JP6285782B2 - Method for producing sodium chromite - Google Patents

Description

  The present invention relates to a method for producing sodium chromite useful as a solid lubricant, a positive electrode active material of a molten salt battery, and the like.

Sodium chromite (NaCrO ₂ ) can be used, for example, as a solid lubricant that can be used in high-temperature liquid sodium or high vacuum (for example, Patent Document 1), or a molten salt in an electrolyte. It is used as a positive electrode active material of a storage battery type molten salt battery to be used (for example, Patent Document 2).

  A molten salt battery using sodium chromite as a positive electrode active material uses sodium ions as conductive ions, and during charging, sodium ions move from the positive electrode side to the negative electrode side, and sodium ions are absorbed by the negative electrode.

  The molten salt battery using sodium chromite as a positive electrode active material has a feature that a sodium-sulfur battery needs to be operated at a high temperature of 280 to 360 ° C., whereas it can be operated at a lower temperature.

  Here, as a conventional method for producing sodium chromite, for example, a method of baking a uniform mixture obtained by mixing chromium oxide and sodium carbonate in vacuum or in an inert gas (Patent Document 1 and Patent Document 2). ), A method of synthesizing chromium hydroxide and sodium hydroxide by reacting them (Patent Document 3) and the like.

JP-A-8-295894 JP 2012-162416 A JP 2012-41237 A

  In the conventional method of obtaining sodium chromite by baking a uniform mixture obtained by mixing chromium oxide and sodium carbonate in a vacuum or in an inert gas, the synthesized sodium chromite converts hexavalent chromium. On the other hand, if the ratio of sodium carbonate is reduced, hexavalent chromium can be reduced, but chromium oxide remains as an unreacted substance, and a heterogeneous phase of chromium oxide is observed by X-ray diffraction. Sodium chromite is difficult to obtain.

  Although a method of adding a reducing agent to sodium dichromate and baking is also conceivable, sodium dichromate is a strong oxidizing agent, and thus has a problem that it is dangerous due to a rapid reaction.

  Accordingly, an object of the present invention is to obtain sodium monochromite which is X-ray diffraction-free and substantially free of hexavalent chromium by an industrially advantageous method using sodium dichromate as a raw material. An object of the present invention is to provide a method for producing sodium chromite.

The object is achieved by the present invention described below.
That is, this invention (1) contains sodium dichromate and carbon powder, and the molar ratio (C / Cr) of C with respect to Cr is 0.7 to 3.0. Is a method for producing sodium chromite that is fired at 750 ° C. or higher in a vacuum or an inert gas atmosphere, and the carbon atom content of the carbon powder is 98 mass% or higher. A method for producing sodium acid is provided.

  According to the present invention, by using sodium dichromate as a raw material, an X-ray diffraction single phase sodium chromite substantially free of hexavalent chromium can be obtained by an industrially advantageous method. The manufacturing method of the sodium chromite which can be provided can be provided.

3 is an X-ray diffraction chart of fired products obtained in Examples and Comparative Examples. 2 is a SEM photograph of the fired product obtained in Example 1. 4 is a SEM photograph of the fired product obtained in Example 2. 4 is a SEM photograph of the fired product obtained in Example 3. 4 is a SEM photograph of the fired product obtained in Example 4. 6 is a SEM photograph of the fired product obtained in Example 5. 6 is a SEM photograph of the fired product obtained in Example 6.

  The method for producing sodium chromite according to the present invention comprises sodium dichromate and carbon powder, and the molar ratio (C / Cr) of C with respect to Cr is 0.7 to 3.0. A method for producing sodium chromite, characterized in that a firing raw material mixture is fired at 750 ° C. or higher in a vacuum or an inert gas atmosphere.

In the method for producing sodium chromite according to the present invention, the firing raw material mixture containing sodium dichromate (Na ₂ Cr ₂ O ₇ ) and carbon powder is fired in a vacuum or an inert gas atmosphere, thereby producing heavy weight. Sodium chromate is reduced to obtain sodium chromite.

