Negative electrode active material and secondary battery

Abstract

Disclosed is a negative electrode active material which is composed of a sodium ferrite that is mainly composed of Na and Fe, which are abundantly present as resources. With this negative electrode active material, it is possible to suppress the manufacturing cost of a negative electrode. If a sodium ion battery is used as the positive electrode, it is not necessary to use a metal such as Li, so that a secondary battery with less environmental impact can be manufactured.

Description

Negative electrode active material and secondary battery

[0001] The present invention relates to a negative electrode active material for a secondary battery. Specifically, the negative electrode active material is sodium ferrite, which is composed mainly of abundant resources such as Na and Fe, and therefore can reduce the cost of producing a negative electrode. Furthermore, if the positive electrode is a sodium-ion battery, the use of metals such as Li is eliminated, thereby providing a secondary battery that has a low environmental impact upon disposal.

[0002] In recent years, there has been an accelerating trend toward smaller and lighter electronic devices such as mobile phones and personal computers, and there is an increasing demand for secondary batteries with high energy density as the power source for these devices.

[0003] Under these circumstances, secondary batteries, which have a large charge / discharge capacity per weight and volume and excellent repeatability in charge / discharge cycles, are attracting attention.

[0004] Commonly used secondary batteries use Li or Co as a positive electrode active material and a carbon-based material such as graphite as a negative electrode active material.

[0005] However, Li and Co are expensive and have the problem of imposing a heavy environmental burden upon their disposal.

[0006] To improve the characteristics of secondary batteries, it is necessary to improve the positive electrode, negative electrode, and electrolyte that constitute the secondary battery.

[0007] An object of the present invention is to provide an anode active material that has high properties as an anode active material, is made of versatile elements, and has a low environmental impact.

[0008] JP 2017-107844 A

[0009] Patent Document 1 describes the use of a compound composed of Li, Fe, and O as the negative electrode active material.

[0010] The negative electrode active material of Patent Document 1 improves the characteristics of the secondary battery, but has the problem that Li is a rare metal and is expensive.

[0011] The inventors of the present invention set out to solve the above-mentioned problems as a technical objective, and as a result of numerous trial and error trials and experiments, they succeeded in producing sodium ferrite containing Na and Fe as its main components. They discovered that the use of the sodium ferrite as a negative electrode active material can improve the characteristics of a secondary battery, that Na and Fe are versatile elements and can reduce the manufacturing cost of the negative electrode, and that the negative electrode has a low environmental impact when disposed of, thereby solving the above-mentioned technical objectives.

[0012] The above technical problems can be solved by the present invention as follows.

[0013] The present invention provides a negative electrode active material for a secondary battery, which comprises sodium ferrite containing Na and Fe as its main components.

[0014] The present invention also relates to the negative electrode active material for a secondary battery, wherein the sodium ferrite is a sodium ferrite having one or a mixed phase of two or more types selected from the following Group A: Group A: Sodium ferrite having a crystal structure in which the space group is R-3m Sodium ferrite having a crystal structure in which the space group is Pna21 Sodium ferrite having a crystal structure in which the space group is P63 / mmc

[0015] The present invention also provides a secondary battery having a negative electrode using the above-mentioned negative electrode active material.

[0016] The present invention also relates to the secondary battery, wherein the secondary battery is a sodium ion secondary battery.

[0017] The negative electrode active material according to the present invention is sodium ferrite containing, as its main components, Na and Fe, which are abundant resources. By using a negative electrode containing the sodium ferrite as the negative electrode active material, a secondary battery having excellent discharge capacity can be produced.

[0018] Furthermore, if the sodium ferrite is one or a mixed phase of two or more selected from sodium ferrite having a crystal structure in which the space group is R-3m (hereinafter referred to as "R-3m structure"), sodium ferrite having a crystal structure in which the space group is Pna21 (hereinafter referred to as "Pna21 structure"), and sodium ferrite having a crystal structure in which the space group is P63 / mmc (hereinafter referred to as "P63 / mmc structure"), a secondary battery with even better discharge capacity can be produced.

