battery. The positive electrode active material layer is located on at least one surface of the positive electrode current collector.

[0120] Examples of materials for the positive electrode current collector include copper, stainless steel, aluminum, nickel, and titanium.

[0121] The method for manufacturing the positive electrode is not particularly limited, as it can be carried out according to known methods. For example, the positive electrode can be manufactured by adding an organic solvent to a mixture containing a positive electrode active material, a binder, a conductive additive, etc., to obtain a positive electrode slurry, which is then coated onto at least one surface of a positive electrode current collector and dried. The resulting electrode is preferably compressed by a method such as a roll press to adjust it to an electrode of appropriate density. More specifically, the method for manufacturing the positive electrode active material layer can be the method described in the examples.

[0122] [Sodium-ion secondary battery] The sodium-ion secondary battery of the second embodiment preferably comprises the positive electrode of the second embodiment, an electrolyte layer, and a negative electrode for a sodium-ion secondary battery (hereinafter also referred to as the negative electrode) from the viewpoint of further improving the discharge capacity. The positive electrode and the negative electrode may be located within the electrolyte layer, or the electrolyte layer may be located between the positive electrode and the negative electrode. The sodium-ion secondary battery may also further include a separator between the positive electrode and the negative electrode.

[0123] As the form of the sodium-ion secondary battery in the second embodiment, a known form of sodium-ion secondary battery can be adopted.

[0124] (Electrolyte layer) The electrolyte layer in the second embodiment may be an electrolyte solution containing an electrolyte and a solvent, or it may be a solid electrolyte layer. Alternatively, the electrolyte layer may be a separator impregnated with an electrolyte solution.

[0125] The electrolyte in the second embodiment is not particularly limited, as it can be a known electrolyte used in sodium-ion secondary batteries.

[0126] (Negative electrode) The negative electrode in the second embodiment is not particularly limited, as a known negative electrode used in sodium-ion secondary batteries can be used.

[0127] (Separator) The separator in the second embodiment is not particularly limited, as it can be a known separator used in sodium-ion secondary batteries or lithium-ion secondary batteries.

[0128] <Method for manufacturing a sodium-ion secondary battery> In the second embodiment, the sodium-ion secondary battery can be manufactured according to a known method.

[0129] Although a second embodiment of the present invention has been described above, these are merely examples of the second embodiment, and various other configurations can be adopted. Furthermore, the present invention is not limited to the second embodiment described above, and modifications, improvements, etc., that do not impair the effects of the present invention are included in the present invention.

[0130] <<Third Embodiment>> The following describes the third embodiment of the present invention, comprising a positive electrode active material for a sodium-ion secondary battery, a positive electrode active material layer for a sodium-ion secondary battery, a positive electrode for a sodium-ion secondary battery, and a sodium-ion secondary battery.

[0131] [Positive electrode active material for sodium-ion secondary batteries] The positive electrode active material for sodium-ion secondary batteries of the third embodiment (hereinafter sometimes simply referred to as "positive electrode active material") includes an olivine-type phosphate transition metal sodium compound represented by the following formula (1): (1): NaMn ａ Fe ｂ M ｃ PO ４In equation (1), M represents a transition metal (excluding Mn and Fe), a and b are independent real numbers between 0 and 1, c is a real number between 0 and 0.5, and a, b, and c satisfy a + b > 0 and 2a + 2b + (valence of M) × c = 2. Furthermore, in the spectrum obtained by X-ray diffraction of the sodium-ion secondary battery positive electrode active material of the third embodiment using CuKα rays as the source, the (111) plane of the olivine-type phosphate transition metal sodium compound is located at a diffraction angle 2θ = 24.2 ± 0.4° and the maximum diffraction intensity is I Ｏ The diffraction peak is located at a diffraction angle of 2θ = 23.7 ± 0.4° and originates from the (111) plane of the malisite-type sodium phosphate transition metal compound, and the maximum diffraction intensity is I Ｍ The diffraction peak is and the peak intensity ratio obtained from it (I Ｍ / (I Ｏ +I Ｍ The value of )) is 0.50 or less.

[0132] The positive electrode active material for sodium-ion secondary batteries of the third embodiment, having the above-described configuration, can provide a positive electrode active material for sodium-ion secondary batteries that can improve the discharge capacity of the sodium-ion secondary battery.

[0133] Our investigations have shown that the above peak intensity ratio (I Ｍ / (I Ｏ +I Ｍ The present invention was completed by discovering for the first time that the discharge capacity of a sodium-ion secondary battery can be improved by keeping the value below a predetermined value.

[0134] (Olivine-type sodium phosphate transition metal compound) The positive electrode active material of the third embodiment includes an olivine-type sodium phosphate transition metal compound, from the viewpoint of being able to improve the discharge capacity of the sodium-ion secondary battery.

[0135] The third embodiment of the olivine-type transition metal sodium phosphate compound includes an olivine-type transition metal sodium phosphate compound represented by the following formula (1), from the viewpoint of improving the discharge capacity of the sodium-ion secondary battery. (1): NaMn ａ Feｂ M ｃ PO ４

[0136] In equation (1), M represents a transition metal (excluding Mn and Fe). a and b are independent real numbers between 0 and 1. c is a real number between 0 and 0.5. Furthermore, a, b, and c satisfy a + b > 0 and 2a + 2b + (valence of M) × c = 2. The composition of the olivine-type transition metal sodium compound represented by equation (1) can be identified, for example, by elemental analysis using energy-dispersive X-ray (EDX) spectroscopy. More specifically, the composition of the olivine-type transition metal sodium compound represented by equation (1) can be identified by measuring the content of each component (sodium, manganese, iron, transition metal M, phosphorus, and oxygen) in the olivine-type transition metal sodium compound using EDX spectroscopy and calculating their mol ratios.

[0137] In formula (1), examples of transition metal M include scandium, titanium, vanadium, chromium, cobalt, nickel, copper, yttrium, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, palladium, silver, and zinc.

[0138] The olivine-type phosphate transition metal sodium compound of the third embodiment preferably contains manganese, from the viewpoint of reducing the cost of raw materials for sodium-ion secondary batteries. In formula (1), a is preferably 0.1 to 1.0, more preferably 0.2 to 0.9, even more preferably 0.3 to 0.8, and even more preferably 0.4 to 0.7.

[0139] The olivine-type phosphate transition metal sodium compound of the third embodiment preferably contains iron, from the viewpoint of reducing the cost of raw materials for sodium-ion secondary batteries. In formula (1), b is preferably 0.1 to 1.0, more preferably 0.2 to 0.8, even more preferably 0.3 to 0.7, and even more preferably 0.35 to 0.6.

[0140] The olivine-type phosphate transition metal sodium compound of the third embodiment preferably contains manganese and iron, from the viewpoint of reducing the cost of raw materials for sodium-ion secondary batteries.

[0141] From the viewpoint of improving the electromotive force of the sodium-ion secondary battery, the manganese content in the olivine-type transition metal sodium phosphate compound of the third embodiment is preferably 30 mol% or more, more preferably 35 mol% or more, even more preferably 40 mol% or more, even more preferably 45 mol% or more, even more preferably 50 mol% or more, and even more preferably 55 mol% or more, when the total content of transition metals in the olivine-type transition metal sodium phosphate compound is taken as 100 mol%. From the viewpoint of improving the output of the sodium-ion secondary battery, the manganese content in the olivine-type transition metal sodium phosphate compound of the third embodiment is preferably less than 100 mol%, more preferably 90 mol% or less, even more preferably 80 mol% or less, even more preferably 75 mol% or less, even more preferably 70 mol% or less, and even more preferably 65 mol% or less, when the total content of transition metals in the olivine-type transition metal sodium phosphate compound is taken as 100 mol%. In the third embodiment, the manganese content in the olivine-type phosphate transition metal sodium compound is preferably 30 mol% or more and less than 100 mol%, more preferably 35 mol% or more and 90 mol%, even more preferably 40 mol% or more and 80 mol%, even more preferably 45 mol% or more and 75 mol%, even more preferably 50 mol% or more and 70 mol%, and even more preferably 55 mol% or more and 65 mol%, when the total transition metal content in the olivine-type phosphate transition metal sodium compound is taken as 100 mol%.

[0142] From the viewpoint of improving the output of the sodium-ion secondary battery, the iron content in the olivine-type transition metal sodium phosphate compound of the third embodiment is preferably more than 0 mol%, more preferably 10 mol% or more, even more preferably 20 mol% or more, even more preferably 25 mol% or more, even more preferably 30 mol% or more, and even more preferably 35 mol% or more, when the total content of transition metals in the olivine-type transition metal sodium phosphate compound is taken as 100 mol%. From the viewpoint of improving the electromotive force of the sodium-ion secondary battery, the iron content in the olivine-type transition metal sodium phosphate compound of the third embodiment is preferably 70 mol% or less, more preferably 65 mol% or less, even more preferably 60 mol% or less, even more preferably 55 mol% or less, even more preferably 50 mol% or less, and even more preferably 45 mol% or less, when the total content of transition metals in the olivine-type transition metal sodium phosphate compound is taken as 100 mol%. In the third embodiment, the iron content in the olivine-type phosphate transition metal sodium compound is preferably greater than 0 mol% and less than or equal to 70 mol%, more preferably between 10 mol% and 65 mol%, even more preferably between 20 mol% and 60 mol%, even more preferably between 25 mol% and 55 mol%, even more preferably between 30 mol% and 50 mol%, and even more preferably between 35 mol% and 45 mol%, when the total transition metal content in the olivine-type phosphate transition metal sodium compound is taken as 100 mol%.