  The sodium dichromate according to the method for producing sodium chromite of the present invention is not particularly limited as long as it is industrially available. The average particle diameter of sodium dichromate is preferably 10 to 1000 μm, particularly preferably 50 to 500 μm, as determined by observation with a scanning electron microscope (SEM). When the average particle size is less than the above range, the handling tends to be poor, and when it exceeds the above range, the reactivity tends to be poor. In addition, in this invention, the average particle diameter calculated | required by the scanning electron microscope observation (SEM) of sodium dichromate is observed with a scanning electron microscope, and 200 pieces are arbitrarily selected from the obtained SEM photograph. It is obtained by extracting particles, measuring the particle diameters of those particles, and calculating the average value of the particle diameters.

  The sodium dichromate according to the method for producing sodium chromite of the present invention may be a hydrate or an anhydride. Further, the sodium dichromate according to the method for producing sodium chromite of the present invention may be obtained by any production method, but in order to obtain high-purity sodium chromite as much as possible. Those with less impurities are preferred.

  The carbon powder which concerns on the manufacturing method of the sodium chromite of this invention is a powder-form particle | grains which consist of carbon atoms. In the carbon powder according to the method for producing sodium chromite of the present invention, the carbon atom content is 95% by mass or more, preferably 98% by mass or more.

  The carbon powder according to the method for producing sodium chromite of the present invention is not particularly limited. For example, natural graphite, artificial graphite, carbon black, acetylene black, furnace black, ketjen black, channel black, thermal black, activated carbon , Carbon nanotube, carbon nanofiber, carbon nanowire, carbon nanohorn, carbon nanoring and the like. The carbon powder may be a single type or a combination of two or more types. Of these, natural graphite is preferable in that it is excellent in reducing ability to reduce sodium dichromate, is substantially free of hexavalent chromium, and is easy to obtain single-phase sodium chromite by X-ray diffraction. Natural graphite, artificial graphite, carbon black, and activated carbon are preferable because they are easily available.

  The average particle size of the carbon powder according to the method for producing sodium chromite of the present invention is preferably 0.01 to 50 μm, particularly preferably 0.01, as determined by observation with a scanning electron microscope (SEM). ˜20 μm. When the average particle size of the carbon powder is in the above range, the reducing ability to reduce sodium dichromate is increased, and hexavalent chromium is substantially not contained and X-ray diffraction single phase sodium chromite is added. This is preferable because it is easy to obtain. In addition, in this invention, the average particle diameter calculated | required by the scanning electron microscope (SEM) observation of carbon powder observes a measuring object with a scanning electron microscope, and 200 particles are arbitrarily selected from the obtained SEM photograph. It is obtained by extracting, measuring the particle diameter of those particles, and calculating the average value of the particle diameters.

  The baking raw material mixture which concerns on the manufacturing method of the sodium chromite of this invention contains sodium dichromate and carbon powder. And the baking raw material mixture which concerns on the manufacturing method of the sodium chromite of this invention is obtained by mixing sodium dichromate and carbon powder. The method for obtaining a baking raw material mixture by mixing sodium dichromate and carbon powder is not particularly limited, and sodium dichromate and carbon powder may be mixed by a wet method or by a dry method. May be. When mixing by a wet method, it can mix using apparatuses, such as a ball mill, a disper mill, a homogenizer, a vibration mill, a sand grind mill, an attritor, a powerful stirrer. Moreover, when mixing by a dry method, mixing can be performed using apparatuses, such as a high speed mixer, a super mixer, a turbo sphere mixer, a Henschel mixer, a Nauta mixer, a ribbon blender, and a V-type mixer. The mixing of sodium dichromate and carbon powder is not limited to the method using the exemplified apparatus. Also, at the laboratory level, mixing can be performed using a home mixer or manually. In addition, if desired, the calcined raw material mixture obtained by mixing sodium dichromate and carbon powder may be pulverized with a jet mill or the like to adjust the particle size.

  In the firing raw material mixture according to the method for producing sodium chromite of the present invention, the molar ratio of carbon atom to chromium atom (C / Cr) is 0.7 to 3.0. When the molar ratio of carbon atom to chromium atom is in the above range, hexavalent chromium is substantially not contained and X-ray diffraction single phase sodium chromite is obtained.