[0019] Furthermore, if the secondary battery is a sodium ion battery, it does not use Li or Co, and therefore the secondary battery places less of a burden on the environment when disposed of.

[0020] 1 is an X-ray diffraction pattern of the sodium ferrite of Example 2. FIG. 2 is an X-ray diffraction pattern of the cobalt-containing sodium ferrite of Example 4. FIG. 3 is an X-ray diffraction pattern of the sodium ferrite of Example 6. FIG. 4 is an X-ray diffraction pattern of the sodium ferrite of Example 9.

[0021] The negative electrode active material according to the present invention is sodium ferrite whose main components are Na and Fe.

[0022] The molar ratio of Na to Fe (Na / Fe) of the negative electrode active material of the R-3m structure or Pna21 structure is preferably 0.8 to 1.1, more preferably 0.9 to 1.1, and the molar ratio of Na to Fe (Na / Fe) of the negative electrode active material of the P63 / mmc structure is preferably 0.5 to 0.8, more preferably 0.6 to 0.7. In addition, when containing a different metal element M such as Ni, Co, or Mn, the molar ratio of Na / (Fe + M) in the R-3m structure and Pna21 structure is preferably 0.8 to 1.1, and the molar ratio of Na / (Fe + M) in the P63 / mmc structure is preferably 0.5 to 0.8.

[0023] In the negative electrode active materials of the R-3m structure and the Pna21 structure, if the molar ratio of Na to Fe is less than 0.8 or more than 1.1, there is a risk of a decrease in discharge capacity. In the negative electrode active material of the P63 / mmc structure, if the molar ratio of Na to Fe is less than 0.5, there is a risk of a decrease in discharge capacity. If it is more than 0.8, there is a risk of a decrease in discharge capacity.

[0024] The negative electrode active material according to the present invention may contain, in addition to sodium ferrite whose main components are Na and Fe, other metals such as Ni, Co, and Mn.

[0025] The content of the different metal is preferably 50 mol % or less relative to Fe. If the content exceeds 50 mol %, the discharge capacity may decrease. The content of the different metal is more preferably 20 mol % or less relative to Fe.

[0026] The negative electrode active material according to the present invention preferably has an R-3m structure (α phase), a Pna21 structure (β phase), or a P63 / mmc structure, or a mixed phase containing two or more structures selected from the R-3m structure, the Pna21 structure, and the P63 / mmc structure, because this allows the production of a secondary battery having excellent discharge capacity.

[0027] The BET specific surface area of ​​the negative electrode active material according to the present invention is 0.1 m ２ / g or more, and more preferably 0.15m ２ / g to 5.0m ２ / g. The BET specific surface area is 0.1 m ２ If it is less than 1 / g, the discharge capacity may decrease.

[0028] When the negative electrode active material according to the present invention has an R-3m structure (α phase), the crystallite size is preferably 120 nm or less, more preferably 30 nm to 100 nm. If the crystallite size exceeds 120 nm, there is a risk of a decrease in discharge capacity. When the negative electrode active material has a Pna21 structure (β phase), the crystallite size is preferably 1000 nm or less, more preferably 30 nm to 700 nm. When the negative electrode active material has a P63 / mmc structure, the crystallite size is preferably 180 nm or less, more preferably 30 nm to 170 nm.

[0029] When the negative electrode active material according to the present invention has an R-3m structure (α phase), the a-axis length is preferably 0.301 nm to 0.303 nm, and more preferably 0.3015 nm to 0.3025 nm. If the a-axis length is less than 0.301 nm or longer than 0.303 nm, the discharge capacity may decrease.

[0030] When the negative electrode active material according to the present invention has a Pna21 structure (β phase), the a-axis length is preferably 0.530 nm to 0.580 nm, more preferably 0.540 nm to 0.570 nm. If the a-axis length is less than 0.530 nm or longer than 0.580 nm, the discharge capacity may decrease. When the negative electrode active material according to the present invention has a P63 / mmc structure, the a-axis length is preferably 0.280 nm to 0.304 nm, more preferably 0.280 nm to 0.301 nm. If the a-axis length is less than 0.280 nm or longer than 0.301 nm, the discharge capacity may decrease.