[0143] In the third embodiment, the total manganese and iron content in the olivine-type phosphate transition metal sodium compound is preferably 50 mol% to 100 mol%, more preferably 60 mol% to 100 mol%, even more preferably 70 mol% to 100 mol%, even more preferably 80 mol% to 100 mol%, even more preferably 90 mol% to 100 mol%, and even more preferably 95 mol% to 100 mol%, when the total transition metal content in the olivine-type phosphate transition metal sodium compound is taken as 100 mol%.

[0144] The positive electrode active material of the third embodiment preferably further contains carbon, from the viewpoint of reducing the electrical resistance of the sodium-ion secondary battery. More preferably, the carbon is present on at least the surface of the positive electrode active material. More preferably, the carbon covers at least a portion of the surface of the positive electrode active material.

[0145] In the third embodiment, the carbon content in the positive electrode active material is preferably 0.1% to 40% by mass, more preferably 1% to 30% by mass, and even more preferably 3% to 25% by mass, when the content of the positive electrode active material is 100% by mass, from the viewpoint of further improving the discharge capacity of the sodium-ion secondary battery.

[0146] In the third embodiment, a known shape of the positive electrode active material can be adopted. The positive electrode active material is preferably in particulate form.

[0147] The following describes the physical properties of the positive electrode active material in the third embodiment.

[0148] In the third embodiment, in the spectrum obtained by X-ray diffraction using CuKα rays as the source, the (111) plane of the olivine-type phosphate transition metal sodium compound is located at a diffraction angle 2θ = 24.2 ± 0.4° and the maximum diffraction intensity is I ＯThe diffraction peak that is defined as diffraction peak O is defined as diffraction peak O. Furthermore, in the spectrum obtained by X-ray diffraction using CuKα rays as the source for the positive electrode active material of the third embodiment, the diffraction peak originates from the (111) plane of the malisite-type phosphate transition metal sodium compound, is located at a diffraction angle 2θ = 23.7 ± 0.4°, and has a maximum diffraction intensity of I Ｍ Let the diffraction peak be diffraction peak M.

[0149] Here, if two peaks are observed in the diffraction angle 2θ range of 23.8° to 24.1°, the higher-angle peak can be identified as diffraction peak O, and the lower-angle peak as diffraction peak M. Also, if one peak is observed in the diffraction angle 2θ range of 23.8° to 24.1°, the peak can be identified using the following procedure. First, if a peak located at diffraction angle 2θ = 16.8 ± 0.4°, originating from the (200) plane of the olivine-type sodium phosphate transition metal compound, is not observed, it means that the olivine-type sodium phosphate transition metal compound is not present, or the content of the olivine-type sodium phosphate transition metal compound is extremely low. Therefore, the peak observed in the diffraction angle 2θ range of 23.8° to 24.1° is a peak originating from the maliscite-type sodium phosphate transition metal compound, and can be identified as diffraction peak M. On the other hand, if a peak originating from the (021) plane of the mallisite-type sodium phosphate transition metal compound, located at a diffraction angle of 2θ = 26.2 ± 0.4°, is not observed, it means that the mallisite-type sodium phosphate transition metal compound is not present, or that its content is extremely low. Therefore, peaks observed in the diffraction angle range of 2θ between 23.8° and 24.1° are peaks originating from the olivine-type sodium phosphate transition metal compound and can be identified as diffraction peak O.

[0150] In the positive electrode active material of the third embodiment, the peak intensity ratio (I) obtained from diffraction peak O and diffraction peak M Ｍ / (I Ｏ +I ＭThe value of (I) is 0.50 or less, preferably 0.40 or less, more preferably 0.30 or less, even more preferably 0.25 or less, even more preferably 0.20 or less, even more preferably 0.10 or less, and even more preferably 0.05 or less, from the viewpoint of improving the discharge capacity of the sodium-ion secondary battery. Ｍ / (I Ｏ +I Ｍ The lower limit of the value of )) is not particularly limited, but for example, it is 0.00 or more. In the positive electrode active material of the third embodiment, the peak intensity ratio (I obtained from diffraction peak O and diffraction peak M) Ｍ / (I Ｏ +I Ｍ The value of )) is preferably 0.00 or more and 0.50 or less, more preferably 0.00 or more and 0.40 or less, even more preferably 0.00 or more and 0.30 or less, even more preferably 0.00 or more and 0.25 or less, even more preferably 0.00 or more and 0.20 or less, even more preferably 0.00 or more and 0.10 or less, and even more preferably 0.00 or more and 0.05 or less, from the viewpoint of further improving the discharge capacity of the sodium-ion secondary battery.

[0151] In the spectrum obtained by X-ray diffraction using CuKα rays as the source, the positive electrode active material originates from the (020) plane of the olivine-type phosphate transition metal sodium compound, is located at a diffraction angle 2θ = 28.2 ± 0.4°, and has a maximum diffraction intensity of I ０２０ It has a diffraction peak A. Furthermore, in the spectrum obtained by X-ray diffraction using CuKα rays as the source, the positive electrode active material originates from the (200) plane of the olivine-type phosphate transition metal sodium compound, is located at a diffraction angle 2θ = 16.8 ± 0.4°, and has a maximum diffraction intensity of I ２００ It has a diffraction peak B. Furthermore, in the spectrum obtained by X-ray diffraction using CuKα rays as the source, the positive electrode active material originates from the (002) plane of the olivine-type phosphate transition metal sodium compound, is located at a diffraction angle 2θ = 36.0 ± 0.4°, and has a maximum diffraction intensity of I ００２It has diffraction peak C. In other words, the positive electrode active material has diffraction peaks A, B, and C in the spectrum obtained by X-ray diffraction using CuKα rays as the source.

[0152] Peak intensity ratio of positive electrode active material (I ０２０ / (I ２００ +I ００２ The value of (I) is, from the viewpoint of improving the discharge capacity of the sodium-ion secondary battery, 0.5 or more, preferably 0.6 or more, more preferably 0.7 or more, even more preferably 0.8 or more, even more preferably 0.9 or more, even more preferably 1.0 or more, even more preferably 1.1 or more, even more preferably 1.2 or more, and even more preferably 1.3 or more. Peak intensity ratio of positive electrode active material (I ０２０ / (I ２００ +I ００２ The upper limit of the value of )) is not particularly limited, but for example it may be 3.0 or less, 2.5 or less, 2.0 or less, 1.8 or less, or 1.5 or less. Peak intensity ratio of positive electrode active material (I ０２０ / (I ２００ +I ００２ The value of )) is preferably 0.5 to 3.0, more preferably 0.6 to 2.5, even more preferably 0.7 to 2.5, even more preferably 0.8 to 2.0, even more preferably 0.9 to 2.0, even more preferably 1.0 to 1.8, even more preferably 1.1 to 1.8, even more preferably 1.2 to 1.8, and even more preferably 1.3 to 1.5.

[0153] The discharge capacity of the positive electrode active material according to the third embodiment (Method 1) is preferably 35 mAh / g or more, more preferably 40 mAh / g or more, even more preferably 45 mAh / g or more, even more preferably 50 mAh / g or more, even more preferably 75 mAh / g or more, even more preferably 90 mAh / g or more, even more preferably 100 mAh / g or more, and even more preferably 110 mAh / g or more. The upper limit of the discharge capacity of the positive electrode active material is not particularly limited, but for example it may be 300 mAh / g or less, 250 mAh / g or less, or 200 mAh / g or less. From the viewpoint of further improving the discharge capacity of the sodium-ion secondary battery, the discharge capacity of the positive electrode active material by the following method (Method 1) is preferably 35 mAh / g or more and 300 mAh / g or less, more preferably 40 mAh / g or more and 250 mAh / g or less, even more preferably 45 mAh / g or more and 200 mAh / g or less, even more preferably 50 mAh / g or more and 200 mAh / g or less, even more preferably 75 mAh / g or more and 200 mAh / g or less, even more preferably 90 mAh / g or more and 200 mAh / g or less, even more preferably 100 mAh / g or more and 200 mAh / g or less, and even more preferably 110 mAh / g or more and 200 mAh / g or less.

[0154] (Method 1) Half cells are prepared according to the method described below (Method for preparing half cells). The discharge capacity of the prepared half cells is measured according to the method described below (Method for measuring discharge capacity). More specifically, the method for measuring the half cells and the method for measuring the discharge capacity can be the method described in the examples.