  And in the manufacturing method of the sodium chromite of this invention, the molar ratio (C / Cr) of the carbon atom with respect to a chromium atom is 0.7-1.5, Preferably it is 0.7-1.0. By doing so, it is possible to obtain sodium chromite which is a sodium chromite having a small residual carbon content derived from the carbon powder and which is substantially free of hexavalent chromium and is X-ray diffracted. On the other hand, if the molar ratio of carbon atom to chromium atom is less than the above range, it is difficult to obtain single-phase sodium chromite by X-ray diffraction, and the content of hexavalent chromium increases. If the above range is exceeded, the carbon content in the sodium chromite increases.

  In addition, when sodium chromite is used as the positive electrode active material of the molten salt battery, the discharge capacity of the molten salt battery can be increased by coating the surface of non-conductive sodium chromite particles with conductive carbon. Can be high. Therefore, in the method for producing sodium chromite of the present invention, the molar ratio (C / Cr) of C to Cr is 1.5 to 3.0, preferably 1.5 to 2.4. Thus, at the time of firing, sodium dichromate in the firing raw material mixture is reduced with carbon powder, and the particle surface of sodium chromite produced by the reduction is carbon content derived from carbon powder not used for reduction. X-ray diffractive single phase sodium chromite substantially free of hexavalent chromium and having a particle surface coated with carbon. Can be advantageously obtained.

  In the method for producing sodium chromite according to the present invention, the firing raw material mixture is fired in a vacuum or an inert gas atmosphere. Examples of the inert gas include argon gas, helium gas, nitrogen gas, and the like. Various physical properties such as purity of these inert gases are not particularly limited.

  In the method for producing sodium chromite of the present invention, the firing temperature when firing the firing raw material mixture is 750 ° C. or higher, preferably 750 to 1200 ° C., particularly preferably 800 to 1000 ° C. When the firing temperature is in the above range, single-phase sodium chromite is obtained in X-ray diffraction. On the other hand, when the firing temperature is lower than the above range, single-phase sodium chromite cannot be obtained. Moreover, when a calcination temperature exceeds 1200 degreeC, there exists a tendency for hexavalent chromium to increase, and there exists a possibility that it may become industrially disadvantageous from a cost viewpoint.

  In the method for producing sodium chromite of the present invention, the firing time when firing the firing raw material mixture is not particularly limited, and is generally 1 hour or longer, preferably 3 to 10 hours. In addition, it is easy to obtain single-phase sodium chromite.

  In the method for producing sodium chromite of the present invention, the number of firings when firing the firing raw material mixture may be one time or two or more times. That is, the firing operation of the firing raw material mixture may be performed only once, or after the first firing operation of the firing raw material mixture is performed and the fired product is cooled, it is obtained again by the first firing operation. The fired product may be refired, or such refired operation may be repeated.

  In the method for producing sodium chromite according to the present invention, the sodium chromite obtained by firing may be crushed or crushed as necessary, and further classified.

Thus to, by performing a method of manufacturing a sodium chromite of the present invention, to obtain a sodium chromite (NaCrO _2).

Sodium chromite obtained by carrying out the method for producing sodium chromite according to the present invention is a compound represented by the chemical formula NaCrO ₂ and is single-phase in terms of X-ray diffraction. Further, the content of hexavalent chromium in sodium chromite obtained by carrying out the method for producing sodium chromite of the present invention is 30 ppm or less, preferably 10 ppm or less. It is sodium chromite containing substantially no (Cr ₂ O ₃ ).

  The average particle diameter obtained by the laser scattering / diffraction method of sodium chromite obtained by carrying out the method for producing sodium chromite of the present invention is preferably 1 to 100 μm, particularly preferably 1 to 40 μm. The average particle diameter of sodium chromite is the average particle diameter of secondary particles. Further, the residual carbon derived from the raw carbon powder is preferably 1% by mass or less, particularly preferably 0.5% by mass or less. Here, in the case of using sodium chromite as the positive electrode active material of the molten salt battery, when covering the particle surface of sodium chromite with conductive carbon, the content of residual carbon is preferably 1 to It is good also as 15 mass%, Most preferably, it is good also as 3-10 mass%.

  Sodium chromite obtained by carrying out the method for producing sodium chromite of the present invention is suitably used as a positive electrode active material of a sodium ion secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte.