[0031] Next, a method for producing the negative electrode active material according to the present invention will be described.

[0032] The method for producing sodium ferrite, which is the negative electrode active material according to the present invention, is not particularly limited and may be a solid phase method, a wet method, or the like, but it is preferable to produce it by the solid phase method.

[0033] Specifically, it can be produced by mixing Na and Fe to a predetermined molar ratio and firing at 150°C to 1100°C. A more preferred firing temperature is 200°C to 1050°C. For example, when producing the R-3m structure and the Pna21 structure, it is preferable to mix Na and Fe to a molar ratio of 0.5 to 1.1.

[0034] If necessary, the film may be further annealed at 400°C to 1000°C.

[0035] The raw material of Fe is preferably iron oxide. Iron oxide is FeOOH, Fe ２ O ３ , Fe ３ O ４ The specific form such as above is not required, and a part of Fe may be replaced with other elements.

[0036] Examples of the source of Na include sodium hydroxide, sodium carbonate, sodium hydrogen carbonate, sodium nitrate, sodium nitrite, sodium oxalate, and sodium amide.

[0037] Next, a non-aqueous electrolyte secondary battery using the negative electrode active material particles according to the present invention will be described.

[0038] Although a sodium ion secondary battery will be described as an example of a non-aqueous electrolyte secondary battery, the present invention is not limited to this.

[0039] (Positive Electrode) The positive electrode is composed of a positive electrode active material, a current collector, a binder that binds the electrode active material to the current collector, and, if necessary, a conductive material.

[0040] The positive electrode active material is not particularly limited as long as it is capable of inserting and extracting sodium ions, but a sodium-containing transition metal composite oxide is preferred.

[0041] The sodium-containing transition metal composite oxide is not particularly limited, but examples thereof include sodium manganese composite oxide, sodium iron composite oxide, sodium nickel composite oxide, sodium cobalt composite oxide, sodium manganese titanium composite oxide, sodium nickel titanium composite oxide, sodium nickel manganese composite oxide, sodium iron manganese composite oxide, sodium iron phosphate compound, sodium manganese phosphate compound, and sodium cobalt phosphate compound.

[0042] It is known that the R-3m structure (α phase) according to the present invention can be used as a positive electrode active material, and therefore a battery may be constructed using the sodium ferrite of the present invention as both the positive electrode active material and the negative electrode active material.

[0043] (Negative Electrode) The method for producing the negative electrode is not particularly limited, but a slurry method is exemplified.

[0044] In the slurry method, a binder, a solvent, and optionally a conductive material such as a carbon material are added to the negative electrode active material and kneaded to prepare an electrode slurry, which is then applied to a current collector and dried to produce a negative electrode.

[0045] The content of the negative electrode active material in the electrode slurry is preferably 40% by weight or more. If it is less than 40% by weight, the discharge capacity may decrease.

[0046] As the binder, known fluorine-containing polymers such as vinylidene fluoride polymers and copolymers thereof, acrylic acid polymers such as polyacrylic acid, its sodium salt and copolymers thereof, and cellulose derivatives such as carboxymethyl cellulose can be used.

[0047] In the present invention, the negative electrode can also be produced by a gas deposition method.

[0048] An example of the gas deposition method is the technology described in Japanese Patent Application Laid-Open No. 2016-96016 (FIG. 13).

[0049] (Electrolyte) As the electrolyte, a non-aqueous electrolyte in which an electrolyte is dissolved in an organic solvent is used.

[0050] The organic solvent is not particularly limited, but examples thereof include one solvent selected from cyclic carbonates, cyclic esters, and chain carbonates, or a mixed solvent of two or more solvents.

[0051] Examples of cyclic carbonates include ethylene carbonate and propylene carbonate.

[0052] An example of the cyclic ester is γ-butyrolactone.

[0053] Examples of chain carbonates include dimethyl carbonate and diethyl carbonate.