[0155] (Method for preparing half-cells) A cathode slurry with a solid content of 53% by mass is prepared by mixing 88.0 parts by mass of cathode active material, 4.0 parts by mass of carbon black, 2.0 parts by mass of multi-walled carbon nanotubes, and 6.0 parts by mass of polyvinylidene fluoride in N-methyl-2-pyrrolidone. Next, the cathode slurry is applied to one side of aluminum foil, with a basis weight of 50 g / m² after drying. ２The material is coated in the manner described above. Next, it is dried at 80°C for 3 hours. Then, it is pressed with a force of 20kN using a roll press. Next, it is cut into a disc shape with a diameter of 10 mm and dried in a vacuum at 120°C for 15 hours to obtain the positive electrode. In addition, a counter electrode is obtained by cutting metallic sodium foil into a disc shape with a diameter of 14 mm and a thickness of 0.5 mm in a glove box under a nitrogen atmosphere. An electrolyte is prepared by dissolving sodium hexafluoride phosphate at a ratio of 1 mol / L in a mixed solvent of ethylene carbonate and diethyl carbonate in a volume ratio of 1:1. Next, a half cell is made in a glove box under a nitrogen atmosphere using the positive electrode, counter electrode, electrolyte, and cellulose nonwoven fabric separator. (Method for measuring discharge capacity) The half cell is charged with a constant current at a temperature of 25°C and a current of 0.01C until the voltage reaches 4.5V. Next, the capacity of the half-cell is measured when it is discharged at a constant current of 25°C and 0.01C until the voltage reaches 1.0V, and this capacity is defined as the discharge capacity.

[0156] The resistance value of the positive electrode active material in the third embodiment, determined by the following method (Method 2), is preferably 1.0 × 10⁻¹⁰, from the viewpoint of further improving the discharge capacity of the sodium-ion secondary battery. １０ It is less than or equal to Ω, and more preferably 5.0 × 10 ９ It is less than or equal to Ω, and more preferably 2.0 × 10⁻¹⁰. ９ It is less than or equal to Ω, and more preferably 1.0 × 10⁻⁶. ９ It is less than or equal to Ω, and more preferably 1.0 × 10⁻⁶. ８ It is less than or equal to Ω, and more preferably 1.0 × 10⁻⁶. ７ It is less than or equal to Ω, and more preferably 5.0 × 10 ６ It is less than or equal to Ω, and more preferably 4.5 × 10 ６ It is less than or equal to Ω. The lower limit of the resistance value of the positive electrode active material in the third embodiment is not particularly limited, but for example, 1.0 × 10 ―３ It is greater than or equal to Ω, and 1.0 × 10⁻⁶ ―２ It may be greater than or equal to Ω, and 1.0 × 10 ―１ It may be greater than Ω, 1.0 × 10 ０ It may be greater than or equal to Ω, and 1.0 × 10 １It may be Ω or greater. The resistance value of the positive electrode active material of the third embodiment according to the following (Method 2) is preferably 1.0 × 10 ―３ Ω or more 1.0×10 １０ It is less than or equal to Ω, and more preferably 1.0 × 10⁻⁶. ―３ Ω or more 5.0×10 ９ It is less than or equal to Ω, and more preferably 1.0 × 10⁻⁶. ―３ Ω or more 2.0×10 ９ It is less than or equal to Ω, and more preferably 1.0 × 10⁻⁶. ―３ Ω or more 1.0×10 ９ It is less than or equal to Ω, and more preferably 1.0 × 10⁻⁶. ―３ Ω or more 1.0×10 ８ It is less than or equal to Ω, and more preferably 1.0 × 10⁻⁶. ―３ Ω or more 1.0×10 ７ It is less than or equal to Ω, and more preferably 1.0 × 10⁻⁶. ―３ Ω or more 5.0×10 ６ It is less than or equal to Ω, and more preferably 1.0 × 10⁻⁶. ―３ Ω or more 4.5×10 ６ It is less than or equal to Ω.

[0157] (Method 2) 5.0 g of positive electrode active material, under a pressure of 100 kgf / cm² ２ Sample 1 is prepared by pressurizing the material for a pressurizing time of 1 minute and shaping it into a 34 mm diameter form. Next, the resistance value of Sample 1 is measured using a resistivity meter under the conditions of a temperature of 25°C, an applied voltage of 10 V, and a holding time of 30 seconds. More specifically, the method for measuring the resistance value of the positive electrode active material can be the method described in the examples.

[0158] <Method for producing positive electrode active material> The method for producing positive electrode active material according to the third embodiment comprises the following step (A). The method for producing positive electrode active material according to the third embodiment may further comprise the following step (B). Step (A): A step of heating a mixed solution 1 containing a sodium source, a transition metal source, and a phosphoric acid source. Step (B): A step of preparing the mixed solution 1. More specifically, the method for producing positive electrode active material can be the method described in the examples.

[0159] (Step (A)) In step (A), a mixed solution 1 containing a sodium source, a transition metal source, and a phosphoric acid source is heated.

[0160] Step (A) is preferably carried out at a pressure of 200 kPa or less, and more preferably at atmospheric pressure. The heating temperature in step (A) is preferably 100°C or more and less than 600°C, and more preferably 100°C or more and 450°C or less.

[0161] Examples of sodium sources include sodium oxides, sodium hydroxides, sodium oxyhydroxides, sodium carbonates, sodium sulfates, sodium nitrates, sodium acetates, sodium halides, sodium oxalates, and sodium alkoxides. The sodium source preferably comprises one or more selected from the group consisting of sodium carbonate and sodium hydroxide.

[0162] Examples of transition metal sources include transition metal oxides, transition metal hydroxides, transition metal oxyhydroxides, transition metal carbonates, transition metal sulfates, transition metal nitrates, transition metal acetates, transition metal halides, transition metal ammonium salts, transition metal oxalates, and transition metal alkoxides. The transition metal source preferably comprises one or more selected from the group consisting of iron(II) chloride and manganese(II) chloride.

[0163] Examples of phosphate sources include phosphoric acid, sodium phosphate, transition metal phosphate, and ammonium phosphate. The phosphate source preferably contains ammonium ions. Examples of ammonium phosphate salts include triammonium phosphate, diammonium hydrogen phosphate, and ammonium dihydrogen phosphate. The phosphate source more preferably contains diammonium hydrogen phosphate.

[0164] In step (B), a mixed solution 1 containing a sodium source, a transition metal source, and a phosphoric acid source is prepared. Preferably, step (B) includes step (B1) of adding the transition metal source to the mixed solution containing the sodium source and the phosphoric acid source in installments or continuously. In step (B1), for example, the solution containing the transition metal source may be added dropwise to the mixed solution containing the sodium source and the phosphoric acid source continuously.

[0165] Furthermore, step (A) may further comprise the following step (A1). Furthermore, step (A) may further comprise the following step (A2). Step (A1): A step of evaporating the mixed solution 1 to dryness. Step (A2): A step of heating the mixture containing the solids obtained in step (A1) and the carbon source.

[0166] In step (A1), the mixed solution 1 is evaporated to dryness. This yields a solid component in step (A1). The solid component preferably contains an olivine-type phosphate transition metal sodium compound.

[0167] Step (A1) is preferably carried out at a pressure of 200 kPa or less, and more preferably at atmospheric pressure. The heating temperature in step (A1) is preferably 100°C or more and less than 600°C, and more preferably 100°C or more and 450°C or less.

[0168] Step (A1) may further include a step of washing the solid obtained by evaporation to dryness before step (A2). In the washing step, for example, water-soluble impurities in the solid can be removed by washing the solid with pure water.

[0169] In step (A2), the mixture containing the solids obtained in step (A1) and the carbon source is heated. Note that step (A2) may be performed together with step (A1) instead of after it. In this case, step (A2) may be performed by adding the carbon source during the evaporation of the mixed solution 1 to dryness.

[0170] Step (A2) is preferably carried out at a pressure of 200 kPa or less, and more preferably at atmospheric pressure. The heating temperature in step (A2) (hereinafter sometimes referred to as "heat treatment temperature") is preferably 100°C or more and less than 600°C, more preferably 100°C or more and 500°C, even more preferably 100°C or more and 490°C, even more preferably 100°C or more and 480°C, even more preferably 100°C or more and 470°C, even more preferably 100°C or more and 460°C, even more preferably 100°C or more and 455°C, and even more preferably 100°C or more and 450°C, from the viewpoint of suppressing the reduction of olivine-type phosphate transition metal sodium compounds and shortening the time required for step (A2).