  A positive electrode active material for a sodium ion secondary battery according to the present invention comprises sodium chromite obtained by performing the method for producing sodium chromite according to the present invention. It is.

  The positive electrode active material is one raw material of a mixture composed of a positive electrode mixture of a sodium ion secondary battery described later, that is, a positive electrode active material, a conductive agent, a binder, and, if necessary, a filler. Since the positive electrode active material for a sodium ion secondary battery of the present invention contains sodium chromite obtained by performing the method for producing sodium chromite of the present invention, it is mixed with other raw materials to prepare a positive electrode mixture. Kneading is easy, and the coating property when the obtained positive electrode mixture is applied to the positive electrode current collector becomes easy.

  The sodium ion secondary battery of the present invention is a sodium ion secondary battery characterized in that the positive electrode active material for sodium ion secondary batteries of the present invention is used. From the positive electrode, the negative electrode, the separator, and the electrolyte Become.

  The positive electrode according to the sodium ion secondary battery of the present invention is formed, for example, by applying and drying a positive electrode mixture on a positive electrode current collector. The positive electrode mixture includes a positive electrode active material, a conductive agent, a binder, and a filler that is added as necessary. In the sodium ion secondary battery of the present invention, the positive electrode active material for the sodium ion secondary battery of the present invention is uniformly applied to the positive electrode. Therefore, the sodium ion secondary battery of the present invention has high battery performance, particularly high capacity and high safety.

  Content of the positive electrode active material contained in the positive electrode mixture which concerns on the sodium ion secondary battery of this invention is 70-100 mass%, Preferably it is 90-98 mass%.

  The positive electrode current collector according to the sodium ion secondary battery of the present invention is not particularly limited as long as it is an electronic conductor that does not cause a chemical change in the constituted battery, for example, stainless steel, nickel, aluminum, Examples include titanium, baked carbon, aluminum, and stainless steel whose surfaces are treated with carbon, nickel, titanium, and silver. The surface of these materials may be oxidized and used, or the current collector surface may be provided with irregularities by surface treatment. Examples of the current collector include foils, films, sheets, nets, punched ones, lath bodies, porous bodies, foam bodies, fiber groups, nonwoven fabric molded bodies, and the like. The thickness of the current collector is not particularly limited, but is preferably 1 to 500 μm.

  The conductive agent according to the sodium ion secondary battery of the present invention is not particularly limited as long as it is an electron conductive material that does not cause a chemical change in the constituted battery. For example, graphite such as natural graphite and artificial graphite, carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, carbon black such as thermal black, conductive fibers such as carbon fiber and metal fiber, Examples include metal powders such as carbon fluoride, aluminum and nickel powder, conductive whiskers such as zinc oxide and potassium titanate, conductive metal oxides such as titanium oxide, and conductive materials such as polyphenylene derivatives. Examples of graphite include scaly graphite, scaly graphite, and earthy graphite. These can be used alone or in combination of two or more. The compounding ratio of the conductive agent is 1 to 50% by mass, preferably 2 to 30% by mass in the positive electrode mixture.

  Examples of the binder according to the sodium ion secondary battery of the present invention include starch, polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose, hydroxypropylcellulose, regenerated cellulose, diacetylcellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, Polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butadiene rubber, fluororubber, tetrafluoroethylene-hexafluoroethylene copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroethylene- Perfluoroalkyl vinyl ether copolymer, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-chlorotrifluor Ethylene copolymer, ethylene-tetrafluoroethylene copolymer, polychlorotrifluoroethylene, vinylidene fluoride-pentafluoropropylene copolymer, propylene-tetrafluoroethylene copolymer, ethylene-chlorotrifluoroethylene copolymer, Vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer, vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene copolymer, ethylene-acrylic acid copolymer, ethylene-methacrylic acid copolymer, ethylene-acrylic Acid methyl copolymers, ethylene-methyl methacrylate copolymers, polysaccharides such as polyethylene oxide, thermoplastic resins, polymers having rubber elasticity, and the like. These may be used alone or in combination of two or more. It can be. The blending ratio of the binder is 1 to 50% by mass, preferably 5 to 15% by mass in the positive electrode mixture.