[0054] The electrolyte of the sodium ion secondary battery is not particularly limited, but may be NaPF ６ , NaBF ４ , NaClO ４ , NaAsF ６ , NaCF ３ SO ３ , Na(CF ３ SO ２ ) ２ N, Na(C ２ F ５ SO ２ ) ２ N, NaN(SO ２ F) ２ and Na(CF ３ SO ２ ) ３ Examples of the electrolyte include one or more electrolytes selected from C.

[0055] The salt concentration of the electrolyte is preferably 0.5 mol / l to 3 mol / l.

[0056] Instead of the non-aqueous electrolyte solution, a polymer gel electrolyte containing an ionic liquid electrolyte solution or the non-aqueous electrolyte solution, or a polymer solid electrolyte in which the above electrolyte is contained in a polymer solid electrolyte having sodium ion conductivity may be used.

[0057] Instead of the non-aqueous electrolyte solution, a polymer gel electrolyte containing the non-aqueous electrolyte solution or a polymer solid electrolyte in which the above electrolyte is contained in a polymer solid electrolyte having sodium ion conductivity may be used.

[0058] In the present invention, a saturated cyclic carbonate having a fluoro group may be added to the electrolyte solution, because this can improve the cycle characteristics.

[0059] The saturated cyclic carbonate having a fluoro group is not particularly limited, but examples thereof include fluoroethylene carbonate and difluoroethylene carbonate.

[0060] The proportion of the saturated cyclic carbonate having a fluoro group is preferably at least 1% by volume, more preferably 5% to 30% by volume, of the electrolyte.

[0061] (Separator) A microporous film or a nonwoven fabric can be used as the separator.

[0062] The composition of the microporous membrane or nonwoven fabric is not particularly limited, but examples include polyester polymers, polyolefin polymers, ether polymers, and glass fibers.

[0063] (Method for producing sodium ion secondary battery) A sodium ion secondary battery can be produced using the negative electrode of the present invention.

[0064] A sodium ion secondary battery is composed of at least a positive electrode, a negative electrode, a separator separating the positive electrode and the negative electrode, an electrolyte, and a battery container.

[0065] The method for producing the sodium ion secondary battery of the present invention is not particularly limited, and the battery can be produced using a known method.

[0066] As an example of a method for manufacturing a sodium ion secondary battery, a method is given in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween to form a planar laminate or a wound body, and the laminate or wound body is housed in a metal or resin battery container and sealed, and then an opening is provided in the sealed portion, an electrolyte solution is injected, and the opening is sealed to produce a secondary battery.

[0067] Examples of the present invention will be described below, but the present invention is not limited to these.

[0068] (BET Specific Surface Area) The specific surface area of ​​the sample was measured by the BET method using a specific surface area measuring device, Macsorb (manufactured by Mountec Co., Ltd.).

[0069] (Composition: ICP) The composition ratio of the sample was measured using an inductively coupled plasma emission spectrometer (manufactured by Thermo Fisher Scientific) after the sample powder was completely dissolved.

[0070] (Crystalline Structure: XRD Measurement) The layers were identified using an X-ray diffractometer (D8 ADVANCE, manufactured by Bruker Japan Co., Ltd.), and the crystallite size and lattice constants (a-axis length, b-axis length, c-axis length) of the sample were refined by Lieveld analysis.

[0071] (Production of Negative Electrode) The sodium ferrite or sodium-containing iron oxide of the example was deposited on an aluminum foil current collector by gas deposition to produce a negative electrode.

[0072] The conditions for the gas deposition method are as follows: Nozzle-substrate distance: 10 mm Pressure difference: 5.0×10^5 Pa Nozzle diameter: 0.5 mm Carrier gas: He (6N) Mask diameter: 10 mm Deposition amount: 20 μg to 50 μg

[0073] (Preparation of Coin Cell) A 2032-type coin cell was prepared using the prepared negative electrode, a metallic sodium foil having a thickness of about 1 mm as a counter electrode, and a glass separator ND420 (manufactured by Asahi Kasei Corporation) as a separator, and an electrolyte was poured into it.