[0171] The carbon source preferably comprises one or more selected from the group consisting of sugars, resins, organic acids, organic bases, petroleum pitch, coal pitch, carbon black, carbon fibers, fullerenes, carbon nanofibers, carbon nanotubes, alcohols, and hydrocarbons, more preferably comprising sugars, even more preferably glucose, and even more preferably D(+)-glucose. Examples of sugars include glucose, fructose, sucrose, saccharose, dextrin, starch, cellulose, or derivatives thereof. Examples of resins include polyethylene glycol, polyvinyl alcohol, polyethylene, polypropylene, polyvinyl chloride, phenolic resin, epoxy resin, furan resin, poly(meth)acrylic acid, poly(meth)acrylonitrile, polyglycol, polyaniline, etc. Examples of organic acids include carboxylic acids, carboxylic acid derivatives, etc. Examples of organic bases include pyridine, triethylamine, or derivatives thereof. Examples of carbon blacks include acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, etc. Examples of carbon nanotubes include single-walled carbon nanotubes and multi-walled carbon nanotubes. Examples of hydrocarbons include hydrocarbon gases. Examples of hydrocarbon gases include acetylene gas and propylene gas.

[0172] In step (A2), the amount of carbon source added is preferably 0.1% to 50% by mass, more preferably 1% to 40% by mass, and even more preferably 10% to 35% by mass, when the total amount of positive electrode active material and carbon source added is taken as 100% by mass, from the viewpoint of further improving the discharge capacity of the sodium-ion secondary battery.

[0173] The method for producing the positive electrode active material of the third embodiment preferably does not include a step of heating at a temperature above 600°C, more preferably above 550°C, even more preferably above 500°C, even more preferably above 490°C, even more preferably above 480°C, even more preferably above 470°C, even more preferably above 460°C, even more preferably above 455°C, and even more preferably above 450°C, from the viewpoint of improving the yield of the olivine-type phosphate transition metal sodium compound.

[0174] According to the third embodiment of the method for producing a positive electrode active material, the heating temperature and pressure values ​​during the production process of the positive electrode active material can be reduced compared to conventional methods. Therefore, according to the third embodiment of the method for producing a positive electrode active material, olivine-type transition metal sodium phosphate compounds can be produced under mild conditions.

[0175] [Positive electrode active material layer for sodium-ion secondary battery] The positive electrode active material layer for sodium-ion secondary battery of the third embodiment (hereinafter also referred to as the positive electrode active material layer) includes the positive electrode active material of the third embodiment, from the viewpoint of further improving the discharge capacity of the sodium-ion secondary battery.

[0176] In the third embodiment, the content of the positive electrode active material in the positive electrode active material layer is preferably 50% by mass or more and 100% by mass or less, more preferably 60% by mass or more and 100% by mass or less, even more preferably 70% by mass or more and 100% by mass or less, even more preferably 75% by mass or more and 100% by mass or less, even more preferably 80% by mass or more and 100% by mass or less, and even more preferably 85% by mass or more and 100% by mass or less, from the viewpoint of further improving the discharge capacity of the sodium-ion secondary battery, when the total amount of the positive electrode active material layer is 100% by mass or less.

[0177] The positive electrode active material layer of the third embodiment further includes a binder from the viewpoint of further improving the discharge capacity of the sodium-ion secondary battery. Examples of the binder include thermoplastic resins. Examples of thermoplastic resins include fluororesins and olefin resins. Examples of fluororesins include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene / hexafluoropropylene / vinylidene fluoride copolymer, hexafluoropropylene / vinylidene fluoride copolymer, and tetrafluoroethylene / perfluorovinyl ether copolymer. Examples of olefin resins include polyethylene and polypropylene. From the viewpoint of further improving the discharge capacity of the sodium-ion secondary battery, the binder preferably includes a fluororesin and more preferably includes PVDF.

[0178] From the viewpoint of further improving the discharge capacity of the sodium-ion secondary battery, the binder content in the positive electrode active material layer of the third embodiment is preferably 0% to 20% by mass, more preferably 0% to 18% by mass, even more preferably 0% to 16% by mass, even more preferably 0% to 14% by mass, even more preferably 0% to 12% by mass, and even more preferably 0% to 10% by mass, when the total amount of the positive electrode active material layer is 100% by mass. The lower limit of the binder content in the positive electrode active material layer is not particularly limited, but for example it may be 0% by mass or more, 1% by mass or more, 2% by mass or more, 3% by mass or more, 4% by mass or more, or 5% by mass or more.

[0179] The positive electrode active material layer of the third embodiment further includes a conductive additive from the viewpoint of improving the output of the sodium-ion secondary battery. Examples of conductive additives include carbon materials. Examples of carbon materials include natural graphite, artificial graphite, coke, carbon black, graphitized carbon fibers, and carbon nanotubes. The carbon material may be crystalline or amorphous. From the viewpoint of improving the output of the sodium-ion secondary battery, the conductive additive preferably includes a carbon material, and more preferably includes one or more selected from the group consisting of carbon black and carbon nanotubes.

[0180] In the third embodiment, the content of the conductive additive in the positive electrode active material layer is preferably 0% to 20% by mass, more preferably 0% to 18% by mass, even more preferably 0% to 16% by mass, even more preferably 0% to 14% by mass, even more preferably 0% to 12% by mass, and even more preferably 0% to 10% by mass, when the total amount of the positive electrode active material layer is 100% by mass, from the viewpoint of further improving the discharge capacity of the sodium-ion secondary battery.

[0181] From the viewpoint of further improving the discharge capacity of the sodium-ion secondary battery, the mass per unit area of ​​the positive electrode active material layer in the third embodiment is preferably 10 g / m² per electrode side. ２ More than 500g / m ２ The following is more preferable: 25 g / m² ２ More than 400g / m ２ The following, and more preferably 40 g / m² ２ More than 300g / m ２ The following applies:

[0182] [Positive electrode for sodium-ion secondary battery] The positive electrode for a sodium-ion secondary battery of the third embodiment (hereinafter also referred to as the positive electrode) comprises a positive electrode active material layer and a positive electrode current collector, from the viewpoint of improving the output of the sodium-ion secondary battery. The positive electrode active material layer is located on at least one surface of the positive electrode current collector.

[0183] Examples of materials for the positive electrode current collector include copper, stainless steel, aluminum, nickel, and titanium.

[0184] The method for manufacturing the positive electrode is not particularly limited, as it can be carried out according to known methods. For example, the positive electrode can be manufactured by adding an organic solvent to a mixture containing a positive electrode active material, a binder, a conductive additive, etc., to obtain a positive electrode slurry, which is then coated onto at least one surface of a positive electrode current collector and dried. The resulting electrode is preferably compressed by a method such as a roll press to adjust it to an electrode of appropriate density. More specifically, the method for manufacturing the positive electrode active material layer can be the method described in the examples.

[0185] [Sodium-ion secondary battery] The sodium-ion secondary battery of the third embodiment preferably comprises the positive electrode of the third embodiment, an electrolyte layer, and a negative electrode for a sodium-ion secondary battery (hereinafter also referred to as the negative electrode) from the viewpoint of further improving the discharge capacity. The positive electrode and the negative electrode may be located within the electrolyte layer, or the electrolyte layer may be located between the positive electrode and the negative electrode. The sodium-ion secondary battery may also further include a separator between the positive electrode and the negative electrode.

[0186] As the form of the sodium-ion secondary battery in the third embodiment, a known form of sodium-ion secondary battery can be adopted.

[0187] (Electrolyte layer) The electrolyte layer of the third embodiment may be an electrolyte solution containing an electrolyte and a solvent, or it may be a solid electrolyte layer. Alternatively, the electrolyte layer may be a separator impregnated with an electrolyte solution.

[0188] The electrolyte in the third embodiment is not particularly limited, as it can be a known electrolyte used in sodium-ion secondary batteries.

[0189] (Negative electrode) The negative electrode in the third embodiment is not particularly limited, as a known negative electrode used in sodium-ion secondary batteries can be used.

[0190] (Separator) The separator in the third embodiment is not particularly limited, as any known separator used in sodium-ion secondary batteries or lithium-ion secondary batteries can be used.

[0191] <Method for manufacturing a sodium-ion secondary battery> In the third embodiment, the sodium-ion secondary battery can be manufactured according to known methods.

[0192] Although a third embodiment of the present invention has been described above, these are merely examples of the third embodiment, and various other configurations can be adopted. Furthermore, the present invention is not limited to the aforementioned third embodiment, and modifications, improvements, etc., that do not impair the effects of the present invention are included in the present invention.