  The filler relating to the sodium ion secondary battery of the present invention suppresses the volume expansion of the positive electrode in the positive electrode mixture, and is added as necessary. As the filler, any fibrous material can be used as long as it does not cause a chemical change in the constructed battery. For example, olefinic polymers such as polypropylene and polyethylene, and fibers such as glass and carbon are used. Although the addition amount of a filler is not specifically limited, 0-30 mass% is preferable in a positive mix.

  The negative electrode according to the sodium ion secondary battery of the present invention is formed by applying and drying a negative electrode material on a negative electrode current collector. The negative electrode current collector according to the sodium ion secondary battery of the present invention is not particularly limited as long as it is an electronic conductor that does not cause a chemical change in the constituted battery. For example, stainless steel, nickel, copper, Examples include titanium, aluminum, calcined carbon, copper and stainless steel whose surfaces are treated with carbon, nickel, titanium, silver, and an aluminum-cadmium alloy. Further, the surface of these materials may be used after being oxidized, or the surface of the current collector may be used with surface roughness by surface treatment. Examples of the current collector include foils, films, sheets, nets, punched ones, lath bodies, porous bodies, foam bodies, fiber groups, nonwoven fabric molded bodies, and the like. The thickness of the current collector is not particularly limited, but is preferably 1 to 500 μm.

  The negative electrode material according to the sodium ion secondary battery of the present invention is not particularly limited, for example, hard carbon, natural graphite, artificial graphite, soft carbon, carbon black, fullerene, carbon nanotube, carbon nanofiber, Carbon materials such as carbon nanohorn, carbon fibril, polyacene, Si, Ge, Sn, Pb, In, Zn, H, Ca, Sr, Ba, Ru, Rh, Ir, Pd, Pt, Ag, Au, Cd, Hg, Elemental elements that alloy with sodium such as Ga, Tl, C, N, Sb, Bi, O, S, Se, Te, Cl, etc., metal materials such as sodium metal, such as oxides and carbides containing these elements, sodium -Sodium-transition metal composite oxides such as titanium composite oxide. These negative electrode materials may be used alone or in combination of two or more.

  As the separator according to the sodium ion secondary battery of the present invention, an insulating thin film having a large ion permeability and a predetermined mechanical strength is used. Sheets and non-woven fabrics made of olefin polymers such as polypropylene, glass fibers or polyethylene are used because of their organic solvent resistance and hydrophobicity. The pore diameter of the separator may be in a range generally useful for batteries, for example, 0.01 to 10 μm. The thickness of the separator may be in a general battery range, and is, for example, 5 to 300 μm. In addition, when solid electrolytes, such as a polymer, are used as electrolyte mentioned later, a solid electrolyte may serve as a separator.

  The non-aqueous electrolyte containing a sodium salt according to the sodium ion secondary battery of the present invention is composed of a non-aqueous electrolyte and a sodium salt. As the non-aqueous electrolyte according to the sodium ion secondary battery of the present invention, a non-aqueous electrolyte, an organic solid electrolyte, or an inorganic solid electrolyte is used. Examples of the non-aqueous electrolyte include N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, tetrahydroxyfuran, and 2-methyl. Tetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 3-methyl -2-oxazolidinone, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, diethyl ether, 1,3- Ropansaruton, methyl propionate, and a solvent obtained by mixing one or more aprotic organic solvents such as ethyl propionate.

  Examples of the organic solid electrolyte according to the sodium ion secondary battery of the present invention include a polyethylene derivative, a polyethylene oxide derivative or a polymer containing the same, a polypropylene oxide derivative or a polymer containing the same, a phosphate ester polymer, polyphosphazene, polyaziridine, Examples thereof include a polymer containing an ionic dissociation group such as polyethylene sulfide, polyvinyl alcohol, polyvinylidene fluoride, polyhexafluoropropylene, a mixture of a polymer containing an ionic dissociation group and the non-aqueous electrolyte.