[0074] The salt of the electrolyte was sodium bisfluorosulfonylamide (NaFSA). The solvent of the electrolyte was N-propyl-N-methylpyrrolidinium / bisfluorosulfonylamide (Py13-FSA). The concentration of the electrolyte was 1M.

[0075] All coin cell fabrication was carried out in a glove box in an argon atmosphere with a dew point of −80° C. or less and an oxygen concentration of 1 ppm or less.

[0076] (Charge / Discharge Measurement) In the case of a sodium ion secondary battery, the potential range is 0.005 V to 2.800 V (vs. Na / Na) at room temperature. ＋ ), and the current density was 50 mA / g relative to the amount of negative electrode active material.

[0077] <Method for producing sodium ferrite particles and measurement> (Example 1) Fe was used as the Fe raw material. ３ O ４ Fe was used. ３ O ４ Sodium hydroxide particles (Na raw material) were weighed out to 10 parts by weight so that the molar ratio of Na / Fe was 0.95. The raw materials were mixed and pulverized in a sample mill, and the mixed and pulverized product was placed in a crucible and subjected to a solid-state reaction at 400°C for 8 hours. The mixture was then cooled to room temperature and pulverized in a sample mill to obtain sodium ferrite particles of Example 1.

[0078] The sodium ferrite particles of Example 1 had a BET specific surface area of ​​2.0 m ２ The peak observed by XRD measurement was a peak attributed to R-3m (α phase).

[0079] The elements contained in the obtained sodium ferrite particles were analyzed by ICP, and it was confirmed that the Na / Fe molar ratio was 0.94, which was almost the same as the ratio of the raw materials used.

[0080] The obtained sodium ferrite particles were deposited on an aluminum foil by gas deposition to prepare a negative electrode.

[0081] Using the prepared negative electrode, a coin cell was assembled with a glass separator, using metallic sodium as the positive electrode, and 1M NaFSA / Py13-FSA adjusted to a concentration of 1M as the electrolyte and electrolytic solution.

[0082] The charge / discharge measurements were carried out by placing the battery in an incubator controlled at 30°C, and measuring the current density of 50 mA / g and the potential range of 0.005 V to 2.800 V (vs. Na / Na ＋ ), under the measurement conditions of constant current (hereinafter referred to as "CC") charge and CC discharge.

[0083] (Example 2) The same Fe as in Example 1 was used as the Fe raw material. ３ O ４ Fe was used. ３ O ４ Sodium hydroxide particles as a Na raw material were weighed out to 10 parts by weight so that the molar ratio of Na / Fe was 1.06. The raw materials were mixed and pulverized in a sample mill, and the mixed and pulverized product was placed in a crucible and subjected to a solid-state reaction at 400°C for 8 hours. The mixture was then cooled to room temperature and pulverized in a sample mill to obtain sodium ferrite particles. The obtained sodium ferrite particles were further annealed at 600°C for 8 hours, cooled, and pulverized in a sample mill to obtain sodium ferrite particles of Example 2.

[0084] The Na / Fe molar ratio of the sodium ferrite particles of Example 2 measured by ICP was 1.06, which was confirmed to be substantially the same as the ratio of the raw materials used in Example 1.

[0085] The sodium ferrite particles of Example 2 had a BET specific surface area of ​​0.2 m ２ The peak observed by XRD measurement was a peak attributed to R-3m (α phase).

[0086] (Example 3) An iron (II) sulfate solution and nickel sulfate were adjusted so that Ni / (Ni + Fe) was 2 mol%, and then a neutralization reaction was carried out with a sodium hydroxide solution until Na / (Ni + Fe) = 1.00. Air was then introduced into the mixed solution to oxidize it, thereby obtaining nickel-containing iron oxide, which was used as an Fe raw material.

[0087] Sodium hydroxide particles as a Na raw material were weighed out relative to 10 parts by weight of nickel-containing iron oxide so that the molar ratio of Na / (Ni+Fe) was 0.95. The raw materials were mixed and pulverized in a sample mill, and the mixed and pulverized product was placed in a crucible and subjected to a solid-state reaction at 400°C for 8 hours. The mixture was then cooled to room temperature and pulverized in a sample mill to obtain nickel-containing sodium ferrite particles. The obtained nickel-containing sodium ferrite particles were further annealed at 600°C for 8 hours, cooled, and pulverized in a sample mill to obtain nickel-containing sodium ferrite particles of Example 3.