[0193] Although embodiments of the present invention have been described above, these are merely examples of the present invention, and various other configurations can be adopted. Furthermore, the present invention is not limited to the embodiments described above, and modifications, improvements, etc., within the scope that can achieve the objectives of the present invention are included in the present invention. Examples of reference embodiments are listed below. 1. A method for producing a positive electrode active material for a sodium-ion secondary battery containing an olivine-type phosphate transition metal sodium compound, comprising step (A) of heating a mixed solution 1 containing a sodium source, a transition metal source, and a phosphate source, wherein when the total content of the transition metal in the mixed solution 1 is 100 mol%, the manganese content in the mixed solution 1 is 25 mol% or more. 2. The method for producing a positive electrode active material for a sodium-ion secondary battery according to 1., wherein step (A) is performed at 200 kPa or less. 3. The method for producing a positive electrode active material for a sodium-ion secondary battery according to 2., wherein step (A) is performed under atmospheric pressure. 4. The method according to 1. to 3., wherein the heating temperature in step (A) is 100°C or more and less than 600°C. A method for producing a positive electrode active material for a sodium-ion secondary battery as described in any of the above. 5. A method for producing a positive electrode active material for a sodium-ion secondary battery as described in 4, wherein the heating temperature in step (A) is 450°C or less. 6. A method for producing a positive electrode active material for a sodium-ion secondary battery as described in any of the above 1 to 5, wherein step (A) includes a step (A1) of evaporating the mixed solution 1 to dryness. 7. A method for producing a positive electrode active material for a sodium-ion secondary battery as described in 6, wherein step (A) further comprises a step (A2) of heating a mixture containing the solids obtained in step (A1) and a carbon source. 8. A method for producing a positive electrode active material for a sodium-ion secondary battery as described in 7, wherein the heating temperature in step (A2) is 100°C or more and less than 600°C. 9. The carbon source includes one or more selected from the group consisting of sugars, resins, organic acids, organic bases, petroleum pitch, coal pitch, carbon black, carbon fibers, fullerenes, carbon nanofibers, carbon nanotubes, alcohols, and hydrocarbons. Alternatively, the method for producing a positive electrode active material for a sodium-ion secondary battery as described in 8.10. A method for producing a positive electrode active material for a sodium-ion secondary battery according to any one of 1 to 9, further comprising a step (B) of preparing the mixed solution 1, wherein step (B) includes a step (B1) of adding the transition metal source in installments or continuously to the mixed solution containing the sodium source and the phosphoric acid source. 11. A method for producing a positive electrode active material for a sodium-ion secondary battery according to any one of 1 to 10, which does not include a step of heating at a temperature of 600°C or higher. 12. A method for producing a positive electrode active material for a sodium-ion secondary battery according to any one of 1 to 11, wherein when the total content of the transition metals in the mixed solution 1 is 100 mol%, the content of the manganese in the mixed solution 1 is 99 mol% or less. 13. A method for producing a positive electrode active material for a sodium-ion secondary battery according to any one of 1 to 12, wherein the phosphoric acid source contains ammonium ions. 14. A method for producing a positive electrode active material for a sodium-ion secondary battery according to any one of 1 to 13, wherein when the total content of transition metals in the mixed solution 1 is 100 mol%, the content of iron in the mixed solution 1 is 0 mol% or more and 75 mol% or less. 15. In the spectrum obtained by X-ray diffraction using CuKα rays as a source, the positive electrode active material for the sodium-ion secondary battery originates from the (111) plane of the olivine-type phosphate transition metal sodium compound, is located at a diffraction angle 2θ = 24.2 ± 0.4°, and has a maximum diffraction intensity of I. Ｏ The diffraction peak is located at a diffraction angle of 2θ = 23.7 ± 0.4° and originates from the (111) plane of the malisite-type sodium phosphate transition metal compound, and the maximum diffraction intensity is I Ｍ The diffraction peak is and the peak intensity ratio obtained from it (I Ｏ / (I Ｏ +I ＭA method for producing a positive electrode active material for a sodium-ion secondary battery according to any of 1 to 14, wherein the value of )) is 0.5 or more. 16. A raw material solution used in a method for producing a positive electrode active material for a sodium-ion secondary battery containing an olivine-type phosphate transition metal sodium compound, comprising a sodium source, a transition metal source, and a phosphate source, wherein when the total content of the transition metal is 100 mol%, the manganese content is 25 mol% or more, a raw material solution for a positive electrode active material for a sodium-ion secondary battery.

[0194] In addition, the present invention also includes configurations that combine the configurations of each of the embodiments described above.

[0195] The embodiments of the present invention will be described in detail below with reference to examples and comparative examples. However, the embodiments of the present invention are not limited in any way to those described in these examples.

[0196] <<Examples 1A to 10A and Comparative Examples 1A to 3A, and Examples 1a to 18a>> The first embodiment of the present invention will be described in detail below with reference to Examples 1A to 10A, Comparative Examples 1A to 3A, and Examples 1a to 18a. However, the first embodiment of the present invention is not limited in any way to the descriptions in these examples.

[0197] [Manufacturing of Cathode Active Material] The cathode active material was manufactured using the following <raw materials for cathode active material> and the following <method for manufacturing cathode active material>.

[0198] <Raw materials for positive electrode active material> ・Sodium source: Sodium carbonate (manufactured by Kishida Chemical Co., Ltd., special grade reagent) ・Phosphorus source: Diammonium hydrogen phosphate (manufactured by Kishida Chemical Co., Ltd., special grade reagent) ・Manganese source: Manganese(II) chloride tetrahydrate (manufactured by Kishida Chemical Co., Ltd., special grade reagent) ・Iron source: Iron(II) chloride tetrahydrate (manufactured by Kishida Chemical Co., Ltd., special grade reagent) ・Nickel source: Nickel(II) chloride hexahydrate (manufactured by Nacalai Tesque Co., Ltd., JIS special grade reagent)

[0199] <Method for preparing positive electrode active material> (Example 1A) A sodium carbonate aqueous solution was prepared by dissolving 14.31 g of sodium carbonate in 66 g of ultrapure water. Next, a diammonium hydrogen phosphate aqueous solution was prepared by dissolving 15.85 g of diammonium hydrogen phosphate in 66 g of ultrapure water. Next, solution A1 was prepared by dissolving 4.77 g of iron(II) chloride tetrahydrate and 7.13 g of manganese(II) chloride tetrahydrate in 66 g of ultrapure water. Next, solution A2 was prepared by mixing the sodium carbonate aqueous solution and the diammonium hydrogen phosphate aqueous solution. Next, solution A1 was added dropwise to solution A2 using a dropping funnel at a rate of 0.8 ± 0.4 mL / second. Mixed solution 1A was prepared by adding the entire volume of solution A1 dropwise. Next, mixed solution 1A was stirred at 25°C for 30 minutes. After that, mixed solution 1A was evaporated to dryness using a dryer under atmospheric pressure at 170°C for 15 hours. Next, the obtained solid was ground in a mortar. Then, the ground solid was washed with 50 mL of pure water, and the solid components were recovered by centrifugation. This process was repeated a total of five times. This removed water-soluble by-products, mainly sodium chloride. Thus, the positive electrode active material of Example 1A (olivine-type iron manganese sodium compound (NaMn) ０．６ Fe ０．４ PO ４ )) obtained.

[0200] (Examples 2A to 10A, Comparative Examples 1A to 3A) The positive electrode active materials were prepared in the same manner as in Example 1A, except that the amounts of the manganese source, iron source, and nickel source were changed so that the manganese content, iron content, and nickel content of mixed solution 1A were as shown in Table 1.

[0201] [Evaluation of Cathode Active Materials] <Measurement of X-ray Diffraction Spectra> The diffraction spectrum of each example of the cathode active material was determined by X-ray diffraction analysis using an X-ray diffractometer (MiniFlex 600, manufactured by Rigaku Corporation). CuKα radiation was used as the radiation source.

[0202] For diffraction peak O originating from the (111) plane of the olivine-type sodium phosphate transition metal compound and located at a diffraction angle 2θ = 24.2 ± 0.4°, the average intensity from diffraction angle 2θ = 23.0° to 23.3° was used as the baseline intensity. The maximum diffraction intensity of diffraction peak O was set to I. Ｏ Furthermore, for diffraction peak M originating from the (111) plane of the malisite-type sodium phosphate transition metal compound and located at a diffraction angle 2θ = 23.7 ± 0.4°, the average intensity from diffraction angle 2θ = 23.0° to 23.3° was used as the baseline intensity. The maximum diffraction intensity of diffraction peak M was set to I Ｍ That's what I decided.

[0203] <Evaluation of the Formation of Olivine-Type Transition Metal Sodium Phosphate Compounds> For each example of the positive electrode active material, the presence of a diffraction peak O was confirmed. If a diffraction peak O was present, it was evaluated as A, indicating the formation of olivine-type transition metal sodium phosphate compounds. If a diffraction peak O was not present, it was evaluated as B, indicating that no olivine-type transition metal sodium phosphate compounds were formed.

[0204] <Peak intensity ratio (I Ｏ / (I Ｏ +I Ｍ )) Calculation > For each example of positive electrode active material, the peak intensity ratio (I Ｏ / (I Ｏ +I Ｍ The following values ​​were calculated for each of the following comparative examples: Here, since no diffraction peak O was observed for the positive electrode active materials of Comparative Examples 1A to 3A, I Ｏ = 0 for the peak intensity ratio (I Ｏ / (I Ｏ +I Ｍ The following was calculated: In addition, for the positive electrode active materials of Examples 1A to 10A, if the diffraction peak M was not observed, I Ｍ = 0 for the peak intensity ratio (I Ｏ / (I Ｏ +I Ｍ The following was calculated:

[0205]

[0206] Furthermore, the effect of heat treatment on the olivine-type iron manganese sodium phosphate compound on its crystal structure was evaluated. The results are shown in Table 2. Examples 1a to 18a will be described in detail below.