As an inorganic solid electrolyte according to the sodium ion secondary battery of the present invention, a nitride, halide, oxyacid salt, sulfide, or the like of Na can be used. For example, Na ₃ N, NaI, Na ₅ NI ₂ , _{Na 3 N-NaI-NaOH,} NaSiO 4, NaSiO 4 -NaI-NaOH, Na 2 SiS 3, Na 4 SiO 4, Na 4 SiO 4 -NaI-NaOH, P 2 S 5, Na 2 S or _Na 2 S- _{P 2 S 5, Na 2 S} -SiS 2, Na 2 S-GeS 2, Na 2 S-Ga 2 S 3, Na 2 S-B 2 S 3, Na 2 S-P 2 S 5 -X, Na 2 _{S-SiS 2 -X, Na 2} S-GeS 2 -X, Na 2 S-Ga 2 S 3 -X, Na 2 S-B 2 S 3 -X, ( wherein, X is _NaI, B 2 _{S 3} or selected from _Al 2 _{S 3} At least one or more), and the like that.

As the sodium salt according to the sodium ion secondary battery of the present invention, those dissolved in the non-aqueous electrolyte are used. For example, NaCl, NaBr, NaI, NaClO ₄ , NaBF ₄ , NaB ₁₀ Cl ₁₀ , NaPF ₆ , NaCF ₃ SO ₃ , NaCF ₃ CO ₂ , NaAsF ₆ , NaSbF ₆ , NaB ₁₀ Cl ₁₀ , NaAlCl ₄ , CH ₃ SO ₃ Na, CF ₃ SO ₃ Na, (CF ₃ SO ₂ ) ₂ NNa, chloroborane sodium, lower fat Examples thereof include salts obtained by mixing one or more of group 1 sodium carboxylates and imides.

  Moreover, the compound shown below can be added to a nonaqueous electrolyte for the purpose of improving discharge, a charge characteristic, and a flame retardance. For example, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivative, sulfur, quinoneimine dye, N-substituted oxazolidinone and N, N-substituted imidazolidine, ethylene glycol dialkyl ether , Ammonium salt, polyethylene glycol, pyrrole, 2-methoxyethanol, aluminum trichloride, conductive polymer electrode active material monomer, triethylenephosphonamide, trialkylphosphine, morpholine, aryl compounds with carbonyl group, hexamethylphosphine Examples include hollic triamide and 4-alkylmorpholine, bicyclic tertiary amines, oils, phosphonium salts and tertiary sulfonium salts, phosphazenes, and carbonates. That. In order to make the electrolyte nonflammable, a halogen-containing solvent such as carbon tetrachloride or ethylene trifluoride can be included in the electrolyte. In addition, carbon dioxide gas can be included in the electrolytic solution in order to make it suitable for high-temperature storage.

  The sodium ion secondary battery of the present invention is a sodium ion secondary battery having a high capacity per volume and excellent safety, cycle characteristics and operating voltage. The shape of the battery is a button, sheet, cylinder, corner, coin Any shape such as a mold may be used.

  The use of the sodium ion secondary battery of the present invention is not particularly limited. And electronic devices such as memory cards and video movies, and consumer electronic devices such as automobiles, electric vehicles, game machines and electric tools.

  In addition to the applications described above, sodium chromite obtained by performing the method for producing sodium chromite of the present invention is suitable for, for example, a solid lubricant that can be used in high-temperature liquid sodium or high vacuum. Used.

  Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited thereto.

Example 1
100 g of sodium dichromate dihydrate having an average particle size of 300 μm (manufactured by Nippon Chemical Industry Co., Ltd., sodium dichromate) and 6.1 g of natural graphite having an average particle size of 15 μm (manufactured by Wako Pure Chemical Industries, Ltd., Wako Special Grade) The mixture was sufficiently mixed while pulverizing with an electric coffee mill to obtain a mixed powder. The obtained mixed powder was put in an alumina sagger and baked at 900 ° C. for 5 hours in a nitrogen atmosphere. The reaction conditions are shown in Table 1, and the obtained results are shown in Table 2.

(Example 2)
100 g of sodium dichromate dihydrate having an average particle size of 300 μm (manufactured by Nippon Kagaku Kogyo Co., Ltd., sodium dichromate) and 7.3 g of natural graphite having an average particle size of 33 μm (RP99-150, manufactured by Ito Graphite Industries, Ltd.) The mixture was sufficiently mixed while pulverizing with an electric coffee mill to obtain a mixed powder. The obtained mixed powder was put in an alumina sagger and baked at 900 ° C. for 5 hours in a nitrogen atmosphere. The reaction conditions are shown in Table 1, and the obtained results are shown in Table 2.