[0088] The molar ratio of Na / (Ni+Fe) of the nickel-containing sodium ferrite particles of Example 3 was measured by ICP and was found to be 0.94.

[0089] The sodium ferrite particles of Example 3 had a BET specific surface area of ​​0.7 m ２ The peak observed by XRD measurement was a peak attributed to R-3m (α phase).

[0090] Example 4 After adjusting an iron (II) sulfate solution and cobalt sulfate so that Co / (Co+Fe) was 4 mol%, a neutralization reaction was carried out with a sodium hydroxide solution until Na / (Co+Fe) became 1.00, and air was introduced into the mixed solution to oxidize it, thereby obtaining cobalt-containing iron oxide, which was used as an Fe raw material.

[0091] Sodium hydroxide particles as a Na raw material were weighed out relative to 10 parts by weight of cobalt-containing iron oxide so that the molar ratio of Na / (Co+Fe) was 0.95. The raw materials were mixed and pulverized in a sample mill, and the mixed and pulverized product was placed in a crucible and subjected to a solid-state reaction at 400°C for 8 hours. The mixture was then cooled to room temperature and pulverized in a sample mill to obtain cobalt-containing sodium ferrite particles. The obtained cobalt-containing sodium ferrite particles were further annealed at 600°C for 8 hours, cooled, and pulverized in a sample mill to obtain cobalt-containing sodium ferrite particles of Example 4.

[0092] The molar ratio of Na / (Co+Fe) of the cobalt-containing sodium ferrite particles of Example 4 was measured by ICP and was found to be 0.92.

[0093] The sodium ferrite particles of Example 4 had a BET specific surface area of ​​1.2 m ２ The peak observed by XRD measurement was a peak attributed to R-3m (α phase).

[0094] Example 5 An iron (II) sulfate solution and manganese sulfate were adjusted so that Mn / (Mn + Fe) was 6 mol %, and a neutralization reaction was carried out with a sodium hydroxide solution until Na / (Mn + Fe) = 1.00. Air was then introduced into the mixed solution to oxidize it, thereby obtaining manganese-containing iron oxide, which was used as an Fe raw material.

[0095] Sodium hydroxide particles as a Na raw material were weighed out relative to 10 parts by weight of manganese-containing iron oxide so that the molar ratio of Na / (Mn+Fe) was 0.95. The raw materials were mixed and pulverized in a sample mill, and the mixed and pulverized product was placed in a crucible and subjected to a solid-state reaction at 400°C for 8 hours. The mixture was then cooled to room temperature and pulverized in a sample mill to obtain manganese-containing sodium ferrite particles. The obtained manganese-containing sodium ferrite particles were further annealed at 600°C for 8 hours, cooled, and pulverized in a sample mill to obtain manganese-containing sodium ferrite particles of Example 5.

[0096] The molar ratio of Na / (Mn+Fe) of the manganese-containing sodium ferrite particles of Example 5 was measured by ICP and found to be 0.94.

[0097] The manganese-containing sodium ferrite particles of Example 5 had a BET specific surface area of ​​2.9 m ２ The peak observed by XRD measurement was a peak attributed to R-3m (α phase).

[0098] (Example 6) The same Fe as in Example 1 was used as the Fe raw material. ３ O ４ Fe was used. ３ O ４Sodium hydroxide particles as a Na raw material were weighed out to 10 parts by weight so that the molar ratio of Na / Fe was 1.00. The raw materials were mixed and pulverized in a sample mill, and the mixed and pulverized product was placed in a crucible and subjected to a solid-state reaction at 400°C for 8 hours. The mixture was then cooled to room temperature and pulverized in a sample mill to obtain sodium ferrite particles. The obtained sodium ferrite particles were further annealed at 980°C for 8 hours, cooled, and pulverized in a sample mill to obtain sodium ferrite particles of Example 6.