[0207] (Examples 1a to 8a) Similar to Example 1A, an olivine-type iron manganese phosphate sodium compound (NaMn ０．６ Fe ０．４ PO ４ The following values ​​were obtained for each example. These were used as the positive electrode active material raw material A1 for each example. Next, the positive electrode active material for each example was prepared by heat-treating the positive electrode active material raw material A1 for each example using a ring-shaped electric furnace at the heat treatment temperature, heat treatment time, and under nitrogen atmosphere conditions described in Table 2. In Example 1a, no heat treatment was performed, and the positive electrode active material raw material A1 was used as the positive electrode active material. Next, for the positive electrode active material of each example, the peak intensity ratio (I) was obtained using the same method as in Example 1A. Ｏ / (I Ｏ +I Ｍ The following values ​​were calculated for each example. Furthermore, the formation of olivine-type phosphate transition metal sodium compounds was evaluated for each positive electrode active material using the same method as in Example 1A.

[0208] (Examples 9a to 17a) Similar to Example 1A, an olivine-type iron manganese phosphate sodium compound (NaMn ０．６ Fe ０．４ PO ４) were obtained. Next, a mixed solution 2A2 was prepared by mixing the obtained olivine-type iron manganese phosphate sodium compound and D(+)-glucose in 10 mL of pure water so that the total amount of the obtained olivine-type iron manganese phosphate sodium compound and D(+)-glucose was 25% by mass. Next, the mixed solution 2A2 was evaporated to dryness using a dryer at atmospheric pressure, 120°C, and for 15 hours to prepare the cathode active material raw material A2 for each example. D(+)-glucose (Kishida Chemical Co., Ltd., special grade reagent) was used as the D(+)-glucose. Next, the cathode active material raw material A2 for each example was prepared by heat-treating it using a cyclic electric furnace at the heat treatment temperature, heat treatment time, and under nitrogen atmosphere conditions as described in Table 2. In Example 9a, no heat treatment was performed, and the obtained cathode active material raw material A2 was used as the cathode active material. Next, for each example of the positive electrode active material, the peak intensity ratio (I) was calculated in the same manner as in Example 1a. Ｏ / (I Ｏ +I Ｍ The following was calculated. Furthermore, for each example of the positive electrode active material, the formation of olivine-type phosphate transition metal sodium compounds was evaluated using the same method as in Example 1A.

[0209] (Example 18a) Similar to Example 1A, an olivine-type iron manganese phosphate sodium compound (NaMn ０．６ Fe ０．４ PO ４Next, a mixture 2A3 was prepared by mixing the obtained olivine-type iron manganese phosphate sodium compound and D(+)-glucose in 10 mL of pure water so that the total amount of D(+)-glucose was 100% by mass of the obtained olivine-type iron manganese phosphate sodium compound and D(+)-glucose was 50% by mass. Next, the mixture 2A3 was evaporated to dryness using a dryer at atmospheric pressure, 120°C, and for 15 hours to prepare the cathode active material raw material A3. D(+)-glucose (Kishida Chemical Co., Ltd., special grade reagent) was used. Next, the cathode active material raw material A3 was heat-treated using a cyclic electric furnace at the heat treatment temperature, heat treatment time, and under nitrogen atmosphere conditions as described in Table 2 to prepare the cathode active material of Example 18a. Next, the peak intensity ratio (I) of the cathode active material of Example 18a was measured in the same manner as in Example 1a. Ｏ / (I Ｏ +I Ｍ The following was calculated. Furthermore, the formation of olivine-type phosphate transition metal sodium compounds was evaluated for the positive electrode active material of Example 18a using the same method as in Example 1A.

[0210]

[0211] In Table 2, in the row for "Presence or Absence of Heat Treatment," "A" indicates that heat treatment using a circular electric furnace was performed, and "B" indicates that heat treatment using a circular electric furnace was not performed. Also, in Table 2, in the row for "Presence or Absence of Carbon Source," "A" indicates that a treatment was performed in which an olivine-type sodium iron manganese phosphate compound and a carbon source (D(+)-glucose) were mixed and evaporated to dryness, and "B" indicates that a treatment was performed in which an olivine-type sodium iron manganese phosphate compound and a carbon source (D(+)-glucose) were mixed and evaporated to dryness.

[0212] <<Examples 1B to 3B and Comparative Example 1B>> The second embodiment of the present invention will be described in detail below with reference to Examples 1B to 3B and Comparative Example 1B. However, the second embodiment of the present invention is not limited in any way to the descriptions in these examples.

[0213] [Manufacturing of Cathode Active Material] The cathode active material was manufactured using the following <raw materials for cathode active material> and the following <method for manufacturing cathode active material>.

[0214] <Raw materials for positive electrode active material> ・Sodium source: Sodium carbonate (manufactured by Kishida Chemical Co., Ltd., special grade reagent) ・Phosphorus source: Diammonium hydrogen phosphate (manufactured by Kishida Chemical Co., Ltd., special grade reagent) ・Manganese source: Manganese(II) chloride tetrahydrate (manufactured by Kishida Chemical Co., Ltd., special grade reagent) ・Iron source: Iron(II) chloride tetrahydrate (manufactured by Kishida Chemical Co., Ltd., special grade reagent) ・Carbon source: D(+)-glucose (manufactured by Kishida Chemical Co., Ltd., special grade reagent)

[0215] <Method for preparing positive electrode active material> (Example 1B) A sodium carbonate aqueous solution was prepared by dissolving 14.31 g of sodium carbonate in 66 g of ultrapure water. Next, a diammonium hydrogen phosphate aqueous solution was prepared by dissolving 15.85 g of diammonium hydrogen phosphate in 66 g of ultrapure water. Next, solution B1 was prepared by dissolving 4.77 g of iron(II) chloride tetrahydrate and 7.13 g of manganese(II) chloride tetrahydrate in 66 g of ultrapure water. Next, solution B2 was prepared by mixing the sodium carbonate aqueous solution and the diammonium hydrogen phosphate aqueous solution. Next, solution B1 was added dropwise to solution B2 using a dropping funnel at a rate of 0.8 ± 0.4 mL / second. Mixed solution 1B was prepared by adding the entire volume of solution B1 dropwise. Next, mixed solution 1B was stirred at 25°C for 30 minutes. After that, mixed solution 1B was evaporated to dryness using a drying oven under atmospheric pressure at 170°C for 15 hours. Next, the obtained solid was ground in a mortar. Then, the ground solid was washed with 50 mL of pure water, and the solid components were recovered by centrifugation. This process was repeated a total of five times. This removed water-soluble by-products, mainly sodium chloride. Thus, an olivine-type sodium iron manganese phosphate compound (NaMn) was obtained. ０．６ Fe ０．４ PO ４Next, a mixture 2B was prepared by mixing the obtained olivine-type iron manganese phosphate sodium compound and D(+)-glucose in 10 mL of pure water so that the total amount of the obtained olivine-type iron manganese phosphate sodium compound and D(+)-glucose was 100% by mass. Next, the mixture 2B was evaporated to dryness using a dryer at atmospheric pressure, 120°C, and for 15 hours to prepare the positive electrode active material raw material B1. Next, the positive electrode active material raw material B1 was heat-treated using a cyclic electric furnace at the heat treatment temperature, heat treatment time, and under a nitrogen atmosphere as described in Table 3 to prepare a positive electrode active material in which at least a portion of the surface was covered with carbon. Thus, the positive electrode active material of Example 1B was obtained.

[0216] (Example 2B) In Example 2B, the positive electrode active material was prepared in the same manner as in Example 1B, except that the heat treatment temperature and heat treatment time were changed to the values ​​shown in Table 3. (Example 3B) In Example 3B, the positive electrode active material was prepared in the same manner as in Example 1B, except that the heat treatment temperature and heat treatment time were changed to the values ​​shown in Table 3, and the amount of D(+)-glucose was changed to 33.3% by mass relative to 100% by mass of the total of olivine-type iron manganese phosphate sodium compound and D(+)-glucose.

[0217] (Comparative Example 1B) Commercially available sodium iron phosphate-based cathode active material (NaFePO ４ (Manufactured by MSE Supplies) was used.

[0218] [Evaluation of Cathode Active Materials] <Measurement of X-ray Diffraction Spectra> The diffraction spectrum of each example of the cathode active material was determined by X-ray diffraction analysis using an X-ray diffractometer (MiniFlex 600, manufactured by Rigaku Corporation). CuKα radiation was used as the radiation source.