(Example 3)
100 g of sodium dichromate dihydrate (manufactured by Nippon Chemical Industry Co., Ltd.) with an average particle size of 300 μm and 7.9 g of artificial graphite (AG6T made by Ito Graphite Industries Co., Ltd.) with an average particle size of 6 μm were pulverized with an electric coffee mill. While mixing well, a mixed powder was obtained. The obtained mixed powder was put in an alumina sagger and baked at 900 ° C. for 5 hours in a nitrogen atmosphere. The reaction conditions are shown in Table 1, and the obtained results are shown in Table 2.

Example 4
100 g of sodium dichromate dihydrate (manufactured by Nippon Kagaku Kogyo Co., Ltd.) with an average particle size of 300 μm and 7.9 g of activated carbon (manufactured by Junsei Chemical Co., Ltd., first grade reagent) with an average particle size of 12 μm were pulverized with an electric coffee mill While mixing well, a mixed powder was obtained. The obtained mixed powder was put in an alumina sagger and baked at 900 ° C. for 5 hours in a nitrogen atmosphere. The reaction conditions are shown in Table 1, and the obtained results are shown in Table 2.

(Example 5)
100 g of sodium dichromate dihydrate (manufactured by Nippon Kagaku Kogyo Co., Ltd.) having an average particle size of 300 μm and 7.9 g of acetylene black (manufactured by Strem Chemicals, Inc.) having an average particle size of 0.04 μm are ground in an electric coffee mill. While mixing well, mixed powder was obtained. The obtained mixed powder was put in an alumina sagger and baked at 900 ° C. for 5 hours in a nitrogen atmosphere. The reaction conditions are shown in Table 1, and the obtained results are shown in Table 2.

(Comparative Example 1)
The same method as in Example 1 was performed except that 4.0 g of natural graphite was mixed instead of 6.1 g of natural graphite. The reaction conditions are shown in Table 1, and the obtained results are shown in Table 2.

(Comparative Example 2)
Instead of setting the baking temperature to 900 ° C., the same method as in Example 1 was performed except that the baking temperature was set to 700 ° C. The reaction conditions are shown in Table 1, and the obtained results are shown in Table 2.

<Analysis of average particle size>
(1) Average particle diameter of sodium dichromate and carbon powder A measurement object is observed with a scanning electron microscope to obtain an SEM photograph of the measurement object, and then 200 particles are arbitrarily selected from the obtained SEM photograph. Were extracted, the particle diameter of those particles was measured, the average value of the particle diameters was calculated, and the average particle diameter was determined.
(2) Average particle diameter of sodium chromite The average particle diameter of the measurement target was determined by laser scattering / diffraction method using Microtrac MT3000 (manufactured by Nikkiso Co., Ltd.).

<Evaluation of physical properties of the fired product>
The fired products obtained in Examples 1 to 6 and Comparative Examples 1 and 2 were subjected to X-ray diffraction analysis. The obtained results are shown in FIG. A Rigaku X-ray diffractometer (Ultima IV) was used for the measurement. Moreover, the scanning electron micrograph of the sodium chromite obtained in Examples 1-6 is shown in FIGS.
Moreover, the average particle diameter, chromium oxide content, carbon content (TC), and hexavalent chromium content of the fired products obtained in Examples 1 to 6 and Comparative Examples 1 and 2 were evaluated. The results are shown in Table 2.
(Evaluation method of chromium oxide (Cr ₂ O ₃ ))
Quantification was performed by the WPPF method using the integrated powder X-ray analysis software PDXL2 manufactured by RIGAKU.
(Evaluation method of carbon content (TC))
Measured with a TOC total organic carbon meter (TOC-5000A manufactured by Shimadzu Corporation).
(Evaluation method of hexavalent chromium content)
Analysis was performed by a colorimetric method. For the determination, an ultraviolet-visible spectrophotometer (UV-1800) manufactured by Shimadzu Corporation was used.

  From Table 2, in Examples 1-6, it turns out that it is a single phase in X-ray diffraction and the sodium chromite which does not contain hexavalent chromium substantially is obtained. On the other hand, in Comparative Examples 1 and 2, a heterogeneous phase is observed in X-ray diffraction, and it can be seen that hexavalent chromium is detected at a high concentration.