[0099] The molar ratio of Na / Fe of the sodium ferrite particles of Example 6 measured by ICP was 1.00.

[0100] The sodium ferrite particles of Example 6 had a BET specific surface area of ​​0.1 m ２ The peak observed by XRD measurement was a peak attributed to Pna21 (β phase).

[0101] (Example 7) The same Fe as in Example 1 was used as the Fe raw material. ３ O ４ Fe was used. ３ O ４ Sodium hydroxide particles as a Na raw material were weighed out to 10 parts by weight so that the molar ratio of Na / Fe was 1.06. The raw materials were mixed and pulverized in a sample mill, and the mixed and pulverized product was placed in a crucible and subjected to a solid-state reaction at 400°C for 8 hours. The mixture was then cooled to room temperature and pulverized in a sample mill to obtain sodium ferrite particles. The obtained sodium ferrite particles were further annealed at 980°C for 8 hours, cooled, and pulverized in a sample mill to obtain sodium ferrite particles of Example 7.

[0102] The molar ratio of Na / Fe of the sodium ferrite particles of Example 7 measured by ICP was 1.06.

[0103] The sodium ferrite particles of Example 7 had a BET specific surface area of ​​0.1 m ２ The peak observed by XRD measurement was a peak attributed to Pna21 (β phase).

[0104] (Example 8) The same Fe as in Example 1 was used as the Fe raw material. ３ O ４ Fe was used. ３ O ４ Sodium carbonate particles as a Na raw material were weighed out to 10 parts by weight so that the molar ratio of Na / Fe was 1.03. The raw materials were mixed and pulverized in a sample mill, and the mixed and pulverized product was placed in a crucible and subjected to a solid-state reaction at 700°C for 4 hours. The mixture was then cooled to room temperature and pulverized in a sample mill to obtain sodium ferrite particles. The obtained sodium ferrite particles were further annealed at 900°C for 8 hours, cooled, and pulverized in a sample mill to obtain sodium ferrite particles of Example 8.

[0105] The molar ratio of Na / Fe of the sodium ferrite particles of Example 8 measured by ICP was 1.03.

[0106] The sodium ferrite particles obtained in Example 8 had a BET specific surface area of ​​1.8 m ２ The peak observed by XRD measurement was a peak attributed to Pna21 (β phase).

[0107] Example 9 Manganese-containing iron oxide was obtained as an Fe raw material in the same manner as in Example 5, except that Mn / (Mn+Fe) was set to 50 mol %.

[0108] Sodium hydroxide particles as a Na raw material were weighed out relative to 10 parts by weight of manganese-containing iron oxide so that the molar ratio of Na / (Mn+Fe) was 0.66. The raw materials were mixed and pulverized in a sample mill, and the mixed and pulverized product was placed in a crucible and subjected to a solid-state reaction at 400°C for 8 hours. The mixture was then cooled to room temperature and pulverized in a sample mill to obtain manganese-containing sodium ferrite particles. The obtained manganese-containing sodium ferrite particles were further annealed at 950°C for 8 hours, cooled, and pulverized in a sample mill to obtain manganese-containing sodium ferrite particles of Example 8.

[0109] The molar ratio of Na / (Mn+Fe) of the manganese-containing sodium ferrite particles of Example 9 was measured by ICP and was found to be 0.66.

[0110] The sodium ferrite particles of Example 9 had a BET specific surface area of ​​1.1 m ２ The peak observed by XRD measurement was a peak attributed to P63 / mmc.

[0111] (Example 10) An iron (II) sulfate solution was mixed with manganese sulfate so that Mn / (Mn + Co + Fe) was 40 mol % and cobalt sulfate so that Co / (Mn + Co + Fe) was 10 mol %, and then a neutralization reaction was carried out with a sodium hydroxide solution until Na / (Ni + Co + Fe) = 1.00. Air was then introduced into the mixed solution to oxidize it, thereby obtaining manganese-cobalt-containing iron oxide, which was used as an Fe raw material.