[0219] <Peak intensity ratio (I ０２０ / (I ２００ +I ００２Calculation of ))> For diffraction peak A originating from the (020) plane of the olivine-type iron manganese sodium phosphate compound and located at a diffraction angle 2θ = 28.2 ± 0.4°, the average intensity from diffraction angle 2θ = 29.0° to 29.3° was used as the baseline intensity. The maximum diffraction intensity of diffraction peak A was set to I ０２０ Furthermore, for diffraction peak B, which originates from the (200) plane of the olivine-type iron manganese sodium phosphate compound and is located at a diffraction angle 2θ = 16.8 ± 0.4°, the average intensity at diffraction angles 2θ = 17.5° to 17.8° was used as the baseline intensity. The maximum diffraction intensity of diffraction peak B was set to I ２００ Furthermore, for diffraction peak C originating from the (002) plane of the olivine-type iron manganese sodium phosphate compound and located at a diffraction angle 2θ = 36.0 ± 0.4°, the average intensity from diffraction angle 2θ = 36.5° to 36.8° was used as the baseline intensity. The maximum diffraction intensity of diffraction peak C was set to I ００２ This was done. Furthermore, regarding diffraction peak C, if a strong peak of the marisite-type iron manganese sodium phosphate compound appeared at diffraction angles 2θ = 36.5° to 36.8°, the average intensity at diffraction angles 2θ = 36.3° to 36.6° was used as the baseline intensity instead.

[0220] For each example of the positive electrode active material, the peak intensity ratio (I ０２０ / (I ２００ +I ００２ The following values ​​were calculated for each. In this case, diffraction peaks A to C were not observed for the positive electrode active material of Comparative Example 1B.

[0221] <Measurement of Resistance> 5.0 g of the positive electrode active material from Example 1B was measured at a pressure of 100 kgf / cm². ２ Sample 1 was prepared by pressurizing the material for a pressurizing time of 1 minute and shaping it into a 34 mm diameter form. Next, the resistance value of Sample 1 was measured using a resistivity meter (Hyrester UP, manufactured by Mitsubishi Chemical Analytec Co., Ltd.) and its attached URS probe under the conditions of a temperature of 25°C, an applied voltage of 10V, and a holding time of 30 seconds. The same procedure was performed to measure the resistance values ​​of the positive electrode active material of Example 2B and the positive electrode active material of Comparative Example 1B. The resistance value of the positive electrode active material of Example 3B was not measured.

[0222] <Measurement of Discharge Capacity> Half cells were prepared using the positive electrode active material for each example according to the method described below (Method for preparing half cells). The discharge capacity of each half cell was measured according to the method described below (Method for measuring discharge capacity).

[0223] (Method for preparing half-cells) A cathode slurry with a solid content of 53% by mass was prepared by mixing 88.0 parts by mass of cathode active material, 4.0 parts by mass of carbon black (Denka Black, manufactured by Denka Co., Ltd.), 2.0 parts by mass of multi-walled carbon nanotubes (VGCF, manufactured by Resonac Co., Ltd.), and 6.0 parts by mass of polyvinylidene fluoride (Soleph PVDF, manufactured by Solvay S.A.) in N-methyl-2-pyrrolidone. Next, the cathode slurry was applied to one side of a 20 μm thick aluminum foil (manufactured by Hosen Co., Ltd.), and the basis weight after drying was 50 g / m². ２ The material was coated in the manner described above. After coating, the cathode slurry was dried at 80°C for 3 hours. Next, the obtained cathode active material layer was pressed using a roll press with a force of 20 kN. Then, the pressed cathode active material layer was cut into a disc shape with a diameter of 10 mm and dried at 120°C for 15 hours under vacuum to produce the cathode. In addition, a counter electrode was produced by cutting metallic sodium foil into a disc shape with a diameter of 14 mm and a thickness of 0.5 mm in a glove box under a nitrogen atmosphere. Furthermore, an electrolyte was prepared by dissolving sodium hexafluoride phosphate at a ratio of 1 mol / L in a solvent (a mixture of ethylene carbonate and diethyl carbonate in a volume ratio of 1:1). Using the above cathode, counter electrode, electrolyte, and cellulose nonwoven fabric separator (manufactured by Nippon Kodo Paper Industry Co., Ltd.), a 2032 type coin cell shaped half cell was produced in a glove box under a nitrogen atmosphere.

[0224] (Method for measuring discharge capacity) Each half-cell was charged with a constant current at a temperature of 25°C and a current of 0.01C until the voltage reached 4.5V. Then, the capacity of each half-cell was measured when it was discharged with a constant current at a temperature of 25°C and a current of 0.01C until the voltage reached 1.0V, and this was defined as the discharge capacity.

[0225]

[0226] <<Examples 1C to 3C and Comparative Example 1C>> The third embodiment of the present invention will be described in detail below with reference to Examples 1C to 3C and Comparative Example 1C. However, the third embodiment of the present invention is not limited in any way to the descriptions in these examples.

[0227] [Manufacturing of Cathode Active Material] The cathode active material was manufactured using the following <raw materials for cathode active material> and the following <method for manufacturing cathode active material>.

[0228] <Raw materials for positive electrode active material> ・Sodium source: Sodium carbonate (manufactured by Kishida Chemical Co., Ltd., special grade reagent) ・Phosphorus source: Diammonium hydrogen phosphate (manufactured by Kishida Chemical Co., Ltd., special grade reagent) ・Manganese source: Manganese(II) chloride tetrahydrate (manufactured by Kishida Chemical Co., Ltd., special grade reagent) ・Iron source: Iron(II) chloride tetrahydrate (manufactured by Kishida Chemical Co., Ltd., special grade reagent) ・Carbon source: D(+)-glucose (manufactured by Kishida Chemical Co., Ltd., special grade reagent)

[0229] <Method for preparing positive electrode active material> (Example 1C) A sodium carbonate aqueous solution was prepared by dissolving 14.31 g of sodium carbonate in 66 g of ultrapure water. Next, a diammonium hydrogen phosphate aqueous solution was prepared by dissolving 15.85 g of diammonium hydrogen phosphate in 66 g of ultrapure water. Next, solution C1 was prepared by dissolving 4.77 g of iron(II) chloride tetrahydrate and 7.13 g of manganese(II) chloride tetrahydrate in 66 g of ultrapure water. Next, solution C2 was prepared by mixing the sodium carbonate aqueous solution and the diammonium hydrogen phosphate aqueous solution. Next, solution C1 was added dropwise to solution C2 using a dropping funnel at a rate of 0.8 ± 0.4 mL / second. Mixed solution 1C was prepared by adding the entire volume of solution C1 dropwise. Next, mixed solution 1C was stirred at 25°C for 30 minutes. After that, mixed solution 1C was evaporated to dryness using a drying oven under atmospheric pressure at 170°C for 15 hours. Next, the obtained solid was ground in a mortar. Then, the ground solid was washed with 50 mL of pure water, and the solid components were recovered by centrifugation. This process was repeated a total of five times. This removed water-soluble by-products, mainly sodium chloride. Thus, an olivine-type sodium iron manganese phosphate compound (NaMn) was obtained. ０．６ Fe ０．４ PO４ Next, a mixture 2C was prepared by mixing the obtained olivine-type iron manganese phosphate sodium compound and D(+)-glucose in 10 mL of pure water so that the total amount of the obtained olivine-type iron manganese phosphate sodium compound and D(+)-glucose was 100% by mass. Next, the mixture 2C was evaporated to dryness using a dryer at atmospheric pressure, 120°C, and for 15 hours to prepare the positive electrode active material raw material C1. Next, the positive electrode active material raw material C1 was heat-treated using a cyclic electric furnace at the heat treatment temperature, heat treatment time, and under a nitrogen atmosphere conditions as described in Table 4 to prepare a positive electrode active material in which at least a portion of the surface was covered with carbon. Thus, the positive electrode active material of Example 1C was obtained.

[0230] (Example 2C) In Example 2C, the positive electrode active material was prepared in the same manner as in Example 1C, except that the heat treatment temperature and heat treatment time were changed to the values ​​shown in Table 4. (Example 3C) In Example 3C, the positive electrode active material was prepared in the same manner as in Example 1C, except that the heat treatment temperature and heat treatment time were changed to the values ​​shown in Table 4, and the amount of D(+)-glucose was changed to 33.3% by mass relative to 100% by mass of the total of olivine-type iron manganese phosphate sodium compound and D(+)-glucose.

[0231] (Comparative Example 1C) Commercially available sodium iron phosphate-based cathode active material (NaFePO ４ (Manufactured by MSE Supplies) was used.

[0232] [Evaluation of Cathode Active Materials] <Measurement of X-ray Diffraction Spectra> The diffraction spectrum of each example of the cathode active material was determined by X-ray diffraction analysis using an X-ray diffractometer (MiniFlex 600, manufactured by Rigaku Corporation). CuKα radiation was used as the radiation source.

[0233] <Peak intensity ratio (I Ｍ / (I Ｏ +I ＭCalculation of ))> For diffraction peak O originating from the (111) plane of the olivine-type sodium phosphate transition metal compound and located at a diffraction angle 2θ = 24.2 ± 0.4°, the average intensity from diffraction angle 2θ = 23.0° to 23.3° was used as the baseline intensity. The maximum diffraction intensity of diffraction peak O was set to I Ｏ Furthermore, for diffraction peak M originating from the (111) plane of the malisite-type sodium phosphate transition metal compound and located at a diffraction angle 2θ = 23.7 ± 0.4°, the average intensity from diffraction angle 2θ = 23.0° to 23.3° was used as the baseline intensity. The maximum diffraction intensity of diffraction peak M was set to I Ｍ That's what I decided.