Using the samples obtained in Examples and Comparative Examples as positive electrode active materials, battery performance tests were performed as follows.
<Production of sodium ion secondary battery>
80% by mass of the positive electrode active material obtained in Examples 1 to 6 and Comparative Examples 1 to 2, 10% by mass of acetylene black powder, and 10% by mass of polyvinylidene fluoride were mixed to obtain a positive electrode material, which was N-methyl-2 A kneaded paste was prepared by dispersing in pyrrolidinone. The kneaded paste was applied to an aluminum foil, dried, pressed and punched into a disk with a diameter of 15 mm to obtain a positive electrode plate.
Using this positive electrode plate, a coin-type sodium ion secondary battery was manufactured using each member such as a separator, a negative electrode, a positive electrode, a current collector plate, a mounting bracket, an external terminal, and an electrolytic solution. Of these, sodium metal was used for the negative electrode, and 1 mol of NaPF ₆ dissolved in 1 liter of a 98: 2 kneaded solution of propylene carbonate and fluoroethylene carbonate was used as the electrolyte.
Subsequently, performance evaluation of the obtained sodium ion secondary battery was performed. The results are shown in Table 3.

<Battery performance evaluation>
The produced coin-type sodium ion secondary battery was operated at room temperature under the following test conditions, and the cycle characteristics of the battery were evaluated.
First, charging was performed at 0.1 C to 3.5 V over 2 hours, and then constant current / constant voltage charging (CCCV charging) charging was performed at 3.5 V for 3 hours. Then, discharge which made constant current discharge (CC discharge) to 2.5V at 0.1C was performed, and these operations were made into 1 cycle, and discharge capacity was measured for every cycle. This cycle was repeated 10 times. The measurement results of the following (1) to (4) are shown in Table 3.
(1) Charging capacity A value obtained by constant current / constant voltage charging (CCCV charging) in cycle characteristic evaluation was defined as a charging capacity.
(2) Discharge capacity A value obtained by constant current discharge (CC discharge) in the cycle characteristic evaluation was defined as a discharge capacity.
(3) Coulomb efficiency The coulomb efficiency was calculated by the following formula from the charge capacity and discharge capacity in the cycle characteristics evaluation.
Coulomb efficiency (%) = (discharge capacity / charge capacity) × 100
(4) Capacity maintenance rate From each discharge capacity of the 1st cycle and 10th cycle in cycle characteristic evaluation, the capacity maintenance rate was computed by the following formula.
Capacity retention rate (%) = (discharge capacity at the 10th cycle / discharge capacity at the first cycle) × 100

  From the results in Table 3, it can be seen that the example is superior to the comparative example.

  According to the present invention, X-ray diffraction single phase sodium chromite substantially free of hexavalent chromium can be provided in an industrially advantageous manner. In addition, the sodium chromite is useful, for example, as a solid lubricant that can be used in high-temperature liquid sodium or high vacuum, or as a positive electrode active material of a sodium ion secondary battery.

Claims (5)

A firing raw material mixture containing sodium dichromate and carbon powder and having a molar ratio of C to Cr (C / Cr) of 0.7 to 3.0 in a vacuum or an inert gas atmosphere , A method for producing sodium chromite to be baked at 750 ° C. or higher, wherein the carbon powder has a carbon atom content of 98% by mass or more .   The carbon powder is natural graphite, artificial graphite, carbon black, acetylene black, furnace black, ketjen black, channel black, thermal black, activated carbon, carbon nanotube, carbon nanofiber, carbon nanowire, carbon nanohorn and carbon nanoring. The method for producing sodium chromite according to claim 1, wherein the method is one or more selected from the group consisting of:   3. The sodium chromite according to claim 1, wherein a molar ratio (C / Cr) of C to Cr in the calcined raw material mixture is 0.7 to 1.5. Manufacturing method.   3. The sodium chromite according to claim 1, wherein a molar ratio (C / Cr) of C to Cr is 1.5 to 3.0 in the firing raw material mixture. Manufacturing method.   The average particle diameter of the said carbon powder is 0.01-50 micrometers, The manufacturing method of the sodium chromite of any one of Claims 1-4 characterized by the above-mentioned.

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