[0112] Sodium hydroxide particles as a Na raw material were weighed out relative to 10 parts by weight of manganese-cobalt-containing iron oxide so that the molar ratio of Na / (Mn+Co+Fe) was 0.66. The raw materials were mixed and pulverized in a sample mill, and the mixed and pulverized product was placed in a crucible and subjected to a solid-state reaction at 400°C for 8 hours. The mixture was then cooled to room temperature and pulverized in a sample mill to obtain sodium ferrite particles. The obtained sodium ferrite particles were further annealed at 950°C for 8 hours, cooled, and pulverized in a sample mill to obtain sodium ferrite particles of Example 10.

[0113] The molar ratio of Na / (Mn+Co+Fe) of the sodium ferrite particles of Example 10 measured by ICP was 0.66.

[0114] The sodium ferrite particles of Example 10 have a BET specific surface area of ​​1.4 m ２ The peak observed by XRD measurement was a peak attributed to P63 / mmc.

[0115] (Example 11) An iron (II) sulfate solution was mixed with nickel sulfate so that Ni / (Ni + Mn + Fe) was 10 mol % and manganese sulfate so that Mn / (Ni + Mn + Fe) was 40 mol %, and then a neutralization reaction was carried out with a sodium hydroxide solution until Na / (Ni + Mn + Fe) = 1.00. Air was then introduced into the mixed solution to oxidize it, thereby obtaining nickel-manganese-containing iron oxide, which was used as an Fe raw material.

[0116] Sodium hydroxide particles as a Na raw material were weighed out relative to 10 parts by weight of nickel-manganese-containing iron oxide so that the molar ratio of Na / (Ni + Mn + Fe) was 0.66. The raw materials were mixed and pulverized in a sample mill, and the mixed and pulverized product was placed in a crucible and subjected to a solid-state reaction at 400°C for 8 hours. The mixture was then cooled to room temperature and pulverized in a sample mill to obtain sodium ferrite particles. The obtained sodium ferrite particles were further annealed at 950°C for 8 hours, cooled, and pulverized in a sample mill to obtain sodium ferrite particles of Example 10.

[0117] The molar ratio of Na / (Ni+Mn+Fe) of the sodium ferrite particles of Example 11 was measured by ICP and was found to be 0.66.

[0118] The sodium ferrite particles of Example 11 had a BET specific surface area of ​​1.6 m ２ The peak observed by XRD measurement was a mixed phase containing P63 / mmc.

[0119] Table 1 shows the composition, synthesis conditions, and properties of the sodium ferrite for each example.

[0120]

[0121] Electrodes were prepared for the powders of Examples 2 to 10 in the same manner as in Example 1, and charge / discharge measurements were carried out under the same conditions. Charging and discharging were confirmed in all sodium ion secondary batteries using the sodium ferrite powder in the negative electrode. The sodium ferrite particles obtained in Example 11 also had a BET specific surface area of ​​1.6 m. ２ / g, it is expected that the secondary battery will have a high initial capacity.

[0122] The negative electrode active material according to the present invention is a sodium ferrite mainly composed of Na and Fe, which are abundant resources, and therefore can reduce the cost of producing the negative electrode, and if the positive electrode is a sodium ion battery, it will not require the use of metals such as Li, and therefore the negative electrode active material according to the present invention can provide a secondary battery that has a low environmental impact when disposed of. Thus, the present invention has high industrial applicability.

Claims

1. A negative electrode active material for secondary batteries consisting of sodium ferrite, the main components of which are Na and Fe.

2. The negative electrode active material for secondary batteries according to claim 1, wherein the sodium ferrite is one or more mixed phases selected from the following group A: Group A: Sodium ferrite having a crystal structure with a space group of R-3m Sodium ferrite having a crystal structure with a space group of Pna21 Sodium ferrite having a crystal structure with a space group of P63 / mmc 3. A secondary battery having a negative electrode using the negative electrode active material according to claim 1 or 2.

4. The secondary battery according to claim 3, wherein said secondary battery is a sodium ion secondary battery.

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