[0234] For each example of the positive electrode active material, the peak intensity ratio (I Ｍ / (I Ｏ +I Ｍ The following values ​​were calculated for each of them. Here, since no diffraction peak O was observed for the positive electrode active material of Comparative Example 1C, I Ｏ = 0 for the peak intensity ratio (I Ｍ / (I Ｏ +I Ｍ The following was calculated: In addition, in the positive electrode active materials of Examples 1C to 3C, if the diffraction peak M was not observed, I Ｍ = 0 for the peak intensity ratio (I Ｍ / (I Ｏ +I Ｍ The following was calculated:

[0235] <Peak intensity ratio (I ０２０ / (I ２００ +I ００２ Calculation of ))> For diffraction peak A originating from the (020) plane of the olivine-type iron manganese sodium phosphate compound and located at a diffraction angle 2θ = 28.2 ± 0.4°, the average intensity from diffraction angle 2θ = 29.0° to 29.3° was used as the baseline intensity. The maximum diffraction intensity of diffraction peak A was set to I ０２０ Furthermore, for diffraction peak B, which originates from the (200) plane of the olivine-type iron manganese sodium phosphate compound and is located at a diffraction angle 2θ = 16.8 ± 0.4°, the average intensity at diffraction angles 2θ = 17.5° to 17.8° was used as the baseline intensity. The maximum diffraction intensity of diffraction peak B was set to I ２００Furthermore, for diffraction peak C originating from the (002) plane of the olivine-type iron manganese sodium phosphate compound and located at a diffraction angle 2θ = 36.0 ± 0.4°, the average intensity from diffraction angle 2θ = 36.5° to 36.8° was used as the baseline intensity. The maximum diffraction intensity of diffraction peak C was set to I ００２ This was done. Furthermore, regarding diffraction peak C, if a strong peak of the marisite-type iron manganese sodium phosphate compound appeared at diffraction angles 2θ = 36.5° to 36.8°, the average intensity at diffraction angles 2θ = 36.3° to 36.6° was used as the baseline intensity instead.

[0236] For each example of the positive electrode active material, the peak intensity ratio (I ０２０ / (I ２００ +I ００２ The following values ​​were calculated for each. In this case, no diffraction peaks A to C were observed for the positive electrode active material of Comparative Example 1C.

[0237] <Measurement of Resistance> 5.0 g of the positive electrode active material from Example 1C was measured at a pressure of 100 kgf / cm². ２ Sample 1 was prepared by pressurizing the material for a pressurizing time of 1 minute and shaping it into a 34 mm diameter form. Next, the resistance value of Sample 1 was measured using a resistivity meter (Hyrester UP, manufactured by Mitsubishi Chemical Analytec Co., Ltd.) and its attached URS probe under the conditions of a temperature of 25°C, an applied voltage of 10V, and a holding time of 30 seconds. The same procedure was performed to measure the resistance values ​​of the positive electrode active material of Example 2C and the positive electrode active material of Comparative Example 1C. The resistance value of the positive electrode active material of Example 3C was not measured.

[0238] <Measurement of Discharge Capacity> Half cells were prepared using the positive electrode active material for each example according to the method described below (Method for preparing half cells). The discharge capacity of each half cell was measured according to the method described below (Method for measuring discharge capacity).

[0239] (Method for preparing half-cells) A cathode slurry with a solid content of 53% by mass was prepared by mixing 88.0 parts by mass of cathode active material, 4.0 parts by mass of carbon black (Denka Black, manufactured by Denka Co., Ltd.), 2.0 parts by mass of multi-walled carbon nanotubes (VGCF, manufactured by Resonac Co., Ltd.), and 6.0 parts by mass of polyvinylidene fluoride (Soleph PVDF, manufactured by Solvay S.A.) in N-methyl-2-pyrrolidone. Next, the cathode slurry was applied to one side of a 20 μm thick aluminum foil (manufactured by Hosen Co., Ltd.), and the basis weight after drying was 50 g / m². ２ The material was coated in the manner described above. After coating, the cathode slurry was dried at 80°C for 3 hours. Next, the obtained cathode active material layer was pressed using a roll press with a force of 20 kN. Then, the pressed cathode active material layer was cut into a disc shape with a diameter of 10 mm and dried at 120°C for 15 hours under vacuum to produce the cathode. In addition, a counter electrode was produced by cutting metallic sodium foil into a disc shape with a diameter of 14 mm and a thickness of 0.5 mm in a glove box under a nitrogen atmosphere. Furthermore, an electrolyte was prepared by dissolving sodium hexafluoride phosphate at a ratio of 1 mol / L in a solvent (a mixture of ethylene carbonate and diethyl carbonate in a volume ratio of 1:1). Using the above cathode, counter electrode, electrolyte, and cellulose nonwoven fabric separator (manufactured by Nippon Kodo Paper Industry Co., Ltd.), a 2032 type coin cell shaped half cell was produced in a glove box under a nitrogen atmosphere.

[0240] (Method for measuring discharge capacity) Each half-cell was charged with a constant current at a temperature of 25°C and a current of 0.01C until the voltage reached 4.5V. Then, the capacity of each half-cell was measured when it was discharged with a constant current at a temperature of 25°C and a current of 0.01C until the voltage reached 1.0V, and this was defined as the discharge capacity.

[0241]

[0242] This application claims priority based on Japanese Patent Application No. 2024-217328, No. 2024-217329, and No. 2024-217330, filed on December 12, 2024, and incorporates all of their disclosures herein.

Claims

1. A method for producing a positive electrode active material for a sodium-ion secondary battery containing an olivine-type phosphate transition metal sodium compound, comprising step (A) of heating a mixed solution 1 containing a sodium source, a transition metal source, and a phosphate source, wherein when the total content of transition metals in the mixed solution 1 is 100 mol%, the manganese content in the mixed solution 1 is 25 mol% or more, and step (A) is performed at 200 kPa or less.

2. The method for producing a positive electrode active material for a sodium-ion secondary battery according to claim 1, wherein step (A) comprises a step (A1) of evaporating the mixed solution 1 to dryness, and further comprises a step (A2) of heating a mixture containing the solids obtained in step (A1) and a carbon source.

3. The method for producing a positive electrode active material for a sodium-ion secondary battery according to claim 2, wherein the heating temperature in step (A2) is 100°C or more and 490°C or less.

4. The method for producing a positive electrode active material for a sodium-ion secondary battery according to claim 2 or 3, wherein the carbon source comprises one or more selected from the group consisting of sugars, resins, organic acids, organic bases, petroleum pitch, coal pitch, carbon black, carbon fibers, fullerenes, carbon nanofibers, carbon nanotubes, alcohols, and hydrocarbons.

5. A method for producing a positive electrode active material for a sodium-ion secondary battery according to claim 1 or 2, further comprising the step (B) of preparing the mixed solution 1, wherein the step (B) includes the step (B1) of adding the transition metal source in installments or continuously to the mixed solution containing the sodium source and the phosphoric acid source.

6. A method for producing a positive electrode active material for a sodium-ion secondary battery according to claim 1 or 2, wherein step (A) is performed under atmospheric pressure.

7. A method for producing a positive electrode active material for a sodium-ion secondary battery according to claim 1 or 2, which does not include a step of heating at a temperature exceeding 490°C.

8. The method for producing a positive electrode active material for a sodium-ion secondary battery according to claim 1 or 2, wherein when the total content of transition metals in the mixed solution 1 is 100 mol%, the content of manganese in the mixed solution 1 is 99 mol% or less.

9. The method for producing a positive electrode active material for a sodium ion secondary battery according to claim 1 or 2, wherein the phosphate source contains ammonium ions.

10. The method for producing a positive electrode active material for a sodium-ion secondary battery according to claim 1 or 2, wherein when the total content of transition metals in the mixed solution 1 is 100 mol%, the content of iron in the mixed solution 1 is 0 mol% or more and 75 mol% or less.

11. In the spectrum obtained by X-ray diffraction using CuKα rays as the source, the positive electrode active material for the sodium-ion secondary battery originates from the (111) plane of the olivine-type phosphate transition metal sodium compound, is located at a diffraction angle 2θ = 24.2 ± 0.4°, and has a maximum diffraction intensity of I Ｏ The diffraction peak is located at a diffraction angle of 2θ = 23.7 ± 0.4° and originates from the (111) plane of the malisite-type sodium phosphate transition metal compound, and the maximum diffraction intensity is I Ｍ The diffraction peak is and the peak intensity ratio obtained from it (I Ｏ / (I Ｏ +I Ｍ A method for producing a positive electrode active material for a sodium ion secondary battery according to claim 1 or 2, wherein the value of )) is 0.5 or more.

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